RU2348749C2 - Method for manufacture of pulp and black lye recycling - Google Patents

Abstract

FIELD: textile fabrics, paper.

SUBSTANCE: methods are related to manufacture of pulp from grass raw material and recycling of black lye, especially the one produced in process of pulp boiling and may be used in pulp and paper industry. Methods of black lye recycling are intended for production of inorganic material and synthesis-gas. Methods for recycling of black lye include supply of heated fluidising gas into reactor process area provided with mass of loose material from oxide of alkali earth metal or containing oxide of this metal. Vortical flow of loose material is generated in this area of reactor so that loose material takes the shape of compact strip and circulates around axis of mentioned process area in the form of turbulent flow, creating fluidisated later. At that fluidising gas contains substoichiometric amount of oxygen for partial oxidation of black lye organic material and for transformation of other organic material of black lye into synthesis-gas. Then black lye is supplied into compact strip of loose material so that it is heated up to the temperature of 650-725°C and gasified. From this layer synthesis-gas is extracted in the form of spent gas, as well as inorganic material in the form of hard substance. Method may be used separately or as part of method for transformation of grass raw material into pulp for production of paper or cardboard. Method for production of pulp from grass raw material includes the following stages: (a) digestion of mentioned raw material with application of white lye that mainly contains sodium hydroxide and calcium hydroxide in amount sufficient for complete transformation of silica available in mentioned raw material into calcium silicate; (b) extraction of pulp and black lye free from free silica; (c) heating of black lye in reactor with fluidisated layer that contains calcium oxide for catalytic transformation of organic substance contained in mentioned black lye into gas and for production of regenerated hard substances, which include precious sodium components of mentioned white lye and calcium oxide; and regeneration of mentioned white lye with application of mentioned regenerated hard substances.

EFFECT: higher safety of processes due to exclusion of explosions, their efficiency, lower environmental pollution and equipment contamination, and also possibility to recycle straw of wheat, rice and other materials with high silica content without production of black lye, which is difficult to be recycled.

39 cl, 5 dwg

Description

Field of use of the invention

The present invention relates to a method for the manufacture of pulp from fiber of herbaceous plants, as well as to a method for processing black liquor, which may be a by-product of the aforementioned method for the manufacture of pulp or can be obtained in any other way, for example, in the form of black liquor during sulfate pulping , or it can be a mixture of black liquor for soda pulping and black liquor for sulphate pulping.

BACKGROUND OF THE INVENTION

Sulphate pulping

The main component of wood is long, straight, transparent cellulose fibers, consisting mainly of chains of glucose molecules, which make up about 42 wt.% Softwood and 45 wt.% Hardwood. Hemicellulose is an additional component of wood, and it is a short, branched chain of glucose and other sugar molecules, relatively soluble in water, and it is removed during the pulping process. Cellulose fibers are connected to each other by lignin, which is a three-dimensional phenolic polymer network, through which cellulose fibers are held in a state connected to each other and which gives rigidity. Lignin makes up about 28 wt.% Softwood and about 20 wt.% Hardwood. It is selectively removed during pulping and subsequent bleaching without significant destruction of the cellulose fibers. Extractives comprise about 3 wt.% Softwood and about 5 wt.% Hardwood. These include plant hormones, resins, and fatty acids.

A kraft process, or sulphate pulping, is a more preferable method for chemical pulping of wood, since this method can effectively cope with the resin component of a large number of types of wood. In this method, sodium hydroxide is used as the main cooking chemical and sodium sulfide as a catalyst, and as a result, a more solid finished pulp is obtained than when pulp cooking using only sodium hydroxide. Anthraion is often used as an auxiliary catalyst in both sulphate pulping and soda pulping. During sulphate pulping, wood chips are cooked in a digester under heating and pressure using a “white liquor” (sulphate cooking liquid) (in this case, an aqueous solution of sodium hydroxide and sodium sulfide) to selectively dissolve lignin. After 2-4 hours, the pulp mixture, the cooked chemicals and wood waste are discharged from the digester. The resulting pulp is separated from the mixture of cooking chemicals and waste called "black liquor".

The chemicals used for processing (sodium sulfide and sodium hydroxide) are then recovered from black liquor through a process in which the main unit of equipment is the so-called “Tomlinson furnace”. Black liquor with a content of about 65% dry matter is sprayed in an oven. During sedimentation, droplets of black liquor lose water residues due to its evaporation, and the solid particles are pyrolyzed to form a coal layer in the lower part of the furnace. The coal layer is burned under reduction conditions at a temperature of 750-1050 ° C, and the recovered chemicals, mainly Na ₂ CO ₃ and Na ₂ S, are removed from the furnace in the form of a melt, which is dissolved in water, to produce the so-called “green liquor” , the precursor of white liquor. The gases generated during the pyrolysis and combustion of coal are completely burned in a higher position in the furnace. Flue gases must be thoroughly cleaned to remove mercaptans that are formed under process conditions. The furnace is equipped with an appropriate heat exchanger for the selection of heat from hot gaseous products of combustion, to produce steam and electric energy.

Although the recovery of suitable chemicals and energy can be achieved with commercial use, the use of the Tomlinson furnace is associated with a number of problems. For example, accidental contact of water or diluted black liquor with a melt of inorganic substances can lead to an explosion. In addition, the high temperature of the coal layer leads to increased release of sodium salts and intense pollution of steam lines in the upper part of the furnace. In addition, the technology currently used to process the black liquor stream may, depending on local economic conditions, be viable only with the production of at least 60,000 tons of pulp per year, which can be compared with the typical scale of a modern wood pulp mill , which is more than 360,000 tons of pulp per year. The processing of straw and other grassy materials is, of course, carried out on a much smaller scale, inter alia due to the fact that transportation of bulk agricultural residues, such as straw, over long distances is not economically feasible.

Fluidized bed reduction for sulphate pulping

A number of recovery processes have been described during sulphate pulping, aimed at solving these problems, as well as reducing investment and improving energy efficiency in the recovery of products in which the risk of explosion was excluded when the water contacted with the melt and the level of release of sodium salts by retaining inorganic chemicals in the solid state, not in the form of a melt.

This principle is described in US patent US-A-3309262 (authors: Kopelend et al.), Which discloses a process for processing black liquor in a reactor, which form a fluidized bed of solid particles consisting essentially entirely of residual inorganic materials obtained from black liquor. The process includes the following steps:

(a) concentrating the black liquor by evaporation to a solid content of 20-45 wt.%, where the said liquor contains combustible substances in an amount sufficient to maintain self-combustion;

(b) spraying concentrated black liquor in the free space above the fluidized bed to achieve substantial evaporation in said free space, the remaining additional concentrated atomized black liquor falling into the fluidized bed;

(c) maintaining fluidity of the fluidized bed by introducing, at a rate of 30/150 cm / s, a fluidizing gas containing oxygen in an amount sufficient to achieve complete removal of the organic material in the form of exhaust gas by essentially complete self-combustion in the fluidized bed;

(d) maintaining the fluidized bed at a temperature not leading to melting below the eutectic temperature of the residual chemical mixture in the fluidized bed, but in a temperature range of about 540-982 ° C, to form gaseous products of combustion over the fluidized bed and agglomerates in the fluidized bed of residual inorganic materials contained in black liquor, which are of sufficient weight to prevent their involvement in the fluidizing gas;

(e) unloading agglomerates from the fluidized bed;

(f) withdrawing exhaust gas from a zone above the fluidized bed.

The maximum recommended temperature of the fluidized bed of spent liquor based on sodium is 760 ° C (although the authors believe that this value was exceeded in practice). The introduction of black liquor in the form of a mixture of large and small drops is recommended in order to combine the rapid evaporation of water, an effective cleaning effect, which reduces dust sedimentation, and promotes and controls the agglomeration of particles in the fluidized bed. Oxidation conditions are maintained in the reactor to prevent the formation of hydrogen sulfide (gas), and the conversion of the organic material to combustible gas is not disclosed. The final products are Na ₂ CO ₃ and Na ₂ SO ₄ , which must be recastified to obtain white liquor. Although the patent literature indicates that attempts have been made to commercialize the Kopelend process, the authors believe that it is prone to strong agglomeration of the fluidized bed, especially in the processing of black liquor, which has a relatively low calorific value, resulting from the cooking of straw, and that the process is not applied from due to insufficient technical and commercial viability. The authors' experience tells them that simple fluidized beds of the type such as those proposed by Kopelend are subject to unacceptable agglomeration, which makes this method impractical immediately after a short initial period.

US Pat. No. 3,523,864 to Osterman discloses a method for recovering black liquor from sulphate pulping using a reactor in which the lower, middle and upper fluidized beds are arranged one above the other, each of which formed from CaO granules. The lower fluidized bed operates at a temperature of 704-760 ° C and contains the reaction products in the solid state, where Na ₂ SO _{4 is} reduced to Na ₂ S. The middle layer operates at a temperature of 648-704 ° C, and black liquor and preheated are fed into it air in an amount of about 30% required for complete combustion to produce Na ₂ CO ₃ and Na ₂ SO ₄ , which are deposited on the surface of CaO granules together with the gases formed by the combustion products and organic substances. The upper fluidized bed accepts reusable CaCO ₃ , which is calcined to regenerate CaO and provide material for the fluidized beds, which are gradually lowered from the upper to the lower layers. The upper gases formed by the combustion products are partially recirculated in the form of fluidizing gases, and after processing in a cyclone, they are partially sent to a steam generator. Again, all three fluidized beds are simple fluidized beds of a boiling type, with the middle layer being the site of unacceptable agglomeration for the reasons already mentioned.

There are two additional reasons for the lack of commercial use of these low-temperature fluidized bed processes: firstly, the relatively high temperature required for the rapid and complete conversion of Na ₂ SO ₄ to Na ₂ S, and secondly, the easy formation of H ₂ S upon contact Na ₂ S with gases formed by the products of combustion at a temperature below the melting point of inorganic salts. Thus, while high temperatures are preferred for reduction, the alternative methods described above require a relatively low temperature, just below the melting point of the inorganic salt mixture. The consequence of this is that in processes that use the fluidized bed in the recovery mode, most of the formed Na ₂ S is quickly converted to H ₂ S (and a certain amount of COS) according to the total reaction:

Na ₂ S + CO ₂ + H ₂ O → Na ₂ CO ₃ + H ₂ S,

leading to a low level of recovery of solid Na ₂ S.

For the sake of completeness of explanation, mention should be made of US patent US-A-4011129 (author: Tomlinson), which describes a method for increasing the ability of chemical regeneration when using a regenerative Kraft furnace by injection of solid granules of sodium sulfate and sodium carbonate directly into the coal layer in the recovery zone of the furnace at maintaining the temperature and reducing gas environment in this zone, thereby obtaining a melt containing sodium sulfide and sodium carbonate from the injected granules. These granules can be made from an additional amount of black liquor in an auxiliary calcination furnace, for example, in a fluidized bed incinerator, the use of which allows to increase the regeneration performance without the need for an additional regeneration furnace.

Production of non-wood pulp

The use of agricultural waste from herbaceous plants remaining after annual harvesting can solve many of the problems associated with obtaining pulp and providing it with the paper industry, including the provision of fiber, as well as problems related to the interests of farmers regarding the cost and availability of agricultural waste, as well as problems related to consumer interests in connection with limited forest resources.

In the broad sense of the word, grassy crop residues are materials remaining after the annual harvest of agricultural crops that implements their main or intended purpose. These residues include: straw from crops, such as wheat, rice, barley, oats; straw of seed grasses, for example flax, rye; crushed stalks of sugarcane, known as "bagasse"; sorghum, corn stalks, and other agricultural residues, such as cotton fluff (short fiber) on cotton seeds after cotton picking. In countries where there is little or no wood stock, pulp from straw and bagasse are used in large volumes for the manufacture of paper (up to 90%) for high-quality printing and writing paper. For example, there have been reports that more than 85% of pulp is used in China to make paper made from non-wood raw materials, with straw being the dominant source. It has been reported that in India, approximately 55% of the pulp is used to make paper from non-wood raw materials, about half from agricultural residues. Since the burning of agricultural waste is prohibited by law on an increasing scale, new incentives are emerging for the development of alternative uses of these resources. With proper land use, farmers can provide small pulp mills with continuous sources of fiber, while at the same time supporting grain production.

Agricultural residues, such as wheat and rice straw, contain pulp and can serve as good raw materials for paper production. As mentioned above, these raw materials are bulky, and the cost of their transportation and logistics suggests that they are best processed into pulp on site and, therefore, on a relatively small scale, about 10-100 tons of pulp per day. Pulp mills produce industrial waste in the form of black liquor, which, if discharged into local rivers, water bodies, will lead to serious environmental pollution. The absence of an economically viable technology, the use of which would allow to process the flow of black liquor resulting from a pulp mill capacity of up to 60,000 tons / year, means that many existing small pulp mills are forced to shut down in order to stop pollution of rivers and water bodies. This lack of suitable technology also hindered the creation of new small pulp mills, in particular new mills where agricultural waste could be used. Due to the inadequate need for small pulp mills, there has been little research and technology improvement for small pulp mills. As a result of this, the production technology for small pulp mills and, in particular, the technology for the production of pulp from straw, as was not the case for technology for large-scale production of wood pulp, was not developed.

Straw can be processed into pulp chemically and in a combined mechanical and chemical way (chemical-mechanical pulp production). For cooking non-wood raw material, it is recommended to use sodium hydroxide alone as an active chemical substance, since most of the non-wood fiber does not contain adhesive resins, and sodium sulfide is not necessary as a catalyst. For this reason, most of the chemical pulp from the raw material of this class is obtained by a method called "soda pulping", in which the raw material is heated together with a cooking liquid with a high alkali content, including sodium hydroxide, to a temperature of 140-170 ° C under pressure. Under these conditions, the bulk of the lignin dissolves. Sodium hydroxide from the black liquor obtained in the process can be reduced, and the organic substances present in the black liquor can be used as fuel for generating energy. In contrast to sulphate pulping, which requires sulphate to be converted to sulphide for recovery, the black liquor produced by sintering pulp can be burned even under highly oxidizing conditions. Chemical reduction thus includes the steps of evaporating black liquor to an appropriate dry matter content and burning the evaporated liquor by using excess oxygen. Inorganic residual combustion products, consisting mainly of sodium carbonate, are dissolved in water and recustored with quicklime to regenerate sodium hydroxide, which is reused. In an embodiment, slaked lime Ca (OH) _{2 was} used in a mixture with NaOH as active chemicals in white liquor, as it is also used as a cooking reagent, and it is cheaper. However, a method for processing black liquor in such a way that NaOH / Ca (OH) ₂ can be reused is not described, and such types of black liquor were simply disposed of in the past without recycling.

Silica in a pulp of grassy starting materials

Due to the relatively high silica content in straw and other non-wood cellulosic agricultural products, difficulties are associated with chemical reduction. Wheat straw contains 4-10 wt.% Silica in the form of small crystals embedded in the straw. Rice straw contains even more silica, namely 9-14 wt.%. Straw of other cereal crops, such as barley, oats and rye, contains 1-6 wt.% Silica. Wood, in contrast, contains less than 1 wt.% Silica. In the pulping process used for boiling straw, most of the silica contained in the straw reacts with sodium hydroxide to form water-soluble sodium silicate, which remains in the black liquor in addition to lignin and other organic compounds. Processing black liquor with a high content of silica leads to an increase in the formation of scale (covering the equipment with a glassy substance), especially during the evaporation process. You can use a modified recovery method designed for wood processing, if the silica content in the straw of cereal crops is less than 5-6 wt.%, But at the same time both capital costs and production costs increase. However, to date, no method has been developed for processing products with a higher content of silica, especially rice straw, which would be technically and commercially viable.

Our international application WO 03/014467 (the contents of which are incorporated into this application by reference) describes a method for processing elongated raw materials suitable for use in a paper mill, comprising the following steps:

- extraction of extraneous materials from raw materials;

- crushing of the raw material from which extraneous materials are removed, for the destruction of nodes;

- splitting the crushed raw material in the longitudinal direction;

- feeding the split raw material into a screw conveyor with rotary screws rotating in one direction, divided into many zones, and processing said material in said conveyor to obtain pulp and a stream of black liquor;

- supply of the processed material in at least one zone;

- regulation of temperature and / or pressure in at least one zone;

- spraying concentrated black liquor in a process tank in the form of a reactor in which a fluidized bed is formed for processing said black liquor, said process tank being part of the means for regenerating the material and energy being processed. The alkali supplied to the screw conveyor with the screws rotating in the same direction to carry out the pulping process may contain sodium hydroxide and additionally calcium hydroxide, with the help of which silica is deposited on cellulose fibers and prevent silica from entering the black liquor in the form of calcium silicate.

International application WO 03/014467 further describes a black liquor processing process in which a black liquor stream discharged after a pulping process is collected in a cooking liquid storage container and concentrated to a solid content of 30-70% using a standard evaporator designed to increase concentration. If the black liquor stream contains 30% or more solids, it can be processed directly in the process tank, bypassing the evaporation step. Concentrated black liquor is sent to the reactor at a temperature above 90 ° C using a pipeline or a hermetically sealed twin-screw conveying device. A hermetically sealed conveying device is used to minimize the loss of organic components due to evaporation. A temperature above 90 ° C is required to reduce the viscosity of the black liquor so that it can be transported without resistance. Black liquor is treated in a reactor in which a toroidal fluidized bed is formed. Although a temperature of 650 ° C is indicated as the upper limit of the temperature of the fluidized bed, in practice the maximum temperature ever used was only 610 ° C. This was because, as was shown, at temperatures above 600 ° C, the evaporation of inorganic particles of alkali metals present in black liquor (for example, Na and K) took place in other processes. When these compounds are in the vapor phase, additional processing equipment is required to restore them, which leads to an increase in the total cost.

SUMMARY OF THE INVENTION

The use of a higher reaction temperature provides the advantage of an increased reaction rate in the processing of black liquor, and therefore, productivity can be improved while maintaining the quality of the product. We found that in the above-mentioned black liquor recovery process, a temperature above 650 ° C can be used. Recent experiments have shown that the loss of inorganic substances contained in black liquor when heated to a temperature in the range of 650-700 ° C or even 725 ° C in the fluidized bed was minimal, i.e. the losses were not economically significant and, therefore, no additional equipment was needed to restore them.

According to the invention, a method for processing black liquor to produce an inorganic material and synthesis gas containing, as components, CO ₂ , CO, H ₂ O and H ₂ together with methane and C ₂₊ components, comprising the following steps: the availability of a reactor containing a technological field equipped with a mass of granular (granular) material consisting of alkaline earth metal oxide or containing alkaline earth metal oxide; supplying heated fluidizing gas to the process area to generate a vortex flow of bulk material so that the bulk material takes the form of a compact strip and circulates around the axis of the process area in the form of a turbulent flow, forming a fluidized bed; moreover, the fluidizing gas contains a substoichiometric amount of oxygen to partially oxidize the organic material present in the black liquor and to convert the other organic material present in the black liquor to synthesis gas; feeding black liquor into said compact strip consisting of granular material, so that it is heated to a temperature in the range of 650-725 ° C and gasified; recovering said synthesis gas in the form of exhaust gas from said layer containing <1 vol.% oxygen; recovering inorganic material from black liquor as a substance from said layer. Black liquor can be obtained from soda-type white liquor, which may contain calcium hydroxide in an amount effective to convert harmful silica of the grass material into calcium silicate. Black liquor can be pre-mixed with calcium oxide in the ratio of calcium oxide to the dry substance contained in black liquor, from 0.1: 1 to 1: 1 to obtain a granular brittle material, and this material is then fed into a fluidized bed reactor. Sodium hydroxide and / or sodium carbonate and calcium hydroxide and / or calcium carbonate are obtained in a reactor in which a fluidized bed is formed.

In the reactor, black liquor can react with an alkaline earth metal oxide to form a mixture of sodium hydroxide, sodium carbonate, alkaline earth metal carbonate, a volatile gas and a liquid component containing a combustible component, which can be used as a fuel, as, for example, in a conventional processing process, for example in the boiler.

An alternative aspect of the invention is the creation of a method for processing grassy materials, which reduces the effect or eliminates the disadvantages associated with the high content of silica in the resulting black liquor.

The invention further provides a method for converting grassy raw material into pulp for the manufacture of paper or paperboard, comprising the following steps: digesting said raw material using white liquor containing mainly sodium hydroxide and additionally containing calcium hydroxide in an amount effective to substantially completely convert silica present in said raw material in calcium silicate; the selection of pulp obtained at the stage of cooking pulp; recovering the black liquor obtained in the pulping step by washing said boiled pulp and, optionally, also by extracting black liquor directly in said pulping step; wherein said isolated black liquor is substantially free of soluble silicate; heating the black liquor in a reactor in which a fluidized bed containing calcium oxide is formed to catalyze the conversion of the organic substance contained in the black liquor into gas and to produce reduced solids including the valuable sodium components of white liquor and calcium oxide; white liquor regeneration using said recovered solids.

The grassy raw material in this case is wheat straw, rice straw, bagasse. In this method, the recovery of black liquor includes the step of combining a stream of black liquor coming from the pulping step with a stream of black liquor coming from the washing step of the pulp. The black liquor is concentrated by evaporation before heating in said fluidized bed reactor. Black liquor is concentrated by evaporation to a solids content of 20-40 wt.%. Fluidizing gases contain at least stoichiometric amounts of free oxygen to completely oxidize the organic material contained in the black liquor. Fluidizing gases contain substoichiometric amounts of free oxygen to partially oxidize the organic material contained in the black liquor and convert the other organic material contained in the black liquor into hot exhaust gas. The method further includes the step of supplying fluidizing gases and black liquor so as to create off-gas above said fluidized bed containing <1% oxygen. Under certain conditions, there is a risk that substantial amounts of silicate can enter the liquor streams during the pulp washing process. However, the inclusion of calcium hydroxide liquor in the cellulose pulp used leads to the fact that the silicate is included in the black liquor in the form of predominantly calcium silicate than in the form of sodium silicate, or with essentially the exclusion of sodium silicate. The presence of calcium silicate is much less likely to cause downstream processing problems than the presence of sodium silicate. In an additional aspect, the invention relates to the use of calcium hydroxide as an additive in the pulping of pulp from a grass source material for the production of pulp while inhibiting scale formation during the concentration and regeneration of black liquor in the processing of black liquor, partially at least obtained by washing the pulp . In this application, black liquor is obtained partly from a digester and partly from a pulp washing process where silica is contained in said black liquor in the form of calcium silicate, while substantially eliminating the presence of silica in the form of sodium silicate. The above use is whereby a flocculant is added during washing to suppress dispersion of calcium silicate, wherein said flocculant is polyacrylamide.

Description of drawings

The invention is further described in more detail by way of example with reference to the drawings, in which:

figure 1 is a General block diagram of a process for the manufacture of pulp from wheat straw according to the invention;

figure 2 is a schematic view of a roller device for use in the pre-processing of raw materials that are part of the manufacturing process of the pulp shown in figure 1;

figure 3 is a schematic view of the design of a self-cleaning annular shaft, which can be used in the roller device shown in figure 2;

figure 4 is a schematic view of a possible embodiment of a twin-screw conveyor with rotatable screws in one direction, which can be used to turn straw into pulp in the process illustrated in figure 1;

figure 5 is a block diagram of a preferred apparatus for processing a stream of black liquor, which can be used in the process illustrated in figure 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An overview of the processing method of wheat or rice straw

The present method is described by way of illustration, with reference to the processing of wheat straw, which is usually chopped before pulping and which contains nodes on the stems, usually remaining intact, if the straw is chopped before pulping. This is a serious drawback in the production of high quality pulp for paper production, resulting in poor quality paper. For this reason, it is preferable to use a method in which the nodes are destroyed by opening the stalks of straw in the longitudinal direction in a gentle manner, and the raw material is fed into the digester in a forced dosed and continuous process. The straw to be processed is fed by a feed conveyor 9 to a pre-processing unit 10, where the stems are crushed between the shafts, foreign material is removed, and the stems are split in the longitudinal direction. Conditioned-quality straw is then fed to the cooking apparatus 12, where it is subjected to mechanical processing in the presence of an aqueous solution of alkali (white liquor), while at the same time subjecting it to elevated temperature and pressure. The resulting black liquor is then sent to a stream processing unit 14, where it is processed by exposure to heat to form a solid, from which the so-called “green liquor” can be obtained. This liquor, in turn, is brought into contact with the lime obtained from the CaCO ₃ feed and regenerated to white liquor for reuse in the pulping plant 12. The waste gas captured during the thermal processing of black liquor can be used to produce steam and heat. Solids flowing out of CaCO ₃ lime sludge are removed to prevent excessive accumulation of trace metals in white liquor.

Pre-processing

If the raw material from which the pulp is to be made is straw, then it can be used in the form of chopped straw, straw that has been chopped in the longitudinal direction or chopped, or straw that has been chopped in the longitudinal direction and / or chopped and cabin.

According to a preferred embodiment of the pre-processing method, after loosening the straw bale, the straw is fed to the conveyor belt 101 (see FIG. 2), where dust, heavy objects, such as stones, and other foreign objects, such as plastic ropes, are removed from it. Then the straw is sent to the hopper feeder 103, from which the straw is fed to the installation containing corrugated shafts 105 and 107, where the nodes are crushed in the straw stalks, and into the shafts with pegs, by means of which the straw stalks are split in the longitudinal direction in a gentle manner. Thus, straw is passed between the first and second corrugated crushing rollers 105 and 107 rotated in opposite directions to crush the nodes in the straw stalks. The crushed material is then passed through a pair of intermediate shafts 109 and 111 rotated in opposite directions, with the help of which they prevent damage to the shafts located below, due to the ingress of any foreign materials into them. Then the straw is passed through a pair of shafts 113 and 115, rotated in this case in one direction. These last shafts are provided with pegs, by means of which they chop and chop straw in the longitudinal direction and act on it together with the feed shoe. As a result of the action of this mechanism, straw is turned into a short, loosened and crushed material without knots. This allows for improved and faster penetration of chemicals and steam, and thus faster production of a more uniform pulp and, at the same time, allows the fiber to be processed in a gentle manner to maintain its length. This results in the manufacture of improved pulp mass, including a very significant reduction in visible “spangles” in the paper sheet due to dispersion of parenchyma cells, improved drainage, higher tensile and tear strength, higher pulp yield and lower chemical demand.

The treated straw is then dumped from the annular shafts 113 and 115 into a hopper feeder 117, from which it is sent either to a conveyor or to a pneumatic pipe (not shown), with which the processed straw is fed to the lower reserve hopper for intermediate accumulation of the prepared material for pulping. The ring and grooved shafts of the ripper and feed unit mentioned above are specially designed for straw processing, but, with minimal modification, can be used for processing any other suitable raw materials, including flax, hemp, bagasse and wood chips or sawdust.

Ring shafts can also be made in such a way that they are self-cleaning when used to process cellulosic raw materials containing longer fibers, such as hemp and flax. This is done to prevent fiber winding around the shafts and clogging the installation. A diagram of the working part of the annular shaft is shown in FIG. The annular shaft 120 has an outer surface provided with a large number of radially protruding pegs 122. They are used with interconnected perforated or woven tape 124, on which the processed material 126. The pegs 122 capture material 126, and when the tape 124 comes off the pegs, they are removed with it the processed material, leaving the annular shaft free from entangled fibers.

Tests of the process described above were carried out in a pilot laboratory installation and improved the process for straw, flax and hemp. In addition, the above-described method of processing raw materials using chippings can be used to process wood raw materials if the wood is processed into shavings rather than wood chips, which was verified by using wood fibers treated with chippings instead of wood chips. This method of preparation of raw material is especially suitable when the wood contains short fiber, since the combination of the processes of processing wood into chips and processing with its splits allows you to save the length of the fiber.

Cooking of grassy raw materials and other cellulosic starting materials

Although chopped straw can be used as the feed product, the preferred feed material of the pulping plant of the present invention is straw or other stem material from herbaceous plants that has been longitudinally split and / or pulverized. In such a material, the white liquor used for cooking pulp easily comes into contact with stem material from herbaceous plants and with any other material containing nodes, dissolving silica in it and digesting lignin and other substances susceptible to alkali.

For cooking non-wood raw material as an active chemical, only one sodium hydroxide is required, and for this reason, most of the pulp from these raw materials is obtained using a process called soda pulping. In this process, the raw material is heated together with cooking liquor containing sodium hydroxide to a temperature in the range of 140-170 ° C or, in some cases, to 180 ° C under pressure. Cooking liquor should have a high concentration of alkali. Under these conditions, most of the lignin contained in the raw material dissolves; however, the bulk of the silicon in any form contained in the raw material reacts with sodium hydroxide to form a water-soluble sodium silicate. Thus, the black liquor resulting from cooking additionally contains, in addition to lignin compounds and other organic substances, silicate ions. Thus, white liquor is preferred for use with a soda liquor (i.e., without sodium sulfide), which additionally contains calcium hydroxide in an amount sufficient to effectively precipitate silica. Calcium hydroxide is either already present in the white liquor used to treat the herbaceous raw materials, or calcium hydroxide is added some time after the introduction of sodium hydroxide, but in any case, the amount of calcium hydroxide should be sufficient to precipitate silica on straw fibers in the form of calcium silicate to reduce soluble content silicate in the resulting black liquor. Calcium hydroxide must be present in an effective amount to convert essentially all of the silica, or a desired portion thereof, into soluble silicate, most of which is deposited on straw fibers before the extraction of black liquor from partially or completely digested straw.

Straw or other non-wood cellulosic material can be digested using a continuous digester, for example a single-cylinder or multi-cylinder screw-fed digester, for example a Pandia digester supplied by Lenzing Technik GmbH & Co FG. The use of high-speed cooking of vegetable fiber in a horizontal cylindrical continuous digester with a screw feeder is considered in the article by J. Atchison. “Continuous horizontal cylindrical cooking kettle for high-speed cooking is currently the standard equipment for cooking pulp from non-wood fiber”, Materials of the conference on the production of pulp, 1990. The article explains that this technology was first developed for pulping from bagasse, but the possibilities of using it for other forms of non-woody plant materials, including wheat and rice straw, were also evaluated. It is argued that the technology allows you to bring the cooking time to only 10-15 minutes when processing bagasse, straw and most other non-wood vegetable fibers, in contrast to previously used cooking methods using rotary loading cooking boilers, for cooking cycles in which four cooking cycles were required hours and more. According to the experience of the authors of the present invention, these indicators mentioned are overestimated, and it is impossible to produce a semi-cellulosic mass in less than 20 minutes unless economically ineffective amounts of cooking chemicals are used. Using a screw feed allows you to increase the density of the feed material and, thus, increase the productivity of the digester, as well as improve the continuous mixing of white liquor and feed material. Again, according to the experience of the inventors, the digester usually contains 2, 3 or 4 series-connected cylinders, each of which has a diameter of about one meter, so that the installation has significant dimensions and cost.

Due to the rapid absorption by straw or bagasse under conditions when these materials are subjected to pressure and elevated temperature in a horizontal cylindrical digester, especially after the splitting operation described above, pulping starts immediately and proceeds quickly. White liquor can be a type of soda or sulphate cooking liquid, and 12-14 wt.% NaOH or 6-7 wt.% NaOH and 6-7 wt.% Na ₂ S, based on dry slurry mass, at a pulping temperature of 170-180 ° C and a pressure of 7-9 bar; however, it is alleged that the cooking time is 10-15 minutes for the production of pulp and 3-5 minutes for the production of half cellulose. As explained above, these values are, in the opinion of the authors of the present invention, unattainable in practice.

In a preferred method of cooking pulp from bagasse or straw (including rice straw), a horizontal cylindrical digester is used, in which the straw is transported by a conveyor based on the rotation of two screws in one direction, the turns of which go one between the other. Such a digester can be created in such a way that it has physically small dimensions in relation to its productivity and, therefore, has a lower cost in comparison with equipment for implementing competing technologies. A significant advantage is that pulping can be done using a small amount of water, which allows to obtain black liquor of high concentration and reduce or eliminate the need for subsequent evaporation of black liquor. Chopped and / or chopped straw is continuously fed from the storage to the digester, into which white liquor and steam are injected through the holes in the boiler or, instead of steam injection, the boiler is heated from the outside by electric means. During the rotation of two augers with turns mutually overlapping one another, the straw and black liquor are mixed and the straw is processed mechanically and cooked. A pair of screws rotate in one direction, which reduces the mechanical impact during processing, reported fiber, and thus minimize damage to the fiber. In a twin-screw unit, transportation along the boiler is carried out mainly due to the mutual entry of the turns of one screw between the turns of the other, while in a single-screw device, transportation is carried out due to the material being trapped between the forward turns and the stationary walls of the boiler. A single-screw installation is thus less efficient due to friction, and increased slippage can occur in it (pressure in the boiler causes straw to slip between the screw and the housing wall) and ripple. The material in the digester with two screws rotating in the same direction moves along a figure-eight trajectory and thus travels a longer path than if the screws were rotated in opposite directions, which provides better mixing of straw and black liquor. The state of the art in relation to twin screw cookers used for the manufacture of pulp paper is shown in US Pat. No. 4,088,528 to Berger et al. And US Pat. No. 4,214,947 to Berger, contents which is incorporated into this application by reference. It should be noted that the inventions disclosed in both of these patents are directed to the processing of wood chips, and not to the processing of grassy material.

The tests showed that using a single digester such as a twin-screw boiler, it is possible to convert chopped and chopped straw with a length of about 25 mm into a pulp or semi-pulp mass having a Kappa number in the range of 30-70 and suitable, for example, for the production of cardboard or boxes, corrugated packaging material, and the like. In a series of experiments, straw was fed into the inlet of a digester with two screws with a diameter of 40 mm, where the pitch of the screws was variable along the length of the screws, and five processing zones were determined by temperature ranging from 90 ° C at the inlet to 165 ° C in the penultimate zone. Alkali was introduced using a metering pump with enough water to achieve a certain ratio of water to solid in the digester. It was found that the temperature in the cooking zones downstream of the digester is preferably maintained at a level not lower than 165 ° C. to obtain a well-disintegrated pulp. It was concluded that it is possible to achieve Kappa numbers in the lower quarters of a relatively short twin-screw digester with a diameter of 40 mm for a period of time less than one minute, and when caustic is added less than 10%, which represents a quality corresponding to a semi-cellulosic mass. It was estimated that on a full-scale machine, the cooking temperature can be 170 ° C, which is common for the manufacture of a pulp using a continuous screw digester.

Table Experiment No. one 2 3 four 5 6 7 Temperature 165 165 165 165 165 165 165 Screw speed 120 120 120 120 120 120 fifty Straw feed 6 3.75 6 6 6.3 6 3.75 The ratio of liquid / fiber (to 1) 1,6 2.5 1,6 1,6 1,5 3.2 3.36 Caustic / Fiber Ratio (%) 9,4 fifteen 9,4 9,4 9 9,4 8.3 Kappa number 45.8 45.8 43.7 43.9 39.9 60.5 42.3

It is predicted that when using a twin-screw extruder of a commercial scale, it is possible to obtain fully digested pulp suitable for the production of printing and writing paper, without an additional cooking step with a holding time of, for example, about 1 minute and a cooking pressure of 7 bar. In an alternative embodiment, a twin-screw extruder can be used to make semi-cellulosic pulp in the first stage, and this pulp can be converted into pulp in the second cooking step, for example, by cooking for 30 minutes at a pressure of 7 bar before dewatering the pulp.

An embodiment of a twin-screw digester 131 with screws rotating in the same direction is shown in FIG. 4. Grassy raw material (straw, flax, hemp, bagasse), wood chips or any other cellulosic raw material from the backup storage can be processed into pulp. For this purpose, the raw material is fed into the digester 131, in which the screws are specially made with two end sections 133 and 134 containing turns going in the first direction, while in the middle section 135 the direction of the turns is reversed. The turns of the conveying screws are made of hardened steel with processing of turns at a large cutting depth to improve the size of the area in which the raw material is forcibly transported, according to the above explanation, and are specially designed to minimize fiber damage. The use of this particular design leads to the fact that a reduced amount of energy is required, which means that a drive shaft and smaller gearbox can be used, which also leads to a reduction in capital costs. The use of the design of the profile of the screw and the drive shaft of a reduced size also allows you to increase the skip speed of the raw material (productivity), which should be increased by the expected 400% compared with the performance of a conventional twin-screw device with screws rotating in one direction.

As shown schematically in FIG. 4, one embodiment of the conveyor comprises a first zone 137 into which raw material is fed through a hopper feeder 139. The turns of the conveyor screws in zone 1 are made so that they are opened as much as possible so that they can be opened with their help it was possible to power the device with material. In the second zone 141, cooking liquor can be supplied through inlet 143, which is preferably white liquor for the soda brewing process, not containing a significant amount of sulfide, but containing Ca (OH) ₂ ; and steam can be introduced into the second zone of the digester through the inlet 145. The length of zone 141 and the exposure time of the raw material in this zone can vary depending on the nature of the material, but they must be sufficient to digest significant amounts of lignin located in the areas of the starting material, easily available for cooking liquor (light lignin) and for dissolving other readily soluble material.

As explained above, it is desirable to convert the silica present in the straw or other grassy starting materials into insoluble silicate, most of which is deposited on the cellulose fibers when the straw is digested, to prevent harmful amounts of silica from entering the black liquor stream in the form of soluble silicates, the presence of which may lead to an increase in scale in evaporators or other parts of downstream chemical recovery equipment. For this purpose, when cooking straw or other grassy materials, 141 calcium hydroxide can be added to the second zone with a consumption of 4%, based on the dry weight of the raw material (straw), and with a sodium hydroxide consumption of 8%. In general, about one mass part of calcium hydroxide is used per two mass parts of sodium hydroxide. This method is applicable to any alkali-based pulping process, and consists in re-precipitating sodium silicate on cellulose fibers in the form of calcium silicate, with some, most or most of the calcium silicate remaining in place these fibers (depending on the conditions of the subsequent processing during pulping), when they are separated in the form of pulp and during subsequent washing, bleaching and turning into paper. In a mixed alkaline composition, in which both NaOH and Ca (OH) ₂ are present, a reaction with the formation of insoluble CaSiO ₃ is very desirable in comparison with a competing reaction with the formation of soluble NaSiO ₃ in which silica is retained on the fiber in the form of calcium silicate or gets into black liquor in the form of insoluble calcium silicate. As a result of this, there is no significant difference, in terms of soluble silica content, between the black liquor from the herbaceous material treated with NaOH / Ca (OH) ₂ and the black liquor obtained by cooking wood pulp, which can be adjusted during processing by known methods. Precipitated calcium silicate, once formed, is not prone to re-dissolving under the conditions encountered in subsequent pulp processing operations, including bleaching, dehydration and the manufacture of paper or paperboard, none of which use low pH (pH <4) ; and it just goes into the harmless part of the ash from the pulp, being chemically similar to wollastonite, which can be used as a filler in the manufacture of paper, and also like kaolin, which is a complex silicate. The only difference is that, if further processing is envisaged, the pulp from grassy materials may, for example, contain 3-4% ash, while the ash content in wood pulp is usually about 1%. It should be noted that in the installation 14 for processing the stream, described below, in which black liquor is gasified using a CaO fluidized bed, a white liquor is obtained that naturally contains Ca (OH) ₂ and also regenerated NaOH; this black liquor is obtained by digesting the feed, and in this case, calcium silicate is deposited on the cellulose fiber so that it does not enter the black liquor in harmful quantities.

The partially digested raw material is transferred to the third zone 147, where the conveying screws have a reverse direction of turns 135, which acts as a braking zone for the advanced raw material, forming a tube from the processed material and thus creating a high pressure zone upstream of the tube. In this zone, the wall of the housing contains a perforated region 149, through which a certain amount of cooking liquor is pressed. The effect of white liquor on the raw material is to quickly dissolve all or most of the easily accessible lignin of the raw material, which dissolves with soluble hemicellulose and other soluble organic solids. In the third zone, a portion of the white liquor, which may typically correspond to approximately half of the originally introduced white liquor, leaves region 149 in the form of a stream of black liquor with a high solids content, typically comprising about 30% by weight of solids. The removal of light lignin, soluble hemicellulose and other soluble organic substances with this stream of black liquor means that these substances are no longer contained in the pulp processing area and do not interfere with the alkali attack on the lignin remaining in the partially digested raw material in subsequent zones, which contributes to the performance later stages of pulping, and also provides a stream of black liquor with a high solids content, which is a contribution to the regeneration of the final unit ennogo black liquor stream of relatively high solids content. The rest of the hull and conveying augers define the fourth zone 151 and the fifth zone 132, leading to the release of 153 pulp. The temperature and pressure in the fourth zone are elevated, as mentioned above, so that pulping continues in the white liquor remaining in the pulp, which also serves to lubricate the partially digested source material as it moves through the digester body. In the fifth zone, the temperature and pressure are lowered in order to prepare the material for release from the twin-screw digester. The material is passed through a twin-screw device for 2-3 minutes. The screw speed can be 200 rpm. It should be borne in mind that although four to five zones are formed in the twin-screw digester shown, any number of zones can be created, suitably from three zones, to fulfill any desired processing mode.

The digester is preferably modular in design, which allows the upgrade of both the screw structure and the housing structure. This should be an effective, from the point of view of investment, way of using one standard twin-screw device for processing many different types of cellulosic raw materials and / or processing different grades of pulp obtained by changing the configuration of the screws and the casing. You can use the machine speed in the range of 50-500 rpm. In practice, a speed of 50-250 rpm was used. The speed needs to be regulated depending on the raw material used and on the quality of the pulp required. A twin-screw digester can be made so that chemicals and liquids can be injected, and liquids or steam can be removed in each zone, which is a standard feature of a twin-screw extruder. It was also found that there is no need to use a complex gearbox and drive of the type commonly used in twin-screw extruders to process the corresponding pulp. A simple gearbox and drive can be used, thereby reducing equipment costs and energy consumption. It is contemplated that pulp cooking equipment should consume less than half the energy that is consumed using a conventional twin-screw device used for this purpose. In another embodiment of a digester (not shown), the feed zone of the conveying screw device is enlarged in comparison with other zones to allow free supply of raw material to said zone to increase conveyor productivity. As the raw material moves forward into the processing zone and into the first and second pressure zones, the space in a twin-screw conveyor with screws rotating in the same direction can be gradually reduced, as a result of which the pressure in these zones will gradually increase.

The used twin-screw conveyor with one-way rotating screws, with a housing size of 100 mm, described below, was a twin-screw extruder with one-way rotating screws and turns of screws, one between the other, in which five zones were defined.

Table Zone one 2 3 four 5 Type of Straw feed Recycling:
steam / alkali Extraction
source liquor Cooking Cooking/
unloading Temperature ° C 65 one hundred 130 150 130 Pressure bar 0 0 2-3 4-5 2-3

Pulp discharged from a twin-screw device may contain approximately 50% moisture and is expected to have a Kappa number of 30-40. This is an unbleached pulp ready for bleaching using standard methods and suitable for the production of printing and writing paper. A semi-cellulose pulp with a higher Kappa number, suitable for use as corrugated packaging material, can also be made, if required. The result of the operation of this device is a function of speed (r / min) and the design of the turns or the time spent by the twin-screw extruder, along with the pressure, temperature and amount of chemicals used in the pulping process. It is expected that Kappa numbers of the order of 20 can be achieved using a single screw extruder.

If full pulp pulp is not achieved in the twin-screw cooker 131, as expected, then it may be necessary to additionally cook the pulp, for example, in an additional twin-screw cooker or in a single-screw digester of the type described by Atchison, as described above, for example, using steam at a pressure of 1-2 bar (120 ° C) for an additional 20-40 minutes and, optionally, by introducing an additional amount of white liquor. Achison described the use of two horizontal cylindrical digesters, located one above the other, with the release of the product from the upper digester and the supply of partially boiled material to the inlet of the second digester. In practice, in the prior art, it was often necessary to use three such digesters arranged in series. However, when using the twin-screw digester described above, significant decomposition of the grassy or other starting material is achieved, and it is expected that only one additional twin-screw or single-screw digester may be required to produce pulp ready for bleaching, so that the size of the installation and its cost can be shortened. After using this additional digester, you can achieve a Kappa number of 14-20.

The pulp discharged from the twin-screw digester or obtained after the subsequent stage of additional cooking is then washed to further remove black liquor from it, before further processing, for example, bleaching in the case of pulp production. Washing is usually a multi-stage operation in which the pulp is subsequently brought into contact with, for example, a washing liquid and passed through a plurality of dewatering plants arranged in series, for example 2-4 such plants, and usually three such plants are used. A certain amount of calcium silicate deposited on the fiber can be dispersed in the washing liquid, and at the exit of a multi-stage washing operation under certain conditions, for example, about 50% of calcium silicate can be re-dispersed in the washing liquid. The rescattering of calcium silicate can be controlled by introducing a flocculant, for example polyacrylamide, into the washing liquid. However, under the conditions used in the evaporation of black liquor, re-dispersed calcium silicate in any quantity is substantially less prone to the formation of harmful precipitation than sodium silicate. In addition, calcium silicate is relatively water-insoluble and has a high melting point (melting point of wollastonite 1540 ° C), significantly higher than the temperature of the fluidized bed during gasification of black liquor using a fluidized bed, and significantly higher than the melting temperature of sodium silicate. It is expected that under the conditions of the process in question, any amount of calcium silicate contained in the black liquor will remain in the form of discrete particles, and it will neither volatilize nor promote agglomeration in the fluidized bed used in the reduction process.

Each stage of dewatering can be performed using a screw-type press, in which an elongated rotary screw is mounted in a perforated sleeve, which, in turn, is located inside the housing for collecting washing liquid passed through the sleeve, the pulp being advanced in the longitudinal direction of the mesh by rotating the screw and subjected to a pressing action, for example, due to the cross-sectional area of the channel, determined by the thread of the screw or the space between adjacent threads, decreasing from the inlet end of the mesh to its outlet end so that the accumulated pulp is compressed, and the liquid is gradually squeezed out of the pulp. In US patents US-A-6792850, US-A-6393728, US-A-6736054 and US-A-3256808 disclosed some of the common features of presses of this type. The washing liquid is usually used in the reverse order with respect to the pulp, so if there are first, second and third screw presses connected in series, then water is supplied to the mixing tank for washing the pulp, which is fed to the third screw press. The washing liquid extracted from the third press is fed into an additional mixing tank for washing the pulp, which is fed into the second screw press; and the washing liquid extracted from the second screw press is fed into a mixing tank for washing the pulp supplied to the first screw press; and the liquid extracted from the first screw press, which may now contain more than 10% solids, for example, about 12-15 wt.%, can form a stream of relatively concentrated black liquor, which can be combined with a stream of even more concentrated black liquor at point 149 and direct into the process to extract alkali from it. Some or even most of the silica that has been converted to calcium silicate will go into black liquor. However, calcium silicate does not tend to dissolve under the conditions of the recovery process and, in contrast to soluble sodium silicate, there will be a tendency to the absence or reduction of harmful deposits in the form of spangles during the concentration of black liquor by evaporation. It should be borne in mind that in-line mixers can be used instead of mixing containers and that other known methods of pulp dehydration can be used instead of screw presses. Screw presses are relatively small compared to drum washers, which can also be used, and are preferred for smaller operations associated with the pulping process from grassy materials.

Black liquor recovery

The present invention has created a process for the extraction of organic and inorganic chemicals and energy in the processing of a stream of black liquor coming from a pulping process from cellulosic paper-making raw materials. The proposed method is specifically intended for use with the pulping process described above, but it can be used separately for processing black liquor obtained from other pulp preparation processes. The proposed method is intended to ensure the economic viability of enterprises with small volumes of production.

Due to the absence of sulfides, black liquor can be processed so as to vaporize the organic component present in it in a fluidized bed reactor under oxidizing conditions in the presence of a stoichiometric amount of oxygen or a gas containing oxygen, such as air. It is preferable, however, to gasify (partially oxidize) the black liquor to synthesis gas containing, among other things, CO ₂ , CO, H ₂ O and H ₂ as components, usually together with methane and C ₂₊ components, under pyrolysis or partial oxidation in the presence of a substoichiometric amount of oxygen or a gas containing free oxygen. Such a gas may be a mixture of steam and gas formed by the products of combustion from a natural gas boiler, or a mixture of steam and gas formed by the products of combustion from a boiler provided with purified recycle synthesis gas supplemented with natural gas, if necessary. Gases are used as a fluidizing medium for the formation of a fluidized bed, and the fluidized bed material consists of CaO or contains CaO, which serves as a catalyst for the gasification process, as well as gasification of any charred substance, which can be formed as a by-product in the fluidized bed. In this process, the amount of oxygen in the mixture of gases supplied to the fluidized bed should be sufficient to maintain partial oxidation and to maintain the temperature of the fluidized bed, but not sufficient to convert sodium hydroxide and / or calcium hydroxide contained in the black liquor to carbonate, the authors consider it possible to carry out the reaction so that at least some NaOH remains as such in the fluidized bed. Thus, the oxygen content in the fluidizing gas can be <5%, and usually about 1.5-2.0% oxygen, leading to the oxygen content in the exhaust gas above the fluidized bed <1%, usually about 0.8% ( everything is expressed in volume fractions).

In one embodiment of the process, sodium hydroxide and / or sodium carbonate and calcium hydroxide and / or calcium carbonate are produced in a reactor in which a fluidized bed is formed.

The nominal linear velocity of the (pseudo) fluidizing gases supplied to the fluidized bed to maintain it in a fluidized state is usually> 2 m / s, for example, 10 m / s.

Thermal decomposition of organic substances contained in black liquor begins at a temperature above 200 ° C and in this case are formed: water vapor, CO ₂ , CO, H ₂ , light hydrocarbons, resin and, in the case of using sulfate black liquor and other cooking liquids, containing sulfur - light sulfur compounds (for example, mercaptans). At 600 ° C, the removal of volatile components is especially difficult to carry out from charred residues containing bound carbon, a certain amount of hydrogen and most of the inorganic substances. The charred composition can vary within wide limits and depends both on the processing conditions (for example, temperature) and on the characteristics of the fuel. Black liquor obtained from straw processing has a lower calorific value than black liquor obtained from wood processing, which should be taken into account when designing a fluidized-bed reactor.

A series of nonisothermal experiments was carried out using thermogravimetric analysis (TGA) with a linear change in temperature. This technique allows you to quickly determine the temperature curve of the decomposition of the material and the subsequent determination of the kinetics of thermal decomposition of the material. When testing black liquor obtained after processing straw, heated to N ₂ at a rate of 20 ° C / min, five individual peaks were determined: (a) in the temperature range 25-105 ° C - moisture loss; (b) in the temperature range 105-250 ° C - the main peak of the release of volatile substances; (c) in the temperature range 300-350 ° C - peak yield of less volatile substances; (d) in the temperature range 425-500 ° C - peak yield of less volatile substances; (e) at 650 ° C, the yield of volatiles from inorganic substances (e.g. Na and K). From these tests, it was not possible to determine which organic components of the liquor were associated with specific peaks, although it is likely that three main organic components — lignin, hemicellulose, and carboxylic acids — could be associated with the three main peaks of volatiles. Using these tests, it was also determined that the operating temperature of the industrial reactor should preferably not exceed 750 ° C in order to enable the recovery of most of the chemicals used in the pulping process (i.e. Na and K) in the preferred solid form; moreover, reaction temperatures, for example, in the range of 675-725 ° C are preferred, for example 675-700 ° C.

Experiments were carried out on the use of a fluidized bed in a batch process using a conventional or boiling fluidized bed of quartz sand, the inner diameter of which was 0.1 m, in order to determine the typical composition of the exhaust gases and the yield of charred material, exhaust gas and resins during black pyrolysis liquor; as well as gasification and combustion experiments. The trends in the processes of mixing and agglomeration in the fluidized bed were also investigated. Black liquor with a solids content of up to 29-45% was fed into a fluidized bed operating at 500-700 ° C with U / U _mf = 4 (i.e., a highly fluidized bed) supplied with a mixture of N ₂ and O ₂ . The analysis of the exhaust gas showed typical compositions of the main product gases, which were in the range of: H ₂ 0-5%; CH ₄ 7-12.5%; WITH 7.5-15%; СО ₂ 55-89%; Particles C ₂₊ 0-8%. The agglomeration in the bed was strong in all cases, despite the high U / U _mf ratio, and usually led to the completion of the experiment after less than 20 minutes due to the loss of bed fluidization. The gas yield was usually very low (about 9% at 550 ° C and increased to 25% at 700 ° C). The low gas yield and gas compositions (ie, a high content of [CO ₂ ] and a low content of [CO] and [H ₂ ]) suggest that the conversion reactions with water vapor of the charred substance did not occur as they should have happened in gasification conditions. This is most likely due to the weak contact between the gas and the solid that took place in the agglomerated fluidized bed. However, increasing the gas yield with increasing temperature is important, and this suggests that in an industrial reactor (designed so that there is a significantly higher permissible agglomeration limit in the fluidized bed), the temperature should be kept as close to the melting point of K and Na so that maximize synthesis gas production. In pyrolysis experiments (using only N ₂ ), when a condenser was added to the exhaust gas line, the resin yield (condensable fraction) was 30% and 38% at 550 ° C and 700 ° C, respectively. The carbonized substance yield was 45% at 550 ° C and decreased to 39% at 600 ° C and to 31% at 700 ° C, suggesting that temperature is an important variable in the process.

The gushing fluidized bed was different from the boiling fluidized bed in that it was formed using a central jet, which forced the material to move from the base of the bed along the central axis before being allowed to pour back down along the walls of the tank. The layer usually has a conical rather than cylindrical shape, which contributes to the volumetric circulation of the material. A gushing fluidized bed may have greater stability and less tendency to agglomerate than a fluidized bed.

The reactor is preferably a reactor in which a toroidal fluidized bed is formed; such reactors are described in US patent No. 4479920 and No. 4559719 (the contents of which are incorporated into this application by reference) and are supplied by Torftech Limited of Reading (United Kingdom,). Such a reactor is intended to overcome the problems of conventional fluidized bed reactors associated with controlling the temperature and rate of heat transfer within the bed, resulting from the chaotic nature of the lateral movement of particles in the bed, which remains essentially static and is fluidized by a vertical stream of gas and air mixture. Both the fluidized bed and the gushing fluidized bed have these disadvantages. According to the experience of the authors of the present invention, agglomeration occurs when there are hot spots in the fluidized bed. In a toroidal fluidized bed, this risk is minimized, and the formation of a layer with active CaO also contributes to this.

The solution proposed by Torftech and corresponding to a preferred aspect of the invention as applied to black liquor processing includes the following steps:

- ensuring the presence of a reactor containing a technological area equipped with a mass of granular (granular) material consisting of calcium oxide or containing calcium oxide;

- the supply of heated gas to the technological region to form a fluidized bed in such a way as to generate a vortex fluid flow in the aforementioned technological region; moreover, the fluid of the said vortex fluid flow forces the bulk material to take the form of a compact strip and circulate around the axis of the aforementioned technological region in the form of a turbulent flow; moreover, the fluidizing gas includes oxygen for at least partial combustion of the organic material contained in the black liquor;

- feeding black liquor into said compact strip of bulk material and processing black liquor in said layer in such a way as to gasify the organic materials contained in said black liquor;

- extracting organic material from said black liquor in the form of exhaust gas from said fluidized bed;

- extraction of inorganic material from said black liquor in the form of solid particles from said fluidized bed.

There is confidence that each particle moves back and forth within the technological area along the entire periphery of the compact strip, so that uniform technological conditions can be achieved. Particle movements in a granular mass are determined by the combined action of the fluid flow, gravity and centrifugal forces created by the vortex fluid flow; the result is complete and continuous mixing of these particles and the substance that is being processed, when such a substance is fed into a strip of particles. Therefore, a very efficient processing can be achieved by using only a flat strip of particles. In addition, the process gas stream strikes the microscopic layer of gas around each particle and minimize isolation. As a result, it turns out that the rate of heat and mass transfer is greater than in other types of reactors, which allows for faster and more efficient processing.

Tests were conducted using a semi-industrial scale reactor of the type mentioned above in which a toroidal fluidized bed was formed. When the black liquor to be processed was solidified, the solid particles were fed using a two-stage screw feeder (one screw was dosing, the other feeding). When the black liquor to be processed was in the form of a fluid, it was supplied using a centrifugal pump either through the top of the reactor or through a two-phase nozzle (into which Ar or N ₂ was supplied) directly on top and perpendicular to the surface of the fluidized bed. Exhaust gas and some solid material were discharged through the top of the reactor and piped into the cyclone and Venturi scrubber before being passed into the afterburner operating at 850 ° C. Solids from the fluidized bed were removed through a central discharge port. Particles of solid particles and gas were taken from various installation sites. The reactor chamber was made of heat-resistant ceramics (not required for this work) and had a size of 400 mm at the base and gradually increased up to 500 mm at the top and had a height of 850 mm. The distributor, through which fluidized gas was supplied to the reactor chamber from below the fluidized bed, contained a series of parallel plates, angled, between which fluidized gas passed. The free surface area of the distributor was 30%. The distributor was used to communicate a strong vortex motion to the fluidized bed, which caused large local gas velocities and greater turbulence without blowing particles from the top of the reactor. The nominal fluidizing linear velocity used in all experiments was 10 m / s; and with it, a layer of calcium oxide particles with an average size of 1.5 mm was adequately supported.

Two different technological regimes for processing black liquor were considered during these experiments: (a) preparation of a preliminary mixture containing black liquor and calcium oxide in the indicated ratio; (b) direct spraying of black liquor (with different solids content) onto a resident fluidized bed of quicklime in a reactor. Operationally supplying the dried solid to the toroidal fluidized bed is the simplest alternative solution, but there are engineering sources that warn that this method is not the preferred solution. The reaction product with CaO is a plastic mass with thermoplastic properties (even when using a significant excess of lime), which must be additionally heated to achieve complete dryness, and then cooled to achieve curing before it can be crushed and fed through a sealed device, preventing the passage of gas (for example, by means of a screw feeder or rotary valve) into the reactor. It is expensive and uncomfortable. In addition, equilibrium modeling shows that with an excess of CaO, the formation of hydrocarbons (especially CH ₄ ) is prevented due to the reaction with the formation of CaCO ₃ . An alternative method is to inject black liquor directly onto a resident CaO fluidized bed (mixed with inert material: either silica sand or alumina) inside the gas generator by using a conventional spray nozzle to spray black liquor. Swelling black liquor when it reaches 200 ° C is a difficult problem in a semi-industrial installation, but the authors expect that this will not be a problem when using larger equipment used on an industrial scale.

Solid particles of a fluidized bed containing inorganic components present in the black liquor, as well as calcium from the catalyst, exit the fluidized bed through a central discharge, where they can be dissolved to form a mixture of Na ⁺ , Ca ²⁺ , CO ₃ ^2- and (OH) ^- in solution according to the equation:

CaO + H ₂ O + Na ₂ CO ₃ → 2NaOH + CaCO ₃ .

This solution is then treated as described above.

The processing temperature used in these experiments was in the range of 550-725 ° C. In general, an increase in temperature led to an increase in the rate of conversion of black liquor to reaction products. Most of the inorganic content of black liquor could be recovered in solid form at higher temperatures than previously thought possible. The typical amount of extracted materials was more than 90%, and the experimental data were determined with an accuracy of 95% confidence interval.

The effect of temperature on the quality of the exhaust gas was not noticeable. The concentration of solids in the black liquor also did not have a noticeable effect on the extraction of sodium or on the composition of the exhaust gas, but, as expected, the exhaust gas contained more water when more diluted liquors were processed (containing 16% solids). It is expected that the optimal solids content in the black liquor, intended for feeding it into the processing process, will be dictated by the conditions of the upstream processes and economic constraints, i.e. the solids content in the pulp obtained during the cooking process and the analysis of the cost of evaporating water in a reactor in which a toroidal fluidized bed is formed, in comparison with the evaporation performed in advance in a standard evaporator battery.

As for the oxygen concentration in the fluidizing gas, in previous experiments with an oxygen content of 5-20% [O ₂ ] in the resulting exhaust gas mixtures, the contents of N ₂ , CO ₂ and H ₂ O corresponded to the expected values under these oxidation conditions. Inorganic matter regeneration, i.e. Na, C and K, was good. Samples of the lower fluidized bed product and cyclone contained: Ca 39-60 wt.% And Na 2.5-13.0 wt.%. In experiments conducted with an oxygen content of <2% [O ₂ ], the resulting gas mixtures contained mainly N ₂ , CO ₂ , H ₂ O and hydrocarbons, and the content of H ₂ or CO was difficult to determine by chromatography, since their presence was obscured by a peak of nitrogen.

Equilibrium modeling has shown that CaO is important for the gasification reaction. Its presence also prevents the formation of solid carbon (i.e. coal and resins), and it forms complexes with sulfur (especially CaS), although this could be considered a very beneficial side reaction. It is preferable to add CaO in an amount of from 0.1: 1 to 1: 1 in the ratio of dry matter, more preferably 0.2: 1-0.4: 1, and most preferably about 0.35: 1. At a ratio of 1.2: 1, the amount of hydrocarbons formed is substantially reduced by binding carbon in the form of CaCO ₃ .

Thus, in a preferred embodiment, black liquor with a solids content of 10-40%, for example 15-30%, can be introduced directly into the reactor, which form a toroidal fluidized bed containing either only calcium oxide or calcium oxide and an inert material , and it can be supplied together with steam and gas formed by the combustion products obtained from the combustion chamber fed by synthesis gas recirculated from the reactor and replenished with natural gas, if necessary. An evaporation plant and kiln for the recovery of CaO from CaCO _{3 are} desirable, and a small stream (about 10%) of CaCO ₃ is required to be disposed _of to prevent the accumulation of heavy elements that are naturally absorbed by the plants from the soil. This material can, however, be sent for local recycling and use in the manufacture of bricks or in cement work.

A preferred embodiment of the flow processing process is described below (see FIG. 5).

A stream of black liquor coming from the pulping process is collected in a tank 301 for storing liquor coming from the pulping process having a solids concentration of 30-70% using a standard evaporator 302 designed to provide a concentration. If the flow of black liquor comes from a twin-screw conveyor with screws rotating in the same direction at a solids concentration of 30% or more, it can be processed directly in the process vessel, excluding the evaporation step. Concentrated black liquor is fed into the reactor vessel 304 at a temperature above 90 ° C. using a hermetically sealed twin-screw conveying device 303. A hermetically sealed conveying device is used to minimize losses of organic components during evaporation. A temperature above 90 ° C is required to reduce the viscosity of the black liquor so that it can be easily transported. Black liquor is processed in a reactor vessel 304 according to one of two methods.

In the first method, black liquor is introduced into a reactor 304 in which a toroidal fluidized bed is formed by spraying concentrated black liquor in a reactor chamber in which a bed of fluidized material is supported. The material may be an alkaline earth metal oxide, for example lime, at a ratio of 0.3: 1 lime to dry matter black liquor. The average particle size of the alkaline earth metal oxide may be in the range of 1-4 mm. As mentioned earlier, the reactor can operate under stoichiometric or substoichiometric conditions.

In the second method, the black liquor stream is pre-mixed in a twin-screw conveying conveyor 303 with an alkaline earth metal oxide, for example lime (CaO), in a ratio of, for example, 0.3: 1 lime to dry black liquor substance to turn black liquor into a granular brittle material, which can then be fed by a screw into a reactor 304, in which a toroidal fluidized bed is formed. Again, when black liquor is converted to a dry solid before being fed into the fluidized bed, the reactor can be operated under stoichiometric or sub-stoichiometric conditions. In an embodiment of both methods, the ratio of alkaline earth metal oxide, for example lime, to the dry substance of black liquor can be in the range of 0.2-1.3: 1. An alkaline earth metal oxide can be supplied from a standard kiln 308. In both cases, the temperature in the range of 300-750 ° C, and preferably 650-750 ° C, is maintained in the reactor chamber 304 in which the toroidal fluidized bed is formed, where the necessary chemical reaction takes place in a few seconds. In a further possible embodiment of the process, part of the solid particles in the reactor 304 in which the toroidal fluid bed is formed can be recycled by feeding the screw feeder 303 back to the reactor 304.

Black liquor is converted through a chemical reaction into:

(1) sodium hydroxide and carbonate and lime in a fluidized bed reactor 304. The fluidized bed overflows through the central discharge point, and the overflow material then dissolves in the dissolution tank 305, with the extraction of sodium hydroxide in the form of green liquor in the traditional manner known as "recustication". The green liquor is then filtered using a known filter 306 to form a slurry of calcium carbonate and white liquor (containing sodium hydroxide and calcium hydroxide) for reuse in the pulping process. However, in one embodiment of the process, if the temperature is carefully controlled, then the recastification can occur in the reactor. In this case, sodium carbonate does not form, and sodium hydroxide can be reduced without using a dissolution tank (306);

(2) gas and fluids with a combustible component that can be used to generate energy. Gas is collected to drive the boiler 309, which generates energy and steam for use in the processing line at the pulp mill. In a further possible embodiment of the process, the gas containing combustible components may be recycled to the fluidized bed reactor to transfer heat for chemical reduction.

The calcium carbonate sludge can be dried to remove some water and sent to a second kiln 308, which can be a reactor in which a toroidal fluidized bed is formed. This reactor can operate at a temperature of about 1100 ° C, where calcium carbonate CaCO _{3 is} converted back to calcium oxide CaO for recycling into the process of chemical regeneration of substances contained in the black liquor stream. It may be necessary to permanently remove from the process approximately 10% of the material generated in the fluidized bed to prevent the accumulation of heavy metals and other materials involved in the process. If required, a black liquor stream with a solids content of less than 30% can also be processed using this method (has been tested). However, in this case, the energy consumption is greater, and therefore this process is not preferred.

It should be borne in mind that in the embodiment described above, various changes can be made without departing from the essence of the invention. For example, the black liquor processed in the fluidized bed may be a sulphate cooking liquid or a mixture of black liquor from soda / Ca (OH) ₂ and a sulphate cooking liquid, or black liquor from a soda / anthraquinone process.

Claims (39)

1. A method of processing black liquor to obtain inorganic material and synthesis gas containing, as components, CO ₂ , CO, H ₂ O and H ₂ together with methane and C ₂₊ components, comprising the following steps:
ensuring the presence of a reactor containing a technological region equipped with a mass of granular (granular) material consisting of alkaline earth metal oxide or containing alkaline earth metal oxide;
supplying heated fluidizing gas to the process area to generate a vortex flow of bulk material in said process area so that the bulk material takes the form of a compact strip and circulates around the axis of the process area as a turbulent flow, thereby forming a fluidized bed, wherein the fluidizing gas contains a sub-stoichiometric amount of oxygen for the partial oxidation of organic material present in black liquor, and for the conversion of other organic m the material present in the black liquor in said synthesis gas;
feeding black liquor into said compact strip consisting of granular material so that it is heated to a temperature in the range of 650-725 ° C and gasified;
recovering said synthesis gas in the form of exhaust gas from said layer containing <1 vol.% oxygen;
recovering inorganic material from said black liquor as a solid from said layer. 2. The method according to claim 1, in which black liquor is obtained as a result of the process of cooking pulp from grass material. 3. The method according to claim 2, in which the grassy material is selected from straw of wheat, rice, barley, oats, flax, rye, bagasse, sorghum or corn. 4. The method according to claim 3, in which black liquor is obtained from white liquor of the soda type. 5. The method according to claim 4, in which the black liquor is obtained from a white soda liquor type containing calcium hydroxide in an amount effective to convert the harmful silica present in the said grassy material into calcium silicate. 6. The method according to any one of the preceding paragraphs, in which the black liquor contains 10-70 wt.% Solid. 7. The method according to claim 6, in which black liquor contains 15-30 wt.% Solid. 8. The method according to claim 1, comprising the step of heating the black liquor to a temperature of 675-700 ° C. 9. The method according to claim 1, in which the bulk material consists of calcium oxide or contains calcium oxide. 10. The method according to claim 9, in which black liquor and calcium oxide are fed into the fluidized bed in such a way as to maintain the ratio of calcium oxide to the dry solid contained in black liquor in the range from 0.1: 1 to 1: 1. 11. The method according to claim 10, in which the black liquor and calcium oxide are fed into the fluidized bed in such a way as to maintain the ratio of calcium oxide to the dry substance contained in the black liquor in the range from 0.2: 1 to 0.4: 1 . 12. The method according to claim 9, in which the black liquor is pre-mixed with calcium oxide in the ratio of calcium oxide to the dry substance contained in the black liquor, from 0.1: 1 to 1: 1 to obtain granular brittle material, and this granular brittle the material is then fed into a fluidized bed reactor. 13. The method according to claim 9, in which sodium hydroxide and / or sodium carbonate, and calcium hydroxide and / or calcium carbonate are obtained in a reactor in which a fluidized bed is formed. 14. The method according to any one of claims 9 to 13, in which the average particle size of calcium oxide is from 1 to 4 mm 15. The method according to claim 1, in which black liquor is introduced into the fluidized bed reactor by spraying concentrated liquor in the reactor chamber in which the fluidized bed is formed. 16. The method of claim 15, wherein the nominal linear velocity of the fluidizing gases supplied to said fluidized bed is> 2 m / s. 17. The method of claim 15, wherein the nominal linear velocity of the fluidizing gases supplied to said fluidized bed is about 10 m / s. 18. The method according to claim 1, in which the fluidized bed is poured through the Central discharge point and the poured material is then dissolved in the dissolution tank to highlight sodium hydroxide in the form of green liquor. 19. The method according to p, in which the green liquor is filtered to obtain a slurry of calcium carbonate and white liquor containing sodium hydroxide, for reuse in the pulping process. 20. The method according to claim 19, in which essentially no sodium carbonate is formed. 21. A method of converting grassy raw material into pulp for the production of paper or paperboard, comprising the following steps:
digesting said raw material using white liquor containing mainly sodium hydroxide and additionally containing calcium hydroxide in an amount effective to substantially completely convert the silica present in said raw material into calcium silicate;
the selection of pulp obtained at the stage of cooking pulp;
recovering the black liquor obtained in the pulping step by washing said boiled pulp and optionally also by extracting the black liquor directly in said pulping step, said black liquor recovered being substantially free of soluble silicate;
heating black liquor in a reactor in which a calcium oxide-containing fluidized bed is formed to catalytically convert the organic matter contained in said black liquor to gas and to produce reduced solids including valuable sodium components of said white liquor and calcium oxide;
regenerating said white liquor using said recovered solids. 22. The method according to item 21, in which said grassy raw material is wheat straw. 23. The method according to item 21, in which said grassy raw material is rice straw. 24. The method according to item 21, in which said grassy raw material is bagasse. 25. The method according to PP.21-24, in which the recovery of black liquor includes the step of connecting the stream of black liquor coming from the pulping stage, with the stream of black liquor coming from the washing stage of pulp. 26. The method according A.25, in which the black liquor is concentrated by evaporation before heating in the aforementioned fluidized bed reactor. 27. The method according to p, in which the black liquor is concentrated by evaporation to a solids content of 20-40 wt.%. 28. The method according to item 21, in which the fluidizing gases contain at least stoichiometric amounts of free oxygen to completely oxidize the organic material contained in the black liquor. 29. The method according to item 21, in which the fluidizing gases contain substoichiometric amounts of free oxygen to partially oxidize the organic material contained in the black liquor, and the conversion of other organic material contained in the black liquor, into a combustible exhaust gas. 30. The method according to clause 29, further comprising the step of supplying fluidizing gases and black liquor so as to create an exhaust gas above said fluidized bed containing <1% oxygen. 31. The method according to item 21, comprising the following steps:
ensuring the presence of a reactor containing a technological area equipped with a mass of granular (granular) material consisting of calcium oxide or containing calcium oxide;
supplying heated fluidizing gas to a process region to generate a vortex fluid stream in said process region so that the fluid of said vortex fluid stream causes the bulk material to take the form of a compact strip and circulate around the axis of said process region as a turbulent stream, the fluidizing gas comprising oxygen to at least partially burn the organic material contained in the black liquor;
feeding black liquor into said compact strip of bulk material and processing black liquor in said fluidized bed so as to gasify the organic materials contained in said black liquor;
recovering organic material from said black liquor in the form of exhaust gas from said fluidized bed;
recovering inorganic material from said black liquor as a solid from said fluidized bed. 32. The method according to item 21, in which the raw material supplied for cooking pulp, was crushed to remove nodes from it and split in the longitudinal direction. 33. The method according to p, in which the crushing is performed using a pair of corrugated shafts, rotated in opposite directions, between which the raw material is passed. 34. The method according to p, in which the splitting is performed using a pair of ring shafts between which crushed material is passed. 35. The use of calcium hydroxide as an additive in the pulping of pulp from grassy raw materials to obtain pulp for inhibiting scale formation during the concentration of black liquor and for the regeneration of components in the processing of black liquor obtained, at least in part, by washing the pulp. 36. The application of clause 35, where the black liquor is obtained partly from the digester and partly from the washing process of the pulp. 37. The application of claim 35 or 36, wherein the silica is contained in said black liquor in the form of calcium silicate, with the substantial exclusion of the presence of silica in the form of sodium silicate. 38. The application of claim 35, wherein a flocculant is added during washing to suppress dispersion of calcium silicate. 39. The use of claim 38, wherein said flocculant is a polyacrylamide.

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