US2921880A - Method of obtaining pulps usable in the paper-making or other industries, from wood or annual plants, and the pulps resulting therefrom - Google Patents

Description

2,921,880 Patented Jan. 39, 1960 IVIETHOD OF OBTAINING PULPS USAF-LE @l THE PAPER-MAKES OR OTHER INDUSTRIES, FROM WQOD ()R ANNUAL PLANTS, AND THE PUMPS RESUL'HNG 'HEREFRQM Louis Grand, Casteljaloux, France No Drawing. Application August 2, 1956 Serial No. 601,623

Claims priority, application France April 12, 1951 4 Claims. (Cl. 162-89) The present application is a continuation-in-part application, of application Serial No. 231,428, filed on lune 13, 1951, now abandoned.

The present invention relates to a method for totally chlorinating wood lignin with a view to obtaining pulps that can be used in paper-making, cardboard-making or other industries, from wood or annual plants.

It is known that all plants, of the annual type, such as straw, corn, bagasse, etc., or of the leafy or resinous, soft or hard wood type, are formed of cellulose, lignin, hemicellulose and various constituents such as resins, waxes or greases, coloring matter, tannin, etc., in variable quantities according as to whether they are resinous, leafy or annual plants.

To obtain pulps suitable for use in paper-making or in the manufacture of artificial textiles, these various constituents must be eliminated, other than cellulose and more particularly ligm'n.

In methods known up till now, wood is delignitied by applying conventional delignification processes such as bisulphite, soda, sulphate, etc., processes. Chlorine is only used for converting into chlorolignins the slight quantity of residual lignin still contained in the product after the delignification treatment and this constitutes a first stage of bleaching; the quantities of chlorine employed are a minimum and the degradation of the raw material by acid hydrolysis is weak.

When chlorine is employed as the sole agent for de lignification, to the exclusion of any lye treatment or preliminary treatment, the quantities employed are considerable and, owing to this fact, the material is degraded by acid hydrolysis resulting from the action of HCl produced during chlorination.

The applicant has discovered a method, which is the subject of the invention, enabling the total chlorination of the lignin to be carried out in a single operation, without harming the cellulose, th's method being essentially characterised by the fact that owing to the reagents used, the pH of the chlorinating solution remains constant throughout the chlorination operation, the value of this pH being comprised between pH=3 and pH=3.5, this chlorination occurring practically in the absence of hyrochloric acid, which is harmful for cellulose, owing to the lowering of the pH which its presence involves.

According to another essential feature of this invention, and whereas other methods are operated hot and under pressure, this method enables operation at ambient temperature and at atmospheric pressure.

According to another feature of the invention, chlorination is done by nascent chlorine, set up by the chlorinating solution itself.

According to another feature of the invention, the method in question enables the treatment of all lignocellulosic materials, no matter where they come from, these materials being in a veryfinely divided state. The method can also be applied to the mixture of these materials, which result could not be obtained with the other methods.

It should be noted that the method according to the invention is not a bleaching process, but a process for delignifying cellulosic raw materials, in order to convert them into pulps capable of being used in paper-making or the manufacture of artificial textiles, after having undergone a more or less complete bleaching treatment properly so called. This bleaching operation may be of any known type having no connection with the present delignification process; it simply forms an extension thereof.

The method according to the invention will be described in detail below.

As already stated above, it has been recognised for some considerable time that chlorine forms a very good reagent, for preparing pulps for use in paper-making or other industries, from various plants, by forming chlorolignins, which are soluble, as is known, in alkaline media.

To this end, chlorine is utilised either in a state dissolved in water, or in a gaseous state, on moist material. It should be noted that as soon as chlorine is put into the presence of water, the following equilibrium reaction sets in:

the hydrochloric acid formed being ionised. On the other hand, the phenomena of chlorination employing chlorine under its various forms are characterised by the following schematic reaction:

. ions is increased.

It thus appears that all these phenomena together tend to degrade the cellulose, which gives products of poorer quality that are not adapted to all piuposes.

As stated above, the process according to the invention is a chlorination process for preparing pulps which eliminates the aforementioned primordial disadvantages, by limiting at any moment of the chlorination and at the temperature of use, the acidity, that is to say the concentration of 1-H ions to a value such that the degradation of the cellulose is made impossible.

According to this process, chlorinating solutions which produce nascent chlorine are obtained that are practically free of hydrochloric acid; the hydrochloric acid that forms during chlorination is destroyed by the coexistence in the chlorine solution, of hypochlorous acid and a chloride, preferably a chloride of a weak base, the hypochlorous acid acting as a regulator of the pH, as will be seen farther on.

The presence of an excess of hypochlorous acid enables, in the presence of Clions, the chemical balance of the above Equation 1 to change from right to left,

i.e., thus limiting at any moment the H+ ions concen- CaCO +2Cl +H O=CaCl +2HOCl+CO (A) If the chlorine is still caused to pass through the abovementioned resulting solution when all the calcium carbonate (CaCO has disappeared, the chlorine dissolves 3 and the pH is a function of the concentration of HOCl and CaCl present in the solution, said concentratio'i' being itself a function of the starting quantity of CaCO this pH may vary between pH:5.5 which is the pH of a solution of pure hypochlorous acid, and 'pI-I=1.5 which is the pH of a solution of chlorine water.

A solution of chlorine in water is itself in the following equilibrium:

Cl +H OZHCl+HOCl (B) This Relation B is superimposed on the Equation A and finally the following equilibrium is obtained:

7 nci2+n2o balances zCaClz 2113001 o yHOCl if x molecules of CaCO have been put in suspension and if n chlorine molecules have been introduced; CaCl ionising in the same conditions as HCl, it is seen that the quantity of HCl and consequently of H ions may be whatever is desired and consequently the pH will always be comprised between 5.5, which is the pH of a hypochlorous acid solution according to (A), and 1.5 which is the pH of the equilibrium (B) (pH of a chlorinated water).

Everything therefore occurs as if, on the one hand, a solution of HOCl+CaCl were prepared by causing chlorine in the requisite quantity to pass into a suspension of CaCO to obtain exactly a solution of HOCl-l-CaCl and that to this solution a solution of chlorinated water were added, on the other hand, by causing chlorine to pass into water. The pH of the adjusted mixture is a function of the quantity of the solution of HOCl+CaCl added. The quantity of CaCO for obtaining preferably a pH comprised, according to the invention, between 3 and 3.5, corresponds to a starting aqueous suspension containing from 0.2 to 1 part of CaCO for 100 parts of water.

It is to be observed that the equilibrium such as (C) produces chlorine in a nascent state in as much as the disappearance of C1 to the left and the formation of HCl according to (2) which is added to that existing at (a) of (C) means that this equilibrium (C) moves from right to left according to:

HCl+HOCl=H O +Cl (nascent) It is known that in the nascent state bodies possess a particular reactivity, and it is this nascent chlorine which is the active chlorination agent.

The Works of Clibbens and Ridg (I. Text. Ind. 18, 135T (1927)) confirmed by other research workers, have shown that the degradation of cellulose by oxidation was at a maximum in the vicinity of neutrality, very reduced and nearly constant between pH 2.5 and 4.5 and that it increased below 2.5.

It is therefore in this zone of pH between 2.5 and 4.5, and preferably between 3 and 3.5, that chlorolignins will form without risk of degradation of the cellulose, either by oxidation or by hydrolysis.

The mixture corresponding to the equilibrium (C) can .also be made in a single operation as stated above,

namely, by passing chlorine into an aqueous suspension of CaCO and instead of stopping the passage of thechlorine when the solution of HOCl-|'CaCl hasformed, to continue the passage of the chlorine. The mixture is automatically at the required pH according to the quantity of HOCl+CaC1 present in the solution, said quantity being a function 'of the initial quantity of CaCO The preferred percentages given of CaCO are selected so as to obtain a pH comprised between 3 and 3.5 which is the most favorable pH for effecting the operation according to the invention.

During the chlorination of lignin by nascent chlorine (a reaction which is known to be very rapid, if not instantaneous), hydrochloric acid is still for-med, as shown by the equation mentioned above, namely:

In the equilibrium (C) the disappearance of chlorine C1 following the chlorination of the lignin and the formation of HCl, means that this equilibrium moves from right to left while regenerating active chlorine (in a nascent state) according to the diagram:

We thus see that the CaCOg, having disappeared from the solution, the latter no longer intervenes and cannot neutralize the HCl. The CaCO has thus only served to pre-form the hypochlorous acid solution (H001) and the calcium chloride (CaCl this solution playing the part of a buffer solution for the hydrochloric acid formed, and maintaining, according to its concentration, the pH of the final solution at suitable values. Moreover, the Ca Cl present in the solution, ionises in the same way as the hydrochloric acid, HCl, and helps to provide the chlorine, C1 required for chlorination.

In order still better to describe the phenomena that take place in the chlorinating solution, the following example shall be considered, always bearing in mind that the most favorable pH for cellulose should be comprised between 2.5 and 4.5.

If a solution is made up whose pH equals 3, there are times less H+ ions than at a pH='1 with a pH=4 there are 1.000 times less H+ ions, from which it can be stated that the solution is practically free of hydrochloric acid. Furthermore, the hydrochloric acid formed during chlorination is destroyed by the hypochlorous acid (HOCl) just as long as the solution contains some. Finally, the solution only contains a number of HCl molecules equal to the number of elementary chlorine molecules that it initially contained.

The equilibrium such as (C) thus enables chlorinating solutions to be obtained that are very rich in active chlorine, and whose pH has a value comprised in the zone which is favorable for cellulose, whereas in the ordinary chlorination formerly used, the hydrolysis of the chlorine causes the pH of the solution to drop to 1.5 and the chlorination reaction itself causes it to drop still further. This chlorination thus takes place in a strongly acid medium, which has the efiect of deteriorating the cellulose by hydrolysis.

An example will enable the phenomenon illustrated by the equilibrium (C) to be better understood.

If in this equilibrium (C) for example, 0.05 molecule of a chlorine molecule is present, it gives rise, by chlorinating the lignin, to 0.05 of a molecule of hydrochloric acid HCl reacting on 0.05 molecule of hypochlorous acid HOCl according to the following equation:

0.05 molecule of C1 shall be released in a nascent state, at the same time as 0.05 molecule of HOCl disappears.

When all the HOCl has disappeared, there finally remains in the solution 0.05 molecule of HCl and the pH of the solution drops by the corresponding value; the HOCl is thus actually a regulator of the pH and more exactly the regulator is the ratio Cl /HOCl which is always greater than zero.

It is thus seen that in the process according to the invention, chlorination takes place by means of nascent chlorine, the latter being regenerated during chlorination by the disappearance of the HCl'resulting from the chlorinatioh itself.

With the same pH, it is possible to obtain different Cl /HOCl ratios, the latter closely depending on the quahtityof CaCO put into suspension inthe Water. It is thus easy to check the operation of this chlorination reaction and foresee the evolution of the pH during this reaction.

In this manner, it is very easy, after having determined by a test what is the type of solution best suited for the material to be treated and the result to be obtained, to check the composition of the solution by simply reading a pH meter.

It should be stated that at no time during the reaction is there any hypochlorite formed,

That the hydrochloric acid formed during chlorination as explained above, is not destroyed by the calcium carbonate;

That there is solely a chlorination reaction and at no time is there an oxidation reaction.

This being the case, it should again be stated that the preparation of the mixture, hypochlorous acid-chlorine chloride of a weak base, may be obtained:

Either by dissolving chlorine in water containing the required quantity of a weak base in suspension, such as calcium carbonate;

Or preparing separately a solution of chlorinated water, on the one hand, and a solution of hypochlorous acid and an ionisable chloride, on the other hand, and by mixing them in the requisite proportions.

In a practical manner, the use of one or other of the above-mentioned solutions is of interest, owing to the fact that the chlorination reaction takes place in the cold state.

In actual practice, the quantity of calcium carbonate required, preferably applied in the process according to the invention, is put into suspension in water in a tank provided with a stirrer.

This suspension, taken up by a pump, is poured at the top of a sandstone or protected steel absorption tower filled with Raschig rings. The chlorine reaches the base of the tower. Absortion thus takes place by countercurrent and the solution flows continously to the base of the tower from whence it is directed towards a storage tank.

The regularity of the composition of the solution is automatically ensured by a pH meter, which regulator may operate according to circumstances, either on the input of the chlorine to the absorption tower, or on the rate of flow of the carbonate suspension. Labor is thus limited to supplying the CaQO which, moreover, only amounts to a small quantity (a few kilogrammes per cubic metre of solution).

It has been stated above that the evolution of the equilibrium (C) from right to left supplies chlorine in a nascent state, whose properties are known as being extremely active, such properties being much more active than those of the molecular chlorine usually applied in chlorination operations.

The process according to the invention is especially applicable to materials already in a sufiiciently divided state. It is not applicable to the treatment of large wood chips, for, owing to the rapid attack (rapid chlorination), this attack would only be superficial, and delignification would not be complete even if the chlorinating solution were made to continue to act. The fact of operating on finely divided material enables this material to be homogenised and, owing to this fact, enables woods as different as poplar, beech, oak, etc. to be treated together, especially if the material is divided down to a state of elementary fibre (Asplund fibre, for example) for there will only be ligno-cellulose fibres present having a chemical composition almost identical whatever their origin may be.

In actual practice, this finely divided material is first of all treated hot and in free air by an alkaline solution, for example, a 2% soda solution, to free it from resinous or waxy materials that it contains, and also to facilitate its ability to become wet; the matter thus treated is then subjected to one, two or three cycles, according to the raw material dealt with and the required quality of the pulp to be obtained.

Each of the afore-mentioned cycles consists of:

1. A chlorinationby 'a chlorinating solution according to the invention;

2. A treatment by a weak alkaline lye for dissolving the chloro-lignins thus formed;

3. A washing in water after the treatments 1 and 2.

Chlorination takes place at ambient temperature and at atmospheric pressure and takes from 3 to 5 minutes according to cases, which is 'very rapid owing to the great activity of the chlorinating solution.

Straw, annual plants, hard or soft woods and their mixture, can be delignified with success and provide, according to the number of cycles applied and the raw material treated, unbleached pulps or easily bleachable pulps.

In this manner, it has been possible to prepare paper pulps with raw materials as diverse as annual plants (straw, corn, sugar-cane, bagasse, alfa), resinous woods (pine, fir), leafy woods (poplar, eucalyptus), hard woods (oak, chestnut, beech).

These raw materials may be treated mixed together provided the physical form under which the mixture is treated is homogeneous; in other words, a pulp obtained with a defibrator can be formed of a mixture of hard and soft woods; this mixture can be treated as easily as a pulp obtained from a single Wood, for as the matter is submitted in a homogeneous form, the chlorinating and dissolving reactions act in an identical manner on the fibres whatever be their origin.

The conventional processes applied to the raw material in this form do not give good results and it is not rational to treat hard wood'a'nd soft wood chips in a mixture, with an alkaline or acid lye. If the characteristics of the lye are correct for soft wood, the hard wood gives a high proportion of insufliciently digested chips; if the characteristics are more severe, the hard wood chips will be correctly treated, but the soft wood chips will be damaged; a mean treatment will only give a mixture of insufficiently or over-lyed pulps. In all cases, the pulp obtained will be of poor quality.

On the contrary, the process according to the invention, by enabling a mixture of the most diverse woods so treated, is suitable in all cases where the forest is not homogeneous, thus authorising the rational working of wooded clumps.

The delignified pulp can be subjected to bleaching operations in the usual manner.

By way of example, and in no restrictive sense, it is here mentioned that with straw as a raw material, there has been obtained:

An unbleached first quality pulp after applying one cycle and a slight mechanical action, such as that of a disintegrator;

A pulp that can easily be bleached and of considerable strength, after applying two cycles.

With sufiiciently divided leafy woods as raw material, a very high quality bleachable pulp has been obtained, after applying two cycles. With conifers, it is preferable to apply three cycles.

Some examples will be given below of the practical operation of the process according to the invention. In these examples, the raw material is subjected to the action of the chlorinating solution in a sufliciently divided state.

The dryness of this material is preferably comprised between 3 and 4%.

Example 1 Chlorine is made to pass into a solution containing 0.16 mol of hypochlorous acid and 0.08 mol of CaCl which corresponds to an initial quantity of CaCO of 0.08 mol (being 0.8 part of CaCO for parts of water). The pH of the solution lowers progressively. If the passage of the chlorine is stopped at pH 3.5, a solution is obtained according to the equilibrium (C) which contains 0.168 mol of active chlorine per litre. If

disappears, and

the passage of the chlorine is stopped at pH 3, the solution contains 0.193 'mol of active chlorine per litre.

In actual practice, it is simpler for example, to put in suspension in water, 0.08 mol of CaCO or 8 gr. per litre of water.

The chlorine reacts first of all with the CaCO which the solution becomes limpid; it then contains 0.161 mol of HOCl and 0.08 of CaCl according to (A). The chlorine is further admitted and one gets back to the preceding case. The chlorine is stopped when the required pH has been reached.

At pH 3, the solution contains 0.193 mol of active chlorine and has necessitated the dissolving of the same quantity of gaseous chlorine, i.e. 13.7 gr. per litre of solution.

If only half the CaCO is used, or 0.04 mol per litre, the solution will titrate at pH 3.5, 0.083 mol and at pH 3, 0.09 mol of active chlorine, i.e. 6.39 gr. per litre.

By working with quantities of CaCO comprised between 0.2 and 1 part for 100 parts of water, giving 0.4 to 2 parts of hypochlorous acid and 0.2 to 1 part of calcium chloride, to adjust the pH between 3 and 3.5, the concentration in active chlorine will be comprised, respectively between 0.043 and 0.25 mol per litre and between 0.040 and 0.25 mol per litre.

Example 2 The raw material formed by a single species of leafy wood or a mixture of woods of different species is first of all divided by any kind of means, the Asplund defibrator, for example.

The ligno-cellulose fibres obtained, containing all the original lignin, are treated for from half-an-hour to one hour by a weak sodium hydroxyde solution at atmospheric pressure (85 to 90); they are then washed.

The pulp is then subjected to the action of the chlorinating solution made up according to the equilibrium (C) containing, for example, 13.7 gr. of active chlorine per litre and at a pH of 3, at the rate of 7.5 kg. of active chlorine per 100 kg. of pulp, i.e. 547 litres of solution. The dryness of the pulp should be about 5% before it is mixed with the chlorinating solution and the mixture should be made with vigorous stirring so that, in spite of its rapidity, the reaction is uniform. The reaction is completed in from 5 to 7 minutes.

The pulp freed from the solution is washed and then subjected to the action of a weak alkaline solution to dissolve the chloro-lignins formed. The washed pulp is again subjected to the same quantity (7.5%) of solution adjusted to pH 3 and containing 13.7 gr. of active chlorine per litre, is washed, alkalinised and washed. It can then be bleached by any of the known bleaching processes.

Example 3 The pulp composed of ligno-cellulose fibres, subjected, as above, to the action of a weak alkaline solution, is treated by achlorinating solution adjusted to pH 3 8 at the rate of 5 kg. of active chlorine per kg., i.e. 365 litres. After alkalinisation and washing as above, the second chlorination may be carried out with 10 kg. of active chlorine per 100 kg. of pulp, i.e. 730 litres of chlorinating solution.

This method, which uses the same quantity of active chlorine as in the preceding example, is more efiicacious and provides highly delignified pulps which, during subsequent bleaching, reach a high degree of whiteness.

What I claim is:

l. in a process of delignifying cellulosic raw materials such as wood and annual plants which have not been previously chemically treated for the removal of lignin therefrom, the steps of dividing the raw material into a ver finely divided state, treating the finely divided material with a hot, weak aqueous solution of sodium hydroxide to remove resins and greases, treating the resulting material with a chlorinating solution at atmospheric temperature and pressure, for a period of three to seven minutes, the pH of said solution being between 3 and 3.5, the pH of the solution remaining in this range during the treatment with said chlorinating solution, and removing the chloro-lignins formed during the treatment with said chlorinating solution by subjecting the material to the action of a weak alkaline solution to dissolve the chloro-lignins.

2. In a process in accordance with claim 1 again treating the material from which the chloro-lignins formed have been removed with the same chlorinating solution, and again removing the additional chloro-lignins formed during the treatment with said chlorinating solution by subjecting the material to the action of a weak alkaline solution to dissolve the chloro-lignins.

3. in a process in accordance with claim 1 wherein the chlorinating solution contains in equilibrium a mixture of chlorine, hypochlorous acid, a chloride of a weak base and a small amount of hydrochloric acid.

4. In a process in accordance with claim 1 wherein the chlorinating solution is prepared by dissolving chlorine in water containing calcium carbonate in suspension until the calcium carbonate has totally disappeared, the quantity of calcium carbonate utilized being between 0.2 and 1 part for 100 parts of water, and continuing to introduce the chlorine until a pH of between 3 and 3.5 is attained.

References Cited in the file of this patent UNITED STATES PATENTS 1,970,065 Tomlinson Aug. 14, 1934 2,021,612 Sconce Nov. 19, 1935 2,049,676 Tomlinson Aug. 4, 1936 2,070,893 Glass Feb. 16, 1937 OTHER REFERENCES Coster et a1.: Some Aspects of the Chlorination of Sulphite Pulp, Tech. Assn. Papers, 1941, Series XXIV, pages 204-208.

Claims (1)

1. IN A PROCESS OF DELIGNIFYING CELLULOSIC RAW MATERIALS SUCH AS WOOD AND ANNUAL PLANTS WHICH HAVE NOT BEEN PREVIOUSLY CHEMICALLY TREATED FOR THE REMOVAL OF LIGNIN THEREFROM, THE STEPS OF DIVIDING THE RAW MATERIAL INTO A VERY FINELY DIVIDED STATE, TREATING THE FINELY DIVIDED MATERIAL WITH A HOT, WEAK AQUEOUS SOLUTION OF SODIUM HYDROXIDE TO REMOVE RESINS AND GREASES, TREATING THE RESULTING MATERIAL WITH A CHLORINATING SOLUTION AT ATMOSPHERIC TEMPERATURE AND PRESSURE, FOR A PERIOD OF THREE TO SEVEN MINUTES, THE PH OF SAID SOLUTION BEING BETWEEN 3 AND 3.5, THE PH OF THE SOLUTION REMAINING IN THIS RANGE DURING THE TREATMENT WITH SAID CHLORINATING SOLUTION, AND REMOVING THE CHLORO-LIGNINS FORMED DURING THE TREATMENT WITH SAID CHLORINATING SOLUTION BY SUBJECTING THE MATERIAL TO THE ACTION OF A WEAK ALKALINE SOLUTION TO DISSOLVE THE CHLORO-LIGNINS.