MX2007005357A - Alkaline electro-hydrometallurgical process for zn extraction from electric arc furnace dust. - Google Patents

Abstract

An electro-hydrometallurgical process that extracts zinc from electric furnace dust (EAF dust) to produce zinc of high purity and fine particle size, including leaching EAF dust with an alkaline solution to form a zincates solution, separating the liquid and gangue in the zincate solution; inertizing the gangue, purifying the liquid by cementation and separating the liquid from precipitated solids; purifying the zincate solution obtained from the cementation by adsorption, at least partially evaporating the resulting pure solution, bleeding at least a fraction of the concentrated solution obtained; crystallizing at least a fraction of the concentrated solution, electro-depositing zinc from the concentrated solution; and separating washing and drying deposited Zn.

Description

ALKALINE ELECTRO-HYDROMETALURGICAL PROCESS FOR THE EXTRACTION OF ZN FROM ELECTRIC OVEN POWDERS DESCRIPTION OF THE INVENTION The subject of this patent is a hydrometallurgical process that incorporates improvements to the basic technology known as "REZEDA Procedure", which refers to the extraction and production of Zinc dust in alkaline medium from steel mill dust ( EAF dust). The invention relates to the methods of treating dust from electric steelworks, and which is also applicable to other raw materials or residues containing zinc and leachable lead in an alkaline environment. The REZEDA process is represented by the patent FR 2.510.141 of 1983, but it is necessary to point out that the alkaline leaching processes denominated CEBEDEAU, CARDIFF and AMAX (1, 2, 3) are also referents. For the benefit of the simplification of the exhibition, from now on, we will only refer to the dust of electric steelworks. The steel mills mainly use pressed scrap, sheets, galvanized elements, etc., which contain heavy metals such as Zn, Pb and Cd, which evaporate at the high temperature at which these kilns operate (1600 ° C), then they condense as oxides when the fumes cool down and are captured in bag filters. This powder is very fine and difficult to handle, its average chemical composition is shown in Table 1, which highlights 20-21% of Zn, 20-35% of Fe, 3 to 5% of Pb, 3% of chlorides, etc. The presence of Cd is registered no more than 800 ppm. Table 1. Average composition of steel dust (EAF dust),% w / w dry base.

Refs. (4, 5) When the zinc content of the powder is at the level of 20 to 21%, it happens that 20 to 30% of it is in the form of zinc ferrite, ZnFe204, which unlike Zn oxide, ZnO, is hardly soluble in hydrometallurgical treatments, both in acidic and alkaline medium to which it is submitted to recover zinc. The environmental legislation of practically all countries, which is increasingly rigorous, classifies dust from the electric steelworks as hazardous waste due to its heavy metal content, which means that the waste can not be disposed of without prior treatment (5). On the other hand, the possibility of recycling this powder in the same steelworks that produces it, to recover the iron it contains (25-35%), is difficult due to the operational problems caused by the fineness of the dust and the presence of the contaminants marked. The dangerousness of steelworks dust is due to the presence of Pb, Cd and hexavalent Cr, which is measured by the EPA (Environmental Protection Agency of the USA) SW-846 Method-1311"TCLP- Toxicity Characteristic Leaching Procedure" , Characteristic toxicity by Leaching Procedure (7, 8). Table 2 shows the leaching limits that the waste must comply with. Table 2. Leaching Limits (ppm) for EAF dust.

Pb Cd Cr As Ag Ba Hg Se U US EPA 5 1 5 5 5 ND 0.2 1 ND Canada 0.5 0.5 0.5 0.5 0.5 50 0.01 0.1 200 Italy 0.2 0.02 2 0.5 0.5 ND ND ND ND Germany 2.0 0.5 10.0 1 ND ND ND ND ND Refs .: (7, 8 and 9) In Chile and most of the Latin American countries, the above rule applies. In recent years, environmental regulations in the world have advanced a lot, for example, in the USA in 1988, the EPA classified steel dust as dangerous (10) and in 1995 it was established that treatment technologies had to comply with the test of TCLP for 14 elements (11). Table 3 shows typical analyzes of TCLP for dust from Chilean steel mills, in which it is observed that it exceeds the national regulations for the Cd. Table 3. Typical results of the TCLP test for Chilean steelworks dust, mg / L.

Consequently, environmental considerations are key in the research and development of technologies to process this type of waste. Thus, the strategy to follow must be to recover the metals of value (Fe, Zn, Pb) and generate a non-hazardous waste that can be stored without problem, or valued through some use. Environmental considerations mean that the industrial processes to be applied must comply with the environmental regulations that each country establishes, and that are translated into regulations and standards that are expressed in compliance with certain environmental standards of air quality, emission of particulate material and disposal. of liquid and solid waste. For recycling, different processes are available. Firstly, the pyrometallurgical processes represented by the Waelz process (12), which is the main current technology of the so-called High Temperature Metal Recovery (HTMR), is a robust technology, well established and exhibits the following characteristics: 1) Operates a high temperature with intensive use of energy. 2) Minimum economic size is over 50,000 tons per year (13), which has forced the establishment of regional plants in the USA given that the average generation of steel dust (EAF dust) is 8,000 tons per year per plant (10 ). This is facing government pressure to restrict the transport of hazardous materials. 3) Generates low value products that require a hydrometallurgical treatment (washing) to value them. 4) Produces an inert slag used in road stabilization. 5) Releases large volumes of combustion gases into the atmosphere. Several of the other HTMR technologies (10) have been oriented to solve the problems presented by the Waelz process, in the sense of installing small plants, processing the powders without having to agglomerate them, recycling the iron and generating a non-hazardous waste. However, almost all these processes are also energy intensive and present problems in the condensation of volatilized metals (Pb, Zn, Cd, and salts such as chlorides), so many of them have been abandoned. Hydrometallurgical technologies are less developed, there are plants at pilot level or small commercial scale. These technologies are becoming important for the following reasons (14): 1) They provide environmental benefits since they do not generate air pollution. 2) They allow the installation of much smaller plants, which in comparative terms are of lower investment and operational cost. 3) They have the ability to produce products with higher added value (pure metals). 4) Ability to separate halides easily and easily dispose of spills. 5) They generate a recyclable solid waste to the EAF or HTMR. The problems that remain to be resolved are the following (14, 5): 1) Low recovery of Zinc due to the fact that the zinc contained in the ferrite does not dissolve in practically none of the leaching media used, including mineral acids and strong bases. 2) Metal separation is expensive. 3) The solid residue of the leaching is not inert, which makes its disposal more expensive and limits its applications.

Research work continues with the process in an alkaline medium for the following advantages (15), especially in the acid process: 1) Separation of iron in the leaching stage, since iron oxides are insoluble in alkaline medium (16), unlike Zinc, Lead and Cadmium oxides, which are rapidly dissolved. In an acid medium, a significant amount of iron dissolves, complicating economically the stages of separation of iron and its final disposal. 2) Produce a product with higher added value (high quality Zinc powder). 3) More favorable for better management and minimization of waste. The improvements that have been investigated in the alkaline process refer to the release of zinc from ferrite by melting with soda and subsequent leaching, with a recovery of 95% (17) and leaching with soda through microwaves (4), which increases in 5 to 10% the recovery of Zinc in relation to the conventional system. In sum, from the point of view of environmental sustainability, hydrometallurgical processes are superior to pyrometallurgical processes, and within hydrometallurgical processes the alkaline process has clear advantages over the process in acid medium, hence the importance that this process is having despite that to date there are no known successful industrial achievements. The first and main patents that formalize the hydrometallurgical process in an alkaline medium (REZEDA procedure) are the US patents 3,326,783 of 1967 and the FR 2,510,141 of 1984. The first, applied to the calcine from the roasting of oxidized zinc ores, and the second applied to the powders of electric steelworks. In the 70's, Amax Co. of New Yersey, USA, operated a pilot plant based on the alkaline process. The development was abandoned due to technical and economic problems (17). An industrial plant was later built in France, also on the basis of the alkaline process, and was also abandoned (17). The improvements introduced to the described patents try to solve economically the elimination of the chlorides and sulphates present in the steelworks powders, whose concentration in the electrodeposition stage is subject to restrictions. Next, and in order to have a better understanding of the state of the art of this technology, the base process will be described (REZEDA procedure) and then the improvements introduced later by other patents. The basic hydrometallurgical process, in an alkaline medium, described in the preceding patents, comprises a stage of leaching the steelworks dust with a soda solution, followed by a filtration step in which a solid residue with a high content of iron is separated. to be washed. The filtrate is purified by cementing with zinc dust, precipitating lead, cadmium, copper and other heavy metals, which are separated by filtration. The filtrate passes to the electrodeposition stage, depositing Zinc powder, which is filtered, washed and dried to obtain the dry powder. The spent solution of the electrodeposition stage is recycled to the leaching stage, after elimination of the salt impurities by evaporation. The exhausted solution of recycle, circulating in a closed circuit, is progressively enriched in salt impurities, specifically chlorides and sulphates that are contributed by the powder feed of steel mills in treatment, by soda, water and other inputs. These impurities must be eliminated since a certain level of concentration of them must be maintained in the electro-deposition stage. These salt impurities are removed as a brine obtained by evaporation of the recycle solution. US Pat. No. 3,326,783 of 1967 does not provide information on the quality of the Zinc powder obtained, nor on the disposal of the waste or how the salt impurities and the washing waters are treated, important issues in the economics of the process, which is explained because that time the environmental regulations were rather elementary. Patent FR 2,510,141, details more precisely the stages of leaching, cementing and electrodeposition. It claims a leaching in two stages to solubilize both the zinc present in oxides and in the ferrites, however, it points out that only the leaching of the oxides is economical, and proposes the evaporation step to eliminate the salts. Regarding the leaching residue, it only indicates that it is sterile, without presenting evidence or indicating how it is available. On the quality of the obtained Zinc powder, it only refers to the granulometry, which places 82% between 200 and 325 microns, and does not report the impurities contained or their metallic content, an important issue to establish the profitability of the process . The patent FR 2,757,540 of 1998 proposes to subject the powders of steel mills to a leaching treatment with aqueous liquor to eliminate them, by dissolution, the chlorides and sulfates present to, subsequently, to alkaline leach the wet waste thus washed, cement the solution alkaline and electro-deposit the zinc from it to finally recycle the spent solution directly to the leaching stage. On the other hand, a fraction close to 70% of the washing solution, loaded with chlorides and sulphates, is recycled to the washing stage with bleach, while the rest is subjected to neutralization with acid to precipitate mainly lead hydroxide, which is sent to alkaline leaching together with the wet residue from the bleach wash. The filtrate is treated in an evaporator to separate a brine and water that is re-processed. The analysis of this patent is as follows: 1) The desalination pretreatment of the steelworks powder forms an autonomous process line (L1) independent of the line (L2) of alkaline treatment itself, linked only because the latter is fed with the wet residue and the hydroxide precipitates generated by the first one, thus losing the degree of separation that they already had. 2) The patent states that the washing water that is treated in the evaporator are only those generated in L1. Nevertheless, in L2 the following situation is found: the wet waste that feeds this line has a chloride content of not more than 0.03% (18) and an estimated humidity between 10 and 30%, humidity level that depends on the type of liquid solid separation that is used. Since the line operates with a leaching solution in a closed circuit, there will be a cumulative effect of salts and water. The content of salts is limited by the concentration imposed by the electrodeposition step, as also the water content is limited by the concentration of the soda in the recycle stream, which must be kept between 300 and 400 g / L. On the other hand, it must also be considered that the washing waters in the filtration stages of the process line are charged with soda and sodium zincate, which is necessary to recover, all of which makes it necessary to carry out continuous or periodic purges and treat the waters of washing in an evaporator. However, the patent proposes to recycle a fraction of the electrodeposition solution to the leaching stage, and neutralize the rest with external acid discharges from galvanization operations. The final result is that evaporation is eliminated but soda is lost and liquid waste is generated. Finally, patent FR 2,770,229 of 1999 refers to the conditioning of the zinc powder obtained by electrodeposition in an alkaline medium. Basically, the pulp containing the Zinc powder from the electrodeposition is filtered in an inert atmosphere. The washing of the Zn cake soaked in the zincate solution is carried out with demineralized and deoxygenated water, finally drying under vacuum (65 mm Hg) and at low temperature (20-30 ° C). All these operations are carried out in a single team. It raises the use of ultrasound to accelerate the washing operation or to improve the faster segregation of the final powder. The zinc powder registered in this patent has a total zinc content of 99.6%, of which 97-98% is metallic Zn and 2 to 3% is zinc oxide, and exhibits a granulometry of the order of 6 microns. In all the patents discussed above, conventional electrodeposition cells of rectangular design, open to the atmosphere on their surface, are used, so they suffer from the disadvantage of producing alkaline mist due to the rupture of hydrogen and oxygen bubbles. In addition, their hydrodynamics determine that they produce Zn of coarse granulometry, flake type, which tend to adhere to the surface of the cathodes, making difficult its removal, making necessary the use of complex systems of vibration or sweeping. The proposed invention patent application refers to a traditional alkaline hydrometallurgical processing (REZEDA procedure plus the innovations proposed by later patents), to which several process steps are introduced as well as improvements and modifications that allow Zinc powder to be obtained. high purity and fine granulometry from electric oven powder (EAF dust), avoid the generation of liquid waste and minimize gaseous emissions, contributing to environmentally sustainable process by reducing environmental pollution to levels below those required by regulations, and also reduce the consumption of water and alkali and make the gangue inert, valorizing it as construction material or material to be recycled to steel mills, all of which supports the profitability of the process.

Next, the improvements, modifications and new operations are summarized, which make the difference with respect to the patents analyzed above, and which aim to ultimately solve the economy of the process and the environmental sustainability that this technology requires in order to achieve industrial recognition that is not has achieved to date: a) Inertization and recovery of the waste to which the Zinc has been extracted, so that it becomes a by-product for the construction industry or in a material suitable for recycling to steel mill furnaces. b) Elimination of arsenic from liquid streams by means of an adsorption operation in stages with chemical reaction, in cross currents, carried out by percolating the solution on a fixed bed of Ba (OH) 2. c) Purification of the leaching solution by fractional crystallization by evaporation, maintaining its concentration of impurities, such as chlorides, sulfates and others, at a certain level required by electro-deposition. d) Use of operational practices that reduce the emission of particulate material, both inside and outside the process vessel, especially during the transport of powders. e) Use of specially designed equipment to eliminate or at least reduce gaseous emissions (alkaline mist) inside the ship and minimize those that are discharged to the outside, in the leaching and cementing operations. f) Use of an unconventional electro-deposition cell to obtain finely divided zinc powder of high purity, without generating polluting emissions (alkaline mist). g) Reduction of water consumption, washing all solids in cross-current or counter-current stages and segregating the solutions, so that the low-concentration ones are recycled in the washing stages and the more concentrated ones are evaporated to recover the water and reconcentrate the soda, thus eliminating the generation of liquid waste ("dry plant"). h) Use of best practices and operating conditions for the washing of Zinc dust in stages, cross currents or countercurrent, i) Use of a drying equipment and another cooling Zn powder specially designed for this purpose, which allow get a high purity. For a better explanation of the improvements and modifications, a description of a preferred embodiment will be made below, based on Figure 1, which shows a generic, preferred scheme of the process.

As shown in Figure 1, the electric oven powder 1 is fed, together with an alkaline solution 2, to a leach reactor 3. The resulting suspension is passed to a solid-liquid separator 4, from which a solution of 5 cinnates (mainly Pb and Zn) and a wet solid (or gangue) 6. The latter is treated with an inerting mixture 7 to be then spread on a drying court, being removed as an inert by-product 8 after a period of curing. The solution of zincates 5 is purified with Zn 9 powder in a cementing reactor 10. The resulting suspension is passed to a solid-liquid separator 11, from which a solution is obtained mainly of zin-12 cincate and a Pb-13 paste. Zinc zincate 12 solution is mixed with recycle zincate solution 14 to continue purifying, this time by adsorption with chemical reaction in a bed of Ba (OH) 2 15, which is carried out by percolation in stages in cross currents. Once the bed 15 is saturated, 16 is discarded and replaced by a fresh one. For its part, the arsenic-free solution 17 is stored in a pond 18, from which a fraction of it is constantly extracted 19 to condition both its moisture content and its salt content, in an evaporator 20 followed by a crystallizer 21. The steam 22 withdrawn from both units is condensed and recycled as process water, recovering the thermal energy in the operation; likewise, the concentrated solutions of zincate 23 and 24 are recirculated to the crystallizer and the evaporator, respectively, while the salts produced in the crystallizer are disposed as solid waste.

In this way, in pond 18, a pure and rich solution in Zn zincate is permanently maintained, which continuously feeds the electro-deposition circuit 26, from which a finely divided Zn pulp 27 is obtained, suspended in solution of Zn. zinc zincate. This pulp is passed through a solid-liquid separator 28, from which a solution 29 is obtained, pure and depleted in Zn cincate, which is stored in the tank 34, and a Zn cake 30 embedded in Zn cincate which, then washed and dried in the dryer 31 under an inert atmosphere 32, constitutes the main product of the plant, a powder of Zn 33, fine and of high purity. A first approach of this patent is that, to simplify the handling of the powders, and to avoid the emission of particulate material, both inside and outside the process vessel, the moisture content of the electric oven powders it must be conditioned between approximately 5 and 10%, preferably around 8%, and maintained at this value during transport from the source supplier, as well as during its subsequent storage and internal management in the plant. To do this, sensors and sprinklers must be available in both the collection equipment and transport vehicles. According to the base process used, the extraction of Zn from the electric oven powder (EAF dust) is carried out in the leach reactor 3, mixing them with an alkaline recycle solution, pure and depleted in zinc, with a Zinc content. less than 10%, with 20% to 50% alkali, preferably NaOH, in a powder to solution ratio between 1: 3 and 1:15, preferably 1: 7, at a temperature between about 70 and 120 ° C, preferably about 90 ° C, under mechanical agitation between about 50 and 150 rpm, preferably about 120 rpm, for about 30 to 240 minutes, preferably about 60 minutes. In a second approach of this patent, it is established that the reactor must be of closed design, to avoid the emanations of alkaline fog both inside and outside the process vessel, which includes an agitator and an external heating system, and whose cover is provided with a condenser followed by an extractor and a vent that leaves the process vessel. A negative pressure is maintained in its interior, between approximately 30 and 60 mm of Mercury, by means of a blower that passes the vapors through a condenser previously to discharge them to the atmosphere through the vent. In addition, the same type of equipment and operational practice should be used in cementation. The suspension resulting from the leaching is passed to a solid-liquid separator 4 which, depending on the production scale, can consist of a filter press, a band filter, a centrifuge, or a system that also includes settling tanks and / or decanters. , it being possible to wash the cake in the same equipment, or separately. The same is valid for the other solid-liquid separation operations of the proposed process (equipment 11, 15 and 28). A third approach of this patent is that the operation of washing the cake, in any of these separators, must be carried out in stages, preferably in countercurrent or cross currents. For example, the washing of the leaching cake (wet gangue, embedded in zincates) is carried out with several successive loads, which must be the first of a soda solution and the last of water. The use of soda in the first loads is essential because it prevents the precipitation of Zn (OH) 2, in which case the Zinc is lost when it is retained in the gangue cake. The number of loads, the volume to be used in each and the concentration of soda in them, will depend mainly on the concentration of soda in the leaching solution and the percentage of suspended gangue. For example, if filter press is used to filter a pulp of 8% suspended solids in 32% soda, when using volumes of loads equal to one volume of cake in each wash, it requires about 6 to 10 loads to reduce the soda content in the cake to insignificant levels, the first 2 to 3 loads of a soda solution and the last of water. This practice can reduce more than 10 times the consumption of washing water, compared to the washing operation by continuous injection of water to the filter. On the other hand, segregating the washing waters, so that the low concentration, for example up to 5%, are recycled, and the more concentrated evaporate to recover the water and reconcentrate the soda, it is possible to optimize the water consumption and eliminate the generation of liquid waste, transforming the facility into a "dry plant". The solution used in leaching has 32% soda and is recirculated in closed circuit, incorporating fresh solution only for the replacement of minor losses. Therefore, any amount of water added to that main stream must be removed to restore the concentration of the soda solution to its original level. A fourth approach of this patent is that the gangue, consisting mainly of Fe, washed and wet, is treated with an inert mixture, consisting of a source of Si02 (pozzolan, bentonite or silica gel), between approximately 0.02 and 0.5% of dry bargain, Ca (H2PO) H20 between about 0.5 and 3%, and Ca (OH) 2 between about 0.2 and 0.8%. These components must be added to the wet gangue cake so that the dough remains with a moisture content between about 15 and 25%, preferably about 20%. The dough is subjected to mixing in a mixer type equipment until homogeneity. The bargain thus obtained, after a curing period of about 3 to 10 days, and dried to a moisture content between about 6 and 12%, preferably about 10%, is inert (improvement a.-) according to Environmental standard EPA SW-846 according to Method TCLP-1311, typical results being around 0.15 mg / L of Cd and around 0.28 mg / L of Pb, when the maximum limits of the EPA standard are 1 and 5 mg / L, respectively, as shown in Table 2. This safe gangue constitutes approximately 63% of the original mass of the electric oven powder and can be valued, among other ways, by its application as a construction or filling material, in the manufacture of refractory bricks, heat accumulators or pigments, or recycled to electric furnaces.

The solution of zincates 5, to which it is convenient to incorporate the most concentrated washing waters, is purified in a cementing reactor 10 with Zn 9 powder, added in a proportion between approximately 2% and 12% higher than the stoichiometric (preferably around 8%) to reduce the metals that are under Zinc in the voltage scale. The reactor operates at a temperature between about 70 ° and 120 ° C, preferably around 80 ° C, under mechanical stirring between about 50 and 100 rpm, preferably around 80 rpm, for about 30 to 180 minutes, preferably about 60 minutes. The resulting suspension is passed to a solid-liquid separator 11, from which a solution of pure Zn 12 zincate and a paste mainly of Pb 13 is obtained. A fifth approach of this patent is that, preferably following the cementation, must introduce in the process an arsenic abatement operation, since it is concentrated in the liquid streams, despite its low content in the electric oven dust, and may exceed the permissible limit in the electro-deposition (0.002 g / L) and also constitute a risk to the health of staff. The elimination of arsenic (improvement b.-) is carried out by adsorption with chemical reaction on solid Ba (OH) 2, according to the following stoichiometry (19): As205 + 2 Ba (OH) 2 = 2 BaHAs04 + H20 In the In practice, it is convenient to carry out the operation in stages, countercurrent or cross currents, percolating the cemented solution on a fixed bed of Ba (OH) 2. This bed should be operated with a Ba (OH) 2 load between about 3 and 10, preferably about 5, times greater than the stoichiometric required by each batch of the cementer, for safety purposes. By way of example, if 1000 L of 32% soda solution and 0.023 gpL of Ace are treated, that is, containing 0.023 kg of As, according to the stoichiometry 0.0487 kg of Ba (OH) 2 will be required to knock it down , and operating at 5 times the stoichiometric requirement, 0.244 kg is needed. The filter purifies the solution of the stock tank of pure solution and rich in Zinc Cincate 18, and even controlling the flow of recirculation 14 can significantly increase the retention time, which allows a great flexibility to face eventual increases in concentration of As The temperature of the storage tank solution is maintained between approximately 30 and 90 ° C, preferably around 35 ° C. A sixth approach of this patent is that the alkaline solution thus purified, is electro-deposited in a Cylindrical cell, type E EW, of Electrometals Technologies Ltd., whose design allows to confine the generated gases, and to wash them prior to their evacuation if necessary. Since it is a closed circuit, the phenomenon of alkaline fog will not occur inside the ship, and the emission to the atmosphere will be a gas composed of approximately 70% oxygen and 30% hydrogen. Typically, it is possible to obtain Zn of fine granulometry, less than about 100 microns and where 50% of the particles is less than about 40 microns, which is separated and washed in a solid-liquid separation equipment. However, by managing the operating conditions of this cell, such as the solution flow through it, the soda concentration thereof, and the current intensity, it is possible to regulate the granulometry of the zinc. Unlike this approach, traditional processes employ a seventh approach of this patent refers to the design of a special drying equipment 31 for Zn powder, and the use of optimal operational practices and conditions for solid-liquid separation 28 and the washing of zinc dust, which must be carried out in stages in countercurrent or in cross currents, under conditions similar to the third approach, since in that way they allow to obtain Zn of high purity. In this case, the washing with water is inconvenient because it will induce the decomposition of the zincate and the consequent precipitation of zinc oxide, which will be retained in the wet Zinc cake, contaminating it. Once the concentration of alkali in the cake has been lowered to insignificant levels, by means of the use of loads of soda solution, it is possible to finish washing with water, thus minimizing the presence of zinc oxide in the final product. Next, a description will be made of a preferred embodiment of the drying equipment 31, based on Figure 2, which shows a generic, preferred scheme of the equipment. The actual drying chamber 1 must be of cylindrical, static design, provided with the following elements: a rotating screw 2 arranged axially inside it, externally driven by a motor 3; a tilting mechanism; a condenser 4 connected at the top of one of its ends, which includes an indoor cooling coil 5 with inlet 6 and outlet 7 for cooling water, an outlet through a water seal 8 that allows the continuous removal of the condensate, maintaining the inert atmosphere inside the dryer; an inert gas outlet 9 connected to a blower 10 which follows the condenser and which is connected to the upper part of the other end of the cylinder; a hopper 11 including an endless screw 12 driven by a motor 13 external to it, connected to the cylinder through a rotary valve 14 in the vicinity of the connection of the condenser, provided with nitrogen injection in its base, for the supply of Wet zinc cake excluding air; a rotary valve 15 located in the lower part of the cylinder, at the opposite end to that of the hopper, also provided with nitrogen injection to remove the dry powder; an indirect heating system 16, indirectly, by means of electric resistances, steam or heating oil or combustion gases; a nozzle for nitrogen injection. The dryer operates at a slight positive pressure of an inert gas inside, for example nitrogen, at temperatures between approximately 100 and 400 ° C, then the powder is cooled to room temperature and finally reduced in its granulometry to less than 6 microns and packaging, also under inert atmosphere. For the cooling of the powder, equipment of the same characteristics of the dryer is used, that is to say, of cylindrical design, provided with the following elements: a rotary auger arranged axially inside it, externally driven by a motor; a tilting mechanism; a rotary valve in the upper part of one of its ends and another in the lower part of the opposite end of the cylinder; an external refrigeration system, indirect, by means of a refrigerant fluid; and a nozzle for nitrogen injection. A final approach of this patent consists in the use of a crystallizer by evaporation, after the multi-effect evaporator, to obtain crystallized salts that are eliminated as a purge. The purpose of this last operation is to purify the leaching solution, maintaining the concentration of impurities, such as chlorides, sulfates and others, at a certain level required by electrodeposition. Next, results of a typical operation at pilot scale of the process of the invention are exposed, on a treatment basis of 22 ton EAF / day. Table 4 shows the main input and output currents, from which it is possible to establish the percentage distribution of the main elements in the different product streams, which is shown in Table 5. In this last table it is observed, for example , that the arsenic that enters the process as the content of the electric oven powder, which in Chile is between 0.01 and 0.03% dry base, is distributed in a 25% to the gangue, 73% is killed in the Ba filter ( OH) 2 and 0.15% goes as a contaminant in Zn dust.

Table 4. Main flows of a typical operation (22 ton EAF / day).

The current numbers refer to those in Figure 1 Table 5. Distribution of the elements in the different products and waste.

The numbers of currents refer to those of Figure 1. Next, the improvement (g) described above, in relation to the consumption and recycling of water is illustrated. The EAF powder is leached with 32% soda solution, in a mass proportion of "soda / powder solution" = 7 (equivalent to a pulp of 40.5% suspended and dissolved solids). The required leaching solution is prepared by adding to the recycled solution, depleted in Zn, a small amount of soda solution to replenish the leach and separate the gangue by filtration, the solution is cemented and then separated from the Pb cement. The purified solution is thus obtained, which is collected in a pond, from which a flow is continuously extracted for the operation of the EMEW, which extracts the Zn in powder form, thus generating the 32% solution of soda depleted in Zn. , which is recycled. The leaching is always carried out with soda at 32% by weight, which constitutes the main stream of the plant, therefore, any water that is introduced into it has to be removed. The water is removed in an evaporator-crystallizer, in which the evaporated water is recovered in a condenser. This condenser uses cooling water from a cooling tower. From the evaporator a condensate and a 32% soda solution are obtained; part of this solution is recycled and the rest goes to the crystallizer, from which a condensate is obtained, a suspension that is purged and a concentrated soda solution that is recycled after being concentrated at 32%. Thus, the plant does not generate liquid waste that must be collected externally to it. The sources of income and water losses of the main stream mentioned are shown in Table 6, together with the destination indicated, which are quantified in Table 7, on a treatment basis of 22 tons EAF / day. It is observed that the daily consumption of fresh water amounts to 18.58 tons, that is, only 0.85 tons of water / ton EAF treated. Table 6. Sources of income, discharge and destination of water.

Table 7. Daily water balance (base 22 t EAF / day) Balance H20, t / day comes out recycles Water Powder EAF 07 Water in leaching solution, recycled 1048 104.8 Water in soda solution 32%, replenishment 3.2 Wash water in 32% soda solution. recovered in evaporator. recycled 2.4 2.4 Water for washing filter cakes, recycled by evaporation 14.0 14.0 Water for washing filter cakes, recycled in circuit. of washing 23.6 23.6 Water for washing filter cakes, lost by evaporation (98.5% recovery) 03 Condensed water lost in Evaporator-Crystallizer (98.5% recovery) 0.2 Evaporated water to obtain the purge, recycled 5.7 57 Water eliminated with the purge 02 Water that goes away in the gangue for the hydroxides, formed 0.4 Evaporated water from the gangue (without recovery) 34 Evaporated water from the Pb Cement (without recovery) 0.3 Evaporated water from the recycled Zn dust 0.7 0.7 Water for washing floors and equipment, recycled. by evaporation 19 19 Water for washing floors and equipment, lost by evaporation 01 Cooling water, recycled 110995 110995 Cooling water lost by evaporation (1.5%) 166 Heating steam, recycled 47.5 475 Heating steam, lost (1.5%) 0.7 ^ SUBTOTAL 1314.9 225 1310.9 fresh water consumed 18.5 Balance, 13335 13335 The effectiveness of modifications d), e) and f), in relation to the emissions of particulate material in the process on a processing basis of 22 tons of EAF dust / day is illustrated below.

The unit operations of the alkaline process that emit particulate material were classified into 10 stages, as shown in Table 8, together with the corresponding emission factors (a) and emissions of particulate material (b), with and without mitigation. The main conclusions of the study of emissions of particulate material are the following: • The emissions of particulate material with mitigation measures, show that these emissions do not exceed 1 kg / day and 0.25 ton / year, therefore, they are not significant The calculations of particulate matter emissions for plant operations were made without considering mitigation measures to obtain a maximum and maximum level of emissions. • At each point of emission of particulate material, mitigation measures will be implemented to minimize the emission.

These mitigation measures are humidifiers and dust collectors with efficiencies of no more than 90%. • The storage of dust will be carried out in a closed warehouse with humidifiers and natural ventilation current. The literature indicates that for this type of mitigation, yields of more than 75% can be achieved. • The stages of the process are considered to be carried out in barn buildings with natural ventilation, which for the base case exemplified has an area of 6,100 m2 and a height of 10 m. • Calculations of emissions using emission factors have an implicit error that varies between 2 and 10%, depending on the degree of accuracy with which they were generated. Next, alkaline mist emissions are estimated taking into account all sources, without considering mitigation measures, in order to obtain a maximum and maximum level of emissions that would occur in the operation of the plant. Table 9 summarizes the calculations for the fog emission estimates. The concept of fog refers to the evaporation of solutions in the leaching, purification and electro-winning processes.

The emissions of haze in the process of electro-obtaining in the EMEW cells are null because this is a closed process. The solution treated in this stage is returned to leaching, so that the fog emissions are only in the leaching and cementing stages. The estimation of the fog emissions was made considering the following assumptions: • The evaporated mixture of the alkaline solution is considered in equilibrium at the temperature of the process. • For all the calculation effects, the mist is considered as an ideal gas. • There is no forced ventilation in the plant enclosure, it is considered a natural ventilation current of 0.2 (m / s) (0.72 km / h, typical value in closed works). • A relative humidity of 50% and a plant ambient temperature of 25 ° C are considered. Table 10 shows that the total level of haze emissions during the operation of the processes with NaOH is 0.00044 [mg / m3]. The total amount of evaporated NaOH is 0.596 mg / day, therefore, the standard of Environmental Hygiene is not exceeded. Having described the invention in detail, as can be appreciated, various changes and modifications can be made without departing from the spirit of the invention. All of those changes and modifications are considered included within the scope of the present invention as defined in the appended claims.

Table 8. Emissions of particulate material in the process 22 ton EAF / day). (a) Issue factors. pn. | Emission Factor | Units | Observations Dry Area Dc discharged material (?? 42 C 11 S 12) 1 0.0017 kg / Mg Assimilated to unloading of raw material in cement manufacture Of stored material (?? 42 C13S2-4) U = 0.75 m / s M = 5% '> 0.00050. { (U / 2.2)? · 2 g / ton U = Average velocity in the zone -V (M / 2) > ! M = Containment of the material's smoke content Assimilated to vehicle emissions on the fine-tuning road (42 C13S2-2) (kilometers / Kilometer Vehicle K = Particle size factor, 4.6 for PM l 0 si. = Fines load in folder, calculated? * (1./2)? 0.65? (g / KTV co ntid d of fine material desc rg to w / 3) "i .5 - c pi o of the warehouse of unit operations 3 of the process. Circulation area 200 m2, 0.823 g / m < i + VV = that average of vehicles circulating, 0.8 ton c kg / Mg C = vehicle emission agent circulating 0.053 for wear of brakes and tires, 0.2119 Assimilated to material transport n front loader (F 1 R \ 'i 6.24 SCC 3-05-025-06, modified by unit conversion factor). From unloaded material (?? 42 C11S12) 4 0.0017 kg / Mg Assimilated to discharge of raw material in cement manufacture. Of ground material (?? 42 C11S19-2) Assimilated to grinding fines of the 0.0075 kg / Mg grinding processes in the aggregate industry. Of processed material. Assimilated to 6 0.03 kg / Mg Processing of metallic minerals (?? 42 C11S24). Loading and transp os of material. Of processed material. A imilado a 7 0.03 kg / Mg Metallic mineral processing (? 42 C11S24). Loading and transfer of material. Of processed material. Assimilated to 8 0.03 kg / Mg Process of metallic minerals (AP42 C11S24). Loading and transfer of material.

Humid Area Of rubbed material. Asimil do a 9 0.03 kg / MK Processing of metallic miners (?? 2 C11 S24). Carry and transfer of material. Electro procurement of packaged product Assimilated to production of C carbon (?? 42 0.14 C6S1, I · I I -: 6.24 SCC 3 01 -005-08, modified by unit conversion factor). Knuckle of final product. a: Average annual speed, b: Data given in description of the process, c: It has been estimated that the mini front loader weighs 450 kg and loads 250 kg at a time, d: The value obtained is calculated assuming all the processes found in the dry area of the plant for both processes and that there is an area of displacement of 200 m2, fines load = 2.1 * emissions of particulate material in the process. (b) Emissions of particulate material. * k g / m e s Operations Dry Area Operation Wet Area 1 Unloading of oliv tic 6 Transportation in stra ns 9 P u r i fe n tio n, cerias da s i 1 > Discharge of pvvo 2 Storage to Zinc Fine Storage Dust Bulk 7 D Pol. Charge Electro 3 Operation After Cutting Achanger in Silo Obtaining (Clergy I * Rum Tl) Storage 10 Kn v sa do de 4 Download from 8 to 8 Product transport end powder l crusher material a re ctor di 5 (Glider (m lienda li liviación terci rí) Table 9. Summary of calculations for the estimation of alkaline mist emissions.

Table 10. Alkaline Haze emission.

Claims (13)

1. CLAIMS 1. An electro-hydrometallurgical process in an alkaline medium for the extraction of Zn from electric oven powder (EAF dust) to produce high purity Zn and fine granulometry, which comprises lixivizing the electric oven powder with an alkaline solution, separating the spent gangue liquid, wash the latter, purify the liquid by cementation and separate it from the precipitated solids, wash the latter, partially evaporate the pure solution and purge a fraction of the concentrated solution obtained, electro-deposit the zinc from this solution; It also comprises separating, washing and drying the deposited zinc, characterized in that it also comprises making the leach gangue inert, purifying the solution obtained from the cementation by adsorption, evaporating a fraction of this purified solution, subsequently crystallizing a fraction of the above concentrated solution, conditioning the humidity of electric oven powder for transport and handling; Separate the Zn powder from the zincate solution, wash in successive stages the separated solids after leaching, cementation and electro-deposition.
2. 2. A process according to claim 1, characterized in that the step of rendering the leach gangue inert comprises treating said wet gangue with a mixture of Si02 (pozzolan, bentonite or silica gel), between about 0.02 and 0.5% of the dry bargain, Ca (H2PO) 4H20 between approximately 0.5 and 3%, and Ca (OH) 2 between approximately 0.2 and 0.8%, these components are added to the wet gangue cake so that the dough remains with a moisture content between approximately 15 and 25%, preferably around 20%, this mass is subjected to mixing in a betonera type equipment until homogeneity and then dried to a moisture content of about 10%.
3. 3. A process, according to claim 1, characterized in that the alkaline solution is adsorbed by percolation with Ba (OH) 2 to remove the arsenic, where said alkaline solution, rich in zinc and partially purified by cementation, is percolated continuously on a fixed bed of Ba (OH) 2, preferably operating in stages, countercurrent or cross currents, with a Ba (OH) 2 load between about 3 and 10, preferably about 5 times, greater than the stoichiometric required for each batch of the cement, for safety purposes; once the adsorption capacity of the bed is exhausted, it is removed, dried and disposed of in a landfill.
4. 4. A process according to claim 1, characterized in that it also comprises evaporating crystallization of the leaching solution, after evaporation in a multi-effect evaporator, to obtain crystallized salts that are eliminated.
5. A process according to claim 1, characterized in that it includes conditioning the moisture content of the electric oven powders between about 5 and 10%, preferably about 8%, which must be maintained during transport from the source supplier, as well as during its subsequent storage and internal management in the plant, for which sensors and sprinklers must be arranged both in the collection equipment and in the transport vehicles.
6. 6. A process according to claim 1, characterized in that the alkaline leaching solution is concentrated in a multi-effect evaporator, up to about 32% soda, to be recycled to the leaching stage, and also to recover the water, which It is reused in the washing processes.
7. A process according to claim 1, characterized in that the washing of solids embedded in zincates, which is required in the solid-liquid separation operations of the leaching, cementing and electrodeposition stages, is carried out in one or more stages , preferably in countercurrent or cross currents, using between 3 and 10 successive loads of washing solutions, the first 2 to 5 loads of a 200 and 400 gpL sodium chloride solution and the last of water, with volumes of the order of approximately one to three volumes of cake to be washed; then, the washing waters used are segregated, in such a way that the most diluted ones are reused in washing operations, in a closed circuit, and those of greater concentration are incorporated into the main alkaline current, prior to the cementation, to later be concentrated in the evaporator according to the process of claim 6, thus recovering both the water and the concentration of the alkali from the leaching solution.
8. 8. A process according to claim 1, characterized in that the purified alkaline solution is subjected to electro-deposition in a cylindrical cell, EMEW type, of Electrometals Technologies Ltd., whose design avoids the generation of alkaline mist and allows washing the gases generated prior to evacuation. In this cell a zinc pulp with a fine granulometry of less than about 100 microns is obtained, where 50% of the particles is less than about 40 microns, which is separated and washed in a liquid solid separator, and then dried. cooled and packed.
9. 9. A process according to claim 1, characterized in that it is necessary to filter, in the solid-liquid separating operation of the Zn pulp, by means of a filter press to separate the zinc from the zincate solution and obtain a zinc cake, which is washed in situ, in cross currents, with successive loads first of pure soda, between 200 and 400 gpL of NaOH, to free it from the cincate, and then of water to free it from the soda, according to the claim 7.
10. 10. A dried Zinc pulp drying equipment, useful in an electro-hydrometallurgical process in an alkaline medium for the extraction of Zn from electric oven powder (EAF dust) to produce Zn of high purity and fine granulometry. according to claim 1, characterized in that it comprises a cylindrical body provided with an endless rotary screw, axially disposed therein and externally driven by a motor; a tilting mechanism; a condenser connected in the upper part of one of its ends, which includes an inner cooling coil and an outlet through a water seal; a blower that follows the condenser and is connected to the upper part of the other end of the cylinder; a hopper that includes an auger driven by an engine external to it, connected through a rotary valve in the vicinity of the connection of the condenser to the cylinder; a rotary valve located in the lower part of the cylinder, at the opposite end of the hopper; an external heating system, indirect, by means of electric resistances, steam or heating oil; and a nozzle for nitrogen injection.
11. 11. Equipment for the cooling of zinc powder useful in an electro-hydrometallurgical process in an alkaline medium for the extraction of Zn from electric oven powder (EAF dust) to produce Zn of high purity and fine granulometry according to claim 1 , characterized by being of cylindrical design, provided with the following elements: a rotary auger, arranged axially inside and externally driven by a motor; a tilting mechanism; a rotary valve in the upper part of one of its ends and another in the lower part of the opposite end of the cylinder; an external refrigeration system, indirect, by means of a refrigerant fluid; and a nozzle for nitrogen injection.
12. 12. Equipment for carrying out both leaching and cementing in an electro-hydrometallurgical process in an alkaline medium for the extraction of Zn from electric oven powder (EAF dust) to produce Zn of high purity and fine granulometry according to claim 1 , characterized in that it comprises a closed reactor, which includes a stirrer and an indirect heating system, whose lid is provided with a condenser followed by an extractor and a vent that leaves the process vessel, a system that maintains a small negative pressure at the inside the reactor, in order to eliminate any leakage of alkaline mist both inside and outside the process vessel.
13. 13. Zinc powder, of high purity and fine granulometry, characterized in that it is obtained by any of the processes of claims 1 to 9. ABSTRACT The process object of this patent, produces zinc dust of high purity and fine granulometry from electric oven powder (EAF dust) through the conventional stages of alkaline leaching, cementing and electroplating, but introducing improvements, modifications and new operations. that, in addition, minimize water consumption, avoid the generation of liquid waste and minimize the generation of solids and gaseous emissions, reducing environmental pollution to levels lower than those required by regulations, which is essential for the materialization of the project. The fine granulometry of Zinc powder is achieved with the use of an unconventional electroplating equipment, which also eliminates gaseous contamination in this operation. Also contributing to the latter are equipment and operational practices specially introduced for this purpose, both in the transport and leaching of dust and in the purification of the zincate solution. Water consumption is minimized by carrying out all the solid washes in the process, in countercurrent stages or in cross currents, and by recovering and recycling the water, which also eliminates the generation of liquid waste ("dry plant"). In addition, a procedure is introduced to render the gangue inert, which values it as a construction material, and another for the elimination of arsenic from liquid streams, making the operation safer. Finally, an equipment and operation for the washing and drying of zinc dust is proposed.