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:
RELATED APPLICATION 
     This application claims priority of Chilean Patent Application No. 1824-2006, filed Jul. 12, 2006, herein incorporated by reference. 
     TECHNICAL FIELD 
     This disclosure relates to electro-hydrometallurgical processes, more particularly to a process for extracting zinc from electric furnace dust (EAF dust). 
     BACKGROUND 
     Electric steelworks mainly use pressed scrap, sheets, galvanized elements and the like, which contain heavy metals such as Zn, Pb and Cd, which evaporate at a high temperature to which these furnaces operate (1600° C.), then condense as oxides when the smoke gets cold and are captured in sleeve filters. This dust is very fine and difficult to handle, its average chemical composition is shown in Table 1, where a 20-21% of Zn, 20-35% of Fe, 3 to 5% of Pb, 3% of chlorides, etc. are highlighted. Cd presence is only recorded in no more than 800 ppm. 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Average Composition of steelworks dust (EAF dust), % p/p 
               
             
          
           
               
                   
                 Element 
                 Chile 
                 USA 
                 France 
                 Spain 
               
               
                   
                   
               
             
          
           
               
                   
                 Fe 
                 34.4 
                 28.5 
                 21.8 
                 25.9 
               
               
                   
                 Zn 
                 20.0 
                 19.0 
                 21.2 
                 18.6 
               
               
                   
                 Pb 
                 4.68 
                 2.1 
                 3.6 
                 3.63 
               
               
                   
                 Cd 
                 0.076 
                 &lt;0.01 
                 ND 
                 0.10 
               
               
                   
                 Cr 
                 0.2 
                 0.39 
                 0.37 
                 0.31 
               
               
                   
                 As 
                 0.01-0.03 
                 ND 
                 ND 
                 ND 
               
               
                   
                 Ca 
                 1.91 
                 1.85-10.0 
                 12.8 
                 3.50 
               
               
                   
                 Ni 
                 0.017 
                 0.01-0.02 
                 0.1 
                 0-0.7 
               
               
                   
                 Mo 
                 &lt;0.01 
                 &lt;0.02 
                 ND 
                 ND 
               
               
                   
                 Mn 
                 1.83 
                 0.08 
                 2.5 
                 2.81 
               
               
                   
                 Mg 
                 0.71 
                 2.46-4.60 
                 ND 
                 1.53 
               
               
                   
                 Cu 
                 0.2-03 
                 0.77-2.93 
                 0.25 
                 0.54 
               
               
                   
                 Si 
                 1.5 
                  0.6-2.32 
                 ND 
                 1.65 
               
               
                   
                 Cl 
                 3.5 
                 1.35-2.49 
                 1.75 
                 3.43 
               
               
                   
                 F 
                 — 
                 0.51-2.36 
                 ND 
                 ND 
               
               
                   
                 K 
                 2 
                 0.01-0.88 
                 2.06 
                 1.23 
               
               
                   
                 Na 
                 1.37 
                 0.06-1.12 
                 2.23 
                 1.27 
               
               
                   
                 Al 
                 0.27 
                 0.29-2.31 
                 ND 
                 0.44 
               
               
                   
                 S 
                 1.08 
                 ND 
                 ND 
                 ND 
               
               
                   
                   
               
               
                   
                 Refs. (4, 5) 
               
             
          
         
       
     
     When the Zinc content in the dust is at a level of 20 to 21%, it happens that a 20 to 30% of the same is found in the form of zinc ferrite, ZnFe 2 O 4 , that dissimilar to zinc oxide, ZnO, is hardly soluble in hydro-metallurgic treatments, both in acid and alkaline media, to which it is subjected to retrieve zinc. 
     Environmental regulation from almost any country in the World, which is increasingly rigorous, classifies the electric steelwork dust as a hazardous waste due to its content of heavy metals, which means that the waste can not be disposed without a previous treatment (5). On the other hand, the feasibility of recycling this dust to the same steelwork which produces it, to retrieve the iron it contains (25-35%), is difficult due to operational problems it causes because of the fineness of the dust and the presence of the above-mentioned contaminants. The hazardous character of steelworks dust is because of the presence of Pb, Cd and hexavalent Cr, which is measured by the Environmental Protection Agency of USA EPA SW-846 test, Method-1131 “TCLP—Toxicity Characteristic Leaching Procedure” (7,8). Table 2 shows lixiviation limits which wastes must meet. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Lixiviation 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 in most of Latin American countries, the above regulation is applied. Recently, environmental regulations have progressed around the world, for example, in the USA since 1988, the EPA has classified steelworks dust as dangerous (10) and in 1995 it was established that treatment technologies must comply with the TCLP test for 14 elements (11). Table 3 shows typical TCLP analysis for Chilean steelworks dusts, where it is observed that the national regulation for Cd is exceeded. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Typical results of TCLP Test for Chilean steelwork dusts, mg/L 
               
             
          
           
               
                 Sample 
                 P 
                 Cd 
                 Cr 
                 A 
                 Ag 
                 B 
                 Hg 
                 Se 
               
               
                   
               
             
          
           
               
                 1 
                 0.5 
                 4.0 
                 0.05 
                 &lt;0.001 
                 &lt;0.02 
                 0.4 
                 &lt;0.001 
                 0.10 
               
               
                 2 
                 1.8 
                 24.6 
                 &lt;0.03 
                 &lt;0.001 
                 &lt;0.01 
                 0.5 
                 &lt;0.001 
                 0.14 
               
               
                 Norm 
                 5 
                 1 
                 5 
                 5 
                 5 
                 100 
                 0.2 
                 1 
               
               
                   
               
             
          
         
       
     
     Consequently, environmental considerations are key in the research and development of technologies to process this type of wastes. Thus, the strategy to follow should recover the valuable metals (Fe, Zn, Pb) and generate a no hazardous waste that can be stored without problems, or otherwise valorized through some other use. 
     Environmental considerations mean that the industrial processes to be applied should meet the environmental regulations established by each country, which are translated in rules and regulations expressed in the accomplishment of certain environmental standards regarding the quality of the air, emission of particulate matter and disposition of liquid and solid wastes. 
     For recycling, a number of processes are available nowadays. In the first place are the pyre-metallurgical processes represented by the Waelz process (12), which is the main technology in use of those called High Temperature Metal Recovery (HTMR). It is a robust and well established technology, having the following features:
         1) It operates at high temperatures and makes an intensive use of energy.   2) Minimum economic size is of over 50,000 tons/year (13), which necessarily has led to set up regional plants in USA since the average dust generation in steelworks (EAF dust) is about 8,000 tons/year per plant (10). This faces the governmental pressure of restraining transportation of hazardous materials.   3) It generates low value products which require a hydrometallurgical treatment (washing) to be valorized.   4) It produces an inert slag which is used in roadways stabilization.   5) It releases important volumes of combustion gases into the atmosphere.       

     Several other HTMR (10) technologies have been oriented to solve problems of the Waelz process, like installing small plants, processing dusts without having to agglomerate them, recycling iron and producing a no hazardous waste. Nevertheless, almost all of these processes also make an intensive use of energy and present problems in the condensation of volatile matter (Pb, Zn, Cd and salts as chlorides), reasons why many of them have been abandoned. 
     Hydro-metallurgical technologies are less developed, existing plants at pilot level or at small commercial scale. These technologies are becoming important due to the following reasons (14):
         1) They provide environmental benefits as they do not generate air pollution.   2) They make possible the installation of much smaller plants, which in comparative terms need less investment and lower operational costs.   3) They have the ability to produce higher added value products (pure metals).   4) They have the ability to easily separate halures and provide an easy disposition of dumping.   5) They generate a solid recyclable to the EAF or HTMR.       

     Problems still seeking solutions are the following (14, 5):
         1) Low Zinc recovery because the Zn contained in the ferrite can not be dissolved in virtually any of the lixiviation media used, including mineral acids and strong basis.   2) Separation of metal is expensive.   3) The solid waste from the lixiviation stage it is not inert, what makes expensive its landfill dumping and also limits its applications.       

     Research works are being performed with the process in alkaline media due to its advantages, especially, in front of the process in acid media (15). Advantages are:
         1) Separation of iron in the lixiviation stage, since iron oxides are not soluble in alkaline media (16), different from zinc, lead and cadmium oxides, which are quickly dissolved. In acid media, an important amount of iron is dissolved, economically complicating its separation stages and its final disposition.   2) It produces a higher added value product (high quality zinc powder).   3) It is more favorable because of better handling and minimization of wastes (15).       

     Improvements investigated in the alkaline process are referred to the release of zinc from the ferrite by fusion with soda and later lixiviation, with a recovery of a 95% (17) and lixiviation with soda through micro-waves (4), thus increasing zinc recovery in 5 to 10% in relation to the conventional system. 
     In short, from the point of view of environmental sustainability, hydro-metallurgical processes are better than the pyre-metallurgical ones, and within the hydro-metallurgical processes, the alkaline process has clear advantages over the acid media process, that is why then the importance this process is having nowadays, in spite of the fact that up to date no successful industrial realizations are known. 
     The first and main patents that formalize the alkaline hydro-metallurgical process (REZEDA procedure) are U.S. Pat. No. 3,326,783 and FR 2,510,141. The former is applied to the calcine coming from toasting of oxidized zinc minerals and the latter is applied to electric steelworks dusts. 
     In the 1970&#39;s, Amax Co. of New Jersey, USA, operated a pilot plant based on the alkaline process. Development of the same was abandoned due to technical and economic problems. An industrial plant was built years later in France, also on the basis of the alkaline process, and it was also abandoned. 
     Improvements tried to economically solve the elimination of chlorides and sulfates present in the steelworks dusts, whose concentration levels are limited by restrictions imposed in the electro-deposition stage. 
     Below, and in order to have a better understanding of the background, it is described the basic process (REZEDA procedure) and, afterwards, the improvements incorporated by others. 
     The basic hydro-metallurgical process in alkaline media, described in the above patents, includes a lixiviation stage of steelworks dust in soda solution, followed by filtration in which the solid waste with high iron content is separated after washing. Filtrate is purified through cementation with zinc powder, precipitating lead, cadmium, copper and other heavy metals, which are separated by filtration. Filtrate then goes to an electro-deposition stage, where the zinc powder is deposited and then filtered, washed and dried to obtain the dry powder. The stripped solution from the electro-deposition stage is recirculated to the lixiviation stage, previous elimination of saline impurities by evaporation. The stripped solution recirculating in a closed circuit is progressively enriched in saline impurities, specifically chlorides and sulfates which are added by the feeding of steelworks dusts to be treated, like soda and water, among others. These impurities must be eliminated since a determined concentration level of them must be maintained in the electrowinning stage. These saline impurities are eliminated like brine obtained through evaporation of part of the recirculating solution. 
     U.S. Pat. No. 3,326,783 does not provide information on the quality of the zinc powder obtained, or the waste disposition, neither as how saline impurities and washing waters are disposed of, important things in the economy of the process, which is explained because, by that time, environmental regulations were rather elemental. 
     FR 2,510,141 provides more precise details on the lixiviation, cementation and electrowinning stages. It claims a two stage lixiviation to make soluble the zinc present in both oxides and ferrites. Nevertheless, it indicates that just lixiviation of oxides is affordable and proposes the evaporation stage to eliminate salts. Regarding lixiviation waste, it only indicates that this is sterile, without presenting evidence or mentioning how it is disposed of. Regarding the quality of the zinc powder obtained, it only refers to the particle size distribution, wherein 82% is between 200 and 325 micron. It does not mention the impurities it contains or its metallic content, which impacts the profitability of the process. 
     FR 2,757,540 proposes to submit steelworks dust to a lixiviation process with aqueous bleach to eliminate chlorides and sulfates by dissolution and then subjecting the washed waste thus obtained to an alkaline lixiviation, cementation of the resulting alkaline solution and electrowinning of Zn from it to, finally, recirculate the stripped solution directly to the lixiviation step. On the other side, a fraction of near 70% of the washing water, charged with chlorides and sulfates, is recirculated to the washing stage with bleach, while the rest is submitted to a partial neutralization with acid to precipitate mainly lead hydroxide, which is sent to the alkaline lixiviation together with the wet waste produced by washing with bleach. The filtrate is treated in an evaporator to separate brine and water which is returned to the process. An analysis is as follows:
         1) The desalinization pre-treatment of steelworks dusts consists of a process line (L1) autonomous and independent from a line (L2) of alkaline treatment itself, only tied because the latter is fed with both wet waste and precipitated hydroxides generated by the former, thus losing the degree of separation they already had.   2) The washing waters treated in the evaporator are only those generated in L1. Nevertheless, in L2 the following situation is present: the wet waste fed to this line carries a content of chlorides not over 0.03% (18) and a moisture content estimated between 10 and 30%, a level that depends on the kind of solid-liquid separation employed. Since the process line operates with a lixiviating solution in a closed circuit, there will be a cumulative effect of salts and water. The salt content is limited by the concentration imposed in the electrowinning stage and, in the same manner, the water content is limited by the soda concentration in the recirculating solution which should be kept around 300 to 400 g/L. On the other hand, in addition, it must considered that the washing waters of the different filtration stages in the process line get soda and zincate which must be recovered, making it necessary to have continuous or periodic purges and treat the washing waters in an evaporator. Nevertheless, the process recirculates to the leaching stage only a fraction of the solution that comes out of electrowinning, and neutralizes the rest with external acid spills coming from operations of galvanization. The end result is that the evaporation is eliminated, but soda is lost and liquid wastes are generated.       

     Finally, FR 2,770,229 refers to conditioning of zinc powder obtained by electrowinning in an alkaline medium. Basically, the pulp containing zinc powder from electrowinning is filtered in an inert atmosphere. Washing of the Zn cake soaked in zincate solution is performed in demineralized and deoxygenated water, drying the Zn powder under vacuum (65 mm of Hg) at low temperature (20-30° C.). These operations are performed with just one equipment. It sets forth the use of ultrasound to accelerate the washing operation or to improve a quicker segregation of the powder. Zinc obtained is claimed to have a total zinc content of 99.6%, out of which 97-98% is metallic zinc and 2 to 3% is zinc oxide, and exhibits a particle size on the order of 6 microns. 
     The traditional processes described in all the above analyzed patents use conventional electrodeposition cells of rectangular design, whose surface is open to the atmosphere, amenable to alkaline fog formation and dissemination due to the rupture of hydrogen and oxygen bubbles. In addition, its hydrodynamics causes the production of Zn in the form of flakes which tend to adhere to the cathode surfaces, thus making necessary the use of complex vibration or sweeping systems. 
     SUMMARY 
     This disclosure relates to 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 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; and crystallizing at least a fraction of the concentrated solution, electro-depositing zinc from the concentrated solution; separating, washing and drying deposited Zn. 
     This disclosure also relates to a system for drying washed zinc cake produced according to the electro-hydrometallurgical process, including a substantially cylindrical body having an interior space provided with a rotary worm screw, arranged axially in the interior space and driven externally by a motor; an inclination mechanism; a condenser connected to an upper portion of one portion of the body end, an inner cooling coil and an exit through a water seal; a blower following the condenser and connected to an upper portion of an other end portion of the cylinder; a hopper that includes a worm screw driven by an external motor, connected to the cylinder through a rotary valve proximate to the connection of the condenser; a rotary valve located in lower portion of the cylinder, in an end portion opposed to one end portion of the hopper; an external heating system; and a nozzle for nitrogen injection. 
     This disclosure further relates to a system for cooling the zinc powder, produced according to the electro-hydrometallurgical process, including a substantially cylindrical body having an interior space; a rotary worm screw arranged axially in the interior space and driven externally by a motor; a tilting mechanism; a rotary valve in an upper part of one of an end portion of the body and another valve in a lower portion of an opposite end of the cylinder; an external cooling system utilizing a cooling fluid; and a nozzle for nitrogen injection. 
     This disclosure still further relates to a system for leaching and cementation according to the electro-hydrometallurgical process, including a closed reactor, implemented with a stirrer and an indirect system of heating; and a lid provided with a condenser followed by an extractor and a ventilation duct, a device that maintains a small negative pressure inside the reactor, thus avoiding leaks of alkaline fog from the reactor. 
     This disclosure further also relates to Zn powder of high purity and fine particle size obtained by the electro-hydrometallurgical process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic flow diagram of an electro-hydrometallurgical process. 
         FIG. 2  is a schematic diagram of drying equipment used in the process of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     It will be appreciated that the following description is intended to refer to specific embodiments of the invention selected for illustration in the drawings and is not intended to define or limit the invention, other than in the appended claims. 
     We provide processes that enable production of zinc powder of high purity and fine particle size as from EAF dust, that avoid generation of liquid wastes and minimize particulate and gaseous emissions, thus contributing to make the process environmentally sustainable by reducing pollution at levels lower than those demanded by regulations and that, in addition, reduce water and alkali consumption and make the gangue to become inert, valuing them as building material or material to be recycled in steelworks, thus supporting the cost-effectiveness of the project: 
     a) Inertization and valuation of waste from where zinc has been extracted, so as it becomes a by-product for building industry, or a material that can be recycled in steelworks furnaces. 
     b) Elimination of arsenic from liquid streams through an operation of adsorption with chemical reaction, in crosscurrent stages, done by percolating the solution on a fixed bed of Ba(OH) 2 . 
     c) Purification of the lixiviation solution by means of fractional crystallization by evaporation, maintaining its concentration of impurities, such as chlorides, sulfates and others, at a certain level required by electrowinning. 
     d) Use of operational practices that reduce the emission of particulate matter, both inside and outside of the processing building, especially during transportation of dusts. 
     e) Use of operating equipment specially designed to eliminate or at least to reduce gaseous emissions (alkaline fog) inside the processing building and minimize those discharged outside, in the lixiviation and cementation operations. 
     f) Use of a no conventional electrowinning cell to obtain zinc powder of high purity and fine particle size, without polluting emissions (alkaline fog). 
     g) Reduction of water consumption by performing all solids washings in crosscurrent or countercurrent stages and segregating solutions in such a way that those of low concentration are recycled to the washing operations, and those more concentrated are evaporated to retrieve water and re-concentrate soda, thus eliminating generation of liquid wastes (“dry plant”). 
     h) Use of optimal operational practices and conditions for washing of zinc powder, in crosscurrent or countercurrent stages. 
     i) Use of specially designed equipment both for drying and cooling of Zn powder, that allows for high purity. 
     For a better explanation of the disclosure, we provide a description of an example, in relation to  FIG. 1 , which shows one generic scheme of a process. 
     As it is seen in  FIG. 1 , EAF dust  1  is fed, together with an alkaline solution  2 , to a lixiviation reactor  3 . The resulting suspension is taken to a solid-liquid separator  4 , out of which a zincates solution  5  is obtained (mainly of Pb and Zn) and a wet solid (or gangue)  6 . This latter is treated with an inertizing mixture  7  to be then spread in a drying court, removing it as an inert by-product  8  after a curing period. Zincates solution  5  is purified with Zn powder  9  in a cementation reactor  10 . The resulting suspension is taken into a solid-liquid separator  11 , from which a solution, mainly of zincate  12  and a Pb paste  13  are obtained. The zincate solution  12  is mixed with a recirculated zincate solution  14  for further purification by adsorption in a chemical reaction on a bed of Ba(OH) 2    15 , which is carried out by percolation in crosscurrent stages. Once the bed  15  is saturated, it is discarded  16  and replaced by a fresh one. The solution  17  free of arsenic is stored in a tank  18 , from which a fraction of it  19  is constantly extracted to condition both its humidity content and its salts content in an evaporator  20  followed by a crystallizer  21 . Steam  22  removed from both units is condensed and recycled as process water, retrieving thermal energy in the operation. Likewise, concentrated solutions of zincate  23  and  24  are recirculated into the crystallizer  21  and the evaporator  20 , respectively, while salts produced in the crystallizer  21  are disposed as solid waste  25 . In this way, in the tank  18 , there is permanently kept a pure solution rich in zincate which continuously feed the electrowinning circuit  26 , from where a pulp  27  of finely divided zinc suspended in zincate solution is obtained. This pulp is passed through a solid-liquid separator  28 , from which there is obtained both a solution  29 , being pure and depleted in zincate, which is stored in the tank  34 , and a cake  30  of Zn soaked in zincate that, after being washed, dried in the dryer  31  under inert atmosphere  32 , and cooled, becomes the main product of the plant, a Zn powder  33  of high purity and fine particle size. 
     To simplify the handling of powders and to avoid the emission of particulate material, as much inside as outside the processing building, the moisture content of EAF dust is set between about 5 and about 10%, preferably about 8%, which is kept at this value during transportation from the supplier source, as well as during its later storage and internal handling in the plant. To accomplish this, sensors and sprinklers must be implemented in stockpile equipment and transport vehicles. 
     According to the base process, extraction of zinc from EAF dust is performed in lixiviation reactor  3 , mixing it with a recirculated alkaline solution, pure and stripped in zincate, with a zinc content below about 10%, with about 20% to about 50% of alkali, preferably NaOH, in a ratio of dust to solution between about 1:3 and about 1:5, preferably about 1:7, at a temperature between about 70 and about 120° C., preferably at about 90° C., under mechanical stirring between about 50 to about 150 rpm, preferably about 120 rpm, over about 30 to about 240 minutes, preferably 60 minutes. 
     The reactor should be of substantially air-tight design to avoid emanations of alkaline fog both in and out of the processing building, it must include a mixer and an external heating system, and whose lid is provided with a condenser followed by an extractor and a ventilation duct that discharges outside the processing building. A negative pressure is kept in its interior, between about 30 and about 60 mm of Hg, by a blower that passes the vapors through a condenser before discharging them to the atmosphere. In addition, the same type of equipment and operational practice should be used in cementation. 
     Suspension resulting from lixiviation is taken to a solid-liquid separator  4  which, depending on the production scale, can be a filter press, a band filter, a centrifuge, or a system including in addition decanters and/or thickeners, being able to do the washing of the cake in the same equipment, or separately. The same is valid for the other operations of solid-liquid separation of the process (equipment  11 ,  15  and  28 ). 
     The cake washing operation, in whichever separator, should be performed in stages, preferably countercurrent or crosscurrent. For example, the moist cake from lixiviation (moist gangue, imbibed in zincates) is washed with several successive loads, the first ones being of a soda solution, and the later of water. The use of soda in the first loads is imperative since it avoids precipitation of Zn(OH) 2 , wherein zinc is lost when it is retained in the gangue cake. The number of loads, the volume to be used in each of them, and their soda concentration, will mainly depend on the soda concentration of the lixiviation solution and on the percentage of suspended gangue. For instance, if a filter press is used to filter a pulp of 8% suspended solids in soda 32%, when using volumes of washing loads equal to one volume of cake in each washing, from 6 to 10 loads are required to reduce the soda content in the cake to insignificant levels, wherein the first 2 to 3 loads are of a soda solution and last ones of water. This practice can reduce more than 10 times the washing water consumption with respect to the washing operation by continuous water injection to the filter. 
     Importantly, the processes and systems of the disclosure can further comprise washing of solids soaked in zincates (e.g., moist cake from lixiviation), in one or more stages, in a counter current or cross current operations, with between about three and about ten successive washing loads, wherein the first 2 to 5 washing loads comprise a solution of about 200 to about 400 g/L of NaOH and the last washing loads comprise water and the washing load volume is on the order of about one to three volumes of the volume of the solids to wash. 
     On the other hand, by segregating washing waters to ones of low concentration, for example, of up to about 5%, are recycled and the more concentrated ones are evaporated to retrieve water and re-concentrate soda, water consumption is optimized and generation of liquid waste is eliminated, thus transforming the premises in a “dry plant.” The solution used in the leaching has 32% of soda water and is recirculated in a closed circuit, with a make up of fresh solution only for replacement of minor losses. Because of this, any amount of water that is added to that main stream is substantially eliminated to restitute the concentration of the soda solution at its original level. 
     The gangue, washed and moist and constituted mainly of Fe, is treated with an inertization mixture, consisting of a SiO 2  source (puzzolane, bentonite or silica gel), between about 0.02 and about 0.5% of the dry gangue, Ca(H 2 PO) 4 H 2 O between about 0.5 and about 3%, and Ca(OH) 2  between about 0.2 and about 0.8%. These components are added to the moist gangue cake so that the mass maintains a humidity content of between about 15 and about 25%, preferably about 20%. Mass is mixed. in an equipment of the cement mixer type until reaching substantial homogeneity. Gangue obtained in this way, after a curing period of about 3 to about 10 days, and being dried until the moisture content reaches between about 6 and about 12%, preferably about 10%, is inert according to the environmental regulation EPA SW-846 Method TCLP-1311, being typical results about 0.15 mg/L of Cd and about 0.28 mg/L of Pb, when the maximum limits of norm EPA are 1 and 5 mg/L, respectively, as it is observed in Table 2, This innocuous gangue approximately constitutes 63% of the original mass of the EAF dust and can be valorized, among other ways, as filling or construction material, in the manufacture of refractory bricks, heat accumulating bricks or pigments, or recycled to the electrical furnaces. 
     Zincates solution  5 , to which it is convenient to incorporate the more concentrated washing waters, is purified in a cementation reactor  10  with Zn powder  9 , added in a proportion of about 2% to about 12% in excess of the stoichiometric ratio (preferably about 8%) to reduce metals found under the zinc in the tension scale. The reactor operates at a temperature between about 70 and about 120° C., preferably about 80° C., under mechanical stirring between about 50 and about 100 rpm, preferably about 80 rpm, over about 30 to about 180 minutes, preferably about 60 minutes. The resulting suspension is taken to a solid-liquid separator  11 , from which there is obtained a pure zincate solution  12  and a paste, mainly of Pb  13 . 
     Preferably following cementation, there is an arsenic removal operation since it is continuously concentrated in liquid streams, in spite of its low content in the EAF dust, becoming a risk for the operators&#39; health and eventually exceeding the permissible limit in electrowinning (0.002 g/L). 
     Elimination of arsenic is performed by adsorption with chemical reaction on solid Ba(OH) 2 , according to the following stoichiometry (19):
 
As 2 O 5 +2Ba(OH) 2 =2Ba HAsO 4 +H 2 O.
 
     In practice, it is convenient to carry out the operation in countercurrent or crosscurrent stages, percolating the cemented solution on a fixed bed of Ba(OH) 2 . This bed should be operated with a load of Ba(OH) 2  between about 3 and about 10, preferably about 5 times over the stoichiometric amount required for each cementator batch, for security reasons. As an example, if 1000 L of solution of 32% soda and 0.023 gpL of As, that is to say, containing 0.023 kg of As, it would be required 0.0487 kg of Ba(OH) 2  to remove it according to stoichiometry and, operating at 5 times that amount, 0.244 kg are needed. The percolator runs 24 hours a day, purifying the stock solution of pure and rich zincate solution  18 , and the recirculation flow  14  can be used to increase the retention time significantly, allowing flexibility to deal with eventual increases of As concentration. Temperature for the stock solution is kept between about 30 and about 90, preferably about 35° C. 
     The alkaline solution thus purified is submitted to electrowinning in a cylindrical cell of the EMEW type, from Electrometals Technologies Ltd., for example, whose air-tight design allows confining the gases generated there and washing them prior to their evacuation, thus avoiding the emission of alkaline fog to the processing building and evacuating to the atmosphere a mixture of approximately 70% oxygen and 30% hydrogen. Typically, it is possible to obtain Zn of fine particle size, less than about 100 microns and with a 50% less than about 40 microns, which is separated and washed in a solid-liquid separation equipment. Nevertheless, by handling the operating conditions of this cell, as the solution flow through it, its soda concentration, and the current density, it is possible to regulate the zinc particle size. Unlike this process, the traditional processes use electro deposition cells of rectangular design, whose surface is open to the atmosphere, amenable to alkaline fog formation and dissemination, and produces Zn in the form of flakes instead of powder. 
     We now refer to the design of a special drying equipment  31  for the Zn powder, and to the use of operational practices and conditions which are the optimum for the solid-liquid separation  28  and the washing of Zinc powder, which have to be performed in countercurrent or crosscurrent stages, in similar conditions to those mentioned above since, in this way, a Zn of high purity can be reached. In this case, washing with water is not convenient because it will induce decomposition of the zincate and consequent precipitation of Zinc oxide, which would be retained in the humid zinc cake, contaminating it. Once the alkali concentration in the cake is reduced to marginal levels by means of washing loads of soda solution, it is possible to finish washing with water, thus minimizing the presence of zinc oxide in the final product. 
     Next, there is a description of one example of drying equipment  31  (in  FIG. 1 ), in relation to  FIG. 2 , which shows a generic scheme of the equipment. The drying chamber  100  is a substantially cylindrical, static design, equipped with the following elements: a rotary worm screw  200  arranged axially in its interior, driven externally by a motor  300 ; a tilting mechanism; a condenser  400  connected to the upper part of one of its ends, that has an inner cooling coil  500  with inlet  600  and outlet  700  for cooling water, and an exit through a water seal  800  that allows substantially continuous elimination of the condensate while maintaining the inert atmosphere inside the dryer; an exit  900  of inert gases connected to a blower  105  that follows the condenser and it is connected to the upper part of the other end of the cylinder; a hopper  110  that includes a rotary worm screw  120  driven by an external motor  130 , connected to the cylinder through a rotary valve  140  in the proximities of the connection of the condenser, provided with a nitrogen injection in its base for feeding the humid zinc cake with exclusion of air; a rotary valve  150  located in the lower part of the cylinder, in the end opposed to the one of the hopper, also provided with nitrogen injection to remove the dry powder; an external and indirect heating system  160 , by means of electrical resistances, heating steam, oil or combustion gases; a nozzle for nitrogen injection. The dryer operates with a slight positive pressure of an inert gas in its interior, for example nitrogen, at temperatures between about 100 and about 400° C. After drying, the powder is cooled to room temperature and finally reduced in its particle size to less than about 6 microns and bagged, also under inert atmosphere. 
     For cooling of the powder, an equipment of the same characteristics of the dryer may be used, that is to say, of substantially cylindrical design, provided with the following elements: a rotary worm screw arranged axially in its interior, driven externally by a motor; a tilting mechanism; a rotary valve in the upper part of one of its ends and another one in the lower part of the opposed end of the cylinder; an external and indirect cooling system, by means of a cooling fluid; and a nozzle for nitrogen injection. 
     A crystallizer by evaporation is used, following the multi-effect evaporator, to get crystallized salts that are eliminated as purge. This operation purifies the lixiviating solution, keeping the concentration of impurities such as chlorides, sulfates and others, at a determined level required by electrowinning. 
     Below, results are provided for a typical pilot scale operation of the process on a processing base of 22 ton EAF/day. Table 4 shows the main inlet and outlet streams, from which it is possible to establish the percentage distribution of the main elements in the different product streams, shown in Table 5. In this last table it is observed, for example, that the arsenic which enters the process as part of the EAF, which in Chile is between 0.01 and 0.03% dry weight basis, is distributed in a 25% to the gangue, 73% is eliminated in the filter of Ba(OH) 2  and 0.15% goes away as polluting agent in the Zn powder. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
               
                   
               
               
                 Main streams of a typical operation (22 ton EAF/day) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Zn 
                 Cu 
                 Pb 
                 Fe 
                 Al 
                 Sb 
                 As 
                 S 
                 Ba 
                 Be 
                 Bi 
                 Cd 
               
             
          
           
               
                 N o   
                   
                 ton/day 
                 Element, kg 
               
               
                   
               
             
          
           
               
                   
                 In 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                  1 
                 EAF dust 
                 22.034 
                 3543.785 
                 81.085 
                 948.192 
                 7484.181 
                 108.701 
                 3.401 
                 3.033 
                 188.023 
                 6.169 
                 0.011 
                 0.742 
                 8.512 
               
               
                  9 
                 Zn powder for cementation 
                 0.882 
                 881.311 
                 0.044 
                 0.044 
                 0.089 
                 0.000 
                 0.000 
                 0.002 
                 0.000 
                 0.000 
                 0.000 
                 0.004 
                 0.044 
               
               
                   
                 Out 
               
               
                  8 
                 Gangue with 10% moisture 
                 15.350 
                 1222.650 
                 78.747 
                 86.576 
                 7183.932 
                 65.622 
                 1.796 
                 0.760 
                 0.525 
                 6.148 
                 0.010 
                 0.553 
                 4.904 
               
               
                 13 
                 Pb Cement with 5% 
                 1.318 
                 88.156 
                 1.986 
                 861.382 
                 299.662 
                 0.000 
                 0.000 
                 0.000 
                 0.000 
                 0.000 
                 0.000 
                 0.000 
                 3.373 
               
               
                   
                 moisture 
               
               
                 16 
                 Ba cake with As, 
                 0.031 
                   
                   
                   
                   
                   
                   
                 2.215 
               
               
                   
                 10% moisture 
               
               
                 25 
                 Crystallized purge, 
                 3.031 
                 0.000 
                 0.000 
                 0.000 
                 0.000 
                 42.961 
                 1.546 
                 0.050 
                 186.911 
                 0.000 
                 0.000 
                 0.010 
                 0.000 
               
               
                   
                 10% moisture 
               
               
                 33 
                 Zn powder with 1% 
                 2.322 
                 2321.135 
                 0.117 
                 0.117 
                 0.234 
                 0.001 
                 0.001 
                 0.005 
                 0.000 
                 0.000 
                 0.000 
                 0.012 
                 0.117 
               
               
                   
                 moisture 
               
               
                   
               
             
          
           
               
                   
                 Ca 
                 Cl 
                 Co 
                 Cr 
                 Sn 
                 Sr 
                 P 
                 Mg 
                 Mn 
                 Hg 
                 Mo 
                 Ni 
               
             
          
           
               
                 N o   
                   
                 ton/day 
                 Element, kg 
               
               
                   
               
             
          
           
               
                   
                 In 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                  1 
                 EAF dust 
                 22.034 
                 492.825 
                 947.46 
                 0.389 
                 44.288 
                 12.890 
                 0.749 
                 39.661 
                 195.367 
                 380.452 
                 0.472 
                 2.269 
                 4.524 
               
               
                  9 
                 Zn powder for cementation 
                 0.882 
                 0.000 
                 0.000 
                 0.000 
                 0.000 
                 0.004 
                 0.004 
                 0.004 
                 0.000 
                 0.000 
                 0.004 
                 0.004 
                 0.000 
               
               
                   
                 Out 
               
               
                  8 
                 Gangue with 10% moisture 
                 15.350 
                 476.626 
                 0.000 
                 0.345 
                 41.446 
                 8.542 
                 0.000 
                 0.000 
                 0.000 
                 290.120 
                 0.018 
                 0.345 
                 1.727 
               
               
                 13 
                 Pb Cement with 5% moisture 
                 1.318 
                 0.000 
                 0.000 
                 0.044 
                 0.000 
                 4.230 
                 0.000 
                 0.000 
                 0.000 
                 0.000 
                 0.442 
                 0.000 
                 2.797 
               
               
                 16 
                 Ba cake with As, 10% moisture 
                 0.031 
               
               
                 25 
                 Crystallized purge, 
                 3.031 
                 16.199 
                 947.34 
                 0.000 
                 2.725 
                 0.000 
                 0.691 
                 39.544 
                 195.367 
                 90.332 
                 0.000 
                 1.807 
                 0.000 
               
               
                   
                 10% moisture 
               
               
                 33 
                 Zn powder with 1% moisture 
                 2.322 
                 0.000 
                 0.000 
                 0.000 
                 0.000 
                 0.012 
                 0.012 
                 0.012 
                 0.000 
                 0.000 
                 0.012 
                 0.012 
                 0.000 
               
               
                   
               
               
                 Stream numbers refer to those of FIG. 1. 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 Distribution of the elements in different streams and residues 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Zn 
                 Cu 
                 Pb 
                 Fe 
                 Al 
                 Sb 
                 As 
                 S 
                 Ba 
                 Be 
                 Bi 
                 Cd 
               
             
          
           
               
                 N o   
                   
                 Distribution of element in the stream, % p/p 
               
               
                   
               
             
          
           
               
                  8 
                 Gangue with 10% moisture 
                 34.50 
                 97.12 
                 9.13 
                 95.99 
                 60.37 
                 52.81 
                 25.05 
                 0.28 
                 99.65 
                 94.05 
                 74.50 
                 57.61 
               
               
                 13 
                 Pb Cement with 5% moisture 
                 0.00 
                 2.45 
                 90.84 
                 4.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 39.63 
               
               
                 16 
                 Ba cake with As, 10% moisture 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 73.02 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 25 
                 Crystallized purge, 10% moisture 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 39.52 
                 45.46 
                 1.65 
                 99.41 
                 0.00 
                 0.03 
                 1.35 
                 0.00 
               
               
                 33 
                 Zn powder with 1% moisture 
                 65 
                 0.14 
                 0.01 
                 0.00 
                 0.00 
                 0.03 
                 0.15 
                 0.00 
                 0.00 
                 0.21 
                 1.58 
                 1.37 
               
               
                   
               
               
                   
                   
                 Ca 
                 Cl 
                 Co 
                 Cr 
                 Sn 
                 Sr 
                 P 
                 Mg 
                 Mn 
                 Hg 
                 Mo 
                 Ni 
               
             
          
           
               
                 N o   
                   
                 Distribution of element in the stream, % p/p 
               
               
                   
               
             
          
           
               
                  8 
                 Gangue with 10% moisture 
                 96.71 
                 0.00 
                 88.73 
                 93.58 
                 66.27 
                 0.00 
                 0.00 
                 0.00 
                 76.26 
                 3.81 
                 15.22 
                 38.17 
               
               
                 13 
                 Pb Cement with 5% moisture 
                 0.00 
                 0.00 
                 11.27 
                 0.00 
                 32.82 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 93.71 
                 0.00 
                 61.83 
               
               
                 16 
                 Ba cake with As, 10% moisture 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
               
               
                 25 
                 Crystallized purge, 10% moisture 
                 3.29 
                 99.99 
                 0.00 
                 6.15 
                 0.00 
                 92.17 
                 99.70 
                 100.00 
                 23.74 
                 0.00 
                 79.61 
                 0.00 
               
               
                 33 
                 Zn powder with 1% moisture 
                 0.00 
                 0.00 
                 0.00 
                 0.00 
                 0.09 
                 1.56 
                 0.03 
                 0.00 
                 0.00 
                 2.48 
                 0.51 
                 0.00 
               
               
                   
               
               
                 Stream numbers refer to those of FIG. 1. 
               
             
          
         
       
     
     The improvement described above, related to consumption and recycling of water, is outlined in the following. EAF dust is leached with a 32% soda solution in a mass proportion of “soda solution/EAF powder”=7 (equivalent to a pulp of 40.5% of suspended and dissolved solids). The lixiviating solution is prepared by adding to the recirculated solution, depleted in Zn, a small make up of soda solution. After leaching and separation of the gangue by filtration, the filtrate is cemented and the Pb cement separated. Thus, a purified solution is obtained, a small proportion of which is substantially continuously passed through the EMEW cell, where Zn is extracted in the form of powder, therefore being regenerated the 32% soda solution depleted in Zn that is recirculated. 
     The leaching operation takes place with a 32% soda solution, which constitutes the main stream of the plant. Therefore, water that is introduced to it should be removed. The removal of water takes place in a multi stage evaporator-crystallizer, and it is recovered as condensate in condensers that use cold water from a cooling tower. From the evaporator, a condensate and a 32% soda solution are obtained. Part of this solution is recirculated and the rest goes to the crystallizer from which are obtained a condensate, a suspension that is purged and a 32% soda solution that is recirculated. Thus, the plant does not generate liquid wastes. 
     The inlet and outlet water sources from the main stream are shown in Table 6, together with their corresponding disposition, and Table 7 quantifies them on a 22 ton EAF/day basis. It is observed that the daily fresh water consumption amounts to 17 tons, i.e. only 0.77 ton of water/ton EAF. 
     
       
         
               
             
               
               
             
           
               
                 TABLE 6 
               
               
                   
               
               
                 Inlet and outlet water sources from the main stream of soda solution, 
               
               
                 and their fate 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Inlet water Sources 
                 Destiny of inlet water 
               
               
                   
               
               
                 Water in EAF (3.5% moist 
                 It is evaporated and recovered in the 
               
               
                 basis) 
                 evaporating crystallizer 
               
               
                 Water in recirculated soda 
                 It is evaporated and recovered in the 
               
               
                 solution 
                 evaporating crystallizer 
               
               
                 Water for washing of gangue 
                 A 41% is recovered in the evaporating 
               
               
                 cake 
                 crystallizer and it is recycled as pure 
               
               
                   
                 water, the rest is reused in the circuit of 
               
               
                   
                 cake washing. 
               
               
                 Water for washing of Pb cement 
                 It is reutilized in the circuit of cake 
               
               
                 cake 
                 washing. 
               
               
                 Water for washing of Zn powder 
                 It is reutilized in the circuit of cake 
               
               
                   
                 washing. 
               
               
                   
               
               
                 cake 
               
               
                 Outlet water Sources 
                 Destiny of outlet water 
               
               
                   
               
               
                 Water in washed gangue cement 
                 It is evaporated without recovery. 
               
               
                 Water in washed Pb 
                 It is evaporated without recovery. 
               
               
                 Water in washed Zn 
                 It is evaporated, condensed and 
               
               
                   
                 recycled. 
               
               
                 Evaporated water to generate the 
                 It is evaporated, condensed and 
               
               
                 purge 
                 recycled. 
               
               
                 Water in the purge 
                 It is evaporated without recovery. 
               
               
                 Water for washing of floor and 
                 10% is lost by evaporation, the rest is 
               
               
                 equipment 
                 evaporated, condensed and recycled. 
               
               
                 Cooling water 
                 A 2% is lost in the cooling tower 
               
               
                 Vapor from steam boiler 
                 A 2% is lost as condensate 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Daily water balance (22 ton EAF/day basis) 
               
             
          
           
               
                   
                 Water Balance, t/day 
               
             
          
           
               
                   
                 in 
                 out 
                 recirculate 
               
               
                   
                   
               
             
          
           
               
                 Water in EAF 
                 0.8 
                   
                   
               
               
                 Water in lixiviation solutions, recirculated 
                 104.9 
                   
                 104.9 
               
               
                 Water in reposition soln. of soda 32% 
                 3.1 
               
               
                 Washing water recuperated in evaporator as 
                 2.5 
                   
                 2.5 
               
               
                 32% soda soln., recycled 
               
               
                 Washing water for filtration cakes, recovered 
                 14.4 
                   
                 14.4 
               
               
                 by evaporation, recirculated 
               
               
                 Washing water for filtration cakes, recycled in 
                 23.7 
                   
                 23.7 
               
               
                 washing circuit 
               
               
                 Washing water for filtration cakes, lost by 
                   
                 0.5 
               
               
                 evaporation (2%) 
               
               
                 Condensate lost in Evaporator-Crystallizer 
                   
                 0.3 
               
               
                 (2%) 
               
               
                 Water evaporated to obtain the purge, recycled 
                 5.8 
                   
                 5.8 
               
               
                 Water eliminated with the purge 
                   
                 0.1 
               
               
                 Water lost by hydroxides formed in the gangue 
                   
                 0.5 
               
               
                 Water evaporated from the gangue (lost) 
                   
                 3.4 
               
               
                 Water evaporated from Pb Cement (lost) 
                   
                 0.4 
               
               
                 Water evaporated from Zn powder, recirculated 
                 0.7 
                   
                 0.7 
               
               
                 Washing water for floor and equipment, treated 
                 2.0 
                   
                 2.0 
               
               
                 and recycled 
               
               
                 Washing water for floor and equipment, lost by 
                   
                 0.2 
               
               
                 evaporation 
               
               
                 Cooling water, recirculated 
                 730.0 
                   
                 730.0 
               
               
                 Cooling water lost by evaporation (2%) 
                   
                 14.6 
               
               
                 Steam, recycled 
                 42.2 
                   
                 42.2 
               
               
                 Steam, lost (2%) 
                   
                 0.8 
               
               
                 SUBTOTAL 
                 929.9 
                 20.8 
                 926.0 
               
               
                 fresh water consumption 
                 17.0 
               
             
          
           
               
                 Balance 
                 946.9 
                 946.9 
               
               
                   
               
             
          
         
       
     
     Next, is shown the effectiveness regarding the emissions of particulate matter in the process, on a 22 ton EAF/day processing basis. 
     The unitary operations of the alkaline process that emit particulate matter were classified in 10 stages, shown in Table 8, together with the corresponding factors of emission (a) and the emissions of particulate matter (b), with and without mitigation. 
     The main conclusions regarding emissions of particulate matter are the following:
         The emissions of particulate matter with mitigation measures show that these emissions do not exceed 1 kg/day and 0.25 ton/year, being no significant emissions therefore. The particulate matter emission calculations for the plant operations were made without considering mitigation measures to obtain a superior and maximum level of the emissions.   In each point of emission of particulate matter, mitigation measures may be implemented to diminish the emission. These measures of mitigation are water sprays and dust collectors with efficiencies of about 90%.   The dust storage may be a closed warehouse with sprays and natural ventilation. After this type of mitigation, it is possible to reach more than about 75% emission reduction   The process stages are considered to be installed in shed type buildings with natural ventilation, 10 m height and with a surface of 6,100 m 2  for the basic case exemplified.   The calculations of emissions using emission factors have an implicit error that varies between about 2 and about 10%, depending on the degree of accuracy with which they were generated.       

     Next, the alkaline fog emissions are estimated taking into account all the sources, without considering mitigation measures, to obtain a superior and maximum level of the emissions that would take place in the plant operations. Table 9 summarizes the calculations for the estimations of the fog emissions. The fog concept refers to the evaporation of the solutions in the processes of leaching, purification and electrowinning. 
     Fog emissions in the process of electrowinning in EMEW cells are null because it is a closed process. The solution treated in this stage is recirculated to the leaching stage, and therefore the fog emissions evolve only in the stages of leaching and cementation. The estimation of the fog emissions was made considering the following assumptions:
         The alkaline solution and the mixture evaporated from it are considered in equilibrium at the process temperature.   For calculation effects, the fog is considered as an ideal gas.   It was considered a natural ventilation of 0.2 m/s (0.72 km/h, a typical value in closed work), in absence of forced ventilation.   A relative humidity of 50% and a room temperature of 25° C. in the plant building was considered.       

     Table 10 shows that the total level of fog emissions during the operation of the alkaline stages is 0.00044 [mg/m 3 ]. The total amount of evaporated NaOH is 0.596 mg/day. Therefore, the norm of Environmental Hygiene is not exceeded. 
     Having described the invention in detail, as it is possible to be appreciated, diverse changes and modifications can be carried out while keeping within the spirit of the disclosure. The totality of those changes and modifications is considered included within the scope of the disclosure as it is defined in the annexed claims. 
     
       
         
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
             
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
               
             
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 8 
               
               
                   
               
               
                 Emission of particulate matter in the process (22 ton EAF/day basis) 
               
               
                   
               
             
             
               
                 (a) Emission Factors 
               
             
          
           
               
                 Opn. 
                 Emission Factor 
                 Units 
                 Observations 
               
               
                   
               
             
          
           
               
                 Dry Area 
               
             
          
           
               
                 1 
                 0.0017 
                 kg/Mg 
                 From unloaded material (AP42 C11S12) 
               
               
                   
                   
                   
                 Assimilated to raw material discharge in cement manufacture 
               
               
                 2 
                 0.00056 
                 g/ton 
                 From unloaded material (AP42 C13S2-4) 
               
               
                   
                 {(U/2.2) 1.3 /(M/2) 1.4 } 
                   
                 U = 0.75 m/s a  ,M = 5% b   
               
               
                   
                   
                   
                 U = average speed in the zone 
               
               
                   
                   
                   
                 M = Moisture content of the stockpile 
               
               
                 3 
                 K * (sL/2){circumflex over ( )}0.65 * 
                 g/KTV 
                 Assimilated to emissions of vehicles on paved road and with 
               
               
                   
                 (W/3){circumflex over ( )}1.5 − C + 
                   
                 fine material load (AP42 C13S2-2) 
               
               
                   
                 0.053 
                 kg/Mg 
                 Grams/Kilometer Moving Vehicle 
               
               
                   
                   
                   
                 K = size particle factor, 4.6 for PM10 
               
               
                   
                   
                   
                 sL = load of fines in carpet, calculated as amount of fine 
               
               
                   
                   
                   
                 material discharged on the warehouse floor of the processing 
               
               
                   
                   
                   
                 unit operations, Circulation Area: 200 m 2 , 0.823 g/m 2d   
               
               
                   
                   
                   
                 W = average weight of vehicles, 0.8 ton c   
               
               
                   
                   
                   
                 C = Emission Factor due to traffic, brake and tire wear, 0.2119 
               
               
                   
                   
                   
                 Assimilated to material transport in a pile loader (FIRE 6.24 
               
               
                   
                   
                   
                 SCC 3-05-025-06, modified by unit conversion factor). 
               
               
                 4 
                 0.0017 
                 kg/Mg 
                 From unloaded material (AP42 C11S12) 
               
               
                   
                   
                   
                 Assimilated to raw material discharge in cement manufacture. 
               
               
                 5 
                 0.0075 
                 kg/Mg 
                 From milling material (AP42 C11S19-2) 
               
               
                   
                   
                   
                 Assimilated to fine milling of the process of milling in the 
               
               
                   
                   
                   
                 industry of barren. 
               
               
                 6 
                 0.03 
                 kg/Mg 
                 From processing material. Assimilated to Processing of 
               
               
                   
                   
                   
                 metallic mineral ores (AP42 C11S24). Material loading and 
               
               
                   
                   
                   
                 moving. 
               
               
                 7 
                 0.03 
                 kg/Mg 
                 From processing material. Assimilated to Processing of 
               
               
                   
                   
                   
                 metallic mineral ores (AP42 C11S24). Material loading and 
               
               
                   
                   
                   
                 moving. 
               
               
                 8 
                 0.03 
                 kg/Mg 
                 From processing material. Assimilated to Processing of 
               
               
                   
                   
                   
                 metallic mineral ores (AP42 C11S24). Material loading and 
               
               
                   
                   
                   
                 moving. 
               
             
          
           
               
                 Wet Area 
               
             
          
           
               
                 9 
                 0.03 
                 kg/Mg 
                 From processing material. Assimilated to Processing of 
               
               
                   
                   
                   
                 metallic mineral ores (AP42 C11S24). Material loading and 
               
               
                   
                   
                   
                 moving. 
               
             
          
           
               
                 Electrowinning 
               
             
          
           
               
                 10  
                 0.14 
                 kg/Mg 
                 From product packaging 
               
               
                   
                   
                   
                 Assimilated to Coal Production (AP42 C6S1, FIRE 6.24 SCC 
               
               
                   
                   
                   
                 3-01-005-08, modified by factor of conversion of units). End 
               
               
                   
                   
                   
                 product packaging. 
               
               
                   
               
             
          
           
               
                   a Average annual speed, 
               
               
                   b Data given in description of the process, 
               
               
                   c It has been considered that the mini pile loader weight 450 kg and load 250 kg, 
               
               
                   d Value calculated by considering all the processes in the dry area of the plant, a displacement area of 200 m 2 , 
               
               
                 Load of fine = 2.1 
               
               
                 *emissions of particulate matter in the process. 
               
             
          
           
               
                 (b) Emissions of particulate matter 
               
             
          
           
               
                   
                 With mitigation 
               
             
          
           
               
                   
                 Without mitigation measures 
                   
                 Emission 
               
             
          
           
               
                   
                 Emission 
                   
                 Rate 
                   
                   
                   
                 to the 
               
               
                 Opn. 
                 Factor 
                 Units 
                 Activity 
                 Units 
                 Emission* 
                 Efficiency 
                 atmosphere* 
               
               
                   
               
             
          
           
               
                 1 
                 0.0017 
                   
                 kg/ton 
                   
                 542 
                 ton/mo. 
                   
                 0.92 
                 25 
                 0.69 
               
               
                 2 
                 0.0242 
                   
                 kg/ton 
                   
                 542 
                 ton/mo. 
                   
                 13.12 
                 75 
                 3.28 
               
               
                 3 
                 0.1824 
                 0.053 
                 g/VKT 
                 kg/ton 
                 216.8 
                 km/mo. 
                 ton/mo. 
                 39.58 
                 90 
                 3.958 
               
               
                 4 
                 0.0017 
                   
                 kg/ton 
                   
                 542 
                 ton/mo. 
                   
                 0.92 
                 90 
                 0.092 
               
               
                 5 
                 0.0075 
                   
                 kg/ton 
                   
                 542 
                 ton/mo. 
                   
                 4.07 
                 70 
                 1.221 
               
               
                 6 
                 0.03 
                   
                 kg/ton 
                   
                 542 
                 ton/mo. 
                   
                 16.26 
                 90 
                 1.626 
               
               
                 7 
                 0.03 
                   
                 kg/ton 
                   
                 542 
                 ton/mo. 
                   
                 16.26 
                 90 
                 1.626 
               
               
                 8 
                 0.03 
                   
                 kg/ton 
                   
                 542 
                 ton/mo. 
                   
                 16.26 
                 90 
                 1.626 
               
               
                 9 
                 0.03 
                   
                 kg/ton 
                   
                 12.93 
                 ton/mo. 
                   
                 0.39 
                 70 
                 0.117 
               
               
                 10  
                 0.14 
                   
                 kg/ton 
                   
                 85.76 
                 ton/mo. 
                   
                 12.01 
                 90 
                 1.201 
               
             
          
           
               
                   
                 Total emissions of particulate matter 
                 119.79 
                   
                 15.437 
               
               
                   
                   
               
             
          
           
               
                 * kg/mo. 
               
             
          
           
               
                 Dry Area Operations 
                 Wet Area Operation 
                   
               
             
          
           
               
                 1 
                 EAF discharge 
                 6  
                 Transport in closed 
                 9 
                 Purification, fine zinc 
               
               
                 2 
                 Dust storage in bulk 
                   
                 dray to storage silo 
                   
                 powder unloading 
               
               
                 3 
                 Transport to Crusher 
                 7 
                 Dust unloading in 
                   
                 Electrowinning 
               
               
                   
                 (Frontal loader) 
                   
                 storage silo 
                 10 
                 Packaging of powder 
               
               
                 4 
                 Dust unloading to Crusher 
                 8 
                 Transport of 
                   
                 final product 
               
               
                 5 
                 Crusher (tertiary milling) 
                   
                 particulate matter 
               
               
                   
                   
                   
                 to leaching reactor 
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
               
                 Summary of calculations for the estimation of alkaline fog emissions 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                   
                   
                   
                   
                   
                 Molar 
                   
                   
               
               
                   
                   
                 Vapor 
                   
                   
                   
                 Fraction 
                 Partial 
                 Mixture 
               
               
                   
                 Mixture 
                 Pressure of 
                 Molecular 
                   
                 N° 
                 in the 
                 Vapor 
                 Vapor 
               
               
                   
                 Temperature 
                 component 
                 Weight 
                 Concentration 
                 Moles 
                 mixture 
                 Pressure 
                 Pressure 
               
               
                 Component 
                 [C.] 
                 [KPa] 
                 [g/mol] 
                 [g/l] 
                 [moles] 
                 [mol] 
                 [KPa] 
                 [KPa] 
               
               
                   
               
             
          
           
               
                 LIXIVIATION 
               
             
          
           
               
                 NaOH 
                 80 
                  1.E−07 
                 39.997 
                 430 
                 10.751 
                 0.195 
                 1.95E−08 
                 38.098 
               
               
                 Water 
                 80 
                 47.3428 
                 18.015 
                 798.1 
                 44.303 
                 0.805 
                 38.098 
               
             
          
           
               
                 CEMENTATION 
               
             
          
           
               
                 NaOH 
                 60 
                 1.5E−08 
                 39.997 
                 430 
                 10.751 
                 0.802 
                 1.20E−08 
                 3.9345 
               
               
                 Water 
                 60 
                 19.917  
                 18.015 
                 47.7 
                 2.647 
                 0.198 
                 3.934480 
               
               
                   
               
             
          
           
               
                   
                   
                   
                 Molar 
                 Latent 
                 Molar 
                   
                   
               
               
                   
                   
                   
                 Fraction 
                 Heat 
                 Fraction 
                   
                   
               
               
                   
                   
                 Heat of 
                 in the 
                 of the 
                 in the 
                 Concentration 
                 Vapor 
               
               
                   
                   
                 vaporization 
                 Vapor 
                 mixture 
                 evaporation 
                 in Vapor 
                 Density 
               
               
                   
                 Component 
                 [KJ/kg] 
                 Phase 
                 [KJ/kg] 
                 [mol/m 3 ] 
                 [g/m 3 ] 
                 [g/m 3 ] 
               
               
                   
                   
               
             
          
           
               
                 LIXIVIATION 
               
             
          
           
               
                   
                 NaOH 
                 209.2151 
                 5.126E−10 
                 2259.749 
                 6.654E−09 
                 2.66E−07 
                 233.87 
               
               
                   
                 Water 
                 2259.7484 
                 0.9999995 
                   
                 12.981908 
                 233.873 
               
             
          
           
               
                 CEMENTATION 
               
             
          
           
               
                   
                 NaOH 
                 209.2151 
                 3.059E−09 
                 2259.749 
                 4.348E−09 
                 1.74E−07 
                 25.603 
               
               
                   
                 Water 
                 2259.7484 
                 0.9999969 
                   
                 1.4211716 
                 25.6028 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
             
               
             
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                 TABLE 10 
               
             
             
               
                   
               
               
                 Alkaline Fog emission 
               
             
          
           
               
                   
                 Lixiviation 
                 Cementation 
                 Units 
               
               
                   
               
             
          
           
               
                 ESTIMATION OF TOTAL FOG EVAPORATION 
               
             
          
           
               
                 Processing Time 
                 60 
                 60 
                 [min] 
               
               
                 Total area of interchange 
                 84.82 
                 21.21 
                 [m 2 ] 
               
               
                 Mass evaporation of 
                 477.69 
                 7.70 
                 [Kg/day] 
               
               
                 Mixture 
               
               
                 Volumetric evaporation 
                 2042.47 
                 300.73 
                 [m3/day] 
               
               
                 of Mixture 
               
             
          
           
               
                 FOG CONCENTRATIONS 
               
             
          
           
               
                 Concentration of emission 
                 0.000266 
                 0.000174 
                 [mg/m 3 ] 
               
               
                 of NaOH 
               
               
                 Concentration of water 
                 233.87 
                 25.60 
                 [g/m 3 ] 
               
               
                 emission 
               
               
                 □ Evaporated mixture 
                 233.87 
                 25.60 
                 [g/m 3 ] 
               
               
                 Evaporation of NaOH 
                 0.5436 
                 0.0523 
                 [mg/day] 
               
               
                   
               
             
          
         
       
     
     The following publications and the subject matter therein are incorporated herein by reference: 
     1) Frenay, J., S. Ferlay and J. Hissel (1986) Zinc and Lead Recovery from EAF Dusts by Caustic Soda Process. 44 th  Electric Furnace Conference Proceedings, Iron and Steel Society of the AIME, Warrendale, Pa., USA. pp. 417-421. 
     2) Wheatley, B. I. (1990) Production of Zinc Powder from Arc and Smelter Flue Dusts. In: Recycling of Metalliferous Materials, IMM, London. pp. 291-299. 
     3) Dreisinger, D.B., E. Peters and Morgan, G. (1989) Hydrometallurgical Treatment Options for Carbon Steel Electric Arc Furnace Dusts, In: M.H.I. Baird and S. Vijayan (editors), Proceedings of 2 nd  International Conference on Separations Science and Technology, Canadian Society for Chemical Engineering, Ottawa, Canada, pp. 578-585. 
     4) Xia, D.K. and C.A. Pickles (2000) Microwave Caustic Leaching of Electric Arc Furnace Dust. Department of Materials and Metallurgical Engineering, Queen&#39;s University.  Minerals Engineering  13(1), 79-84. 
     5) Department of Chemical Engineering, Universidad de Santiago de Chile. 
     6) Bourdeau, K. (1994) Update on Regulations and Enforcement. Proc. of the CMP Electric Furnace Dust Treatment Symposium (Pittsburgh, Pa. EPRI Center for Materials Production). 
     7) Kola, R. (1991) The Treatment of EAF-Dust in Europe. Proceedings: Recycling Lead and Zinc. The Challenge of the 1990&#39;s. Rome, Italy, June 11-13, 279-295. 
     8) QUESTOR ENGINEERING LIMITED (1983) Caustic Leaching-Electrowinning Process for Treating Electric Arc Furnace Baghouse Dust, Final Report, Queen&#39;s University Micromedia, Req. No. 1-9092, July 5, pp. 7-12. 
     9) Aota, J., S. A.Mikhail, D. T. Liang and W. N. Howell (1994) Low Temperature EAF Dust Vitrification Process, CANMET Mineral Science Laboratories, 33 rd  Conference of Metallurgists, Aug. 20-25, Toronto, Canada. 
     10) Center for Materials Production (1993) Electric Arc Furnace Dust-1993 OveMew, CMP Report No. 93-1, (prepared by) Arthur D. Little, Inc. for Center for Materials Production, Pittsburgh, USA. 
     11) Zunkel, D. (1996) What to do with your EAF Dust.  Steel Times International , July, 46-50. 
     12) Moser, W. S., G. T. Mahler, T. R. Kneooer, R. M. Kuba and J. E. Pusateri (1991) Metals Recycling from Steelmaking and Foundry Waste by Horsehead Resource Development. 
     13) Nyirenda, RL. (1991) The Processing of Steelmaking Flue Dust: A Review.  Minerals Engineering  4(7-11), 1003-1025. 
     14) Xia, D.K. (1997)  Recovery of Zinc from Zinc ferrite and Electric Arc Furnace Dust . Thesis Department of Materials and Metallurgical Engineering. Queen&#39;s University. Kingston, Ontario, Canada pp. 1-23. 
     15) Palencia, I.; R. Romero, N. Iglesias and F. Carranza (1999) Recycling EAF Dust Leaching Residue to the Furnace: A Simulation Study.  JOM , August 1999. 
     16) Merril, C.C. and R.S. Lang (1995)  Experimental Caustic Leaching of Oxidized Zinc Ores and Minerals and the Recovery of Zinc from Leach Solution . Report of Investigations 6576, Bureau of Mines, United States Department of the Interior. 
     17) Youcai, Z. and R. Stanforth (2000) Integrated hydrometallurgical process for production of Zinc from electric arc furnace dust in alkaline medium.  Journal of Hazardous Materials  80, 223-240. 
     18) CA-A-12000701 
     19) Chukhlantscv, V.G. (1956), The Solubility Product of Metal Arsenates,  Zhur. Neorg. Khim ., I: 1975-82, II: 529-35;  Zhur. Anal. Khim . 11, 529-35;  J. Anal. Chem . 11, 565-71.