Patent Application: US-59520006-A

Abstract:
an electro - hydrometallurgical process that extracts zinc from electric furnace 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:
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 fig1 , which shows one generic scheme of a process . as it is seen in fig1 , 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 ): 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 fig1 ), in relation to fig2 , 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 . 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 . 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 . 3 ) dreisinger , d . b ., e . peters and morgan , g . 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