CA1046288A - Purification of zinc sulphate solutions - Google Patents
Purification of zinc sulphate solutionsInfo
- Publication number
- CA1046288A CA1046288A CA218,818A CA218818A CA1046288A CA 1046288 A CA1046288 A CA 1046288A CA 218818 A CA218818 A CA 218818A CA 1046288 A CA1046288 A CA 1046288A
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- CA
- Canada
- Prior art keywords
- stage
- zinc
- cement
- purification
- solution
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
- C22B19/00—Obtaining zinc or zinc oxide
- C22B19/20—Obtaining zinc otherwise than by distilling
- C22B19/26—Refining solutions containing zinc values, e.g. obtained by leaching zinc ores
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
- C01G9/00—Compounds of zinc
- C01G9/06—Sulfates
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
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- Environmental & Geological Engineering (AREA)
- General Life Sciences & Earth Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Inorganic Chemistry (AREA)
- Manufacturing & Machinery (AREA)
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- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Manufacture And Refinement Of Metals (AREA)
Abstract
"PURIFICATION OF ZINC SULPHATE SOLUTIONS"
ABSTRACT OF THE DISCLOSURE:
A method is provided for purifying zinc sulphate solutions containing copper, cadmium and cobalt impurities, which comprises, in a first stage, treating the impure zinc sulphate solution with zinc dust and an antimony or arsenic compound so as to form a cement containing essentially all the copper and cadmium and a portion of the cobalt and separating this cement from the partially purified solution, then, in a second stage, treating the partially purified solution with zinc dust and an antimony or arsenic compound to form a cement containing essentially metallic zinc as well as the remaining cobalt and other incidental impurities and separating this cement from the purified solution and recycling at least the major portion of this cement to the first stage where it is used instead of zinc dust to effect further treatment of a fresh impure zinc sulphate solution. The purified solution from the second stage may also be subjected to a third purifi-cation stage for control purposes. This method provides an improved procedure for the purification of zinc sulphate solutions.
ABSTRACT OF THE DISCLOSURE:
A method is provided for purifying zinc sulphate solutions containing copper, cadmium and cobalt impurities, which comprises, in a first stage, treating the impure zinc sulphate solution with zinc dust and an antimony or arsenic compound so as to form a cement containing essentially all the copper and cadmium and a portion of the cobalt and separating this cement from the partially purified solution, then, in a second stage, treating the partially purified solution with zinc dust and an antimony or arsenic compound to form a cement containing essentially metallic zinc as well as the remaining cobalt and other incidental impurities and separating this cement from the purified solution and recycling at least the major portion of this cement to the first stage where it is used instead of zinc dust to effect further treatment of a fresh impure zinc sulphate solution. The purified solution from the second stage may also be subjected to a third purifi-cation stage for control purposes. This method provides an improved procedure for the purification of zinc sulphate solutions.
Description
fàZ88 Th~s inventlon relates to a method for purifylng zinc sulphate solutlons containing copper, cadmium and cobalt as maJor lmpurities and particularly for purifying zinc sulphate solutions obtained from leaching zinc calcines.
~/ec~r~I nn~ 4 In the clcctrolytio rc~ining of zlnc3 a zinc sulphate solution is normally prepared by leaching a roasted zinc sulphide concentrate (also known as zinc calcine) which contains iron compounds and other impurities including copper, cadmium, cobalt, etc. Iron is then precipitated out of the solution by various known procedures, such as oxidation and neutralization techniquesJ jarosite precipitation or the like.
Together with iron, other hydrolysable impuritiesJ such as arsenic and antimony,-are also usually removed by these techniques. After this initial removal of iron and other hydrolysable impurities which co-precipitate with iron, the obtained zinc sulphate solution must be further purified to eliminate impurities such as Cu, Cd and Co which are the most - important impurities remaining Other incidental impurities, such as Ni, Ge, etc. may also be present in such solution and they must equally be removed to a satisfactory level. However, the applicant has found that, as a normal practice, when cobalt has been removed to a satisfactory level (usually below 0.1 mg/l), all other impurities are also usually already reduced to an acceptable degree.
It is already known to purify zinc sulphate solutions by first removing Cu and Co by cementatlon with zinc dust at elevated temperatures (preferably above 90C) in the presence of arsenic or antimony compounds and of CuS04 and then by removing Cd by cementation with zinc dust and CuS04 at relatively low temperatures (preferably below 8Q C). This conventional method has several disadvantages~ First, this is a batch process. Also it utilizes great amounts of metallic ~s, 104t;288 zinc in the form of zinc dust, which must consequently be removed from production and transformed into dust for use in the purification treatment. Moreover it utllizes substantial amounts of CuS04 and requires stringent operating controls in order to avoid Cd cementation in the first stage of the operation where Cu and Co are eliminated. Normally, however, at least some Cd, and often a subs~tantial proportion thereof, is co-precipitated with Cu and Co in the first stage of the operation a~d constitutes an important commercial loss.
More recently a continuous purification method has been disclosed in Belgian patent No. 783,549 of June 15, 1972 issued to SOCIETE DES MINES ET FONDERIES DE ZINC DE LA VIEILLE
MONT~GNE, according to which Cu and Cd are first eliminated by precipitation with excess zinc dust and then Co as well as other incidental impurities are removed by cementation with zinc dust and an antimony compound at an elevated temperature between 80C and the boiling point of the solution. Although this method avoids the use of CuS04 it still requires great amounts of zinc dust which must either be purchased or removed from production as already indicated above.
It is an ob~ect of the present invention to provide an lmproved purification process for zinc sulphate solutions.
A further ob~ect of,the present invention is to provide a continuous purification process for zinc sulphate solutions in which the lmpurities are satisfactorily removed, while producing only one cake for further treatment.
A still further ob~ect of the present invention is to provide a method in which the cake or cement obtained at the end o~ the purification system can be recycled to the beginning of the system and reused instead of the zinc dust, thereby reducing the amount of zinc dust required and increasing the final overall zinc production.
~/ec~r~I nn~ 4 In the clcctrolytio rc~ining of zlnc3 a zinc sulphate solution is normally prepared by leaching a roasted zinc sulphide concentrate (also known as zinc calcine) which contains iron compounds and other impurities including copper, cadmium, cobalt, etc. Iron is then precipitated out of the solution by various known procedures, such as oxidation and neutralization techniquesJ jarosite precipitation or the like.
Together with iron, other hydrolysable impuritiesJ such as arsenic and antimony,-are also usually removed by these techniques. After this initial removal of iron and other hydrolysable impurities which co-precipitate with iron, the obtained zinc sulphate solution must be further purified to eliminate impurities such as Cu, Cd and Co which are the most - important impurities remaining Other incidental impurities, such as Ni, Ge, etc. may also be present in such solution and they must equally be removed to a satisfactory level. However, the applicant has found that, as a normal practice, when cobalt has been removed to a satisfactory level (usually below 0.1 mg/l), all other impurities are also usually already reduced to an acceptable degree.
It is already known to purify zinc sulphate solutions by first removing Cu and Co by cementatlon with zinc dust at elevated temperatures (preferably above 90C) in the presence of arsenic or antimony compounds and of CuS04 and then by removing Cd by cementation with zinc dust and CuS04 at relatively low temperatures (preferably below 8Q C). This conventional method has several disadvantages~ First, this is a batch process. Also it utilizes great amounts of metallic ~s, 104t;288 zinc in the form of zinc dust, which must consequently be removed from production and transformed into dust for use in the purification treatment. Moreover it utllizes substantial amounts of CuS04 and requires stringent operating controls in order to avoid Cd cementation in the first stage of the operation where Cu and Co are eliminated. Normally, however, at least some Cd, and often a subs~tantial proportion thereof, is co-precipitated with Cu and Co in the first stage of the operation a~d constitutes an important commercial loss.
More recently a continuous purification method has been disclosed in Belgian patent No. 783,549 of June 15, 1972 issued to SOCIETE DES MINES ET FONDERIES DE ZINC DE LA VIEILLE
MONT~GNE, according to which Cu and Cd are first eliminated by precipitation with excess zinc dust and then Co as well as other incidental impurities are removed by cementation with zinc dust and an antimony compound at an elevated temperature between 80C and the boiling point of the solution. Although this method avoids the use of CuS04 it still requires great amounts of zinc dust which must either be purchased or removed from production as already indicated above.
It is an ob~ect of the present invention to provide an lmproved purification process for zinc sulphate solutions.
A further ob~ect of,the present invention is to provide a continuous purification process for zinc sulphate solutions in which the lmpurities are satisfactorily removed, while producing only one cake for further treatment.
A still further ob~ect of the present invention is to provide a method in which the cake or cement obtained at the end o~ the purification system can be recycled to the beginning of the system and reused instead of the zinc dust, thereby reducing the amount of zinc dust required and increasing the final overall zinc production.
- 2 -104~Z88 Other objects and advantages of the present invention will be made apparent from the following more detailed description thereof.
Basically, this invention provides a method for the purification of zinc sulphate solutions containing copper, cadmium and cobalt as major impurities, which comprises:
(a) in a first stage, treating the impure zinc sulphate solution with zinc dust in the presence of an antimony compound and/or As203, so as to form a cement containing essentially all the copper and cadmium and a portion of the cobalt and separating said cement from such partially purified solution;
(b) in a second stage, treating said partially purified solution with zinc dust in the presence of an antimony compound and/or As203, to form a cement containing essentially zinc as well as the remaining cobalt and other incidental impurities, and separating said cement from the purified solution; and (c) recycling at least the major portion of said cement from the second stage to the first stage to be used therein instead of zinc dust for the treatment of a fresh impure zinc sulphate solution.
The amount of the zinc reagent (zinc dust and/or recycled cement) to be used in the first stage of this puri-fication system and the amount and type of the antimony or arsenic compound, can be readily determined and controlled by those familiar with the art to achieve an essentially complete copper and cadmium precipitation together with a portion, preferably up to 75%, of the cobalt cementation during the first stage treatment. It has been found, for example, that between about 2 and 5 grams of zinc dust or recycled cement (on dry basis) per liter of impure feed LL~
~04f~288 solution are satisfactory for the treatment in the first stage. Initially, zinc dust is introduced into the first stage, however, as soon as the recycling of the second stage cement commences, very little, if any,additlonal zinc dust would be required. The antimony is normally used in the form of antimony oxide Sb2O3, however, antimony salts such as that of potassium and even pure antimony metal powder are also suitable, and all these reagents are included within the meaning of "antimony compound" as used herein. The amounts of the anti-mony compound can also be readily determined by those familiarwith the art to effect the desired cementation of substantially all Cu and Cd and of a portion of Co (which should be as large as possible) in the first stage of the purification treatment.
The applicants have found, for example, that between about 0.1 and 1.0 mg of Sb2O3 per litre of impure feed solution is a suit-able amount for the first stage operation. Instead of the anti-mony compounds As2O3 can also be used. The amount of this com-pound is, again, such as to remove a portion of Co together with substantially the totality of Cu and Cd. The applicant found that it is economically advantageous to use Sb2O3 rather than As2O3, which will only be used-if the antimony compounds become, for some reason, too expensive or unavailable.
The first stage purification treatment is carried out under normal agitation (about 40-50 rpm) and preferably at a temperature of between about 50C and 80C. The pH of the solution is also normally maintained between about 4 and 5, preferably between 4.4 and 4.8. The desired pH value can be maintained by addition of spent electrolyte into the first state purification system. These conditions are, however, not restrictive since they depend on many factors such as the type and amounts of the antimony compound added, the type of solution to be purified, the time used to achieve such lO~f~Z88 purification, etc. A man of the art will be able to select the most appropriate temperature and pH conditions for his specific operation to achieve essentially total removal of Co and Cd impurities and a portion (preferably as high as possible) of the cobalt in the first purification stage.
The first stage purification treatment is normally carried out on a continuous basis in a plurality of tanks. For instance, three tanks may be used for this purpose with the impure feed and other reagents being preferably introduced into the first tank which would overflow into the second tank which, in turn, would overflow into the third tank. The first stage purification treatment would proceed in all three tanks on a continuous basis taking from about one to three hours. Obviously, if it is found desirable, some of these reagents can also be introduced into tanks other than the first.
Then, a liquid-solid separation is effected to remove the obtained cement from the partially purified solution. This is normally done by transferring the first stage mixture into suitable thickeners from which the underflow is filtered, for instance, in vacuum drum filters, such as Do~rco filters, to remove the solids from the filtrate. The overflow from the thickeners, as well as the filtrate from the filters are then sent to the second stage purification treatment. The second stage purification treatment is carried out in the presence of zinc dust and an antimony compound or As2O3 to remove the remain-ing impurities to a satisfactory level. The amount of zinc dust added should again be in excess with respect to the impurities present and will normally be in the same concentration range as in the first stage or slightly greater. For example, between 3 and 7g of zinc dust per liter of the partially purified solution entering the second stac3e has been found satisfactory. The 104~288 amount of the antimony compound or As203 is also preferabl~
slightly higher than in the first stage and, for instance, between 0.4 and 1.0 mg of Sb20~ has been found quite adequate.
With respect to the amounts of these ingredients to be added in the second stage, the same general considerations as discussed above with reference to the first stage also apply, namely the man of the art will have no difficulty in adjusting and controlling these amounts to satisfy his specific require-ments.
The temperature of the second stage operation is normally higher than that of the first stage and temperatures between 85C and the boiling point of the solution have been found to be quite suitable. The pH is very similar to the one used in the first stage, namely between about 4 and 5, preferably between 4.1 and 4.7. This pH may again be adjusted by addition of spent electrolyte if this becomes necessary. The second stage operation is also usually conducted in a plurality of tanks with suitable agitation (about 40-50 rpm) and on a continuous basis. The total time of the second stage purification treatment is usually between about 2 and 5 hours. The various tanks, just as in the first stage, are normally connected in series.
The mixture obtained from this second stage purific~tion treatment is then forwarded to a liquid-solid separation step, The solids from this liquid-solid separa-tion, for instance in the form of a cake, are then recycled back to the first stage purification treatment for treating freshly introduced impure feed. The liquid phase from this liquid-solid separation should normally be sufficiently ~0 pure to be used in the electrowinning of zinc.
If, however, the solution from the second stage 104~;Z88 puri~ication treatment is found to be insufficiently pure to be used as electrolyte, or in order to provide a better control of the final electrolyte solution, an optional third purification stage can be carried out, in which the solution from the second stage is again treated before it is sent to electrolysis. This final treatment stage can normally be considered as a control stage and it is highly preferred to incorporate it as part of the process of the present invention.`
The operating conditions of this third control stage can be adJusted in such a way as to effect the final impurity control of the electrolyte. Thus, if it is found that the solution from the second stage purification treatment still contains some impurities then it can further be treàted in the third stage to remove these impurities to an acceptable level.
For instance, if it is found that the solution ~rom the second stage purification treatment still contains cobalt over the desired limit, it can be removed by addition of zinc dust at a temperature of between about ~0 and 90C and a pH
~ OI' between about ~ and 5 in the 9~co~ stage. After completion of the third stage purification treatment, which will normally be carried out in only one tank, a liquid-solid separation will again be effected and the solids which will contain at le&st 9~% of zinc, will be recycled back to the first stage, together with the sollds from the second stage, to be used there instead of zlnc dust. It should be noted that it is preferred to recycle all the solids from the second stage purification treatment and all the solids from the optional third stage purification treatment back to the first stage purification treatment where they are reused instead Or zinc dust, However, if, for some reason, it is desired to withhold some of these sol:lds from the second and third stages and add zinc dust lnstead into the first stage, this obviously can be done.A
10~;2~8 Also, the solids from the third stage could be recycled back into the second stage rather than the first. However at least the major proportion of the solids from the second and third stages should be recycled in order to obtain the desired advantageous effect of the present invention, namely the saving in zinc dust consumption and the increase of the overall capacity of electrolytic zinc production.
The invention will now further be described with reference to the appended drawings in which:
Figure 1 is a flow sheet of the basic embodiment of the present invention using a two stage purification system;
Figure 2 is a flow sheet of a preferred embodiment of the present invention having a third stage purification treatment; and Figure 3 is a more detailed flow sheet of the system as it has been designed by the applicants.
According to figure 1, the impure zinc sulphate solution feed containing Cu, Cd and Co as major impurities is treated in the first purification stage 1. In addition to the impure feed, there is introduced into this first stage zinc dust and an antimony compound or As2O3. As the process proceeds, zinc dust is replaced by the cement cake from the following purification stage, which is recycled back to the first stage. If desired, a minor proportion of additional zinc dust can always be introduced into the first purification stage.
This first purification stage 1 is carried out under such operational conditions that essentially all copper and cadmium impurities as well as a substantial portion of the cobalt impurity are coprecipitated.
Then, a liquid-solid separation step 2 is effected and the solids from this liquid-solid separation, which contain 1C~4~i288 essentially all copper and cadmium as well as a portion of the cobalt, are treated separately in a known manner to extract cadmium while also producing a copper cake containing some cobalt in it. This copper cake may be treated, in a known mannerJ to extract copper and cobalt or it can be sent to a copper smelting operation. These additional treatments of solids or cakes are well known in the art and they do not form an actual part of the present invention except to the extent that these are the only solids produced according to this invention which require further treatment.
The liquid phase from the liquid-solid separation step 2 is then treated in the second purification stage 3 with appropriate addition of zinc dust and an antimony com-pound or As203 to remove cobalt to a satisfactory le~el.
Spent electrolyte may also be added to either stage for pH
control. Then, a~other liquid-solid separation step 4 is effected in such a way that the final solution resulting therefrom is essentially free of solids. The solid phase from this step is recycled back to the first stage 1 to be used therein as the zinc reagent and the liquid phase constitutes a pure solution suitable for electrolysis.
In the embodiment of figure 2, the first purification stage 1 and the liquid-solid separation step 2 as well as the second purification stage 3 are basically identical to the same procedures described with reference to figure 1. The liquid-solid separation step 4 can be effected by using cyclones, thickeners or presses. However, the liquid phase from the liquid-solid separation 4 is then treated in a third purifica-tion stage ~. In this case, the third stage treatment is effected only with zinc dust and is followed by a liquid-solid separation step ~ froM which the solids are recycled bac~. to the first purification stage 1 and the liquid phase, _ g _ ,, ~04~Z88 -- whiCh should be essentlally free of solids, constitutes the pure solution suitable for electrolytis.
Figure ~ represents a flow sheet of a ~pecific but non limitative example which will now be described in gre~ er detail.
- EXAMPLE
The impure feed is introduced into the first purification stage 1 which consists of three tanks connected in series. The impure feed is introduced continuously into the first tank at the rate of 1000 - 1500 UOS. gallons per minute. This impure feed has the following typical analysis of zinc and impurities:
Zn : 140 - 150 g/l Cu : 300 - 800 mg/l Cd : 400 - 1000 mg/l Co : 2-12 mg/l the remainder being other incidental impurities that may be present in the zinc sulphate salution.
~ Initially,with the impure feed, there is introduced lnto the first tank o~ the first purification stage 1 some 25-35 short tons per day of zinc dust and 4-5 lbs per day of Sb20~ as well as a necessary volume of spent electrolyte conslsting essentlally of about 60 g/l Or Zn and 185-200 g/l of H2S~4 to maintain the pH at\the desired value. After the lnltlal treatment, ba~ically no new æinc dust is introduced into the rirst purification stage 1, but instead solids rrom the second purlfication stage are recycled back into stage 1.
The overall treatment in the three tanks Or the flrst purification stage 1 lasts approximately two hours and the obtained mixture is then forwarded to thickeners 2a and 2b ln which the initial liquid-s~lid separation ls effected.
The underflow from the thickeners which has a solids content of about 200 g/l is filtered preferably ln vacuum drum 104f~288 filters 2c of Dorrco type to remove solids for further treatment while the filtrate together with the overflow from the thlckeners pass through the heat exchanger 7 in which they are heated to over 90C and constitute the pa~tially purified solution which is conducted to the second purification stage ~. Zinc dust is introduced into this second purification stage 3 at a rate of about 30-1~0 short tons per day and spent electrolyte may be introduced at a variable rate ~or pH control. Sb20~ is introduced into this second stage at a rate of 8 to 10 lbs/day.
The second purification stage 3 preferably consists of five tanks connected in series and the total time of the second stage purification treatment is between about 2.5 and
Basically, this invention provides a method for the purification of zinc sulphate solutions containing copper, cadmium and cobalt as major impurities, which comprises:
(a) in a first stage, treating the impure zinc sulphate solution with zinc dust in the presence of an antimony compound and/or As203, so as to form a cement containing essentially all the copper and cadmium and a portion of the cobalt and separating said cement from such partially purified solution;
(b) in a second stage, treating said partially purified solution with zinc dust in the presence of an antimony compound and/or As203, to form a cement containing essentially zinc as well as the remaining cobalt and other incidental impurities, and separating said cement from the purified solution; and (c) recycling at least the major portion of said cement from the second stage to the first stage to be used therein instead of zinc dust for the treatment of a fresh impure zinc sulphate solution.
The amount of the zinc reagent (zinc dust and/or recycled cement) to be used in the first stage of this puri-fication system and the amount and type of the antimony or arsenic compound, can be readily determined and controlled by those familiar with the art to achieve an essentially complete copper and cadmium precipitation together with a portion, preferably up to 75%, of the cobalt cementation during the first stage treatment. It has been found, for example, that between about 2 and 5 grams of zinc dust or recycled cement (on dry basis) per liter of impure feed LL~
~04f~288 solution are satisfactory for the treatment in the first stage. Initially, zinc dust is introduced into the first stage, however, as soon as the recycling of the second stage cement commences, very little, if any,additlonal zinc dust would be required. The antimony is normally used in the form of antimony oxide Sb2O3, however, antimony salts such as that of potassium and even pure antimony metal powder are also suitable, and all these reagents are included within the meaning of "antimony compound" as used herein. The amounts of the anti-mony compound can also be readily determined by those familiarwith the art to effect the desired cementation of substantially all Cu and Cd and of a portion of Co (which should be as large as possible) in the first stage of the purification treatment.
The applicants have found, for example, that between about 0.1 and 1.0 mg of Sb2O3 per litre of impure feed solution is a suit-able amount for the first stage operation. Instead of the anti-mony compounds As2O3 can also be used. The amount of this com-pound is, again, such as to remove a portion of Co together with substantially the totality of Cu and Cd. The applicant found that it is economically advantageous to use Sb2O3 rather than As2O3, which will only be used-if the antimony compounds become, for some reason, too expensive or unavailable.
The first stage purification treatment is carried out under normal agitation (about 40-50 rpm) and preferably at a temperature of between about 50C and 80C. The pH of the solution is also normally maintained between about 4 and 5, preferably between 4.4 and 4.8. The desired pH value can be maintained by addition of spent electrolyte into the first state purification system. These conditions are, however, not restrictive since they depend on many factors such as the type and amounts of the antimony compound added, the type of solution to be purified, the time used to achieve such lO~f~Z88 purification, etc. A man of the art will be able to select the most appropriate temperature and pH conditions for his specific operation to achieve essentially total removal of Co and Cd impurities and a portion (preferably as high as possible) of the cobalt in the first purification stage.
The first stage purification treatment is normally carried out on a continuous basis in a plurality of tanks. For instance, three tanks may be used for this purpose with the impure feed and other reagents being preferably introduced into the first tank which would overflow into the second tank which, in turn, would overflow into the third tank. The first stage purification treatment would proceed in all three tanks on a continuous basis taking from about one to three hours. Obviously, if it is found desirable, some of these reagents can also be introduced into tanks other than the first.
Then, a liquid-solid separation is effected to remove the obtained cement from the partially purified solution. This is normally done by transferring the first stage mixture into suitable thickeners from which the underflow is filtered, for instance, in vacuum drum filters, such as Do~rco filters, to remove the solids from the filtrate. The overflow from the thickeners, as well as the filtrate from the filters are then sent to the second stage purification treatment. The second stage purification treatment is carried out in the presence of zinc dust and an antimony compound or As2O3 to remove the remain-ing impurities to a satisfactory level. The amount of zinc dust added should again be in excess with respect to the impurities present and will normally be in the same concentration range as in the first stage or slightly greater. For example, between 3 and 7g of zinc dust per liter of the partially purified solution entering the second stac3e has been found satisfactory. The 104~288 amount of the antimony compound or As203 is also preferabl~
slightly higher than in the first stage and, for instance, between 0.4 and 1.0 mg of Sb20~ has been found quite adequate.
With respect to the amounts of these ingredients to be added in the second stage, the same general considerations as discussed above with reference to the first stage also apply, namely the man of the art will have no difficulty in adjusting and controlling these amounts to satisfy his specific require-ments.
The temperature of the second stage operation is normally higher than that of the first stage and temperatures between 85C and the boiling point of the solution have been found to be quite suitable. The pH is very similar to the one used in the first stage, namely between about 4 and 5, preferably between 4.1 and 4.7. This pH may again be adjusted by addition of spent electrolyte if this becomes necessary. The second stage operation is also usually conducted in a plurality of tanks with suitable agitation (about 40-50 rpm) and on a continuous basis. The total time of the second stage purification treatment is usually between about 2 and 5 hours. The various tanks, just as in the first stage, are normally connected in series.
The mixture obtained from this second stage purific~tion treatment is then forwarded to a liquid-solid separation step, The solids from this liquid-solid separa-tion, for instance in the form of a cake, are then recycled back to the first stage purification treatment for treating freshly introduced impure feed. The liquid phase from this liquid-solid separation should normally be sufficiently ~0 pure to be used in the electrowinning of zinc.
If, however, the solution from the second stage 104~;Z88 puri~ication treatment is found to be insufficiently pure to be used as electrolyte, or in order to provide a better control of the final electrolyte solution, an optional third purification stage can be carried out, in which the solution from the second stage is again treated before it is sent to electrolysis. This final treatment stage can normally be considered as a control stage and it is highly preferred to incorporate it as part of the process of the present invention.`
The operating conditions of this third control stage can be adJusted in such a way as to effect the final impurity control of the electrolyte. Thus, if it is found that the solution from the second stage purification treatment still contains some impurities then it can further be treàted in the third stage to remove these impurities to an acceptable level.
For instance, if it is found that the solution ~rom the second stage purification treatment still contains cobalt over the desired limit, it can be removed by addition of zinc dust at a temperature of between about ~0 and 90C and a pH
~ OI' between about ~ and 5 in the 9~co~ stage. After completion of the third stage purification treatment, which will normally be carried out in only one tank, a liquid-solid separation will again be effected and the solids which will contain at le&st 9~% of zinc, will be recycled back to the first stage, together with the sollds from the second stage, to be used there instead of zlnc dust. It should be noted that it is preferred to recycle all the solids from the second stage purification treatment and all the solids from the optional third stage purification treatment back to the first stage purification treatment where they are reused instead Or zinc dust, However, if, for some reason, it is desired to withhold some of these sol:lds from the second and third stages and add zinc dust lnstead into the first stage, this obviously can be done.A
10~;2~8 Also, the solids from the third stage could be recycled back into the second stage rather than the first. However at least the major proportion of the solids from the second and third stages should be recycled in order to obtain the desired advantageous effect of the present invention, namely the saving in zinc dust consumption and the increase of the overall capacity of electrolytic zinc production.
The invention will now further be described with reference to the appended drawings in which:
Figure 1 is a flow sheet of the basic embodiment of the present invention using a two stage purification system;
Figure 2 is a flow sheet of a preferred embodiment of the present invention having a third stage purification treatment; and Figure 3 is a more detailed flow sheet of the system as it has been designed by the applicants.
According to figure 1, the impure zinc sulphate solution feed containing Cu, Cd and Co as major impurities is treated in the first purification stage 1. In addition to the impure feed, there is introduced into this first stage zinc dust and an antimony compound or As2O3. As the process proceeds, zinc dust is replaced by the cement cake from the following purification stage, which is recycled back to the first stage. If desired, a minor proportion of additional zinc dust can always be introduced into the first purification stage.
This first purification stage 1 is carried out under such operational conditions that essentially all copper and cadmium impurities as well as a substantial portion of the cobalt impurity are coprecipitated.
Then, a liquid-solid separation step 2 is effected and the solids from this liquid-solid separation, which contain 1C~4~i288 essentially all copper and cadmium as well as a portion of the cobalt, are treated separately in a known manner to extract cadmium while also producing a copper cake containing some cobalt in it. This copper cake may be treated, in a known mannerJ to extract copper and cobalt or it can be sent to a copper smelting operation. These additional treatments of solids or cakes are well known in the art and they do not form an actual part of the present invention except to the extent that these are the only solids produced according to this invention which require further treatment.
The liquid phase from the liquid-solid separation step 2 is then treated in the second purification stage 3 with appropriate addition of zinc dust and an antimony com-pound or As203 to remove cobalt to a satisfactory le~el.
Spent electrolyte may also be added to either stage for pH
control. Then, a~other liquid-solid separation step 4 is effected in such a way that the final solution resulting therefrom is essentially free of solids. The solid phase from this step is recycled back to the first stage 1 to be used therein as the zinc reagent and the liquid phase constitutes a pure solution suitable for electrolysis.
In the embodiment of figure 2, the first purification stage 1 and the liquid-solid separation step 2 as well as the second purification stage 3 are basically identical to the same procedures described with reference to figure 1. The liquid-solid separation step 4 can be effected by using cyclones, thickeners or presses. However, the liquid phase from the liquid-solid separation 4 is then treated in a third purifica-tion stage ~. In this case, the third stage treatment is effected only with zinc dust and is followed by a liquid-solid separation step ~ froM which the solids are recycled bac~. to the first purification stage 1 and the liquid phase, _ g _ ,, ~04~Z88 -- whiCh should be essentlally free of solids, constitutes the pure solution suitable for electrolytis.
Figure ~ represents a flow sheet of a ~pecific but non limitative example which will now be described in gre~ er detail.
- EXAMPLE
The impure feed is introduced into the first purification stage 1 which consists of three tanks connected in series. The impure feed is introduced continuously into the first tank at the rate of 1000 - 1500 UOS. gallons per minute. This impure feed has the following typical analysis of zinc and impurities:
Zn : 140 - 150 g/l Cu : 300 - 800 mg/l Cd : 400 - 1000 mg/l Co : 2-12 mg/l the remainder being other incidental impurities that may be present in the zinc sulphate salution.
~ Initially,with the impure feed, there is introduced lnto the first tank o~ the first purification stage 1 some 25-35 short tons per day of zinc dust and 4-5 lbs per day of Sb20~ as well as a necessary volume of spent electrolyte conslsting essentlally of about 60 g/l Or Zn and 185-200 g/l of H2S~4 to maintain the pH at\the desired value. After the lnltlal treatment, ba~ically no new æinc dust is introduced into the rirst purification stage 1, but instead solids rrom the second purlfication stage are recycled back into stage 1.
The overall treatment in the three tanks Or the flrst purification stage 1 lasts approximately two hours and the obtained mixture is then forwarded to thickeners 2a and 2b ln which the initial liquid-s~lid separation ls effected.
The underflow from the thickeners which has a solids content of about 200 g/l is filtered preferably ln vacuum drum 104f~288 filters 2c of Dorrco type to remove solids for further treatment while the filtrate together with the overflow from the thlckeners pass through the heat exchanger 7 in which they are heated to over 90C and constitute the pa~tially purified solution which is conducted to the second purification stage ~. Zinc dust is introduced into this second purification stage 3 at a rate of about 30-1~0 short tons per day and spent electrolyte may be introduced at a variable rate ~or pH control. Sb20~ is introduced into this second stage at a rate of 8 to 10 lbs/day.
The second purification stage 3 preferably consists of five tanks connected in series and the total time of the second stage purification treatment is between about 2.5 and
3.5 hours. The pH of the second stage is controlled between
4.1 and 4.7.
The mixture from the second purification stage 3 is conducted through liquid-solid separation cyclones 4 from which the overflow is forwarded to the third purification stage ~
and the underflow having a solids content of 40 to 50 g/l is sent back to the first stage 1.
The retention time in the third purification stage 5, whlch i8 normally carried out in one tank, is usually less than one hour and in addition to the overflow from the cyciones 4 there i8 also added to this stage ~inc dust in an amount o~
The mixture from the second purification stage 3 is conducted through liquid-solid separation cyclones 4 from which the overflow is forwarded to the third purification stage ~
and the underflow having a solids content of 40 to 50 g/l is sent back to the first stage 1.
The retention time in the third purification stage 5, whlch i8 normally carried out in one tank, is usually less than one hour and in addition to the overflow from the cyciones 4 there i8 also added to this stage ~inc dust in an amount o~
5-10 tons per day and spent electrolyte to maintain the pH
between about ~ and ~.
After the third purification stage, the obtained mixture i8 subJected to liquid-solid separation in filter presses 6. ~rom these filter presses, the cake 8 amounting to ~0 about 5 to 10 short dry tons per day is repulped at 9 with impure feed and sent back to the first purification stage 1.
Tho liquid phase from the filter presses 6 constitutes th~
104b~
- so-called "pure neutral" solutiorl 10 which ls forwarded to check ~ank~ 11 and then to neutral storage 12 from whiCh the electrolyte for zinc electrolysis is withdrawn after cooling at 13.
The range of solids analysis from the first purifica-tion stage 1 was found to be:
25 to 45% Zn ~ - 15% Cu 10 - 24%'Cd , about - 0.1% Co.
. The typical impurity content o~ the filtrate after the first purification stage contained:
less than 1 mg/l Cu .
0.1 - 1.5 mg/l Cd 0,01 - 0.02 mg/l Sb 0.5 - 2.5 mg/l Co.
After the second stage purification, the lmpurity content of the cyclones overflow analysed typically:
less than 0.4 mg/l Cu less than 0.5 mg/l Cd less than 0,1 mg/l Co about 0.01 mg/l Sb , The ~olids (dry basi,s) analysis of the underflow solids w~s typlcally:
. 93 - 98% Zn 0.2 - 0.3% Cu 0.3 - 0.4% Cd 0.02 - 0.05~ Co.
After the optional third stage, the analysis of the final solution wa~ substantially similar to that of the overflow solution from cyclone 4. Only slightly better purlty 104f~88 was achieved. The cake 8 contained over 95% zinc.
It is also possible to add to any purification stage some CuS04 if this is required to improve the cementation of impurities. Under~certain conditions, it is known that to eliminate "dragging" of Cd that may still remain, some copper should be added and such copper can be added in the~form o~
CuS04 crystals or solution. When desired, a concentration o~ CuS04 of about 10-30 mg/l has been found quite adequate.
However, this is only an optional measure and not an essential feature of the present invention.
Thus, basically, the idea behind applicant's improved process is to remove all impurities progressively at the various stages of the purification treatment. This can be done in a continuous operation while recycling to the first ~tage at least the ma~or part and preferably all the solids from subsequent stages, these solids to be used in the first stage instead of zinc dust for purification of a fresh impure solultion. In view of the fact that these solids from 6ubsequent stages are not in the form of pure metallic zinc but contain zinc in a proportion of about 90 to 98%, the applicant was concerned that some of the impurities present in the solids, particularly after the second stage purifica-tion treatment, would be reintroduced back into the first stage and would thereby increase the total impurities and be detrimental to the overall purification process. This concern proved to be un~ustified since a pilot plant operation of the process carried out a Canadian Electrolytic Zinc Limited, Valleyfield, Quebec, clearly demonstrated that no adverse effect occured on the final electrolytic solution due to the recycle Or the solids from the second and eventual third stages back to the first purification stage.
10~i288 ! It is clear, also that the novel process is slmple, vcrsatile and efficient. Obviously, the two or three stages of the process can be monitored as desired to remove most effectively the various impurities. If for some reason some other reagents or ingredients need to be added in such purification system, they can readily be added at any desired stageJ and the various conditions can be appropriately controlled at the various stages to achieve optimum purification results.
It is therefore believed that the novel method produces a clear advance in the art of purification of zinc sulphate solutions.
.
;
~0
between about ~ and ~.
After the third purification stage, the obtained mixture i8 subJected to liquid-solid separation in filter presses 6. ~rom these filter presses, the cake 8 amounting to ~0 about 5 to 10 short dry tons per day is repulped at 9 with impure feed and sent back to the first purification stage 1.
Tho liquid phase from the filter presses 6 constitutes th~
104b~
- so-called "pure neutral" solutiorl 10 which ls forwarded to check ~ank~ 11 and then to neutral storage 12 from whiCh the electrolyte for zinc electrolysis is withdrawn after cooling at 13.
The range of solids analysis from the first purifica-tion stage 1 was found to be:
25 to 45% Zn ~ - 15% Cu 10 - 24%'Cd , about - 0.1% Co.
. The typical impurity content o~ the filtrate after the first purification stage contained:
less than 1 mg/l Cu .
0.1 - 1.5 mg/l Cd 0,01 - 0.02 mg/l Sb 0.5 - 2.5 mg/l Co.
After the second stage purification, the lmpurity content of the cyclones overflow analysed typically:
less than 0.4 mg/l Cu less than 0.5 mg/l Cd less than 0,1 mg/l Co about 0.01 mg/l Sb , The ~olids (dry basi,s) analysis of the underflow solids w~s typlcally:
. 93 - 98% Zn 0.2 - 0.3% Cu 0.3 - 0.4% Cd 0.02 - 0.05~ Co.
After the optional third stage, the analysis of the final solution wa~ substantially similar to that of the overflow solution from cyclone 4. Only slightly better purlty 104f~88 was achieved. The cake 8 contained over 95% zinc.
It is also possible to add to any purification stage some CuS04 if this is required to improve the cementation of impurities. Under~certain conditions, it is known that to eliminate "dragging" of Cd that may still remain, some copper should be added and such copper can be added in the~form o~
CuS04 crystals or solution. When desired, a concentration o~ CuS04 of about 10-30 mg/l has been found quite adequate.
However, this is only an optional measure and not an essential feature of the present invention.
Thus, basically, the idea behind applicant's improved process is to remove all impurities progressively at the various stages of the purification treatment. This can be done in a continuous operation while recycling to the first ~tage at least the ma~or part and preferably all the solids from subsequent stages, these solids to be used in the first stage instead of zinc dust for purification of a fresh impure solultion. In view of the fact that these solids from 6ubsequent stages are not in the form of pure metallic zinc but contain zinc in a proportion of about 90 to 98%, the applicant was concerned that some of the impurities present in the solids, particularly after the second stage purifica-tion treatment, would be reintroduced back into the first stage and would thereby increase the total impurities and be detrimental to the overall purification process. This concern proved to be un~ustified since a pilot plant operation of the process carried out a Canadian Electrolytic Zinc Limited, Valleyfield, Quebec, clearly demonstrated that no adverse effect occured on the final electrolytic solution due to the recycle Or the solids from the second and eventual third stages back to the first purification stage.
10~i288 ! It is clear, also that the novel process is slmple, vcrsatile and efficient. Obviously, the two or three stages of the process can be monitored as desired to remove most effectively the various impurities. If for some reason some other reagents or ingredients need to be added in such purification system, they can readily be added at any desired stageJ and the various conditions can be appropriately controlled at the various stages to achieve optimum purification results.
It is therefore believed that the novel method produces a clear advance in the art of purification of zinc sulphate solutions.
.
;
~0
Claims (12)
1. Method for the purification of zinc sulphate solutions containing copper, cadmium and cobalt as major impurities, which comprises:
(a) in a first stage, treating the impure zinc sulphate solution with zinc dust and an antimony compound and/
or As2O3 so as to form a cement containing essentially all the copper and cadmium and a portion of the cobalt, and separating said cement from such partially purified solution;
(b) in a second stage, treating said partially purified solution with zinc dust and an antimony compound and/
or As2O3 to form a cement containing essentially zinc as well as the remaining cobalt and other incidental impurities and separating said cement from the purified solution; and (c) recycling at least the major portion of said cement from the second stage to the first stage to be used therein instead of zinc dust for the treatment of a fresh impure zinc sulphate solution.
(a) in a first stage, treating the impure zinc sulphate solution with zinc dust and an antimony compound and/
or As2O3 so as to form a cement containing essentially all the copper and cadmium and a portion of the cobalt, and separating said cement from such partially purified solution;
(b) in a second stage, treating said partially purified solution with zinc dust and an antimony compound and/
or As2O3 to form a cement containing essentially zinc as well as the remaining cobalt and other incidental impurities and separating said cement from the purified solution; and (c) recycling at least the major portion of said cement from the second stage to the first stage to be used therein instead of zinc dust for the treatment of a fresh impure zinc sulphate solution.
2. Method according to claim 1, wherein the purified solution from the second stage is further subjected to a third purification stage wherein a cement is formed comprising at least 95% zinc, said cement being then separated from the obtained pure solution and at least the major portion thereof being recycled back to the first or second stage to be used therein instead of zinc dust.
3. Method according to claim 1, wherein essentially all the cement from the second stage is recycled back to the first stage.
4. Method according to claim 2, wherein essentially all the cement from the third purification stage is recycled back to the first stage.
5. Method according to claim 1, wherein the first stage treatment is carried out at a temperature between about 50°C and 80°C and a pH of between about 4 and 5.
6. Method according to claim 1, wherein the first and second stage treatments are carried out with addition of Sb2O3, an antimony salt or antimony metal powder as the antimony compound.
7. Method according to claims 1 or 5, wherein the first stage treatment is carried out with addition of about 0.1 - 1.0 mg of Sb2O3 as the antimony compound per liter of impure feed solution.
8. Method according to claim 1, wherein the second stage treatment is carried out at a temperature between about 85°C and the boiling point of the solution and a pH of between about 4 and 5.
9. Method according to claims 1 or 8, wherein the second stage treatment is carried out with addition of about 0.4 - 1.0 mg of Sb2O3 as the antimony compound per liter of solution.
10. Method according to claim 2, wherein the third purification stage is used as a control stage for specifically removing any impurity that may not have been adequately removed in the previous stages.
11. Method for the purification of zinc electrolytes which comprises:
(a) in a first stage, treating under agitation the impure electrolyte with zinc dust and an antimony compound at a temperature in the range of about 50°C to 80°C and while keeping the pH between about 4 and 5, until essentially all copper and cadmium and a substantial portion of cobalt are cemented as solids, and separating said solids from such partially purified electrolyte;
(b) in a second stage, treating under agitation said partially purified electrolyte with zinc dust and an antimony compound at a temperature between about 85 and 100°C
and while keeping the pH between about 4 and 5, until the remaining impurities are cemented as solids and separating said solids from the purified electrolyte;
(c) recycling the solids from the second stage to the first stage to be used therein for the treatment of a fresh impure electrolyte;
(d) in a third stage, treating the purified electro-lyte with zinc dust at a temperature of between about 70 and 90°C and a pH of between about 3 and 5 to form a cement containing at least 95° zinc and separating said cement from the pure electrolyte; and (e) recycling the cement from-the third stage to the first or second stage.
(a) in a first stage, treating under agitation the impure electrolyte with zinc dust and an antimony compound at a temperature in the range of about 50°C to 80°C and while keeping the pH between about 4 and 5, until essentially all copper and cadmium and a substantial portion of cobalt are cemented as solids, and separating said solids from such partially purified electrolyte;
(b) in a second stage, treating under agitation said partially purified electrolyte with zinc dust and an antimony compound at a temperature between about 85 and 100°C
and while keeping the pH between about 4 and 5, until the remaining impurities are cemented as solids and separating said solids from the purified electrolyte;
(c) recycling the solids from the second stage to the first stage to be used therein for the treatment of a fresh impure electrolyte;
(d) in a third stage, treating the purified electro-lyte with zinc dust at a temperature of between about 70 and 90°C and a pH of between about 3 and 5 to form a cement containing at least 95° zinc and separating said cement from the pure electrolyte; and (e) recycling the cement from-the third stage to the first or second stage.
12. Method according to claims 1 or 11, comprising carrying out said purification in a continuous operation.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA218,818A CA1046288A (en) | 1975-01-28 | 1975-01-28 | Purification of zinc sulphate solutions |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA218,818A CA1046288A (en) | 1975-01-28 | 1975-01-28 | Purification of zinc sulphate solutions |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1046288A true CA1046288A (en) | 1979-01-16 |
Family
ID=4102155
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA218,818A Expired CA1046288A (en) | 1975-01-28 | 1975-01-28 | Purification of zinc sulphate solutions |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1046288A (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0025466A1 (en) * | 1979-09-13 | 1981-03-25 | Kidd Creek Mines Limited | A process for the recovery of arsenic as zinc arsenate from cement copper cake and a process for the purification of an impure zinc electrolyte using zinc arsenate so obtained |
| US5200042A (en) * | 1990-06-08 | 1993-04-06 | Noranda Inc. | Process to recover pure copper during purification of zinc sulfate solution |
| EP0641741A1 (en) * | 1986-08-18 | 1995-03-08 | Noranda Inc. | Process for the removal of cobalt impurities from zinc sulphate solutions |
-
1975
- 1975-01-28 CA CA218,818A patent/CA1046288A/en not_active Expired
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0025466A1 (en) * | 1979-09-13 | 1981-03-25 | Kidd Creek Mines Limited | A process for the recovery of arsenic as zinc arsenate from cement copper cake and a process for the purification of an impure zinc electrolyte using zinc arsenate so obtained |
| EP0641741A1 (en) * | 1986-08-18 | 1995-03-08 | Noranda Inc. | Process for the removal of cobalt impurities from zinc sulphate solutions |
| US5200042A (en) * | 1990-06-08 | 1993-04-06 | Noranda Inc. | Process to recover pure copper during purification of zinc sulfate solution |
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