CA1133228A

CA1133228A - Purification of zinc sulphate solution

Info

Publication number: CA1133228A
Application number: CA363,167A
Authority: CA
Inventors: Robert C. Kerby
Original assignee: Teck Metals Ltd
Current assignee: Teck Metals Ltd
Priority date: 1980-10-24
Filing date: 1980-10-24
Publication date: 1982-10-12

Abstract

ABSTRACT OF THE DISCLOSURE

A process is disclosed for removal of cobalt, nickel, cadmium and copper impurities from zinc sulphate solution by cementation with zinc dust alloy consisting of, by weight, 0.001 to 0.06%, preferably 0.001 to 0.03% aluminum, 0.05 to about 2.0%, preferably 0.05 to 1.0% lead, and the balance zinc. The alloy can additionally contain 0.02 to 0.1% copper.

Description

11~3~

The present invention relates to a process for the removal of impurities from zinc sulphate solutions intended for use in the electrolytic recovery of zinc and, in particular, relates to the preparation and use of zinc dust for the purification of said zinc sulphate solutions.
It is customary in the purification of zinc sulphate solution to use atomized zinc, which has a low level of impurities, to cement impurities for removal from the solution.
Desirable promoters of cementation are alloyed with high grade zinc prior to atomization or are added to the process as soluble salts. Aluminum normally is considered an undesirable impurity in zinc dust used for cementation of the metals cadmium, copper, cobalt and nickel and, accordingly, the use of zinc dust containing aluminum usually is avoided.

The present invention permits economic use of a substantial proportion of scrap zinc-aluminum alloy, such as obtained from preparation of metal for galvanizing and die casting processes, in the preparation of the atomized zinc.
Although there is a limit to the amount of aluminum that can be present in zinc for effective cementation, substantial savings can be made by blending inexpensive aluminum-containing scrap with high grade zinc in the preparation of the zinc dust.
Aluminum, which may he present in zinc ores or concentrates, is removed from zinc sulphate solution in an iron precipitation step which precedes-the zinc dust cementation step for removal of such impurities as cadmium, copper, cobalt and nickel. Aluminum, therefore, is an impurity which has not been considered in studies of solution purification after iron removal. Also, in that the zinc dust used in cementation normally is obtained by atomizing aluminum-free zinc that is 11;~;~228 produced by the process in which it is to be used, aluminum impurity would not be introduced into or accumulate in the zinc sulphate purification system.
~ owever, relatively inexpensive zinc containing alumi-num may be available and unsatisfactory cementation of impuri-ties with this zinc dust, as will hereinafter be explained, can be overcome by appropriate additions of lead or lead and copper to the molten zinc before it is atomized.
Lead has long been known as a beneficial additive in the zinc dust purification of zinc sulphate solutions. United States Patent 1,920,442 discloses a method for removal of cadmium, cobalt, and germanium by zinc dust cementation from zinc sulphate solution to which a tellurium salt has been added. More rapid precipitation is obtained with a zinc alloy containing 1% tin, copper or lead.
United States Patent 2,503,479 discloses the purifi-cation of zinc electrolyte solutions wherein 50 to 200 mg/L lead in the form of a so~uble compound are used with zinc dust.
Copper and antimony salts may be added. Use of small amounts of lead increases the precipitation of cohalt and antimony impurities and slows their re-solution.
United States Patent 3,579,327 provides for the removal of cobalt from zinc sulphate solution by the addition of zinc dust that is alloyed with 0.002 - 5~ antimony and 0.05 - 10% lead, best results being obtained with zinc dust containing 3% lead. - -United States Patent 3,826,648 relates to zinc dust cementation of impurities, particularly cobalt, by addition of antimony and lead to zinc sulphate solution. This patent teaches that the lead may be added as a lead salt after 32~8 separation of copper and cadmium impurities, Example 3, or zinc powder containing some lead may be supplemented by addition of lead sulphate, Example 5.
In the method of British Patent 1,546,640, zinc sulphate solution is purified by adding zinc to remove copper and cadmium and then precipitating cobalt with 1 to 4 g/L
zinc containing 0.5 to 2.5 weight percent lead, an antimony -compound and a soluble copper compound, at a temperature in the range between 65C and the boiling point of the solution.
It is a principal object of the present invention to provide a process for the removal of impurities from zinc sulphate solutions by cementation of the impurities by the use of a zinc dust which contains aluminum.
In general, my invention relates to a process for removal of impurities from zinc sulphate solution by cementation with zinc dust/ said process comprising adding to said solution zinc dust alloy containing, by weight, 0.001 to 0.006% aluminum, preferably 0.001 to 0.003% aluminum, 0.05 to about 2.0% lead, preferably 0.05 to about 1.0% lead, and the balance zinc. The presence of up to 0.1% copper has been found beneficial in the cementation process.
The process of the invention will now be described in detail, with reference to the accompanying drawing, in which:
Figure 1 is a 150x photomicrograph of ternary Zn-Pb-Al alloy dust from the cobalt removal stage;
Figure 2 is a 500x photomicrograph of the dust illustrated in Figure l;
Figure 3 is a 150x photomicrograph of Special High Grade zinc from the cobalt removal step; and Figure ~ is a graphical illustration of cadmium concentration in solution relative to time.
Product zinc made by electrodeposition is melted and cast into ingots, either as zinc metal or as an alloy containing such additives as lead, aluminum or copper. A portion of the product zinc, scrap alloy and alloy drosses which contain additives which are not detrimental to zinc dust purification of zinc sulphate solution, or to the electrolysis of treated solution, may be atomized from a melt or condensed from zinc vapour and used for cementation of impurities in the solution.
The zinc sulphate solution to be purified is generally treated in two main stages, a first purification stage essen-tially for cadmium removal wherein attention is directed primarily to cementation of substantially more abundant and generally more readily removed impurities such as cadmium and copper, and a second purification stage essentially for cobalt removal wherein attention is directed primarily to cementation of less abundant but more difficult to remove impurities such as cobalt and nickel. Operating conditions in the two stages may differ. In particular, cementati`on at an elevated temperature is required for the removal of cobalt and nickel. In the following tests, effects on cementation by the use of zinc dusts prepared from different alloys were studied, first in the cobalt removal stage, second in the cadmium removal stage and finally in a countercurrent operation. According to the countercurrent operation at least a portion of the residual zinc dust moves from the cobalt removal second stage to the cadmium removal first stage and zinc sulphate solution entering the system is first treated for removal of the cadmium group of impurities and then treated for removal of the cobalt group of impurities.
This two-stage procedure, described broadly by N.A. Popov in i~3~28 Tsvetnye Metally, Vol. 2, No. 2, ~ebruary, 1961, pages 83-84, is advantageous in providing effective utilization of the zinc dust.
In a first group of tests, the effects of alloying additives on the efficiency of zinc dust cementation of substantial quantities of cobalt and nickel from zinc sulphate solution were determined. Tests were made in 2 litre batches of zinc sulphate solution containing 150 g/L Zn, 25 mg/L Co, 25 mg/L Ni, 20 mg/L Cu and 2 mg/L Sb wherein the solution was continuously stirred with 2 g/L atomized zinc alloy, or 4 grams per test, at 75C, which is a convenient temperature within the operable range of 60C to the boiling point of the solution.
The solution was maintained at pH 5.2 by sulphuric acid additions to avoid precipitation of basic zinc sulphate.
Comparisons were made with Special High Grade (SHG) zinc, a standard grade of zinc containing 99.99% Zn by weight and having defined impurity limits, e.g., 0.003% Pb. SHG zinc containing 0.002% Pb and less than 0.0005% Al was used to prepare the alloys. ~These minimum percentages of lead and aluminum were present in all the alloys tested. Zinc dust used was m;nus 200 plus 325 mesh, Tyler Screen Series. Samples of slurry, each 20 ml, were withdrawn at the time intervals shown in Table 1 and filtered immediately. The solutions were then assayed. Nitrogen atmospheres were maintained over the solutions during testing.

1133Z~8 Removal of Co and Ni from ZnSO4 Solution with Atomized Zn Alloys Atomized zinc Solution Assays (mg/L) Residue alloy (wt %)Element 0.5 h 1.0 h 1.5 h 2.0 h weight (g) SHG zinc Co 17.513 11 11.5 1.85 Ni 9.44.5 3.2 3.9 0.03 Pb Co 17 13 10 7.6 2.85 Ni 136.3 3.9 2.6 0.09 Pb Co 159.2 6.4 4.8 3.24 - Ni 105.8 4 1.5 0.47 Pb Co 148.4 5.2 4.3 3.36 Ni 116.2 3.5 2.4

2.0 Pb Co 9.65.1 3.1 2.1 2.99 Ni 62.5 1.4 1.2 0.10 Cu Co 15.510.5 7.5 6.5 2.61 Ni 104.7 2.8 2.8 0.001 Al Co 15 14 15 16.5 0.62 Ni 9.98.8 8 8 0.01 Al Co 19 15 13.5 13.5 2.57 Ni 169.7 7.7 7.7 0.03 Al Co 20 16 14 14 2.60 Ni 15.510 8.9 7.7 0.70 Al Co 2016.5 16 15 2.22 Ni 16.512 12 10 0.01 Al Co 19 14 10 7.8 3.18 O.Oq Pb Ni 14.57.7 3.3 4.4 0.01 Al Co 18.512.5 8.5 7 3.35 0.097 Pb Ni 114.7 2.4 1.7 0.02 Al Co 19 13 11 8.7 3.18 0.09 Pb Ni 157.7 4.4 3.6 0.03 Al Co 17 12 8 5.9 3.28 0.097 Pb Ni 104.7 1.7 0.01 Al Co 20 14 11.5 8 3.17 0.10 Pb, 0.06 Cu Ni 12 5.5 3.1 1.3 0.02 Al, 0.07 Pb, Co 21 14 10 8 3.26 0.02 Cu Ni 146.4 3.9 2 0.02 Al, 0.95 Pb, Co17.5 12 8.5 5.9 3.33 0.05 Cu Ni 104.7 2.7 1.3

3~:~8 Table 1 data show that improvement in removal of cobalt and nickel with increasing amounts of lead up to 2~ Pb in Zn-Pb alloy containing less than 0.0005 ~ Al is consistent with prior art. Without lead addition, increasing the Al content of the alloy, even to the level of only 0.001% (seventh test), resulted in earlier and more extensive re-solution of Co than with SHG zinc dust (first test), a finding confirmed by the excessive consumption of zinc dust occurring in the seventh test. With lead additions, the amounts of zinc dust used, as shown by residues ranging between 2.85 and 3.36 g, were substan-tially less than in tests in which other binary zinc alloys (except for SHG level impurities) were used. For example, at the 0.10% Cu level, use of Zn-Cu alloy appears to be beneficial.
However more zinc dust was required than with the Zn-Pb alloys.
At higher copper levels, not tabulated, dissolving of cemented cobalt and nickel was apparent after 1 hour, and low residue weights showed rapid consumption of the zinc dust. With the Zn-Al alloys containing 0.002~ Pb, cobalt and nickel in solution either increased or decreased very slowly with time up to 2 hours, and the consumption of zinc dust, as shown by residues ranging between 0.62 and 2.22 g, was substantially greater than in any of the tests in which Zn-Pb alloy was used.
Although ternary Zn-Pb-Al alloys did not remove as much cobalt and nickel as Zn-Pb alloys in the 2 hour period, high residue weights, 3.18 to 3.35 g, and continuing declines in the amounts of cobalt and nickel impurities in solution, were obtained. The same trends were observed when Zn-Pb-Al-Cu alloys containing 0.10 to 0.95% Pb, 0.01 to 0.02% Al and 0.02 to 0.06% Cu were used.
Microscopic examination of the residues of zinc dust 11~3;~ '~

and cemented impurities obtained in the foregoing tests ; showed a -tendency for cemented particles to be in the form of compact, spherical coatings on zinc alloy cores when zinc dust alloys containing lead were used in the cobalt removal stage.
This is illustrated at 150x magnification in Figure l wherein spongy, non-reflective coatings lO containing cobalt, nickel, copper and traces of lead and antimony and comprising particles 12 (Figure 2) formed around particles 14 of unconsumed zinc alloy, exposed portions of which are smooth and glossy. As shown more clearly in cross-section at 500x magnification in Figure 2, coatings 10 had shapes similar to those of original zinc alloy particles, remaining portions of which are white. In the absence of lead in the zinc alloy, dendritic cement residues 16 grow out of zinc particles and become detached from spongy masses 18 as the zinc is consumed.
This is shown in Figure 3 for SHG zinc comprising 99.99% Zn and containing only 0.002~ Pb. High zinc dust consumption occurs when cemented impurities dissolve rapidly from detached cement residues and use additional zinc in re-cementation.
A second group of tests was made in which zinc sulphate solution containing 150 g/L Zn, l90 mg/L Cd, 75 mg/L
Cu, 2 mg/L Sb and l mg/L Co was treated with 2 g/L zinc alloy as previously described. Use of Zn - 0.03% Al alloy resulted in marked re-solution of cadmium. Cadmium in solution decreased to less than 0.1 mg/L in the first hour, then increased to reach 30 mg/L in 2 hours. The residue weighed 0.62 g. When 1133;~

aluminum-containing zinc was alloyed with lead, this re-solution of cadmium within 2 hours did not occur. For example, use of alloys containing 0.01% Al and 0.15% Pb, 0.02% Al and 0.09% Pb, and 0.03% Al and 0.97% Pb, respectively, resulted in solutions ' all of which contained only 0.2 mg/L Cd after 2 hours stirring with the zinc dust. Residue weights, indicating unconsumed zinc dust after 2 hours, were 0.79, 0.88 and 1.02 g respectively, considerably higher than the 0.62 g residue obtained for the Zn-Al alloy. Improvements due to use of lead-containing alloys were not as marked as those obtained with comparable alloys in the removal of nickel and cobalt, and the compact, spherical deposits noted in the previous group of tests were not evident in photomicrographs.
In a third group of tests, residues like those obtained in the tests recorded in Table 1 were obtained in the same manner, i.e., by cementation of cobalt and nickel from solutions containing 25 mg/L of each impurity. These residues we~e used to treat solutions that were high in cadm~um and copper impurities. Under these conditions, binary alloys con-taining 0.05% and 2% Pb, respectively, and ternary alloy con-taining 0.01% Al and 0.09~ Pb were effective in removing cadmium and copper. Their effectiveness in the cadmium removal stage was largely due to high residue weights in the cobalt removal stage, 2.65 g, 3.69 g and 2.94 g respectively, there being sufficient unused zinc dust transferred to the cadmium removal stage to inhibit re-solution of cemented impurities. Also for lead additions in excess of 0.05 percent, a trend towards higher residue weights, i.e., less zinc dust consumption, after cadmium removal was observed, even though part of the initial ~ g zinc dust per test was used to remove the il~3'~Z~

cobalt and Nickel. With Zine - 0.03% ~1 binary alloy, marked re-solution of cobalt in the first cementation stage left only 0.62 g residue, which was not enough for effective removal of eadmium in the second cementation stage.
The foregoing tests indicated that use of zinc dust ~ COntAining elemental lead and having some impurity cemented on - the surface would be advantageous in cementation of eadmium and eopper. The tests also indieated that the presenee of lead in zinc alloy overcomes detrimental effects due to aluminum whieh may be in the zine used to prepare the zinc dust.
In the foregoing tests in which eobalt and nickel were first removed, the zine sulphate solution initially contained substantial amounts of these impurities. ~urther counter-current eementation tests were made in whieh the solution treated initially eontained substantially less eobalt and niekel impurities to determine if the eompaet eementation of smaller amounts of eobalt and niekel impurities using zine dust alloyed with lead would be benefieial to the eementation of eadmium and copper. Solution A containing 149 g/L Zn, 0.4 mg/L Cu, 1.6 mg/L Cd, 1.2 mg/L Co, 0.9 mg/L Ni and 0.02 mg/L Sb was treated with 2 g/L zine or zine alloy. Then solution B containing 147 g/L Zn, 200 mg/L Cd, 36 mg/L Cu, 1.6 mg/L Co, 1.8 mg/L Ni and 0.75 mg/L Sb was treated with residue reeovered from the treatment of solution A. These compositions approximate those encountered in a plant operation.
In eaeh step, 2 litres of solution were stirred with the zine dust at 75C under a nitrogen atmosphere for 2 hours, and samples of solution obtained by immediate filtration after eaeh time interval and residues remaining after treatment of solution B were analyzed. Typical analyses are shown in Table 2.

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.' In considering the cobalt and nickel assays shown in -~ the first cementation stage (second purification stage in countercurrent cementation), it may be noted that in Tests 1 to 16 the composition of solution A represents typical purified solution entering the second purification stage. The quantities of copper and antimony present were not sufficient for them to act as promoters that are generally required to get very low cobalt and nickel levels in purified solution. Without such promoters, all the ternary Zn-Pb-Al alloys containing 0.09% Al or more, Tests 13 to 16, failed to remove cobalt from solution A. Ability of ternary alloys containing added lead and up to 0.03% Al to remove cobalt and nickel may be noted in Tests 9 and 11 and in Table 1. Therefore, as far as first stage cementation is concerned, a preferred upper limit of aluminum in zinc used for cementation is between 0.03% Al and 0.09% Al. In view of some first stage cementation activity in Test 12, a preferred upper limit of about 0.06% Al is indicated. Since cobalt and nickel removals increase progressively as the lead in zinc dust increases to 2% Pb, an alloy containing up to 0.06% Al and up to 2% Pb is preferred in this stage. Addition of copper to the ; alloy, up to 0.06% Cu, Tests 17 and 18, was beneficial in the - cobalt removal stage.
Differences in effectiveness of the alloys in the i cadmium removal stage (second cementation stage) are not as clearly apparent. In the absence of aluminum impurity, Tests 1 to 5, the amount of cadmium removed from solution in the first 0.25 hour of stirring increases with increasing lead in the alloy to peak between 0.04% Pb and 0.3% Pb, and then falls off with further increases of lead in the alloy. This observation led to the derivation of "Initial Rate" of cementation which is shown in Ta~le 2 for each test. These rates were determined 11;~3'~
empirlcally by plotting m~/L cadmium in solution on a logarithmic scale vs. time in hours on a linear scale, as shown in Figure 4, and determining in each case the absolute value of the slope of an initial linear portion of the graph. This linear portion of the graph represents a first order reaction wherein cementation of the cadmium on the zinc dust is not inhibited by depletion of either the cadmium or the zinc dust.
In Figure 4, point 20, (0, log 200), represents the concentration of cadmium in solution at the beginning of Test 1 in which SHG
zinc was used. Linear portion 22 of line 24, which represents data for Test 1, was conveniently extended to intersect the x-axis (0.1% Cd) at point 26, (1.02, log 0.1). Therefore (Test 1) (log 200 - log 0.1)/(0-1 02) = (2.30 + 1)/(-1.02) = 3.2 h In the same manner, for example, the data for Test 8 were plotted on line 2~ and linear portion 30 was extended to inter-sect the x-axis at point 32, (0.60, log 0.1), to provide an initial rate of 5.5 h 1 Except for the SHG zinc used in Test 1, the initial cementation rates for the Zn-Pb alloys containing less than 0.0005% Al impurity all exceeded 4.0 h 1 Plotting of these rates on a linear scale vs. %Pb in alloy on a logarithmic scale provides initial cementation rates that are substantially greater than 4.0 h 1 when the alloy contains between about 0.02% and about 1.0% Pb. In cementation with the Zn-Pb-Al alloys, lower initial rates were obtained than with alloys having comparable lead levels. However, initial rates in excess of 4.0 h I were obtained in Tests 8 and 10. At low aluminum impurity levels, particularly at 0.01% Al, there is an indication that the 11;~3Z28 initial cadmium cementation rates follow the same trend, peaking within the 0.02 to 1.0% Pb range which is preferred when alumi-num impurity is less than 0.0005% Al. To ensure presence of sufficient lead in the ternary alloy to overcome the detri-mental effects of aluminum on cementation, a range of O.Q5 to 1.0% Pb in zinc dust eontaining aluminum impurity is preferred.
Choice of a preferred maximum of 0.03% aluminum in zinc dust used for cadmium cementation will ensure presence of sufficient lead in the alloy to obtain a rapid initial cementation rate.
This preferred maximum Is within the 0.06% Al preferred limit proposed for zinc dust used in the cobalt removal stage. It is therefore preferred to limit to about 0.03 percent the amount of aluminum ln zine that is atomized for cementation in both the cobalt removal and cadmium removal stages.
Inclusion of small amounts of copper, e.g., up to 0.1% Cu, in Zn-Pb-Al-Cu alloys in Tests 17 and 18 were beneficial in the cadmium removal stage. In each ease, the initial eadmium eementation rate was greater than 4.0 h 1. As shown in Table 1, use of about 0.02 to about 0.1% eopper in the alloy is benefieial r 20 in the cobalt removal stage.
As previously stated, a zinc producing plant may have aluminum-eontaining zine serap from the preparation of alloys sueh as Speeial Zine No. 50 for galvanizing which contains 0.67 to 0.83% Al and 0.04 to 0.08% Pb. This alloy scrap may be diluted with substantially aluminum-free zine such as SHG zine to provide zine dust for eementation whieh eontains,for example, - 0.03% aluminum. If 10 kg zine dust are needed for solution purifieation in the manufaeture of 1000 kg zinc, the zinc dust may contain up to 0.40 kg Special Zinc No. 50 containing 0.75 Al, i.e. about 4 percent by weight of the zinc dust required ~133Z28 may be provided as the aluminum-containing alloy, without detriment to the cementation process. In this case, lead in the alloy would be diluted to a less than effective amount, and an addition to the cementation alloy to obtain a preferred lead content of 0.05% to 1.0~ Pb should be made.
It will be understood of course that modifications can be maae in the embodiment of the invention illustrated and described herein without departing from the scope and purview of the invention as defined by the appended claims.

Claims

The embodiments of the invention in which exclusive property or privilege is claimed are defined as follows:

1. A process for removal of impurities from zinc sulphate solution by cementation with zinc dust, said process comprising adding to said solution zinc dust alloy consisting essentially of, by weight, 0.001 to 0.06% aluminum, 0.05 to about 2.0% lead, and the balance zinc.

2. A process as claimed in Claim 1, in which said zinc dust alloy consists essentially of, by weight, 0.001 to 0.03%
aluminum and 0.05 to about 1.0% lead, the balance zinc.

3. A process for removal of cobalt, nickel, cadmium and copper impurities from zinc sulphate solution by cementation with zinc dust in a two-stage countercurrent purification comprising, adding zinc dust alloy consisting essentially of, by weight, 0.001 to 0.06% aluminum, 0.05 to about 2.0% lead, and the balance zinc to the second purification stage for the precipitation of cobalt and nickel as cement with the zinc dust alloy, separating said cement from the solution, adding at least a portion of said separated cement to the first purifica-tion stage for the precipitation of cadmium and copper as cement and separating said cement from the solution.

4. A process as claimed in Claim 3, in which said zinc dust alloy consists essentially of, by weight, 0.001 to 0.03%
aluminum and 0.05 to about 1.0% lead, the balance zinc.

5. A process as claimed in Claim 2, 3 or 4, wherein said zinc dust alloy additionally contains about 0.02 to about 0.1%
copper.

6. A process as claimed in Claim 1 or 3, wherein said zinc dust alloy is prepared by adding lead and substantially aluminum-free zinc to aluminum-containing zinc alloy.