CA1075912A

CA1075912A - Process for the purification of an aqueous solution of zinc sulphate

Info

Publication number: CA1075912A
Application number: CA252,309A
Authority: CA
Inventors: Jan W. Meulemans
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 1975-06-04
Filing date: 1976-05-11
Publication date: 1980-04-22
Also published as: NL182494C; DE2624762A1; GB1546640A; FI761553A; AU498044B2; NL7506598A; AU1455776A; DE2624762C2; IT1060887B; FI69872B; NO147153B; ES448460A1; AT355323B; NO761873L; FR2313453A1; NL182494B; BE842179A; FI69872C; JPS51148696A; NO147153C

Abstract

ABSTRACT OF THE DISCLOSURE

The invention relates to a process for the purification of a zinc sulphate solution in which, after the separation of the iron, Cu and Cd are substantially completely precipitated with zinc and separated and subsequently, using a limited quantity of zinc, traces of antimony compound and a copper compound in such a quantity that the copper/cobalt weight ratio is at least 0.2, but with a maximum corresponding with 200 mg of Cu per liter of solution, the Co still present in the solution is precipitated and separated; to zinc sulphate solution obtained in this manner and to zinc prepared therefrom.

Description

~07S91'~

-2-I`he process relates to the purification of aqueous solutions of zinc sulphate intended for t~le production of zinc by electrolysis. Such solutions usually contain 100-180 grams of zinc per litre.
Such solutions are generally p~epared by leaching roasted zinc ore with sulphuric acid. Roasted zinc ore consists principally of zinc oxide (readily soluble in sulphuric acid) and zinc ferrites (not readily soluble in sulphuric acid), and further contains minor quantities of other metal compounds soluble in sulphuric acid, as well as insoluble component~ (such as PbS04, AgCl, SiO2, CaS04).
During leaching, the latter components do not enter into the solution and consequently play no further role.
Various methods are serviceable for the leaching. In general, it is endeavoured to cause the maximum amount of zinc from the roasted ore to dissolve, use being made of, inter alia, hot sulphuric acid in order to cause the not readily soluble zinc ferrites to dissolve. The result of this, however, is that iron also enters into solution.
For the preparation of zinc by electrolysis of zinc - sulphate solutions, zinc sulphate solutions of comparatively high purity are required. It is primarily of importance to remove the dissolved iron from the solution since iron disturbs the electrolysis.
Various methods ar~ known for the removal of the iron.
It is possible, for example, largely to precipitate the iron as jarosite or goethite and subsequently precipitate the final residue of iron as hydroxide. The precipitationscan be removed .

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107S9lZ

by filtration. During the removal of the iron, the elements Pb, Ag, As and Sb also disappear at least partly from the solution.
After the removal of the iron the solution generally still contains impurities which can be classified in two groups. T~e first group comprises the elements Cu, Cd, Ni, Pb, Co and Tl, which adversely affect the current efficiency during electrolysis. The second group comprises elements such as Na, Ca, Mg, Mn, C1 and F, which are not detrimental provided that their concentration in the solution is not too high. Usually, however, these elements are not present in harmful concentrations, The solution must, therefore, be further purified in order to remove the elements Cu, Cd, Ni, Pb, Co and Tl as far as possible. The invention relates to this further purification.
; Since the elements in question are more electro-positive than zinc, it should be possible to precipitate those elements from the solution by adding zinc powder to the solution. It has not been found possible, however, to precipitate Co to a sufficient degree in this manner.
According to the Netherlands patent application 7208722, the precipitation of Co with zinc powder has, however, been found possible if the solut~n contains Cu together with As, Sb or Sn. The most customary combinations are Cu ~ Sb and Cu + As. The removal of Co then requires considerable quantities of copper, namely more than 200 mg . , ' ~ .

~07591Z

of Cu per litre of solution if tne Cu + Sb combincition is applied, and more than 500 mg/l if the Cu + As com-bination is applied.
In the Netherlands patent application 7208722, a process is consequently described as prior art, in which process zinc powder is added to the solution in a first step in order to precipitate Cu, while care is - taken that the copper does no precipitate completely.
200 mg/1 or more is held in solution. The consequence is that elements which are more electro-negative than copper, such as cadmium, also remain in solutDn. In a second step, an excess of zinc powder and Sb is then added and Co is precipitated at comparatively high temperature (70-100C).
; At the same time the other elements also disappear.
As a possible variant of this known process, the Nether-lands patent application 7208722 mentions the possiblity of completely precipitating Cu and Cd in the first step. Before ` proceeding to the second step (the precipitation of Co), it is then necessary first to add copper in soluble form (e.g., as copper sulphate) again, According to the Nether-lands patent application 7208722, however, this is an expensive and complicated method, which has never been applied in practice Now the invention described in the Netherlands patent application 7208722 is based on the theory that under certain conditions it is possible to precipitate Co without Cu being present. The invention consists in completely prebipitating 1075~12 Cu and Cd from the zinc sulphate solution in a first step by the addition of zinc powder. In a second step an antimony compound and zinc powder, in quantities required for the removal of cobalt and other impurities, are added to the solution, at a temperature between 80C and the boiling point of the solution.
This process, however, has a number of drawbacks, such as the high temperature required in the second step and the large consumption of zinc powder.
The present invention is now based on the theory that the process which has been rejected in the Netherlands patent application 7203722 as being expensive and com-plicated, can in fact be very advantageous provided that it is carried out in the correct manner.
The invention relates to a process for the puriflcation of an aqueous solution of zinc sulphate, obtained by leaching roasted zinc ore with sulphuric acid and separating the iron from the resultant solution, which process comprises in a first step substantially completely precipitating-Cu and Cd from the solution by the addition of zinc and separating the precipitate and in a second step, at a temper~
ature of from 65C to the boiling point of the solution, precipitating Co from the solution by the addition of zinc in a quantity of at least 1 g per litre of solution, an antimony compound in a quantity corresponding with 0.4-10 mg of Sb per litre of solution and a soluble copper compound in such a quantity that the copper/cobalt weight ratio is at least 0,2, but with a maximum corresponding with 200 mg of Cu per litre of solution, and seperating the pre-cipitate.

This process has the following advantages:
1) The quantity of zinc required is lower than in the known process. This is very important, because it is an expensive matter to have to pulverize a proportion of the zinc which has been produced by the electrolysis of a certain quantity of zinc sulphate solution and to use it for the purification of a subsequent quantity of solution.
2) The second step of the process can also be carried out at temperatures below 80C.

3) The zinc sulphate solution can be purified in a short period.

4) A very low antimony concentration in the zinc sulphate solution can be achieved, sometimes even a conce~tration as low as 0.002 mg/l.
If Cu and Cd are substantially completely precipitated in the first step of the process, Ni, Pb and Tl will largely accompany them. The precipitate is separated from the solution~
for example by filtration. Approximately twice the equivalent quantity of zinc is sufficient for the precipitation, which usually amounts to a consumption of 1.5-2 g of zinc per litre of solution. A suitable temperature is approximately 65C. Higher t~?mperatures, such as 80-goc, are also possible;
a proportion of the cobalt then also precipitates.

107S9~2 After the precipitation of the iron, a zinc sulphate solution can, for example, contain per litre:
Co 35 mg Cd 330 mg Cu 4 lo mg Sb 0.03 mg Zn 153 grams As 0.09 mg Fe 1 mg Mn 4.5 grams Ni 12 mg Pb 35 mg After the first treatment with zinc, the quantity of - Co has then, for example, fallen to 31 mg, that of Cd to 3 mg, that of Cu to 1 mg, and that of Sb to < 0.01 mg, all per litre of solution.
This solution is now subjected to the second step, in which Co and the residues of Cd, Cu and Sb are removed.
The non-detrimental Mn remains in the solution. After the 20- second step, the solution is suitable to be subjected to electrolysis, in which a prcportion of the zinc sulphate is converted into metallic zinc and sulphuric acid. This sulphuric acid can be re-used as recycle sulphuric acid for the leaching of roasted zinc ore.
A zinc sulphate solution can be regarded as suitable to be subjected to electrolysis if the solution does not contain more than approximately 0.2-0.3 mg of Co and not ':

10759~2 more than 0.01 mg of Sb per litre and moreover if the solution does not contain more ;han 0,1 mg of Cd and 0 1 mg of Cu per litre.
In order to achieve this degree of purity, the fol-lowing points must be observed in the second step.
1) The quantity of Co present in the solution will depend on the nature of the zinc ore from which the solution is prepared, but is frequently between 10 and 80 and usually between 10 and 70 mg of Co per litre of solution, No relationship has been found between the quantity of cobalt present in the solution and the quantity of zinc required in the second step. It has been established, how-ever, that zinc must be added in a quantity of at least 1 g (and preferably more than 1 g) per litre of solution. As has already been stated, it is a major advantage of the present process that only a comparatively small quantity of zinc is required.
Usually not more than 4 g of zinc per litre of solution will therefore be added. Preferably, the added quantity is between 1.3 and 2.5 g per litre of solution.
Zinc is usually added in the form of a powder. The particles in the powder generally have a particle size of less than 500 ~, and preferably of less than 75 ~. It is advisable to wet the powder with water before adding it to the zinc sulphate solution. The 1(~'7591Z
, g powder may be added to the solution for example in the form of an aqueous slurry.
Preferably, the zinc contains a small quantity of lead, for example 0.5-2.5% by weight.
:5 2) The addition of an antimony compound, such as Sb203 or antimony tartrate, is necessary. Presumably, the antimony reduces the hydrogen overpotential on zinc and thereby activates the zinc. The antimony compound ~ is added in a quantity corresponding with 0.4-10 mg 1() of Sb per litre of zinc sulphate solution. Good results are generally achieved by adding the antimony compound in a quantity correspondin~ with 0.5-2 mg of Sb per litre. -Since the presence of dissolved antimony in the sulphate solution is undesirable, it is intended that the adde~
antimony should precipitate with the cobalt and be removed, For that reason the added quantity of antimony compound must be kept within narrow limits. Otherwise, the zinc sulphate ~ ution might contain too much dis- -solved antimony.
3) By also adding a soluble copper compound (such as CuS04) in the second step, both the added quantity of zinc and the temperature can be kept comparatively low. It is assumed that copper and cobalt form intermetallic com-pounds, which are nobler than copper and are therefore more readily precipitated by zinc. The second step can be carried out at a temperature as low as 65C. This means that it is not necessary for the solution to be drastic~lly heated after the first step in order ta be able to carry out the second step. If desired, it is possible, however, to operate in the second step at temperatures above 65C up to the boiling point of the solution.
~) The quantity of copper compound which must be added depends in the first place on the quantity of cobalt present in the solution and in the second place on the temperature.
In general, it may be stated that an increase in the quantity of copper and a rise in temperature both con-tribute to a faster and better precipitation of th~ cobalt.
From this it follows that less copper is sufficient at a higher temperature than at a lower temperature. The minimum quantity of copper compound which must be added is such that the copper/cobalt weight ratio is at least 0.2. This small quantity of copper, however, is only sufficient at a comparatively high temperature (e.g., around 85C). At lower temperatures it is necessary to operate with a higher copper/cobalt weight ratio.
In order to ensure good precipitation of the cobalt, in general somewhat more copper compound will be added than the minimum required for the cobalt just to be precipitated to the desired degree at the selected temperature.
Generally, the process will be operated at copper/cobalt weight ratios between 0.5 and 1Ø

lG7591Z

5) Although it has been stated above that an increase in the amount of copper contributes to faster and better precipitation of the cobalt, the quantity of copper can nevertheless not be raised indefinitely.
If the Cu concentration is more than 200 mg/l, the copper is able to precipitate completely or partially at a fast rate, for example within half an hour, but it then tends to dissolve again quickly afterwards.
The system of solution + precipitate is then said to be unstable. Working with ~ch an unstable system has major drawbacks in practice. It would require watching exactly at what moment the cobalt has precipitated and then immediately filtering the solution, in the hope that during the time required for filtration no consider-- 15 able quantities of cobalt enter into solution again.
In practice, it is therefore desired to operate with a stable system, i.e., a system in which the cobalt once precipitated does not t~nd to enter into solution again.
It is then much easier to work with a standardized pre-cipitation time and there is no hurry with filtration.
For that reason a Cu-concentration of more than 200 mg/l is not suitable.
This limit also is dependent on the temperature. The said maximum of 200 mg of Cu/l applies at comparatively low temperatures (approximately 65-70C). At higher temper- -atures, 200 mg of Cu/l will be too much and the maximum allowable quantity of copper is somewhat lower.

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107~ 2 . , .
With the process of the invention, the entire purification with z;inc can be carried out in two steps, that is to say that after two precipitations and removal of the Fecipitates, the zinc sulphate solution is suitable to be subjected to electrolysis.
It is known that zinc fabrication in the art frequently re-quires 3 and sometimes even 4 purification steps.
In the process of the invention it is not necessary to add the entire required quantity of zinc or other ingredients to the solution at once in each step. It is also possible to add the zinc or the other ingredients in partions or continuously during the entire required precipitation time or a part thereof.
EXAMPLES
The following tests, the results of which are shown in Tables A-C, demonstrate the results and the advantages of the process according to the invention.
All the zinc sulphate solutions used as starting material in the tests had been obtained by leaching roasted zinc ore with sulphuric acid, precipitating the iron from the resultant solution, and then substantially completely precipitating Cu and Cd by the addition of zinc powder (1.5 g per litre of solution). Thus, the solutions had already undergone the first purification step with zinc, and the tests consequently de-monstrate only the second purification step with zinc, in which the main objective is to remove the cobalt still present.
The zinc sulphate solutions were brought to the desired temperature in a beake~ and subsequently an aqueous solution of antimony tartrate, an aqueous soluti~n of copper sulphate l~S9lZ
-13- :

and an aqueous slurry of zinc powder (0.9% by weight plu~ferous) were added thereto, The temperature, quantities of the ingredients adced and duration of the tests are stated in the Tables. During the tests the mixtures were continuously stirred in order to prevent the zinc particles from settling. Care was taken to allow the minimum amount of air to be admitted to the test mixtures. At set times (as indicated in the Tables) samples were taken and analyzed for dissolved cobalt and antimony.

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10759~2 The tests of which the results are represented in Table A show the effect ~f the application of copper. The tests 1-7 demonstrate the process of the invention. The tests lA-7A demonstrate the!known process which is operated with Zn and Sb but without Cu. It will be seen that in the tests 1-7 a considerably smaller quantity of zinc was in-variably required and the solutions were purified faster, although the temperature was invariably 10C lower than in the tests lA-7A.
The tests shown in Table B demonstrate the effect of temperature in relationship to the amount of copper.
The first series of tests comprises the tests 27a, 27b, 27c, 29, 28a, 28b and 28c. In the 1ests 27a, 27b and 27c, the Cu/Co ratio falls from 2.0 via o . 8 to 0,4 and it can be seen that a Cu/Co ratio of 0.4 in test 27c at 70C is not sufficient to achieve a good result. However, upon increasing the temperature from 70C to 80C, namely in test 29, the ratio of 0.4 is sufficient. The tests 28a, 28b and 28c show that a temperature of 60C is too low; compare these tests with tests 27a, 27b and 27c, The next series of tests comprises tests 24a, 24b and 24c. At a temperature of 80C, Cu/Co ratios of between 2.0 and 0.5 are of course good.
The next series of tests comprises tests 62, 63 and 64.
These show in principle the same as the tests 27a, 27b and 27c, At 75C, in test 62 a Cu/Co ratio of 0.7 is good, in test 63 a ratio of 0,5 is a border line case, and in test 64 a ratio of 0.3 is too low, ~0'759i2 Hitherto the Table included no tests in which the quantity of copper came in the neighbourhood of 200 mg/1, In the next series of tests (87, 88, 93, 94, 108, 99, 89, 91, 95, 97, 107, go and 92) this quantity is reached or exceeded in some cases, Test 87 was doomed to failure because the temperature of 60C is too low and the quantity of copper of 310 mg/l is too high. The tests 88 and 93 show that at 65C a Cu/Co ratio of 1.0 in test 88 is sufficient and a ratio of 0.7 in the test 93 is not sufficient. From the tests 94 and 108 it appears that at 70C a quantity of copper of 310 mg/l in test 94 is too high and a quantity of 200 mg/l in test 108 is allowable. Passing to the tests 99, 89 and 91, which were carried out at 75c, we see that a Cu/Co ratio of 0.3 in test 91 is a border-line case, in this case because of the rather high final concentration of Sb, although the final concentration of Co is good. In the tests 95, 97 and 107, which were carried out at 80C, not only 250 mg/l but also 200 mg/l of copper is too much, 170 mg/l, however, is acceptable.
Test 97 contains a clear demonstration of an unstable system (< 0,1 mg/l of Co after 2 hours, but 15.5 mg/l of Co after 2~ hours. Finally, the tests 90 and 92 show that at 85C the Cu/Co ratio can drop as low as 0.2 before arriving at the border-line.
The tests 48 and 50 show once again what difference a temperature of 75c or 80c makes at a Cu/Co ratio of 0 3 The next series of tests (116, 111, 115 and 112) comes up to expectations. The only thing that may arouse surprise - ~Q7~91Z

is that in test 111 a Cu/Co ratio of 1.0 is not sufficient at 65C, whereas the same ratio at the same temperature in test 88 was sufficient. However, it must be considered that in test 111 there was more cobalt to be removed and that after 3 hours' reaction time the cobalt content of the solution was still falling. Perhaps test 111 would have yielded an ac-ceptable result if the test had been continued longer.
The tests 105 and 106 show once again that 230 mg/l of Cu is too much.
lOThe tests 38, 52b, 55 and 117 were not intended to be comparative tests among each other. After the foregoing, the results of these tests are easily explainable.
The tests shown in Table C are intended to show how much Zn and Sb is required. In these tests the temperature was 15held constant at 75C and the Cu/Co ratio at 0.75. Considering the tests shown in Table B, this ratio must be ample.
As regards the quantity of zinc, it is seen from Table C
that good results can be achieved with 1.5 g or more of zinc per litre of solution. Test 100, in which only 1 g of zinc per litre was added, did not give a good result. At least 1 g of zinc per litre must therefore be added, and prefer-ably slightly more ~an 1 g.
As regards the quantity of antimony, we need only discuss the tests in which this quantity was very low (0.5 mg/l) or very high (10 mg/l).
In the tests with a low quantity (tests 101, 47b, 114, 104 and 46b) the result was sometimes good, sometimes moderate - ~07S9~2 and intwo cases bad. The conclusion is that here we begin to approach the lower limit for the quantity of antimony~
so that the level should not fall below 0.4 mg/l.
In the tests with a high quantity (tests 44, 98 and 110) the result was bad. In the tests the final concentration of Sb was too high in two cases and only in one case was it acceptable, so that 10 mg/l appears to be the upper limit.
The results stated in Table B are graphically represented in Figs. 1 and 2.
In Fig. 1, the temperature in C is plotted on the horizontal axis and the copper/cobalt ratio on the vertical axis. The vertical axis has a logarithmic scale. This Figure is designed to show the relationship between the temperature and the minimally required copper/cobalt ratio. For that reason there was no point in incorporating in this Figure those tests in which the Cu-concentration was very high. The result of the tests is shown in the Table as "good, "moderate"
or "bad" and in the Figure by a dot, an encircled dot and a cross, respectively A difficulty arose in representing the result of tests 88 and 111. Both tests were carried out at a temperature of 65C and a Cu/~o ratio of 1.0, so that both tests occupy the same point in the Figure. According to the Table the result of test 88 was "good" and of test 111 "bad". By way of a compromise the dot concerned is indicated as "moderate" in the Figure. In the Figure a line is drawn dividing the tests with a good result from the tests with a bad result. The path of this line shows that as the temperature .

- ~0759iZ

falls, the Cu/Co ratio required increases.
In Fig. 2, the temperature in C is plotted on the horizontal axis and the copper concentration in mg/l on the vertical axis. The vertical axis has a logarithmic scale.
This Figure is designed to show the relationship between the temperature and the maximum allowable copper concentrat~n.
For this reason onlythose tests are shown in this Figure in which the Cu-concentration was very high, i.e., the very tests which are absent in Fig. 1. In this Figure 2 another line is drawn separating the tests with a good result from the tests with a bad result. The path of this line shows that as the temperature rises, the maximum allowable copper con-centration decreases.

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Claims

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

1. A process for the purification of an aqueous solution of zinc sulphate, obtained by leaching roasted zinc ore with sulphuric acid and separating the iron from the resultant solution, which process comprises in a first step substantially completely precipitating Cu and Cd from the solution by the addition of zinc and separating the precipitate and in a second step, at a temperature of from 65°C to the boiling point of the solution, precipitating Co from the solution by the addition of zinc in a quantity of at least 1 g per litre of solution, an antimony compound in a quantity corresponding with 0.4-10 mg of Sb per litre of solution and a soluble copper compound in such a quantity that the copper/cobalt weight ratio is at least 0.2, but with a maximum corresponding with 200 mg of Cu per litre of solution, and separating the precipitate.

2. A process as claimed in claim 1, in which in the second step the zinc is applied in a quantity of more than 1 g per litre of zinc sulphate solution.

3. A process as claimed in claim 1, in which in the second step the zinc is applied in a quantity of not more than 4 g per litre of zinc sulphate solution.

4. A process as claimed in claim 2 or 3, in which in the second step the zinc is used in a quantity of from 1.3 to 2.5 g per litre of zinc sulphate solution.

5. A process as claimed in claim 1, in which the zinc used contains lead in an amount of 0.5-2.5% by weight.

6. A process as claimed in claim 1, in which in the second step the antimony compound is applied in a quantity corresponding with 0.5-2 mg of Sb per litre of zinc sulphate solution.

7. A process as claimed in claim 1, in which in the second step the soluble copper compound is applied in such a quantity that the copper/cobalt weight ratio is between 0.5 and 1Ø

8. A process as claimed in claim 1, in which the zinc sulphate solution to be purified contains 10-80 mg of cobalt per litre.