NO147153B - PROCEDURE FOR PURIFICATION OF A HIGHLY SOLUTION OF ZINC SULPHATE OBTAINED BY LINING OF RUSTED ZINC ORE WITH SULFUR ACID - Google Patents

Description

This invention relates to a method for purifying aqueous solutions of zinc sulphate suitable for the production of zinc by electrolysis. Such solutions usually contain 100-180 g of zinc per litres.

Such solutions are generally prepared by leaching

of roasted zinc ore with sulfuric acid. Roasted zinc ore consists mainly of zinc oxide (easily soluble in sulfuric acid) and zinc ferrites (not easily soluble in sulfuric acid) and also contains smaller amounts of other metal compounds that are soluble in sulfuric acid, as well as insoluble constituents (such as PbS04, AgCl, SiO,,, CaSO 4 ). During leaching, the latter components do not pass into the solution and therefore no longer play any role.

Different methods can be used for the leaching.

In general, one tries to bring as much of the zinc in the roasted ore to dissolve as possible, using e.g. hot sulfuric acid to cause the insoluble zinc ferrites to dissolve. The result of this, however, is that iron also dissolves.

In the electrolytic production of zinc from zinc sulphate solutions, these solutions must be of relatively high purity. It is particularly important to remove the dissolved iron from the solution, as iron interferes with the electrolysis.

Various methods are known to remove the iron. For example, it is possible to precipitate most of the iron as jarosite or goethite and then precipitate the rest of the iron as hydroxide.

The precipitated materials can be removed by filtration. During the removal

of the iron, the elements Pb, Ag, As and Sb will also completely or partially disappear from the solution.

After the iron is removed from the solution, it contains

in the rule pollutants that can be classified into two groups. It

the first group includes the elements Cu, Cd, Ni, Pb, Co and Tl, which have an unfavorable effect on the current yield during electrolysis. The second group includes elements such as Na, Ca, Mg, Mn, Cl and F, which are not harmful, 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 with the aim of removing the elements Cu, Cd, Ni, Pb, Co and Tl as far as possible. The invention relates to this further purification.

As the elements in question here are more electropositive than zinc, it should be possible to precipitate these elements from the solution by adding zinc powder to it. However, it has not been found possible to precipitate cobalt to a sufficient extent in this way.

However, according to Norwegian patent no. 133,327

the precipitation of cobalt with zinc powder has been found possible if the solution contains Cu together with As, Sb or Sn. The most common combinations are Cu + Sb and Cu + As. The removal of cobalt then requires significant amounts of copper, namely over 200 mg Cu per liter of solution if the combination Cu + Sb is used, and over 500 mg/l if the combination Cu + As is used.

In Norwegian patent no. 133,327 it is consequently

described a previously known method where zinc powder is added to the solution in a first step for precipitation of copper, it being ensured that the copper is not precipitated completely. 200 mg/l or more is kept in solution. The consequence is that elements that are more electronegative than copper, such as cadmium, also remain in the solution. In another step, an excess of zinc powder and Sb is added, and cobalt is precipitated at a relatively high temperature (70-100°C). At the same time, other elements will also be precipitated.

As a possible variant of this known method, Norwegian patent no. 133,327 indicates the possibility of completely

precipitation of copper and cadmium in the first stage. Before proceeding to the second step (precipitation of cobalt), one must first add copper in soluble form (e.g. copper sulphate) again. According to the aforementioned Norwegian patent, this is an expensive and complicated method, which has never been used in practice.

The invention described in Norwegian patent no.

133,327 is based on the theory that under certain conditions it is possible to precipitate cobalt without copper being present. The invention

consists in completely precipitating copper and cadmium from the zinc sulphate solution in a first step by adding zinc powder;

in another step, an antimony compound and zinc powder are added in sufficient quantities to remove cobalt and other impurities in the solution at a temperature between 80°C and the boiling point of the solution.

However, this method has several disadvantages, 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 method which in Norwegian patent no. 133,327 has been rejected as expensive and complicated, can actually be very advantageous, provided that it is carried out in the right way.

The invention relates to a method for purifying an aqueous solution of zinc sulphate obtained by leaching roasted zinc ore with sulfuric acid and separating the iron from the resulting solution, in which method copper and cadmium are in a first step mainly completely precipitated from the solution by the addition of zinc powder and separated from the solution, and cobalt in a second step is precipitated from the solution by adding zinc powder, a small amount of an antimony compound and a small amount of a soluble copper compound, after which the precipitated cobalt is separated from the solution, characterized in that the second step as in and for known is carried out at a temperature between 65 and 85°C, that the zinc powder in the second step, which is also known in and of itself, is added in an amount of 1-2.5 g per liter of solution, that the copper compound is added in an amount of between 5 and 200 mg Cu per liter of solution, so that the copper/cobalt weight ratio in the solution is between 0.5 and 1.0, and that the temperature and the copper addition are regulated so that they correspond to values on the right side of it in fig. 1 drawn curve.

This procedure has the following advantages:

1) The required amount of zinc is lower than in the known process. This is very important, as it is expensive to have to pulverize part of the zinc that has been produced by electrolysis of a certain amount of zinc sulfate solution and use

the for purification of a subsequent quantity of solution.

2) The second step of the process can also be carried out at temperatures below 80°C.

3) The zinc sulphate solution can be cleaned in a short time.

4) A very low antimony concentration in the zinc sulphate solution can be achieved, sometimes even a concentration as low as 0.002 mg/l.

If copper and cadmium are mainly completely precipitated in the first stage of the process, Ni, Pb and Tl will mostly follow them. The precipitated material is separated from the solution, e.g. by filtering. About twice the equivalent amount of zinc is sufficient

for the precipitation, which usually amounts to a consumption of 1.5-2 g of zinc per liter of solution. A suitable temperature is 65°C. Higher temperatures, such as 80-85°C, can also be used; some cobalt will then also precipitate.

After the precipitation of the iron, a zinc sulphate solution can for example contain, per liters:

After the first treatment with zinc, the amount of cobalt has fallen to, for example, 31 mg, the amount of cadmium to 3 mg, the amount of copper to 1 mg and the amount of antimony to less than 0.01 mg, all per liter of solution.

This solution is now processed in the second stage, where cobalt and residues of cadmium, copper and antimony are removed. The manganese, which does not cause any inconvenience, remains in the solution. After the second step, the solution is ready to be electrolysed, during which part of the zinc sulphate is converted into metallic zinc and sulfuric acid. This sulfuric acid can be recycled and used again for the leaching of roasted zinc ore.

A zinc sulphate solution can be considered suitable for electrolysis if the solution does not contain more than approx. 0.2-0.3 mg Co and no more than 0.01 mg Sb per liter and further if the solution does not contain more than 0.1 mg Cd and 0.1 mg Cu per litres.

To achieve this degree of purity, the following points must be observed in the second step: 1) The amount of cobalt in the solution will depend on the nature of the zinc ore used, but is often between 10 and 80 and usually between 10 and 70 mg Co per liter of solution. No relationship has been found between the amount of cobalt present in the solution and the amount of zinc required in the second step. However, it has been found that zinc must be added in an amount of at least 1 g (and preferably more than 1 g) per liter of solution. As already mentioned, it is a significant advantage of the present method that only a relatively small amount of zinc is required. The amount that is added is preferably between 1.3 and 2.5 g per liter of solution.

Zinc is added in the form of a powder. The powder particles generally have a particle size below 500 pm, preferably below 75 pm. It is advisable to moisten the powder with water before adding it to the zinc sulphate solution. The powder can be added to the solution, for example in the form of an aqueous slurry.

The zinc preferably contains a small amount of lead, e.g.

0.5-2.5% by weight.

2) The addition of an antimony compound, such as Sb2O3 or antimony tartrate, is necessary. Presumably the antimony reduces the hydrogen overvoltage on zinc and thereby activates the zinc. The antimony compound is added in an amount corresponding to 0.4-10 mg Sb per liter of zinc sulfate solution. Good results

is usually achieved by adding the antimony compound in an amount corresponding to 0.5-2 mg Sb per litres.

As the presence of dissolved antimony in the sulphate solution is undesirable, the aim is for the added antimony to be precipitated together with cobalt and removed. For this reason, the added amount of antimony compound must be kept within narrow limits. The zinc sulphate solution could otherwise contain

too much dissolved antimony.

3) By also adding a soluble copper compound (e.g. CuS04) in the second step, both the added amount of zinc and the temperature can be kept relatively low. It is believed that copper and cobalt form intermetallic compounds that are more noble than copper and are therefore more easily precipitated by zinc. The second step can be carried out at a temperature as low as 65°C. This means that it is not necessary to give the solution a strong heating after the first step in order to be able to carry out the second step. If desired, however, it is possible to carry out the second step at temperatures above 65°C up to 85°C. 4) The amount of copper compound that must be added depends primarily on the amount of cobalt present in the solution, and secondly also on the temperature. In general, it can be said that an increase in the amount of copper and a rise in temperature both contribute to a faster and better precipitation of cobalt. It follows from this that less copper is sufficient at a higher temperature than at a lower temperature. The smallest amount of copper compound that is added is such that the weight ratio between copper and cobalt is at least 0.5. However, this small amount of copper is sufficient only at relatively high temperatures (e.g. close to 85°C). At lower temperatures, a higher weight ratio between copper and cobalt must be used. To ensure good precipitation of cobalt, as a rule, somewhat more copper compound is added than the minimum amount necessary for precipitation of cobalt to a certain desired degree at the chosen temperature. The method is carried out as mentioned with a copper/cobalt weight ratio of between 0.5 and 1.0. 5) Although it was said above that an increase in the amount of copper contributes to faster and better precipitation of cobalt, the amount of copper cannot be increased indefinitely. If the Cu concentration is above 200 mg/l, cobalt can precipitate rapidly, completely or partially, e.g. within 1/2 hour, but it will then quickly dissolve again afterwards. The system solution + precipitate is then said to be unstable. Working with such an unstable system has many disadvantages in practice. One then had to observe exactly at what point cobalt was precipitated and then immediately filter the solution, in the hope that significant amounts of cobalt would not dissolve again during filtration. In practice, it is therefore desirable to work with a stable system, i.e. a system where cobalt that has been precipitated does not tend to dissolve again. It is then far easier to work with a standardized precipitation time, and there is no rush with the filtration. For this reason, a Cu concentration above 200 mg/l is not suitable.

This limit is also temperature dependent. The aforementioned maximum of 200 mg Cu/l applies at relatively low temperatures (approx. 65-70°C). At higher temperatures, 200 mg Cu per liter be far too much, and the permissible maximum amount of copper is somewhat lower.

With the method according to the invention, the entire purification with zinc can be carried out in two stages, i.e. that after two precipitations and removal of the precipitate, the zinc sulphate solution is suitable for electrolysis. It is known that when producing zinc of this kind, you often have to use 3 and sometimes even 4 purification steps.

In the method according to the invention, it is not necessary

adding the entire required amount of zinc or other ingredients to the solution at once in each step. Furthermore, it is possible to add the zinc or the other ingredients in portions or continuously during the total required precipitation time or part thereof.

Examples

The following experiments, the results of which are indicated in tables A-C, show the results and advantages obtained by the method according to the invention.

All the zinc sulphate solutions that were used as starting material in the experiments were obtained by leaching roasted zinc ore with sulfuric acid, the iron being precipitated from the resulting solution, after which copper and cadmium were mainly completely precipitated by the addition of zinc powder (1.5 g per liters of solution). The solutions were thus already subjected to the first purification step with zinc, and the experiments consequently only show the second purification step with zinc, where the main purpose is to remove cobalt still present.

The zinc sulphate solutions were heated to the desired temperature in a beaker, after which an aqueous solution of antimony tartrate, an aqueous solution of copper sulphate and an aqueous slurry of zinc powder (0.9 wt% lead) were added. The temperature, the quantities of the ingredients and the duration of the experiments will appear in the tables. During the experiments, the mixtures were stirred continuously to prevent deposition of zinc particles. Precautions were taken to prevent air access to the test mixtures. At specified times (cf. the tables), samples were taken and analyzed with regard to dissolved cobalt and antimony.

The experiments whose results are reproduced in Table A show the effect of the application of copper. Trials 1-7 represent the method according to the invention. Trials 1A-7A represent the known process where zinc and antimony are used, but without copper.

It will be seen that a considerably smaller amount of zinc was required in

trials 1-7, and the solutions were purified more quickly despite the fact that the temperature was 1G°C lower than in trials 1A-7A.

The experiments in table B show the effect of temperature in relation to the amount of copper.

The first test series includes tests 27a, 27b, 27c, 29, 28a, 28b and 28c. In experiments 27a, 27b and 27c, the Cu/Co ratio falls from 2.0 via 0.8 to 0.4, and it will be seen that a Cu/Co ratio of 0.4 in experiment 27c at 70°C is not sufficient to give a good result. After the temperature has been increased from 70°C to 80°C, namely 1 trial 29,

however, the ratio of 0.4 is sufficient. Trials 28a, 28b and 28c show that a temperature of 60°C is too low; compare these experiments with experiments 27a, 27b and 27c.

The next series of experiments includes experiments 24a, 24b and 24c. At a temperature of 80°C, a Cu/Co ratio between 2.0 and 0.5 is of course satisfactory.

The next series of experiments includes experiments 62, 63 and 64. These show in principle the same as experiments 27a, 27b and 27c. At 75°C

a Cu/Co ratio of 0.7 is satisfactory in trial 62, a ratio of 0.5 in trial 63 is a borderline case and a ratio of 0.3 in trial 64 is too low.

The table has so far not included any experiments where the amount of copper comes close to 200 mg/l. In the next series of experiments

(87, 88, 93, 94, 108, 99, 89, 91, 95, 97, 107, 90 and 92) this amount is reached or exceeded in some cases. Trial 87 necessarily had to give poor results, as the temperature of 60°C is far too low and the amount of copper of 310 mg/l is far too high. Trials 88 and 93 show that at 65°C a Cu/Co ratio of 1.0 (trial 88) is sufficient, and a ratio of 0.7 (trial 93) is insufficient. From tests 94 and 108, it will be seen that a copper quantity of 310 mg/l (test 94) at 70°C

is too high, and an amount of 200 mg/l (trial 108) is acceptable. From trials 99, 89 and 91, which were carried out at 75°C, it will be seen that a Cu/Co ratio of 0.3 (trial 91) is a borderline case, in this case due to the relatively high final concentration of antimony , although the final concentration of cobalt is satisfactory. In experiments 95, 97 and 107, which were carried out at 80°C, not only 250 mg/l but also

200 mg/l copper too much; However, 170 mg/l is acceptable. Experiment 97 contains a clear demonstration of an unstable system (•^0.1 mg/l Co after 2 h, but 15.5 mg/l after 2.5 h. Finally, experiments 90 and 92 show that the Cu/Co ratio can go as far down as 0.2 at 85°C before the limit is reached.

Tests 48 and 50 show again what a difference it makes if the temperature is 75°C or 80°C at a Cu/Co ratio of 0.3.

The next series of tests (116, 111, 115 and 112) show approximately what one would expect. The only thing that could be surprising is that in trial 111 a Cu/Co ratio of 1.0 is not sufficient at 65°C, while the same ratio at the same temperature was sufficient in trial 88. However, it must be taken into account consideration that in experiment 111 there was more cobalt to be removed, and that the cobalt content of the solution was still falling after 3 hours of reaction. Perhaps experiment 111 would have given an acceptable result if the experiment had been continued for another time.

Experiments 105 and 106 also show that 2 30 mg/l Cu is too much. Experiments 38, 52b, 55 and 117 were not intended to be comparison experiments with each other. After the above, the results in these experiments are easy to explain.

The experiments shown in Table C are intended to show how much zinc and antimony is required. In these experiments the temperature was kept constant at 75°C and the Cu/Co ratio at 0.75. Considering the experiments shown in Table B, this ratio must be more than high enough.

With regard to the amount of zinc, it will be seen from table C that good results can be achieved with 1.5 g or more zinc per liter of solution. Experiment 100j in which only 1 g of zinc was added per litres, did not give a good result. At least 1 g of zinc per liter must therefore be added and preferably somewhat more than 1 g.

With regard to the amount of antimony, we only need to discuss the experiments in which the amount was very low (0.5 mg/l) or very high (10 mg/l).

In the experiments with a low amount (experiments 101, 47b, 114, 104 and 46b) the result was sometimes good, sometimes good and in two cases poor. The conclusion is that here we begin to approach the lower limit for the amount of antimony, so that the amount should not be

. lower than 0.4 mg/l.

In the trials where the antimony content was high (trials 44, 98 and 110), the results were poor. The final concentration of antimony in the experiments was too high in two cases, and in only one case was it acceptable, so that 10 mg/l seems to be the upper limit.

The results given in table B are shown graphically in fig. 1 and 2.

In fig. 1, the temperature in °C is plotted along the abscissa and the copper/cobalt ratio along the ordinate. The latter has a logarithmic scale or division. Fig. 1 shows the relationship between the temperature and the minimum required copper/cobalt ratio. For this reason, there was no purpose to include in fig. 1 the experiments where the Cu concentration was very high. The results of the experiments are indicated in the table as "good", "good" and "bad", and in the figure by a dot, a circled dot and a cross, respectively. A difficulty arose in indicating the result of runs 88 and 111. Both runs were carried out at a temperature of 65°C and a Cu/Co ratio of 1.0, so that both runs occupy the same point on the figure. According to the table, the result of trial 88 was sufficient for the designation "good" and in trial 111 "bad". As a compromise, the dot in question is indicated as "good" in the figure. In the figure, a line has been drawn that separates the trials with good results from the trials with poor results. This line shows that the Cu/Co ratio required increases with decreasing temperature.

In fig. 2, the temperature in °C is indicated along the abscissa and the copper concentration in mg/l along the ordinate. The ordinate has a logarithmic division. Four. 2 shows the relationship between the temperature and the maximum permissible copper concentration. For this reason, only the trials included in fig. 2 in which the Cu concentration was very high, i.e. precisely the experiments that are not included in fig. 1. In fig. 2, a line has been drawn that separates the experiments with a good result from the experiments with a bad result. This line shows that the maximum permissible copper concentration decreases with increasing temperature.

Claims (1)

Process for purifying an aqueous solution of zinc sulfate obtained by leaching roasted zinc ore with sulfuric acid and separating the iron from the resulting solution, in which process copper and cadmium are in a first step mainly completely precipitated from the solution by adding zinc powder and separated from the solution, and cobalt in a second step is precipitated from the solution by adding zinc powder, a small amount of an antimony compound and a small amount of a soluble copper compound, after which the precipitated cobalt is separated from the solution, characterized in that the second step, which is known per se, is carried out at a temperature between 65 and 85°C, that the zinc powder in the second step, which is also known in and of itself, is added in an amount of 1-2.5 g per liter of solution, that the copper compound is added in an amount of between 5 and 200 mg Cu per liter of solution, so that the copper/cobalt weight ratio in the solution is between 0.5 and 1.0, and that the temperature and the copper addition are regulated so that they correspond to values on the right side of it in fig. 1 drawn curve.