In the wet process of refining zinc, cobalt is a very harmful impurity elements, if not the purification process, in addition to the effect of cobalt, cobalt-lead content of the electrolyte rises abruptly. When the cobalt content exceeds a certain concentration, cobalt will precipitate on the cathode, which has a great influence on the zinc electrowinning: on the one hand, due to the low super-voltage of hydrogen deposited on the cobalt, the hydrogen is easily precipitated, which reduces the zinc current efficiency; aspect of the cobalt in combination with zinc, forming a micro cell, anti-solvent causes precipitation of zinc, zinc cause corrosion of the cathode ( "burn plate"), that the corrosion of zinc black spot, the closer and more serious side of the aluminum plate, forming the horn Round hole. Some high-zinc concentrates purchased by our company in recent years are mainly high in harmful impurities such as cobalt, copper and silicon. The high cobalt ore referred to in this paper refers to the mass fraction of cobalt in the zinc concentrate greater than 0.0079/5.
First, the problems caused by the high cobalt zinc concentrate to the purification process
There are many purification processes for zinc sulphate solution, among which zinc powder and xanthate purification process is a relatively traditional process. The principle of the cobalt removal method is: in the presence of copper sulfate, the cobalt sulfate in the solution acts with the xanthate to form a poorly soluble cobalt yellow cobalt precipitate, where Cu 2 + can oxidize Co 2 + to Co 3 compared with a zinc + antimony salt method, the biggest advantage of this method is the ability of the secondary inter strong cobalt, cadmium reconstitution but the drawback is serious.
The high cobalt content in the supernatant will cause a high purity of the cobalt in the purification solution, which makes the secondary purification difficult, and the purification time is relatively prolonged, the cadmium re-solubility is intensified, the labor intensity is increased, and the yield and quality are affected.
(1) The supernatant contains high cobalt
1, cobalt leaching rate
Zinc hydrometallurgy leaching is zinc electrowinning waste liquid generated as a solvent, the dissolved valuable metals into the zinc-containing solution, a process feedstock. The use of hydrolysis purification to remove some of the harmful impurities, can reduce the burden of the purification process. According to the potential-pH diagram of the Zn-M-H 2 O system (not shown), when a (Co = 2 Ã— 10 -4 mol / L, the boundary of the stable region of Co 2 + and Co (OH) 2 is at pH. =8.15. Co 2 + becomes Co 3 + under the action of oxidant, and the pH when precipitated in the form of Co(OH) 3 is 6.6, so under the condition of neutral leaching (pH is controlled at 5.0 to 5.5), Cobalt is still partially dissolved, and the leaching rate of cobalt is about 70%.
2, cobalt system internal circulation
The fluctuation of cobalt content in zinc concentrate is the main reason for the fluctuation of cobalt in the middle and upper supernatants, and the circulation of cobalt in the system increases the cobalt content of the supernatant. There are six main destinations for the circulation of cobalt in the system: supernatant in zinc oxide, indium and thorium extract, cadmium-depleted solution, cobalt residue after pickling, residual liquid and cadmium concentrated supernatant and indium replacement solution. Among them, the cobalt in the raw material has the greatest influence on the cadmium-depleted liquid.
According to the cobalt content of zinc concentrate of 0.0084%, and the supernatant of the ore powder containing 155mg/L of zinc, the supernatant of the ore powder contains cobalt of l3mg/L. However, in actual production, the cobalt in the supernatant can reach l6mg / L. If the high-cobalt ore is concentrated, the cobalt content of the supernatant in the ore fines is above 20 mg/L. This is due to the circulation of a portion of the cobalt in the solution. Table l is the cobalt content of the supernatant in the company's mineral powder for a certain year. As can be seen from Table 1, the batch containing cobalt in excess of 20 rng/L was 7.95%.
Table 1 Cobalt content of supernatant in mineral powder of the company in a certain year
(2) The rate of removal of cobalt in one purification
A purification process, that is, a replacement process, is to replace a metal ion whose standard electrode potential is corrected by a metal having a negative electrode potential.
Table 2 Standard electrode potentials of some elements
According to the reaction formula of zinc powder replacing cobalt (Zn + Co 2 + = Zn 2 + + Coâ†“), when the reaction reaches equilibrium, Ï† = 0, a (Co 2 + ) = 10 - 16.47 a (Zn 2 + ). From this, it can be seen that Co 2 + can be completely replaced by zinc, but in actual production, |Ï†(Zn)| is almost equal to |Ï†(Co)|, and there is no activator, and the reaction is difficult to carry out. Only when Ï†|(Zn)|>|Ï†(Co)|, the reaction will continue to carry out the purification of the company once a day, the removal rate of cobalt is listed in Table 3.
Table 3 Cobalt content in a part of the purified material
Second, the treatment measures
It can be seen from the above analysis that the difficulty in processing high-cobalt ore is mainly reflected in the following three aspects: one is that the leaching rate of cobalt is about 70%%; the other is the internal circulation of cobalt; and the third is that the removal rate of cobalt is not high.
(1) Increasing the removal rate of cobalt in one purification
1. Replace the self-produced zinc powder with alloy zinc powder.
Without the action of an activator, the reaction of zinc powder to replace cobalt is difficult to carry out. The production practice proves that the strontium salt purification method and the arsenic salt purification method can be used to maintain a certain amount of arsenic arsenic in the supernatant in the zinc powder-xanthate purification process, or use the alloy zinc powder containing arsenic bismuth to help Increase the removal rate of cobalt in one purification. The purification indicators of the two zinc powders are listed in Table 4.
Table 4 Comparison of the use of alloy zinc powder and self-produced zinc powder
2. Control the temperature of the appropriate supernatant
Only when Ï†|(Zn)|>|Ï†(Co)|, the zinc powder replaces the anti-cobalt reaction. Ï†|(Zn)| and |Ï†(Co)| vary with the concentration of the ion in the solution, the temperature of the solution, and the nature of the cathode metal. The effect of solution temperature and ion concentration on Ï† is shown in Table 5.
It can be seen from Table 5 that as the temperature of the solution increases, both Ï†|(Zn)| and |Ï†(Co)| decrease, but the value of |Ï†(Co)| decreases by Ï†|(Zn)| The value is much larger. That is, as the temperature increases, the difference of Ï†|(Zn)|-|Ï†(Co)| increases, which is beneficial to the replacement of cobalt by zinc powder. However, it should be noted that cadmium re-dissolution is an endothermic reaction, and cadmium re-dissolution is significantly aggravated with increasing temperature. Therefore, in actual production, if the supernatant contains high cadmium and the cobalt is normal, the temperature of the supernatant should be controlled at 63-65 Â° C; if the supernatant contains higher cobalt (greater than 18 mg / L), The temperature of the supernatant can be appropriately increased, and it can be generally controlled at 65 to 67 Â°C.
Table 5 Effect of temperature and ion concentration on Ï†(Zn) and medium Ï†(Co)
(2) System cobalt open circuit
The purified cobalt is removed into the cadmium recovery process along with the copper cadmium residue, and then enters the main system together with the cadmium-depleted liquid, so that the cobalt continuously circulates and accumulates inside the system, which affects the effect of one purification. The copper cadmium residue in the cadmium-depleted liquid after the recovery of cadmium and copper is open to the cobalt is an important means to solve the low cobalt removal rate.
Since there are some disadvantages in the removal of cobalt by the xanthate method and the arsenic salt method, the cobalt in the depleted cadmium solution is removed by the cerium salt method. The main process is: the cadmium-depleted liquid is sent to the cobalt removal tank (the cadmium and cobalt are sampled and analyzed in the upper part of the tank), and when the temperature is raised to 8.-95 Â° C, the strontium salt is added according to the cobalt content in the cadmium-depleted liquid (according to 8mg / L added), stirring for 5min, slowly adding zinc powder (added 8 ~ 12g / L), stirring for 1 ~ 2h, sampling and analysis of cadmium and cobalt, cadmium and cobalt content after passing the pressure filtration, the filtrate is leached, cobalt slag Foreign transportation. Another production practice proves that temperature and acidity are two important factors that can not be ignored in the effect of cobalt open circuit.
1, the effect of temperature on the removal of cobalt
When the temperature rises, the difference of Ï†|(Zn)|-|Ï†(Co)| increases, which is beneficial to the removal of cobalt by zinc powder. Since the temperature of the solution before the removal of cobalt is about 40 Â° C, it takes a long time to heat the solution to about 85 Â° C with steam, so some operators start to add zinc powder when the temperature is less than 85 Â° C. In addition, the current temperature measurement method is that the operator uses a sling to hang the solution in the cobalt removal tank, and the test temperature is significantly different from the actual solution temperature (especially in winter), resulting in inaccurate control of the purification temperature. In addition to cobalt effect. Therefore, changing the temperature measurement method and increasing the temperature rate help to increase the cobalt removal rate.
2. Effect of acidity on removal of cobalt
When the pH of the solution is >5, the solution turns white and becomes cloudy, and the cobalt is low. The Zn(OH) 2 precipitate formed by Zn shake hydrolysis adsorbs on the surface of the electrode, hindering further conversion of Co 2 + ; if the pH is low, the consumption of zinc powder is increased, and the production cost is also increased. Practice has shown that the pH of the solution is controlled to be 3.5 to 4.0.
Third, the conclusion
(1) Keeping the mineral powder supernatant containing a certain amount of arsenic and antimony, and controlling the temperature of the appropriate supernatant, helps to improve the removal rate of cobalt in one purification.
(2) In order to avoid the accumulation of cobalt in the internal circulation, part of the system cobalt can be opened. To ensure open circuit ratio, temperature and acidity control are key.
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