After the initial purification of the post-removal liquid and magnesium oxide slurry under certain conditions, that is, to generate cobalt hydroxide precipitation, the reaction is as follows:
Mg(OH)2+CoSO4=Co(OH)2↓+MgSO4
In order to get high grade cobalt hydroxide product and high cobalt recovery, the experiment of active magnesium oxide usage was carried out to find out its effect on cobalt grade, cobalt recovery and pH, and to determine the best amount of magnesium oxide. The experimental results are shown in Figure 2, 3 and 4.
Experimental conditions:temperature 60℃, constant stirring during the reaction, magnesium oxide slurry concentration 10%, magnesium oxide addition amount was 1.0, 1.2, 1.4 and 1.6 times of cobalt metal in solution.
It can be seen from Fig. 2 that the grade of cobalt in the cobalt hydroxide product decreased gradually with the increase of magnesium oxide. The cobalt grade reaches the highest value of 36.4% when the amount of magnesium oxide used is 1.0tMgO/tCo. As can be seen from Figure 3, the more magnesium oxide is added, the higher the cobalt recovery rate of a section of cobalt precipitation, and when the amount of magnesium oxide added is 1.6tMg0/tCo, the cobalt recovery rate almost reaches 100%, but the cobalt grade of the cobalt hydroxide product is only 26% at this time. As can be seen from Figure 3, the solution pH increases with the prolongation of reaction time, and the solution pH rises quickly within 1 hour at the beginning of the reaction, the pH of the end of the reaction increases with the increase of MgO addition, and the end pH is 7.3 when the MgO addition is 1.0tMgO/tCo, and the end pH reaches above 8.0 when other multiples are added.
In the industrial trial production, the amount of magnesium oxide used was 1.0 times the amount of cobalt metal in the solution, and the industrial production data were as follows: the end-point pH of one section of cobalt precipitation reaction was 7.2, and the cobalt recovery was about 81.5%, and the higher grade cobalt hydroxide product was obtained, and its cobalt grade reached 32.96%.
The typical composition of one stage cobalt precipitation solution is shown in Table 3, and the typical composition of dried cobalt hydroxide product is shown in Table 4.
From Table 3, it can be seen that the concentration of cobalt in the liquid after one stage of cobalt precipitation is still 0.67g/L, so it is necessary to carry out the second stage of cobalt precipitation to recover all the cobalt. From Table 4, it can be seen that the magnesium content in the cobalt hydroxide product is high, reaching 5.75%. After inspection and analysis, it is found that about 38% of the magnesium in the product exists in the form of magnesium sulfate, which is caused by the co-precipitation of magnesium sulfate with cobalt hydroxide and magnesium hydroxide during the reaction of cobalt precipitation, so the washing process can be added to remove the soluble magnesium sulfate and reduce the content of magnesium and other impurities in the cobalt product.