An Experimental Study On Cobalt Cementation By Zinc Dust For Deep Purification Of Zinc Sulfate Solution

Deep purification is the major trend in purifying zinc sulfate solution, which can not only improve the current efficiency and quality of the electrowined zinc, but also is the prerequisite of realizing mechanization of zinc ripping. There is still a certain gap for purification technology in our country compared with the international advanced level, such as lower purity of zinc sulfate solution, larger zinc dust consumption and so on, therefore, there is of great significance in strengthening research in this area.Antimony salt process is the most widely used for cobalt removal from zinc sulfate solution. In order to strengthen the process, many factories abroad and at home adopt the method of adding a certain amount of copper sulfate to the solution or retaining a small amount of Cu(Ⅱ) ions and Cd(Ⅱ) ions in the stage of removing copper and cadmium, but the addition amounts of Cu(Ⅱ) ions and Cd(Ⅱ) ions, as well as their effects on cementation of cobalt are still in dispute. In addition, the activeness of zinc dust present in the solution reduces gradually along with the time in the purification process, which always leads to the rate of cobalt removal slow and the dosage of zinc dust increase. Generally speaking, this is due to the basic zinc sulfate formed on the surfaces of zinc dust hinder the process of mass transfer in the purification process, therefore enhancing mass transfer is one idea to solve this problem. To deal with these two problems, the effects of adding Cu(II) ions and Cd(II) ions were systematically investigated in the paper, and also a preliminary study was carried out on the effect of ultrasonic wave on the cobalt cementation process by zinc dust.Experiments on cobalt removal using antimony salt as additive in the absence of Cu(Ⅱ) ions and Cd(Ⅱ) ions were carried out firstly. The effects of such factors, as types of zinc dust, dosage of zinc dust, dosage of antimony salt, temperature, stirring speed and initial pH value were examined on the rate of cobalt removal, as well as the purities of the zinc sulfate solution. The results showed that the characteristics of zinc dust used have a large effect on the purification of cobalt. Using zinc dust with finer particle size and containing a little amount of Pb, Sb, can not only increases the purities of zinc sulfate solution, saves the dosage of zinc dust used, but also accelerates the cobalt removal rate; Increasing temperature can increase the cobalt removal rate significantly, and the apparent activation energy for the process was calculated equal to 90 kJ/mol according to experimental results, suggesting that the cobalt removal process is controlled by a surface chemical reaction; The optimal conditions for cobalt removal without Cu(II) and Ca(Ⅱ) ions addition were obtained to be as follows:temperature 85℃, zinc dust dosage 2 g/L, antimony salt dosage 3 mg/L, stirring speed 300 r/min, initial pH 4.The effects of adding Cu(Ⅱ) ions and Cd(Ⅱ) ions on cobalt removal were investigated under the optimal conditions described above. The result showed that adding solely a certain amount of Cu(II) ions not only increases the cobalt removal rate, but also decrease zinc dust dosage added to obtain a deep purification, the optimal addition amount of Cu(Ⅱ) ions was determined to be 50 mg/L, which will result [Co2+] bellow 0.1 mg/L after purifying for 90 minutes; adding solely a certain amount of Cd(II) ions was beneficial to the process, but a Cd(II) ions concentration up to 150 mg/L would be harmful to cobalt removal; It was also validated that adding Cu(Ⅱ) and Cd(Ⅱ) ions at the same time behaves a better effects of cobalt removal than that of adding Cu(II) ions solely, and furthermore, compared with that in the absence of Cu(Ⅱ) ions and Cd(Ⅱ) ions, can obtain a low [Co2+] under a lower temperature. The optimal conditions for cobalt removal were obtained to be as follows:[Cu2+]015 mg/L, [Cd2+]015 mg/L, antimony salt dosage 3 mg/L, zinc dust dosage 2 g/L, temperature 85℃and stirring speed 300 r/min, initial pH 4.A primary investigation on the effect of ultrasonic wave on cobalt removal by zinc dust cementation was carried out, and the main conclusions were as follows:the rate of cobalt removal can be accelerate to a certain extent in the presence of ultrasonic wave; the higher the power of ultrasonic wave up to 160 W in the experiments, the higher the cobalt removal rate; but further increase of the ultrasonic power beyond this range was deleterious to the cobalt removal; the presence of ultrasonic wave with a higher frequency (e.g.100 kHz) was beneficial to the cobalt removal process, while the presence of ultrasonic wave with a lower frequency (e.g.45 kHz) accelerated the redissolution of cobalt deposited after an effecting duration for 60 min.