Study On Preparation Of High Purity Cobalt

High purity cobalt has excellent semiconductor properties,magnetic properties and electrical conductivity.It has become an important material for many applications,such as magnetic recording media,magnetic recording heads,optoelectronic devices and magnetic sensors and integrated circuits,etc.Due to technological limitations,a large amount of 5N and higher purity cobalt must be imported.Therefore,it is very important to develop a high purity cobalt production process with own intellectual property.In this study,cobalt sulfate solution was purified by ion exchange method.High purity cobalt was prepared by electrolysis using purified cobalt sulfate solution as electrolyte.A new chelating resin Monophos was used to remove Fe from cobalt sulfate solution.Optimal process conditions were determined through experiments for Fe removal using Monophos resin as well as for cobalt electrorefining.Effects of pH,resin to liquid volume ratio,reaction temperature and reaction time on the adsorption of iron were investigated through static adsorption experiment.The results have shown that the chelating resin Monophos has a very strong adsorption ability for Fe.With pH = 2~3,temperature = 30~60oC,resin to liquid volume ratio = 1:5~1:8,adsorption time > 90 min,a Fe removal rate of up to 70% has been obtained.The content of Fe in the solution has been decreased from about 2.0 mg/L to about 0.4 mg/L.Three different kinds of acid(HEDPA,HCl and H2SO4)solutions were used to elute resin Monophos.The results have shown that H2SO4 had no analytical ability to Monophos resin,and the desorption effect of 0.5 mol/L HEDPA is better than that of 6 mol/L HCl.Eluting using HEDPA leads to a lower adsorption capacity of the resin comparing with the eluting using hydrochloric acid.The HCl concentration has a large effect on the desorption of resin.Desorption effect is better when HCl concentration is higher than 5mol/L.The Monophos resin has a good repetition performance,a strong regeneration ability and a long service life.It is an ideal ion exchange resin for removing Fe in cobalt salt solution.Study of Fe adsorption mechanism by Monophos resin reveals that the main adsorption group in the resin is phosphoric acid group.An infrared spectrum analysis has confirmed that the metal ions replace the hydrogen in the P-O-H and are bounded with the oxygen atoms through ionic bond.The pseudo first order kinetic equation and the pseudo second order kinetic equation were used to fit the experimental data of the dynamic adsorption curve.The results have shown that Fe adsorption by Monophos resin is in accordance with the pseudo second order kinetics.The control step for Fe adsorption by Monophos resin is the particle diffusion.Langmiur and Freundlich adsorption isotherm models were used to fit the isothermal equilibrium adsorption data.The results have shown that the Langmiur equation can more accurately reflect the Fe adsorption process by Monophos resin.Thermodynamic study of adsorption process has shown that the Fe adsorption process by Monophos resin is spontaneous(?G<0),endothermic(H>0),and with a positive entropy change(?S>0).Therefore,an increase of temperature is favorable to the adsorption process.Results of electrolysis experiments have shown that optimal process conditions for preparing high purity cobalt from cobalt sulfate system are as follows: the cobalt concentration = 60~70g/L;pH = 3~4;the cathode current density = 150~300A/m2;the electrolysis temperature = 40~60oC.The cathode cobalt prepared by using these parameters has a purity of 5N as measured by GDMS analysis.