Technical And Basis Studies On The Extraction Of Nickel And Cobalt From Laterite In Choride Medium

To avoid disadvantange of traditional leaching conditions and extract nickel and cobalt more effectively, this paper studied some extraction conditions in chloride medium, such as the atmospheric (acid) leaching, chloridizing roasting and chloridizing segregation. The effect and mechanism of mineral phase reconstruction on the extraction behavior of nickel and cobalt from laterite in these extraction conditions are discussed in detail.Based on the analysis of thermodynamics, the atmospheric HCl acid leaching conditions are studied. The optimal conditions are that the particle size of laterite is-0.15 mm, initial hydrochloric acid concentration is 8 mol/L, reaction temperature is 353 K, solid/liquid ratio is 1:4, rotation rate is 300 r/min, and the leaching time is 2 h, the results show that the dissolution yields of Ni, Co, Mn, Fe and Mg can be 93.94%, 60.5%,94%,56%and 94%, respectively. Because Ni in the laterite mainly exists in the ferric mineral phase and the leaching of Ni exhibits linear correlation with iron extraction. Due that part of Co exists in silicate, the leaching of Co does not exhibit good linear correlation with iron extraction. The results of kinetics of leaching laterite show that the dissolution rates of Ni, Co and Mn accord with unreacted shrinking core models for solid film control. And the activation energy of Ni, Co and Mn are 11.56 kJ/mol,11.26 kJ/mol and 10.77 kJ/mol. And the dissolution order of different mineral phases in the laterite is that geothite>lizatdite> magnetite≥hematite.On the basis of thermodynamic analysis, Ni and Co can be extracted selectively from laterite ore through chloridizing roasting, and impurities extraction can be suppressed. Experimental results show that low temperature chloridizing roasting by hydrogen chloride gas can effectively extract the nickel and cobalt and inhibit the iron extraction. The optimal conditions are as follows:mineral particle size is-0.074 mm, the roasting temperature is 300℃, hydrogen chloride gas flow rate is not less than 80 mL·min-1, the roasting time is not less than 30 min, nickel leaching rate is up to about 80.6% and cobalt leaching rate is about 60%. Ni/Fe molar ratio is about 15 times than that of raw ore, and Ni/Mg molar ratio is about 8 times than that of raw ore. The chloridizing roasting experiments in the intermediate temperature show that nickel and cobalt can be extracted effectively with solid chlorinating agent used and the leaching of iron can be inhibited. The optimum conditions are that mineral particle size is-0.074 mm, the mixture of NaCl and MgCl2·6H2O is chlorinating agent (mass ratio 0.4), the addition of chlorinating agent are 18% of the ore, the roasting temperature is 900℃, the roasting time is 1.5 h, the leaching rate of nickel is up to about 85% and the leaching rate of cobalt is about 60%. The Ni/Fe ratio is about 20 times than that of raw ore, the Ni/Mg ratio is 15 times than that of raw ore. Kinetic experiments show that the chloride reaction of nickel and cobalt belongs to the unreacted shrinking core model. The chloride reaction apparent activation energy of nickel and cobalt are 16.469 kJ/mol and 32.792 kJ/mol, respectively. The mixture of NaCl and MgCl2·6H2O (mass ratio 0.4) used as chloride reagent has eutectic point and can melt at relative low temperature, it favours the uniform mixing and permeation into the ore. And the chloride reagent can produce hydrogen chloride gas in a wide range temperature which avoids the release of excess hydrogen chloride gas.Results show that chloridizing segregation of nickel is the main reaction with control of reductant dosage, chlorinating agent dosage and reaction temperature. Through a series of experiments of magnetic field strength, the desired concentrate of nickel and cobalt can be obtained through two times magnetic separation with different magnetic field strength which are 3000 Gs of the first time and 1000 Gs of the second time. The optimum conditions of chloridizing segregation and magnetic separation are that mineral particle size is-0.074 mm, the mixture of NaCl and MgCl2·6H2O is chlorinating agent (mass ratio 0.4), the addition of chlorinating agent are 6% of the ore, the particle size of bituminous coal reductant is-0.2 mm, the reductant dosage is 2% of the ore, reaction temperature is 1000℃, reaction time is 90 min, and magnetic field strength are 3000 Gs of the first time and 1000 Gs of the second time, the grade of nickel and cobalt can be 5.78% and 0.187% respectively and the extraction of nickel and cobalt could reach 87.69% and 69.02% respectively.The roasting of laterite ores under atmospheric pressure at different temperatures can lead to the release of free water and bound water, dehydroxylation of goethite at 300℃and lizardite at 610℃respectively, which can lead to the change and reconstruction of the structure of original mineral and more fine pores and scraps on the surface of ore roasted can be found. As roasting temperature increases to 700℃and even 800℃, some fine particles reunite together which lead the decrease of specific surface area, and the amorphous magnesium silicate phase appears to have recrystallised as forsterite (Mg2Si04) and enstatite (MgSiO3). Compared with difference of leaching rate of nickel and cobalt between raw ore and roasted ore in atmospheric (acid) leaching, chloridizing roasting and chloridizing segregation, it can be seen that ore roasted at 300℃can make more nickel is exposed to the reaction interface which benefit the leaching of nickel and cobalt, at the same time, the leaching of iron is suppressed. And raw ore is roasted at 610℃, the main mineral lizardite (Mg3Si2(OH)4O5) begins to discompose and leads to mineral structure destroyed, the specific surface area attains the maximum, which benefit gas-solid reaction in the low temperature chloridizing roasting by hydrogen chloride gas and increase the leaching rate of nickel and cobalt. There is no significant effect on the extraction of nickel and cobalt through pre-roasting at 300℃and 610℃in chloridizing roasting at intermediate temperature and chloridizing segregation because the mineral phase have reconstructed before chloridizing reaction. When roasting temperature increases to 700℃and 800℃, it lead to the incorporation of nickel into the magnesium silicate phase (Mg,Ni)3Si02 followed by recrystallisation into the forsterite phase which appears to be detrimental to the nickel recovery. Thereafter, pre-roasting at 700℃and 800℃do not benefit for nickel recovery in the atmospheric (acid) leaching, chloridizing roasting and chloridizing segregation.In conclusion, the present processes proposed for nickel and cobalt extraction from laterite in chloride medium have been studied comprehensively in this paper. Three kinds of technology prototype are established for different target products. And the nickel and cobalt can be extracted selectively through chloridizing roasting and chloridizing segregation-magnetic separation which can decrease the addition of reagent and disposal in subsequent purification process. It is particularly significant in the exploitation of rich reserves of laterite.