A Fundamental Study On Selective Chlorination Of Pre-reduced Limonitic Nickel Laterite

As the declining global reserves of nickel sulphides, nickel laterite ores have become the major nickel reserves for nickel industry. A limonite ore in the upper of a typical nickel laterite deposit is an important reserve of nickel and cobalt. It is rich in iron in the form of goethite and hematite, which are bearing most of metal values. To extract nickel and cobalt, it is usually processed by hydrometallurgical methods, including high pressure acid leaching (HPAL) and reductive roasting-ammonia leaching (Caron process), in which nickel and cobalt are leached into solution for further recovery while most iron is rejected in residue as oxides or hydroxide. A new reductive roasting-selective chlorination-water leaching process for limonitic nickel laterite is presented in this work. Thanks to the development of the Caron process, the reductive roasting of limonitic nickel laterite has been studied and discussed in many publications. On the foundation, this research focuses on the selective chlorination of pre-reduced limonitic nickel laterite, to give a detail description of mechanisms of the process and valuable reference for the treatment of nickel laterite.Limonitic nickel laterite ore was characterized by chemical analysis, X-ray diffraction (XRD), scanning electron microscope (SEM) and thermogravimetric-differential thermal analysis (TG-DTA). Then reduced ores was obtained from the reductive roasting of the ore with various activated carbon additions, which were used as raw material for selective chlorination experiments. The reduced ores were characterized by chemical analysis, XRD, optical microscope and SEM. Nickel and cobalt in limonitic nickel laterite can be selectively reduced to metal state in the form of iron alloys while iron oxide is mostly reduced to wustite (FeO). Fine-grains of iron alloys with diameter of about1μm were uniformly distributed in the substrate particle of iron oxides. With activated carbon addition of10wt%, the metallization degrees of iron, nickel and cobalt reached8.34%,82.20%and82.86%respectively.Basis on the analysis results above, thermodynamics on the selective chlorination of pre-reduced limonitic nickel laterite in HCl-H2O-O2atmosphere were studied. Equilibrium phase diagrams of Fe(Ni, Co)-HCl-H2O-O2system at temperatures of400K,600K,800K and1000K were calculated and drawn. From the diagrams, an obvious thermodynamic stable region of NiCl2, CoCl2and Fe2O3can be found and gets smaller as temperature increasing. And then, Gibbs free energy change, equilibrium partial pressures of hydrogen chloride, oxygen and chlorine of potential reactions for pre-reduced limonitic nickel laterite in HCl-H2O-O2atmosphere were calculated in the temperature range of298-1200K. From the thermodynamic equilibrium analysis, the selective chlorination of nickel, cobalt and iron as metal or wustite can be performed at500-800K while magnetite and hematite are uncreative to hydrogen chloride; chlorinated iron can be rejected as hematite by the oxydrolysis of ferrous chloride; the oxidation of pre-reduced ores and the chlorination of other impurites could also take place. It suggests a relative high reduction degree of pre-reduced ore to ensure the metal state of values and the main phase of iron as wustite, a relative high partial pressure ratio of HC1to H2O and a properly low partial pressure of oxygen for the selective chlorination process.The kinetics study on selective chlorination process of the pre-reduced ore was conducted in HCl-O2-H2O-N2atmosphere. The effects of variations in temperature, the partial pressures of hydrogen chloride, oxygen and vapor, and total gas flow rate were all studied. It was found that the chlorination of nickel and cobalt could be described by two stages; the rate was fairly fast in the initial stage, but slow for the second stage; and both of them were controlled by gas diffusion through product layer. Four apparent activation energies, including20.43kJ/mol and12.89kJ/mol for the first and second stage of nickel chlorination,14.95kJ/mol and13.02kJ/mol for the first and second stage of cobalt chlorination respectively, were obtained in the temperature range of420-460℃. And the apparent reaction order of hydrogen chloride was also obtained. During the selective chlorination, the oxidation of pre-reduced ore could take place with the selected conditions; chlorinated iron was mostly rejected as hematite and thus nickel and cobalt were selectively chlorinated.The thermal decomposition of A1C13·6H2O, FeCl3·6H2O, FeCl2·4H2O, NiCl2·6H2O and CoCl2·xH2O were studied using TG-DTA. It was found that the chlorides, including AlCl3·6H2O, FeCl3·6H2O and FeCl2·4H2O can be used as solid chloride agent to generate hydrogen chloride gas for the selective chlorination of pre-reduced ore. Chlorides of nickel and cobalt are stable while iron chloride transform to hematite (Fe2O3) at～450℃with selected test conditions. The thermal stability differences indicated that nickel and cobalt can be selectively chlorinated while iron is rejected as hematite during the selective chlorination. The selective chlorination roasting process was investigated using AICl3·6H2O, FeCl3·6H2O or FeCl2·4H2O as solid chloride agent. The effects of roasting temperature, chloride agent addition, roasting time and the reduction degree of pre-reduced ores were studied. All the selected chlorides showed good performance for the selective chlorination. Under the optimum experimental conditions,～91%Ni,～90%Co and only<5%Fe were extracted by water leaching at80℃.A chlorinated ore was obtained from the selective chlorination with ferrous chloride tetrahydrate addition of40wt%and time of150min at460℃. Then the leaching behavior of metals from the chlorinated ore and catalytic oxidation of Fe(II) in the acidic chloride solution were studied. It showed a fast kinetic during the leaching of nickel and cobalt from the chlorinated ore. Liquid/solid ratio in leaching process, leaching time and temperature had little influence on metal extraction. As the initial pH value of leaching solution decreasing from6.85to1.00, the extractions of Fe, Ni, Co, Al, Cr and Mn increased slightly while that of Mg increased obviously.91.78%Ni,87.64%Co,<2%Fe,2.53%Al,<1%Cr,36.98%Mg and63.04%Mn in the ore were leached by water; and a leaching solution (pH value=2.35) containing4.43g/L Ni,0.27g/L Co,2.52g/L Fe,1.40g/L Mn and other impurities was obtained. The leaching residue with Fe65.68wt%and S0.02wt%shows a great potential for its utilization as ironmaking raw materials. The oxidation of Fe(II) in the acidic chloride solution at oxygen atmosphere was effectively prompted by addition sodium nitrite as catalyst. After Fe(II) oxidation-goethite precipitation, the removal fraction of iron in the leaching solution reached98%.