چكيده به لاتين
Extraction of nickel from laterites, especially low-grade limonitic ores has been facing challenges due to the nickel dispersion in host minerals and the subsequent lack of effective beneficiation methods. However, the depletion of enrichable sulfidic ores in the last decade has lead to growth in the share of nickel extraction from laterites. Therefore, finding a solution to nickel concentration from low-grade laterites is a key to make great resources of lateritic nickel which have been useless until today operational. Beside the concentration efforts, atmospheric leaching and production of nickel pig iron (NPI) are in the spotlight of research works as comprehensive processing solutions for nickel laterites. The aim of the present work is developing a novel method for nickel extraction from low-grade limonitic laterites comprising several steps: (1) controlled crushing, (2) chemo-physical concentration by oxalic acid reverse leaching, (3) ammoniacal leaching of the oxalate concentrate, (4) deammoniation of the ammoniacal pregnant solution, and (5) pyrolysis to NiO products with particular characteristics and added value. A limonitic laterite sample from Sarchahan deposit (Bavanat, Fars province, Iran) containing 0.8% Ni and 27.3% Fe was used for the experiments with a particle size of -2.8 mm. The parameters of oxalic acid leaching were temperature (50-90 °C), oxalic acid dihydrate initial concentration (60-100 g/L), L-ascorbic acid additive (0-1 g/L), and time (24 h including 13 sampling) at constant conditions of 50 g/L pulp density and rotation speed of 400 rpm. The results of leaching tests and physical separation of pulps showed that unlike other observing constituents (Fe, Cr, Mn), nickel precipitated as Ni(ox).2H2O in the fine fraction of leaching residue. The nickel build-up in the fine residues up to 4% produced a concentrate or intermediate enriched product containing about 12% nickel oxalate and decresed amounts of iron. On the contrary, iron dissolved as trioxalatoferrate(III) (Fe(ox)33-) in the solution except at low ORPs caused by either high temperature or ascorbic acid which make it precipitate as Fe(ox).2H2O. In the second phase of experiments, an adequate amount of the concentrate was produced in a bench-scale test (80 °C, 90 g/L oxalic acid dihidrate, 27 h) to be served as feed material for a selective ammoniacal leaching. Amongst the studied factors namely initial ammonia concentration (2.5-25 wt.%), temperature (20-50 °C), pulp density (50-200 g/L), time (90 min including 7 sampling), and sonication only time and ammonia concentration increased the nickel dissolution yield and the other parameters just enhanced the rate. It was found that under the conditions of 25% NH3, T=50 °C, S/L=200 g/L, and 15 minutes of sonication, about 93% of nickel is dissolved resulting in a pregnant leach solution (PLS) with a nickel content of 7.4 g/L as hexaaminenickel(II)oxalate Ni(NH3)6(ox). The kinetic investigation of the first phase (oxalate leaching) showed that the activation energy of iron dissolution from its minerals as the main racting component is about 80 kJ/mol (chemical control) and data area adaptable with both shrinking particle model and avrami equation, however, at lower ORPs the fitting of avrami model on the dimensionless diagram are better and the avarami’s parameter of n decreased in the reducing condition which indicates a shift to diffisuion control. Also, an activation energy of 73 kJ/mol was obtained for the nickel oxalate dissolution in ammonia solution (the second phase) while the degree of reaction to initial ammonia concentration was calculated to be 0.6. The Ni(NH3)6(ox) PLS was subsequently deammoniated by either boiling (90 °C) or partial vacuum-assisted gentle heating to reclaim the nickel as nanoscale oxalate precursors. The pyrolysis of the precursors resulted in mesoporous nanorods of 97% purity NiO (Co: 1.1%, Mg: 0.64%, Mn: 0.08%, Cu: 0.09%, Zn: 0.03%, Fe: 0.02%) with a width of 100 nm and 30 nm, specific area of 45 and 70 m2/g, mean pore diameter of 8 and 4.8 nm respectively for the boiling (NiO-B) and partial vacuum (NiO-V). The supercapacitor characterization of the nanoparticle products showed that NiO-B with a well-shaped morphology and bigger pore size in assistance wirh the natural doped elements has an unexampled specific capacity of 3352 F/g (90% of theoric value) at 1 A/g current density, while NiO-V with a same chemical composition has a specific capacity of 2089 F/g. Both products having powr density of 204 and 184 W/kg and energy density of 75 and 47 Wh/kg respectively, can be studied for power supply applications after cycle durability tests. The present results can lead to development of a direct production method for NiO supercapacitors from laterites with no purification requirements with high power desity and energy density which makes them alternatives for batteries.
