ABSTRACT:
Mineral ores are usually processed by means of pyrometallurgical or hydrometallurgical processes. Myriad of deficiencies surrounding pyrometallurgical process have spurred research interests geared toward low temperature and lixiviant concentration hydrometallurgical process. Leaching of copper, zinc and iron from Nigerian chalcopyrite, sphalerite and ilmenite ores using binary solutions (HCl-KCl, HCl-KClO3 and HCl-NaNO3) as lixiviants was investigated. The ore samples were characterized using instrumental techniques (scanning electron microscopy, SEM, X-ray diffraction spectroscopy, XRD, energy dispersive x-ray, EDX and X-ray flourescence, XRF). The effect of acid concentration, oxidant concentration, particle size, solution temperature, stirring speed and liquid-to-solid ratios on the percentage copper, zinc and iron dissolution was evaluated. The experimental data obtained at various process parameter conditions were fitted in eight kinetics models: diffusion through liquid film model(DTLF), diffusion through product layer model (DTPL), surface chemical reaction model (SCR), mixed kinetics model (MKM), Jander (three dimensional) model, Kröger and Ziegler model, Zhuravlev, Lesokhin and Templeman model and Ginstling-Brounshtein model. Thermodynamic parameters, Gibbs free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) were estimated. Modeling and optimization of the leaching process was achieved using response surface methodology (RSM) and adaptive neuro fuzzy inference system (ANFIS) – Particle swarm optimization technique. The predictive ability of the RSM and ANFIS models were evaluated in terms of root mean square error (RMSE), chi-square (χ2), model predictive error (MPE) and coefficient of determination (R2). SEM micrographs before and after dissolution displayed modifications in the morphology of the ore samples. XRF results established the dominance of copper, iron and sulphur in chalcopyrite, zinc and sulphur in sphalerite, titanium and iron in ilmenite. XRD revealed that the chalcopyrite, sphalerite and ilmenite exist mainly as CuFeS2, ZnS and FeTiO2 respectively confirming the originality of the ores. Results of the leaching studies disclose that all the controllable variables had synergetic effect on the response variable except particle size. HCl-KClO3 marginally outperformed HCl-KCl and HCl-NaNO3. The dissolution kinetics of the processes mostly conformed to the diffusion controlled Kröger and Ziegler model. The corresponding activation energy values estimated were generally <21kJ/mol therefore affirming that the processes are diffusion controlled. The average ΔG values of -5.42kJ/mol, -1.25kJ/mol and -4.54Jk/mol; -29.97kJ/mol, -5.23kJ/mol and -27kJ/mol; -1.26kJ/mol, -52.12kJ/mol and -1.30kJ/mol for chalcopyrite, sphalerite and ilmenite dissolution in HCl-KCl, HCl-KClO3 and HCl-NaNO3 lixiviants suggest the feasibility and spontaneity of the process. The positive enthalpy values (ΔH) of 10.41 kJ/mol, 5.97 kJ/mol and 9.63 kJ/mol; 45.56 kJ/mol, 10.07 kJ/mol and 42.86 kJ/mol; 6.53 kJ/mol, 67.73 kJ/mol and 6.42 kJ/mol for chalcopyrite, sphalerite and ilmenite dissolution in HCl-KCl, HCl-KClO3 and HCl-NaNO3 lixiviants indicate that the reactions were endothermic in nature. RSM model summary results showed that quadratic model compared with linear, 2FI and cubic models, best approximated the experimental data. ANFIS recorded lower values of RMSE, χ2, MPE and values closer to unity compared to RSM. The results showed the superiority of ANFIS in capturing the nonlinear behaviour of the leaching systems. The ANFIS-PSO optimal predictions of 96.95%, 97.85% and 95.74%; 95.40%, 97.72% and 90.91%; 98.83%, 95.57% and 92.85% for chalcopyrite, sphalerite and ilmenite in HCl-KCl, HCl-KClO3 and HCl-NaNO3 binary solutions were in close agreement with the experimental 95.10%, 95.92% and 94.78%; 93.53%, 96.98% and 88.24%; 96.95%, 96.68% and 90.90% obtained at the same process conditions. The results obtained corroborate the potential capability of HCl-KCl, HCl-KClO3 and HCl-NaNO3 binary solutions as lixiviants for copper, zinc and iron recovery from chalcopyrite, sphalerite and ilmenite ores.
