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Title: Chemical investigation of leaching and electrochemical behaviour of sensor-based pre-concentrated copper ores with alkaline lixiviant
Author: Amos, Idzi Ambo
ISNI:       0000 0004 5922 2935
Awarding Body: University of Exeter
Current Institution: University of Exeter
Date of Award: 2015
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While the production of copper metal from primary ores is still increasing, the gradual depletion of high-grade ores implies that the recovery of the metal from low-grade ores is presenting a challenge. A major problem associated with processing of low-grade copper ores can be their high calcite content and concentration of other metals such as Fe, Mn, Co, As, Pb, and Ni. These other metals make the processing of such ores expensive due to the high cost of the leaching chemicals. Therefore, in this study a novel, integrated copper processing method is developed to enhance the economic extraction of copper from such ores. This research investigated the chemical leaching behaviour of porphyry copper ore samples from Chile, South America. Ores were pre-concentrated and classified with near infrared sensing into product, middling and waste fractions. Mineralogical analysis of the ore was conducted using QEMSCAN®, XRD and NIR. The elemental investigation of the ore was carried out using PXRF, ICP-MS and SEM. Analyses revealed that the major copper-bearing mineral is chrysocolla and that the ore is composed mainly of silicate, oxide and carbonate gangue. Calcite in the ore is considered problematic due to acid consumption. In terms of abundance of gangue in the classified ore the order is waste > middling and product. A systematic comparison of the complexometric behaviour of the ore was investigated with Na2EDTA reagent while the leaching behaviour of the classified ore was investigated in NH4Cl and H2C2O4 lixiviants. The influence of process variables such as lixiviant concentrations, particle sizes, solid-to-liquid ratio, temperature, time and stirring speed on the behaviour of the classified ore were investigated. The rate of Cu dissolution and formation of Cu-EDTA complex in Na2EDTA was higher in the product than the middling while that of the waste was found to be insignificant at 0.01 M, even when the concentration of the complexing agent was increased from 0.01 M to 0.05 M. The rate of Cu-EDTA complex formation was found to increase with decreasing particle sizes and solid-to-liquid ratio across the ore categories. Ammonium chloride leaching of the product and middling revealed contrasting behaviour. The rate of Cu extraction was found to increase significantly from 21.5 % to 75.0 % and 27.3 % to 89.0 % when the temperature was increased from 40 ˚C to 90 ˚C, respectively. On increasing the concentration of NH4Cl from 0.5 M to 5 M, the extraction of Cu was found to increase from 20.0 % to 65.0 % and 26.5% to 83.3 %, respectively. It was found that the leaching yield of Cu increased substantially when the particle size was decreased from –125+90 μm to –90+63 μm and –63+45 μm and with decreasing solid-to-liquid ratio (middling and product), respectively. A steady decrease in Cu extraction was obtained when the stirring speed was increased from 300 rpm to 800 rpm. Similarly, the effectiveness of the leaching process was investigated over an extended time period from 2 h to 4 h with an NH4Cl concentration range of 0.55 M to 1.65 M, a temperature of 70 ˚C to 90 ˚C, and at a constant stirring speed of 300 rpm, particle size fraction of -64+45 μm and a solid to liquid volume of 6 g/ 250 mL. It was found that Cu extraction was enhanced by about 90 % during the experiment. The estimated activation energy of the leaching process was characterized using the shrinking core model under the experimental conditions. It was found to range between 45 and 71 KJ/mol in the first and second batch experiments, which is indicative of a chemically controlled leach process. XRD and ICP-MS characterization of mineralogical and chemical composition of residues suggested that the NH4Cl lixiviant leaching is selective for Cu. Examination of leachate with ICP-MS for co-extraction of Mn, Co, Ni and Zn indicated insignificant solubilisation of the metals during leaching. Comparison of Cu extraction in NH4Cl and H2C2O4 under the same experimental conditions revealed that NH4Cl is a better extractant than H2C2O4. Furthermore, the electrodeposition of Cu metal was studied with ore leachate containing Cu(NH3)42+ complexes and with Cu(NH3)4SO4 synthetic electrolyte. Cyclic voltammetric measurements were conducted across a range of cathodic potentials from 0.8 V to – 1.0 V for selected scan rates of 20, 30, 50, 100 and 200 mV/s. Chemical reduction and electrodeposition of Cu from the complexes was found to proceed via two reversible electrochemical processes, each involving the transfer of a single electron. Cu(NH3)42+ complexes are first reduced to Cu(NH3)2+, which is in turn reduced to metallic Cu. The result is compared with the behaviour of synthetic Cu(NH3)4SO4 electrolyte. It was observed that the reduction of Cu(NH3)4SO4 to metallic Cu proceeds as two sequential, single electron transfer processes. The Cu/Cu(NH3)42+ redox reaction was observed to be fast compared to Cu/Cu2+ redox reaction in the Cu(NH3)4SO4 solution. Investigation of the electrochemical kinetics shows that the cathodic peak current varied linearly with the square root of the scan rate, which is indicative of the Cu(NH3)2+ and Cu(NH3)4SO4 reduction to Cu proceeding through a diffusion-controlled process. Assessment of the effect of calcite for leaching of copper from classified ore fractions indicated the potential of NH4Cl lixiviant for the leaching application. Three processing routes to handle the ore fractions on the basis of variation in calcite, gangue and copper content are proposed.
Supervisor: Hylke, J. Glass Sponsor: Nigerian Tertiary Education Trust Fund (TETFund)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available