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The large-scale production of mine wastes and their secure disposal has been a problem of global importance. In this work, the mine waste from Merelani, crude overburden pyrite ore was converted into a value added chemical, sulphuric acid. XRF, XRD, AAS and modified ASTM D-2492 tests were used to investigate the mineral and chemical composition of the ore. It was revealed that the ore comprised about 60% w/w of mineral pyrite (FeS2). The evaluation of XRD pattern shows that the pyrite is of good quality according to the International Centre for Diffraction Data (ICDD).
The elucidation of chemical kinetics of roasting of the ore to form a key precursor in the production of sulphuric acid, sulphur dioxide, revealed pseudo first order kinetic with respect to solid reactant in gas-solid system and optimum roasting temperature of 750° C. The activation energy (Ea) value of 15 kJ/mol was deduced from Arrhenius equation suggesting that the roasting reaction was controlled by diffusion of oxygen through the ash layer to the reacting surface.
The main gaseous product of roasting crude pyrite ore was then converted into sulphuric acid, which apart from providing proper mitigation to the environment, but also serves as a social˗economic income to local people. The grade of sulphuric acid obtained was 40% w/w which is sufficient for battery acid use.
The techno-economic evaluation of a small-scale plant of converting crude pyrite into sulphuric acid was done with the aid of SuperPro Designer simulator. The plant involves five major sections namely: size reduction, pyrite roasting, gas cleaning, formation of NO gas, formation of NO2 gas and acid chamber. Both the capital investment and operating cost
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were estimated by standard engineering cost estimation methods in evaluating chemical process economic viability. The economic indicators showed that the project is feasible with estimated value of NPV (3,503,000 USD), IRR 18.05% and PBP of about 3 years. Sensitivity analysis results were comparable with theory where the unit production cost decrease; the NPV and IRR increase with increasing plant production scale of maximum limit of 1.4. |
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