The design of a mobile concentrator plant for gold extraction form tailings in Tanzania

The advantages brought by a mobile concentrator plant are many. Nevertheless, the knowledge of the materials to be processed is of great importance for the process design, which in turn is used for plant design. Tailings samples from five different locations, which include Tanga, Morogoro, Dodoma, Mawemeru and Mgombani in Mwanza region, were characterized by x-ray diffraction (XRD), optical microscopy, dry sieve and cyanide leaching analyses. Subsequently, the characterization results were used to design the process route for gold extraction from tailings and equipment selection of a mobile plant. Using MODSIM digital simulator and a simple model developed for prediction of free gold recovery, mass balances and optimal process were obtained, which form a base for the economic evaluation of the process. It was found that gold occurs as free grains of 5 – 20 µm size within both fine and coarse fractions of the tailings deposits at a magnification size of 200 – 500x. It was noted that leaching by cyanide reaches its equilibrium and optimum gold dissolution after 23 hours. The recoveries of gold were 80 % and 91 % for composite and Tanga tailings deposits, respectively. Recovery of 91 % gold by cyanide leaching confirmed the presence of free gold observed by optical microscopy. Gold was also associated with silicate minerals, more especially quartz and feldspars and in some cases with oxides such as goethite. Other minerals found in the tailings deposits were chalcopyrite, pyrite, pyrrhotite and hematite for Tanga, Dodoma, Mawemeru and Mgombani, whereas silver telluride and chromium telluride showed a unique character for Morogoro tailings sample. The presence of chalcopyrite and silver minerals showed potentiality of copper and silver production in the gold concentrate. An uncertain character of gold smeared onto other mineral grains was noted, but the exact composition has to be verified by electron microprobe or scanning electron microscopic analyses. Moreover, particle size analysis showed that only Mgombani tailings (24.61 g/t Au) contains high percentage of fairly fine particle fraction whose P. 80 % is 138 µm with high gold concentration in the fine fraction ( 678 µm. The model predicted the optimum concentrate grade ranges from 93 – 5007 g/t for materials with lower feed grade (0.54 g/t) and highest grade (24.6 g/t), respectively, at the cut size of 160 µm. A feed grade of 3.41 g/t Au gives a concentrate grade of 510 g/t Au and was used to evaluate the economic viability of the designed process. Low concentrate grade from the configuration of the spiral and shaking table is due to inability of the MODSIM models to predict the performance of the spiral concentrator for gold deposits. The proposed mobile concentrator plant of capacity 5 t/h is economically viable with a payback period of one year. The economic evaluation shows that NPV at 10 % interest rate is estimated to be $ 26,323,483 which is much higher for investing the same amount of money in the bank ( NPV $ 296,403). It means that it is better for an investor to invest in the designed plant for gold extraction from tailings rather than to deposit money for the bank investment. The economic analysis identified that the most severe risk parameters for the designed process are concentrate grade, gold price and operating costs. Sensitivity analysis showed that the minimum concentrate grade should be 155 g/t for investing on a designed plant. A pilot study is suggested in order to increase the knowledge of the proposed plant and resources.Validerat; 20101217 (root