Pilot scale testing of the SepAir on Witbank coal seams

Abstract

There has been an increase in focus on dry beneficiation technology over the last couple of years due to water scarcity and the environmental impact of wet beneficiation. Dense medium separation uses a large amount of process water and is very expensive. The process water in use poses a significant environmental risk if not treated, which consequently adds to the high cost associated with dense medium separation. There has been extensive research into a variety of processes, and all have their benefits and limitations. The SepAir technology, which is pneumatic separation, has been showing promising efficiencies. The material is fed to the SepAir unit over a perforated mesh conveyor that screens out the ultrafine particles with the remaining material separated by means of density. The separation takes place when an upwards draft is induced through suction into a vortex section. Although the SepAir has performed well on Northern hemisphere coal, it is not sure how this technology will perform on Southern hemisphere coal. South Africa forms part of the Southern hemisphere coal deposits, which is difficult to process in comparison with the Northern hemisphere coals. This is due to the high near-density material present in Southern hemisphere coals. A SepAir pilot plant was purchased to test South African coal, and more specifically the Witbank coal deposits. The tests were conducted using the aforementioned pilot plant which has a capacity of 40tph. Various seams of the Witbank coal deposits were treated on four different particle size ranges, namely, 10x25mm, 10x50mm, 25x50mm, and 0x50mm. The tests considered three independent variables, which were: feed to plant, the speed of the mesh conveyor, and the speed of the suction fan. The performance of the SepAir was measured in terms of yield, cut-point density, Epm, and organic efficiency. Samples were taken for each test and sent for float and sink analysis consisting of fractional analysis done for proximate analysis, calorific value (CV), sulphur, and abrasive index. The aim of the project was mainly to test the maximum feed quality upgrade achievable by making use of the SepAir. Desirable results were achieved from the test work conducted as significant CV upgrades and ash, sulphur, and abrasive index reductions were observed. Cut-point densities varied from 1.4 kg/m3 to 2.2 kg/m3, depending on the product quality specification aimed for, while Epm values of 0.06 to 0.24 were obtained. The SepAir's efficiency is however highly dependent on the amount of near density material present and at the low cut-point densities and consequently high feed quality upgrades, the yield was compromised. It was also found that high amounts of misplacement were present, but fortunately more misplacement of the product (floats) was observed in the discards (sinks) than vice versa. It was therefore decided that due to the high amount of misplacement present and the low yields obtained that further test work was required on testing the capability of the SepAir to perform double stage beneficiation. The primary stage consisted of treating the feed material for a maximum upgrade and minimum yield to achieve export coal requirements. The discards were then retreated to run for a maximum yield and achieve domestic market specifications. This resulted in desirable upgrades as well as an increase in the overall yield. The results indicated that the SepAir can treat South African coal at high feed quality upgrades and at a desired efficiency.