The use of magnetite as a medium in the wet processing of coal has been used since the early days of dense medium separation. The high magnetic susceptibility and density of magnetite make it an ideal medium to use in wet coal beneficiation because it is relatively easily and successfully recoverable. Owing to the need for more sustainable technologies, Coaltech has been investigating alternative dry processing processes: the Bohou process (developed in China) was identified as a possible feasible option. The Bohou process comprises dry dense medium separation using magnetite as the medium. The recovery and re-use of magnetite are, however, problematic. The aim of this investigation was to determine how efficiently magnetite can be recovered and to identify the factors influencing the magnetite losses during this dry processing.
The test work for the project was divided into two phases. The aim of Phase 1 was to identify the magnetite losses to the oversize coal fraction for different moisture conditions of the coal and magnetite. In Phase 2, magnetite and high-titanium magnetite (an alternative source of magnetite) were used to conduct test work to determine which medium could be successfully recovered from fine coal. Magnetite or high-titanium magnetite was mixed with the coal sample as a medium. During Phase 2, the effects of using different screens and different moisture conditions were investigated. For both phases, the samples received were divided into three categories containing different moisture contents: dry coal and dry magnetite, dry coal with wet magnetite (4% to 4.4%), and wet coal (3.5% to 6.5%) with dry magnetite. In Phase 1, the coal samples were screened at 13.2 mm, the oversize mixed with magnetite, and then screened again with a 13.2 mm screen: the magnetite losses were then recorded. For Phase 2, the prepared samples were screened at 3 mm, 13.2 mm, and with a 3 mm high-frequency screen. The undersize was passed through a low-intensity magnetic separator. The recovered magnetite was then passed over a magna chute to recover additional magnetite.
The results for both phases indicated that the highest recovery of magnetite occurred when dry magnetite and dry coal samples were used. The samples with wet magnetite also gave high recovery, but the samples with wet coal were detrimental to recovery and significant losses were observed. It was found that the magnetite stuck to the surface moisture of the coal. The use of a high-frequency screen improved recovery of the magnetite from the wet coal samples from 45.38% to 74.27%. Recovery from the high-frequency screen for both dry and wet magnetite samples was lower than that achieved with a conventional 3 mm screen.
The test results indicated that magnetite can be recovered in the dry beneficiation of coal when the surface moistures of both the coal and magnetite are controlled. Use of a high-frequency screen can improve recoveries only for conditions where the surface moisture of the coal is high.
Dissertation (MSc (Metallurgy))--University of Pretoria, 2021.