Integrated Mining and Processing

Fine coal was screened at small apertures while surface moisture stayed below about 5 per cent. Above that, fines adhered to the coarser particles and screening became very difficult. The dual vibrating polyurethane mats stopped blinding and clogging, so screening area stayed available, but the screen action still could not dislodge fines from very damp coal.

A rating and ranking system was applied to South African coalfields and, separately, to the coal bearing countries elsewhere in the Southern African Development Community. The Waterberg ranked highest in South Africa and is the coalfield most worth detailed investigation as Witbank output declines. Botswana ranked highest among the other member states on contained resources and exploitation potential.

Tonnage estimates for the remaining low phosphorus resource could not be produced, because the phosphorus analyses supplied by mining companies did not state what had been measured. Distribution trends were still mapped. Coal at or below 0.010 per cent phosphorus is widespread in the Witbank Coalfield, restricted to the far northwestern Highveld, and follows a northwest to southeast band in eastern Ermelo.

Electrical and electromagnetic ground surveys were run over a Waterberg profile with borehole logs and three dimensional seismic data as control, to test whether cheaper methods can match seismic reflection for locating coal and faults. Resistivity tomography at two electrode spacings, frequency and time domain electromagnetics, controlled source audio magnetotellurics and magnetics were all acquired. Results are preliminary.

A compound dry cleaning machine de-stoned low grade raw coal without using water, but separation efficiency was low, with high relative density cut points and EPM values in the order of 0.3. Misplaced material was heavy and coal below 6 mm reported to product almost indiscriminately. The unit is cheap to buy and run, needs only crushing to 50 mm, and gives high throughput.

Dense medium cyclones on the fine coal fraction lifted product yield well above the spiral circuit they were compared against and gave closer control of product quality. Cut point density control proved the hardest part of running the plant, medium properties being critical. Magnetic separator capacity had to match the duty, and a basement was needed to contain and recover medium spillage.

Air blowing converted coal tar into a pitch with 11.27% volatile matter and 87.68% fixed carbon, which bound high ash coal fines into structural composites. Compressive strengths ran from 106.58 to 344.71 MPa and flexural strengths from 48.75 to 159.30 MPa. Flammability was low across all composites and corrosion rates stayed below those of some commercial ceramic tiles.

Geotechnical factors affecting secondary extraction were identified from interviews with collieries already re-mining pillars and with people experienced in stooping, then quantified and ranked against the different aspects of the work. No single factor governs the outcome. Factors interact, so choosing a method and working safely in old workings depends on managing them together across disciplines.