Integrated Mining and Processing

Ground based radar mapped rehabilitation soil profiles well enough to replace much of the auger sampling normally needed, and a 100 MHz system picked out anomalies at about 5 to 6 m depth that were likely old mining voids and related subsidence. Penetration below roughly 6 m needs lower frequencies, which cost resolution and require larger antennas.

The fine coal dense medium pilot plant was rebuilt at South Witbank Coal Mine and run on fine coal, with tests covering single, double and three stage processing, the effect of recirculating load, magnetite size and losses, separation efficiency and continuous operation. The fast screen could not deliver the required capacity and was replaced by dewatering cyclones to deslime the feed.

Microgravity, resistivity based methods including self potential, and time domain electromagnetics are identified as able to detect and delineate shallow old workings, with controlled source audio magnetotellurics useful below about 30 metres. How well each performs depends on target depth, the material filling the void and the overburden, and has never been quantified. Field trials were planned to close that gap.

Micro-gravity, electrical resistivity and time domain electromagnetics located the boundary between old workings and virgin ground, but no ground technique tested could resolve individual bords and pillars at the depths typical of the Witbank and Highveld coalfields. Airborne thermal infrared surveys worked well as a low cost regional reconnaissance tool for shallow sinkholes, fires and possible pollution plumes.

No operating full scale frictional ignition test facility was found anywhere in the world, and building one was judged expensive, impractical and unlikely to justify its cost against laboratory scale testing. Effort should instead go to small scale laboratory equipment using South African rocks, and to expanding geological data such as silica distribution and petrographic analysis.

No standard technique for predicting, controlling or extinguishing spontaneous combustion emerged, and the methods in use proved expensive and not routinely successful. Because every mine differs, the work sets out a sequence of locating the fire, assessing the risk, choosing a strategy and method, then monitoring it. Separate control measures are given for old workings, exposed coal and spoil heaps.

No flameproof mechanised system for cleaning under and around moving conveyors was found to exist anywhere. Of more than 200 mini machinery manufacturers identified worldwide, few build for mining, and only one or two products were near ready for surface use. Manufacturer interest in mining applications has since risen, but the industry has not settled on which technology to back.

Horizontal air winnowing separated fine coal by particle size and density in laboratory experiments, supported by equation of motion modelling and computational fluid dynamics. Modelling indicated that a chamber 0.6 m high and 1 m long is sufficient to displace any minus 6 mm particle at air velocities of 10 to 30 m/s.

Numerical modelling produced upper and lower bound strength formulae for rib pillars, the lower bound covering weak roof and floor contacts. The formulae apply between width to height ratios of 0.5 and 2.5, with linear interpolation below that, and give conservative designs above 3. Adjustments were suggested where regular joint sets occur.

Back analysis of 77 collapsed and 245 uncollapsed pillar cases showed that much of the scatter in pillar strength comes from differences between coalfields, so a single national formula is unrealistic. Seam specific formulae were derived for the major coalfields; Free State and KwaZulu Natal data remained too sparse. The lognormal safety factor distribution was retained.

A 30 tonne per hour pilot Allair dry jig upgraded run of mine coal to thermal coal specification, but could not effectively beneficiate finer duff. Cut point densities and EPM values matched other dry processing equipment, though the jig appeared to give better control of the parameters governing separation. Four tests ran on run of mine coal and two on duff.