Mining

Geophysical logging of boreholes above three Witbank seams identified clay rich matrices, stiffness changes, bedding planes and the principal stress direction, and picked up fractures in 30 to 75 per cent of cases. Core logging found most of the same features faster, so routine use was not recommended unless stiffness and stress direction matter.

Practical guidance for opencast operations covers early detection with low cost thermal cameras, prevention, and the treatment methods in current use: water sprays and cannons, sand cladding of highwalls and stockpiles, removal of burning coal by dragline, tipping into water ponds and the treatment of sinkhole areas. A three month full scale trial of sprayed fly ash as an antioxidant is reported.

No surface geophysical method can map individual bords and pillars at the depths typical of the Witbank coalfield, a limit the review attributed to physics rather than to technology, since the targets are small relative to their depth and conductive layers often lie above the seam. Borehole laser scanning remains the workable option, with sonar suited to flooded workings.

Conveyance legislation promulgated in 2008 and enforced soon after left the coal industry without a lawful way to clean near and under running conveyors. The first phase defined the problem, recorded the cleaning practices mines already use successfully, and built a structured database of mechanised platform options worth pursuing. High risk areas remain drives, take ups, snub and tail pulleys and transfer points.

A single float sink test destroys the sample and gives one deterministic answer, so it cannot show how washability changes with particle size distribution. Simulating washability from petrographic images and tomography allows many size distributions to be tested statistically from one sample. Running simulation alongside float sink work during exploration lets the prediction error of each be compared.

Existing thermal imagery of the Witbank coalfield was traced to its holders and assessed for format, quality, resolution and coverage, then brought into one geographic information system with supporting metadata. Satellite thermal bands give a regional picture only; detailed mapping of coal fires or acid drainage discharge needs high resolution aerial surveys.

A three dimensional model of the northern Witbank and Highveld coalfield was built from more than 1100 borehole logs, with strata sorted into 13 major groupings and surface topography taken from satellite elevation data. It runs on open source visualisation software rather than a commercial package. Delays limited coverage to about a quarter of the coalfield.

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.