Coal Processing

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.

Co-treating coal discards with sewage sludge by hydrothermal carbonisation gave hydrochars with better physicochemical properties than treating either waste alone. The blended hydrochar reached a surface area of 20.35 m2/g and a pore volume of 0.38 cm3/g, and removed 97.19% of nevirapine and 93.32% of lamivudine from recycled water. Routes to activated carbon and hydrogen storage were also assessed.

No single method gives a continuous, real time report on how a coal preparation plant is performing. Mass meters converted to yield values, on-line ash and moisture monitors, load cell chutes and automatic medium density control together supply most of what is needed. Measuring discard and slimes tonnages, particularly slimes disposal, remains the weakest point in plant accounting.

Run of mine and discard coal carried total rare earth contents of 260 and 265 ppm, above the 130 ppm cut off grade used for coal. Flotation with an emulsion binder did not raise those concentrations in either the product or the tailings streams. Calcination did, lifting totals to between 285 and 289 ppm, with most rare earths leaching within the first fifteen minutes.

Nine bituminous coals from the Witbank, Waterberg and Soutpansberg coalfields all formed compacts without a binder, and the process worked better with coking coals. Reactive maceral content, free swelling index and Hardgrove grindability index governed briquette strength, with ash and kaolinite contents also contributing to bonding. Briquetting pressure, feed moisture and particle size distribution controlled the outcome.

The separator handled fine coal and produced a 28 MJ/kg product, but its measured separation efficiency was lower than that of conventional dense medium cyclones and it was harder to operate. Two efficiency tests produced no evidence that the unit outperforms cyclones on fine coal, so cyclones were recommended when the plant was rebuilt at its next site.

Four centrifugal concentrators all cut ash in fine coal, with desliming the decisive step. The best unit took ash from 35.2% to 18.47% and sulphur from 0.68% to 0.36% in a single stage, raising calorific value from 23.78 to 26.16 MJ/kg. Pilot work reached 19% ash on thickener underflow and reconstituted spiral feed.