South Africa is one of the world’s leading nations in the mining, preparation and use of coal for its internal energy needs and the export market, says project house DRA Mineral Projects process engineering consultant Mark Cresswell.

From a yearly production of about 250-million tons, about 30% is exported, mainly to Europe, with the balance increasingly going to the Asian markets, where South African coal export first began in the mid-1970s at a modest rate of five-million tons a year.

Owing to its low sulphur and relatively low ash content, the exported coal fraction is highly sought after as a blend coal. This, in turn, has led to an internal industry model where the majority of the 0,5-mm bituminous run-of-mine (ROM) coal in the Witbank, Highveld and Ermelo coal-fields is typically separated at a low density to generate a float export fraction.

This model is being used at the latest large capacity DRA-designed coal preparation plants such as Mafube, a 940 t/h plant for mining giant Anglo American Corporation (AAC) and coal-miner Eyesizwe Coal, commissioned during 2008; Phola, a 2 380 t/h plant for a joint venture between AAC and mining giant BHP Billiton Energy Coal South Africa (Becsa), currently being commissioned; as well as Douglas Middelburg, a 2 200 t/h plant for Becsa that should be for commissioned in May 2010.

Cresswell says that these plants are pioneering the use in South Africa of the next- generation large-diameter dense medium (DM) cyclones, of 1 150 mm diameter, and banana screens that are 4,2 m in width, together with the latest technology in remote supervisory control and data acquisition control systems. Use of large unit equipment and an efficient layout has led to the Mafube plant being the most energy efficient coal plant in the AAC group on a kW/t/h basis.

South African coal occurs as opencast and relatively shallow underground coal deposits, often at the more mature mines. ROM crushing circuits are customised to each mine type with the opencast mines, after primary crushing often making use of a rotary breaker in a secondary crushing role to also take out some of the harder inter-seam burden. Primary and tertiary crushing sections are increasingly being simplified by making use of single, large double roll crushers in an open circuit configuration.

This technology is being used in DRA’s latest plant design for a coal project to supply a low sulphur feed for the Mmamabula power station, in Botswana. The 1 000 t/h Serorome plant will use innovative dry screening technology to enable the 50 mm × 6 mm fraction to be washed in a high density dense medium separ- ation (DMS) cyclone plant while the –6 mm raw coal fines bypass the plant and join the coarse float fraction. Online laser plasma coal analysers will be integrated into the plant control system to control the proportion fed to the DMS plant and washing density.

DMS is the predominant technology for separating the +0,5 mm coal with topsizes increasingly set at 50 mm, but in the past up to 150 mm. This has allowed DRA to design DMS plants which make extensive use of large-diameter pump-fed DM cyclones. This has led to the requirement for ever larger product screens in order to keep to a unit equipment design philosophy, which leads to lower maintenance costs. Recent plants have, therefore, used 4,2-m-width banana screens in common with recent plants in Australia and China.

New fine coal treatment technologies are increasingly being added to the traditional gravity spiral plants, such as teetered bed separators, fine coal DMS, which substitutes for spirals, and flotation. DRA built the first Witbank flotation plant at Greenside, a 60 t/h plant for AAC, in 2004, and followed this up with a 100 t/h plant at AAC’s Goedehoop plant using the same type of flotation cells developed by AAC.

Rather than using solid bowl centrifuges for concentrate dewatering, owing to the increased tonnage, two giant fully automatic Lasta filter presses from Ishigaki, in Japan, are used. These can dewater the flotation concentrate to 20% moisture, which makes it viable as an export coal product.

At the Phola and Mafube plants, the lack of slimes dams and a desire to significantly reduce the raw water consumption meant that slimes dewater- ing plants had to be incorpor-ated into the plant design. At these plants, the filter cake can be directed to either the middlings product or discard product if the quality is too poor. Space has also been left for a future flotation plant and an additional DMS module.

Cresswell says that DRA has been building at least one coal plant since 1986, mainly small DM cyclone or drum DMS plants. In their modern guise, using large diameter cyclones and banana screens, these are typified by the 360 t/h second DMS module at Forzando colliery, which is a twin of the first module built there by DRA in 1995. This pioneered the use of large 1 000-mm- diameter cyclones in South Africa, together with a 3-m-wide banana screen. This technology was shortly followed up by the design and construction of the 1 430 t/h 1 000-mm-diameter gravity- fed cyclone plant for petrochemicals giant Sasol Mining, at Twistdraai during 1996.

The 1 000 mm pump fed DM cyclone modular plant design has since been adapted for use in different plants, such as at the Sebuku colliery, at Inyanda, for Exxaro and, currently, at Dorstfontein for petrochemicals company Total Coal.

Further afield, DRA has been involved in two coal plants in China, which have been designed to produce a coking coal as well as a mid-dlings coal at finer sizes than those encountered in South African coals. At Xiezhuang there are two types of ROM coal with different separation characteristics and the concept is quite different from the standard South African model. A high density cyclone removes a discard sinks fraction while spirals are used as preconcen- trators of the –3 mm fines ahead of a low gravity separation.

Extremely low separation densities are used, down to SG 1,3, which requires the use of fine coal for medium stability to produce a 7% ash coking coal product. There are high levels of near-gravity material and a sharp separation is critical to increasing the efficiency of the recovery. Flotation is included to recover fine and ultra fines while reflux classifiers are used to clean the spiral concentrate 0,6 mm × 0,2 mm fraction.

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