Matla shortwall long on performance

Technical problems have not stopped South Africa's Matla mine from becoming arguably the world's best-performed shortwall operation.

Staff Reporter

South Africa’s Matla shortwall mine produced 3.69 million tonnes of coal last year, a result which would have put it among the top four Australian longwall operations in the same year. Considering the Matla shortwall is half the length of the top echelon of Australian longwalls, its performance was significant.

About 150km east of Johannesburg in the province of Mpumalanga, South Africa, the Matla complex is situated in the Highveld Coalfield and consists of three underground mines: Matla No 1, No 2 and No 3. Between them, the mines produce 12-15Mt per annum of raw coal for the nearby Matla power station.

Coal is produced from three contiguous seams - the 5, 4 and 2 seams - with the average depth to seam below surface of 60m, 90m and 116m, respectively. The average seam thicknesses for the three seams are 1.5m (5), 4m (4) and 5m (2).

One shortwall face, which is in fact a 114m longwall, has been in operation at the Matla No 2 mine since 1997. The mine is preparing to install a second shortwall face, expected to add about 5Mtpa (including development coal) to the 5-6Mtpa already produced by the No 2 mine. The second shortwall will replace existing production capacity from the No 1 and No 3 mines presently exploited by continuous miner bord and pillar methods.

Matla was the first mine in South African to develop a mine based exclusively on continuous miner bord and pillar panels - at the time unproven in the country. The average production per continuous miner section at Matla, including shortwall development, is about 60,000 tonnes per month. Matla No 2 mine is one of only three coal mines employing long/shortwall mining methods in South Africa.

Until 1998 Matla used the longwall mining method in 5 seam, but the operation was stopped in 1998 due to the abrasiveness of the coal as well as production cost considerations.

In 1993 the mine began trialling a thick seam shortwall face on the 4 seam and early in 1997 the present shortwall was installed. Block dimensions were designed with 115m face width and lengths of 3000m. In calendar 1998 the shortwall produced 2.5Mt (excluding development tonnage which totalled 1Mt), rising to 3.7Mt excluding development tonnage) in 1999. This was based on three nine-hour shifts, 5.5 days per week. Forecast production for 2000 for the shortwall is 3.3Mt.

After the success of the 4 seam shortwall, Matla decided to develop a second shortwall in 2 seam to increase the extraction ratio of the reserve. As part of the project the mine’s conveyor system is to be upgraded from the current 1350mm running at 3.8m per second. Each shortwall will have its own coal clearance system consisting of 1650mm conveyors running at 4.5m per second which discharge into underground bins.

One of the challenges with a shortwall is keeping development ahead of mining and Matla achieves average development advance rates of 25m per shift.

“We have high development rates because we don’t have to do as extensive bolting and meshing, and we do no sidewall bolting as you do in Australia,” said production manager, Heine Booysen. “If we leave a coal beam of between 500-600mm below the shale then we encounter almost no roof problems at all. In two of our development sections we cut and bolt simultaneously, and our machine is not held up by the bolting pattern.”

The system of roadway development is two and three chainroad developments. In 4 seam pillar centres are at 20x50m and at 25x60m centres in 2 seam.

Development equipment for 4 seam includes Joy HM9 drum miners with 15t shuttlecars and one roofbolter per section. Voest-Alpine ABM30s are used on 2 seam serviced by 20t Joy D-cars.

Matla’s very hard coal (UCS 35MPa) means there is no free coal and the face does not slaugh or spall. Every tonne must be cut, Booysen said. Matla’s maintenance regime includes tracking tonnes per pick, which average 180t per pick, although but when stone is encountered this drops to 50t/pick. When stone is encountered, picks had to be changed after every web, Booysen said.

An initial problem with the 4m seam and hard cutting conditions was that the ranging arms on the shearer had to be upgraded to handle the conditions. Booysen said the main challenges in introducing a second shortwall were in changing the culture from a predominantly continuous miner mine to a shortwall mine, and finding the right people for the job.

“Everyone at the mine was au fait with the workings of bord and pillar but not everyone understood the workings of a longwall,” he said. “A different level of day to day and long term planning is required as all your eggs are now in one basket, production is cyclical, and when you have shortwall moves few tonnes are produced.

“There are also not many people experienced with shortwall face control, floor control, etc. You can’t afford to spend that kind of capital and end up burying the face.”

Water inflow is also encountered as the reserve is very shallow and there are three goafed zones (5 seam, 4 seam and 2 seam) on top of each other. Booysen does not anticipate pit room problems underground as each set of shortwall equipment will be serviced by two separate development units. In addition the faces will be on two, superimposed seams.

An area of concern for Matla is the fact that there are only four shearers in South Africa. Very little interchangeability of sub-assemblies between shearers is possible. This increases costs and makes ongoing support problematic, according to Booysen.