Published in September 2005 Australian Longwall Magazine
SEVEN years after an ominous beginning, Anglo Coal’s Greenside Colliery now produces 3.6 million ROM tonnes per annum and is one of the company’s star performers, regularly breaking continuous miner records.
The mine, some 110km east of Johannesburg in South Africa, did not get off to the best of starts. When Anglo Coal originally bought the operation from Gold Fields Coal it chopped the workforce from 1200 to 600 by closing down the No. 1 Seam operation. This retrenchment, in the wake of the labour reductions brought about by the introduction of the Joy continuous miners in the No. 4 Seam areas, brought about a total labour reduction of about 75% over a two-year period.
A largely inexperienced continuous miner workforce, ripe with suspicion and insecurity, started full room and pillar operations in the No. 4 Seam areas in 1997. So how does a mine with such troubled origins transform itself?
According to the mine’s engineering manager Derek Laing, success is based on three criteria: a clear production management system, utilization of the best available technology and a labour force with the Simunye (“being as one”) spirit.
Greenside extracts low-volatile steam coal from the 3.4m No. 4 Seam and has a mine life expectancy of 27 years. Most of the output (2.5 million tonnes) is exported via Richards Bay Coal Terminal to European buyers.
The mine has a good sandstone floor and competent roof, but it is currently experiencing difficult conditions, including a soft floor and a laminated shale roof, with the move to the new middle-east block.
Greenside’s 580-strong workforce operates 3.5 production sections with four Joy 12HM31 continuous miners fitted with Megahead cutter drums. Three of the units have been named by the work force: Mapula (new beginning), George (in memory of a mine overseer) and Thandeka (a peace singer during the apartheid struggle). A rotating system of two production and one maintenance eight-hour shifts is utilised. Each production shift is manned by 11 people, while four men crew the maintenance shift.
When production began at the mine, the team aimed for monthly output of 55,000t per section to justify the capital cost of the mine. In its first full year of production, Greenside reached this goal.
“This was great but then we started hearing about some ‘millionaire’ sections in the industry and decided that that was where we wanted to be. The Phambili process was started and Greenside took off. The combination of Phambili (meaning to go forward), reliable equipment and Simunye people produced results that exceeded our greatest expectations,” Laing said.
Production increased from 55,700t per section per month in 1998 to more than 100,006t/section per month in 2002. Last year the mine produced 106,760t/section per month on average.
As well as the continuous miners, Greenside uses a combination of Joy 16t- and 20t-shuttle cars, 20t-Stamler-BH20 battery cars and 16t-DBT 818 battery cars. All four sections are equipped with Fletcher twin boom roof bolters and Stamler or FFE-Buffalo feeder breakers.
The Greenside policy has been to try and stay one or two steps away from the cutting edge and rely on new but tried and tested technologies. The one deviation in this policy has been the purchase of the latest in continuous miners – the high voltage Mark 11 – the first of its kind to be manufactured in South Africa. Greenside took delivery of the unit in February 2005.
The Mark 11 has a 3.3kV supply and cutting motor, allowing a 50mm cable to be used, which is lighter and easily handled in sections, as well as reducing hot connections and circuit breaker failures. The traction motor system has been changed to an AC variable frequency speed controlled drive and upgraded to 60kW. The cutter motors have been upgraded to 220kW, giving more power to the cutting picks, and has the potential to increase cutting rates.
The new Mark 11, which cost 20 million rand (almost $US3 million) produced 140,060t in April. “Great things are expected from this unit,” Laing said.
Greenside can also attribute its equipment reliability to clearly identifying its downfalls and attacking these, together with the OEMs concerned. The mine’s single biggest technology problem was on the continuous miners’ 37kW DC traction motors. Increasing production rates meant increased stress on the miner’s traction system. The less frequent failures from water ingress and commutator problems were fixed with an additional seal on the power plug and socket arrangement, and stall protection software to halt commutator failures.
However, over-temperature accounted for 80% of failure. The mine maintained the thermal rating of the motor and the cooling system was too low for the production rates being achieved. In March 2004, new 50kW traction motors were ordered at R350,000 each. Since the installation of the new motors, Greenside has only experienced one failure.
The mine keeps a very close eye on production, measuring elements such as cutting time and breakdowns on a minute-by-minute basis.
“The production equation is very simple. Production equals cutting time multiplied by cutting rate. Simply put, the longer you cut for and the faster you cut the more tonnes you get. The key to cutting time is utilisation and the key to cutting rate is operating efficiency,” Laing said.
Cutting time is measured by the control system on the continuous miner – programmed to record time elapsed while the cutter drum motors are drawing more than 75% of full load current. The system starts recording time when sumping is commenced, continues through the shearing process and ceases once trimming the floor has been completed. Time not spent in production is accounted for and eliminated or minimised.
The cutting rate is calculated by dividing the estimated tonnes produced by the cutting time in hours. This gives a figure in tonnes per hour, indicative of the overall efficiency of the cutting cycle.
“This figure reflects the skill or efficiency of the continuous miner and coal transporter operators. It also reflects...click here to read on.
