March was the most successful month ever experienced using a shortwall, at the Matla No 3 mine in South Africa. The mine produced an amazing 722,586 tonnes from a 130m wide face, a world record for a shortwall. The same mine achieved the world’s best previous record of 679,043t in April 2003.
The Matla complex, located 150km east of Johannesburg in Mphumalanga, is owned by Eyesizwe Mining and supplies coal to ESKOM, South Africa’s electricity utility. The complex consists of two shortwalls, three development and four bord and pillar sections, to produce some 15 million tonnes per annum.
The No 3 mine shortwall was initially commissioned in April 1997 at the No 2 Mine and then moved to Mine 3 in January 2002, mining the 4 seam at an average depth of 90m below surface. The second shortwall was commissioned in 2002 in the lower (approximately 110m) No 2 seam at Mine 2.
The roof over 4 seam consists of coal, shale and massive sandstone and resistance to caving is compounded by the short face. The coal is very hard due to an average UCS (unconfined compressive strength) of 25Mtpa. Recoverable reserves for the shortwall measure 58Mt.
Since commissioning, the No 3 mine shortwall has achieved generally good results and by March 2004 a total of approximately 26Mt had been produced with the shortwall equipment.
ROM output from the shortwall in calendar 2003, totalled 5.093Mt with development tonnage of 2.277Mt and one bord and pillar section mining 708,000t, for a combined ROM total of 8.087Mt. Shortwall output to development output is about 69%.
The forecast total output for 2004 from the No 3 Mine is 7.2Mt from one shortwall and two development sections.
The March record caps off an excellent run for the operation. Total mine output for the month was 896,458t, including development tonnage, equating to 2,409 tonne per man per month. Shortwall productivity measured 9,032t/man/month during March.
The record was achieved during the mining of the 3km long panel 13, which took a total of four months and two days to complete. In-panel tonnes measured 2.394Mt and average tonnes per month totalled 590,000t.
According to production manager Chris Silver, several factors contributed to the record. These included excellent ground conditions contributing to the high rate of retreat, the pantechnicon’s ability to handle the high rate of linear retreat, an effective underground transport system which includes road building and non-flameproof vehicles, the older, more experienced workforce (average age 44) and excellent labour relations between management and unions, and the experienced and highly dedicated management team.
Some interesting facts relate to the shortwall’s performance – ‘the magic sevens’ as mine personnel call them.
When the record was achieved the Joy 6LS05 shearer had completed 7,000,000t since its last major overhaul and mined the 722,586t that March. The shearer has had seven major overhauls since 1997.
Shearer engineering costs in panel 13 were 77c per tonne (in South African currency, which equates to about $0.20/t). The record of 514,522t held by the No 2 Mine has been improved seven times since the 4 Seam shortwall was relocated in January 2002 from the No 2 Mine to the No 3 Mine.
On the equipment front, Matla No 3 mine has a Joy 6LS05 shearer, with installed power of 1500kW and a drum diameter of 2.29m. The shearer operates at an average cutting height of 3.82m and uses a web cutting cycle known as the Opti-cycle which has been shown to be 15% more productive than bi-directional cutting.
This is based on the principle that the web is extracted in two passes from the face with a half face snaking system and bottom, or top and bottom benches extracted on the second pass. This allows the shearer to cut at a higher haulage speed.
In recent years face move time has been reduced from around 30 to 21 days but the mine is aiming for a further reduction to 14 days. To achieve this a second Joy 6LS05 shearer was purchased and the mine is currently in the process of building and purchasing a second pantechnicon.
The armoured face conveyor is a DBT supplied stage loader crusher, roller curve, pan line and a non- driven return end with installed power of two 400kW motors, rated at 2,500tph. The shields are 2-leg 880t, DBT supports, controlled by a PM4 control system operating within a working range of 2.1-4.2m.
To date these supports have seen approximately half of their rated cycles of 30,000. Structural modifications are being made to the top canopy and backshield by means of mechanical locks and normal repairs.
The Nepean supplied trunk coal clearance system utilises a 1,650mm wide conveyor running at 4.5 metres per second with BOSS (belt optimum soft & slave) drives, designed to handle 4,500tph with a total of 8 x 300 kW powerpacks, hauling coal a distance of 4.4km.
The shortwall panel length of 3,000m and the increased tonnage (3000tph) have necessitated a 1,350mm wide gate conveyor running at 3.8m/s drive powered by 4 x 260kW BOSS drives and a tripper drive powered by 2 x 260kW BOSS drives, Nepean’s proprietary drive technology. This was the first time a single LCDC (long centre distance conveyor) was used at the operation.
Recent upgrades were made to the incline conveyor to incorporate Nepean BOSS drives (4 x 260kW powerpacks), 45-degree trough frames, and class 2000 steel cord belts. Pulley diameters were changed to attain a belt speed of 4m/s and deliver coal at 3,300tph.
Availability of gate conveyors typically averages 98% over a complete panel, partly achieved with clip joints rather than vulcanised splices. This speeds up belt repair, removal and maintenance.
March was also the first month the mine worked a new three shift cycle on the development sections. This was introduced because development advance rates were failing to stay ahead of the shortwall. All production crews now work the same type of shift rotation, which facilitates better utilisation of the maintenance window, mine-wide.
Prior to this Matla development crews worked an average of two shifts in 2003, averaging 49,000t per month. The three shift structure has increased tonnage to 84,000tpm, and given a 71% increase in normal time production.
The mine has a four hour maintenance window (from 06:00 to 10:00), five days per week, with a two hour shift overlap taken up by transport to the section, a pre-shift safety group meeting and a production planning meeting with the miner of the operating shift. During the shift rotation/overlap, the machine operators hand over at the machines to affect a “hot seat” change.
Shearer motors and gear cases are monitored by the OEM (original equipment manufacturer) and the results are made available to mine personnel. Preventative maintenance is carried out to avoid costly failures of critical components.
The mine’s condition monitoring department monitors development sections and the condition of conveyors. Condition monitoring covers vibration, oil and debris analysis.
Equipment used in the two development sections, driving the three development roadways, are two Joy 12HM9 continuous miners, supported by six Joy 10SC 22 D CAR (20t) shuttle cars, two Ram roofbolters and two Buffalo type feeder breakers.
Available cutting time averages 90-100 hours per week for production units and sections, Monday to Friday. System engineering availability averages 92%.