Underground » Mining Technology and Production
This project has utilised several proprietary and commercially available modelling packages along with field observations and expert opinion to investigate the assessment of LTCC for Australian coal mining conditions. The project conducted detailed investigations into the various parameters known to influence the performance of LTCC in China and adapted these observations and findings to develop an approach that can be readily applied to a potential Australian LTCC mine resource or reserve.
LTCC is a method of extracting thick seams greater than 4.5m. The method employs both coal cutting of the lower portion of the coal seam accompanied by caving and reclamation of the ‘top’ coal. Coal is first cut from the longwall face using a conventional shearer and AFC arrangement working under hydraulic face supports that incorporate a rear coal conveyor and cantilever / flipper arrangement. Face cutting heights are generally in the range of 2.8 to 3.0m. As the support is advanced forward after the shear the rear conveyor remains in place in preparation for the caving sequence. The caving sequence allows the broken coal at the rear of the supports to flow from the goaf onto the rear conveyor and to the gate end transfer. The flow of coal onto the rear conveyor is controlled by retracting the rear cantilevers of selected supports exposing the rear conveyor to the goaf coal which ‘caves’ into the free space. Once an area has been caved the rear cantilever is extended back out into the goaf stopping any further influx of goaf material. The caving process may be repeated at the same position (secondary caving) if further coal is present before the rear conveyor is finally advanced forward under the rear of the support ready for the next shearer cycle.
Depending on the conditions in the mine various caving sequences are employed to maximise the top coal recovery. In many cases the top coal caving is the primary production mechanism from the face rather than coal cutting, and overall face cycle times depend entirely on caving rates rather than shearing rates.
Research conducted previously on coal resources has identified that over 6.4 billion tonnes of measured resources of coal in Australia lies in thick seams, i.e. seams of 4.5 m or greater. There is an urgent need for implementation of effective thick seam extraction technologies in Australia to improve resource recovery and productivity levels in underground coal mining.
The Longwall Top Coal Caving (LTCC) method, which is being used successfully in China, offers a great potential for thick seam extraction in Australia. The LTCC method can be used to successfully extract up to 12 meters in thick seams. Production peaks up to 7 MT per face were reported in Chinese mines with this method. However, successful application of this method in Australia requires a detailed understanding of the caving mechanics of the LTCC method, gas and dust control options over the rear AFC and the effect of top coal caving on the overall performance of the system.
The major benefits of the effective introduction of LTCC to the Australian Coal Industry include higher coal recovery rates and less development per longwall tonne mined, which has substantial economic benefits. The reduced cutting height of initial extraction offers a much safer working environment with improved face control. It also reduces the risk of spontaneous combustion with less coal remaining in the goaf. LTCC will offer an alternative method to mine thick coal seams economically and safely in Australian conditions.
The main objective of the project is to develop design tools and technology to facilitate the application of the LTCC method in Australia. The project has developed a geotechnical top coal caving model and assessment procedure, an understanding of the environmental hazards and an optimum top coal mining process model. The project has built on previous work carried out on LTCC by CSIRO, the University of NSW and the Yankuang Coal Group and can offer a suite of tools to provide detailed caving assessment and LTCC design services.
The project work involved detailed investigations into the three key technical components of the LTCC method. Geotechnical assessments, environmental studies and LTCC processes will be assessed in detail, design tools developed and optimum parameters determined to assist in the successful introduction of the LTCC method into Australia.
The project has characterised various coal seam parameters on the basis of caving and fragmentation characteristics and developed methods to understand the actual top coal breakage mechanism and the effect of various mining and shield parameters on the top coal caving operation. The project developed CFD models of LTCC faces to investigate gas accumulations and dust flows over the rear AFC area. The project also determined optimum heights for primary and caving cycles and optimum operational processes for various Australian site conditions.
In Australia there is a large potential for an underground thick seam mining method such as LTCC to be introduced as currently there are measured thick seam resources of 6.4Bt situated in both New South Wales and Queensland (Cai Y et al 2003). ACARP recognised this potential and funded an initial study completed in 2003 (ACARP Project No. 1137F) into the feasibility of introducing LTCC into Australia.