Longwall Top Coal Caving

Longwall top coal caving (LTCC) is a special type of longwall mining applicable to very thick seams (greater than 4.5m) where good quality coal is being left because "conventional" longwall equipment has not yet been designed to operate successfully beyond around 5m mining height. It enables an increased recovery for only an incremental additional cost.
The method originated in Europe but has been developed in China in more recent years and is used there quite extensively and successfully.
The lower section of the seam is cut by a conventional longwall set-up except that the longwall supports have a longer rear canopy extending past the base into the goaf. The extended canopies have a sliding door fitted into them.
An additional AFC is attached to the rear of the chocks and runs directly below the canopy openings.
As the face moves forward, the coal left above the section cut by the machine falls onto the extended canopies, providing the goaf is caving normally. The sliding doors in the canopies are sequentially opened, and the coal falls through onto the rear mounted AFC. The maingate stage loader is extended beyond the face conveyor to enable the rear mounted AFC to discharge coal directly onto it and carry coal to the maingate conveyor system.

Diagramatic view of principle of longwall top coal caving

The sliding canopy doors are opened and closed in a controlled manner to ensure the conveyor is loaded efficiently and to prevent stone being taken out when all the coal has been recovered from a particular section.
Not all the coal would be recovered, but recoveries of 75-80% of the full seam in the block are achievable. The system also allows the initial mining height to be reduced to a preferred working height without losing coal.