Hydraulic Sluiced Longwall Mining without Supports

A concept for a mining method developed for a series of thin soft sloping seams between competent roof and floor strata is presented. The method proposed has the form of a retreating longwall where the mining method is by hydraulic sluicing. The face is essentially formed by drilling a directionally controlled borehole between gateroads. The directional drilling assembly is then removed from the drill-string and replaced by a jetting bit which is used to sluice the coal from between the borehole and the goaf. The process may then be repeated by drilling another hole parallel to the first and the process of mining operation is repeated. BACKGROUND Present longwall practice Longwall mining is a capital intensive inflexible system which can yield very high production when things go right. It is generally suitable for operation in geologically uniform situations where the panel width can be maintained and where the panel length is adequate to justify the cost of moving the machinery between panels. It is limited in the range of height in which it can be used to those that permit miners to work safely. This means about 1.5 to 4.0 m, or higher if top level caving is used. Longwall mining has been used on slopes to about 25 but at these angles there is a constant battle to prevent the equipment walking down hill with the consequence that production is diminished. Hydraulic mining Hydraulic mining has been in existence for many years. One of the most notable examples of its use was Mitsui Sunagawa mine in Hokkaido, Japan in an approximately 6 m thick weak (3 MPa uniaxial compressive strength) seam at a dip of approximately 70. There the coal was sluiced by the use of a monitor via adits driven in rock, which intersected the seam to form monitor bays and draw points. The water used for this operation was delivered to the 22 mm diameter monitor nozzle at pressures of 10 to 15 MPa and a flow rate of 3 m/min, (Gray, 1980). This corresponds to a nozzle velocity of 132 m/s. This was a gassy mine was a gassy mine which had a very strict regime of gas drainage to avoid outbursts. Following the closure of Sunagawa mine the pumps and monitor were transferred to the Strongman mine in New Zealand (Duncan, 1998). The use of the monitor was to be from a series of sub horizontal roadways driven in the sloping coal seam. A similar plan was prepared for the underground part of the Burton Mine in North Queensland. However this was never put into operation. The ill fated Pike River mine also planned to use hydraulic extraction (Whittall, 2006). The fundamental limitation of all the hydraulic mining systems is the amount of roadway that must be driven to the volume of coal that is extracted. The reason for this is the limited effective range, approximately 30 m, of the water jet produced by the monitor. This limitation is brought about by dispersal of the jet and by limited visibility for the operator. Directional drilling Over the past 25 years, directionally controlled drilling has been used in underground mining so that holes of 600 m length can readily be drilled in seam. These can be extended to about 1500 m with more effort and a drop in drilling rate (Gray, 1994). The drilling system that has been almost universally adopted for this purpose comprises one that uses a poly crystalline cutter drag bit rotated by a down-hole motor. This bottom hole assembly has as part of it a bent sub which can be orientated so that the drill may drill in a preferred direction chosen by the operator. This orientation is achieved by rotating the drill string Managing Director, Sigra Pty Ltd. http://www.sigra.com.au, an@sigra.com.au 2011 Underground Coal Operators’ Conference The AusIMM Illawarra Branch 10 – 11 February 2011 17 to a chosen tool face position under the real time guidance of an electronic survey system, which contains triaxial magnetometers and accelerometers. Thus it is possible to drill around rolls in the seam or to re-join the seam if it is disrupted by a fault. The angular build rate that may be achieved by such a system commonly lies in the range of 0.5 to 1.0 per metre. Rotation of the drill string may be used to make the bottom hole assembly drill a relatively straight hole. The drill pipe used for this process has, in the Australian context, been almost universally NQ or more recently NRQ type manufactured by Boart Longyear. This high tensile steel pipe (620 MPa yield strength) with heat treated tool joints has an outside diameter of 69.9 mm (2 3/4”) and an inside diameter of 60.3 mm (2 3/8”). It has been generally used to drill boreholes using a 96 mm drill bit. This drill pipe has been found to be particularly useful because it only weighs 23 kg per 3 m Iength and is readily handled by underground drill operators.