HYDRAULIC MINING OF ANTHRACITE IN USA

Reproduced from International Mining Equipment (Vol. 16, No.5 - May 1965) with kind permission of the publishers - The Certificated Engineer March 1966.

In the belief that hydraulic mining holds great potential for mining coal under certain conditions in the U.S.A., the Bureau of Mines bas been investigating and developing this form of underground mechanisation. A fully integrated hydraulic system for mining, transport and hoisting could perhaps be the key to its exploitation. 

Since 1958, when a hydraulic mining project in Pennsylvania proved that bituminous coal can be mined hydraulically at high-production rates, the U.S. Bureau of Mines has conducted a programme of tests to see if the method can be applied successful to the much-harder anthracite. 

Initially tests were made with samples weighing between 1 350 and 6 400 lb each, which were encased in 6 in of reinforced concrete, using various enclosure patterns to simulate mining conditions. A high­pressure pump was used to full capacity, and pressure drop across the nozzle was estimated to be from 3 500 to 3 700 p.s.i. and the quantity of water passing through the nozzle at 260 g.p.m. All samples were completely mined out of the concrete enclosures. 

The results of these were promising enough for a full-scale experiment to be set up in the Bottom Red Ash bed at Sugar Notch anthracite mine, Sugar Notch, Pennsylvania. For this operation water pressure and quantity for mining anthracite were arbitrarily established at 5 000 p.s.i. and 30 g.p.m. (1 000 h.p.); for full-volume flow, hydraulic pressure was controlled by orifice size with a displacement pump. 

The hydraulic jumbo was designed for manoeuvring on pitches from 0° to 20° in any direction and for a coalbed ranging in thickness from 1 ft 6 in to 15 ft. All motions of the jumbo are controlled and actuated by oil hydraulics, and starting and stopping of the jet flow is controlled at the face through an interlocked pushbutton system. One button is in the operator's cab, one is at the pump station and another (a 'safety shut-off) is near the mining face. 

Up until December 162, 7400 tons of bed material had been mined at an average rate of 0·8 ton per minute, with an average power requirement of 14·125 kWh per ton. In January 1963, the objective was changed from an engineering development basis to a research basis-to determine ultimately the most efficient level of operating characteristics for maximum response, as measured by mining-out rate. This research phase of the project is not yet complete, although by December, 1963, nearly 13 000 tons of bed material had been mined, at rates as high as 1·3 tons per minute for a complete face advance. 

The 8-ton jumbo carrying the monitor assembly is skid-mounted and self-propelled. Its design provides for abrupt changes in bed height of from 10 ft 6 in to 14 ft 6 in and it can manoeuvre in the width of the place to be mined, established at 20 ft. In its search for a basic design, the USBM found that the motions of standard drill jumbos were of two distinct types­orbital and linear; the linear-motion type offers the advantage of ease of operation for cutting straightline patterns, so much so that automation is feasible. Further elimination of high-pressure water hose is thought possible only with the linear-motion type. 

The high-pressure pump used is a vertical, nine­plunger, positive-displacement unit, designed to deliver 300 g.p.m. at 500 p.s.i. working pressure. Plunger diameter is 2t in and stroke is 7 in. The pump is driven by a 1000 h.p. 2300 volt a.c. engine-type synchronous motor with a speed of 300 r.p.m. It is a positive-displacement pump, and therefore delivered quantity values of less than 300 g.p.m. can be obtained only through by-passing water. The pump is started unloaded; after attaining speed it may be loaded and unloaded by the monitor operator at will without being stopped. 

In the particular location chosen there was a choice between an underground high-pressure pump installation using water from the main sump of the mine, and a surface location for the high-pressure pumps, using water from a public main nearby. In fact, the fresh water was used to eliminate corrosion problems during the experiment, but the pump used was designed with corrosion in mind. 

Hydraulic mining began on August 30, 1961. By December 31st, 1963, 657 ft of 22 ft wide chamber and 289 ft of 14 ft wide crosscut had been driven, and during this period 12765 tons of material had been mined. The mining-out rate for chambers has averaged 0·821 ton per minute; for crosscuts, 0·714 ton per minute. The electric power requirements have averaged 13·2 kWh per ton for chamber mining and 16·5 kWh per ton for crosscuts. 

Establishing Technique 

In the experiment it was necessary, first, to learn to handle the equipment and, second, to make the face operations as workable as possible. Following this efforts were directed to establish best practice, as measured by mining-out rate, for a number of factors including quantity of flow, jet pressure and pattern. Initially, slot tests were made at a number of levels for quantity and pressure, followed by mining-out of the newly slotted face. No correlation was established between slot-penetration and mining-out rate, as the tests were terminated in favour of a statistically­designed experiment, which is now in progress. 

Operating Performance 

The operating performance is shown in Table 1. Mining-out rate was best during the slot-test period, mainly because it was learned during the previous phase that a swing pattern for jet movement gave the best mining-out response. This experience becomes more significant with evidence that some of the slot tests were made at quantities at pressures below the maximum of 300 g.p.m. and 5000 p.s.i. During the designed-experiment test, the highest mining-out rate recorded (1·3 tons/min) was obtained. It is planned to investigate the effect of water infusion as a means of producing a still higher rate. 

It is generally recognised that the hydraulic method is most efficient if used in mining beds which pitch sufficiently for the face to be cleared of broken material by gravity (on running pitch). With the problems of face loading, or clearance, and jumbo­mobility fully solved, the operation becomes continues except for roof-support phase which is no more serious with hydraulic mining than with any continuous mining method. 

To exploit the system fully, the development of economical, high-productive mining system is considered vital: a fully-integrated hydraulic system (mining, transport and hoisting) might help to answer the need.