WESTRAY’S LOW-SULHUR COAL

Curragh Resources’ Westray coal mine was officially opened Sept. 11, 1991. With hard-to-beat logistics and equipped as a brand new operation, Westray should be Nova Scotia’s most productive coal mine. Financing problems resulted in a one-year development delay, which contributed to a slower than expected buildup to production.

Westray’s proven and probable coal reserves amount to 47 million tonnes. Possible reserves bring the total to 166 million.

The 47 million tonnes will yield 16-17 million tonnes of washed product. An annual output of up to 1.3 million tonnes is scheduled from 1994 onwards. Nova Scotia Power Corp. (NSPC) will take 700,000 tonnes annually over the next 15 years. Because of the coal’s high quality (10,800-12,000 BTU/lb.) and low sulphur content (between 0.5%-0.8%), the company doesn’t expect any problems selling the “excess” 600,000 tonnes over its NSPC contract. Westray coal is well within the U.S. Environmental Protection Agency’s sulphide emission standards.

The mine is entered by twin entries driven 1,300 m on a slope of 23% to intersect the Foord seam. Once arrived at the coal, the entries follow the undulations of the seam. The seam itself ranges to a maximum 15-m thickness with a dip of up to 20deg.

While the overall seam is thick, like most coal seams it carries internal dirt bands that mandate cleaning to produce a saleable product. Washplant design is most efficient when feed has an ash content between 18% and 28% (more on this later). The two final products made by the washplant are “premium” coal carrying 13.5% ash, and “regular” coal, 20% ash. “Ash” in this context is not only the residue left after complete combustion of clean, uncontaminated coal, but also the extraneous rock taken during coal cutting and the above-mentioned internal dirt bands.

The dirt bands can’t be eliminated in the mine; they’re part of the seam and are mined as such. Contamination of the coal was also caused by a well-documented Appalachian phenomenon known as cutter roof, where the roof was unravelling soon after the underlying coal had been cut, and this was mixing with the coal transported to the plant.

What happens is shown in Fig.1 — a steeply dipping fracture develops in the roof and to one side of the drift and despite early (conventional) rockbolting, the roof weakens and pieces may collapse in small portions within the drift.

Why it happens is bound up in the tectonic history of the coal belt. The belt stretches (with major breaks through Maine and New York) from Nova Scotia in the north through Pennsylvania, West Virginia and Tennessee to Alabama in the south.

Horizontal forces in the earth’s crust collided in geological time and pushed up the Appalachian Mountains. Not all the horizontal stresses were dissipated through this mountain building episode. The coalfields draped around the fringes of the mountain chain still retain residual horizontal stresses. They are, in fact, frequently of greater magnitude, near surface, than the vertical stresses due to gravity.

Engineers of the U.S. Bureau of Mines have made a detailed study of stresses in eastern U.S. coal mines. Their work has confirmed that a fracture of the “cutter roof” type would likely form under the conditions found at many of the Appalachian mines and that tunnel orientation and artificial roof support were critical.

Westray encountered “cutter roof” failure early in its mining of the Foord seam. At that time, the operating method was to carry the back of the drifts at the contact of the coal and the overlying rock, and to bolt in the conventional manner (a row of vertical bolts in the back of the drift). The first roof failures spurred changes in technique, and Westray has now re-oriented the direction of the drifts to a more favorable azimuth. The back of the drift is no longer at the coal/rock contact and a system of truss bolting and support straps have been introduced.

A metre of low-grade coal is now left on the back. (The new bolting system is shown in Fig.2.) The loss of coal due to this procedure is acceptable, because the coal is of low quality and can carry up to 50% ash. (This coal has never been included in the official reserve figure.) This current system may not be the final word on roof support. “As the mine matures, changes to the support system are likely to occur,” said Curragh’s Marvin Pelley, President, Corporate Development and Projects.

Conventional room-and-pillar mining (with partial pillar recovery) will be the norm at Westray, albeit with the latest high-productivity equipment. Coal is cut by Joy 12CM continuous miners at the rate of one 15-tonne shuttle car per half minute. The latest rockbolting rigs have shown their paces with three men placing screen and 190 bolts per shift.

Local faulting in some areas will require that mine planners schedule to maintain ash at predictable levels. Overall mine performance was based on 38% coal recovery for study purposes, but practical expectations are into the 50% range.

The mine operates 24 hours per day, seven days per week and 354 days per year. At a production level of 1.3 million tonnes of saleable coal per year, the annual feed from the mine will be 1.473 million tonnes for a daily production of 4,160 tonnes. Mine complement will be 47 staff and 224 hourly rated for a total of 271 at full production.

Gerald Phillips, Vice-president and General Manager of Westray Coal, points out that the current complement is drawn almost entirely from Nova Scotia (91% of the salaried and hourly-rated workforce) and, of the total, 60% are from Pictou County. He is also pleased with the safety record — in 1991, there were 4.24 injuries (none of them severe) per 200,000 man/hours worked. The frequency rate in British Columbia last year was more than 10, he noted. “And I know we’re the best in Nova Scotia. We’ve had that confirmed (by a government representative.)” In fact, Westray was recently informed that they were the recipients of the John T. Ryan Safety Award for coal mining in Canada. “It’s a record we’re pretty pleased with,” Phillips said.

Beyond the underground operation, Westray is also conducting a 2-part bulk test on an estimated 8-million-tonne coal deposit mineable by open pit and near the town of Stellarton. The tests will completed by summer and, “if it looks reasonably economic, we’ll apply for a mining licence,” said Pelley. With its proximity to Stellarton, the deposit will have to be mined under strict dust, noise and water contamination guidelines.

Wash Plant

The wash plant produces two products (noted earlier) and it has the capability of delivering varying ash levels to meet customers’ requirements.

The plant was tested during the initial period of mine production. Because of financing problems, underground development was delayed and access to the lower-ash feed coal was not available. Also, the mine entries encountered a major fault zone on entering the coal. The zone was expected, but the time required to clear the disturbed ground delayed production even further. With NSPC requiring coal by a firm and fast-approaching date, Westray was compelled to mine the coal closest to hand. This particular coal was high in ash content and originally scheduled to be blended with higher quality coal. This was not to be, and the wash plant took a full flow of high-ash coal in order for Westray to meet its contractual arrangements. Fortunately, that challenge is behind it. “We’re in pretty good shape now,” said Pelley.

Raw coal to the plant is screened, passed through a rotary breaker and conveyed to a 10,000-tonne storage pile. It is recovered from the pile and screened to produce simultaneously two products in three different sizes, “coarse,” “small” and “fine,” respectively — minus 100 mm to plus 6 mm, minus 6 mm to plus 0.6 mm and minus 0.6 mm. The “coarse” is cleaned in a drum-type heavy media separator, the “small” in a heavy media cyclone. The “fine” is filtered and centrifuged only. Contrary to the normal rule, the “fine” coal is a high-grade product.

Separation into size fractions is solely for cleaning purposes. After cleaning, the three sizes are reconstituted and blended to produce the final saleable products.