Current research at the Technical University of Nova Scotia (TUNS) Mining Engineering Dept. will soon determine the potential of anhydrite, the dehydrated form of gypsum, for use as an alternative binder material to ordinary portland cement in hardrock mine backfill materials.
Anhydrite-cemented backfills in the hardrock industry could incorporate the use of pneumatically stowed backfills now being tested in underground, soft-rock coal mines operated by Cape Breton Development Corp. (CBDC).
In comparison with the hydraulic fill systems being used extensively in the hardrock industry, this new technology could provide the following advantages:
* Since pneumatically placed anhydrite-cemented fill has a high angle of repose, bulkheads for containing hydraulic fill during placement can be eliminated.
* Reduction or complete elimination of mine dewatering would be possible because anhydrite-based systems use only as much water as is required for the cementation process.
* Desliming hydraulic backfill materials could be eliminated as the fines actually increase the strength of the anhydrite-based backfill.
* Anhydrite backfill will develop a strength much earlier than the hydraulically placed backfill.
Abundant quantities of gypsum (CaSO4 . 2H2O) and anhydrite (CaSO4 . 2H2) occur together in evaporite deposits throughout Nova Scotia. In 1985, gypsum production totalled 5.9 million tonnes. Only 200,000 tonnes of anhydrite were produced. Cape Minerals and CBDC have been testing anhydrite as a cementitious construction material in CBDC underground coal mines. Cbdc operates three underground coal mines (Lingan Colliery, Prince Colliery, Phalen Colliery), extracting about 4.3 million tonnes of coal from the Sydney coal field in Nova Scotia every year.
A research program initiated under the Canada-Nova Scotia Mineral Development Agreement demonstrated that Nova Scotia’s anhydrites are suitable construction materials in coal mines. The research team, headed by Dr. Donald Jones, formerly with the TUNS and now with the Nova Scotia Dept. of Mines & Energy, determined that some crushed anhydrites, when combined with water and chemical accelerators, will produce a cementitious mine “pack” construction material suitable for underground coal mines. The TUNS research will also determine the backfill potential of anhydrite in hardrock mines.
Cape Minerals of Sydney, N.S., recently completed successful, full- scale surface trials of the anhydrite-packing system in a structure that simulated underground conditions at CBDC. CBDC is conducting a large, underground trial in an active longwall at Lingan Colliery. Performance of the system to date has been encouraging.
The use of anhydrite in underground coal mines is practiced extensively in West Germany, and to a lesser degree in Great Britain and France. If the underground anhydrite trials at Lingan Colliery are as successful as laboratory tests and surface trials have been, anhydrite may soon be established in CBDC collieries and anhydrite demand from Nova Scotia evaporite deposits may increase dramatically in the future.
Longwall mining is a high-volume, low-cost underground method of extracting coal. Large panels of coal (up to 300 metres wide, 2,500 metres long, the thickness of the coal seam usually being 1.5 to 10 metres) are extracted by specialized longwall mining machines. The rock strata overlying the mined-out panel will collapse under their own gravitational load until the void fills with the caved rock material, referred to as “gob.” In advance longwall mining, the development of panel access gateroads is concurrent with the coal extraction on the longwall face (Figure 1.3). As a result of this mining sequence, the gateroads are maintained in highly stressed ground next to the gob area and are subject to severe deformation. To alleviate the pressure on the gateroad supports (steel arches), load-bearing structures referred to as “packs” are constructed on the gob-side of the gateroad. These packs replace the extracted coal adjacent to the gateroad and provide roof support around the gateroad. The pack prevents excessive deformation of the gateroad steel arch supports.
Anhydrite is prepared for underground coal mines by dry crushing the run-of-mine anhydrite in an impact crusher equipped with specially sized grates and a dust collection system. The anhydrite is taken underground by a batch system (rail cars) or a centralized, pneumatic, transportation system, following which it is stored in underground storage bins. A pneumatic stowing machine blows the anhydrite through wear-resistant pipes to the application site or packhole. At the packhole, the anhydrite is pneumatically forced through a special nozzle where it is dampened with water. An accelerator consisting of potassium sulphate and ferrous sulphate is added to produce a quick-setting, high-compressive-strength, monolithic material.
There are three methods of introducing the accelerator into the anhydrite-water mixture:
— The accelerator can be mechanically mixed with the anhydrite at the manufacturer’s plant before delivery to the mine;
— the accelerator can be stored in a separate hopper on the pneumatic stowing machine and metered into the dry anhydrite as it is conveyed by the pneumatic stowing machine; or
— the accelerator can be mixed with the gauging water and introduced into the dry anhydrite at the special mixing nozzle.
A second use of anhydrite, as a roadway backfill material in underground coal mines, is also being examined by Cape Minerals and CBDC. The steel arch supports for roadways and gateroads are not always in continuous contact with the rock strata behind the arch. One common method of achieving continuous contact is by driving wooden wedges between the arch and the rock mass to provide blocking points (Figure 1-10). Anhydrite as a cementitious backfill material is an improvement on block pointing, providing continuous blocking of the structure. Thus the load-bearing capacity of the steel arch (Figure 1-11) is used to the fullest.
The combined use of anhydrite pack construction and gateroad backfill work provides competent gateroad structures that will experience limited deformation during active gateroad life. In addition, stronger, fully blocked arch structures usually permit increased arch support spacings or fewer arch support materials.
A wide variety of materials have been used for longwall pack construction, including wood, hand-stowed and machine-stowed rock walls, precast cement blocks, cement based monolithic pumped packs, and anhydrite. Anhydrite is considered to be superior for several reasons:
* It is not a fire hazard in the way that the large volume of wood required for pack construction and lagging behind arch supports is.
* It is less labor-intensive than the manual construction of wooden packs. (However, this advantage is somewhat offset by the substantial capital investment in anhydrite conveyance systems and pneumatic stowing equipment.)
* If a local source of anhydrite is available, lower capital costs accrue.
* Anhydrite packs seal the gateroad from the gob and prevent the leakage of ventilation air through the gob area that occurs when wood packs are employed. Ventilation leakage through the gob is a major problem, especially in mature advance longwall operations. Cement-based monolithic pump pack systems also prevent ventilation leakage; however, these systems have much lower uniaxial compressive strengths and do not generate the support strength provided by anhydrite packs.
* Anhydrite packs have increased load-bearing capacity, so repair costs to roadway arch supports will be greatly reduced.
In the face of all this, Nova Scotia, which boasts billions of tonnes of anhydrite, stands to gain plenty if and when the European practice of using the substance spreads to the coal mines of Nova Scotia and possibly to the hardrock mines across Canada.
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