Hydraulic backfill with long-wall miningby Dr Md Rafiqul Islam
EXTRACTION of the first slices of the Barapukuria coalmine is going to be completed by early 2012. Right now, the coalmine is faced with two major issues — work safety and protection of ground subsidence when carrying on underground mining activities, and enhancement of production or recovery rate, because only 9 per cent of the total reserve (about 377 Mt) is mine-able if the present multi-slice long-wall mining method with descending order is applied. Technically, these two problems could be resolved, i.e. ground subsidence would be controlled and the production rate would be enhanced, if the hydraulic backfill technology is properly applied. In Canada, Poland, Australia and India, for example, backfill with underground long-wall coal mining has been in use as an effective technology over several decades.
Backfill refers to any material that is placed into voids mined underground for the purposes of some engineering functions (Grice, 1998) like control of ground pressure, management of deformation of surrounding rock strata of a mine panel, and prevention of ground movement and fracturing of overburden strata. The purpose of backfill is not to transmit the rock stresses but to reduce the relaxation of the rock mass so that the rock itself will retain a load carrying capacity and will improve load shedding to crown pillars and abutments (Barrett et al, 1978). Backfill leads to less deterioration in ground conditions in the mine, improving economic operations and safety.
There are several types of mine backfilling, including un-cemented hydraulic fill, cemented hydraulic fill, paste fill and gel fill (Bloss, 1992; Rantala, 2012). Generally, there are two fundamental types of backfilling strategies. The first, un-cemented backfilling, i.e. it does not make use of binding agents such as cement. A classic example of un-cemented backfilling is the use of hydraulic fills that are placed in the form of slurry into the underground voids. Hydraulic fills are simply silty sands or sandy silts without clay fraction. The clay fraction is removed through a process known as de-sliming, whereby the entire backfill material is circulated through hydro-cyclones and the fine fraction is removed and then sent to the tailings dam. The remaining hydraulic fill fraction is reticulated in the form of slurry through pipelines to underground voids. The second category, cemented backfilling, makes use of a small percentage of binder such as Portland cement or a blend of Portland cement with another pozzolan such as fly ash, gypsum or blast furnace slag (Sivakugan, et al., 2006). Paste fill is a muddy material that is as solid as toothpaste. Compared to sand and water, it performs better and requires smaller amounts of cement to produce a very strong product. It is also much more uniform in texture after placement. Gel fill is a hybrid of sand fill and paste fill, though it is practically indistinguishable in appearance from sand fill. Gel fill has a chemical additive that thickens the water into a glue-like substance, making it a consistently uniform product. Gel fill’s advantage is that it is a better utilisation of cement. It flows like sand through the pipelines (Rantala, 2012). Practically any material that is cheap, available, and performs well can be used as back fill. Four types of backfill materials as used in Polish coalmines are sand, crushed and milled development waste rock, processing-plant tailings (flotation waste and slime), and power-station ash, fly ash, and slag (Palarski,1994).
According to Rantala (2012), two-thirds to three-quarters of the mines in Canada use backfill. Countries like South Africa may not use backfill at all because of difference in selection of mining methods. Instead, they would put in wooden posts, about one metre in length, or use some sort of cribbing technique. In some cases they may use sacks that are filled with sand to help support the openings. Australia is also doing good things with backfills. A review of Palarski (1994) reveals that the Polish coalmine operations have widely been used hydraulic backfill to both minimise surface subsidence and to enable thick seam extraction methods. The mining methods commonly used in Polish coal mines are room and pillar, ascending order slicing of seam by long-wall with backfill, descending order slicing of seam by long-wall with backfill, caving long-wall with filling through boreholes from surface (Fig-1). The long-wall method is employed in most coal seams in Poland, where coal is extracted from advancing or retreating faces, the faces usually being more than 150m wide and 500m long (Palarski, 1994). The fills used in coalmining operations in Poland have evolved from loosely dumped rock and hydraulically placed sand fills, through pneumatic and throwing stowing. The earliest record of backfilling in Australia was the placement of aggregate at Mount Isa in 1933, where the cut and fill stopes were initially mechanized and then long hole drilling rigs were introduced. Hydraulic backfill, dry aggregate and mine development waste were used. In 1998, Wambo Mining Corporation placed cemented backfill into a series of headings in the path of long-wall No 9 at Homestead Colliery near Singleton. The sand, fly ash and cement mix was designed to 4MPa strength. No ground stability problems were encountered and the cost of the backfill project was revenue positive (Grice, 1998). The need for backfilling is now a major issue in Australia, because 10 million cubic meters of underground voids are generated annually as a result of mining. Considering a wide range of mines in Australia, more than 20 different hydraulic fills were studied at James Cook University (Sivakugan, et al., 2006). According to Rankine et al., (2006) hydraulic backfills used in Australian mines have similar grain size distributions whilst having quite different specific gravity values, typically in the range of 2.7–4.4. China and some other major mining countries are increasingly applied cut-and-fill mining method because of the following advantages: (1) it effectively controls rock pressure and ground movement; (2) it protects the ground environment and (3) disposal of a huge amount of solid wastes is thus made possible (Li et al., 2004). In Indian coalmining, hydraulic sand stowing (HSS) plays a vital role to protect surface subsidence. Among the different methods of stowing, HSS is very effective in Indian coal mining. The maximum subsidence in Indian coalfields with HSS filling is only 5 percent whereas it is 60 percent in the case of caving with respect to extraction thickness of a single seam extraction. Thus, the maximum subsidence can be reduced 12 times by hydraulic filling of voids with sand with respect to caving (Lokhande et al., 2005).
Typical costs of backfill range from $2 to $20 per cubic metre (as per 1998) which can be a significant part to the operating costs of the mine. Where cemented backfills are used, these costs tend to be between 10 and 20 per cent of the total operating cost of the mine and cement represents up to 75 per cent of that cost. The commonest binder that is added to backfill is Portland cement – GP Grade. Adding low percentages of Portland cement of between 3 and 6 per cent by weight, permits the development of cohesive strength and the ability for the backfill mass to be self supporting when exposed in vertical face. Typical addition rates up to 6 per cent by weight are added into hydraulic fill and 4.5 to 5 per cent by weight into paste fill and rock fills (Grice, 1998). It was experimented in the Polish coalmines that a backfill mixture of about 5 per cent fly ash, 85 per cent sand, and 8 per cent cement with 2 per cent water developed a compressive strength of about 0.85 MPa after 28 days. The strength was sufficient to enable the backfill in Polish coalmines (Palarski, 1994).
Barapukuria is the first underground coalmine with multi-slice long-wall mining in north-western Bangladesh. In this coalmine, an extraction height of 3m is being practised where the panel dimension is almost 120m in and about 520m in length. In between 2005 and 2011, there appeared some evidences of ground surface movement (about 0.8m) that was induced by the extraction of the first slice (1101, 1103, 1105, 1107, and so on). It is assumed that the stress redistribution in surrounding rocks resulting from the mining, the existence of the void space and the influence of ongoing mining activities is the key rationale for the occurrence of surface subsidence. According to Li et al (2004) and others, it has been generally believed that underground mining with backfill technology effectively controls ground pressure and prevents the super-incumbent ground movement and fracturing. The backfilling of underground voids improves local and regional stability, enabling safer and more efficient mining of the surrounding areas (Sivakugan et al, 2006). The advantage of hydraulic backfill is the simplicity and low cost of production and delivery. Un-cemented hydraulic backfill can be placed for less than $2 per cubic meter (as per 1998). Increasing strength is a simple matter of adding cement (Grice, 1998).
In my previous research in Japan (Islam et al., 2009), I recommended to apply hydraulic sand stowing (HSS) technology in the Barapukuria coalmine. HSS can be considered as a remedial shield to reduce fracture development, roof subsidence, and the associated potential for water inflow hazards by reducing mining induced stress from 13.3 MPa to 6.5 MPa. The practice is well developed in some Indian coal mines such as the Lachhipur, Samla, and Madhusundanpur collieries (Islam et al., 2009; Lokhande et al., 2005). As mentioned above, the Polish deep (about 480m-1,500m) underground long-wall coalmines use backfill techniques with respect to three orders of extraction, like- ascending order slicing of seam by long-wall with backfill, descending order slicing of seam by long-wall with backfill, and caving long-wall with filling through boreholes from surface. The panel dimension also 150m width and 500m length. Coals seams in Poland have typical thicknesses of 1.8m to 20.0m, with dips from 5 degrees to near vertical. The deepest coalmines are located up to 1500m (Spearing, 1994; Palarski, 1994). Likewise the Polish coalmines, descending slicing (Fig. 2) are being practiced without any backfill in the Barapukuria coalmine. The average thickness of the present mine-able seam VI is 36m at Barapukuria. However, according to the present mine design only about 21m coal could be extractable by using multi-slice mining panel. Huge amounts of coal will stay behind the present mine plan. Regarding this critical situation in the Barapukuria, production rate could be enhanced by using hydraulic backfill technology. The present descending slicing with high recovery rate is possible if the cemented hydraulic backfill is considered. In contrast, for un-cemented backfill, the present mine design should be revised to ascending order extraction. In this case, backfilling through boreholes from surface could enhance production rate.
The primary source of backfill materials will be local to the mine (Grice, 1998). It is reasonable to mention that the huge amounts of sand and silt resource of the Jamuna riverbed, which is not so far from the mine area, could be used as backfill material not only in the Barapukuria coalmine, but also in the others proposed coalmines including Phulbari, Khalaspir, Dighipara of Dinajpur district and a proposed underground coal gasification (UCG) project in the Jamalganj coalfield of Jaipurhat district. Although some rock debris/rock dust/mining tailings remain in the Maddhapara Granite Mine area as stockpile, however, the amount is not so sufficient to recover the basic need for backfill. Only river-bed sand and silt would be the cost effective fill material for coalmine. In addition, thousands of tons of fly ash remain in the Barapukuria mine-mouth thermal power plant. This resource may be used as binder materials of backfill. Moreover, there are two Ordinary Portland Cement (OPC) factories in Jaipurhat and Sirajganj and both of the factories are well-connected to railway through Jamalganj coalfield of Jaipurhat district and Phulbari, Barapukuia coalfield area of Dinajpur district. Railway wagon would be a potential cost effective transport system for the backfill materials from the Brahmaputtra-Jamuna River. In this case, three feasible location along the west-bank of the Jamuna, like (i) Balashi-Phulchuri-Gaibanda point, (ii) Shariakandhi-Bogra point, and (iii) upstream of the Jamuna Bridge-Sirajganj point can be considered as the first dumping ground of the fill materials. After sieving of the fine-gained clay particles, the rest sand and silt particles could be transported through railway wagon to the mining area.
Underground coalmining with Polish-type extraction and backfill would be the best option for Barapukuria and other proposed coalmines in Bangladesh because the overall geology and hydro-geologic conditions of the country is not favourable for open-pit mining. The present study reveals that hydraulic backfilling-based underground long-wall mining would be the commendable option for the extraction of coal in Bangladesh to solve the energy crisis.
The availability of local sand, silt, fly ash, and Portland cement, and mining depths in Bangladesh with compared to that of Poland indicate economic prospect of hydraulic backfill technology with high recovery rate of coal.
As per 1998, un-cemented hydraulic backfill can be placed for less than $2 per cubic metre. Costs of cemented and others typical binders backfill range from $2 to $20 per cubic metre (Grice 1998). To my calculation, cost of hydraulic backfill (both cemented and un-cemented) that can be placed into voids space of the underground coalmine in Bangladesh would be ranged between $7 and $15 per cubic metre because of availability of sand and silt particles in the Jamuna riverbed and fly ash in the Barapukuria mine-mouth power plant.
Typical mix ratio of the backfill materials, as experimented in the Polish coalmines, would be about 5 per cent fly ash, 85 per cent sand, and 8 per cent ordinary Portland cement with 2 per cent water by weight. For a special case, ordinary Portland cement can vary between 3 and 6 per cent by weight.
The hydraulic backfilling technology (both cemented and un-cemented) would be cost effective for Bangladesh because the present market price of the Barapukuria coal is now about $140 to $150/ton.
If the government of Bangladesh agrees with the concept of this article in association with financial support, the author wishes to carry out a fundamental collaboration research work regarding the feasibility study of the hydraulic backfill technology of the Barapukuria coalmine and other proposed coalmines in north-western Bangladesh.
Dr Md Rafiqul Islam is assistant professor, Department of Petroleum and Mining Engineering, Shahjalal University of Science and Technology, Bangladesh
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