In this article we will discuss about the opencast mining (quarrying):- 1. What is Opencast Mining? 2. Methods of Quarrying 3. Quarriable Limit 4. Formation of Benches 5. Manual Opencast Working in Coal 6. Removal of Overburden 7. Extraction of Coal 8. Advantages 9. Disadvantages.
Contents:
- What is Opencast Mining?
- Methods of Quarrying
- Quarriable Limit
- Formation of Benches
- Manual Opencast Working in Coal
- Removal of Overburden
- Extraction of Coal
- Advantages of Quarrying
- Disadvantages of Quarrying
1. What is Opencast Mining? :
Opencast mining or quarrying of minerals is easier than mining by underground methods. During quarrying the alluvium and rocks below which the minerals lie, are removed and dumped, in the initial stages, in a place which is not required in future for quarrying, residential or other purposes.
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The mineral exposed is completely extracted. Opencast mining is also known as open-put mining, open-cut mining, surface mining and also as strip mining, the later term being commonly used in the U.S.A. for opencast mining of coal.
The overburden and the mineral, coal, are excavated in long strips of a few metres thickness and hence the operations are termed strip mining.
2. Methods of Quarrying:
The operation of removing overburden and extracting mineral is done by one of the following methods:
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1. Manual Quarrying:
In this case manual labour is employed. Small drilling machines, drilling 1.2 m to 1.8 m deep holes, 37 mm diameter, are used and the holes are blasted with gun-power or other explosives. The overburden and mineral are manually loaded into tubs which are hauled by rope haulages or locomotives. Tipping trucks are also sometimes employed and manually loaded.
2. Mechanical Opencast Working:
In this method heavy earth moving machinery like draglines, power shovels, rear-dumping trucks (common type being Haulpaks), well-hole drills etc. are used. The blast holes are 6 m to 18 m deep, and 125 mm to 250 mm diameter. The rock is blasted by liquid oxygen, open cast gelignite or other high explosives.
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Mineral and overburden are transported by locomotives, belt conveyors or large trucks known as dumpers. Bucket wheel excavators are used in some mines for soft rocks e.g., at Neyveli lignite project.
A method of surface mining known as placer mining involves mining and washing together of generally unconsolidated or semi-consolidated rock near the ground surface and the method is normally not treated as opencast mining but a variation of it.
Glory hole mining is a method where the mineral is excavated in small open pits but is transported to the surface through underground excavations and transport system.
Nearly 70% of the mineral production in the world comes by opencast mining and in India this method of mining accounts for nearly 75% of our mineral output.
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3. Quarriable Limit:
The cost of removing overburden to extract mineral lying below it goes up as the quarrying operations extend to the dip side of the property and the thickness of overburden increases. The stripping ratio, thickness of overburden; thickness of mineral deposit therefore decides the economic working limit of quarrying, i.e., the quarriable limit.
The softer the rocks, the less are the expense of overburden removal and higher is the stripping ratio.
The wages of labour, the selling price of mineral and the margin of profit and the major considerations in deciding the limiting ratio which is as follows in coal mines:
The maximum depth from the surface in existing mines in our country is 120 m but future mines are planned to reach a depth of nearly 480 m.
4. Formation of Benches:
The overburden and mineral deposit can be extracted by formation of benches or by keeping the surface sloped so that the angle of slope does not exceed 45° from the horizontal. A bench has two elements, the floor and the face (high wall).
The width of the floor should not be less than the height of the face (high wall) and the heights of benches are as follows:
1. Loose Material:
In alluvium, morrum, loose earth, etc. which is likely to slide the bench height should not exceed 1.5 m. During the rainy season, there is possibility of land slide of the loose debris and it is desirable to keep the width of the floor much larger than the height of the bench.
2. Coal:
The height of coal bench, i.e., the coal face, should not exceed 3 metres.
3. Sand Stone and Hard Rock:
Benches in sand stone and hard rocks are rarely vertical but generally sloping at a small angle with the vertical. The mining regulations do not stipulate definite bench heights in hard rocks except that the bench width has to be more than the bench height. In manual quarry the bench height is usually 3 metres to 4.5 metres and in mechanised quarries, more than 5.5 metres and depends upon the height of the boom of shovel above the bench floor.
Suitable bench height for a 2 m3 shovel is 6 to 8 m and for a 3.5 m3 shovel, about 12 m. The slope of the high wall is usually 20° off vertical and depends upon the travel of the bucket during loading. The width of bench floors in mechanised quarries is usually 15 metres and preferably more for movement of dumpers, tractor loaders and other equipment.
Gradients of roads in quarries for tyred vehicular traffic should not exceed 1 to 10.
5. Manual Opencast Working in Coal:
The quarriable area is divided into sections along the strike so that overburden extraction takes place in some sections and coal extraction in others which have the coal already exposed after overburden removal. Small pillars of the rocks excavated are left for measurement.
These are called witness or ‘Sakhi’ in Hindi. They are removed after measurement of excavated area, usually once a week is over. The height of such ‘sakhi’ should not exceed 2.5 m and where the height of such pillar exceeds 1.25 m its base should not be less than 1.5 in diameter.
6. Removal of Overburden:
The soft material like earth and weathered rock is cut by earth cutting picks. A team of workers consists of 3 or 4 members, one cutting and two loading. As female workers are allowed in quarries, a team of 3 workers usually includes one or two females members who are normally permitted to work only between 5 a.m. and 7 p.m. under the Mines Act. Each team is allowed a small plot usually 4.5 m x 4.5 m. The average output per worker in soft rock or earth is about 2.8 m3 (in situ) per day.
In the hard rocks which need blasting holes are drilled.
(a) Manually with the help of hexagonal steel rod with chisel end, or
(b) By compressed air hammer drill (jack hammer).
Method (a) is used in a small quarry where the output is only 50 to 100 te of coal per day, electricity or compressed air is not available and labour is cheap. A hole is 1.2 m to 1.5 m deep and one man can drill 5 to 8 holes, each 1.2 m deep in a shift (8 hrs.) Method (b) is now- a-days commonly employed.
Compressed air is supplied by steel pipes 50 to 100 mm dia, up to central places and branch pipes supply air to drills through lose pipes. Two workers (one driller and one helper) drill 40 to 50 holes, each 1.5 m deep, in one shift. The holes one placed 1.2 to 1.5 m apart and are blasted with gun powder or other suitable explosives. Blasting is done during the rest interval of the workers to prevent frequency interruption of work.
The overburden is loaded into tipping tubs (0.73 m3 capacity) which are hauled by direct or endless haulages. In seams of mild gradients locomotives may be used. The haulage track is taken to each bench or alternate benches. Blasted overburden of higher bench is sometimes dropped on the lower bench for loading into tubs.
Overburden is dumped to the rise of the outcrop or beyond the quarriable limit but as the coal extraction proceeds, the overburden may be dumped in the area from which coal is extracted – an operation known as back filling. The overburden should be so dumped that it does not roll down at the coal benches when it assumes its angle of repose, nor should it choke water courses or damage paddy fields, other agricultural area or water reservoirs.
7. Extraction of Coal:
The coal which is exposed after removal of overburden is blasted after drilling holes. The same drills which are used for stone may be utilised for coal also but if the compressor has a limited capacity electrically operated drills are used for coal due to their relative lightness and better performance.
The spacing of holes in coal is 1.5 m to 2.2 m, the depth varying from 1.2 m to 2 m. The coal available per kg of high explosive like special gelatin (60 to 80%) is generally 10-12 tonnes. With blasted coal, the average loading performance per loader is nearly 4 tubs (1.1 m3 capacity).
After it has advanced some distance along the dip. The entrance to the working places is by steps and inclined roads. The permanent installations like haulages, compressors, power transmission lines, etc. are installed in such places that their frequent shifting is not necessary when the stone benches or coal benches advance.
On a level track hand pushing of empty tubs having pedestal bearing is not uncommon for a distance of 100 m. If the same track is used for loads as well as empties, the gradient of the track should be nearly 1 in 80 in favour of loads. These factors limit the length of benches.
The property is divided into blocks, each 120 to 180 m long along the strike. Each block has a direct or endless haulage in the middle and the benches are formed along the strike on either side of the haulage. Block A which is shown more advanced than Block B, raises coal from the coal benches CB1, CB2, CB3, CB4.
Each bench is 3 m high and has a haulage track on its floor. From the clipping point PI to the junction P2 the haulage track is either level or slightly rising in bye at 1 in 80 or so. The stone benches SB1, SB2, are ahead of the coal benches so that the exposed coal lasts for 2-4 weeks. Stone from the higher bench SB2, is dropped on the lower bench SB1 which has haulage tracks on its floor.
The stone is taken in tipping tubs along a level track TA by hand pushing for dumping in the de-coaled area. The track TA is along the barrier. The tubs cross the direct haulage track over a bridge K1 to fully utilise the decoaled area for dumping.
As the stone benches advance the position of the bridge K1 has to be shifted to the dip side. Another level haulage track is taken from the stone bench SB1 to the decoaled area over a ledge LL1. This ledge is 3m wide at the top and is a solid barrier of coal and stone left between blocks A and B. The coal of the ledge is not recoverable.
In block B, there is emphasis on removal of overburden. The direct haulage track is along the floor of the coal seam and more stone benches than in block A are provided for employment of a large number of workers on overburden removal.
The stone benches SB5, SB6, SB8 are served by the central direct haulage. Stone benches SB3 and SB4 are worked by a level track Tb and another level track passing over the ledge LL1. stone is taken over these level tracks in tipping tubs for dumping in the decoaled area in the same manner as described for block A. The bridge K2 serves the same purpose as bridge K1 in block A and is advanced towards the dip at intervals.
During rainy season coal raising may be suspended in block A which is on the dip side, and only overburden removal may take place from the higher stone benches which are free from water. From block B, only coal raising may take place during monsoon and the block can be kept free from water and making a through connection 1.8 m high in the ledge LL1 for the water to gravitate to block A.
Drains for water, called garland drains are cut to minimise inflow of surface water into the quarry. Pumps are installed at possible places of heavy accumulation of water. The installation of compressed air pipes and the overhead power lines.
8. Advantages of Quarrying:
There is no problem of roof control or ventilation. Full extraction of mineral is possible. No mineral is blocked in shaft pillars, support of main roadways, etc., as in underground mining. Quick return on capital and early extraction are possible without the need to wait for a long period of development work or unproductive work like shaft sinking, etc. Large output is available from a small area and supervision is easy owing to concentrated work. Artificial lights are necessary only after dark.
Dangers and hazards are less as compared to underground mining. There is no risk of gas explosion. Very few stringent mining regulations are applicable to quarries. For example, compared to underground mining less number of competent persons is to be appointed and less number of statutory inspections is necessary.
Better sanitary conditions can be maintained.
High efficiency of mine workers; ease in loading the tubs in unconfined space and natural light gives high O.M.S. (output per man shift).
Female labour can be employed. All the members of a family are often employed and the accommodation problem is simplified.
Once the mineral is exposed, output can be easily varied to meety wagon supplies, or consumer demand.
Training of operatives is easier. Large scale mechanisation is possible as there is no restriction on the dimensions of machines to be used. Unlike in underground mines, machinery working at high voltage can be employed.
9. Disadvantages of Quarrying:
Among the disadvantages of quarrying are:
Work is affected by weather. During winter nights, and summer mid-days efficiency of workers is very low. During rainy season unless effective steps are taken to dewater the mine mineral which is at the lower levels, cannot be worked and mining comes practically to a standstill.
Surface land is destroyed and is rendered unfit for agriculture and residential purposes.
Mining lease gives only underground rights; surface rights have to be acquired for quarrying.
The method is uneconomic for working mineral beds at depth.
Where quarrying aims at quick return on capital, outcrop mineral is also mined. As it is inferior in quality due to weathering and percolating of water, the overall quality available to the consumers is affected in earlier phases of mining.
The quarried area and the OB heaps present an unpleasant sight. In some foreign countries the mining law requires that the quarried area should be filled up with overburden and restored to the pre-quarry state fit for agriculture.
Marshy land, after extraction of underlying mineral and restoration of surface has, in a few cases, resulted in a good agricultural area. Such low requiring surface restoration to pre-quarry stage does not exist in India.
The overall O.M.S. is low due to a large labour force engaged in OB removal in manual quarrying.
A property with extensive area on the strike and containing a thick mineral bed moderately inclined, lying at shallow depth, is ideal for quarrying. In India coal seams with inclination as steep as 1 in 3 have been worked by mechanised opencast mining methods in Karanpura field.
Seams at shallow depths which are actively gassy, liable to spontaneous heating or with bad roof should preferably be extracted by quarrying as the mining legislation for underground working of such seams is stringent.
Before the quarrying operations are undertaken it is necessary to vacate buildings and divert electric overhead lines, aerial ropeways, water mains, telephone lines, roads, railway lines, streams, etc. from the area which has to be quarried.
The trees have to be cut. Sufficient space for dumping of overburden, not far from the quarry, has to be considered. Where mechanised opencast mining is to be adopted, plans should be prepared to show the contour lines and the thickness of coal and overburden and good roads without steep inclinations and sharp curves should receive attention.
Dumpting yard for OB should be so selected that wind does not carry the dust to residential colony. By dumping of OB if the ponds, paddy fields, mango/coconut groves, not under the ownership of the mine owner, are likely to be affected, the compensation payable should not be ignored.
If heavy explosive charges have to be blasted as in mechanised quarries, the quarry site has to be far away from residential area (beyond 300 m). Such heavy blasting may cause cracks in old buildings resulting in demand for compensation.
On the surface reference lines have to be marked on a square pattern, every 30 m apart, for monthly measurement of the excavation and they should extend 50 m beyond the limits of the proposed quarry. Junctions of the squares should be marked by permanent pegs in brick pillars.