MECHANISED OPENCAST MINING

MECHANISED OPENCAST MINING

Opencast mining is the oldest method of excavating minerals but the mining operations have been mechanised by the use of heavy earth moving machinery during the last 50 years resulting in excavations on a scale which was unthinkable half a century ago.

Some of the coal mines planned for large production (million tonnes per year) are: Kusmunda - 6.0, Mukunda -15.1, Gevra -14.0, Nighai-12, Rajmahal - 10.5, Jayant/Dhudhichuva/Khadia -10.0 each, Dipka expansion/Anant - 8.0

For mechanised quarries, employing heavy earth moving machinery the DGMS makes byelaws covering bench sizes, roads etc. These have to be studied before planning a mechanised quarry. Mechanised opencast mining is preferred when there is a thick mineral bed of mild inclination, practically continuous and not in pockets, at a low depth and the reserves are plentiful. For coal, a seam of less than 6 m thickness and with less than one million tonnes of quarriable reserves will not justify the heavy capital expenditure, large amount of interest on il and the depreciation charges.

The overburden may be removed by a combination of dozers and scrapers if the rocks are soft. If they are hard, blast holes arc drilled by wagon drills or well hole drills and blasted with explosives. The blasted rock is loaded into dumpers by dipper shovels or tractor shovels. Draglines are also used where the overburden is alluvium, sand or soft rock, but if it consists of hard rock it is loosened by sparse blasting for loading.

The equipment commonly used in a mechanised quarry for drilling, loading and transporting is briefly described below.

Crawler chain vs. pneumatic tyred equipment

The machines employed in a mechanised quarry are mounted on pneumatic tyred wheels if they have to be towed from one place to another e.g. compressors, wagon drills, etc. Self propelling units having their own engines (like tractors, dozers, shovels, cranes, etc.) may however be mounted on pneumatic tyres or crawler chains. Table 1 shows the approximate rolling resistance of various road surfaces to pneumatic tyres and crawlers.

TABLE 1 

Rolling resistance

Type of surface	Pull in kgf per 1,000 kgf of gross weight
	Low pressure tyres	Crawler chains
Smooth concrete	18	28
Good macadem	30	35
Earth roads, dry, dusty	40	43
Unploughed earth	60	55
Earth road, rutted, uneven	90	70
Loose sand and gravel	110	85
Construction haul roads or roads in loose soil	145	112

A resistance of 9 kgf per 1,000 kgf is roughly equivalent to that offered by an up gradient of l% i.e. a road rising 1 m vertical through 100 m horizontal.

It will be seen that the advantage of pneumatic tyres recedes rapidly as the ground conditions worsen and for rough roads a crawler unit is at an advantage. Machines using crawler chains are not allowed to cross public roads which are damaged by the cleats of grouser plates and the latter should be replaced by plain plates preferably with rubber soles, 'for such crossing. Speed of crawler chains is slower than that of tyres. Where public roads have to be crossed often, or where the flitting is frequent, it is an advantage to use pneumatic tyred equipment. In the Held crawler mounted equipment scores over the tyred one for climbing steep gradients and has better traction on rough roads and better gripping capacity on bench floor when the machine has to dig hard, e.g. when extracting toe. Crawlers can negotiate sharp turns, a big advantage in a quarry of limited size. The quantity of mineral and rock that remains frozen in the roadways of a quarry where tyre mounted dozers and tractor shovels are used is much larger compared to that when similar equipment on crawlers is deployed.