Mining is one of the core sectors driving the growth of the Indian economy as it accounts for 2.5 per cent of the country’s GDP and acts as a catalyst for the growth of other core sectors such as power, steel and cement, among others. Primarily, there are two types of mining methods being used for the extraction of minerals and ores – surface/opencast mining and underground mining. The choice of method is largely determined by factors such as depth, geology of the mineral deposit and the cost of equipment. In India, surface mining is more prevalent, producing most of the metallic ores and minerals (excluding petroleum and natural gas) which are situated at shallow depths, while the share of underground mining has been declining as it requires greater technical expertise and massive investments to extract ores from deep mines.
To streamline the process of extraction, choosing the right mining technology is a prerequisite. However, challenges pertaining to demand for high quality minerals, high land acquisition costs, and heavy dependence on hired equipment (without appropriate manpower for operations) create impediments to the introduction of appropriate technologies. Differentiating each technology and determining its efficiency enables a mining company to select the most suitable one.
Surface mining is used when deposits of the mineral or ore are found near the surface, where the overburden (gravel and sand) is relatively thin. It is a broad category as it includes different types of mining methods such as strip mining, open-pit mining, mountaintop removal and dredging. It has become the predominant method for the Indian mining industry and is expected to continue to remain the mainstay of mining operations. In most surface mining methods, heavy equipment is used to extract the mineral. This includes surface miners, in-pit crushers and conveying systems, dragline excavators and the shovel-dumper combination.
A surface miner is basically used in opencast mining as it leads to selective mining which helps in improved separation of mineral ores from waste materials. It consists of cutting drums, scraper blades with primary conveyors and crawler track assemblies. This technique offers good recovery and productivity. Moreover, since no drilling and blasting is required, it is an eco-friendly mining solution. Further, it ensures improved coal recovery in areas which are unsuitable for blasting. Even though these machines are considered highly efficient, production is dependent on the stripping ratio (amount of waste removed to recover the ore). Further, this technique is suitable for gentle gradients and softer soil formations and cannot function in hard rock areas.
In-pit crushing and conveying
In-pit crushing and conveying systems are primarily used for hard rock mining as they allow the ore to be crushed in the pit and transported using a conveyor system. In a fully mobile in-pit crushing and conveying system, an excavator located on the muck pile loads the material directly on to the hopper of a mobile crushing plant, instead of feeding a dump truck. Crushed rock is then transported to an in-pit belt conveyor via mobile conveyors. This conveyor carries the crushed rock from the mobile crusher to a fixed secondary crushing plant for further processing thereby eliminating the need for dump trucks. In-pit solutions are applicable to both greenfield and mine expansion projects. These in-pit crushers are economical in the sense that they require less road and truck maintenance, along with negligible fuel and labour costs, thereby significantly reducing input costs. Further, they help in removing waste from the work site by providing a continuous transportation system. These systems are reliable as they are not impaired by bad weather. The technique works well where there is horizontal movement of the mine face, but is not feasible for deep mines with several vertical levels.
Dragline excavators are used for digging softer material and are mainly suitable for loose soil and marshy areas containing water. Therefore, they proves to be efficient for side casting and dredging. It is a cost-effective technology and is cheaper than the shovel-dumper combination. The technique of using dragline excavators is advantageous as it does not have to move inside the pit and can extract minerals by using a bucket operating adjacent to the pit.
Over the years, the demand for domestic output of high quality expensive minerals has been increasing. Rocks rich in these minerals are buried deep within the hard earth crust and cannot be extracted using opencast mining techniques. In underground mining, the overlying rock is left in place and minerals are removed through shafts or tunnels. However, due to the high cost and safety issues involved in its implementation, there is a declining trend in underground mineral production. The demand for high quality coal can be met by adopting underground production methods such as cut-and-fill, bord-and-pillar, shrinkage stopping and longwall coal mining. These methods are supported by highly mechanised equipment. Key among these are continuous miners, longwall mining, load haul dumper and sub-level stoping.
This technique is a mass production technology that can be used for underground mining using the bord-and-pillar and shortwall methods. These continuous miners cut and load the mineral deposit into a shuttle car. The shuttle car then hauls the mineral to the feeder breaker which feeds the sized mineral to the conveyor at a consistent rate. A continuous conveying system helps in reducing the time lag between loading and crushing, thus resulting in higher productivity. Moreover, no drilling and blasting is required, which makes it an eco-friendly option.
Longwall mining is an old technology and is suitable for deep-seated mines with thick seams. It involves the mining of long walls of mineral deposits in a single slice. Longwall mining equipment has a powered roof support, a shearer for cutting walls, and a conveyor belt for transporting the mineral. The operation of this machine requires geological continuity in order to prevent roof cave-ins. Though this technique is highly productive in terms of extraction, numerous pillars containing minerals are left untouched in order to support the overlaying strata. As this equipment is operated manually and works in deep walls within the earth’s crust, proper ventilation is required.
Load haul dumper
The load haul dumper has been designed to fit in restricted heights and narrow areas within a mine. It is a specialised loading machine for underground mining. It travels at a modest speed and consists of internal and external components such as a bucket, engine, ventilator and loading card. The extracted ore is loaded onto the cart using the bucket and is dumped at the bottom of the mine for primary crushing before being hoisted to the surface. Even though these machines provide an optimum mining solution, they involve extensive use of labour to provide guidance while loading the muck pile and to prevent the machine from hitting the walls. Besides, the technique involves high operation and maintenance costs of equipment parts like tyres.
Sustainable mining production
India produces 95 types of minerals and in terms of the quantity produced, most of this is coal. The increase in coal production envisaged in the coming years necessitates the adoption of appropriate technologies for sustainable mining production. To do so, proper resource estimation and mine planning processes are required. This can be done by gathering drill hole and mine
survey information to assess the geological structure. Mining constraints have to be acknowledged for defining the resource requirement. For efficient mine scheduling, designing a three-dimensional mining block is of great relevance. Based on survey information and model studies, estimates of the resource quantity and quality contained in the mining blocks can be used to develop a planned production schedule factoring in the mining constraints.
Moreover, the techniques should ensure mass production and must be capable of working in remote locations. Another critical factor to consider is reduction of the carbon footprint in the mining industry by adopting techniques which are environment friendly and do not require blasting. Given that mining is a risky operation, proper safety of labour and good ventilation need to be ensured.
The choice of the mining method is largely determined by factors such as depth, geology of the mineral deposit and the cost of equipment.