History of the Deployment of Surface Miners
In 1980 the Wirtgen Company made a prototype of a road miller machine, which is the present-day surface miner. The first surface miner was introduced by Wirtgen in 1983 for the mining of gypsum in South Africa. In India, the first application of a surface miner (Wirtgen model - 1900SM) was reported from Gujarat in 1995, where it was used to excavate limestone deposits from the mines of the Ambuja Cement company. Later, the use of surface miners was limited by the geological safety aspects of region, and places having Uniaxial Compressive Strength (UCS) less than 50 MPa (at a particular time) were identified as suitable ones for the application of surface miners. In the context of coal mining, Mahanadi Coalfields Limited (MCL) deployed the first surface miner (Wirtgen model - 2100SM) in 1999, in an opencast mining project at IB valley Coalfields in Lakhanpur town. After successful applications in other coal mines, Hindalco Industries also deployed surface miners with rippers in Talabira coal mine to mine coal and overburden (Pradhan, 2009; Pradhan et al., 2014). Afterwards, in 2010 National Aluminium Company, India introduced a surface miner on a trial basis for hard rock excavation. Today, Wirtgen and L&T are the two dominant manufacturers deploying surface miners in Indian conditions. Apart from these, Voest Alpine, Krupp, Huron and KSM make surface miners which are also deployed in India with varying degrees of success.
Types of Surface Miners and Operating Methods
The different categories, on the basis of machine travel mode, of surface miners are illustrated in Figure 2.3. The operating methods are classified as a) empty travel back method, (b) turn back method and (c) continuous mining method (Pradhan et ah, 2014).
FIGURE 2.3 Types of surface miners.
In ‘empty travel back method’, the material is first cut by the surface miner from one end of the pit and then it returns to the starting end. It returns empty, as there is no cutting during the backward movement of surface miners. In the ‘turn back method’ the material is first cut by the surface miner from one end of the area, and after completion of the cut the cutting drum is generally raised and the surface miner travels back empty to its original position. In ‘continuous mining’ mode the surface miner operates on an even field, and material is cut continuously (Nandan, 2015).
Another important aspect related to surface miner application is the type of loading method. Generally, three distinct types of loading method are reported for surface miners: conveyor loading, windrowing and side casting. In the case of conveyor loading, the machine is set to cut the material and its cutting drum is lowered at a predetermined depth. It starts excavation with its forward movement, and the excavated material is transported to a discharge conveyor. In windrowing method, loading of cut material to a dumper is done by various loading equipment such as front-end loader and scraper. In the third method (side casting), the dumping of cut material is allowed by the discharge belt on the side of the miner (Prakash et al., 2013).
Performance Parameters of a Surface Miner
There are certain performance parameters (intact and rock mass related) that influence the design, selection and operation of surface miners. Intact rock parameters include rock density, moisture content, uniaxial compressive strength, tensile strength, point load strength index, seismic wave velocity, abrasiveness and petrography etc. On the other hand, rock mass parameters include discontinuities, rock quality designation, Schmidt rebound hardness number and rock mass rating etc. Moreover, the performance of the surface miner is also dependent on some machine parameters such as cutting tool configurations, drum weight, drum width, engine power and nature of coolant for tips etc. (Prakash et ah, 2013). The other considerations highlighted and discussed in literature are:
- a) Coal production (t/h),
- b) Diesel per production of 1000 t, and pick consumption.
Typically, the performance of a surface miner depends on the operating mode, in which the surface miner has to be operated, and on the cuttability index of the material being excavated. When the cuttability index increases, the performance of the surface miner decreases. However, if the cuttability index exceeds 80, then the surface miner should not be deployed. This cuttability index depends on a large number of influencing parameters, as described below:
Coal mass parameters: Compressive strength, moisture content, point load index, Young’s modulus, fracture energy, specific energy of cuttability, toughness index, stickiness of material, density, brittleness etc.
Configuration based parameters: Cutting tool configuration, number of picks, weight, engine power etc.
Application-oriented parameters: Mode of operation (traveling and loading options), operating area and level of skill required to operate.