Abstract:
Near-density material is material with similar densities to that of the chosen cut-point density during the separation process of coal into product and discard. The problem is usually classified as a beneficiation problem and not as a reserve problem. To overcome the risk associated with near-density material in the beneficiation process, the constraints of the separators must be merged with the constraints of the coal resource. The problems caused by the presence of near-density material in the beneficiation process of raw coal to a certain percentage ash product, are commonly associated with South African coals. Near-density material is caused by the presence of finely dispersed mineral matter in the coal that can not be separated from the coal by current liberation methods. The mineral matter referred to is directly linked with the original depositional environment of the coal. The depositional environment of coal formation determined the ash, the mineral matter present and the distribution within the coal matrix. A closer look at the ash distribution reveals that all types of coal, irrespective of depositional environment, has intrinsic ash content grouped around a certain percentage and that the amount of near-density material present in the beneficiation process depends on the percentage ash, in the clean coal product, required. As the ash distribution is the controlling factor on near-density material, any external factor that effects the ash distribution will affect the coal’s washability characteristics. There are various methodologies to define a coal’s amenability to being washed to a certain clean coal ash product. All of the methods generate an empirical value of near-density material or the coal’s “difficulty” in being washed. All of these methods have their advantages and disadvantages, but the method used is of no consequence if the information is not applied correctly to the coal resource or reserve. At Leeuwpan Coal Mine a risk matrix is used to relate the values from near-density material calculations to the coal reserves. The risk values from the matrix is incorporated into the mining blocks so that during the mine planning phase the risk of near-density material can be quantified and minimized. At Leeuwpan Coal Mine the application of the risk matrix into the planning and exploitation of the coal seams indicated that through resource/reserve management quality problems, due to near-density material can be minimized and that yield can then be optimized. The optimisation of yield leads to a financial gain that increases the value of the reserves. Therefore by pro-active planning and a good understanding of the resource/reserve the risk associated with near-density material can be managed.
