The effect of the mode of applied breakage on coal yield

Abstract

Some coal resources, for example, those in Southern Africa such as the Waterberg and Mozambique Coalfields, have a highly interlayered morphological structure consisting of coal macerals along with mineral matter. The latter is incombustible, and becomes altered during combustion remaining as an undesirable ash by-product. It therefore lowers the economic value by reducing the calorific (heating) value of the coal. Furthermore, coal processing plants experience low yields due to the large quantities of mineral matter. Yields can be improved by liberating the valuable components of coal from the mineral matter. It is of economic importance to maximize the amount of valuable coal recovered from raw coal; since different coal value fractions are intermingled and cannot be used directly as mined. Size reduction of coal particles may, however, be required to increase the liberation of different coal quality fractions from the mineral material. This would enable the separation of the undesirable incombustible material from the valuable coal after mining, in order to produce a low ash value coal product. Size reduction of coal leads to the generation of fine coal. Not only has fine coal higher moisture retention than coarse coal, but processing of fine coal is also generally more expensive; and less efficient. Despite these difficulties in fine coal processing, crushing of certain coal sources to smaller particle sizes prior to processing may be required to attain higher degrees of liberation, resulting in an increase in the plant yield. Since it is unlikely to eliminate all fines in comminution processes, a balance between sufficient liberation of coal to attain a certain yield, and the formation of excessive fines must be achieved. The intention of this study was to investigate the liberation of coal to attain a certain yield; and to determine the breakage modes, or combination of breakage modes, that will be best suited to optimize liberation at the coarsest possible coal particle size, whilst keeping fines formation to a minimum. The yield with respect to liberation for a market specification for a low ash product was investigated, with a view to achieving the optimal liberation size. Run-of-mine (ROM) coal from the Moatize Coalfield in Mozambique was chosen because of its highly layered morphological structure consisting of coal seams interlayered with mineral matter. Coal breakage modes investigated were: i) Impact breakage followed by attrition breakage, and ii) Compressive breakage followed by attrition breakage. Compressive breakage was performed by applying a compressive force from a garden roller (80 kg weight) to break the coal sample of approximately 6 kg to certain top sizes. The chosen top sizes were 6.7 mm and 13.2 mm, and were based on the required sizes for industrial application, and also to minimize fines generation. These top sizes were obtained by pushing a garden roller over a single layer (to prevent inter-particle breakage) of multiple particles located within a steel frame. A drop weight impact rig was used to perform impact breakage according to the SANS 401:2010. Top sizes for impact breakage were similar than that for compressive breakage. After the initial breakage mode, compressive breakage or impact breakage, some samples were also subjected to attrition breakage. Attritioning was performed in a tumbling mill at residence times of 1, 2 and 5 minutes respectively. The tumbling mill was rotated at 20 revolutions per minute (rpm) (30 % of its critical speed), ensuring that only cascading motion occurred to prevent further impact breakage. The coal yield improvement attained by the various combinations of breakage modes, and the washability properties, were investigated by a floatsink analysis. A particle size analysis was performed before and after the breakage to evaluate the particle sizes formed due to breakage. Other analyses performed were the proximate analysis, free swelling index, and calorific value determination, performed according to the relevant standards. Focus was set on material with a relative density (RD) of lower than 1.4. This is because of the relatively “clean” coal, consisting mainly of liberated coal macerals, at these relative densities (RD<1.4). Results obtained during this research study have indicated that impact breakage as well as compressive breakage to a top size of 13.2 mm had an insignificant effect on the coal yield, compared to the yield of the raw coal prior to any testing. Both impact breakage and compressive breakage to a top size of 6.7 mm resulted in a coal yield enhancement. Particularly, the coal yield was substantially increased by impact breakage to a top size of 6.7 mm. It was concluded that impact breakage resulted in a greater coal yield enhancement of material with the RD<1.4, compared to compressive breakage. Impact breakage thus resulted in a larger proportion of liberated coal with the RD<1.4. Attrition breakage additional to impact breakage, i.e. impact breakage followed by attritioning, to a top size of 6.7 mm resulted in an even further increase in the coal yield. This is because impact breakage to a smaller top size of 6.7 mm exposed more coal macerals than breakage to a top size of 13.2 mm. Exposed coal macerals were then chipped off from the mineral matter during attrition breakage resulting in a higher amount of material with RD<1.4. Attrition breakage additional to breakage to a top size of 13.2 mm did not show any noticeable effect on the coal yield. A particle size of 4.75 mm was investigated to be an optimum liberation size. It was seen that the cumulative yield of liberated material (at RD<1.4) was an optimum for this particle size. A particle size of 2 mm resulted in a lower cumulative yield of material with the RD<1.4. This was because of the high value vitrinite that was liberated first from the gangue, where after finely disseminated mineral matter began to liberated, leading to a reverse effect of the increase in the cumulative yield at a particle size of 4.75 mm. Breakage to a smaller top size of 2 mm will therefore not result in a significant coal yield increase. Sufficient liberation for the coal used during this study was defined as the largest particles size with the maximum allowable ash yield obtained to produce a product as required by market specifications. The maximum allowable ash yield was defined as 10 %, since this is the maximum ash yield allowed for coal to be suitable for coking purposes. The largest particle size with a maximum ash yield of 10 % was 4.75 mm, which indicated a yield of 50 %. A hypothetical target ash yield of 10 % was investigated by using Microsoft Excel’s FORECAST function to predict the overall cumulative yield that can be obtained, and also to estimate the required relative density for separation to obtain the hypothetical ash yield. Impact breakage to a top size of 6.7 mm followed by attritioning for 1 minute resulted in a cumulative yield of 61 % compared to that of the raw coal which was 26 %. It was concluded that the most effective combination of breakage modes applied to optimize the coal yield was impact breakage to a top size of 6.7 mm followed by attrition breakage for 1 minute. The residence time for attritioning must be a minimum to minimize fines generation