Factors Influencing the Moisture Content of Coarse Coal After Centrifugation

Abstract

High moisture contents in export coals increase transport and handling costs and can result in contract penalties. Considerable efforts have been focussed on fine coal dewatering, but the significant variations in the moisture content of the coarse product are also important. Very little work has been carried out to determine the magnitude and causes of these variations. An AMIRA funded project has been conducted to investigate this problem. An on-line moisture monitor was installed on the product belt immediately after the first coarse coal centrifuge at two coal preparation plants to continuously measure the moisture content of the coal from a single centrifuge. Samples were taken over a range of operating conditions and at times of low and high product moisture contents. The coal samples were characterised by a range of techniques and those properties correlating with moisture content variations have been identified. Basic studies were also conducted with a laboratory centrifuge specifically designed to characterise various coarse coal samples in terms of their water retention capacity. It has been established for the plants investigated, that the moisture of the coarse coal (typically 6 to 9%) can vary by at least one percent at a particular plant, and a number of percentage points between plants. An empirical model has been developed which can explain about 80% of the variation of the moisture content of the coarse coal from the 25 samples investigated. The rank of the coals ranged from 0.7 to 1.65 mean reflectance. The model identified four principle controlling factors: (i) The amount of moisture held internally within the coal, as measured by the new laboratory centrifuge technique. (ii) The specific surface area of the coal being presented to the centrifuge. (iii) The rank of the coal as measured by reflectance. (iv) The amount of ultra-fine (slimes) material associated with the coarse coal, which can affect the drainage of water from the coal particle surface. The hydrophobicity (rank) of the coal and the ultra-fine particles hinder the water flow from the coarse particles. This model provides an estimate of the moisture level which can be achieved by centrifugation of coarse coal.