Open Cut » General
Coal moisture, and particularly in situ moisture (Mis), is a parameter which is often examined inadequately during exploration programs, and the resulting limited knowledge can easily impact negatively producing inaccurate definition of reserves, incorrect estimations of product moisture levels, and poor tonnage reconciliations. Mis is very difficult to determine. This project set out to determine the means to estimate Mis.
Coal samples were collected from 17 different sites, with a reflectance range of 0.40-2.49%. Four types of samples were collected, (i) large lumps (generally 5-10 kg) from freshly exposed underground and above ground seams, (ii) lumps from 200 mm deep holes in seams cut with a 200 mm diameter concrete drill, (iii) borecore samples (up to 100 mm core) and (iv) washed coal samples. All samples were packed and sealed as quickly as possible after collection to avoid any moisture loss. Lumps were selected from the raw samples and treated as follows: saturation in water under reduced pressure; determination of the relative density, and moisture, of the saturated lumps; and determination of a range of coal quality parameters. A survey of the coal industry was carried out, seeking data on the various moisture parameters and other general quality characteristics.
The moisture of the as received lump coal was taken as an approximation of Mis. Lump samples increased in moisture during saturation by an average 0.4%. It is believed that, during the saturation process, moisture enters tiny fissures and cracks in the coal that were generated during the relief of seam stresses prior to the exposure of the working face.
Offsets between moisture parameters from the sampling program were generally supported by those from the survey, although the survey offsets were indicative only. The difference between in situ moisture and moisture holding capacity (MHC) type parameters was found to be significant contrary to international research.
The Preston & Sanders equation for the calculation of relative density (RD) from one moisture basis to another has been tested, and the results support the validity of the equation.
Results from moisture holding capacity and equilibrium moisture have been compared, and no significant differences were observed for values below 10%. There appears to be little difference in reliability between MHC (for higher rank coal) and EM.
A procedure for the estimation of the in situ moisture is described which involves the use of equations to calculate values for Mis using a range of parameters. The calculated results are then considered in the light of other knowledge of the deposit, such as geological data, other analysis data, data from other local mines, etc, to determine final estimated values. RDis can then be calculated using the Preston & Sanders equation.
Multi-variable regression was also used to develop an equation for washed product moisture levels. The parameter determined is Mcent,best which is a measure of the lowest product moisture likely to be obtained in a properly maintained and operated coal dewatering plant. Likely product moistures will range upward from this value.