Coal Preparation » Gravity Separation
Coal Grain Analysis (CGA) characterisation by size and RD provides an amazing insight as to the nature of each particular coal type.
Maceral analysis quantifies a coal in terms of its coal matter components and mineral matter. From that point of view it provides an absolute theoretical delineation of 'marketable reserve' within a coal resource. However, in practice, the coal matter does not exist as discrete particles from the mineral matter. The 'ore' is composite in nature, with coal in the mineral matter, and vice versa.
Existing practice is that float and sink testing is used to estimate the yield of recoverable coal at some particular quality or separation criterion. Float and sink testing (F/S testing) is subject to poor reproducibility and its result is sensitive to the particle topsize and particle size distribution.
CGA assesses the maceral and mineral composition of individual grains (particles). Detailed information about each grain type is able to be deduced such as the relative density (RD) and analytical data such as proximate and ultimate.
Because CGA assesses individual particles, it addresses the short-coming of conventional maceral analysis and allows delineation of recoverable coal matter. Coal preparation requires separation of particles. The key strength of CGA technology is its ability to provide in-depth information on the individual particles (size and component makeup).
It is recommended that CGA should become an integral and fundamental tool for the purpose of definition of marketable reserves pursuant to the Coal Guidelines and The JORC Code as relevant to The VALMIN Code. This may require the CGA method to be formalised in some further manner.
Calculations in this report demonstrate that use of CGA should reduce the cost of resource assessment (drilling and analytical costs) by the order of 25% to 33%.
CGA is routinely undertaken on samples prepared to minus 1 mm. The key challenge for the project was to develop a way to interpret how to 'upscale' the information from -1 mm particles to 50 mm particles. That was achieved by using characterisation (CGA on every RD fraction of every size fraction from 63 mm down) to investigate how the coal grains distributed by size and RD for a given seam. Subsequently, that information was used in reverse as partition tables to provide prediction of both size and washability distribution using only a single raw coal CGA result.
Validation samples were based on the same seam but from totally different sample locations. The results from the validation samples were extremely encouraging.
The grain partition tables have been found to be seam-specific. It is not yet known whether the partition tables may be able to be correlated with rank (or some other parameter), so the technique relies on seam-specific characterisation in the first instance. It remains a potential area for further research to identify whether the CGA characterisation information for a particular seam in one locality may be extended to adjacent coal measures in the same locality.
This project has highlighted the difficulties that result due to the measurement uncertainty of F/S testing. That finding needs to be more widely understood, since coal industry personnel tend to think of float and sink testing as providing absolute data, which is not the case. Information included in an ASTM Standard provides an indication of the large magnitude of F/S testing measurement uncertainty, particularly at RDs of 1.40 and below where there is typically a large proportion of near-gravity material.
For the first time, the efficacy of F/S testing has been able to be directly assessed for the
-2+1 mm and -1+0.038 mm size fractions using CGA. The assessment has identified that fine float and sink testing exhibits severe misplacement errors.
The measurement uncertainty associated with F/S testing needs to be considered by Practitioners and Competent Persons who issue reports pursuant to The VALMIN and JORC Codes.
The recommendations arising from this report include:
· The 'power' of CGA needs to be imparted to the coal industry as a whole, preferably through a series of interactive workshops;
· A technology implementation Strategy and Task Plan need to be prepared so that industry users may adopt a structured and managed approach to implementation;
· Any specific requirements relating to use of CGA-generated data for the purposes of applying the Coal Guidelines to JORC and VALMIN reports needs to be coordinated with the relevant professional bodies and/or committees;
· The measurement uncertainty associated with float and sink testing for particles in the size range of 6.3 to 63 mm needs to be quantified;
· The CGA washability prediction methodology needs to be assessed for its potential to extend to adjacent coal measures within a locality (but that does not prevent its immediate implementation for seams that have been subject to CGA characterisation); and
· Model development, based on fundamental rather than numerical-data-fit methodology, is required in order to extend the prediction methodology beyond seams that have already been characterised.