Coal Preparation » Fine Coal
Galvin and Pratten (1999) have previously reported the technique of utilizing
the density segregation effect of a narrow size range of particles in a fluidised bed to determine
washability data. This has been verified with replication of yield density information, but to date
there has been no investigation of the performance of the method in generating yield-ash distributions.
This project examines the procedure of using water fluidisation for attaining coal washability data with
a view to providing a rapid, accurate and safe alternative to the
current laboratory method of float-sink testing.
In this study, the primary focus has been on using water fluidisation to obtain
data for the particle size range of -4.0+0.25mm. We have identified that separation is a
combination of the techniques of fluidisation and particle sizing. This study has shown
that the segregation of particles is independent of whether the material is initially
sized into narrow particle size ranges for fluidising or if a broad size range is fluidised
and then the subsamples sized into individual fractions for analysis. This second
option allows a simplification of the analytical technique for obtaining washability
data by water fluidisation.
The new method has been tested using a variety of Australian coals.
Results indicate good agreement between the particle density distribution data
generated by float-sink testing and the new method, although, comparison of
particle ash distribution information shows obvious variation mainly in the
low ash region. This was especially true for one of the coals tested, which we
conclude was atypical. This deviation is due to particle dispersion, which is
an integral part of the fluidisation process and cannot be entirely eliminated.
The application of a universal correction factor to washability information
generated by the new method has been employed to effectively
remove any discrepancy with float-sink data.
Although it is desirable to minimise any discrepancy
between results produced by floatsink testing and the new method,
the wash column data may in fact be an indication of the optimal
performance of a particular coal under autogenous separation conditions,
similar to jigs, teetered bed separators and spirals processing.
The difference may represent the true coal washability performance
characteristics that can be expected from a normal plant processing
environment, in the same way the tree curve characterises
optimum flotation performance.
The major advantage of the new water-based fluidisation
technique is the significant reduction in the use of heavy organic
liquids utilised in the float-sink procedure, and hence considerable
health and environmental benefits. The new method also provides
information on a size-by-size basis. Finally, application of the new
method to fine particles (-0.250 +0.045mm) has also produced results
comparable to the standard laboratory technique of tree flotation.