Quality Of Dense Medium

The objective of this project was to quantitatively detail the impact the particle characteristics constituting the dense medium in a coal preparation plant have on the properties which are key in defining the performance of a dense medium circuit; that is the viscosity and medium stability.  Secondary objectives were:

• The critical evaluation of methods to quantitatively measure the values for viscosity and medium stability; and

• The development of a Handbook for assisting plant staff in the operation of their dense medium cyclone circuits.

Background

The use of a dense medium provides the most efficient and cost effective approach for the separation of clean coal from raw coal. Finely ground magnetite has become the medium of choice for the Australian coal preparation industry. While the magnetite being supplied to the plant has a nominal size distribution, the actual characteristics of the steady state medium found in the plant will be a balance between a number of activities occurring within the circuit :

• Some of the material in the feed magnetite has poor magnetic susceptibility and is quickly lost by the magnetic separators.

• The operation of the magnetic separators is such that complete recovery of magnetite is not possible.
• The desliming screens do not remove all of the particles less than the nominal bottom size set for the DMC circuit.
• Some of the magnetite is not recovered from the drain and rinse screens.

The clay bands and shale tend to disintegrate in the water as they pass through the DMC circuit and, since the majority of the medium reports to the 'correct' medium stream from the drain and rinse screens, there is accumulation of this material.

To control the level of 'build up' of material, some of the 'correct' medium is also sent to the magnetic separators to reduce the level of contamination from non-magnetic material.

In some plants water carry over on the drain and rinse screens can cause detrimental effects to the medium as the non-mags within the medium are diluted which can lead to an unstable medium.

The presence of slimes in the medium will increase its stability by virtue of the particle size and low mineral density, and on operating plants the level of build up of these slimes is normally closely controlled. In some cases a build up of slimes to increase suspension stability is desirable, and clay is sometimes added for this purpose.  

Conclusions and recommendations

The F5 settling test was found to be unreliable for the medium suspensions used in Australia with dense medium cyclones. It may have relevance to suspensions used in dense medium baths. Comment on this test should be removed from the technical information used in the industry for the education of new practitioners.

From the extended Einstein equation the calculated viscosities for plant samples were expected to be in the range of 1.4 cp to 2 cp. This indicates that the amount of material in the colloidal size range is relatively small and thus would have little effect on the flow of solids in a dense medium cyclone. The measured viscosities of the synthetic medium samples were within this range and showed no correlation with the stability as measured with settling tests.

Settling tests on the synthetic medium slurries showed some distinct linkages to the measured parameters, particularly the volume fraction of the non-magnetics present.

The results suggest that the major factors that influence the settling behaviour of the medium are the volume fraction of the non-magnetics and the mean size of the magnetite particles. There is therefore potential for improving stability, and hence performance, through tailoring and controlling the size distribution and the proportions of magnetics and non-magnetics.

These results show that more experimental work is required to produce a handbook for use in the coal preparation industry. This report goes someway to providing the required data but other factors such as the rate of build up of fines in the medium circuit and consideration of differing fines build up within loops is not addressed. Furthermore the effect of plant stoppages on the medium quality is not addressed. The measurement of the DMC underflow density to determine density differential should also be investigated as a means to establish the medium stability.

The potential for improving stability by tailoring and controlling the size distribution and proportions of  magnetics and non-magnetics should also be investigated.