Coal Preparation » Gravity Separation
The use of large diameter (greater than 1 m) dense medium cyclones has become widespread in the Australian coal industry, and in the future is likely to be the dominant processing unit. There is little readily available data on the efficiency of these units, particularly with respect to the impact of medium to coal ratio and their effectiveness in treating -4 mm coal. Access to such information will allow the optimisation of the design and operation of plants that include these units.
This project was essentially a scoping study to identify key operating traits of large diameter dense medium cyclones, and to recommend project areas that would yield benefits to the industry if further R&D were carried out.
Project information was gathered from published data, a survey of unpublished plant audit data, and detailed sampling of two operations. Project outcomes were based on the collation, correlation and evaluation of these data..
Readily available information from the literature has been collated to provide a firm basis for the consideration of the observed operational results. Key points to come from this review were:
- for a given cyclone diameter, there is a particle size, termed the 'breakaway size', below which Ep deteriorates rapidly. As cyclone diameter increases, the breakaway size becomes larger - around 5 mm for 1000 mm cyclones, compared with just over 2 mm for 610 mm cyclones
- below the breakaway size, Ep increases hyperbolically as particle size decreases
- also, below the breakaway size, RD50 increases as particle size decreases, by around 0.05 as particle size falls from 4 mm to 1 mm - termed 'density shift'
- density shift will further contribute to a poorer overall Ep
- medium:coal ratio (M:C) can have a substantial impact on DMC operation, and will have a disproportionately large impact on the finer particles
- if M:C is greater than 3:1 in the DMC overflow, then there is little impact on cyclone operation with low Eps and predictable offset between medium density and cut-point
- if overflow M:C falls below 2:1, then there is substantial loss of low gravity material to the underflow, much poorer Eps and reduced RD50, even below the medium density.
For some time now, there has been a feeling that Ep (75-25) alone does not completely describe the efficiency of cyclone operation, and there have been suggestions of using Ep (90-10) or even Ep (95-5). Error Area was found to be a reliable method of assessing unit efficiency and, together with modern curve fitting techniques, gives a broader and more applicable method of performance assessment, particularly in relation to the fine coal (-4 mm) in the process feed.
Although there are some variations in the data obtained from operations and the plant sampling carried out, the following trends were observed:
- Generally cyclones gave acceptable results for the processing of +4 mm material, despite some very high levels of near gravity material, with Ep around 0.02 and Aer 10-20.
- As particle size decreased, there was an increase in D50 and deterioration in efficiency (Ep and cut-point 'drift').
- Efficiencies were poorer at higher D50s
- The tendency was for efficiencies on finer particles to deteriorate as cyclone diameter increases.
From the data obtained it was difficult to discern any significant findings with respect to medium to coal ratio.
On the basis of Ep only, the 1000 mm diameter DMC is considered to be in the same order of efficiency as the 710 mm diameter cyclone examined in the detailed JK Handbook work carried out by Wood.
The data examined in this preliminary review showed good correspondence to the trends expected from the JK Model. This encourages further collection and collation of data to develop working models for the industry to use in applying large dense medium cyclone technology in optimising coal preparation.
There were insufficient data supplied in relation to the larger cyclones, up to 1300 mm diameter, to provide the detailed trends and data as were generated on the 1000 mm cyclone. The single set of 1300 mm diameter DMC data indicated that the larger cyclone was less efficient than the 1000 mm cyclone. However, significantly more data is needed before any further comments are made. This is an important area for further studies.