Coal Preparation » Fine Coal
Fine coal beneficiation is virtually essential for coking coals unless the raw coal is of extremely low ash value, i.e. 10%. Most steel mills seek to have no more than 12% ash in coke and therefore selectivity in beneficiation is more important for lower ash coking coals than for thermal coals which can tolerate higher ash levels.
The need for highly selective coal flotation is therefore mainly related to the coking coal producing mines. Alternatives to flotation for this purpose are few. Hydraulic processes may compete for processing the coarser end of the fines beneficiation requirement but poor selectivity of finer sizes of and the difficulty in excluding clays from the product makes the availability of effective coal flotation a desirable target for the industry.
To think of run of mine coal as a simple mixture of carbonaceous material accompanied by rock and mineral material is simplistic. It is a convenient mechanism to reduce the level of thinking required on a complex issue such as the flotation process, but it means that the level of understanding is also decreased. Recognising a range of particle types and sizes results in a better understanding of the range of flotation kinetics and the factors influencing the kinetics.
Since the 1980s, fine coal cleaning in Australia has been dominated by column- or 'flash' flotation -style flotation. Mechanical flotation machines were considered to be 'old' technology which can be considered to be an apt title given the technology available at that time (Euston, 2010). The mechanical cells were relatively small capacity and a large number of cells were required with individual motors and air controls and a high level of operator involvement.
The objective of this project was to conduct a detailed analysis of the information available, prepare a comparison between the available technologies, develop conclusions based on the evidence available; and make recommendations on further research projects so that the best possible technology for a particular coal type can be identified. The possibility of mixing the three technologies is also considered.
The project required the identification of the most relevant and reliable information to allow appropriate comparisons to be made between the column, 'flash' flotation and mechanical technologies. This was achieved via the examination of the open literature and a survey of operating mines.
A methodology for describing the information has been described in Firth (2008). This methodology allowed the important aspects of the flotation process to be presented in an organised manner and was used as an aid in the comparative study.
The lists of the health-related issues are given together with the appropriate linkages. The detailed conclusions are effectively the health issues shown in the Appendix item. A summary of these conclusions is as follows:
- All three flotation technologies can provide similar technical performance given optimal operation;
- However, the complex nature of the feed in coal flotation, combined with the broad size distribution and material type, makes it virtually impossible to obtain optimal results from a single-step process;
- Also single-step Microcell and Jameson cells invariably have problems with residual frother and poor residence time distributions in large cells, leading to sub-optimal operation;
- Operating any machine beyond its maximum carrying capacity will result in decreased yield, particularly for coarser particles;
- There is a need for improved (non-blocking) wash water systems;
- There is obviously interest in better operation and control, but there would seem to be scope for improved instrumentation and control approaches and the capture of information required to achieve this. But the amount of finance available for this area is limited and low cost solutions (including maintenance) are desirable where feasible;
- Improved understanding of the impact of slurry subdivision and froth transport is needed.
The summary of these recommendations is:
- The optimisation of the flotation process requires a multi-step process. Work is also required to identify the conceptual design of this process along with experimental investigations to provide information which is not currently available. For example, the removal of residual frother by a secondary step. This should include some cost/benefit analysis in order that the 'affordability' of the circuit concept can be defined;
- The link between residence time distribution and staged reagent addition and the recovery of both fast and slow floating particles needs to be explored;
- Maximum carrying-capacity and the factors controlling it need to be identified. This should include froth movement and launder design;
- Research on wash water systems to improve distribution and maintenance is needed;
- Development of new instruments and control approaches accompanied with the subsequent education of end-users is desirable.
The project recognises the work of Sanders and Williamson (1996) which concluded that, at that time, new column and flash flotation technologies appeared to be 'more efficient' than previous technologies. Since this report was published there has been a considerable volume of work published and a reconsideration of the conclusions of this work is included.
An e-newsletter has also been published for this project, highlighting its significance for the industry.