Coal Preparation » Fine Coal
In a simplistic view, the conerstone of the emerging new flotation technology is the fineness of air bubbles. In contrast with the mechanically agitated cells new technology may be termed collectively as 'microbubble flotation'.
Amongst the commercially available new technology a distinction should be made between two different types of processes. The first type is the true column flotation where air is admitted separately from the feed in a tall vessel. Microcel is an example of this type. The other type uses a much more shallow vessel and the air is admitted with the feed, mixing occurring in the feed inlet passage. These may be called 'shallow columns', or, to note their short retention time, 'flash flotation' type. Both Jameson and Ekof belong to this type.
All published data, laboratory and full scale, show that all of the three processes investigated return invariably, superior performance to that of the mechanically agitated cells. It is a proven fact that the industrial performance of new flotation technology represents a major step forward in metallurgical efficiency.
One of the most significant aspects of microbubble flotation is the capacity of the process to vary product ash within a much wider range and with greater accuracy than was ever possible with mechanically agitated cells.
Reagent consumption, according to conventional wisdom, accounts for 50-60% of flotation operating costs. Column flotation is expected to require more reagents on account of the much larger bubble surface available. This was found to be true, with frother consumption about double the rates used in conventional flotation. Experience so far shows that this can be expected to be substantially reduced as operating accumulates and new reagents may be developed to suit better the new process of dynamics.
New flotation technology is in its adolescence. Process development has reached commercial application.
The next stage of development, to maturity, involves effective integration into the preparation plant. Compatibility with other processes, such as reagent effects, complementary contribution to overall plant performance, impact on operating practices/effectiveness and other effects not yet evident will have to be understood. Wear and tear will need to be assessed much more accurately.
Materials of construction, particularly for high wear components such as nozzles, spargers and pipes need to be optimised. Some may be readily fixed but others may require considerable effort before they will be solved. This integration to maturity can only be achieved via full scale operation.
With three alternative processes already on commercial scale available to the Australian Coal Industry, there is no justification to fund R&D into new processes. The areas where industry funding could bring benefits are in the integration of the new processes into the plant. Areas where such opportunities may exist are:
- investigation of simple means of performance monitoring, such as optical monitoring;
- support/joint funding of the development of flotation reagent;
- fines dewatering;
- concentrates handling.
From the data seen and the discussions with plant management and operators of these technologies on industrial scale and preparation engineers who tested pilot scale units on their plants, the study could not but conclude that these new processes will inevitably take over froth flotation the Australian Coal Industry. It is difficult to visualise new installations of mechanically agitated cells in competition with the performance of microbubble flotation.
Comparison with gravity based processes currently available shows that the metallurgical performance of microbubble flotation is superior to all others on the -0.5mm sizes.
Detailed capital costs were unavailable. Most exciting installations have been retrofits or replacements and were obviously justified by performance. A road review of verbal (mainly confidential) information from plant management and comparison of the equipment and engineering involved, suggest that new technology flotation is probably, similarly based on hard equipment costs. These observations are made on a very few number of new installations. It is expected that with more plants coming on stream more accurate details of the capital costs will become available. Competition between the three processes may also help to lower the costs to the industry.
Although originally developed for the finest sizes, microbubble technology appears to be able to separate and lift coarser sizes as well as the fines. This is highly significant because of its potential impact on future processing strategies. This capability has been studied less and will need to be established. Judging from industry interest, it will be evaluated sooner rather than later. If the capability to treat effectively, sizes above 0.5mm is proven, then the preparation of the future will be more efficient and easier to operate, in every respect. We can see a plant with HM cyclones down to say 0.8 or 1.0mm and a very efficient flotation to follow down to 0mm.
Predicted results of "ideal" performance still vary considerably. The tree procedure should be reviewed and other measures of performance may be researched and developed.
- New flotation processes reviewed in this report are highly efficient and available competitively for the Australian Coal Industry. With commercial suppliers of new technology already well established, any new research, however prospective the ideas may be, is likely to return little or no benefit to the Industry. The study recommends no further research into the process of flotation or in new flotation processes for the foreseeable future.
- Whilst currently not recognised as a serious need, areas of monitoring and control may be present opportunities for useful. An example of such research is the current work on froth characterisation using digital vision technology. These opportunities should be evaluated on their merits at the time.
- The developments of new reagents is likely to be needed to realise the full benefits of new technology. Industry funding - suggest jointly with reagent suppliers - could be used effectively to speed up such development.
- The importance of new reagents is likely to be needed to realise the full benefits of new technology. It is recommended to continue with Industry funding of fines dewatering research.
- A continual program of information transfer/dissemination is considered to be highly desirable and is recommended. This may take the form of a one day seminar at various locations convenient to the coalfields and could be arranged under the aegis of the Coal Preparation Society. Consideration should be given to acquiring well substantiated operating data by ACARP funded performance studies.
- Given that microbubble flotation is just starting to be established on a commercial scale, a review of the status of flotation in about five year's time is recommended.
- Review 'tree procedure' and other measures with the view to a better 'ideal' measure and comparison.