Coal Preparation » Fine Coal
The overall objective of the TurboFlotation project was the development of an inexpensive compact process based on centrifugally enhanced flotation, capable of reducing the material residence time by an order of magnitude. The project was conducted jointly by the CSIRO Energy Technology Coal Preparation Group and Ludowici Mineral Processing Equipment (LMPE). The project was funded by ACARP, LMPE, CSIRO and an AusIndustry Start Grant.
The concept is based on separating the flotation sub-processes from the main vessel, for individual optimisation. A vertical jet ejector is used for rapid air dispersion in the pipeline, followed by an in-line static mixer that promotes particle/bubble contact and attachment. The mixture is then injected tangentially into a cycloid separator, thus imposing a centrifugal force to enhance the separation process. The result is a compact, high capacity system.
The concepts were first tested on a bench scale unit (90mm diameter separator), at which stage acceptable metallurgical results were obtained with four different coal flotation feeds with a volumetric capacity increase of over 10 times compared with conventional flotation systems. This was scaled-up to a 300 mm diameter pilot scale unit, with a capacity of approx 20 m3/h, which gave equally promising results. This work was completed in 1998, and separately reported as ACARP project C5445.
The objective of this phase of the project was to investigate the potential of this new technology at a commercially viable scale, considered to be a 1m-diameter separator with a capacity of about 600 m3/h.
A sequential approach to scale-up was adopted, based on:
- constant residence time of material flowing through the system
- constant Froude number (the centrifugal force on the slurry as it enters the separator), and
- geometric similitude.
Design, Construction and Installation
Based on the scale-up concepts described above and knowledge gained from the previous phase of the project, pertinent dimensions and process parameters for the 1 m TurboFlotation unit were determined by CSIRO personnel. A process flow diagram was developed which allowed for single stage operation, with or without recycle, or two stage operation. Engineering drawings of all equipment components were prepared by LMPE engineers. High wear areas were lined with wear-resist material and sumps and separators were rubber lined. All equipment components were fabricated by LMPE at their Brisbane facilities or were contracted out. Electrical components were contracted to CRISP Technologies.
The 1 m test unit was assembled at the Coppabella minesite, following a comprehensive hazards and operability (HAZOP) study, as a stand-alone unit with feed and return pipelines serving as interface between the test unit and the main plant. Feed consisting of desliming cyclone overflow that normally goes to the thickener was admitted to the test unit by opening a pneumatic-actuated gate valve. The product and tailings from the test unit were recombined and returned to the thickener feed line downstream of the feed supply point.
Prior to the plant trials, feed samples were obtained for characterisation and to determine the flotation potential. The pulp density was determined to be 6.7% solids by weight with ash content of 20% (adb). Size analysis of the sample indicated a very fine feed, with 60% of the material less than 0.038 mm. To assess the flotation response of the coal, a 'tree' flotation analysis and Australian Standards Basic flotation tests were performed. The basic flotation tests gave an average product yield of 80% with an average product ash of 8.5%. Tests conducted with the pilot scale TurboFlotation unit resulted in yield of 80% and 11% ash in two-stage operation.
Large Scale Results
The 1-m unit generated a 5-7% ash product from a 20% ash feed. Product yields were between 60% and 70% for the Creek Pit coal. These yields were slightly lower than the 73-80% obtained from the Australian Standard Basic Flotation test on the initial coal sample, although the product ashes from the TurboFlotation units were lower than the 7.6-9% from the Standard. The Turbo Flotation units were conservatively run with respect to reagent additions in order to minimise the possibility of upsets to the operations of the main coal preparation plant. Yield by size fraction analysis revealed that the vast majority of the uncollected carbonaceous material is in the minus 0.020 mm size fraction, and this material constitutes a significant proportion of the feed being treated.
The results obtained after the resolution of the initial operational problems were good but not to the level where it could be confidently installed as a commercial operating unit, so that further development is required particularly in the pressure and flow relationship of the air supply systems.