Coal Preparation » Fine Coal
The main goal of this project was to examine a new approach to maximising the recovery of product specification coal during coal flotation using new generation reagents complemented by the use of the Coal Grain Analysis tool. The Coal Grain Analysis tool, developed by CSIRO, was used to quantify the flotation response of the different types of grains. The technique enables the flotation response of specific grain types, such as pure component and composite grains to be determined so that appropriate remedial action can be taken. The benefits to the industry should be additional saleable coal and a commensurate increase in economic returns.
Previous work had shown that coarse inertite, inertinite-rich and vitrinite-rich composites are poorly recovered using flotation columns such as Microcells and Jameson cells. If a way was found to recover these components, the amount of saleable combustible material would be boosted considerably. It was considered that the poorly floating coals that are lost could be recovered by the use of targeted reagents with minor modifications to the fines circuit. The groups of reagents used included surfactants from the group of tri-block copolymers of polyethylene oxide (PEO) and polypropylene oxide (PPO) often denoted as PEO/PPO/PEO or the reverse block copolymer PPO/PEO/PPO. Tri-block copolymers represent a fascinating class of materials that can be tailored to meet specific requirement in a wide range of applications by varying block composition and molecular weight.
Laboratory flotation experiments in which the novel reagents were employed as promoters by adding small amounts before adding conventional collector (diesel oil) showed that flotation recovery was significantly increased with only a small product quality (ash%) penalty. Analysis of the flotation products using the grain analysis technique determined that whilst the recoveries of most grain types were improved, the coarse composite grains which were the components targeted for enhancement showed the most improvement.
These reagents act to improve combustibles recovery by the hydrophilic groups of the tri-block copolymers, i.e. the polyethylene groups strongly interacting with the less hydrophobic composite grains with the hydrophobic groups oriented away from these surfaces. This then facilitates the spreading of the conventional collector on these sites and therefore results in increased product recovery. As the composite grains can contain intrinsic minerals there is the potential for reduced product quality. Hence the understanding of the mineral-maceral associations in these composite grains is a fundamental requirement to maximise yield by this approach without excessive product quality reduction.
Plant scale test results confirmed the laboratory findings with remarkable improvements in recovery achieved for all components, especially for coarse inertite and composite grains. The difficult to float coals that are lost at this plant may be recovered without significant modification to the fines circuit by the use of targeted reagents.
Another goal of the project was to generate contact angle data for discrete coal maceral groups that would be useful in flotation modelling and simulation. Advancing and receding contact angles of vitrinite and minerals were independently measured on samples determined to be predominantly vitrinite and mineral and then averaged. For inertinite and liptinite the approach used was to measure sessile drop advancing and receding contact angles on heterogeneous coal surfaces whose component composition information had been determined from the coal grain analysis methodology. Then the inertinite and liptinite contact angles were estimated by least square fitting via Cassie's (1948) equation. This provides a pathway for estimating the hydrophobicity of heterogeneous coal particles. It appears that the relatively low receding contact angle determined for inertinite may account for the reduced flotation response of coarse inertinite and composite particles.