Coal Preparation

Characterisation and Flotation of Oxidised Coals

Coal Preparation » Fine Coal

Published: March 17Project Number: C23039

Get ReportAuthor: Xumeng Chen and Yongjun Peng | University of Queensland

Coal oxidation has deleterious effects on coal flotation. Using the conventional reagents suitable for floating non-oxidised coals, mild coal oxidation reduces the combustible recovery from flotation while severe oxidation makes the flotation impossible. To help the coal preparation plants to better manage and treat the oxidized coals, this project firstly developed both sophisticated surface analysis techniques and easy-to-use practical tools to characterize coal oxidation. Then, the flotation behaviour of oxidized coals in both fresh water and saline water were quantitatively investigated, and solutions were developed to improve the flotation of oxidized coals.


It was identified that XPS can quantitatively analyse the carbon oxidation species on coal surfaces in its "as received" state. However, the direct application of XPS in plant operation is limited by the high cost of purchasing and maintaining the instrument. Therefore, another two tools which can be easily applied in plant operations were developed. The first one is based on the alkaline extraction of coal oxidation products followed by UV-vis analysis of the filtrate. The second one is developed based on measuring the oxygen adsorption kinetics of coal particles when purging oxygen into a coal particle suspension. Both tools were tested in laboratory and proven to be accurate by correlating to the oxidation degree measured by XPS.


The effect of coal surface oxidation on coal flotation was quantitatively investigated in this study. Coal samples with different degrees of surface oxidation were prepared and the oxidation degree was characterized by XPS. Flotation results showed that the combustible recovery decreased with the increase of surface oxidation degree and this relationship was well fitted using a sigmoidal function. A critical degree of coal oxidation, above which the true flotation became impossible, was identified. The flotation behaviour of oxidized coals in saline water was also studied. It was found that, with the same degree of surface oxidation, a higher combustible recovery was achieved in saline water. In addition, the critical oxidation degree increased with the increase of water salinity. The mechanism studies indicate that both changes of surface charge and bubble size in saline water can compensate the negative effect of surface oxidation on flotation.


Solutions were developed in this project to improve oxidized coal flotation. It was found that the increase of diesel dosage only slightly increased oxidized coal flotation recovery. This project indicates that a composite collector consisting of diesel and a polar collector can be formulated to effectively float the oxidised coals with the ratio of individual collectors depending on the degree of coal surface oxidation.


A case study was conducted using the oxidized coal sample collected from a coal preparation plant in Central Queensland. Very poor flotation performance was observed under plant flotation conditions. The increase of MIBC dosage and diesel dosage partly restored the oxidized coal floatability, but the improvement was limited. Six different types of commercial polar collectors were tested and the best polar collector was identified. The addition of a low dosage of polar collector significantly improved the combustible recovery without any negative effect on the product ash content. In addition, the application of a stronger frother also significantly improved the oxidized coal flotation.


The project presents a systematic approach to improve the flotation of oxidized coals from the characterization of coal oxidation, understanding the effect of coal oxidation on flotation, and applying a new collector system to effectively float oxidized coals.


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