Developing Operator Friendly Coal Oxidation Measurement

Flotation of oxidized coals is a challenge confronting many Australian coal preparation plants. Previous projects have demonstrated that polar collectors can be added together with diesel to improve the flotation of oxidized coals, however, the ratio between polar collector and diesel needs to be optimised based on the degree of coal oxidation. An oxidation measurement system has also been developed in previous projects and implemented in the plant laboratory, however, this system has some drawbacks including the use of hazardous chemicals, the requirement of heating the solvent to 90 oC, a long measurement time and also the need for a clean laboratory for the setup of the UV/Vis spectrophotometer. These drawbacks can restrict the application of the measurement system in coal preparation plants. The coal preparation plants require a more operator friendly oxidation measurement system which can be used directly in the plant and can produce results in minutes. This is particularly important for the plants reprocessing historical tailings where the degree of coal oxidation in flotation feeds can vary significantly.

In this project, a new operator-friendly coal oxidation measurement system was developed and tested on a range of flotation feed samples supplied from coal preparation plants. This system uses a new solvent which is safe to handle and can complete the extraction at room temperature in a short time. A portable UV spectrophotometer with a small size was also identified for plant applications. It is powered by battery and is IP 65 rated. A detailed procedure and a heavy duty case containing all measurement materials were also developed for an easy application in plants. In addition, a new slurry sampler which can be directly connected to the plant sampling hose was developed to representatively sample 25mL slurry from the flotation feed stream in plant for the oxidation measurement. With these improvements, the new coal oxidation measurement system can be directly used in coal preparation plants without the need of a laboratory.

This project also identified that, after normalising the UV oxidation measurement value based on the combustible material content in the sample, it could be well correlated with the real concentration of total oxidized carbon measured by X-ray photoelectron spectroscopy (XPS). In addition, all the flotation data from both lab testing and plant trials correlated well with the normalised oxidation value of coal samples from different mine sites. In general, the combustible recovery decreases sharply with the increase of coal oxidation degree until it reaches a critical oxidation value. This normalised oxidation value can be used as a standard for evaluating the oxidation degree of coal samples and predicting the flotation performance from different deposits and mines.

This project tested three polar collectors including two commercial ones and one pure chemical identified by UQ on four coal samples with different degrees of oxidation. It was found that there was an optimal ratio between the polar collector and diesel for delivering the maximum improvement for coal flotation. In addition, the optimal ratio had a direct correlation with the normalized degree of coal oxidation measured by the new oxidation measurement system. A model was developed to predict the optimal polar collector/diesel ratio based on the normalized oxidation value. The rapid coal oxidation measurement system together with the model can be used to guide plant engineers to maximise the flotation of oxidised coals.