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Coal Preparation

Reducing Diesel Consumption While Improving Fine Coal Flotation Through Controlled Diesel Emulsification in Saline Water

Coal Preparation » Fine Coal

Published: February 21Project Number: C28051

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

Diesel collector used in coal flotation is not soluble in water and therefore has a low efficiency in dispersing in coal slurry and adsorbing on coal surfaces. Previous research found that coal flotation can be significantly improved through diesel emulsification in water. However, the application of diesel emulsification in Australian coal preparation plants is still rare mainly due to the lack of:

  • in-situ techniques to characterize the emulsification process in the plant,
  • comprehensive evaluation of feasible technologies for diesel emulsification under plant environments,
  • robust analysis of the benefits of diesel emulsification to plant production.

Furthermore, many Australian coal preparation plants use process water with a high salinity but the salinity was not considered in the previous studies.

In this project, an in-situ system for monitoring diesel emulsification in coal flotation was developed first based on focused beam reflectance measurement (FBRM). Diesel emulsions with a wide range of droplet sizes were prepared and measured by using FBRM and the traditional off-line method, Malvern mastersizer, at the same time. A linear relationship with R2 = 0.96 was identified between the square weighted mean chord length measured by FBRM and the Sauter mean droplet size measured by Malvern mastersizer. This result indicates that FBRM can be used to monitor diesel emulsification in-situ in coal preparation plants.

This project evaluated the performance of rotor-stator mixer, static mixer and power ultrasound in diesel emulsification in water with different salinities. Key operating parameters of each technique were identified. The rotor-stator mixer and static mixer produced a minimum Sauter mean diesel droplet size of around 10 μm in fresh water and low salinity water. However, both were not able to emulsify diesel in high salinity water. In comparison, power ultrasound could produce diesel emulsions with a much finer droplet size, even below 1 μm. In high salinity water, the power ultrasound could still emulsify diesel.

After identifying power ultrasound as the most efficient technique for diesel emulsification, this project developed a continuous ultrasonic emulsification system with a potential to be used in coal preparation plants. It was found that pre-mixing of diesel and water was critical for continuous ultrasonic emulsification, but the droplet size at the pre-mixing stage was not important. Ultrasound power intensity, flow residence time and diesel to water ratio were found to be the important factors in controlling the diesel droplet size. A mathematical model was developed to predict the diesel droplet size based on these factors.

The benefits of diesel emulsification on the flotation of 5 different coal samples from 3 processing plants covering both coking coal and thermal coal were evaluated. For all the coal samples, flotation performance was improved with a decrease in diesel droplet size until an optimum droplet size ranging from 2.4 μm to 6.7 μm depending on the coal type. At a similar flotation performance, emulsified diesel reduced the diesel consumption significantly, compared to un-emulsified diesel.

Overall, this project identified power ultrasound as the most suitable technique for diesel emulsification in coal preparation plants. It was highly efficient in diesel emulsification and was the only technique which could generate a small droplet size to maximize coal flotation. A brief cost-benefit analysis demonstrated that the improvement of flotation yield by applying diesel emulsification would bring an additional revenue of US$8.8M per annum for Plant 1 and US$9.4M per annum for Plant 3. The application of diesel emulsification can also reduce the diesel consumption in the range from 97,412 L to 304,941 L per annum depending on the plant operation conditions.

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