Coal Preparation » Dewatering
The presence of overly stable froths in fine coal flotation products is becoming a common phenomenon in Australian coal preparation plants due to the increasing amount of fine coals which need to be processed and the widespread use of high salinity process water. The overly stable froth causes severe problems in the dewatering process. In thickening, stable coal froths accumulate on top of thickeners, not only resulting in the loss of valuable coals into clarified water but also reducing the thickening efficiency. In filtration, stable coal froths reduce the permeability of water through the filter cakes and also cause the loss of vacuum when vacuum filters are used, all leading to reduced filtration efficiencies.
The use of high salinity water in flotation was identified as a main contributing factor to the presence of overly stable froths. This project investigated the effects of water salinity on the dewatering performance. The results showed that an increase in water salinity significantly decreased the dewatering efficiency due to the increased amount of stable froths. Scanning electron microscope (SEM) analysis indicated an increased particle interaction and aggregation in coal froths with the increase of water salinity, which was responsible for the decreased dewatering performance.
To deaerate the overly stable froth, this project developed both chemical and physical deaeration techniques.
In order to improve the deaeration efficiency while reducing the chemical consumption, a new deaeration strategy was developed. Firstly, the froth in the flotation product was pre-separated from the slurry before conducting any treatment. By doing so, the coal slurry which did not contain stable froths would not need to be treated, which reduced the chemical and energy consumption. Secondly, the separated froth was deaerated using physical methods. After that, the ultrastable froth which could not be broken by physical forces was subject to chemical deaeration. By applying this strategy, the deaeration efficiency was significantly improved and the chemical consumption was significantly reduced.
Case studies were conducted in two plants to test the deaeration techniques developed in the laboratory. Both plants have very stable froths produced in flotation which negatively affect the dewatering process. Plant results showed that surfactant DOSS was the most effective deaeration chemical, which was consistent with the previous lab tests. Spinning basket with 0.81 mm aperture mesh at a rotating speed of 1100 rpm was the most feasible physical method in both plants. Physical deaeration followed by chemical deaeration was proved to be an effective way to improve the overall deaeration efficiency while reducing chemical consumption. Dewatering tests were conducted after deaeration and the results showed a significant improvement in filtration in Plant 1 and in thickening in Plant 2.