Coal Preparation » Dewatering
The presence of persistent froth in fine coal flotation concentrates is increasingly common in Australian coal preparation plants, driven by the rising volume of fine coal requiring processing and the prevalent use of high-salinity process water. This froth poses significant challenges in downstream dewatering processes, particularly in thickening and filtration, and effective deaeration of this froth is essential to improving the dewatering efficiency of coal flotation concentrates.
Deaeration columns and water sprays are the most commonly used methods for froth deaeration, however, they are not effective for persistent froth. As a result, more robust techniques are required. In previous project C24040, two pilot-scale froth deaeration units, vacuum and mechanical deaerators, were designed, manufactured and tested in coal preparation plants to break persistent froth. It was demonstrated that the pilot-scale vacuum deaeration system was highly effective in sucking and breaking viscous froth on top of thickeners and its installation did not require any modification of existing thickeners. It was also demonstrated that the mechanical deaeration system, comprising a froth-slurry separator and a spinning basket, was highly effective in breaking stable froth in the flotation concentrate feeding to filters, thereby enhancing the filtration efficiency and capacity.
This project aimed to scale up the innovative froth deaeration systems by designing and manufacturing large demo-scale deaerators for continuous plant trials, verifying their efficiency, identifying potential issues in industrial prototypes, and providing technical supports for future commercialisation.
The vacuum deaerator was designed to handle a processing capacity of 2 t/h solids in froth on top of a thickener. It consists of three key components: a vacuum system, a vacuum deaeration tank and a slurry discharge pump. Multiple froth feed pipes are connected to the vacuum tank, allowing for direct suction of froth from the thickener's surface. To facilitate seamless on-site operation, the system incorporates an automated PLC control system. This control system regulates vacuum pressure to maintain a predefined setpoint and automatically adjusts the slurry pump's frequency to sustain the liquid level within the desired range. This integrated automation ensures efficient operation by maintaining consistent vacuum pressure and liquid levels, optimising performance, and protecting the equipment from potential damage.
Plant 1 trial was conducted to determine the optimal operating conditions and control strategies for the vacuum deaerator, focusing on its performance in froth deaeration and dewatering efficiency under varying vacuum pressures. Results demonstrated that vacuum pressure was the primary factor influencing the deaeration efficiency.
Plant 2 trials were conducted to efficiently remove stable froth from the flotation concentrate slurry through separation and deaeration. The results demonstrated that mechanical deaeration significantly increased flotation product slurry density while substantially reducing air content. At moderate spinning speeds above 400 rpm, pulp density exhibited notable improvements with a corresponding decrease in air content from the slurry inlet to the outlet. Further increases in spinning speed enhanced both pulp density and air content reduction, confirming the effectiveness of the system. Notably, any untreated froth was recirculated within the inner tank to ensure complete deaeration before discharge to the centrifuge. Filtration tests revealed further enhancements in dewatering performance as water content in the froth decreased progressively with increasing spinning speed.
Froth deaeration significantly enhances filtration efficiency, addressing a major bottleneck in the dewatering process. This improvement increases pumping efficiency, enhances slurry level sensor accuracy, and streamlines dewatering operations, yielding both operational benefits and cost savings.