Ultra-Low Ash Coal by 3D Binder Flotation

The objective of this project was to investigate the potential to produce a coal‐water mixture fuel, ideally less than 1 wt% ash, through liberation by grinding, followed by beneficiation via a novel agglomeration method referred to as 3D Flotation. Different levels of grinding were used to achieve increasingly lower ash product.

Initial experiments focused on a two‐stage agglomeration process, where a coal tailings stream was agglomerated, ground using various grinding media, and then the liberated product was re‐agglomerated. Despite an extensive investigation covering variations in the experimental materials and methods, the agglomeration process was not effective in achieving an ultralow ash product. Previous work on a flotation concentrate, involving no grinding, led to a reduction in the product ash from about 7.3% to 2‐3% with better than 95% combustible recovery, a consequence of rejecting existing slimes from the flotation product. However, when comminution is used to liberate the carbonaceous material from the mineral matter, the agglomeration yields little change in the product ash, but does achieve very high combustible recovery. Overall, the various methods delivered relatively poor product ash values of ~ 8%. Although the binder had the ability to recover particles in an unprecedented size range, and rapidly, the binder was found to recover any particle exhibiting a degree of hydrophobicity. Therefore, the high effectiveness in recovering hydrophobic particles, became a negative factor in its ability to achieve a low ash product.

Following the agglomeration studies, the ability to produce a low ash product using the Reflux Flotation Cell (RFC) was examined. Using a multi‐stage RFC process, combined with grinding in a rod‐mill, a product ash of 4 wt% was generally achieved, with a minimum of 2.7 wt%. Despite the ash content being higher than desired, this result shows the RFC performing remarkably well for such a fine particle feed. A combustible recovery of 88 wt% could be achieved on a coal feed with a Sauter mean diameter of 6.32 μm when operating in a pseudo-continuous system.

This project found the RFC to out‐perform the emulsion binder as a method to produce low ash products. However, it is believed that issues in comminution and particle breakage while grinding, significantly impact the ability to achieve ashes less than 1 wt%. As particles are not cleanly liberated, hydrophobic and hydrophilic material remains as one composite particle. In these situations, the emulsion binder will recover any hydrophobic containing material, resulting in the recovery of the composite material. The coal feeds used in the study also significantly impact the ability to reduce the product ash, with more binary and cleaner systems likely to produce better results.