Effect of Flotation Water Chemistry on Coal Chemistry, Fluidity and Coke Quality

In an effort to minimise fresh water use, coal preparation plants use recycled water. The water lost during recycling is obtained from various sources such as underground and surface water. The use of alternative water sources in coal preparation has significant effects on flotation and coke characteristics since these water sources contain dissolved compounds, both inorganic as well as organic, which can radically alter the chemistry of the system.

Stage 1

Laboratory flotation experiments were conducted to investigate the effect of reagents (including water quality) on the flotation product and its thermoplastic properties. Unfortunately, no high fluidity coal could be sourced for this project. Nevertheless, the yield of particles was strongly dependent on the frother concentration and the water quality through a two-way interaction with the frother. When considering a size-by-size basis, the flotation of coarser particles was strongly dependent on the composition of the water whilst that of finer particles on the frother concentration only. It suggests the quantity and quality of the product, e.g. entrainment vs true flotation, should be closely monitored with respect to the quality of the water and the addition of reagents.

Fluidity tests on coal samples treated in saline water were performed. The tests aimed at studying the effect of salt concentration, filtering efficiency, washing, and coal blending on the thermoplastic properties.

No conclusion could be stated from this work.

Stage 2

In the previous two stages (C25011 and stage 1 of this project), the process water chemistry was found to significantly impact the thermoplastic properties of coal and flotation yield and its ash value. In C25011, dissolved inorganic electrolytes were found to precipitate on the surface of coal upon drying and have a deleterious effect on the maximum fluidity of coal. In stage 1, the maximum yield of flotation was predominantly governed by the frother dosage, while a combined effect between the frother and salt was observed. Salt concentration seemed to decrease the flotation yield at higher frother concentrations. Overall, flotation in saline solution produced a higher ash value at a given yield, indicating a higher clay content, which is detrimental to the thermoplastic properties of the coal product. Unfortunately, due to the lack of high-fluidity coal samples in stage 1, a relationship between flotation results and coal fluidity could not be established. However, flotation results from that stage indicated that there was a strong association between the maximum yield and water recovery, which referred to more salt retention in the coal product and raised another problem (as the increase in yield is associated with clays), that is how the impact of residual salts in the product can be avoided for the sake of the thermoplastic properties of the coal product.

A potential solution in this stage was to evaluate the dewatering process and develop an optimised washing strategy for removing the salts from the filtered products. The objectives of stage were to:

1. Develop a laboratory scale filtration unit to study the removal of residues in the flotation filter cake
2. Optimise the laboratory apparatus under a range of operating parameters and process water received from wash plants
3. Determine the relationship between coal fluidity and flotation filter cake washing
4. Apply the results from objective 2 to on site trials to optimise the efficiency of residue removals from the filter cake

The initially proposed onsite work in objective four was replaced by extending the laboratory investigation on coal and process water samples obtained from the plants. The filtration and washing experiments were carried out with the developed filtration unit. The filter cake structure was examined using a microcomputed tomography (μCT) technique for more insights on the distribution of the pores under the effects of slurry chemistry changes and flocculation.

The effect of particle hydrophobicity on the filtration behaviour was studied. The surface of the coal sample received from the plant was modified with dodecane, which enhanced the surface hydrophobicity of the coal particles, and the combined effect of the flocculation and surface hydrophobicity enhancement on the filtration performance was evaluated.