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

In an effort to minimise fresh water use, a number of Australian coal preparation plants use underground water or recycled water. The use of alternative water sources in coal preparation, especially in flotation, has significant effects on flotation and coke characteristics since these water sources contain many dissolved compounds, both inorganic as well as organic, which radically alter the chemistry of the system. Significant work has been carried out on the role of alternative water sources on flotation performance. However no study has looked into the effect of the use of recycled water on the caking properties (Gieseler plastometry) of coals in preparation plants using recycled water. Whilst some plants have observed a reduction in maximum fluidity, the cause was unknown. This study investigated the effect of recycled water on the caking properties of coals.

Experiments were conducted with a series of inorganic electrolytes which includes: Na₂CO₃, K₂SO₄, Na₂SO₄, CaCO₃, CaCl₂, and MgCl₂ as well as process water. The coal samples were either left to oxidise in a well controlled environment or placed in an inert atmosphere to limit oxidation. The changes occurring on the coals were evaluated by Gieseler plastometry and the surface was characterised by x-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) equipped with an energy dispersive x-ray spectrometer (EDS).

The reduction in fluidity observed due to the use of recycled water may be caused by the oxidation of the coal. The oxidation may be accelerated by the presence of inorganic electrolytes, which were shown by XPS to produce carbon oxides. However, the major effect, which has been observed to reduce fluidity, was from the concentration of inorganic electrolytes in the water. It was shown that even after dewatering a substantially large amount of inorganic electrolytes remains in the water with the coal as evidenced by SEM-EDS. These ions precipitate in the form of solid salts on the surface of the coal particles and affect the caking properties of the coal. De-watering a 0.1 M solution (which is found in coal preparation plants) at 30 % water (w/w) in coal leaves a 2 % (w/w) of salt in the coal. Such amount of salt was sufficient to reduce the maximum fluidity of the coal by approximately 84%. Salts have been known for their de-caking properties. It is speculated that they act as cross-linking agents and produce large macromolecular structures thus combining free radicals which are produced during pyrolysis. These structures are believed to reduce the fluidity of the coal.