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Coal Preparation

Low Toxicity Liquids for Coal Float / Sink Separations

Coal Preparation » Gravity Separation

Published: May 99Project Number: C7047

Get ReportAuthor: Bob van Emden, Brad Smith, Vincent Patrick | Central Chemical Consulting

Low toxicity aqueous solutions were evaluated as potential replacements for white spirit, perchloroethylene, bromoform and tetrabromoethane in laboratory coal float/sink testing. It was confirmed that aqueous caesium formate solutions spanned the required density range for coal float sink testing (1.1 to 2.2 g/mL) and had sufficiently low viscosity (2 to 5cP) to allow effective separations within typical operating times.

Caesium formate solutions were close to neutral (pH 8 to 9), chemically stable under the expected conditions of use, and did not appear to react significantly with coal. Caesium formate and its concentrated solutions are deliquescent. At densities above 1.7 g/mL its solutions gradually absorb moisture, and at lower densities the solutions gradually lose water. Very little change of density was observed for solutions left exposed in the laboratory.

Caesium formate is of low toxicity and appears to be environmentally acceptable. Caesium formate is already commercially available and will be notified to Worksafe Australia to finalise this project.

Coal float sink separations were accomplished effectively with caesium formate solutions at densities from 1.2 to 2.1 g/mL. Caesium formate exhibited a limited hysteresis effect. The effect on measured ash content was coal specific, and a variation of less than one percent would be generally expected.

The use and recovery of a water based reagent is considerably different to the conventionally used volatile organic solvents. Therefore, a significant part of the project was directed at developing coal washing and formate recovery protocols that would allow caesium formate to be used in coal testing laboratories. Recycling procedures involved three steps: (1) washing the caesium formate from the coal, (2) filtering the caesium formate washings, and (3) evaporation to produce caesium formate solutions of the required density.

Batch washing to maximise removal of caesium formate from coal required 7 litres of water for every kilogram of coal washed. A more efficient method was to use a series of countercurrent batch washes, which required only 1.5 litres of water for every kilogram of coal washed. Recoveries of caesium formate using either coal washing method ranged between 50 % for -2 mm coal to 90 % for +4 mm sized coal. Coal immersed in caesium formate solutions retained from 1 mL/kg for coarse coal to 15 mL/kg for fine coal.

Filtration of coal fines from caesium formate solutions was assisted by using an anionic polyacrylamide flocculant. Improvement in filtration times and an increase in the filtrate clarity resulted when the flocculant was used. Evaporation rates of up to 2 litres/hour were obtained with a 2kW stainless steel low watt density immersion heater and an air stream.

Potassium formate costs about one tenth of the price of caesium formate and is commercially available. Its use alone in coal separations is limited by a maximum density of 1.57 g/mL, but potassium / caesium formate blends can be used up to 2 g/mL. Potassium formate solutions are slightly more viscous, but otherwise are similar to caesium formate solutions.

A series of comparative float/sink tests carried out by participating coal testing laboratories showed that coal testing with caesium formate is comparable to that with conventional organic liquids. Some difficulties were experienced in washing and drying of coal, and in the recovery of formate solutions. Most problems were due to changing to an aqueous system. The protocols developed during this project demonstrated that the difficulties could be overcome by experience with such an aqueous system.

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