Cleaning study of a low-rank lignite with DMS, Reichert spiral and flotation

The cleaning potential of a low-rank lignite coal has been evaluated via dense medium separation (DMS), Reichert spiral and flotation techniques. The washability characteristic of the lignite was investigated with laboratory sink and float tests. Solutions of zinc chloride (ZnCl2) with specific gravities ranging from 1.3 to 1.9 were used as dense medium liquid. The as-received raw lignite sample was sieved into three size fractions (−100 + 25 mm; −25 + 1.5 mm; −1.5 mm) and washability characteristics of the former two fractions were separately investigated. The latter fine fraction was cleaned using both DMS and Reichert spiral techniques under different operational parameters. Flotation tests were conducted using the −150 μm fraction. Clean coal characteristics and recoveries were determined after each experiment. tary ash and ±0.1 relative specific gravity curves showed that this low-rank coal can be classified as “difficult to clean” due to its high content of near-gravity materials at specific gravities lower than 1.7. However, about 50% and 40% of the raw coal (3571 kcal/kg) can be obtained as clean coal with calorific values of 5400 kcal/kg for the coarse fraction and 5600 kcal/kg for the fine fraction, by DMS and Reichert spiral methods, respectively. The ash contents (14–18%) and calorific values (6022–5582 kcal/kg) of the cleaned coarse and fine fractions are suitable for use in industry. Low calorific value (about 2500 kcal/kg) middling and tailing can be used in properly constructed thermal electric power plants.