Dense-Medium Cyclone: Plant Experience with High Near-Gravity Material Indian Coals

Abstract

Washery-II of West Bokaro, Tata Steel, India has eight cyclones in a bank treating medium coking coal in the size range of − 13 + 0.5 mm. A three-mode strategy was adopted for increasing the clean coal yield from the washeries. Efforts were made initially to determine the major performance indices that would be used for measuring the separation efficiency. The studies showed that for coals having a high level of near-gravity material (NGM) a new index called the Cone Ratio Feed Rate (CRFR) index—a function of both the cone ratio (ratio of volumetric flow rate of media to the overflow to underflow) and the feed rate — could show better relationships and trends on the Ep, the cutpoint shift, and the coefficient of separation (C.S) than the individual variables themselves. The index was used to obtain the best combination of the feed rate and the cone ratio. The dense-medium or the magnetite suspension, in which the separation takes place, also has a significant impact on the efficiency of the cyclone. The stability and rheology of dense-medium suspensions have shown to influence the performance of the process. High viscosities and high yield stress values result in poor separation efficiency as the coal and gangue are not able to move freely through the suspension to achieve a complete density separation. Addition of a viscosity modifier can reduce the viscosity of the dense medium with minimum effect on stability. It was found that 0.03% of viscosity modifier (based on dry magnetite) gave the best C.S in the dense-medium cyclones (DMCs). Upon implementation of the R&D findings, the coarse clean coal yield increased by 3.8%, thereby increasing the composite clean coal yield by 3%. Alternative techniques that would improve the efficiency of dense-medium separation were also looked into. Laboratory and plant studies conducted with a vorsyl separator (VS) established the superiority of the process over DMC for treating Indian coals.