Challenges in Dewatering of Indian Medium Coking Coals: Case Studies with Chemical Reagents

Abstract

At the West Bokaro Washery of Tata Steel, India, the coarse coal is dewatered in a vibrating basket centrifuge and fine clean coal is dewatered in a screen bowl centrifuge. Due to the higher amount of fine fraction (0.038 mm) treated in the flotation circuit of the washery (about 9% of ROM), the moisture content of the clean coal fines is high. A polyacrylate surfactant reduced the fine coal moisture content by 8% and it was found to be the best among the various reagents tested in the laboratory. The chemical was found very effective for dewatering the coarse clean coal. Laboratory and plant trials showed that through treating only the coarse clean coal by the surfactant, the moisture in the composite clean coal could be reduced from 12.0% to 9.8%. The optimum dosage of the reagent was 0.24 kg=t dry coal. Laboratory-scale filtration using coagulant and surfactant increased filtration rate of centrifuge centrate. Also, the final moisture content was found to be 10% less than that obtained without addition of any reagents. For improving the clarity of thickener overflow, anionic polyacrylate based flocculant was found better than present plant flocculant based on polyamide. The optimum dosage of the recommended flocculant was 0.012-0.016 kg=t of dry fine coal. INTRODUCTIONCoal washeries consume vast quantities of water. After processing, the finished product must be dewatered. Dewatering is accomplished by many devices of which the two most common in practice are: (1) screen-bowl centrifuge and (2) filters operating under vacuum or pressure. The problem of dewatering Indian coals is very challenging due to their high ash content and difficult liberation characteristics. Coal washing in India is carried out at finer sizes leading to high moisture in clean coal. The details of dewatering practice at Tata Steel, India, challenges faced, and the studies conducted with various chemical reagents to overcome this problem are discussed in this paper.Indian coals, specially the medium coking variety, are usually crushed to below 13 mm to achieve optimum liberation. West Bokaro washeries of Tata Steel treat about 4 million tonnes of such coal (30%-35% ash) to produce about 1.8 million tonnes of clean coal having below 16% ash. The coarser fraction (À13 þ 0.5 mm) of the crushed coal is treated in dense medium cyclones (DMC). The finer fraction (À0.5 mm), which is about 20%-25% of the raw coal fed to the washery, is treated with the conventional flotation process. The bulk water associated with the coarse clean coal is first removed through sieve bends and vibratory screens and then subjected to dewatering in vibrating baskettype centrifuges. The fine clean coal in the form of froth is subjected to dewatering in screen-bowl centrifuges. The middlings and the rejects are dewatered through sieve bends and vibrating screens. The tailings (pulp density of about 3%-5%) of froth flotation are first thickened for concentrating the slurry suspension and also to recover clarified water for reuse in the plant. The thickener underflow i...