Extraction of clean coal from washery rejects and its effect on coking properties: an approach toward sustainable development

ABSTRACT

Since coal washeries generate huge quantities of high-ash rejects as waste, effective utilization of coal rejects help in sustainable coal preparation. Out of different coal rejects, tailings from froth-flotation process can be a potential material for the extraction of clean coal due to its fine size, availability in the slurry form, presence of good carbon values, and better liberation properties. In the present study, three different methodologies i.e., chemical method, chemico-physical method, and physical method are explored to separate clean coal with >49.8% yield for their utilization in coke making process. The chemical method of demineralization by alkali-acid leaching shows a high clean coal yield of 65.4%; however, the alteration of vitrinite-group macerals makes it unsuitable for coke making process. Chemico-physical method of clean coal separation by hydrothermal extraction followed by density separation results in clean coal at 49.8% yield with an increase in vitrinite group macerals, which shows a positive impact on rheological and coking properties. The physical separation method by ultrafine grinding followed by density-gradient-centrifugation resulted in 58.2% of clean coal yield with coking properties similar to the clean coal obtained in conventional coal beneficiation plants. By adopting chemico-physical method and physical method of coal cleaning, the overall clean coal yield of the washery can be improved by >5.1%. Also, there is no deterioration in coke properties such as coke strength after reaction and coke reactivity index by using the prepared clean coal in coal blend.

KEYWORDS:

• Coal
• flotation
• rejects
• waste
• demineralization
• hydrothermal extraction
• coke properties

Acknowledgments

The authors would like to thank Mr. A Suresh, Dr. Debjani Nag, Dr. Rashmi, Mr. Bidyut Das, Mr. Ajinkya Meshram, Mr. Sourav Shome, Mr. Arijit Chatterjee of Tata Steel Ltd. for their extended help in material characterization and carbonization study. Special thanks to Dr. Debaprasad Shee, Dr. Raja Banerjee, Dr. Deepak Raikwar, Mr. Tathi Naidu, Mr. V. Babu, and Mr. Asileti Adarsh of IIT Hyderabad for their continuous support in executing the experimental work.