Combination of Microwave and Ultrasonic Irradiations as a Pretreatment Method to Produce Ultraclean Coal

Abstract

Coal samples from concentrate of Shahroud coal preparation plant with an ash content of 13.8% and a total sulfur content of 1.89%, without pretreatment and also with pretreatment by microwave irradiation, ultrasonic waves, and microwave followed by ultrasonic (microwave- ultrasonic), was leached with HF and then HNO3 to produce ultraclean coal (UCC) in a batch reactor. Irradiation pretreatments, in the order of microwave-ultrasonic > ultrasonic > microwave had positive effects on acid leaching with HF, especially for coarse size fractions and early in the leaching process. After HF leaching, the HNO3 complementary leaching was more effective on the microwave-ultrasonic pretreated sample, where, after processing, 95.5% ash content of the sample was removed. A comparison between sulfur content of samples before (1.89%) and after leaching (HF followed by HNO3) shows that the sulfur content of nonpretreated sample decreased to 1.26%, whereas this reduction for microwave-ultrasonic pretreated sample was more noticeable by 0.8%. The results indicated that microwave-ultrasonic irradiation can be considered as the most effective pretreatment method in the chemical leaching of coal to produce UCC.