Abstract:
The common use of air-swept ball mills in coal power plants is largely attributed to the ease of discharge of the pulverised coal by entrainment in the air stream passing through the mill. The efficiency of mill product discharge (which determines the capacity of the power plant), depends on the mill design and operating conditions. Presented in this paper is an investigation of the effect of liner/lifter profile on the discharge capacity of the mill product. The focus is on assessing the improvement in product discharge rate achievable by retrofitting worn liners with cone-lifters. The cases investigated were (i) bevel (45°, face angle) and (ii) bevel (45°, face angle) modified with cone-lifters. The discharge rate of the mill product was found to be 100% higher in the latter than in the former case. It is postulated that the ‘raking’ action by the cone-lifters helped to mix the load as a whole, which ensured fine particles are more exposed to the classifying air. This presumably enhanced the pre-classification efficiency inside the mill, thereby resulting in more fines removal from the mill. Further analysis was performed to assess the efficiency of particle discharge from the mill for different size classes, using an empirical equation, which is based on mass-size balance of material inside the mill and that discharged.