Design and Development of Hydro-Squeeze Classifier Assisted Grinding Ball Mill for Narrow Size Particle Separation

Abstract

In the iron and steel industry, the production of desired particle size distribution (PSD) for pellet feed making from the iron ore is very difficult. This study is carried out to achieve desired pellet feed particle size distribution from the iron ore. The iron ores have been collected from three different sources (mines in Karnataka state) and milled. Iron ore obtained from different sources differ in their chemical and physical properties. These variations make the process of grinding a difficult task. The work carried out in this context focuses on three different samples of iron ore, viz., High Silica High Alumina (HSHA), Low Silica High Alumina (LSHA), and Low Silica low Alumina (LSLA). The grinding process for all the three iron ores is carried out individually in Bond‟s ball mill and the total retention time taken by each iron ore sample is calculated. The present investigation focuses on utilizing the calculated retention time of the iron ore as a standard grinding reference time to the laboratory ball mill for optimizing the grinding time of each ore. The desired P80 (150 μm) with an acceptable range of hematite liberation (>75%) was obtained in laboratory ball mill after reducing 6 min from the total retention time taken in the Bond‟s ball mill. The blend as iron ore feed sample was prepared by using High Silica High Alumina, Low Silica High Alumina, and Low Silica low Alumina iron samples, in various proportions. The iron ore blend feed sample is analyzed in the Optical Microscope (OM) and QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) to understand the PSD and percentage of hematite liberation. A new approach was adapted to identify the retention time (RT) of the iron ore blend in the mill, and the total retention time (TRT) taken for the blend sample in the Bond‟s ball mill (BBM) was considered as the reference grinding time for milling in the Laboratory Ball Mill (LBM). The desired PSD (-150 μm) with acceptable hematite liberation was achieved at an optimal grinding time of 7 min in the LBM. ii The discharge end design of a ball mill plays an important role in discharging the desired particle sizes (-150 + 10 μm) and the percentage of recirculating load from the discharge end of the ball mill. In continuous wet ball mills, the composition of feed (hard ore or soft ore) to the mill varies continuously, leading to uncontrolled grinding in the mill. In view of this, a new design of the discharge mechanism has been implemented to remove the ground particles of desired particle size fraction with minimum recirculating load (+150 μm). The results from the discharge end with lifters (closed and open) show that the particle size fraction obtained from the discharge end has a maximum percentage of desired particle size fraction when the mill is operating at 60% critical speed. Discharge end without lifters has an uncontrolled particle size distribution in the discharge and the percentage of desired-size particles discharged was found to be very less. Also, the percentage of the recirculating load is minimum in the case of discharge end with lifter design compared with discharge end without a lifter. Hence, a new design of lifters in the discharge end leads to the discharge of the desired particle size fraction with minimum recirculating load. A new ball mill hydro-squeeze classifier has been developed for particle size reduction and wet classification of different particle sizes. The mill classifier consists of milling and classification sections with a squeezing disc with a mesh of 150 μm. This study analyses the size of particles discharged from the ball mill and the efficiency of hydro-squeeze classifier in separating oversize particles from undersize particles. The ball mill hydro-squeeze classifier was tested at different iron ore feed slurry concentrations and the ball mill operating discharge end opening time. A significant increase in coarse particle discharge from the mill to the classification section was achieved at a higher (75%) slurry solid concentration. The squeezing of the slurry increases the recirculating load to the mill. The squeezing efficiency is maximum (84.8%) at a solid slurry concentration of 55% at a mill discharge opening time of 150 s. The separated particles in the hydro-squeeze classifier consist of 100% -150 μm particles. Further, these particles can be directly used for downstream processing without any classification. The results obtained are applicable for the pilot- iii scale development of a new ball mill hydro-squeeze classifier unit for wet grinding and classification process.