Coal - Study on the Electrostatic Concentration of Low Ash Coal in Corona Discharge Field

In order to recover coal concentrate of low ash and low iron content, the electrostatic separation of coal was studied. It is more successful in the corona discharge field to recover the coal concentrate of low ash and low iron content than in the field without corona discharge. The effect of humidity on the electrostatic separation of coal was then studied. From the experimental results, it was clear that the electrostatic separation was excellent in about 60% relative humidity of atmosphere. Thus, the separation of vitrite from coal is practicable by electrostatic separation. In recent years the demand for high quality coal containing low ash, low iron, and low sulfur has increased. To fulfill this requirement, a higher grade coal concentrate must be produced. Study on the electrostatic separation of coal has been performed for several years in the author's laboratory. From the results of this research, it was considered that the behavior of coal particles in the electrostatic field relates closely to the ash and iron content in coal. In the present paper, the electrostatic separation of coal in corona discharge field was studied in order to recover the high quality coal concentrate and then the effect of humidity on the electrostatic separation was investigated. PREPARATION OF COAL SAMPLES The coals from the Akabira mine, the Kushiro mine, the Miike mine, and the Oshima mine which are famous coal mines in Japan were used in this study. The ash contents of these coals are shown in Table I. From the results of the analysis of the coal samples by the Norelco X-ray diffractometer, it is recognized that the Miike coal includes mainly pyrite, quartz, kaoline, calcite, and epidote; the Kushiro coal: kaoline, quartz, and montmorillonite, and the Oshima coal: siderite, dolomite, and other substances. For the preparation of samples, the coarse pieces of coal were crushed to about 2 mm in diam and then ground in an agator mortar. The ground products were sieved and fractions of 10 — 14 mesh, 14 — 20 mesh, 20-28 mesh, and 28-35 mesh were prepared, respectively. APPARATUS OF ELECTROSTATIC SEPARATION The principal part of the apparatus of the electrostatic separation used in the experiments is shown in Fig. 1. In order to generate the corona discharge field, a needle-shaped electrode combined with a pole electrode is installed as the high tension electrode. In the case of investigating the effects of the humidity on the electrostatic separation of coal, the needle-shaped electrode was combined with a plate electrode instead of the pole electrode as shown in Fig. 2. The pole or the plate electrode was installed to promote the repelling of the charged particles by producing the electrostatic field between this electrode and the grounded roller. It seems to be very useful on the coal separation to install these combined high tension electrodes. SEPARATION OF COAL PARTICLES IN CORONA DISCHARGE FIELD The separation testings on the coal samples in the corona discharge field were performed by the apparatus as shown in Fig. 1. These experiments were carried out under the following conditions: feed size 20 — 28 mesh, room temperature, normal humidity and applied voltage 15-20 KV, and the combined electrodes including the needle-shaped electrode were negatively charged. The results of these testings are