Integrated drying and screening of small coal particles

Abstract

The recent development of dry beneficiation technologies were motivated by the shortage of water, not only in South Africa, but in many other coal producing countries worldwide. The low capital and operating costs associated with dry coal beneficiation are made even more attractive by the added benefit of saving water. However, a drawback of dry coal beneficiation is the lower achievable efficiencies in relation to wet beneficiation. It is therefore important to maximize the cleaning efficiency of the various dry processes. One way of improving these techniques is finding an efficient dry screening method that would enable a plant to limit the particle size range sent to the dry beneficiation processes, and thus enable these units to operate on more focussed size ranges. Most of the current problems experienced with dry screening are caused by excess surface moisture on the coal particles, which causes particle agglomeration and ineffective screening. The addition of an airflow system or ceramic pre-drying to a standard linear dry screen could improve its efficiency by reducing surface moisture and therefore the influence of the surface moisture on the particles during screening. Allowing fine agglomerated particles to separate from the parent particle. To test this theory, a three-panel linear screen was equipped with an air-blow system at the bottom of the screen, and a dust collection system at the top. A standard coal feed was prepared from crushed and screened coal particles in the -6 mm to +0.5 mm range. The feed was moisture controlled to be at inherent, 5% and 10% moisture content. The prepared coal was fed to the modified screen at different bed depths, subjected to various airflow rates, and pre-dried using ceramic adsorbents in different mass ratios. The efficiency of the process was determined through particle size analysis of the feed, oversize stream and undersize stream, as well as measuring the moisture content of each stream. During the determination of the undersize efficiency of screening -6 mm +0.5 mm (d50 = 3.2 mm) coal particles at screen aperture sizes of 5.6 mm and 2.8 mm the influence of surface moisture on the screening efficiency of the fine coal particles was also tested. An undersize efficiency of 70% was achieved when feeding the 5.6 mm aperture screen with coal at inherent moisture content. The undersize efficiency reduced to only 48% when the aperture of the screen was changed to 2.8 mm. It highlighted the difficulty of dry screening finer coal particles. With the feed coal at 5% surface moisture, the undersize efficiencies for the 5.6 mm and 2.8 mm aperture screens was found to be 68% and 12% respectively, with the greatest effect on particles of 2.8 mm and smaller. At 10% surface moisture in the feed, the undersize efficiency was below 20%

for the 5.6 mm apertures and below 4% for the 2.8 mm apertures, with all particles affected by agglomeration. These results clearly demonstrated the detrimental influence of surface moisture on screening efficiencies. This influence was also exaggerated at deeper bed depths. To increase the screening efficiency of the coal, air was blown from beneath the screen panels to assist in drying the coal and loosen the agglomerates. Under all specified conditions the addition of airflow did not improve the efficiency. The airflow did not have a significant impact on the moisture content of the coal due to the short contact time during screening. A new feed had to be created that was in the -4 mm + 0.5 mm size range to allow easy separation of the 5 mm ceramics by screening. The new feed had a d50 of 1.92 mm. The undersize screening efficiency reduced from 90% at inherent moisture conditions with the new feed to 31% with the addition of 5% moisture content by weight, and to as low as 2% with the addition of 10% moisture content by weight. However, increased screening performance was observed when mixing the feed with ceramic adsorbents prior to feeding the coal with the ceramics to the screen. When using a 1:1 ceramic to coal mass ratio, the undersize efficiency increased to 83% with only 30 seconds of contact prior to screening at 5% moisture content, and to 85% with a 0.5:1 ceramic to coal mass ratio. When screening the adapted feed at 10% moisture content, the efficiencies were increased to 70% and 65% when using a 1:1 ceramic-to-coal mass ratio and a 0.5:1 ceramic-to-coal mass ratio respectively with 30 seconds of contact time. It was determined that contact time was the limiting factor for the reduction of moisture content. This was confirmed by increasing the contact time when screening at 10% moisture content to 120 seconds. The 1:1 ceramic-to-coal mass ratio undersize screening efficiency increased to 89% and the 0.5:1 ceramic-to-coal mass ratio to 86%. In conclusion the airflow did not have any significant impact on the screening efficiency. This was due to too little contact time for the airflow to remove excess surface moisture which is the main cause of ineffective screening. The ceramic adsorbents however improved the screening efficiency by rapidly reducing excess surface moisture prior to and during screening.