Fine Coal Cleaning by Spiral and Water Washing Cyclone

Abstract

The performance of three different types of spirals and a 200 mm diameter water washing cyclone have been assessed in open-circuit on-site testwork at the Norwich Park preparation plant on nominally 500 µ x 75 µm sized coal. The study is part of a program investigating alternative methods to heavy media cyclones and froth flotation for removing ash from fine coal from Central Queensland mines in the nominally 3 mm x 75 µm size range. The two spirals specifically developed for treating fine coal showed better cleaning efficiencies than the water washing cyclone on the +75 µm size fraction in the feed (essentially 1 mm top size). Their single start capacity was about half that of the water washing cyclone for the same clean coal ash content. The spirals readily produced a high ash reject (> 81%) in a single stage and, with the appropriate splitter configuration, a small middlings suitable for retreatment. Middlings splitter setting and vortex finder position (for the water washing cyclone) both provided reasonable control of clean ash at intermediate solids feed rates. For the spirals there was a strong correlation between solids feed rate and both clean coal ash content and yield at constant middlings splitter setting. For small cuts to middlings, separation efficiency remained relatively constant over the full range of feed rates. Relative density partition data indicated that less size classification took place in the spirals although the efficiency of both processes decreased sharply below about 125 µm. For a given clean coal ash the capacity and/or recovery of the spirals and water washing cyclone depended on tshe proportion of -75 µm high ash slimes in the feed.