Dense medium separation in an inverted fluidised bed system

The purpose of this work was to examine the novel concept of performing a wet dense medium separation, utilising magnetite, on a fine (−1.0 mm) coal feed in an inverted fluidised bed system. The feed was a natural mixture of coal and mineral particles and the fluidised bed system was an Inverted Reflux Classifier. The Inverted Reflux Classifier, which consists of a system of inclined channels below a fluidisation zone, was selected for use as it has been previously used for the separation of positively buoyant cenospheres from negatively buoyant fly ash particles in water. In the current work, the dense magnetite medium established an elevated bed density such that lower density coal particles tended to rise relative to the medium and high density mineral matter tended to settle. The magnetite medium was present in both the feed to the unit and the downwards fluidisation. At a low feed throughput of 6.8 t/m2 h and an effective bed density of 1537.4 kg/m3, a combustible recovery of 81.9% at a product ash of 12.2% was obtained. Detailed analysis showed the separation performance was significantly poorer than the results reported for a standard water based Reflux Classifier. It was concluded that the presence of the dense medium slowed the speed of separation, leading to unsatisfactory performance as the particle size decreased, especially below 0.3 mm. It was therefore concluded that the standard water based Reflux Classifier was vastly more effective in achieving efficient gravity separation of the fine coal compared to the novel variation examined here.