Coal Preparation » Fine Coal
The purpose of this work was to examine a novel concept for achieving simultaneous gravity separation and desliming. The novel concept was the application of the dense medium effect within an Inverted Reflux Classifier. This device, which consists of a system of inclined channels below a fluidisation zone, was selected for use as it has been previously used for the successful separation of positively buoyant cenospheres from negatively buoyant fly ash particles. In this application, the lower density coal particles tended to rise relative to the dense magnetite medium, while the mineral matter tended to settle. The magnetite medium was present with the entering feed, and was also present in the downwards fluidization. Conceptually, this downwards fluidization flow of the dense medium through the bed of upward moving fine coal product was used to wash ultrafine particles, including slimes, from the product while promoting the upward transport of the lower density coal. The lower system of inclined channels was used to prevent loss of coal to the underflow reject stream. Here, the low-density coal particles would segregate towards the downwards facing inclined surfaces and hence return to the upper fluidisation zone.
A lab-scale Inverted Reflux Classifier was used in combination with the dense medium consisting of magnetite and water. The work was undertaken in a number of stages. In the first instance, the behaviour of a magnetite-water-only system was investigated. In subsequent runs, the coal feed rate was increased from relatively low levels right up to the pre-defined throughput limit of the system. A lower feed particle size of 0.125 mm was used initially so that the coal could be readily separated from the magnetite. Later, the lower size was reduced to 0.056 mm and 0.063 mm in the higher throughput runs.
The initial work on the dense medium only system showed that a difference in density of 0.49 R.D. units existed between the overflow and underflow, indicating that the magnetite experienced a certain degree of separation within the Inverted Reflux Classifier. However, as the density differential was within an acceptable range, no modifications were made to the dense medium. To overcome this effect, the fluidisation medium was introduced at a higher suspension density than the effective feed slurry density.
The best separation was achieved on a feed with a size range below 1.0 mm, utilising a low feed throughput of 6.8 t/m2h and an effective feed medium density of 1537.4 kg/m3, achieving a combustible recovery of 81.9% at a product ash of 12.2%. This satisfactory performance was consistent with the partition curves which were relatively sharp for particles larger than about 0.3 mm. The separations achieved at the moderate (14 t/m2h) and higher (19 - 20 t/m2h) throughputs, however, were very poor. These poor results were also consistent with the partition curves, which had relatively large Ep values.
It was concluded that in the presence of the dense medium the hindered settling slowed the speed of separation, making the performance vulnerable as the particle size decreased, especially below 0.3 mm. These effects were amplified at the higher solids feed throughputs where the separation performance was very poor. Overall, the performance of the system in the presence of the dense magnetite medium was well below that reported for the conventional water-based Reflux Classifier, in terms of Ep and dependence on the solids throughput.