Dr James Zhou

A 2 m diameter REFLUX¿ Flotation Cell was fed at roughly 210 m3/h, equivalent to a flux of 1.9 cm/s, about twice the maximum rate used in conventional flotation cells. The coal feed slurry had 59¿64 wt% head ash and nominal size range -0.100 mm (Sauter mean size 0.004 mm). The air, wash water and underflow rates were 180 m3/h, 65 m3/h and 235 m3/h respectively, giving a positive downwards wash water bias flux of 0.2 cm/s. Product ashes of 11¿15 wt% were obtained at combustible recoveries of 58¿75 %, with results on or better than the tree curve. These initial results demonstrate that the beneficial hydrodynamics seen at laboratory scale are realised at full-scale. Also demonstrated is the use of a novel oil-agglomeration technique to obtain detailed performance versus size data, showing that high hydrophobic recoveries were being obtained at sizes down to 0.001 mm.

The REFLUX¿ Classifier is a gravity separation device that consists of a system of inclined channels located above a fluidized bed. Here we report for the first time the results obtained processing a minerals sands feed using narrow 6 mm channels that promote a laminar-shear separation mechanism that enhances the separation based on density. The feed had a head grade of approximately 5 wt% heavy minerals of density greater than 2800 kg/m3, with the majority in the size range from 50 to 150 µm. The overall recovery of the heavy minerals component was approximately constant at 85% for solids throughputs of 10-18.5 t/(m2 h). The unrecovered heavy mineral had an average density close to that of the gangue, hence was deemed unrecoverable by gravity separation. The recovery of the denser and more liberated zirconium mineral exceeded 95%. So this work demonstrated the potential for the heavy mineral to be upgraded by a factor of 16 or more in a single stage operation.

This paper is concerned with the separation of cenosphere particles from fly ash. Cenospheres are hollow alumina silicate micro-shells found in fly ash. They are positively buoyant in water, thus allowing gravity-separation to be used to achieve separation from negatively buoyant fly ash particles. In this study an Inverted Reflux Classifier, a combination of parallel inclined channels and a vertical fluidized bed, was used for the first time to recover and concentrate cenospheres from a real fly ash feed obtained from a coal fired power station. The effects of different operating parameters such as the feed rate, product rate, and fluidization rate were investigated. The device was fed at a solids flux of about 2600 kg/(m2 h). A product grade of 76% was achieved from a feed with a grade of only 0.51%, corresponding to an upgrade of 149. Here, the recovery of the cenospheres was 42%. By increasing the overflow product rate, a significantly higher recovery of 64% was achieved, but at a reduced upgrade of 33. In both cases most of the losses were attributed to the relatively fine cenosphere particles being entrained to the underflow.

Abstract The enhanced separation of valuable positively buoyant cenosphere particles from negatively buoyant fly ash particles using an Inverted Reflux Classifier (IRC) was examined. The effect of the suspension density on the recovery and concentration was examined in the IRC by operating at different feed pulp densities ranging from 10 wt% to 46 wt%. Using a sufficiently high fly ash concentration, it was hypothesised that a powerful bulk streaming phenomenon develops (Batchelor and Van Rensburg, 1986) within the inclined channels, driving the segregation between the positively and negatively buoyant species. With the feed flow rate, fluidization rate, and flow split to overflow and underflow fixed, the recovery of the cenospheres increased from 61.7% (at 10.1% solids) through to an optimum recovery of 89.9% (at 38.1% solids), before declining rapidly to a recovery of 60.2% (at 46.4% solids). The performance at the optimum of 38.1% pulp density was remarkable, with 3.1 t/(m² h) solids throughput, a single-stage cenosphere recovery of 89.9% and upgrade of 58.6, and throughput advantage over a conventional fluidized bed of 54. Detailed analysis indicated that the inclined channels produced an underlying throughput advantage of 18, with a further factor of 3 attributed to the bulk streaming phenomenon. The separations were also assessed in terms of the partitioning of the cenospheres between the overflow and underflow exit streams, with the sharpest size classification evident at the optimum feed pulp density, with the d<inf>25</inf> = 31.5 µm, d<inf>50</inf> = 36.5 µm, and d<inf>75</inf> = 50.0 µm. The separation was then investigated using different feed flow rates, providing the basis needed for ensuring optimum performance in future pilot scale investigation of this novel technology.

Cenospheres are hollow spherical particles formed as part of the fly ash waste of coal-fired power stations. In a previous paper Kiani et al. (2015) investigated the recovery and the concentration of these particles using an Inverted Reflux Classifier (IRC) at a laboratory scale, of cross-section 0.100 m × 0.086 m, achieving a throughput advantage over a conventional fluidized bed by a factor of 54. The present paper investigated the potential to achieve scale-up, utilizing a pilot scale device with cross-section 0.3 m × 0.3 m. The product grade and recovery were examined as a function of the solids yield by varying the product volumetric rate relative to the feed volumetric rate. The performance data were compared directly with those obtained at the smaller laboratory scale. Agreement was excellent. The performance was also examined as a function of the feed slurry flux, with good agreement again evident at the laboratory and pilot scales. Overall, the separation performance was excellent, with a cenosphere recovery of about 80% achievable at a high upgrade of 19 while a recovery of 75% was achieved at an upgrade of 38. Here the feed solids flux was 4.2 t/(m² h). It is noted that much higher upgrade was achieved at a recovery of about 80% in the former study by operating at a lower solids feed flux. This paper provides the necessary basis for proceeding with a full scale implementation of this technology.

Cenospheres are hollow, low-density particles found in power station fly ash. They have many commercially-useful properties which make them a valuable by-product. However, recovering cenospheres from fly ash is difficult due to their low concentration and fine size. Experiments were performed to test the novel approach of using an Inverted Reflux Classifier. In this configuration, the particles are fluidised by adding wash water from above which helps to wash any entrained dense material from the overhead product. Inclined channels are mounted at the base to minimise the loss of buoyant cenospheres in the waste underflow stream. Experiments were performed at both laboratory scale (80 mm × 100 mm cross-section) and pilot scale (300 mm × 300 mm cross section) using mixtures of cenospheres and silica, all nominally less than 100 µm in size. In batch tests, the bed expansion behaviour of the positively-buoyant cenospheres in the Inverted Reflux Classifier was found to be analogous to the behaviour of negatively-buoyant particles in the standard configuration. Continuous steady-state experiments were performed using feeds with suspension solids concentration varying from 0.3 to 9.5 wt.% solids and a buoyant cenosphere grade of 0.5 to 65 wt.%, with a range of fluidisation wash water rates, and degree of volume reduction (ratio of volumetric feed to product rate). Both units delivered high recoveries and product grades. An increase in volume reduction (decreasing overflow rate for a given feed rate), caused a drop in recovery and an improvement in grade. The throughput advantage compared to a conventional teetered (fluidised) bed separator was over 30 in some cases. Both laboratory and pilot-scale units displayed similar behaviour and the results were also consistent with existing correlations for negatively-buoyant particles in the standard Reflux Classifier. Hence this technology has clear potential for recovering and concentrating cenospheres from fly ash. © 2014 Elsevier B.V.