Concentration and recovery of positively buoyant cenospheres using an inverted REFLUX classifier

Kiani, Ali

Title: Concentration and recovery of positively buoyant cenospheres using an inverted REFLUX classifier
Creator: Kiani, Ali
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 2017
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: This thesis is concerned with investigating the application of the Inverted REFLUX™ Classifier (IRC™) for separating positively buoyant particles from other negatively buoyant particles. This innovative technology was investigated here for the first time to recover valuable cenospheres, less dense than water, from the fly ash waste of a coal fired power station. Annually, millions of tonnes of fly ash generated from burning coal are discarded to the land surrounding power stations, causing long-lasting environmental and health issues. Fly ash contains valuables components such as cenospheres, unburnt carbon, metals and trace elements. The cenospheres are hollow micro-shells consisting of oxides of silicon and aluminium. These particles represent one of the most valuable components found in fly ash, but at low levels of order 1 wt.%. The particles offer superior properties such as high insulation, high strength, and low density, and hence are valued sometimes up to $ 2000 per tonne. They are positively buoyant in water, and hence wet gravity separation offers the potential for their separation from the negatively buoyant fly ash particles. In this study, wet gravity separation in an Inverted REFLUX™ Classifier was investigated to recover and concentrate cenospheres. The Inverted REFLUX™ Classifier (IRC™) consisted of 1-metre long parallel inclined channels located underneath a 1-metre long vertical liquid fluidized bed. Downwards fluidization was supplied through a distributor at the top for the purpose of washing high density slimes from the low density cenospheres. The inclined channels enhanced the segregation rate of the cenosphere particles leading to a throughput advantage over a conventional fluidized bed. Following preliminary experiments it was shown, using the correlation derived by Laskovski et al. (2006), the throughput advantage of the IRC™ was 32. In other words, for a given separation performance, the feed rate per unit of vessel area to the IRC™ can be increased to a level 32 times higher than for a conventional fluidized bed. These preliminary studies were based on using a model feed, a mixture of commercial cenospheres and silica flour. Then, a real fly ash feed containing around 0.51 wt.% cenospheres was used. At a solids throughput of about 2.3 t/(m² h), a product grade of 76 wt.% and a recovery of about 42 wt.% were obtained, corresponding to an upgrade of about 151. By increasing the product rate, the recovery of cenospheres increased to about 64 wt.%, while the upgrade was reduced to 33. A more systematic study was then conducted using a new feed consisting of about 1 wt.% cenospheres, focussed on the role of the solids concentration in effecting enhanced segregation. Based on the study by Batchelor and Van Rensburg (1986), it was hypothesised that a bulk streaming phenomenon should develop in the inclined channels at sufficiently high cenosphere and fly ash concentrations. Different feed solids concentrations from 10 wt.% to 46 wt.% were used, for a fixed feed flow rate, fluidization rate, and volumetric split between the overflow and underflow. As the feed solids concentration increased from 10.1 wt.% to about 38.1 wt.%, the recovery of the cenospheres increased from 61.7 wt.% to an optimum recovery of 89.9 wt.%, before declining rapidly to a recovery of 60.2 wt.% at a feed solids concentration of about 46.4 wt.%. At the optimum feed solids concentration of 38.1 wt.%, the solids throughput was a remarkable 3.1 t/(m² h), and the upgrade in the cenospheres concentration was 58.6. The overall throughput advantage at the optimum condition was found to be 54, based on a partition curve analysis of the separation size of the cenospheres. More detailed analysis indicated that the inclined channels delivered a throughput advantage of 18, hence it was concluded that a further throughput advantage of 3 was most likely due to the bulk streaming phenomenon. The sharpest size classification was also evident at the optimum feed solids concentration, providing the d₂₅ = 31.5 μm, d₅₀ = 36.5 μm, and d₇₅ = 50.0 μm. The separation performance at the optimum feed solids concentration was further investigated at different feed flow rates and product split ratios, in order to provide the optimum operating conditions to be used in the pilot scale investigation. The potential to scale-up the process by a factor of 10 was investigated using a pilot scale device with cross-section 0.3 m × 0.3 m. The separation performance in the pilot scale IRC™ was compared with that obtained from the laboratory scale performance. The results were found to be consistent. At a solids throughput of about 4.1 t/(m² h), a cenosphere recovery of about 80 wt.% and a high upgrade of 19 were achieved while at a lower product split ratio, a slightly lower recovery of 75 wt.% and a higher upgrade of 38 were achieved. This part of the study provides the necessary basis for justifying a full scale investigation of this technology. The potential benefits of a multi-stage arrangement were also investigated. A fly ash feed with the cenosphere grade of about 0.9 wt.% was subjected to a three-stage IRC™ separation. At the end of the process, a very high grade product of about 97 wt.% (almost pure on a volume basis) was achieved. However, the overall three stage recovery fell to around 50 wt.%, mainly due to the low separation efficiency in Stage 2 of the process. In fact, the second stage involved a very dilute feed, and hence a likely explanation is the lack of the bulk streaming phenomenon under these conditions. It is therefore concluded that the single stage separation offers the best option. A further fly ash feed containing larger cenospheres at an even higher cenosphere concentration was examined in the IRC™. At a high solids throughput of 4.9 t/(m² h), the cenosphere recovery was found to be 93 wt.%, and product grade 80 wt.%. This final study demonstrated the remarkable separation performance that can be achieved, and the fact that in the presence of larger cenospheres high recoveries and upgrades are possible at even higher solids throughputs. The work was also consistent with the earlier findings which show the benefit of a higher cenosphere feed concentration in promoting the bulk streaming phenomenon. This study has investigated for the first time an entirely new technology for separating very low grade buoyant particles from a very high concentration of ultrafine high density particles. The approach is effectively an inverted application of the REFLUX™ Classifier. This thesis has therefore incorporated the analysis developed for the REFLUX™ Classifier, providing a clear basis for assessing this new, inverted, system. Through this approach it has been possible to infer the existence of hydrodynamic benefits that arise from operating at higher concentrations, and in turn elevated solids processing rates. Further investigation of the bulk streaming phenomenon within inclined systems is recommended in order to identify the precise onset of the phenomenon. This benefit has not previously been identified in the separation of cenospheres from fly ash. The overall findings from this study demonstrate a separation performance significantly better than achieved previously by any other technology to date.
Subject: cenospheres; fly ash; inverted reflux classifier; recovery; product grade; thesis by publication
Identifier: http://hdl.handle.net/1959.13/1335572
Identifier: uon:27456
Language: eng
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