Recycling transforms fly ash into 'eco-minerals'

21st February 2013

Philip Michael explains an innovative recycling technology which is transforming coal-fired power station waste (fly ash) into valuable, 'eco-minerals' on an industrial scale.

RockTron's new plant at Fiddler's Ferry in Widnes, Cheshire, UK (Fig. 1), has the capacity to recycle up to 800,000 tonnes of fly ash a year. It is designed to process both fresh and stockpiled fly ash, effectively solving the current problems of large-scale waste storage and removal, making long term fly ash site remediation possible and, through the recovery of useful eco-minerals, enables the conservation of virgin natural resources. For example, in the concrete sector the partial substitution of traditional CEM I (Ordinary Portland Cement) with RockTron Alpha allows concrete companies to cut their costs, increase their margins and maintain their bottom line.

RockTron’s fly ash beneficiation process uses a traditional mining technology, called 'froth flotation' (Fig. 2). This separates and washes the components that make up fly ash to produce new eco-minerals which have many applications. The overall business objective is to process power station PFA (Pulverised Fuel Ash or fly ash) waste from tip, lagoon and fresh arisings in order to produce economically viable products with no waste or effluent. Historically, BS 3892 and BS EN 450 - the British and European Standards stipulating the use of fly ash as a cement substitute - emphasise the key measures of particle size and carbon content. While dry classification and/or selective removal has been successfully employed for the past 20 years, power stations producing fly ash with a high carbon content had no alternative but to stockpile their waste. So RockTron set out to remove the carbon content in order to produce an economically viable cementitious alternative with typically <5 per cent Loss on Ignition (LOI).

Stage One - Feed. RockTron's plant can accept feed from either stockpiled, lagoon or fresh ash from a power station's precipitators. Fresh ash from electrostatic precipitators is sluiced with recycled process water into a pump suction tank where the pulp density is automatically controlled for optimum pumping to the plant. The slurry is then pumped into a specially designed receiving vessel - the cenospheres removal tank. The vessel's design allows gentle agitation and physical separation of the cenospheres under gravity. The resulting CenTron™ product has particular applications in the automotive and aerospace industries.

Stage Two - Flotation circuit. The remaining slurry is then pumped into the flotation circuit, the central hub of the plant where pulp density is established and mixed with reagents to ensure product quality. The whole circuit is fully automated and insensitive to widely fluctuating changes in head grade. The culmination of these processes causes the carbon to float off. A cleaner circuit cleans it to increase the overall grade with a target of >90 per cent. The carbon is then dewatered on a horizontal belt filter and, if required, flash dried. In filter cake form the carbon is ideal for reuse by the power station

Stage Three - Magnetite removal. Following carbon removal, the remaining feed comprises alumino-silicate particles and spherical magnetic particles in the form of Fe3O4-Al2O3-SiO2. This spherical magnetic component may be recovered, dewatered and dispatched. Removal of the magnetite is discretionary. The remaining slurry is eventually separated into the RockTron Alpha and Delta products for the cementitious market.

Stage four - Classification. The remaining alumino-silicates are then pumped into high efficiency hydrocyclones (used in the clay industry) and classified into two particle size groups resulting in the fine 5-9µm (d50) Alpha particles and the coarser 60-80µm (d50) Delta particles. Delta is then dewatered and stored in bulk. Alpha is dewatered to <0.5 per cent moisture and stored in silos.

Un-beneficiated fly ashes have chemical and physical characteristics that limit their substitution levels due to their relatively low cementitious properties when compared with Ordinary Portland Cement (CEM I). RockTron's beneficiated Alpha has significantly lower LOI (typically <5 per cent) and a finer particle size. These properties enable improved water reduction while increasing the cementitious contribution when used with OPC, resulting from a beneficiating process that produces cementitious material, which is ‘manufactured’ to provide consistent quality, deliver greater fineness, a lighter colour, and is available to the market  throughout the year.

These improved characteristics allow higher levels of cement substitution when compared with normal fly ash. Alpha is currently being used and certified in BS 8500 CEM IIB-V cement composition which permits 21-35 per cent Alpha content. Kirton Concrete and other independents are testing BS8500 CEM IVB-V cement combinations which permit substitution at levels 35 per cent - 55 per cent of Alpha. This will allow structural engineers to design using concrete based on what is required. Cement companies can manufacture to the exact requirements of the customer, ensuring a later age strength and durability greater than that produced from OPC/CEM I alone.

This is reinforced by the characteristics of RockTron's Alpha and Delta: BS EN 450 category A: Max LOI 5 per cent: Alpha typically <5 per cent; BS EN450 category S: Max particle size +45 micron of <12 per cent; Alpha typically 5-6 per cent.

RockTron Delta offers a coarser grade of particle size, making it ideal for applications where water reduction is less critical but the technical benefits of a pozzolanic material are needed, such as lean, precast and mass concrete.

This new technology has applications far beyond the first commercial plant in the UK and RockTron is currently negotiating new plants in the US, Asia, Europe and Russia.

Philip Michael is the co-founder and Technical Director of RockTron Ltd, Keynsham, Bristol, UK.

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