Recycling Transforms Fly Ash Into Eco-Minerals

(Engineering Live 4-27-2010) RockTron's new plant at Fiddler's Ferry in Widnes, Cheshire, UK (Fig. 1), can recycle 800,000 tonnes of fly ash a year. It is designed to process both fresh and stockpiled fly ash, effectively solving the problems of large-scale waste storage and removal, site remediation and conservation of natural resources. This GGBS, or CEM I (Portland cement (PC)) substitution proposition, allows companies to cut their costs, increase their margins and maintain their bottom line. RockTron is currently negotiating new plants in the US, Malaysia and Russia.
Essentially, RockTron uses a traditional mining technology, called 'froth flotation'. This beneficiation process separates and washes the components that make up fly ash to produce new eco-minerals which have many applications (Fig. 2). The overall objective was to process power station PFA waste from tip, lagoon and fresh arisings 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 <2 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 (typically 30 per cent solids). Star valves in the fly ash silo automatically control the solids rate. 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. Fly ash recovered from the tip or lagoon (1km away) is repulped to its original particle size.
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 product is alumino-silicates. This slurry is eventually separated into the RockTron Alpha and Delta products for the cementitious market. However, these alumino-silicates also contain spherical magnetic particles in the form of Fe3O4-Al2O3-SiO2, a form of magnetic rich glass spheres. Removal of the magnetite is discretionary.
Stage four - classification. The remaining alumino-silicates are then pumped into high efficiency hydrocyclones (used in the clay industry), classified into two particle size groups resulting in the fine 7µm (d50) Alpha particles and the coarser 45µm (d50) Delta particles. Delta is then dewatered to 15 per cent moisture 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 (<2 per cent). This is much finer and ensures water reduction increasing its cementitious contribution when used with PC, resulting in a beneficiating process that will produce material with controlled LoI, fineness and colour, with low variability available throughout the year, derived from fresh or stored fly ash.
These improved characteristics will allow higher levels of cement substitution when compared with normal fly ash. However, under BS EN 450 fly ash substitution is restricted. Alpha conforms fully to BS EN 450-1:2005+A1:2007 but substitution can exceed 50 per cent. Kirton Concrete and other independents are testing substitution at >60 per cent. 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/CEMI alone.
This is reinforced by the characteristics of RockTron's Alpha and Delta: consistent colour; consistent supply; guaranteed low LoI of 2 per cent; meets and exceeds BS EN 450; category A: LoI [s39]5 per cent: Alpha <2 per cent; category S: Particle size classification of <12 per cent; Alpha <7 per cent.
RockTron Delta achieves <5 per cent LoI, ensuring light colour and strength (ie, carbon is soft and carries water) - 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. 
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