Focus on Coal
Fig. 2. The RockTron process. Picture: RockTron Ltd.
allows gentle agitation and physical separation of the cenospheres under gravity. Te 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. Te remaining
“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.”
Philip Michael, Technical Director, RockTron Ltd.
www.engineerlive.com 44
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. Te whole circuit is fully automated and insensitive to widely fluctuating changes in head grade. Te 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. Te 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 Tree – Magnetite removal. Following
carbon removal, the remaining product is alumino- silicates. Tis 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 4 – classification. Te 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). Tis 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. Tese 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. Tis 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. Tis 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 ≤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. l
Philip Michael is Technical Director, RockTron Ltd, RockTron Ltd, Keynsham, Britsol, UK.
http://rktron.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52