Flyash mixtures

Shih and Lin (2003) investigated the solidification/stabilization of arsenic-rich flyash from an abandoned copper smelter in northern Taiwan. The flyashes (2-40 % total arsenic, mostly as As(III)) were collected from three flue gas discharge tunnels. Extremely high cement dosages (cement/waste mass ratio of greater than 6) were required to stabilize the wastes so that they would pass the US TCLP for arsenic (<5 mg L 1 Appendix E). (The TCLP is often used in research outside of the United States.) Cement dosages could be reduced and the mixtures would still pass the TCLP for both arsenic and lead if municipal waste incinerator flyash was added. Lime alone was able to stabilize arsenic and pass the TCLP however, the leachates exceeded the TCLP lead standard of 5mgL 1. The immobilization of arsenic in lime may be due to the formation of sparsely water-soluble calcium arsenites and arsenates, such as CaHAsC>3 //1LO or Ca3(AsC>4)2 H20, where n > 0 (Shih and Lin, 2003, 692). 

Ottosen, Pedersen and Christensen (2004) investigated the removal of arsenic from ashes with an elec-trodialytic method. The ashes resulted from the incineration of wood that had been treated with CCA preservatives. Using the electrodialytic method over five days, Ottosen, Pedersen and Christensen (2004, 300) were able to extract about 92 % of the arsenic from a 40-g mixture of bottom ash and flyash. The ash mixture initially contained 35 000 mg kg-1 of arsenic (Ottosen, Pedersen and Christensen, 2004, 300). Arsenic was found in both the anode and cathode compartments. In the cathode compartment, arsenic was associated with copper (Ottosen, Pedersen and Christensen, 2004, 307). 

Another approach to the suppression of catalytic activity has been taken by workers at the University of Waterloo.26,42-44 After conducting extensive laboratory trials on flyash obtained from a variety of combustion plants, an amine-based destroyer/inhibitor mixture was formulated. This reactant was injected into the boiler of a MSW incinerator in an amount that represented 7-10% of the flyash loading in the flue gas the destroyer in the temperature window 590 + 50°C, and the inhibitor in the temperature window 375 + 50°C. Overall reductions in PCDD/F formation of 80-94% were claimed. This approach has yet to be made commercially available. 

Solidification This method is a modification of the reserve pit method. When drilhng is completed, a mixture of cement, flyash (from coal-fired utility boilers), and/or lime or cement kiln dust is added to the contents of the pit. The liquid in the pit does not necessarily need to be removed. The contents of the pit solidify into a concrete-like block, which immobilizes the heavy metal components. 

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