| dc.description.abstract | Chemically bonded phosphate ceramics (CBPCs) are produced by acid-base reactions between an inorganic oxide and either phosphoric acid solution or an acid-phosphate solution. By taking the advantage of forming chemical phosphate bond and the ability to capsulate wastes, the CBPCs can be used to solidify and/or stabilize toxic wastes. Typical reactants of this acid-base process (i.e., MgO, Fe2O3, and phosphoric acid) can be provided with by industrial wastes: iron-oxide-containing wastes can be obtained from steel making plants, and waste phosphoric acid from IC foundry plants. Some of the iron oxide may also be obtained from the MSWI fly ash. These industrial wastes provide an excellent opportunity to evaluate the feasibility of developing phosphate ceramics from wastes (referred to as WDPCs), and to assess their further applicability to solidify and/or stabilize MSWI fly ash. Accordingly, this study investigated the feasibility of developing phosphate ceramics from wastes such as electric arc furnace dust (EAF dust), blast furnace wet dust (BF dust), cast iron dust (CI dust), and hot-roll wet dust (HR dust); as well as waste phosphoric acid. In this study, the waste-derived
phosphate ceramics (WDPCs) were further evaluated by their capacity to solidify and/or stabilize MSWI fly ash.
The results indicate that the reactive inorganic oxides, mainly iron (III) oxide, in EAF dust, BF wet dust, CI dust, and HR wet dust were found to be 54, 38, 35, and 54% (w/w), respectively. Each gram of the dusts, stoichiometrically, required 0.84, 0.68, 0,68, and 0.98 gram of phosphoric acid (pure) respectively to produce proper WDPCs, or 1.53, 1.24, 1.24, and 1.77 gram for phosphoric acid of averaged 55% concentration. The WDPCs were fabricated by adding water to the previous formula at a solid-to-liquid ratio as determined by a workable viscosity of the WDPC pastes. The
determination resulted in an appropriate solid- to-liquid ratio of 0.52, 0.60, 0.80, and 0.30, respectively for the tested EAF dust, BF dust, CI dust, and HR dust respectively. Phosphoric acid concentration less than 75% was recommended to avoid vigorous acid-base reaction and the excessive forming, which adversely affected the quality of the WDPC pastes. For all WDPC samples, the leaching concentrations of the target metals resulting from the TCLP test were found to be in compliance with the US EPA’’s regulatory thresholds. The effect of solidification/stabilization for Pb and Cu was especially significant. However, some WDPCs failed to develop sufficient regulatory compressive strength for solidified monolith. One the other hand, of the four WDPCs tested, the
maximum ratio for MSWI fly ash to be solidified was found to be 20, 25, and 30% (w/w) for phosphate ceramics derived from the corresponding EAF dust, BF dust, and CI dust; whereas that derived from HR wet dust failed in
solidifying/stabiling MSWI fly ash.
Solidification/stabilization of MSWI fly ash with WDPCs is economically competitive with other disposal methods such as traditional cement
solidification process and can have the added attractiveness of being beneficial to the waste disposal. | en_US |