The crystal chemistry of ellestadite for the immobilisation of incinerator fly ash
Date of Issue2015
School of Materials Science and Engineering
Incineration is the method of choice for the volume reduction of municipal waste generated in Singapore. While effective, the total amount of ash produced is substantial, and contains significant quantities of toxic metals, such as Cr, Cd, Pb, As, and V. Typically, incinerator ash is deposited directly in controlled landfills, which is costly and, in some circumstances, leads to the gradual spread of toxic metals via the hydrosphere. Consequently, there is a clear need to investigate new materials for stabilization that address two critical issues - waste loading and chemical durability. This thesis is concerned with the design, synthesis and characterization of ceramic waste form oxides, analogous to durable mineral species, that contain a range of ionic acceptor sites which can capture hazardous cations and anions in their least toxic chemical states. The structural adaptability of apatite waste forms has attracted significant interest for the stabilization of municipal incinerator waste (MSW) fly ashes. The thermodynamic and chemical stability of apatites under a wide range of conditions, and relative insolubility in natural environments make them effective materials for remediation. Apatites adopt hexagonal (or pseudohexagonal) symmetry, the most common space group being P63/m, and conform to the general stoichiometry [A(1)4][A(2)6][(BO4)6][X2]. They are framework structures in which A(1)4B6O24 walls, composed of A(1)O6 metaprism columns corner-connected to the BO4 tetrahedra to create tunnels that surround the A(2)6X2 component. As the relative size of the framework and tunnel contents vary, the metaprism twists through an angle _. In MSW ash, metalloids such as Si, S, As, and V are often relatively abundant, as are the halides (F, Cl, Br) found widely in consumer products. Therefore, for this study, ellestadite-type apatites with the ideal formula Ca10[(SiO4)3(SO4)3](Cl, F)2 were chosen for investigation as an immobilization matrices due to their capacity to incorporate cationic and anionic toxic species. To this end, three ellestadite series Ca10[(SiO4)x(BO4)6-2x(SO4)x][(Cl1-yXy)2] (B = P, V and X = F) were synthesized and the crystallographic structures studied by powderX-ray and neutron diffraction. These synthetic materials, unlike mineral specimens that are well equilibrated, show no Si/P/S ordering and conform to P63/m symmetry, except for the vanadium endmember Ca10[(SiO4)x(VO4)6-2x(SO4)x]Cl2 (x = 0) where the rotation of VO4 tetrahedra lowers the lattice metric to triclinic P-1 to achieve more satisfactory bond-valence sums. As expected, the unit cell volume decreases as smaller cations enter the B sites, with a concomitant increase in the CaO6 metaprism twist angle (_). For fluor-chlorellestadite solid solutions Ca10[(SiO4)3(SO4)3][Cl2 − xFx], all compositions conform to P63/m symmetry where F− is located at the 2a (0, 0, 1/4) position, while Cl− is displaced out of the 6h Ca(2) triangle plane and occupies 4e (0, 0, z) split positions with z ranging from 0.336(3) to 0.4315(3). Increasing fluorine content, leads to a progressive shift of Cl− toward the center of the Ca(2) triangle. Calcium and chlorine deficiencies are always observed in ellestadites due to the evaporation of CaCl2 during the synthesis Leach tests found increasing dissolution of apatite as Si/S progressively replaces P, with a significant change in solubility, and therefore free energy of formation _Gf, especially for x _ 1 in Ca10[(SiO4)x(PO4)6-2x(SO4)x]Cl2. Similarly, the incorporation of vanadium (also a proxy for Cr, As) should not exceed a loading of 25 at% V to avoid significant dissolution. The introduction of F in place of Cl improves stability. Moreover, CaF2 precipitation may serve as a barrier to further ellestadite dissolution. Therefore, ellestadites where Si/S less than 33 at% and V less than 25 at% at B sites prove most chemically resistant to TCLP extractions, and this waste loading should not be exceeded for immobilization of silicon, sulphur and toxic metal rich wastes. These synthetic studies, supported by comprehensive characterization tools, create the systemization required to explore tailoring ellestadites to incorporate MSW ash to high waste loadings and with good durability. Future investigations should include the treatment of real fly ash compositions, with validation of the relationship of crystal structure flexibility and chemical conditioning to enhance durability. The central hypothesis of this thesis, that the chlorellestadites Ca10[(SiO4)2(PO4)2(SO4)2]Cl2 possesses sufficient crystallochemical flexibility for exploitation as a ceramic waste form for the immobilization of incinerator fly ash was proven. However, the compositional boundaries to retain the waste are less extensive than originally envisaged.