Calcium Vanadinite: An Alternative Apatite Host for Cl-rich Wastes
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Calcium Vanadinite: An Alternative Apatite Host for Cl-rich Wastes M. R. Gilbert AWE, Aldermaston, Reading, RG7 4PR, UK. ABSTRACT Apatites are often seen as good potential candidates for the immobilization of halide-rich wastes and, in particular, chlorapatite (Ca5(PO4)3Cl) has received much attention in recent years. However, synthesis of chlorapatite waste-forms can produce a complicated multi-phase system, with a number of secondary phases forming, including β-TCP (Ca3(PO4)2), spodiosite (Ca2(PO4)Cl) and pyrophosphate (Ca2P2O7), many of which require elevated temperatures and extended calcinations times to reduce. Calcium vanadinite (Ca5(VO4)3Cl) demonstrates a much simpler phase system, with calcination at 750 °C yielding Ca5(VO4)3Cl together a small quantity of a Ca2V2O7 secondary phase, the formation of which can be retarded by the addition of excess CaCl2. Characterization of compositions doped with SmCl3 as an inactive analogue for AnCl3 show the Cl to be immobilized in the vanadinite whilst the Sm forms a wakefieldite (SmVO4) phase. INTRODUCTION Chlorapatite (Ca5(PO4)3Cl) has been of interest as a waste-form due to its relatively high natural Cl content (6.8 wt. %), which makes it a potential candidate for the immobilization of Clrich wastes arising from the pyrochemical reprocessing of Pu [1,2]. Chlorapatite can be synthesized via conventional solid state synthesis from stoichiometric quantities of CaHPO4, CaCO3, and CaCl2. However, the product formed is not single-phase, but a mix of chlorapatite with a number of secondary phases, including β-TCP (β-Ca3(PO4)2), spodiosite (Ca2(PO4)Cl) and pyrophosphate (Ca2P2O7) [3,4]. These require elevated temperatures and extended calcinations times to reduce, however, to prevent excess volatilization of Cl the maximum calcination temperature possible is 800 °C, and even after 8 hours calcination at this temperature the spodiosite, β-TCP and pyrophosphate phases are still present at levels of approximately 3, 4 and 11 wt. % respectively (as determined by Rietveld refinement) [1,4-5]. Such a varied multi-phase mix presents issues regarding the partitioning of species between different phases and deconvolution of their long-term durability [5]. A simplified system would therefore be beneficial in assessing both its chemical and long-term behavior. Calcium vanadinite (Ca5(VO4)3Cl) is a chlorapatite isomorph, crystallizing in the hexagonal P63/m system with the Cl- anions lying in the [00z] channels. Vanadinites have been viewed as prospective waste-forms for a number of years, with lead iodo-vanadinite (Pb5(VO4)3I) proposed as a potential host for 129I, the iodine able to be immobilized within the channels of the z-axis [6,7]. Synthesis of single-phase Ca5(VO4)3Cl is documented in the literature using a two-step process, firstly reacting CaCO3 and V2O5 in a 3:1 molar ratio at 1100 °C for 10 h to form Ca3(VO4)2, then reacting this with CaCl2 in a 3:1 molar ratio at 900 °C repeatedly for 1 h [8,9]. These processing conditions are too extreme to meet AWE’s requirements (limi
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