Solution-Derived, Chloride-Containing Minerals as a Waste Form for Alkali Chlorides

Sodalite (Na₈(AlSiO₄)₆Cl₂) and cancrinite ((Na,K)₆Ca₂(AlSiO₄)₆Cl₄) are environmentally stable, chloride-containing minerals that are a logical waste form option for the mixed alkali chloride salt waste stream that is generated from a proposed electrochemical separations process during nuclear fuel reprocessing. Here, we discuss a low-temperature, solution-based process to make these phases where sodalite particles are produced in the form of a fine powder with particle sizes on the order of 1–10 μm. Due to the small particle size, these powders require additional treatment to form a monolith. In this study, the powders were pressed into pellets and fired to achieve >90% of the theoretical density of sodalite (2.27 × 10⁻³ kg/m³). The cancrinite structure, identified as the best candidate mineral form in terms of waste loading capacity, was only produced on a limited basis but was converted to sodalite upon firing. Here, we discuss the specifics of the solution-based approach, the chemical durability of select waste forms as well as the steps taken to maximize the chloride-containing phases, decrease chloride loss during pellet firing, and to increase pellet densities.