Cement Based Encapsulation Experiments for Low-radioactive Phosphate Effluent

Abstract

Development work has been carried out for several years on the cementation of phosphate waste from the Tokai Reprocessing Plant Solvent Waste Treatment Facility. The phosphate waste consists of mainly NaH₂PO₄ at a concentration of approximately 440g/ℓ, and has a pH of approximately 4. During attempts to encapsulate this waste in cement the phosphate species interfere with the hydration reaction and retard the setting of the cement and reduce the strength of the wasteform. To mitigate these detrimental effects, pretreatment of the phosphate waste with Ca(OH)₂ prior to cementation has been investigated at small scale. The effect of pre-treating the waste with various amounts of Ca(OH)₂, specified by the Ca(OH)₂/ NaH₂PO₄ molar ratio, was investigated under a range of mixing conditions. The viscosity of the pre-treated waste simulant was recorded and an optical microscope was used to identify the presence of small crystals formed during pre-treated. Cementation of pre-treated phosphate waste simulant was also investigated using various conditions to assess the effects of changing the waste loading, mixing temperature and water/cement ratio. The dimensional stability and strength of the resulting cemented waste was assessed up to 28 days. Results from the trials show that pre-treatment of the waste simulant with Ca(OH)₂ is particularly sensitive to the Ca(OH)₂/ NaH₂PO₄ molar ratio and mixing temperature. These two factors strongly affect the rheology of the pre-treated waste and the acceptability for subsequent cementation. An optimum Ca(OH)₂/ NaH₂PO₄ molar ratio was found to exist and the importance of controlling the waste temperature during pre-treatment was explained. Cementation trials were performed using the pre-treated phosphate waste simulant and slag cement. It has been shown that pre-treated phosphate waste may be encapsulated in slag cement where the wasteform achieved compressive strengths exceeding 10 MPa after 28 days curing at waste loadings exceeding 13wt% (given as a % weight of cement wasteform represented by the NaH₂PO₄ in the untreated waste). Demonstration trials of pre-treatment and encapsulation will be executed at full scale in the future.