No abstract

NOTICE This document contains information of a prelimins y nature It is subject to revision or correction and Therefore does not represent a final report.

A computational model is developed to calculate thermodynamic phase equilibria in aqueous solutions of fluoride, phosphate, and hydroxide up to 100 °C. A variety of data are used, including isopiestic and electromotive force measurements, freezing point data, vapor pressure data at 100 °C, heat capacities, heats of dilution, and solubility measurements. Pitzer's ion-interaction treatment is used to model electrolyte solutions, and many unknown parameters are determined from existing data through nonlinear least-squares fitting. Phase equilibria are determined by minimization of the total Gibbs energy using a modification of the code SOLGASMIX. Results calculated using the model accurately predict phase equilibria from many quantitative experiments. Qualitative experiments are performed to evaluate calculated solubilities in regions of sparse or nonexistent data; the calculated results are reasonable and exhibit a general qualitative agreement with such data. Model predictions are useful in understanding problems that may arise in the treatment of waste streams containing fluoride and phosphate anions in highly caustic solutions.

A thermodynamic model is developed to describe silicate behavior in solutions containing high concentrations of both NaOH and NaNO3. Experimental solubility data in such solutions are also presented. The model itself applies Pitzer's ion-interaction approach and involves several aqueous polymeric species. Model predictions compare well with a variety of experimental datasolubilities, NMR results, and pH measurements.

Additional components such as nitrite and carbonate will be added to the test matrix. The results will be used to develop safe operating windows for safe waste transf=s at Hadord. In addition to the waste transfer, unwanted solids formation can occur during waste processing activities such as the Enhanced Sludge Washing (ESW). Previous work has shown that phosphate gels and particles are formed as the hot leach solutions from the ESW process are permitted to cool. In another study, insoluble aluminosilicates apparently formed during the hot caustic leaching, which significantly reduced the pefiormance of &e ESW process. The aluminosilicates, as well as the phosphate gels from the ESW process, could also forma pipeline plug. This study confirmed the formation of alurninosilicates during the ESWprocess and demonstrated the importance of a thorough water wash prior to the caustic leaches. The water wash improves the performance and the reproducibility of the ESW process. A key difference in the leach tests with washed sludge and unwashed sludge was the higher ionic strength in the unwashed tests due to the presence of water-soluble salts. In response to the alurninosilicate plug at Savannah River, an equilibrium model on the solubilities of silicate vii species at 25 'C was developed. After the solubilities of silicon and aluminum species at higher temperatures are incorporated into the model, safe operating windows for the evaporator system will -be provided to the tank farm operators. Finally, a simulated chemical plug based on the sodium phosphate plug was developed and provided to the Retrieval Program of the Tanks Focus Area to test recovery methods. Additional simulated plugs have been requested.