Doctoral Dissertations

Date of Award

12-1994

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemical Engineering

Major Professor

Robert M. Counce

Committee Members

Milton H. Lietzke, Joseph J. Perona, Jack S. Watson

Abstract

Production operations at many Department of Energy (DOE) sites throughout the United States have resulted in enormous quantities of stored radioactive wastes. Because disposal costs for transuranium bearing wastes are enormous, one option for final disposal of these wastes involves removing the actinides from the waste so the bulk of the waste may be disposed of by less expensive methods. The DOE sponsors development of the transuranium extraction (TRUEX) process for removing the actinides from the waste. The solvent is a mixture of octy(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) and tri-n-butyl phosphate (TBP). The extraction characteristics of CMPO are not as well understood as those of TBP. A study of the extraction characteristics of nitric acid, uranyl nitrate, and bismuth nitrate with CMPO was conducted. In the experimental program, CMPO was dissolved in n-dodecane to produce the organic extracting medium. Three different aqueous systems were used in the tests: (1) nitric acid, (2) uranyl nitrate in nitric acid, and (3) bismuth nitrate in nitric acid. In each experiment aqueous solution was equilibrated with the organic extractant and the concentration of the extract was measured in each phase to obtain distribution data. The objectives of the project were to estimate extraction stoichiometry and equilibrium constants for the extraction of nitric acid, uranyl nitrate, and bismuth nitrate with the CMPO extractant. Experiments were performed over a limited range of concentrations to avoid conditions favoring formation of a third phase. Aqueous nitric acid concentrations were limited to 0.30 M at 25 °C, 1.0 M at 40 °C, and 3.0 M at 50 °C. These limits were lowered by the addition of other nitrates to the system. Uranyl nitrate and bismuth nitrate concentrations were limited to tracer levels. The data indicate that CMPO extracts nitric acid with a 1:1 stoichiometry. The value of the equilibrium constant is 2.660 ± 0.092 at 25 °C. The enthalpy of the extraction is estimated to be -5.46 ± 0.46 kcal/mol. Slope analysis indicates that uranyl nitrate extracts with a mixed equilibria of 1:1 and 2:1 stoichiometries occurring in nearly equal proportion. Over the range of the data the extraction is well modeled by a 2:1 stoichiometry. Effects of nitric acid concentration are well modeled by an aqueous nitrate complexation equilibrium. The equilibrium constant of the 2:1 extraction is 1.213 x 106 ± 3.56 x 104 at 25 °C. The enthalpy of the reaction is estimated to be -9.610 ± 0.594 kcal/mol. The nitrate complexation constant is 8.412 ± 0.579 at 25 °C. The enthalpy of the complexation is estimated to be -10.72 ± 1.87 kcal/mol. Slope analysis studies show that bismuth nitrate also extracts with a mixed equilibria of, perhaps, 1:1 and 2:1 stoichiometries. Over the range of the data a 2:1 extraction equilibrium and a nitrate complexation adequately model the data. The equilibrium constant for the extraction equilibrium is estimated to be 7.847 x 107 ± 4.27 x 106 at 25 °C. The enthalpy of the extraction is estimated to be -18.99 ± 0.82 kcal/mol. The nitrate complexation constant is estimated to be 76.47 ± 12.03 at 25 °C. The enthalpy of the complexation reaction is estimated at -21.75 ± 5.07 kcal/mol.

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