MERCURY EXTRACTION BY THE TRUEX PROCESS SOLVENT .1. KINETICS, EXTRACTABLE SPECIES, AND DEPENDENCE ON NITRIC-ACID CONCENTRATION

Mercury extraction from acidic aqueous solutions by the TRUEX process solvent (0.2 M CMPO - 1.4 M TBP in n-dodecane) has not extensively been examined. Research at the Idaho Chemical Processing Plant (ICPP) is currently being conducted to evaluate the TRUEX process for actinide removal from several acidic waste streams, including liquid sodium-bearing waste (SEW), which contains significant quantities of mercury. Preliminary experiments were performed involving the extraction of Hg-203, added as HgCl2, from 0.01 to 10 M HNO3 solutions. Mercury distribution coefficients (D-Hg) range between 3 and 60 from 0.01 M to 2 M HNO3. At higher nitric acid concentrations, i.e. 5 M HNO3 or greater, D-Hg significantly decreases to values less than 1. These results indicate mercury is extracted from acidic solutions less than or equal to similar to 2 M HNO3 and stripped with nitric acid solutions less than or equal to similar to 5 M HNO3. Experimental results indicate the extractable species is HgCl2 from nitrate media, i.e., chloride must be present in the nitrate feed to extract mercury. Extractions from Hg(NO3)(2) solutions indicated substantially reduced distribution ratios, typically D-Hg < 1, for the range of nitric acid concentrations examined (0.01 to 8 M HNO3). Extraction of mercury, as HgCl2, by the individual components of the TRUEX solvent was also examined from 2 M HNO3. The diluent, n-dodecane, does not measurably extract mercury. With a 1.4 M TBP/n-dodecane solvent, D-Hg similar to 3.4 compared with D-Hg similar to 7 for the TRUEX solvent. Finally, the kinetics of Hg-203 extractions were examined. For contact times varying from 15 seconds to 10 minutes, the distribution coefficient of (HgCl2)-Hg-203 was essentially constant at D-Hg approximate to 18 from 1 M HNO3 solutions. Consequently, mercury extraction by the TRUEX solvent is kinetically rapid.