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 bee n examined. Research at the Idaho Chemical Processing Plant (ICPP) is currently being conducted to evaluate the TRUEX process for actinide r emoval from several acidic waste streams, including liquid sodium-bear ing waste (SEW), which contains significant quantities of mercury. Pre liminary experiments were performed involving the extraction of Hg-203 , added as HgCl2, from 0.01 to 10 M HNO3 solutions. Mercury distributi on 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 simil ar to 2 M HNO3 and stripped with nitric acid solutions less than or eq ual to similar to 5 M HNO3. Experimental results indicate the extracta ble species is HgCl2 from nitrate media, i.e., chloride must be presen t in the nitrate feed to extract mercury. Extractions from Hg(NO3)(2) solutions indicated substantially reduced distribution ratios, typical ly D-Hg < 1, for the range of nitric acid concentrations examined (0.0 1 to 8 M HNO3). Extraction of mercury, as HgCl2, by the individual com ponents of the TRUEX solvent was also examined from 2 M HNO3. The dilu ent, 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 const ant at D-Hg approximate to 18 from 1 M HNO3 solutions. Consequently, m ercury extraction by the TRUEX solvent is kinetically rapid.