Enhancement of Uranium Loading on Ion Exchange Resin From Carbonate Leachate Lowering pH From 8 to 6.5
Authors: Otto J.B. 
Published: December 2007

Otto Jr., J.B., SPE, Mobil R and D Corp.

This paper discusses a laboratory study that shows the saturation ion-exchange loading of uranium from carbonate leachate can be doubled by lowering the pH of the leachate from 8 to 6.5. Small column and batch resin-loading tests using Dowex 21K ion-exchange resin are described. The leachate contained 3,300 ppm chloride, 2,400 ppm carbonate, and 220 ppm U3O8, and had a pH of 8. Even at this rather mild salinity the saturation ion-exchange loading was found to be only 3 to 4 lbm U3 O8/cu ft resin [48 to 64 g/dm3] because of competition with the chloride ion for exchange sites on the anionic resin. Lowering the pH of the leachate to 6.5 by CO2 gas addition, however, increased loading to 8 Ibm U3O8/cu ft resin [128 g/dm3]. The pH-lowering effect worked especially well at relatively high salt concentration. The same leachate, with its chloride content increased to 12,000 ppm, loaded, only 0.5 lbm U3O8/cu ft resin [8 g/dm3] at pH 8 but loaded 5.5 lbm U3O8/cu ft resin [88 g/dm3] at pH 6.5. The method is applicable to carbonate uranium in-situ leaching processes, which are operated at pH 8 with 1,000 to 2,000 ppm carbonate as sodium bicarbonate. Under these conditions, ion-exchange loading is greatly increased by lowering the pregnant leachate pH to 6.5 before it enters the ion-exchange column. While enhancement of loading occurs at all salinities, the largest relative changes are noted at higher salinities. Control of pH can be implemented by CO2 gas addition in a carbonator before the pregnant leachate enters the ion-exchange column. After the barren leachate leaves the ion-exchange column, its pH can be raised to the leaching pH of 8 before reinjection by CO2 removal in a decarbonator spray tower. In this way, the pH can be controlled without increasing the salinity of the leachate, which would occur when the pH is controlled by mineral acid addition followed by caustic neutralization.

Introduction

Reaction with anion-exchange resin is commonly used in the industry for recovery of uranium from in-situ carbonate leachates. The equation for this reaction is The anion [UO2(CO3)2]2 is also present and reacts similarly. The capacity of commercial anion-exchange resin is about 1.4 meq/mL [1.4 meq/cm3] water- saturated resin. If all sites were occupied by [UO2(CO3)3]4-, resin loading would be about 6.1 Ibm U3O8/cu ft water-saturated resin [98 g/dm3], while, if all sites were occupied by [UO2 (CO3 )2] 2-, resin loading would be about 12.2 Ibm U3O8/cu ft resin [195 g/dm3]. In actual field practice, 5 lbm U3O8/cu ft resin [80 g/dm3] is considered good loading. The immediate objective of this study was to find a practical way that could be used in the field to load resin efficiently at a Duval County (south Texas) location where chloride concentration of the formation water was relatively high, about 3,500 ppm. If the conventional loading procedure (pH 8) was used, a maximum loading of about 3 lbm U3O8/cu ft resin [48 g/dm3] would be expected. Studies on the application of anion-exchange resin to the recovery of uranium from in-situ leachates have been discussed in papers by Boyce, Bossler and Janke, and Traut et al. The entire process of in-situ leaching and ion-exchange recovery of uranium has been described in a patent issued to Hunkin et al. The entire process has also been discussed by Annamalai and McGarvey. In the first experiments performed in an attempt to solve the problem, a leachate was used that contained about 900 ppm of calcium ion, Ca++, and about 500 ppm of carbonate ion, CO3 =. Because the CO3 concentration was thought to be too low, an additional 1,000 ppm CO3 was added as NaHCO3. This action produced a CaCO3 precipitate. The precipitate was brought back into solution by bubbling the solution with CO2 gas to pH 6.2. When this solution was loaded by the column method on an anion resin, the loading was found to be about double. After refinement of the method, it was found that pH 6.5 with 2,000 ppm CO3 present was an optimal condition and that the method was applicable from 0 to 12,000 ppm Cl - or higher. Thus, the method proved a serendipitous discovery. The increase in loading may be explained by considering the carbonic acid/sodium bicarbonate/sodium carbonate system. As the pH is increased by adding base, the CO3 concentration of the system increases from carbonic acid through sodium carbonate.

JPT

P. 1027^