Magnesium hydroxide as the neutralizing agent for radioactive hydrochloric acid solutions

摘要

The current technology at Los Alamos for removing actinides from acidic chloride waste streams is precipitation with approximately 10 M potassium hydroxide. Although successful, there are many inherent drawbacks to this precipitation technique which will be detailed in this paper. Magnesium hydroxide (K(sub sp) = 1.3 x 10(sup -11)) has limited solubility in water and as a result of the common ion effect, cannot generate a filtrate with a pH greater than 9. At a pH of 9, calcium (K(sub sp) = 5.5 x 10(sup -6)) will not coprecipitate as the hydroxide. This is an important factor since many acidic chloride feeds to hydroxide precipitation contain significant amounts of calcium. In addition, neutralization with Mg(OH)(sub 2) produces a more filterable precipitate because neutralization occurs as the Mg(OH)(sub 2) is dissolved by the acid rather than as a result of the much faster liquid/liquid reaction of KOH with the waste acid. This slower solid/liquid reaction allows time for crystal growth to occur and produces more easily filterable precipitates. On the other hand, neutralization of spent acid with strong KOH that yields numerous hydroxide ions in solution almost instantaneously forming a much larger volume of small crystallites that result in gelatinous, slow-filtering precipitates. Magnesium hydroxide also offers a safety advantage. Although mildly irritating, it is a weak base and safe and easy to handle. From a waste minimization perspective, Mg(OH)(sub 2) offers many advantages. First, the magnesium hydroxide is added as a solid. This step eliminates the diluent water used in KOH neutralizations. Secondly, because the particle size of the precipitate is larger, more actinides are caught on the filter paper resulting in a smaller amount of actinide being transferred to the TA-50 Liquid Waste Treatment Facility. Third, the amount of solids that must be reprocessed is significantly smaller resulting in less waste generation from the downstream processes.