Processing nitric acid dissolution

An overview is presented of plutonium process chemistry at Rocky Flats and of research in progress to improve plutonium processing operations or to develop new processes. Both pyrochemical and aqueous methods are used to process plutonium metal scrap, oxide, and other residues. The pyrochemical processes currently in production include electrorefining, fluorination, hydriding, molten salt extraction, calcination, and reduction operations. Aqueous processing and waste treatment methods involve nitric acid dissolution, ion exchange, solvent extraction, and precipitation techniques. 

The Rover Fuels Processing Facility provides a head-end system designed specifically for reclaiming uranium from graphite matrix nuclear rocket fuels. Residual ash from fluidized bed burners is charged to the dissolver and is dissolved tmd complexed in a four step process (a) nitric acid dissolution (b) water dilution (c) hydrofluoric acid dissolution and (d) fluoride com-plexing. Quantities of reagents are determined by foe... 

An initial solution was prepared by dissolving metallic niobium powder in 40% hydrofluoric acid. The dissolution was performed at elevated temperature with the addition of a small amount of nitric acid, HN03, to accelerate the process. The completeness of niobium oxidation was verified by UV absorption spectroscopy [21]. The prepared solution was evaporated to obtain a small amount of precipitate, which was separated from the solution by filtration. A saturated solution, containing Nb - 7.01 mol/1, HF - 42.63 mol/1, and corresponding to a molar ratio F Nb = 6.08, was prepared by the above method. The density of the solution at ambient temperature was p = 2.0 g/cc. Concentrations needed for the measurements were obtained by diluting the saturated solution with water or hydrofluoric acid. 

The tantalum dissolution process takes longer compared to the preparation of the corresponding niobium solution, therefore the solution is heated and a small amount of nitric acid is added. A grey precipitate indicates saturation of the solution. The prepared solution is separated from the precipitate by filtration and used as the initial solution. 

It can be seen from Figure 5.18 that the KD values for zirconium are higher than those for hafnium at all nitric acid concentrations. This is because the dissolution of zirconium nitrate (Zr(N03)4) into zirconyl (Zr02+) and nitrate (NOj) ions takes place to a lower extent as compared to the corresponding dissolution of hafnium nitrate in an aqueous medium. Hence, separation is feasible. However, at higher nitric acid concentrations the separation factor is reduced significantly because the dissociation of hafnium nitrate (Hf(NOs)4) decreases sharply with increasing nitric acid concentration, with the result that the separation factor, p, falls off rapidly. Hence, the separation process calls for the adjustment of the nitric acid concentration to a suitably low value. 

In contrast to the equilibrium electrode potential, the mixed potential is given by a non-equilibrium state of two different electrode processes and is accompanied by a spontaneous change in the system. Besides an electrode reaction, the rate-controlling step of one of these processes can be a transport process. For example, in the dissolution of mercury in nitric acid, the cathodic process is the reduction of nitric acid to nitrous acid and the anodic process is the ionization of mercury. The anodic process is controlled by the transport of mercuric ions from the electrode this process is accelerated, for example, by stirring (see Fig. 5.54B), resulting in a shift of the mixed potential to a more negative value, E mix. 

Fig. 5.54 Mixed potential. (A) Zinc dissolution in acid medium. The partial processes are indicated at the corresponding voltammograms. (B) Dissolution of mercury in nitric acid solution. The original dissolution rate characterized by (1) the corrosion current y a is enhanced by (2) stirring which causes an...

Attempts to obtain nitrocellulose containing more than 12.7% of nitrogen failed. When acid diluted below 77% was used partial dissolution of the product occurred. As an explanation for this, Vieille suggested that the soluble substances represented products which were hydrolysed or oxidized under the influence of the relatively dilute nitric acid. The reactions of hydrolysis and oxidation occur less readily the more concentrated the nitric acid, and the more hydroxyl groups are esterified. It also seems that nitrate groups counteract the side processes (such as oxidation). 

In order to make use of thorium as a nuclear resource for power generation, development of efficient separation processes are necessary to recover 233U from irradiated thorium and fission products. The THORium uranium Extraction (THOREX) process has not been commercially used as much as the PUREX process due to lack of exploitation of thorium as an energy resource (157,180). Extensive work carried out at ORNL during the fifties and sixties led to the development of various versions of the THOREX process given in Table 2.6. The stable nature of thorium dioxide poses difficulties in its dissolution in nitric acid. A small amount of fluoride addition to nitric acid is required for the dissolution of more inert thorium (181). 

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