Fluoride dissolution

C is fafrly easy to dissolve, material calcined at higher temperatures (up to 1700°C) becomes an intractable refractory oxide. Better methods, which can be scaled to a production mode, are needed to convert these refractory oxides to a form amenable to reprocessing. Some advances have been made in aqueous dissolution, using an electrolytic assist to fluoride dissolution, but further chemical and engineering data are needed to convert this into a viable production process.(19)... 

When sodium fluoride is used for a fluoridation system, it should be remembered that, while sodium fluoride is quite soluble, the fluorides of calcium and magnesium are not. Thus, the fluoride ions in solution will combine with available calcium and magnesium ions in the makeup water and form a precipitate that can clog the feeder, the injection port, the feeder suction line, the saturator bed, etc. Therefore, water used for sodium fluoride dissolution should be softened whenever the hardness exceeds 75 ppm, or even if the hardness is less than this value (17). Only the water used for solution preparation must be softened. 

For waste management, cerium-promoted dissolution has another important characteristic relative to fluoride dissolution. 

An early conventional method for plutonium analysis was coprecipitation in acid solution with a rare earth fluoride, dissolution in aluminum nitrate solution with sodium nitrite to maintain the 4-4 oxidation state, and extraction into thenoyl-trifluoroacetone (TTA) in benzene. Plutonium was back-extracted into dilute HCl, the acid was evaporated, and plutonium was taken up in HCI-NH4CI solution... 

Fio. 9.11, Extraction of tantalum (fluoride dissolution, re rystallization of double fluoride, sodium reduction process). 

Titanium(lV) fluoride dihydrate [60927-06-2] TiF 2H20, crystals can be prepared by the action of aqueous HF on titanium metal. The solution is carefully evaporated to obtain the crystals. Neutral solutions when heated slowly hydroly2e and form titanium(lV) oxyfluoride [13537-16-17, TiOF2 (6). Upon dissolution in hydrogen fluoride, TiF forms hexafluorotitanic acid [17439-11-17, ll]TiF. ... 

Niobium Dioxide Fluoride. Niobium dioxide fluoride, Nb02F, is formed on dissolution of niobium pentoxide in 48 wt % aqueous hydrofluoric acid, evaporation of the solution to dryness, and heating to 250°C. 

The rate (kinetics) and the completeness (fraction dissolved) of oxide fuel dissolution is an inverse function of fuel bum-up (16—18). This phenomenon becomes a significant concern in the dissolution of high bum-up MO fuels (19). The insoluble soHds are removed from the dissolver solution by either filtration or centrifugation prior to solvent extraction. Both financial considerations and the need for safeguards make accounting for the fissile content of the insoluble soHds an important challenge for the commercial reprocessor. If hydrofluoric acid is required to assist in the dissolution, the excess fluoride ion must be complexed with aluminum nitrate to minimize corrosion to the stainless steel used throughout the facility. Also, uranium fluoride complexes are inextractable and formation of them needs to be prevented. 

Most mineral acids react vigorously with thorium metal. Aqueous HCl attacks thorium metal, but dissolution is not complete. From 12 to 25% of the metal typically remains undissolved. A small amount of fluoride or fluorosiUcate is often used to assist in complete dissolution. Nitric acid passivates the surface of thorium metal, but small amounts of fluoride or fluorosiUcate assists in complete dissolution. Dilute HF, HNO, or H2SO4, or concentrated HCIO4 and H PO, slowly dissolve thorium metal, accompanied by constant hydrogen gas evolution. Thorium metal does not dissolve in alkaline hydroxide solutions. 

Beryllium fluoride is hygroscopic and highly soluble in water, although its dissolution rate is slow. FluoroberyUates can be readily prepared by crystallization or precipitation from aqueous solution. Compounds containing the BeP ion are the most readily obtained, though compounds containing other fluoroberyUate ions can also be obtained, eg, NH BeF, depending upon conditions. 

Fluorides. Most woddwide reductions in dental decay can be ascribed to fluoride incorporation into drinking water, dentifrices, and mouth rinses. Numerous mechanisms have been described by which fluoride exerts a beneficial effect. Fluoride either reacts with tooth enamel to reduce its susceptibihty to dissolution in bacterial acids or interferes with the production of acid by bacterial within dental plaque. The multiple modes of action with fluoride may account for its remarkable effectiveness at concentrations far below those necessary with most therapeutic materials. Fluoride release from restorative dental materials foUow the same basic pattern. Fluoride is released in an initial short burst after placement of the material, and decreases rapidly to a low level of constant release. The constant low level release has been postulated to provide tooth protection by incorporation into tooth mineral. 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

Similar considerations apply to oxidation. An anion which is considerably more stable than water will be unaffected in the neighbourhood of the anode. With a soluble anode, in principle, an anion only needs be more stable than the dissolution potential of the anode metal, but with an insoluble anode it must be stable at the potential for water oxidation (equation 12.4 or 12.5) plus any margin of polarisation. The metal salts, other than those of the metal being deposited, used for electroplating are chosen to combine solubility, cheapness and stability to anode oxidation and cathode reduction. The anions most widely used are SOj", Cl", F and complex fluorides BF4, SiFj , Br , CN and complex cyanides. The nitrate ion is usually avoided because it is too easily reduced at the cathode. Sulphite,... 

Precipitation of fluoride compounds from solutions of hydrofluoric acid, HF, is performed by the addition of certain soluble compounds to solutions containing niobium or tantalum. Initial solutions can be prepared by dissolving metals or oxides of tantalum or niobium in HF solution. Naturally, a higher concentration of HF leads to a higher dissolution rate, but it is recommended to use a commercial 40-48% HF acid. A 70% HF solution is also available, but it is usually heavily contaminated by H2SiF6 and other impurities, and the handling of such solutions is extremely dangerous. 

It was proposed [445 - 447] that the dissolution of tantalum and niobium oxides in mixtures of hydrofluoric and sulfuric acids takes place through the formation of fluoride-sulfate complexes, at least during the initial steps of the interaction and at relatively low acid concentrations. Nevertheless, it was also assumed that both tantalum and niobium fluoride-sulfate complexes are prone to hydrolysis yielding pure fluoride complexes and sulfuric acid. No data was provided, however, to confirm the formation of fluoride sulfate complexes of tantalum and niobium in the solutions. 

Hydrofluoric acid, at relatively high concentrations and at elevated temperatures, dissolves columbite-tantalite concentrates at a reasonable rate. The dissolution process is based on the fluorination of tantalum, niobium and other metal oxides and their conversion into soluble complex fluoride acids yielding complex fluoride ions. 

In the case of a mixture of hydrofluoric and sulfuric acids, the process is more complex. It can be noted that sulfuric acid most probably interacts mainly with iron and manganese, whereas hydrofluoric acid serves mostly in the dissolution of tantalum and niobium and their conversion into soluble fluoride complexes. Nevertheless, due to the high acidity of the solution, here too the formation of hexafluorotantalate and hexafluoroniobate complex ions, TaF6" and NbF6, is expected. Hence, it is noted that the acid dissolution of tantalum-and niobium-containing raw material leads to the formation of hexafluoro-acids — HTaF6 and HNbF6. 

Ta and Nb containing minerals, 4 Ta and Nb oxides dissolution, 258 Tantalum fluoride properties, 25 Tantalum extraction, 285-288... 

Dissolution. Plutonium is solubilized in nitric acid solutions at Rocky Flats. The feed material consists of oxide, metal and glass, dissolution heels, incinerator ash and sand, slag, and crucible from reduction operations. The residues are contacted with 12M HNO3 containing CaF2 or HF to hasten dissolution. Following dissolution, aluminum nitrate is added to these solutions to complex the excess fluoride ion. 

Chamberlain Powers, 1976 Jendresen Trowbridge, 1972). The addition of stannous fluoride to the cement increases dissolution, but this is an advantage rather than a disadvantage, for the fluoride released is taken up by neighbouring enamel (Bitner Weir, 1973). 

Kuhn, A. T. Jones, M. P. (1982). A model for the dissolution and fluoride release from dental cements. Biomaterials, Medical Devices and Artificial Organs, 10, 281-93. 

Fluoride is found in some zinc phosphate cements, generally as stannous fluoride. The cements are weaker and have less resistance to dissolution than normal zinc phosphate cements (Myers, Drake Brantley, 1978 Williams et al., 1979). They release fluoride over a long period (de Freitas,... 

The dissolution and ion release from dental silicate cement have been the most investigated characteristics with good reason, for they are central to its clinical performance. Erosion limits its life but release of fluoride has important clinical consequences. 

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