Hydrofluoric acid diffusion

To convert naturally occurring uranium oxide, yellow cake or U Og, to the gaseous UF, hydrofluoric acid is first used to convert the U Og to UF. Further fluorination using fluorine (generated from more HF) is employed to convert the UF to UF. The UF is then processed at gaseous diffusion enrichment plants. 

Hydrofluoric acid. An acid that has some very useful and specific applications, but is also very dangerous, is hydrofluoric acid, HF. This acid reacts with skin in a way that is not noticeable at first, but becomes quite serious if left in contact for a period of time, ft has been known to be especially serious if trapped against the skin or after diffusing under fingernails. Treatment of this is difficult and painful. Concentrated HF is about 50% HF (26 M). It is an excellent solvent for silica (Si02)-based materials such as sand, rocks, and glass. It can also be used for stainless steel alloys. Since it dissolves glass, it must be stored in plastic containers. This is also true for low pH solutions of fluoride salts. 

Workers exposed to an airborne fluoride concentration of 5mg/m complained of eye and respiratory tract irritation and nausea. The lethal oral dose of sodium fluoride for humans has been estimated to be 32-65 mg F/kg of body weight. Effects from ingestion are diffuse abdominal pain, diarrhea, and vomiting excessive salivation, thirst, and perspiration painful spasms of the limbs and sometimes albuminuria." Gastrointestinal effects produced after the acute ingestion of toxic amounts of fluoride likely arise from the corrosive action of hydrofluoric acid, which is produced within the acidic environment of the stomach. Cardiac arrest after accidental exposure to high levels of fluoride has been attributed to the development of hypocalcemia and/or hyperkalemia. ... 

Figure 3. Electron micrograph of the carbon shells of tridymite particles. After diffuse coating with carbon in a vacuum, the tridymite was dissolved away by hydrofluoric acid specific surface area of this sample of relatively coarse crystals is 7.7 sq. meters/gram (X24,000)...

Anti-acids, astringents and antiseptic agents may contain a variety of aluminium salts. Organic salts, alumina, the hydroxide and phosphates may be attacked with concentrated hydrochloric acid and diluted to bring the aluminium concentration into the range 10—50 pg ml"1. Alternative procedures for antacids using hydrochloric/nitric acid [67] and extraction with 4M hydrochloric acid [95] have been proposed. For silicates, the sample is best taken up in perchloric/hydrofluoric acid, evaporated to dryness to remove silica, and then the residue dissolved in warm hydrochloric acid [87], In each case the nitrous oxide/acetylene flame is the preferred atom cell, and the method of standard additions may be used to minimise any errors arising from lateral diffusion. 

The reaction of acids with glass may be either a leaching process or a complete dissolution process. Acids such as hydrofluoric acid attack silica glasses by dissolving the silica network. Other acids such as hydrochloric acid or nitric acid may react by dissolving certain glasses. However, the reaction mechanism is by selective extraction of alkali and the substitution of protons in a diffusion-controlled process. 

Considering FA and TFS as consolidation additives, it should be noted that penetration of the SPC compositions modified by FA strongly depends on concentration of an aggressive environment. The polymerization rate of FA in the silicate media under the action of aggressive environments depends on pH, especially in the first reaction stages. Corrosion of silica by hydrofluoric acid occurs rather quickly and leads to loosening of the SPC composition and an increase of diffusive penetration. Consequently, application of the consolidation additive FA is desirable at pH < 5. 

The mechanism of TFS action as a consolidation additive in these environments is a little different there is a hydrolysis of TFS with formation of an orthosilicic acid and FA [4], Inorganic cement fills the saddles between globules of silica gel. An increase of the contact area leads to significant growth of durability, and in so doing, compensates for strength reduction due to etching a matrix by hydrofluoric acid. In that way, TFS essentially reduces the diffusive penetration of water and weak acids and allows use of SPC for reinforced structures without special protection of a reinforcement. 

Corrosion of silica by hydrofluoric acid occurs rather quickly and leads to loosening of SPC composition and increasing of diffusive penetration. Because of this, application of the consolidating additive FA is desirable at pH < 5. 

Figure 6.31 Example material balance in the recovery of nitric acid and hydrofluoric acid from waste acid solution by diffusion dialysis.

J. Sato, M. Onuma, H. Motomura and Y. Noma, Recovery of nitric acid and hydrofluoric acid from the pickling solution by diffusion dialysis, Jitsumu Hyomen Shori Gijutsu, Practical Surf. Treatment Technol.), 1985, 32, 220-226. 

The diffusers work at a temperature of 80°C. The enriched hexafluoride is transformed to uranium oxide. The hydrofluoric acid is set free and re-used. The uranium oxide is pressed into small cylindrical pellets, 10 mm high and 10 mm in diameter. The pellets are filled into 4-5 m long tubes, made of zircaloy. The tubes are filled with helium and welded. They are the fuel of the nuclear power stations. 

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