Question

Why is HF(g) (Hydrogen Fluoride , gas) is not stored in glass bottles ?
Give the name of process,
and reactions involved.

Answer 1

Hydrofluoric acid is a solution of hydrogen fluoride (HF) in water and is a precursor to almost all fluorine compounds. It is a colorless solution that is highly corrosive, capable of dissolving many materials, especially oxide and its ability to dissolve glass has been known since the 17th century. Because of the high reactivity toward glass and moderate reactivity toward many metals, hydrofluoric acid is usually stored in plastic containers (although PTFE is slightly permeable to it). Hydrogen fluoride gas is an acute poison that may immediately and permanently damage lungs and the corneas of the eyes. Aqueous hydrofluoric acid is a contact-poison with the potential for deep, initially painless burns and ensuing tissue death.

Thus, hydrogen fluoride (HF) is produced in two forms, as anhydrous hydrogen fluoride (the gaseous form, HF(g)) and as aqueous hydrofluoric acid (the aqueous solution form, HF(aq)). The predominant form manufactured is anhydrous hydrogen fluoride, a colorless or gas that fumes on contact with air and is water-soluble. Traditionally, hydrofluoric acid (HF) has been used to etch and polish glass. Currently, the largest use for HF is in aluminum production. Other HF uses include uranium processing, petroleum alkylation, and stainless steel pickling. Hydrofluoric acid is also used to produce fluorocarbons used in aerosol sprays and in refrigerants. Although fluorocarbons are heavily regulated due to environmental concerns, other applications for fluorocarbons include manufacturing of resins, solvents, stain removers, surfactants, and pharmaceuticals.

In the process, hydrogen fluoride is manufactured by the reaction of acid-grade fluorspar (calcium fluoride, CaF2) with sulfuric acid (H2SO4):

CaF2+H2SO4→CaSO4+2HF

This endothermic reaction requires 30–60 min in horizontal rotary kilns externally heated to 200–250°C (390–480°F).

To accomplish this, dry fluorspar and a slight excess of sulfuric acid are fed continuously to the front end of a stationary prereactor (to ensure thorough contact by mixing the components prior to charging to the rotary kiln.) or directly to the kiln by a screw conveyor. Calcium sulfate (CaSO4) is removed through an air lock at the opposite end of the kiln, and the gaseous reaction products—hydrogen fluoride and excess sulfuric acid from the primary reaction and silicon tetrafluoride (SiF4), sulfur dioxide (SO2), carbon dioxide (CO2), and water produced in various secondary reactions—are removed from the front end of the kiln along with entrained particulate matter. The particulate matter is removed from the gas stream by a dust separator and returned to the kiln, while sulfuric acid and water are removed by a precondenser. Hydrogen fluoride vapors are then condensed in refrigerant condensers as a crude grade of the product, which is removed to intermediate storage tanks.

The remaining gas stream passes through a sulfuric acid absorption tower or acid scrubber where most of the remaining hydrogen fluoride and some residual sulfuric acid are removed and are also placed in intermediate storage. The gases exiting the scrubber then pass through water scrubbers, where the silicon tetrafluoride (SiF4) and remaining hydrogen fluoride are recovered as fluorosilicic acid (H2SiF6). The tail gases from the water scrubber are passed through a caustic scrubber before being released to the atmosphere but only after further treatment has removed any potential environmental contaminants. The hydrogen fluoride and sulfuric acid are delivered from intermediate storage tanks to distillation columns where the hydrofluoric acid is extracted to a purity of 99.98% (v/v). Weaker concentrations of the hydrofluoric acid (typically 70%–80%, v/v) are produced by measured and careful dilution with water.

Answer 2

Hydrofluoric acid (HF) reacts with silicon dioxide to produce silicon tetraflouride and water.

May be u can call it John Davy process.