Silica, reaction with hydrofluoric acid

It is recognised that vitreous enamel possesses good acid resistance, but an exception occurs with hydrofluoric acid. This is due to the relative ease of reaction between this acid and the silica (which is the largest constituent in the frit) to form silicon tetrafluoride. This reaction is made use of in some de-enamelling plants. 

Detection.—Apart from naturally occurring ores of vanadium, vanadium steels, and ferrovanadium, the commonest compounds of vanadium are those which contain the element in the pentavalent state, viz. the pentoxide and the various vanadates. The analytical reactions usually employed are, therefore, those which apply to vanadates. Most vanadium ores can be prepared for the application of these reactions by digesting with mineral acids or by alkaline fusion with the addition of an oxidising agent. When the silica content is high, preliminary treatment with hydrofluoric acid is recommended. Vanadium steels and bronzes, and ferrovanadium, are decomposed by the methods used for other steels the drillings are, for instance, dissolved in sulphuric acid and any insoluble carbides then taken up in nitric acid, or they are filtered off and submitted to an alkaline fusion. Compounds of lower valency are readily converted into vanadates by oxidation with bromine water, sodium peroxide, or potassium permanganate. 

N. Tschischewsky made the nitride by working at 1400°. About 4 per cent, of the silicon volatilizes during the reaction. The product obtained after heating the silicon in nitrogen for an hour is an amorphous, voluminous, white powder which, under the microscope, seems to consist of several different substances. The composition of the final product is dependent on the mode of purification if boiled with potassium hydroxide soln., followed by treatment with hydrofluoric acid, a product containing silicon hemitrinitride mixed with silica is obtained. The silica could not be removed from the nitride. M. Blix and W. Wirbelauer obtained the nitride by heating silicam, Si2N3H, between 1200° and 1400° and H. St. C. Deville and F. Wohler, by the action of ammonia on silicon chloride. 

Silica is present in the mineral as an impurity, and it reacts with hydrofluoric acid to yield silicon tetrafluoride, which can be converted to fluorosifi-cic acid, an important source of fluorine. More than half of the phosphoric acid that is produced by the reaction of phosphates with sulfuric acid is converted directly to sodium or ammonium phosphates to be used as fertilizer thus, purity is not a concern. 

Blumberg, A. A. Differential Thermal Analysis and Heterogenous Kinetics The Reaction of Vitreous Silica with Hydrofluoric Acid. J. Phys. Chem. 63, 1129 (1959). 

A colourless or white vitreous solid, SiOj, insoluble in water and soluble (by reaction) in hydrofluoric acid and in strong allcall m.p. 1713°C b.p. 2230°C. The following forms occur naturally cristoballte (cubic or tetragonal crystals r.d. 232) trldymlte (rhombic r.d. 2.26) quartz (hexagonal r.d. 2.63-2.66) lechatellerlte (r.d. 2.19). Quartz has two modifications a-quartz below 575°C and fi-quartz above 575°C above 870°C fi-quartz is slowly transformed to trldymlte and above 1470°C this is slowly converted to cristo-balite. Various forms of sillcon(IV) oxide occur widely in the earth s cmst yellow sand for example is quartz with iron(III) oxide impurities and flint is essentially amorphous silica. The gemstones amethyst, op, and rock crystal are also forms of quartz. 

H.13 Hydrofluoric acid is used to etch grooves in glass because it reacts with the silica, Si02(s), in glass. The products of the reaction are aqueous silicon tetrafluoride and water. Write a balanced equation for the reaction. 

FIGURE 15-22 When a mixture of hydrofluoric acid and ammonium fluoride is swirled inside a glass vial (a i, reaction with the silica in the glass frosts the surface of the cover glass as well as the walls of the vial (b)... 

The covalently bonded solids such as silica cannot be easily broken by aqueous solutions. For example, the strong Si-O bonds silica is not dissolvable by boiling with concentrated acids except hydrofluoric acid because of the formation of silicon fluoride which is a gas and expels otherwise else it may form fluosilicic acid by reaction with water. 

Hydrofluoric acid may be prepared by dilution of a concentrated aqueous solution or by reaction of enough ammonium bifluoride with aqueous 15% HC1 to prepare a 12% HCl/3% HF solution. Hydrochloric - hydrofluoric acid blends have the major advantage of dissolving silicaceous mineral including clays and silica fine particles. HCl/HF blends are quite corrosive. 

G. S. Serullas treated potassium chlorate with an excess of hydrofluosilicic acid the clear liquid was decanted from the sparingly soluble potassium fluosilicate, the soln. evaporated below 30°, and filtered through glass powder J. J. Berzelius evaporated the acid liquid mixed with finely divided silica below 30° in air, or over cone, sulphuric acid and potassium hydroxide in vacuo. The excess of hydrofluoric acid was volatilized as silicon fluoride, and the clear liquid was then filtered from the excess of silica. R. Bottger treated sodium chlorate with oxalic acid whereby sparingly soluble sodium oxalate was formed J. L. Wheeler, and T. B. Munroe treated sodium chlorate with hydrofluosilicic acid and M. Brandau treated potassium chlorate with aluminium sulphate and sulphuric acid and precipitated the alum so formed with alcohol. Chloric acid is formed in many reactions with hypochlorous and chlorous acid for example, it is formed when an aq. soln. of chlorine or hypochlorous or chlorous acid decomposes in light. It is also formed when an aq. soln. of chlorine dioxide stands in darkness or in light. A mixture of alkali chlorate and chlorite is formed when an aq. soln. of an alkali hydroxide is treated with chlorine dioxide. 

Mix 0.5 gram of the precipitated silica with 1 gram of powdered calcium fluoride. Place the mixture in a test tube, moisten it with 36 N H2S04, and warm it gently under the hood. Dip a stirring rod in water and lower it, with a drop adhering, into the gas in the test tube. Note the precipitate that forms in the drop of water. Write equations for all the reactions, and state what rather unusual properties are shown by this experiment to be possessed by hydrofluoric acid and by silicon tetrafluoride. 

In a typical hydrofluoric acid manufacturing process, fluorite ore is reacted with 93 wt% aqueous sulfuric acid, supplied 15% in excess of the stoichiometric amount. Ninety-five percent of the ore dissolves in the acid. Some of the HF formed reacts with the dissolved silica in the reaction... 

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. 

The etching of glass by hydrofluoric acid may be represented by the simplified reaction of silica with HE... 

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