Silica minerals hydrofluoric acid

Acid digestion involves the use of mineral or oxidizing acids and an external heat source to decompose the sample matrix. The choice of an individual acid or combination of acids is dependent upon the nature of the matrix to be decomposed. The most obvious example of this relates to the digestion of a matrix containing silica (Si02), e.g. in a geological sample. In this situation, the only appropriate acid to digest the silica is hydrofluoric acid (HF). No other acid or combination of acids will liberate the metal of interest from the silica matrix. 

Spat lackp m. barytes lake, -sandp m. silica sand. Saure, /. hydrofluoric acid, -stein, m. selenite (the mineral). 

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. 

Formation permeability damage caused by precipitation of dissolved minerals such as colloidal silica, aluminum hydroxide, and aluminum fluoride can reduce the benefits of acidizing (132-134). Careful treatment design, particularly in the concentration and amount of HF used is needed to minimize this problem. Hydrofluoric acid initially reacts with clays and feldspars to form silicon and aluminum fluorides. These species can react with additional clays and feldspars depositing hydrated silica in rock flow channels (106). This usually occurs before the spent acid can be recovered from the formation. However, some workers have concluded that permeability damage due to silica precipitation is much less than previously thought (135). 

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. 

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. 

As an element, silicon is a hard brittle metallic-looking substance which crystallizes in the diamond lattice. It is produced commercially by the electrothermal reduction of silica, resulting in a product which contains about 97 per cent silicon. The element shows no visible oxidation or corrosion at ordinary temperatures and oxidizes very slowly below red heat. Halogens attack it more readily, and chlorination proceeds satisfactorily at 250° C. to form the silicon chlorides which are perhaps the best-known volatile compounds of silicon. Whenever the element is exposed to nascent hydrogen, or its metallic compounds are treated with acids, some hydrides usually are formed, as may be noticed by the odor which such hydrides impart to the gas evolved when cast iron is dissolved in dilute acids. A mixture of concentrated nitric and hydrofluoric acids will dissolve pure silicon, but mineral acids singly will not do so. Hot concentrated solutions of alkalies will dissolve it, however, with the evolution of hydrogen. 

The primary use of hydrofluoric acid is for the decomposition of silicate rocks and minerals in the determination of species other than silica. In this treatment, silicon is evolved as the tetrafluoride. After decomposition is complete, the excess hydrofluoric acid is driven off by evaporation with sulfuric acid or perchloric acid. Complete removal is often essential to the success of an analysis because fluoride ion reacts with several cations to form extraordinarily stable complexes that interfere with the determination of the cations. For example, precipitation of aluminum (as AI2O3 XH2O) with ammonia is incomplete if fluoride is present even in small amounts. Frequently, it is so difficult and time-consuming to remove the last traces of fluoride ion from a sample that the attractive features of hydrofluoric acid as a solvent are negated. 

PHYSICAL PROPERTIES white, cubic-system crystals formed from quartz or amorphous silica at temperatures above 1000°C (1832°F) odorless solid transparent and tasteless may be a component of many mineral dusts practically insoluble in water or acid, except hydrofluoric acid MP (1600°C, 2912°F) BP (2230°C, 4046 F) SG (2.66) VD (NA) VP (0 mmHg... 

With metallic oxides, hydrofluoric acid gives rise to water and metallic fluorides HF - - MO = HO -F MF. The afiinity of fluorine for silicon is such, that hydrofluoric acid decomposes all silicious compounds and this explains its corrosive action on glass and porcelain. Berzelius has employed this acid as a means of analysing silicious minerals. As the fluoride of silicium is gaseous, any such mineral, if digested with hydrofluoric acid, soon loses all its silica, and is dissolved, so that the other ingredients may be determined. 

Extraction of columbate-tantalates, titanocolumbates, and titanosilicates may also be initiated by treatment of the mineral with hydrofluoric acid. The procedure has the advantage that columbium, tantalum, uranium(VI), scandium, titanium, zirconium, and hafnium are dissolved, while silica is volatilized as silicon tetrafluoride and the rare earth elements, together with thorium and uranium(IV), remain as slightly soluble fluorides. The residue is then heated with concentrated sulfuric acid to remove hydrogen fluoride and to oxidize uranium (IV), the thorium is separated by precipitation of the phosphate (synthesis 12), and the rare earths are precipitated as oxalates. 

Bergman, in his examination of volcanic minerals, noticed the gelatinisation of silica when some silicates (zeolites) are treated with acids. He showed that the waters of several springs contain dissolved silica. He obtained artificial quartz ciystals by letting powdered quartz stand in a corked bottle for two years with hydrofluoric acid (containing hydrofluosilicic acid), and he emphasised that the precipitate (potassium or sodium fluosilicate) formed from the solution by vegetable or mineral alkali (potash or soda) is a triple salt of a peculiar kind, composed of siliceous earth, fluor acid, and fixed alkali, which dissolves with difficulty in warm water , whilst the precipitate with ammonia is pure silica. ... 

Johann Carl Friedrich Meyer (Stettin 1733-20 February 1811), Court Apothecary in Stettin, besides his papers on colloidal silica (see p. 188), hydrofluoric acid (see p. 215), the non-convertibility of silica into alumina (see p. 568), and the preparation of sodium carbonate from common salt (see p. 562), investigated the solubility of lead in sulphuric acid and the action of acids on strontia, and analysed minerals. He prepared artificial Seltzer water. He found that Siberian native iron dissolves in dilute sulphuric acid to a green solution, which becomes blue with ammonium chloride. Proust later showed that native (Peruvian) and meteoric iron contain nickel. Meyer s supposed new metal, hydrosiderum, he showed himself was iron phosphide (seep. 194). 

The injection volume was 500 pL. Depending on the sampling location, almost all common mineral acids can be detected in clean room air. Concentrations vary between 100 and 900nmol/m The main contaminations are hydrochloric acid and sulfuric acid, both of which can be detected with either of the sampling techniques described above. In contrast, hydrofluoric acid can only be detected with the silica absorber. Presiunably, fluoride is retained in the wash bottle due to interactions with the glass materiaL Separate analysis of the two silica segments allows a statement of the efficiency of the gas absorption by silica, which is 100% for HF and 91-100% for HCL... 

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