Fluosilicic acid solutions

Properties. See how a fluosilicic acid solution acts on various indicators. Study the reaction of this solution with metals, namely, zinc and magnesium. 

With this process, the smelted lead is cast as anode and electrolysed in a fluosilicic acid solution. By this process pure lead is collected as cathode, while any other metals remain on the anode and are recovered as anodic slurry, further treated to recover single metals. 

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. 

The product of the dissolution process of silicon electrodes in HF is fluosilicic acid, H2SiF6. In contrast to HF, H2SiF6 is mostly (75%) dissociated into Sily and 2H+ in aqueous solution at RT. The diffusion coefficient of the SiF at RT decreases from 1.2X10 5 cmV1 for 0.83 mol 1 1 to 0.45 cm2s 1 for 2.5 moll-1, with values of activation energy around 0.2 eV [We7]. 

Benzoic acid Solution s Fluosilicic acid Unknown s... 

In addition to the method outlined under procedure A, hydrazoic acid may be prepared by the action of oxalic1 or fluosilicic acids2 upon solutions of sodium azide or by the treatment of barium azide solutions with dilute sulfuric acid. A method involving the action of perchloric acid upon potassium azide has also been proposed. However, subsequent distillation of the filtrate, after removal of the precipitated potassium perchlorate, is necessary to prepare pure hydrazoic acid. Pure hydrazoic acid has also been obtained by the oxidation of hydrazine in acid solution by hydrogen peroxide.3-"... 

Fluorine is recovered at the evaporator station by scrubbing the vapors leaving the flash chamber. The vapors pass through an entrainment separator to remove fine droplets of phosphoric acid and then into a spray tower where they are scrubbed with a weak solution of fluosilicic acid according to the reaction ... 

The spot-test technique of the reaction utilizes the conversion of the silicic and fluosilicic acids by means of ammonium molybdate into silicomolybdic acid H4[SiMo12O40]. The latter, unlike free molybdic acid, oxidizes benzidine in acetic acid solution to a blue dyestuff and molybdenum blue is simultaneously produced. (DANGER THE REAGENT IS CARCINOGENIC.)... 

Potassium fluosilicate, K2SiF6.—The fluosilicate is precipitated in the form of microscopic crystals of slight solubility by the action of fluosilicic acid on solutions of potassium salts ... 

Some workers (35) have reported that the critical current density for electropolishing certain metals is decreased when the concentration of the metal in the solution is increased. Fluosilicic acid is the filial anode reaction product during the electropolishing of silicon in HF solutions. Turner (29) found no change in ic, within the limits of experimental error, with additions of up to 25% by weight SiF in 5% aqueous HF. 

A recent report by Haver et al. (H16) described the recovery of lead and. sulfur from a lead sulfide concentrate using ferric sulfate as the leaching reagent. Elemental sulfur was produced during the oxidation of lead sulfide to lead sulfate by hot ferric sulfate solution. The lead sulfate in the re.sidue was changed to acid-soluble lead carbonate by treatment with ammonium carbonate, and ammonium sulfate was a by-product. Hydro-fluosilicic acid dissolved the lead carbonate and an electrolysis step regenerated the HjSiF and deposited 99.9% pure lead metal. The recovery for lead was about 90% and for sulfur, 67%, half as elemental sulfur and the other half as ammonium sulfate. 

For the analysis of uranium, the sample is ignited in a platinum crucible and weighed as U30g. Fluoride is determined by titration with a standard solution of thorium nitrate using sodium alizarinsulfonate as an indicator, after steam distillation of fluosilicic acid. ... 

In the prepared solutions, whether it is manual preparation of sodium fluoride solutions, dilution of fluosilicic acid, or the dissolution of the output of dry feeder, it is very... 

Using a paddle with sufficient manual mixing will suffice for the initial preparation of a dilute solution, although it will not be sufficient to prevent stratification. Therefore, a mechanical mixer is preferred. Mixers are available in various sizes, with shafts and propellers made of various materials to meet chemical-resistant requirements for different fluoride chemicals. Depending on the size of the mixing tank, a fractional horsepower mixer with a stainless-steel shaft and propeller will be satisfactory for sodium fluoride solution, and a similar mixer with a corrosion-resistant alloy or plastic-coated shaft and propeller will handle fluosilicic acid (15). 

Fluosilicic acid An unstable acid known only in aqueous solution,... 

To an ice cold solution of 10 g sodium azide in 100 ml water, 41 g of a 27% fluosilicic acid are added. Sodium fluosilicate precipitates, and the filtered solution contains 6.6 g hydrazoic acid, contaminated with 0.7 g fluosilicate [49]. 

Beryllium Fluosilicate.—Berzelius (1823 i) prepared a fluo-silicate by the action of fluosilicic acid on beryllium hydrojdde, but did not analyze or give details, and both Atterberg (1873 7) and Marignac (1873 i) state that it can be m de only in solution. 

Fluorine is usually recovered in the form of an aqueous solution of fluosilicic acid, H2S1F6. The concentration may be as high as 20%-25%. In some countries the fluosilicic acid is used directly for fluoridation of municipal water supplies to prevent tooth decay. The acid is shipped to various municipalities in rubber-lined rail-cars. Salts of fluosilicic acid, such as sodium, potassium, and ammonium fluosilicates, have various uses and can be readily produced from the acid. Sodium fluosilicate is... 

Both are harmful fluoride emissions. The reactor fluorides are usually absorbed in water or in diluted solutions of fluosilicic add. The modern absorbir equipment allows production of concentrated solutions, w ich could be processed to synthetic cryolite, aluminum fluoride, and varioLE fhiorosilicates. If not sold as a byproduct, the fluosilicic acid can be neutraBzed by liming. The neutralized slurry may require further treatment, such as settling, before it is discharged. 

The dried paste is leached in spent electrolyte to dissolve lead in accordance with Equation 11.18. Lead concentration is raised to around 150 g/L or up to 200 g/L but must maintain a free fluosilicic acid concentration of at least 50 g/L to prevent hydrolysis and precipitation of lead compounds. Lead recovery to the leach solution is about 95 per cent and the leach residne containing 30 per cent lead and two to three per cent sulfur can be recycled to the desulfurisation step, or part discarded after conversion to a slag in an electric furnace. 

The electrolyte is a solution of lead fluosilicate and free fluosilicic acid. The lead concentration is normally in the range of 75 to 120 g/L and the free fluosilicic acid 65 to 100 g/L. The total equivalent fluosilicic acid is close to twice the free acid amount. 

Copper Plating Solution (45% Ckipper Eluoioborate 19% Copper Sullale 8% SuKoriC Acid) Copper Matte Dipping Bath (30% Ferric Chloride 19% Hydrochloric) NR NR NR NR R R R R Flue Gas Fluoboric Acid Fluosilicic Acid 0-20% Formaldehyde Formic Acid NR NR NR R R NR NR NR NR NR R R R R R R 120 R R R... 

Food processors are becoming more aware of the severe corrosive nature of cleaning chemicals and disinfectants. Fluosilicic acid is typical of these corrosive agents. A 25% solution of fluosilicic acid at 125° F corrodes CF-8M at a rate of 500 mills per year. ILLIUM PD has a measured corrosion rate of 0.25 mills per year. 

The simple fluosilicic acid probably reacts with additional SiF4 or SiC>2 to form a more complex form of this compound. The second reaction (equation 6-7) is reversible to the point that solutions of fluosilicic acid exert a definite vapor pressure of HF and SiF4. 

Whynes has presented data on the concentration of fluosilicic acid in equilibrium with SiF4 vapors these are reproduced in Figure 6-13. The curves indicate that an exhaust gas containing 0.3% silicon tetrafluoiide can produce a solution containing about 32% fluosilicic acid at temperatures below 70°C. This has been confirmed in a plant wash-tower by continuously recirculating the solution for several hours. 

In silicon tetrafluoride operations, the gas is usually available at moderate temperature (from phosphate-rock addulation) however, relatively high fluoride concentrations may be developed. Depending on whether recirculation of fluosilicic add solution is practiced, the fluosilidc acid concentration in the liquid may range fiom a fraction of 1% to over 10%. Because of the wide range of operating conditions and performance requirements of fertilizer-plant exhaust-gas purifying systems, a considerable variety of construction materials have been utilized. 

---