CS270799B1 - Process for treating waste hydrochloric acid - Google Patents

Abstract

The acidic and salted mother liquor after precipitation is heated to at least 50 °C, thereby accelerating the decomposition of dissolved N82SIF5 into HF, NaF and SiOo.x HoO gel. The latter is separated, the balance part of the obtained clear solution is saturated with NaCl and used for new precipitation. The excess solution is subjected to rectification to increase the HCl content to at least 13% by weight. To the thus concentrated hydrochloric acid is added so much calcium hydroxide, and preferably CaCO3, that the calcium ion content in the solution increases to a value of 20% by weight exceeding the stoichiometric need for complete precipitation of fluoride ions in the form of calcium fluoride. To the thus treated solution is added the reversible deatilization residue from the subsequent operation in such an amount that the total salt content in the mixed solution increases to at least 10% by weight. The solution is then optionally cooled to a temperature of 40 °C or lower and aged at this temperature for at least 3 hours with gentle stirring. During cooling and aging, CaF2 is precipitated, which is then separated and 13 and 18% hydrochloric acid, containing a maximum of 0.1% by weight of hydrofluoric acid, is distilled from the filtrate.

Description

The present invention relates to a process for the treatment of waste hydrochloric acid formed in the production of sodium hexafluorosilicate from dilute hexafluorosilicic acid by precipitation with sodium chloride, which comprises decomposing dissolved Na ₂ SiF _{6 by} thermal hydrolysis using part of the acid thus treated to precipitate back to NaCl saturation. In the second part of the solution, the HCl content is increased by rectification min. to 13%, and the addition of calcium ion, followed by the addition of a residue precipitated upon cooling most of the fluoride ions is CaF ₂ · Of the separated liquid phase is stripped from 13 to 18% hydrochloric acid, the distillation residue was partially recycled into operation the precipitation of CaF _{2 <Second} part is neutralized with sodium hydroxide, after which the excess ca ⁺ ion is precipitated with sodium carbonate. The liquid phase is returned after saturation of the Nac for precipitation, the solid serves as a secondary source of Ca 2+ ions for the precipitation of CaF ₂ . another valuable product - moderately concentrated hydrochloric acid - and a secondary fluorinated raw material - calcium fluoride.

In the production of sodium hexafluorosilicate by precipitation from hexafluorosilicic acid with sodium chloride solution, a dilute and saline hydrochloric acid solution is formed. Such an acid with a HCl concentration of up to 5% by weight is technically practically unusable and is normally neutralized by neutralization with calcium hydroxide to give soluble calcium chloride. Neutral saline wastewater is then discharged into the surface stream or into the sea, depending on the location of the processing plant, but the discharge into freshwater surface water thus increases the undesirable salt content, which is hygienically harmless in the concentrations achieved. Therefore, the problem of processing waste HCl from said production is becoming very topical, especially in landlocked countries. The processes proposed so far, such as the use to decompose phosphates, do not solve this shortcoming, because chlorides in another form are returned to the wastewater, albeit after the double use of the acid potential.

It has now been found that, by a suitable method, said solution can be processed practically without waste into technically usable medium-concentrated hydrochloric acid. Sodium salts contained in the waste acid is utilized to re-precipitation of Na ₂ SiF _&. The procedure includes several follow-up operations described below:

Mother solution (from precipitation of sodium hexafluorosilicate according to the process described with acid and sodium chloride saturated solution), containing up to 10% by weight. HCl and up to 3.5% by weight of salts are first freed of dissolved Na ₂ SiF $ by thermal hydrolysis, which is carried out simply according to the overall reaction:

Na ₂ SiF ₆ + x H ₂ 0 = 2 NaF + 4 HF + SiO ₂ ./x-2/ H ₂ 0, conducted at a temperature of at least 50 ° C for at least 10 minutes.

The SiO ₂ · x H ₂ O gel is separated off, a part of the solution obtained is saturated with NaI and used for new precipitation of Na ₂ SiFg. The balance of the excess solution is adjusted by rectification so that the HCl concentration is increased to at least 13% by weight. _{The calcium salt, preferably CaCO 3} or CaCl ₂ , is added to the solution thus concentrated in such an amount that the excess calcium in the liquid phase * is at least 20% by weight, the stoichiometric requirements according to the reaction:

-1 2 +

F ^x + Ca = CaF ₂

The solution thus prepared is mixed with such an amount of recirculating distillation residue from the subsequent operation that the salt content of the system is increased to at least 10% by weight, then the solution is cooled to at least 40 ° C and left at gentle stirring for at least 3 hours. During cooling and maturation, solid caF ₂ is precipitated in well-separable microcrystals. After separation of the solid phase, the solution is subjected to distillation to obtain up to 18% (m / m) of hydrochloric acid, containing a maximum of

CS 270799 Bl

0.1% by weight of hydrofluoric acid, part of the distillation residue is returned to precipitate caF ₂ . The residue is neutralized with sodium hydroxide and the calcium ion is practically completely removed by subsequent precipitation with sodium carbonate. The suspension thus obtained is separated and the liquid phase is returned to the precipitation of Na 2 SiF _c after possible saturation of NaCl. the separated solid phase, containing caco ₃ and CaF ₂ , is preferably used as an additional source of calcium ion back to the precipitation of CaF ₂ ·

Calcium fluoride, separated after its precipitation operation, can be advantageously used as a highly concentrated secondary raw material for the production of, for example, hydrogen fluoride after washing and possible drying.

By the process described above, the formation of waste salts in the production of sodium hexafluorosilicate by precipitation from a solution of hexafluorosilicic acid with a saturated sodium chloride solution can be practically completely eliminated.

Example 1

After precipitation of Na ₂ SlFg from H ₂ SiF ₆ with a mixture of acidic return mother liquor and treated distillation residue (neutralization with NaOH and precipitation of Ca ²⁺ as CaCO ₃ ) saturated Naci, after separation of crystals, mother liquor was obtained - dilute hydrochloric acid with the following basic composition:

HCl - 7.4% by weight, Na ₂ SiF $ - 0.7% by weight, total salts - 2.8% by weight, in an amount of 1,500 g. The solution was heated to 60 ° C and left at this temperature for 30 minutes. . Excluded gel Si0 ₂ . x H ₂ O was separated by vacuum filtration to give 1,450 g of a solution practically free of SiF 8 ions. the content of acidic components, calculated on HCl, increased to 7.8% by weight. 450 g of the thus prepared mother liquor were returned to saturation with Naci, 1000 g were adjusted by rectification to an HCl content of 14.1% by weight, with a decrease in total weight to 550 g. The salt content increased to 5.1% by weight, of which the fluoride ion content was 0.7% by weight. To the solution was added 12 g of finely ground limestone containing CaC0 _Q 97 weight%, the solution is added 515 g of hot bottoms, which contained 23% of salts and acidic inserts 9%, expressed as hydrochloric acid. The resulting mixture was 82 ° C and cooled to 36 ° C over 30 minutes. As the temperature was reduced to 68 ° C, a solid phase began to precipitate as fine crystals. After reaching the final temperature, the suspension was stirred for 4 hours. The solid was then separated and washed with water. The combined filtrate in an amount of 1,060 g, containing 0.1% by weight of fluoride ions, was subjected to distillation until a solid phase appeared in the distillation residue. 400 g of 16.1% hydrochloric acid containing 0.07% HF were obtained. 515 g were separated from 660 g of distillation residue to precipitate CaF ₂ . The residue was neutralized with 41% NaOH until pH 4.0, after which it was powdered Na ₂ CO ₃ precipitated CaC0 _third At the end of the neutralization, the remainder of the fluoride ion precipitated out of solution as caF ₂ , so that after completion of the whole operation a total of 160 g of 25% NaCl solution was obtained after solid phase separation, which was used to _{reprecipitate Na 2} SiFg together with saturated acidic mother liquor in balance sheet quantity. The solid phase was used to reprecipitate CaF ₂ to utilize the CaCO ₃ present.

Claims (4)

OBJECT OF THE INVENTION A process for the treatment of waste hydrochloric acid from the production of sodium hexafluorosilicate, characterized in that it is first subjected to thermal hydrolysis of dissolved Na ₂ SiFg at a minimum temperature of 50 ° C for at least 10 minutes, after which the resulting SiO ₂ .x H ₂ 0 gel is separated. , then a part of the solution after saturation with sodium chloride is used to _{reprecipitate Na 2} SiFg, the other part is rectified so as to increase the HCl content to at least 13% by weight and to add thus treated calcium salt, preferably CaCO ₃ or cacl ₂ to bring calcium ion in solution CS 270799 B1 was in excess of fluoridated in an excess of at least 20% by weight against the need for reaction Ca ²⁺ + 2 F ¹ ”” ^CaF 2 ' ^and further enough distillation residue from the main separation to increase the salt content in the mixed solution to at least 10% by weight. , then the solution is cooled to 40 ° C or lower, aged at a final temperature for at least 3 hours, then the precipitated CaF ₂ is separated and up to 18% hydrochloric acid containing at least 0.1% by weight of hydrofluoric acid is distilled off from the liquid phase. 2. The process according to item 1, characterized in that the remaining part of the distillation residue 1 is neutralized with sodium hydroxide and the calcium ions present are subsequently precipitated with sodium carbonate, the solid phase is separated off and the solution is used again to precipitate Na ₂ SiF. ₆ . 3. The process according to item 1, characterized in that the solid phase formed during the removal of fluoride ions is washed and optionally dried, whereby further usable calcium fluoride is obtained. 4. The process of claim 2, wherein the separated solid phase is reused to precipitate calcium fluoride.