FR2459204A1 - Cryolite prodn. from fluorosilicic acid - by forming alpha-aluminium fluoride soln. and digesting with slurry of crystalline sodium fluoride, followed by calcination of solid prod. - Google Patents

Abstract

Cryolite is prepd. from fluorosilicic acid and/or sodium fluorosilicate by first forming alpha-aluminium fluoride soln. by a continuous or discontinuous process. Silica is eliminated by filtering or centrifuging, then the soln. is mixed in a digestion vessel with a stoichiometric amt. of a crystalline NaF slurry and digested for at least 60 mins. at 40-80 deg.C, pref. 50 deg.C. The reaction is carried out at an initial solids concn. of 10-50%, esp. 20%. The prod. is filtered off and calcined at above 500 deg.C to below 700 deg.C, esp. at 580 deg.C. Process is esp. useful for prodn. of cryolite from fluorosilicic acid which is obtd. as a by-prod. in industries producing e.g. phosphates and HF.

Description

 1, Process for obtaining cryolite from fluosilicic acid and / or sodium fluosilicate ".

The present invention relates to a process for obtaining cryolite from fluosilicic acid and / or sodium fluosilicate, fruit of the experience acquired over the years by the applicant in the field of the use of acid fluosilicon, a by-product of certain industries.

Indeed, it is to take advantage of fluosilicic acid which constitutes a by-product of the phosphate and hydrofluoric acid industries, by making it react on the one hand with an aluminum salt (for example hydrate alumina, bauxite, clay, etc.), thus obtaining a solution of α -ALF3, from which the precipitated silica is separated, and on the other hand by reacting it with one or more sodium salts (for example Na OH, NaCl-NaOH, Na2CO3, Na2504 -NaOH, NaCl -Na2CO3, etc.) to obtain a pulp of crystallized NaF, from which the silica was previously separated in soluble form such as SiO3Na2. The solution of a-AlF3 is mixed with the pulp of
NaF in stoichiometric quantities, the residue is filtered and calcined to finally obtain the cryolite.

 Obviously, the use of fluosilicic acid is becoming increasingly important every day; in fact, it is a byproduct of the phosphoric acid, superphosphates and hydrofluoric acid manufacturing industries in which recovery processes can be included which make it possible to obtain a concentrated acid at around 20% which is already of acceptable quality for the manufacture of fluorinated fluxes.

 Consequently, the advantage of taking advantage of fluosilicic acid originating from the industries in question consists not only in its economic aspect but also in its aspect in relation to the contamination because its use makes it possible to resolve the problem of the evacuation of effluents from the industries in question.

 The principle of the chemical reactions on which this process is based is well known: decomposition of sodium salts and aluminum from fluosilicic acid.

H2SiF6 + 2A1 (OH) 3 ------- 2A1F3 + SiO2 + 4H20 (1)
H2SiF6 + 8NaOH ----------- 6NaF + SiO3Na 2 + 5H2O (2)
Na2SiF6 + 6NaOH ----------- 6NaF + SiO Na 2 + 3H20 (3)
Reactions (2) or (3) are used without distinction depending on the raw material to be used, fluosilicic acid or sodium fluosilicate.

For the formation of the sodium salt of fluosilicic acid, the reaction of the acid with a soluble sodium salt, for example NaCl or Na2SO4, can be carried out with the following reactions
H2SiF6 + 2NaCl ----------- Na2S iF6 + 2HC1 (4) H2SiF6 + Na S04 ----------- Na2SiF6 + H2SO4 (5)
It should be noted that in the current state of the art various processes are known for obtaining the aluminum fluoride solution (reaction 1). For example, Spanish Patents Nos. 376,226 and 422,982 are discontinuous processes for obtaining aluminum fluoride. Also Spanish patents, Nos 403 234 and 414 546 relate to continuous processes for obtaining aluminum fluoride, in which the precipitated silica is separated by centrifugation or by filtration. British patent No. 1 074 665 consists finally an improvement in the silica filtering operation.

With regard to reactions (2) and (3), there is Spanish Patent No. 406,173, among others, which relates to obtaining sodium fluoride from fluosilicic acid and / or sodium fluosilicate.

 Under these conditions, the subject of this patent is centered on obtaining the cryolite starting from aluminum fluoride and sodium fluoride obtained starting from fluosilicic acid or sodium fluosilicate, according to the reactions indicated above, and in agreement with the state of the art defined by the patents mentioned.

Obtaining the cryolite, according to the process which is the subject of the present invention, is carried out by means of the following phases
1. mixture of the a-AlF3 solution and the pulp of
Crystalline NaF in stoichiometric amounts.

 2. Precipitation of the cryolite according to the proportion of mixture, by the reaction of the crystalline NaF in fine grain and the ions of F - and A13 +.

3. Formation of the cryolite by the reaction of
AlF3.3H2O crystallized and free NaF, by calcination at a temperature above 5000C and below 7000C.

4. During the calcination process, preferably dynamic, of the residue obtained by the mixture of paragraph (1), pyrohydrolysis occurs with an appreciable loss of
SiO2 that the residue could contain, and there remains a final product which comprises approximately 0.3S; of this silica impurity.

 The bibliography related to obtaining cryolite from fluosilicic acid is abundant. Hydrolysel by sodium hydroxide is described in Spanish patent No. 375,325 and in US patent No. 3,061,411, as well as in the article by A.S. Korobitsyn and G.N. Bagachov, Zh. Prikl. Khim.

(Leningrad) 44, 1,635 - 1,640 (1970) with Na2C03 and CO2, etc.

The use of crystalline NaF and a solution of AlF3> in four phases and on a laboratory scale, is described in the work of CN Bogachov (Tr. Ural. Nauch. Issled Khim,
Inst., 1968, 17, 92-5). Another, more recent article is that of Marian Globelny (Journal of Fluorine Chemistry, 8 (1976) 133-144), which relates to research into the conditions for the formation of cryolite, chilite and other double fluorides of aluminum and sodium.

 US Patent No. 2,996,355 uses fluosilicic acid in solution in hydrofluoric acid as the fluorinated raw material.

 In the current state of the art, one would think that there is an industrial process summarizing or simplifying the methods described. In reality, there are different methods, based like this one on the hydrolysis of the sodium or ammonium salt, but which have the disadvantage of giving rise to the obtaining of cryolite containing a significant proportion of silica or phosphorus.

 Precisely, the advantage of the method recommended in this patent is the obtaining of sodium cryolite which can be used as flux in the aluminum industry or in any other industry which requires the use of this fluorinated product, with a very low percentage of silica impurities.

 The separation of the silica in reactions (1), (2) and (3) is acceptable and it decreases substantially when the wet product is calcined at a temperature between 500 and 70 ° C., preferably at 5800 ° C.

Another additional advantage of this method is the possibility of adapting the installation to simultaneously or separately produce the fluxing products: cryolite, sodium fluoride and aluminum fluoride, using an economical and abundant raw material, such as fluosilicic acid, etc. sodium fluosilicate.

 Reactions (1), (2) and (3) can be carried out discontinuously although they are capable of being carried out by a continuous process, by means of a cascade of three or more reactors.

 The aluminum fluoride solution obtained in reaction (1) has a concentration of between 120 and 200 g / l, preferably 150 g / l, and reacts in a digestion tank with sodium fluoride pulp, which has a concentration of solids from 30 to 50%, preferably 45%.

 The digestion time is at least 60 minutes, at a temperature between 400 and 800C, preferably 600C, and then filtered and calcined at a temperature above 500 C and below 7000C, preferably 5800C.

 The humidity of the pulp obtained after filtration which is introduced into the oven must be greater than 30%, for pyrohydrolysis to occur, with the corresponding significant loss of SiO2 in the form of SiF4.

 Another advantage of this process is the recovery of the sodium which occurs in the reactions for obtaining NaF. The mother liquors contain sodium dissolved in the form of sodium silicate, sodium fluoride and even sodium hydroxide, which it may be advantageous to recover by any traditional process.

These mother liquors are treated with lime to produce the precipitation of Si03Ca and CaF2, depending on the reactions.
Si03Na - Ca (OH) 2 ------ SiOCa +? NaOH (6)
2NaF + Ca (OH) 2 --------------- CaF2 + 2NaOH (7)
The sodium recovered in the form of sodium hydroxide is recycled in the process of production of sodium fluoride.

 Of course, various modifications can be made by those skilled in the art to the processes which have just been described solely by way of nonlimiting example, without departing from the scope of the invention.

Claims (4)

 1. Method for obtaining cryolite from fluosilicic acid and / or sodium fluosilicate, essentially characterized in that the solution of a-AlF3 obtained in batch or continuous process from fluosilicic acid and / or sodium fluosilicate, from which the silica has been removed beforehand by filtration or centrifugation, is mixed in a digestion tank with crystalline NaF pulp, in stoichiometric quantities, the digestion time being at least 60 minutes and the reaction temperature included between 40c and 800C, preferably 50C, the reaction occurring between the crystalline and fine sodium fluoride with the aluminum fluoride ions contained in the aluminum fluoride solution, reaction which is carried out with an initial concentration of solids included between 10 and 50%, preferably 20, with filtration and subsequent calcination at a temperature above 5000C and below 700 "C, preferably 5800C.  2. Method for obtaining cryolite from fluosilicic acid and / or sodium fluosilicate, according to claim 1, characterized in that the sodium fluoride that the reaction pulp contains is crystalline, and in that the diameter of its particles are such that 99% of them pass through a 200 mesh Tyler screen. 3. Method for obtaining cryolite from fluosilicic acid and / or sodium fluosilicate, according to claim 1, characterized in that the humidity of the filtered product and which is intended to be calcined thereafter, must be at least 40%.  4. Method for obtaining cryolite from fluosilicic acid and / or sodium fluosilicate, according to claim 1, characterized in that the calcination is preferably carried out dynamically, at a temperature between 50 and 7000C, this process giving rise to an appreciable loss of silica, by pyrohydrolysis of the initial product, with the formation of SiF4 in gaseous form, which is extracted from the calcination phase.