DE1088035B - Process for the production of synthetic cryolite - Google Patents

Description

GERMAN

The invention relates to a method for the production of artificial cryolite, which relates to its degree of purity can be compared with a purified natural product.

Synthetic cryolite is usually produced in such a way that aluminum fluoride is reacted with compounds which provide sodium or fluorine ions. Sodium fluoride and ammonium fluoride are usually considered as compounds which provide sodium and fluorine ions. If ammonium fluoride is used, then another compound, such as sodium chloride or sodium carbonate, which supplies sodium ions, must be added to the reaction mixture in order to form cryolite, Na ₃ AlF ₆ .

However, if raw material of the highest degree of purity is not used for these previously known processes, then the obtained artificial cryolite is for use as a flux for aluminum containing Ores unsuitable for the production of metallic aluminum, the most important field of application. Since on the one hand the supply of natural cryolite is decreasing, on the other hand the Aluminum industry is experiencing considerable expansion, must have large quantities of synthetic Cryolite of high purity can be produced.

A process for the production of artificial cryolite which contains practically no silicon dioxide has already been described. Here, raw aluminum fluoride solution having a crude solution of sodium fluoride which has been acidified to a _pH value of 3.5, was added. The cryolite obtained, going from _s-free silicon dioxide is however determined even by about 0.10 to 0.23% phosphorus as P ₂ O _5, contaminated, depending on the reaction conditions. This apparently insignificant amount of P ₂ O ₅ has an extremely detrimental effect on the efficiency of the current for the reduction process in the aluminum industry. Studies carried out on facilities of the order of magnitude in the industry have clearly shown that 0.01% P ₂ O ₅ contained as impurities in the cryolite flow increases the power efficiency in the troughs by 1 to 1.5% decrease. The presence of P ₂ O ₅ in the aluminum trough is so important that it is common practice in the aluminum industry to reduce the phosphorus content of the alumina transported to the reduction plants to 0.002 to 0.003% P ₂ O ₅ .

Another disadvantage of the artificial cryolite produced by the previously known method is that it is difficult to represent as a practically white product similar to natural cryolite. the Discoloration of the artificial cryolite represents one

Method of manufacture
of synthetic cryolite

Applicant:

WR Grace & Co.,
New York, NY (V. St. A.)

Representative:

Dr. GW Lotterhos and Dr.-Ing. HW Lotterfios,
Patent attorneys, Frankfurt / M., Lichtensteinstr. 3

Dr. Chem. Frederick T. Fitch, Baltimore, Md.,

and James E. Armstrong, Pittsburgh, Pa. (V. St. Α.),

have been named as inventors

empirical yardstick for the general level of trace impurities. If for implementation If a product with a high degree of purity is necessary for processes, then there is hesitation in favor of the natural one Cryolite through this reddish or mouse-gray product, which is usually

3.0 drive is obtained to replace.

It has been found that an artificial cryolite of practically white color and a P ₂ O ₅ content of less than 0.05% can be obtained from starting materials which contain phosphorus oxide or other phosphorus compounds as impurities, if one is at the reaction 'between the aluminum fluoride and sodium and fluorine ions-providing compounds in aqueous solution, the sodium and fluorine-providing compounds, at least in the for the reaction with aluminum fluoride required stoichiometric amount, preferably in large quantities, applying and during the Kryolithfällung the _pH value in the Keeping the reaction mixture below about 2 by adding mineral acid.

Examples of raw materials containing phosphorus oxide and phosphorus compounds are bauxite, a relatively cheap raw material for alumina, and hydrofluoric acid, a by-product which occurs in the acid treatment of phosphate rock and is an important raw material for cheap fluorine. It is known that this acid contains traces of phosphorus either in the form of colloidal P ₂ O ₅ or, more likely, as H ₃ PO ₄ .

009 589/383

Crude hydrofluoric acid and others besides cryolite in aqueous solutions from raw materials Crude aluminum-iron materials suitable for bauxite due to traces of phosphorus or its compounds also contain this impurity. fertilizers and heavy metal salts are contaminated,

The essence of the invention thus lies in the fact that it can be carefully identified.

Fold control of the side reactions that are associated with the 5 The conditions which are associated with the production of a cryolite which is essentially free of impurities in the formation and precipitation of the double salt. Going more specifically, the invention relates to raw materials containing phosphates and heavy metals, the control of the involved side reactions as impurities, in the implementation of * salts and acids of phosphorus and fluorine and such materials in aqueous solutions not heavy metal complexes as well as those that from io agile, the following description of iron and titanium are formed. The double fluorides can be found in connection with the drawing at the beginning as cryolite Na _{3 AlF 6} or as stands. In this drawing, the diagram shows the ammonium _{cryolite (NH 4} ) ₃ Al F _{6 being} precipitated rapidly as a result of the percentage of phosphorus containing impurities in a simple metathesis to sodium aluminum fluoride formation (indicated as P ₂ O ₅ ) in the final cryolite. The method of the inventions 15 product, the final _pH value of the Kryolithdung then is to control the reaction f ällungsstuf e is compared, namely for different con- ditions in impurity-containing aluminum fluoride ammonium fluoride, sodium chloride solutions, in which Aluminum fluoride is brought into contact with a reaction mixture containing different molar ratio compounds or compounds that provide sodium and nits of aluminum fluoride to ammonium fluoride ent-fluoride ions. 20 hold.

Aluminum fluoride will be discussed in detail for the purposes of the invention

dung preferably by reaction between bauxite, it seems appropriate to provide an explanation of the and hydrofluoric acid according to the post-main reaction that leads to the formation of the insoluble Equation I given above: double salt leads, and the many side reactions,

25 associated with it. For reasons

Al ₂ O ₃ + H ₂ SiF ₆ - ^ 2AlF ₃ + SiO ₂ -HH ₂ O. (I) of clarity, this discussion is primarily intended to be based on

the reaction in its preferred course of operation

This method, whereby one is limited to a maximum output, namely to that which is represented in the booty of aluminum fluoride solution, which is represented in relation to equations II and III.
Silica free is obtained in the USA

U.S. Patent 2,842,426. After this AlF ₃ + 3 NH ₄ F-> (NH ₄ ) ₃ AlF ₆ (II)

Processes are bauxite and silicon fluorohydrate (NH) AlF + 3Na + ->

chemical acid for a predetermined time at a * ^{s 6}

predetermined temperature in contact with each other '-> 3 NH ₄ + + Na ₃ AlF ₆ (III)

brought up to the point of maximum solubility 35

is achieved by aluminum fluoride. After that the aluminum fluoride and ammonium fluoride will be in

Solution filtered immediately, reacted with one another from the filter cake of free aqueous solution, with washing, whereby an aluminum fluoride solution obtained monium cryolite precipitates. By simple metathesis which in this state without crystallization becomes ammonium cryolite in the more familiar and of the insoluble aluminum fluoride, about 40 usable sodium salts are converted. The reaction 3 hours left. Obviously, other un- is preferably at elevated temperatures below pure aluminum-iron materials for the presence of the boiling point of the reaction mixture for the purpose instead of bauxite as raw material at about 93 ° C, because ammonium cryolite and be turned. It is equally evident to a limited extent in cold sodium cryolite that other fluorine-containing acids, such as HF, are soluble in water. The reaction mixture should be a Manufacture of aluminum fluoride could be used for a long enough time, generally about 30 to 45 microns. utes, to be left to allow the reaction with

To make cryolite using security stops. Then the solution of an aqueous aluminum fluoride solution is filtered or centrifuged as the base and the artificial cryolite starting material has been isolated from a choice of two. It has been found that side reactions are the known alternatives. According to the one possible phosphorus and heavy metal impurities process, the aluminum fluoride can be brought into contact with sodium and those occurring during the reaction can be brought into contact by fluoride and thus the stoichiometric ratios in which the trium and fluorine ions in the reaction mixture reaction components are present , -and the final delivery *. According to the other method 55 may _pH value of the reaction mixture closely influenced who-AlF ₃ with NH ₄ F to yield a white low 'the. Equation II shows that one mole of aluminum sludge, ammonium cryolite, is reacted and that minium fluoride reacts with 3 moles of ammonium fluoride, with the reaction mixture then reacting with sodium formation of ammonium cryolite. It has been found that containing a compound capable of being one that the amount of phosphorus in the end product, simple metathesis with ammonium cryolite to below 60 specified as P ₂ O ₅ , is significantly lower when the throw is treated, whereby sodium- stoichiometric ratios can be varied slightly, aluminum fluoride forms. General examples of Verdb when using 0.98 mol of aluminum fluoride per 3 mol of compounds that are suitable for this purpose are ammonium fluoride. The adverse effect of sodium chloride and sodium carbonate. While it is preferable to a trace amount of undesirable contamination for economic considerations, 65 has already been described. It has also been found that cryolite can be produced via ammonium cryolite by reducing the pg value of the reaction to the above-mentioned solution reactions or ionic mixture by reducing the O H ion concentration during their usual chemical equivalents to the process which is at a minimum prior to the entire reaction time applicable. The advantages of being able to produce a cryolite, which in the present invention are essentially free of phosphorus-containing contaminants in the production of 7 °

5 6

cleaning is. It should be noted that such heavy metal phosphates represent volumi-

Final pjj value (cf. equation III and the diagram) represent nous, unoriented colloidal precipitates, which are only sparingly soluble in the cryolite precipitation stage below about 2 in the case of acid. While they can be decomposed by the preferred molar ratio of 1 AlF ₃ to concentrated sulfuric acid, 3NO ₄ F, or below about 1 when uncontrolled, such decomposition is in dilute aqueous lated stoichiometric conditions. Not easy to get with the solutions. When the COM-term is intended to about 2 a _pH-value of 2 ± 0.3 sawn phexen iron, titanium or aluminum phosphates are inscribes, the normal deviation are formed corresponds times, it is almost impossible to speak again that when using raw materials, remove. Therefore, acid is added to the reaction switch, larger or smaller amounts of impurity mixture,. in order to contain vacancies during the entire period is to be expected. Under the designation to have hydrogen ions. Apparently, in an uncontrolled manner, the indiscriminate addition of Al F ₃ is supposed to be the simplest way to achieve this goal, to understand _{the acid of one or the other reaction compound to NH 4} F in a molar ratio of almost 1: 3, and with the end- The p _H value is a nenten before the precipitation of ammonium cryolite to the _H value that is added after the addition of all reaction values. If, therefore, one adds to the ammonium fluoride solution participants, in other words in the mixture, an amount of acid which is actually sufficient for the Na ₃ AlF _{6 to be} formed is measured. to the _pH value of the solution be reduced to about 6,

The mechanism given below, that before they are mixed with aluminum fluoride, a final reaction mixture can be formed in a further complicated manner by additional side reactions, serves to explain this point. ₈ α p _H value is about 2 or less. This _pH value is in the phosphorus Ammoniunmuorid- and aluminum is sufficient to prevent the formation of trace amounts of miniumfluoridlösungen in the form of H ₃ PO ₄ or sparingly soluble complex phosphates, appli-phosphates before. In the acidic aluminum fluoride formation, the preferred method of controlled solution, it is probably in the form of H ₃ PO ₄ to prevent stoichiometric ratios. Strong, since in the presence of excess water mineral acids such as hydrochloric acid or sulfuric acid there is a tendency for ionization to be suitable for this purpose. With comparable suppression (cf. equation IV): The pg-value of the A1F _{3 -solution could reduce the amount of acid}

below about 1 if there is

H ₃ PO ₄ -> 3 H + 4- PO ₄ (IV) should be desirable, .this reaction component

30 to be pretreated _{instead of NH 4 F.}

The main amount of phosphorus impurity Further information on the stoichiometric origin of the aluminum fluoride solution, since the proportions of the reaction mixture can be seen, the basic material bauxite often even 2 to 3 ⁰ _{AiT 2} O _{5 so} that it is advisable to keep the amount of ammonium fluoride in. To have the addition of excess ammonium excess. Is aluminum fluoride fluoride, which the presence of a small quantity ₃₅ in excess, so is that * dargefreiem in Equation IX ammonium fluoride in the reaction mixture turned reaction takes place:

, prevents the formation of insoluble ter- λ, ₊₊₊ ι ρ q AIPOl (IK)

tiary phosphates by the formation of soluble complexes ^{4 4} ^ ^ '

Fe ⁺ ++, TiO ⁺ + and other heavy metals, which is colloidal aluminum phosphate except

Otherwise it would precipitate as phosphates and the cryolite- _{40 is} practically insoluble in concentrated acids

and VI): relationships formed. When the stoichiometric

p _e +++ _ | _g jqrjj + - (- gp-_ _> . ratios are not controlled to the

⁴ τ? -pt £ TvTTT + nj \ to suppress formation of this colloidal material,

->. tfe.F ₆ - -T-OiNtI ₄ Ky) _{45 the} end pn value must still be set to less than 1

TiO ⁺⁺ + 2 H ⁺ + 6 F- -> must be made to remove it. The formation of this

_ _> 'j> £ p l_jj Q / yj \ I leave unwanted aluminum salt on the ground

^{62 of} the stoichiometric ratios are not entirely

First, the acid exerts a buffering effect, say, since buffer response may be present, whereby the ionisation of H suppressed ₃ PO ₄ and ₅₀ will be substantially by the reaction between aluminum and phosphate An inclination to make the co-precipitation of the ion harmless. It is known that with heavy metal phosphates with the double salt precipitation to the addition of hydroxy honing to acid solutions that limit. _{Excess NH 4} F then reacts, containing phosphate and alkali ions, causing immediate, preferably heavy metals, with the formation of soluble precipitates of tertiary phosphates. A complex, e.g. B. FeF ₆ -, TiF ₆ ^{- -} complexes, which in 55 such a situation exists when a slightly acidic aluminum acid media can be easily dissolved. In addition to a basic aluminum, it is known that phosphoric acid is added to heavy metals such as eluoride solution, which by its nature reacts to iron, according to equation VII. That contains an extraordinary amount of ammonium hydroxide. All tertiary Hch insoluble iron phosphate and its strongly colored _: Heavy metal phosphates are stable towards water, isomeric form represent the end products. Go Those that are more complex in nature, esp

Iron, especially those of iron, are even in acidic solutions

4Fe 4-12 H + + 4PO ₄ - ^ stable and must, in order to be solved, with strong

- ^ 2 Fe PO ₄ I + Fe [Fe (PO ₄ ) ₂ ] | + 12 H ⁺ (VII) concentrated acids. When a

sometimes a complex heavy metal phosphate precipitated

Hydrolysis of this latter compound yields 65 ', there is a tendency for it to be removed from the cryolite complex molecule that is believed to be absorbed by the precipitate structure. Therefore water des Product of equation VIII. Substance ions during the entire reaction time in the

Mixture in excess, in order to avoid any post-

Fe [Fe (PO ₄ ) ₂ ] + 8HÖH- »partial effect of the hydroxyl ions to be suppressed.

-> ■ Fe (OH) ₃ · H ₃ [Fe (PO ₄ ) ₂ I- SH ₂ O (VIII) 70 Once the reaction can take place, has

subsequent addition of any amount of acid has no demonstrable effect on reducing the amount of contaminants that remain in the cryolite. The amount of trace impurities in the cryolite resulting from uncontrolled reactions in neutral or slightly acidic aqueous solutions is between 0.25 and 0.30 ^fl / o. From the foregoing discussion it can be seen that at such a phosphorus level, a 25 to 50% loss in current efficiency was observed when the cryolite was used to start an aluminum trough.

In curve 1 of the diagram the effect is (expressed as P ₂ O ₅₎ of the final p _H -value of Kryolithfällung on the amount of phosphorus in the specified Kryolithprodukt for a reaction mixture which is rich in AlF. ₃ It has been observed that must be set under these conditions the _pH value to about 1 or even less, if one wants to obtain a product containing less than about 0.05% P ₂ O ₅ has. Such a product is of questionable use for the production of aluminum by fused-salt electrolysis. Curve 2 shows the amount of P ₂ O ₅ in the final product when the reaction mixture was rich in NH ₄ F. Interestingly, under these conditions, with a p 1 of about 2 or less, a product is obtained which has less than 0.03% P ₂ O ₅ . Such a product is comparable to natural cryolite when used in aluminum reduction.

It has been found that sodium fluoride can be used in place of NH ₄ F (see Equation X).

AlF ₃ + 3 NaF -> Na ₃ AlF ₆

(X)

35

If NaF is used in a slight excess in an acidic medium, the reaction of equation XI is found instead of:

Fe +++ + 6 Na + + 6 F- -> FeF ₆ "- - + 6 Na + (XI)

40

Considerations with respect to the p _H -value and the stoichiometric ratios, as has been described above with reference to AlFj-NH ^ F-Na + 'reaction mixtures, apply equally when NaF is used as the reaction component.

When the double salt precipitate is formed under controlled stoichiometric proportions, at a _pH value of about 2 or less, is allowed to the reaction mixture at a temperature of about 93 ° C for about ¹ Zt to ³ A hours are preferably to an end of the Ensure response. If the initial precipitate consists of amonium cryolite, sodium chloride or a comparable compound which provides sodium ions is added to the reaction mixture before it is allowed to stand. The one-part cryolite crystals are then filtered off or centrifuged off from the mother liquor and dried and calcined in a known manner. It is common practice to pre-dry the cryolite crystals at low temperature, preferably around 120 ° C, either in an oven dryer or in a rotary dryer prior to calcination. The material is calcined at about 594 to 650 ° C to remove bound moisture contained therein, which is considered undesirable for aluminum reduction. While the drying and calcination stages of the cryolite process are known, an interesting and overall useful effect of the process according to the invention can be observed after the calcination. The cryolite produced by the usual methods of solvent chemistry has a reddish color when the stoichiometric ratios were such that aluminum fluoride was present in a slight excess. This color is directly due to the formation of complex salts from impurities present in traces, for example the isomeric phosphate Fe [Fe (PO ₄ ) ₂ ]. It has been observed that cryolite, the same amount of * contains trace impurities, according to the stoichiometric ratios and the _pH value of the precipitation stage knows, or can appear red. It is evident that from the standpoint of trace impurities, the color of the product is of little importance.

A general comparison of the commercially available cryolites suitable for reduction appears to be functional here. Table 1 shows the chemical analysis data for cryolite produced after Method of the invention for artificial cryolite made from practically phosphate-free materials for reduction purposes for artificial cryolite, which is made from impure materials according to the previously known method without acidification or control of the stoichiometric ratios, as well as for natural ones Cryolite compiled.

Table of analyzes of typical cryolites

component

Percentage composition

Cryolite
according to "procedure
the invention

More natural
Cryolite

Practically

phosphate-free

Cryolite

Cryolite

after

previously known
procedure

N / A...

Al ...

P ₂ O ₅

Fe ₂ O ₃

TiO ₂ .

SiO ₂ .

52.48
33.18
12.91
0.03
0.021
0.038
0.26

53.25
30.33
12.68
0.03
0.023
0.001
0.21

49.00
29.85
12.80
0.034
0.056

1.40

52.31

32.09

12.80

0.25

0.086

0.52

9 10

It can be seen from Table 1 that the product obtained according to the invention reaction mixture therefore contained an AlF ₃ : NH ₄ F compound with the purified natural molar ratio of 1.02: 3. The final _pH value was the borrowed material and an artificial product mixture by addition of sodium chloride 1.2 and on costly paths, phosphate-free Mate P ₂ O ₅ content of the material 0.05, which requires a considerable rials has been produced, an increase in this contamination can compete with the competition. - product of the process described in example I

Example I showed after which the reaction mixture is rich

NH ₄ F was.

A bauxite which has the addition shown in Table 2 to Beisoiel III

has composition, was for the production io

an aluminum fluoride solution is used. The procedure described in Example II was followed

essentially repeated using a reaction mixture that has a slight stoichiometric

load / see excess of AlF ₃ compared to the existing

Bauxite raw material had 15 amount of NH ₄ F. The ending pjj value of the

Composition mixture was 1.6, and the cryolite product

Al ₂ O ₃ 59.87 »/» held 0.05% P ₂ O ₃ .

SiO ₂ 8.67% Example IV

Fe ₂ O ₃ 0.33%

Τ; ο 1.30% ^ao The procedure described in Example I was carried out in

ρ ο 2.90% substantially repeated using one

Solution containing excess ammonium fluoride (about 0.98: 3) compared to the existing aluminum

753 g of this material were exhibited with 2.714 g of H ₂ SiF ₆ , fluoride. 130 ecm of concentrated salts containing 16.21 percent by weight F, acid was added to the NH ₄ F for 40 minutes before it was added to the solution for a long time at an average temperature of 70 ° C. The final _pH value of the reaction brought contact. After completion of the reaction mixture was 3.6, and the resulting cryolite time, the sludge was filtered immediately and the filter product had a P ₂ O ₅ content of 0.10. It cake washed with 3 liters of water. The result obtained was found that even using the solution which, apart from colloidal particles, the improved technique of stoichiometric control that were present in traces, was clear, weighed 5.637 g for aluminum reduction purposes and contained 11.55 " / O aluminum fluoride. Was unacceptable (compare Table 3 below,

Simultaneously with the aluminum fluoride production, in which the final _H p-value is one of AlF ₃ , 2.714 g H ₂ SiF ₆ (16.7% F content) with 1.508 g mixture [1.04: 3] 2.7 and the P ₂ O ₅ content of an NH ₄ OΗ solution containing 28% ammonia, 35 product was 0.17.) Reacted, the reaction mixture under 30 minutes

closed conditions while maintaining Example V

a pjj value of about 8.9 and off

the ammonium fluoride solution thus obtained. The procedure of Example I was repeated,

The silica is filtered off and the filter cake is washed with 3 liters of water, using an impure hydrated clay. 6.148 g of solution were obtained, which was known in the trade as a precipitation product with 13.4% ammonium fluoride. To this he added 'one is. Identical amounts of the reactants 100 ecm conc. Sulfuric acid to the _pH value of were used with the exception that the reac-8,9 to reduce to 6.1. The ammonium fluoride mixture by adding 100 ecm H Cl to the was mixed with the aluminum ammonium fluoride produced at the same time before mixing the amfluoride, ammonium cryolite tnonium with the aluminum fluoride solution being formed which was left to stand for about 10 minutes. Final _pH value was acidified by 4.9. The Alu-Das AlF ₃ : NH ₄ F molar ratio in the resulting minium fluoride-ammonium fluoride molar ratio was 0.98: 3. For this purpose, 7090 g of a 0.98: 3 and the P ₂ O ₅ content of the product 0.15%. 18.18% NaCl solution added, whereby 50

Forms sodium aluminum fluoride. The final pn value Example VI

the cryolite mixture was 2.0. This mixture

allowed to stand for about 45 minutes, after which the procedure of Example V was repeated,

Cryolite crystals were filtered off and treated with 31 water from the addition of HCl being omitted. Washed free of impurities. The filtered 55 _End-pH value of the reaction mixture was 7.9, and material was dried at 122 ° C in the furnace, and the obtained P ₂ O ₅ content calcium 0.25.

In the previous examples it is a muffle furnace. For the heavy metals, the analysis showed that the cryolite, which was obtained from mixtures of reaction and phosphate impurities in the cryolite product, had the following values: Fe ₂ O ₃ : 0.023%; TiO ₂ : 0.058%; 60 excess of monium fluoride over the P ₂ O _{5 present} : 0.02%. Aluminum fluoride, white or nearly white

Had color against it. Products made from solutions with Example II excess aluminum fluoride buff up to

were dark red.

It was essentially as in Example I 65

drive and 5.479 g of an ammonium fluoride solution with example VII

14.45%> NH ₄ F at a _pH value of about 6.1 with

ecm acidified sulfuric acid. This solution I _n Table 3 further investigations (some

was brought into contact with 5.312 g of an aluminum fluoride solution (used for the curves in the diagram) with 11.9% AlF _3. The acidified 70 put together.

Table 3

Relationship of _pH value and stoichiometric

Relationships to color and purity of the artificial

Cryolite

end
PH value
the
Cryolite
mixture AlF ₃ :
NH ₄ F-
MoI-
relationship colour
of the product ^fl / oFe ° / oP ₂ 0 ₅ 2.2 1.04: 3 Light leather color 0.018 0.074 2.7 1.04: 3 Leather colors 0.028 0.17 4.0 1.04: 3 Leather colors 0.036 0.25 5.7 1.04: 3 Reddish 0.03 0.30 5.4 0.83: 3 Reddish 0.048 0.28 2.9 0.83: 3 Light leather color 0.026 0.086 1.9 0.83: 3 Light leather color 0.028 0.05 8.1 0.77: 3 White 0.050 0.26 6.7 0.77: 3 White 0.046 0.25 3.9 0.77: 3 White 0.045 0.17

10

This shows that the process has a substantial improvement over the previously known methods for producing synthetic cryolite from impure raw materials. The first time a product is made available from such raw materials have the loan with natural or artificially ¹ products from reactant that no impurities can compete. Using the preferred measures of the invention, both the control of the p _H -value also comprise as the stoichiometric ratios, a white substantially free of impurities are produced cryolite from reaction mixtures whose _pH value is below about. 2 Such a technique is apparently superior to acidifying the reaction mixture to a final pg below about 1, since it provides a significant saving in raw materials with a reduction in a severe corrosion problem. Solutions which contain Cl--, F "-, SO ₄ " -, H ⁺ - and Na ⁺ ions increase with regard to their corrosion to the same extent as the H ion concentration. When the P _n value is reduced from 2 to 1, this problem becomes acute and one has to use special, expensive alloys which are resistant to acid as materials for the apparatus for carrying out the process.

Claims (6)

Patent claims: 1. A process for the production of synthetic cryolite by reacting aluminum fluoride with sodium and fluorine ion supplying compounds in aqueous solutions which contain phosphorus compounds as impurities, characterized in that the sodium and fluorine ion supplying compounds at least in that for the reaction with aluminum fluoride required stoichiometric amount, are preferably used in a larger amount during the Kryolithfällung the _pH value in the reaction mixture with mineral acid below about 2 is maintained, and the precipitated cryolite is isolated. 2. The method according to claim 1, characterized in that that as compounds that deliver sodium and fluorine ions, sodium fluoride or ammonium fluoride used in combination with sodium chloride or sodium carbonate. 3. The method according to claim 1 or 2, characterized in that the compounds containing sodium and provide fluorine ions, prior to mixing with aluminum fluoride with a strong Mineral acid are pretreated. 4. The method according to claim 1 to 3, characterized in that the reaction mixture according to the cryolite precipitation for a short time at a higher temperature and then the Cryolite is separated, dried and calcined. 5. The method according to claim 4, characterized in that the solution takes about 30 to 45 minutes is allowed to stand at a temperature below the boiling point of the reaction mixture. 6. The method according to claim 2, characterized in that the ammonium fluoride with a strengthen mineral acid is pretreated before it comes into contact with the aluminum nitride solution brings, and then added sodium chloride or sodium carbonate to the resulting mixture will. 1 sheet of drawings © 009 589/383 8.60