US3128151A - Process for producing a sodium fluoaluminate composition having predetermined naf/alf3 ratio - Google Patents

Description

United States Patent 3,128,151 PROCESS FOR PRODUCING A SODIUM FLUO- ALUMKNATE COMPOSITION HAVING PRE- DETERMINED NaF/AlF RATIO Domenico Zanon and Roberto Trupiano, both of Venice, Italy, assignors to I.C.P.M.--Industrie Chimiche Porto Marghera S.p.A., Milan, Italy, a corporation of Italy No Drawing. Fiied Oct. 4, 1960, Ser. No. 60,306 Claims priority, application Italy Oct. 30, 1959 2 Claims. (Cl. 23-88) The present invention relates to a process for the preparation of sodium fluoaluminates having a NaF/AlF molar ratio fixed at a value between 2.65 and 1.67.

By the term sodium fluoaluminates it is intended to denote, in the present description, those compounds in which the fluorine, aluminum and sodium are chemically bound to one another, that is, in each molecule of which, are always present all three of the elements, as occurs, for example with the known cryolite and chiolite compounds.

In order to characterize hereinafter such compounds, for the sake of simplicity, the NaF/AlF molar ratio, will be adopted, even if said ratio is not in any way related to the actual structure of the molecules.

During long operation periods of a furnace for aluminum electrolysis, the Nai /A11 ratio in the bath tends to vary due to losses deriving from decomposition and volatilization of the various constituents of the bath, losses which are normally higher for the aluminum fluoride than for the sodium fluoride, whereby the NaF/AlF molar ratio in the electrolysis baths is evidences an increase, and is therefore re -established and maintained at the value required by the practice of each plant, by means of periodic additions of aluminum fluoride, in addition to cryolite,

The drawbacks derived from the necessity of adding free aluminum fluoride to the baths are many and commonly known; they are in relation to the different melting point and the different solubility in the baths of the aluminum fluoride and cryolite, and to its high volatility and easy decomposition. In addition the relatively low titer of the products mainly on the market (in which often the AlF content does not reach 90%), the considerable moisture content of these products, and the inconvenience of the operation itself are also disadvantages.

An object of the present invention is to provide a process for the preparation of a sodium fluoaluminate having a definite NaF/AIF molar ratio fixed at a value comprised in the range between 2.65 and 1.67, that is, in other Words, wherein the percentage of Allis higher than that of the normal types of natural and synthetic cryolites. For clarity of explanation it is stated that these new fluoaluminates have a pro-established excess of aluminum fluoride varying from about 5.3 to 35% with respect to the standard reference formula Na AlF depending on the type of the desired fluoaluminate, but wherein the aluminum fluoride is not free nor in mechanical mixture, but is all chemically combined with the sodium fluoride in the fluoaluminate molecule; within this molecule the aluminum fluoride thus acquires the heat stability, solubility and other Well known properties, positive as far as the electrolysis of a chemically bound substance is concerned.

Said iluoaluminates may thus be advantageously used in the aluminum industry for a more regular operation of the electrolysis baths instead of the usual cryolite and aluminum fluoride, because, by means of their use, the drawbacks already mentioned, deriving from the separate use of cryolite and free aluminum fluoride, are completely or partially eliminated, as are particularly the 3,128,15l Patented Apr. 7., 1964 "ice drawbacks deriving from the ready decomposition of the aluminum fluoride by the heat, a fact which always leads to considerable losses, and the considerable difference between the solubilities and the melting point of the aluminum fluoride and the solubility and melting point of the cryolite.

Numerous processes have been heretofore proposed and adopted for the preparation of synthetic cryolite; the difficulties of obtaining a product having a high NaF/ AlF molar ratio that is close to 3, and therefore proximal to the molar ratio of the natural cryolite, are well known above all when the preparation of synthetic cryolite occurs in the presence of free acid. It is also known that the synthetic cryolite, tends during preparation, to degenerate toward the formation of compounds which have in fact a lower NaF/AlF ratio; this is due to the presence of free aluminum fluoride whose quantity, furthermore is not controllable, and varies within very wide ranges depending upon a great number of factors, such as the temperature and concentration of the various components of the solution from which the product is being precipitated, in relation to the complicated phase diagram of aluminum fluoride and the different crystalline forms under which the aluminum fluoride may precipitate; precipitation of free aluminum fluoride, i.e. that which is not bound to the sodium fluoride, gives rise as a direct consequence, to a considerable decrease of the reaction yields, precisely in relation to the sodium fluoride not bound which remains in solution and is then eliminated with the mother liquors. Furthermore, during drying and calcination of the product, the Water which crystallizes with aluminum fluoride at the rate of from 3 to 9 moles for one mole of A11 causes an hydrolysis of the products with a consequent loss of hydrofluoric acid, loss which is the higher the greater is the quantity of unbound aluminum fluoride in the product.

Cryolite Na AlF is prepared by reacting, for example, a sodium salt solution with a fluoaluminic acid solution in a small volume, mixing intimately together the two solutions then removing the slurry containing the cryolite precipitate just formed and finally maintaining the product obtained, under agitation for at least one hour to effect a digestion. The synthetic cryolite thus obtained has a NaF/AIF molar ratio equal to about 3 and physical characteristics (e.g. heat stability) very close to those of natural cryolite, and in addition the precipitation yields are high even if chloride is used as sodium salt, that is, even if the cryolite is precipitated in an atmosphere containing at the free state the hydro chloric acid, which, as known, is formed by the reaction between sodium chloride and fluoaluminic acid, the fluoaluminic acid solution, prepared for example by dissolving aluminum oxide or hydrate in hydrofluoric acid, has an HI /A11 molar ratio equal to 3, that is, not lower than the NaF/AlF molar ratio which it is desired to obtain in the synthetic cryolite.

According to the inventive process, the reaction is carried out with the above-mentioned modalities, between a sodium salt solution, for instance chloride, and a solution containing hydrofluoric acid and aluminum fluoride, said solution having however an HF/A-lF molar ratio lower than 2.65, more precisely, fixed to one of the values comprised between 2.65 and 1.67, having thus a predetermined excess of AlF with respect to the quantity which corresponds to the fluoaluminic acid formula (H AlF It has been found that, by carrying out the reaction between the two solutions it is possible to avoid the presence of aluminum fluoride in the "obtained precipitate, by blocking the Na-Al-F-compound in the initial ratio preestablished as desired, though the precipitate is constituted of a sodium fluoaluminate having an NaF/AlF molar ratio lower than that of cryolite, that is lower than 3, or approximately equal to an HF/AlF molar ratio which was in the aluminum fluoride land hydrofluoric acid solution used for the precipitation. As already said, and as illustrated in the following examples, in the obtained precipitate free aluminum fluoride is not present. Instead, in said precipitate two components coexist: one having the formula Na AlF (cryolite) and the other having the formula Na Al F (chiolite), in different proportions depending on the pro-established ratio. The same result is attained by carrying out the reaction still with the same reaction and conditioning modalities, between an aluminum fluoride and hydrofluoride acid solution having an HF/AlF molar ratio even higher than NaF/A11 molar ratio which it is desired to obtain in the product, and a sodium salt solution, always added, however, in a quantity such that Na+/AlF molar ratio is equal, and in any case not higher than NaF/Al'F molar ratio which it is desired to obtain in the product. Similarly, it is possible to operate by reacting a solution containing aluminum and sodium with a solution containing fluorine, or a solution containing sodium and fluorine with a solution containing aluminum, always complying, however, with the fact that Na+/Al+++ molar ratio should never be higher than NaF/A11 molar ratio which it is desired to obtain in the product, and that the quantity of F must be at least stoichiometrically equal and not lower than the sum of Na+ and Al used.

Example 1 Percent NaF 57 MP 43 NaF/AlF molar ratio of the product obtained is therefore 2.65 (NaF/A11 weight ratio= 1.325) whereby this compound corresponds to a cryolite with 5.3% of AlF in excess with respect to the standard formula of comparison Na AlF A roentgenographic analysis carried out on the product has given the following results:

Percent Cryolite 79.3 Chiolite 20.7

Example 2 250 kgs. of a solution containing 25 kgs. of HF and 39.6 kgs. of AlF in which the HF/AI'F molar ratio is equal to 2.4 are reacted with a sodium chloride solution comprising 25 NaCl.

The slurry obtained is conditioned for 1 hour and 30 minutes at the temperature of 52 C. 92.1 kgs. of fluoaluminate are collected. The chemical analysis carried out on said fluoaluminate has given the following results:

Percent NaF 54.5 AlF 45.5

The NaF/AlF molar ratio of the product obtained is therefore 2.4 (NaF/AIR, weight ratio=1.2) so that this compound corresponds to a cryolite with the 10.2% of AlF in excess with respect to standard formula of comparison Na AlF A roentgenographic analysis carried out on the product has given the following result:

Percent Cryolite 62.1 Chiolite 37.9

4. Example 3 200 kgs. of a solution containing 20 kgs. of HF and 38.2 kgs. of AlF in which the H-F/AlF molar ratio is equal to 2.2, are reacted with a sodium chloride solution comprising 25 NaCl.

The slurry obtained is conditioned for 2 hours at the temperature of 51 C. 80.2 kgs. of fluoaluminate are collected. The chemical analysis carried out on said fluoaluminate has given the following results:

Percent Na'F 52.4 AlF 47.6

The NaF/AlF molar ratio of the product obtained is therefore 2.2 (NaF/AlF weight ratio=l.l), so that this compound corresponds to a cryolite with 14.6% of AlF in excess with respect to the standard formula of comparison Na AlF A roentgenographic analysis carried out on the product has given the following results:

Percent Cryolite 47.6 Chiolite 52.4

Example 4 Percent NaF 54.5 A-lF 45.5

The Na'F/AlF molar ratio of the product obtained is therefore 2.4 (NaF/AlF weight ratio=1.2), so that this compound corresponds to a cryolite with the 10% of AlF in excess with respect'to the standard formula of comparison Na AlF A roerntgenographic analysis carried out on the product has given the following results:

Percent Cryolite 62.1 Chiolite 37.9

Example 5 kgs. of a solution of HF at 70% are reacted with 660 kgs. of a solution obtained by mixing 153.4 kgs. of NaCl and 211.4 kgs. of AlCl 6H O.

The slurry is conditioned for 1 hour and 30 minutes at the temperature of 49 C. 183.8 kgs. of sodium fluoaluminate are collected. The chemical analysis on said fluoaluminate has given the following results:

Percent NaF 45.78 AlF 54.22

The NaF/AIF molar ratio of the product obtained is therefore 1.69 (NaF/AlF weight ratio-=0.845) whereby this compound corresponds to a cryolite with 31% of AlF in excess with respect to the standard formula of comparison Na AlF A roentgenographic analysis carried out on the product has given the following results:

Percent Cryolite 1.8

Chiolite 98.2

We claim:

1. A process for producing a sodium fluoaluminate composition with a predetermined sodium fluoride/aluminum trifluoride molar ratio between 1.67 and 2.65 having substantially all of its aluminum trifluoride content chemically combined with sodium fluoride, comprising the steps of intimately and rapidly mixing together a first solution containing two compounds selected from the group which consists of a sodium halide, an aluminum halide and hydrofluoric acid, and a second solution containing the third compound, thereby forming a suspension, said sodium halide being present in an amount not exceed ing the stoichiometric equivalent of sodium in said sodium fiuoaluminate and being in a molar ratio with the aluminum of said aluminum halide substantially equal to said predetermined molar ratio, said hydrofluoric acid being present in an amount selected so that the total quantity of fluoride ion in said solutions is substantially equal to the stoichiometric amount of fluoride in the sodium fiuoaluminate composition of predetermined sodium fluoride/aluminum trifiuoride molar ratio; and

6 conditioning the suspension by continuously stirring it for a period of at least one hour at a temperature on the order of 50 C.

2. A process as defined in claim 1 wherein said aluminum halide is selected from the group which consists of aluminum fluoride and aluminum chloride.

References Cited in the file of this patent UNITED STATES PATENTS 1,475,158 Howard Nov. 20, 1923 2,058,075 Gaither Oct. 20, 1936 2,182,510 Heiser Dec. 5, 1939 2,196,077 Morrow et al Apr. 2, 1940 2,915,443 Wallace et al Dec. 1, 1959 2,996,335 Kamlet Aug. 15, 1961 3,049,405 Trupiano et a1 Aug. 14, 1962

Claims (1)

1. A PROCESS FOR PRODUCING A SODIUM FLUOALUMINATE COMPOSITION WITH A PREDETERMINED SODIUM FLUORIDE/ALUMINUM TRIFLUORIDE MOLAR RATIO BETWEEN 1.67 AND 2.65 HAVING SUBSTANTIALLY ALL OF ITS ALUMINUM TRIFLUORIDE CONTENT CHEMICALLY COMBINED WITH SODIUM FLUORIDE, COMPRISING THE STEPS OF INTIMATELY AND RAPIDLY MIXING TOGETHER A FIRST SOLUTION CONTAINING TWO COMPOUNDS SELECTED FROM THE GROUP WHICH CONSISTS OF A SODIUM HALIDE, AN ALUMINUM HALIDE AND HYDROFLUORIC ACID, AND A SECOND SOLUTION CONTAINING THE THIRD COMPOUND, THEREBY FORMING A SUSPENSION, SAID SODIUM HALIDE BEING PRESENT IN AN AMOUNT NOT EXCEEDING THE STOICHIOMETRIC EQUIVALENT OF SODIUM IN SAID SODIUM FLUOALUMINATE AND BEING IN A MOLAR RATIO WITH THE ALUMINUM OF SAID ALUMINUM HALIDE SUBSTANTIALLY EQUAL TO SAID PREDETERMINED MOLAR RATIO. SAID HYDROFLUORIC ACID BEING PRESENT IN AN AMOUNT SELECTED SO THAT THE TOTAL QUANTITY OF FLUORIDE ION IN SAID SOLUTIONS IS SUBSTANTIALLY EQUAL TO THE STOICHIOMETRIC AMOUNT OF FLUORIDE IN THE SODIUM FLUOALUMINATE COMPOSITION OF PREDETERMINED SODIUM FLUORIDE/ALUMINUM TRIFLUORIDE MOLAR RATIO; AND CONDITIONING THE SUSPENSION BY CONTINUOUSLY STIRRING IT FOR A PERIOD OF AT LEAST ONE HOUR AT A TEMPERATURE ON THE ORDER OF 50*C.