DE2001675C3 - Method of reducing the burn loss of synthetic cryolite - Google Patents

Description

Synthetic cryolite generally contains bound water or impurities such as compounds formed by the substitution of some of the fluorine atoms of the cryolite (Na ₃ AlF ₆ ) with hydroxyl groups, ie [Na ₃ Al (F-OH ₆ ] or [Na ₃ Al (F-OH) ₆ XH ₂ O], or also contains aluminum hydroxide as an impurity depending on the manufacturing method, therefore, synthetic cryolite has a disadvantage that its burning loss is larger than that of natural cryolite when such synthetic cryolite is used in an aluminum electrolytic reducton cell is, the hydroxyl group or the bound water contained in the synthetic cryolite reacts with the cryolite, and fluorine is converted into hydrogen fluoride, which is volatilized and escaped, which means not only a loss of fluorine but also an air pollution problem Above disadvantages that occur when synthetic cryolite is used to reduce alum inium is used, the method of firing the synthetic cryolite at a temperature of 350 to 700 ^° C. after drying was used in all previously known cases. However, the above-mentioned firing of the synthetic cryolite at an elevated temperature after drying the cryolite is problematic and a special step is required. In addition, hydrogen fluoride volatilizes during this operation.

As far as is known, there is still no other method of reducing the burning loss of synthetic Besides calcination, cryolite has become known.

The German patent specification 453670 relates to a process for the extraction of iron from cryolite, a fluoroaluminium-fluoroalkali double compound which contains iron in large quantities. The cryolite is stirred, optionally with heating and optionally with the addition of mineral acid in the presence of an aluminum salt (AlCl ₃ solution), whereby the Fe-containing contamination is converted into FeCl _{3 with AlCl 3} , so that the iron passes into the mother's eye and can be washed out.

German patent 355303 relates to a method for separating aluminum compounds from other substances, especially iron compounds that consists of cryolite by adding alkali fluorides while avoiding an excess « of fluoride to a solution of aluminum sulfate containing iron in the presence of excess sodium salts fails and the separated double scrap washes with water or highly diluted acid for so long until it is practically free of foreign metal by observing the deposition conditions of cryolite an essentially pure product. The addition of acid to the wash water is an optional measure and in no way stated as strictly necessary.

The invention relates to a process for reducing the burning loss of synthetic cryolite which has a burning loss exceeding 1% by weight, which is characterized in that it is placed in an aqueous solution of hydrochloric acid ^{at 50 to 95 ° C. for 30 minutes to 2 hours} , Nitric acid or sulfuric acid with a pH below 4.

It has been found that the burn loss of synthetic cryolite can be substantially reduced can without subjecting the synthetic cryolite to the above calcination step.

An object of the present invention is to provide a method for reducing the burning loss of synthetic Create cryolite through a simple operation without the calcination of the synthetic Cryolite. Another aim is to find a method for improving the quality of synthetic To create cryolite which makes it possible to reduce the burning loss of synthetic cryolite and essentially removing the contaminants contained in the synthetic cryolite.

The synthetic cryolite used in the present invention to improve its quality includes all those products made from known raw materials by known manufacturing processes can be won. The synthetic cryolite products obtained by known methods differ in their properties depending on the raw materials and the manufacturing process, but they all contain impurities of various kinds.

An example of a synthetic cryolite composition is as follows:

Components (Al ₂ O ₃ ), at most Weight% SiO ₂ , at most 91 Cryolite (Na ₃ AlF ₆ ), at least CaF ₂ 48-52 Fluorine (F) Fe ₂ O ₃ 31 - 34 Sodium (Na) SO ₄ 13-15 Aluminum (Al) P ₂ O ₅ 6.0 Aluminum oxide 0.9 0.04-0.06 0.10 0.1-3 0.005-0.3 0.05-0.15 humidity 1 - 5 Burning loss *)

*) Loss in% by weight as a result of calcination at 800 ° C. for 1 hour. (This definition of the term "Burning loss" is applicable in the following.)

According to the present invention, the synthetic cryolites described above are subjected to slurry treatment in an aqueous solution of an acid such as hydrochloric acid, nitric acid and sulfuric acid. As used in the present specification and claims, the term "slurry" means an operation which consists in converting the synthetic cryolite into a slurry in an aqueous acidic solution with stirring. It is important that the acid concentration in the aqueous acidic solution, in terms of pH, be kept below 4 in reducing the burning loss of synthetic cryolite. The preferred pH for the aqueous acidic solution is in the range of 1 to 2. It is effective to use a small amount, ie not more than 5% by weight, of a sodium salt of either hydrochloric acid, nitric acid or sulfuric acid (ie NaCl, NaNO ₃ or Na ₂ SO ₄ ) to the aqueous acidic solution used as the slurry medium in the present invention. This addition makes it possible to further reduce the burning loss of the synthetic cryolite. It is preferred that the sodium salts be added in amounts of 0.5 to 3% by weight.

In order to achieve the objects of the present invention, it is generally preferred to use the slurry medium in an amount of 1 to 20 parts by weight and preferably 5 to 10 parts by weight to use synthetic cryolite per part by weight.

The temperature in the slurry treatment of the present invention is in the range of 50 to 95 ° C. Normal atmospheric pressure is used with good results to produce the invention To carry out slurry treatment. The time of the slurry treatment may vary depending vary on the pH of the slurry medium and the temperature. It is in the range of 30 minutes to 2 hours.

The slurry treatment is carried out by removing the slurry from synthetic cryolite under the conditions given above, stir using a conventional vessel that comes with is provided with a stirrer. To ensure even contact between the synthetic cryolite and the To secure the slurry medium and to make the slurry effective, the slurry concentration should be used below 50% by weight, and preferably in a range from 5 to

Table I.

30% by weight. The synthetic cryolite can be added to the slurry medium in its original state State to be added, or it can be added after becoming particles of one size has been ground to less than 50 microns.

The slurry treatment can be in a single step or in two or more steps and it can be carried out either continuously or intermittently. For example can

ίο the slurry treatment can be carried out in two steps, wherein in the first stage the synthetic cryolite is added to the aqueous acidic solution and the slurry is carried out, after which the slurry is allowed to stand to separate the supernatant liquid, and then the separated slurry in the second stage, if desired, slurried after a further addition of an aqueous acidic solution. The slurry treatment can be carried out by contacting the synthetic cryolite mil of the aqueous acidic solution either in counter-current or with the current.

In the present invention, the synthetic cryolite used in the slurry treatment such as described above, separated from the slurry medium by a known one Method of separating the solids from the liquid, such as by filtration, centrifugation or Decanting. The synthetic cryolite thus separated is washed with water if necessary. He can then put to its final use after having it to remove the adherent Moisture has merely dried at a temperature of about 100 to 110 ° C.

The following examples are intended to explain the present invention further without, however, restricting it.

example 1

Synthetic cryolite was slurried with ten times the amount of water on a solid basis and then adjusted to the pH shown in Table 1 with hydrochloric acid. Slurrying at different temperatures was performed for 2 hours, after which the treated cryolite separated, washed (21 kg of water per cryolite) and at 110 ⁰ C was dried. The analysis of the treated cryolite and the untreated cryolite is given in Table I.

sample Aufschläm- Analvsenwerle Al of the cryolite (dry basis), weight% SO ₄ SiO ₂ TiO ₂ V ₂ O ₅ Remarks - motivation conditionscn 0.025 1.13 CaF ₂ 4.29%. As he pH Tem- Na FP ₂ O ₅ Fe ₂ O ₃ Brenn result of an X-ray pera- 12.85 - _ loss powder diffraction tur ⁰ C (at analysis was 12.51 - 0.34 0.0013 - 800 ⁰ C) CaF ₂ , Na ₅ Al ₃ F ₁₄ Theoretical 32.87 54.30 - found values Natural product 29.17 53.13 0.004

continuation

Sample Slurry Analysis Values of Cryolite (Dry Basis), Weight%

conditions

pH Tem-Na Al F temperature ° C

Remarks

P ₂ O ₅ SO ₄ SiO ₂ TiO ₂ V ₂ O ₅ Fe ₂ O ₃ Burning loss
(at
800 ⁰ C)

Test sample (before
Slurry)

H ₂ O treat- 90

lung *)

After opening 3-4 9 nights
slurry

0.027 - 0.8] -

0.019 -

the same

1 - 2 90 -

0.13 0.19 -

31.70 12.16 51.65 0.027 0.80 0.82 0.0010 0.0008 0.06 2.90 As a result of a

Bound water was found by infrared spectral analysis

2.88 No change compared to pre-slurry product

1.70 As a result of infrared spectrum analysis, it was bound Water found

0.91 Compared with natural cryolite, no change was found by infrared spectral analysis, DTA and TGA. As a result of X-ray diffraction analysis, only Na ₅ AlF _{6 was} found

1-2 50 32.64 12.82 53.18 0.005 0.14 0.22 0.0008 under- 0.005 0.87 same.

half

0.0001 0 50 - - - 0.003 0.13 0.05 0.0002 under- trace 0.95 The X-ray powder cr-

semi diffraction analysis

0.0001 showed a very

small amount of Na ₅ AIF ₁₄ . The other results were as with natural. Cryolite

*) The procedures for the treatment with H ₂ O were similar to those for the slurry with HCl

the same

the same

From the results, as reported in Table I, it can be seen that the burn loss synthetic cryolite could not only be reduced to an area that of natural Cryolite corresponds, but that also the contained impurities effectively through the slurry treatment according to the invention could be eliminated.

Example 2

The slurry treatment was carried out as in Example 1, but using nitric acid instead of hydrochloric acid when adjusting the pH to 1 to 2. The results are anacaeben in Table II.

Table II

Sample based on analysis values of the cryolite (dry basis), weight% remarks

slurry temperature, ⁰ C

Na Al FP ₂ O ₅ SO ₄ SiO ₂ TiO ₂ V ₂ O ₅ Fe ₂ O ₃ Brenn

loss (at
800 ⁰ C)

Before slurry

Sample A Sample B

31.70 12.16 51.65 0.027 0.80 0.82 0.0010 0.0008 0.06 2.90
31.25 12.87 51.17 0.110 0.24 0.93 - 3.14

After slurry

Sample A

Sample A Sample B

32.51 12.85 53.20 0.004 0.13 0.21 0.0007 below
0.0001

70 70

0.003 0.13 0.19 0.0007

32.58 12.75 52.98 0.008 0.06 0.24 cont.
0.0001

In the case of X-ray powder diffraction

0.006 0.98 analysis only found Na ₃ AlF ₆ . The results. d. DTA, TGA and infrared spectral analysis were like b. Naturally. Cryolite

0.006 0.91 the same

0.89 the same

Example 3

Synthetic cryolite was slurried with ten times the amount of water based on the solids. After adjusting the pH of the slurry to 1 to 2 with either nitric or hydrochloric acid, a sodium salt of each acid, ie a small amount of either NaNO ₃ or NaCl (3% by weight based on the cryolite), became the corresponding cryolite slurries admitted. The slurry treatment was then carried out at 50 ^° C. as described in Example 1. The results are given in Table III.

Table III

Analysis values of the cryolite (dry basis), weight% Na NH ₃ Al FP ₂ O ₅ SO ₄

Remarks

Burning loss (at
800 ⁰ C)

Test sample (before slurry)

30.95 0.78 12.41 51.62 0.025 0.84 4.48

After slurry

ENT treatment 32.68 12.56 53.12 0.003 0.14 0.87

HCl treatment 32.56 0 12.71 53.15 0.003 0.13 0.89

Infrared spectral analysis showed NH ₄ ⁺ . X-ray powder diffraction analysis showed a very small amount of (NH ₄ ) _{2 NaAlF 6}

X-ray powder diffraction analysis _{showed only Na 3 AlF 6} . The results of DTA, TGA and infrared spectral analysis were the same as those of natural cryolite

also 709 611/103

ίο

As stated in the previous examples, 1% by weight or less can be achieved by a simple

according to the present invention, the burn loss operation, which consists in that one the syn-

(at 800 ° C) from synthetic cryolite to a synthetic cryolite of a slurry treatment

be richly reduced, which subjects comparable or equal in an aqueous acidic solution,
that of natural cryolite, that is to say about 5

Claims (4)

Patent claims: 1. Method of reducing the burning loss of synthetic cryolite, which is 1% by weight excess burning loss, characterized in that it is heated to 50 to 95 ° C for 30 minutes to 2 hours in an aqueous solution of hydrochloric acid, nitric acid or sulfuric acid with a pH of slurries below 4. ι ο 2. The method according to claim 1, characterized in that the aqueous solution up to 5% by weight a sodium salt such as sodium chloride, sodium nitrate and sodium sulfate can be added. 3. The method according to claim 1, characterized in that a pH value of the aqueous acidic Solution in the range from 1 to 2 is observed. 4. The method according to claim 1, characterized in that the aqueous acidic solution in one Amount of 1 to 20 parts based on 1 part of the synthetic cryolite. is used.