FR2587021A1 - PROCESS FOR RECOVERING SODIUM ALUMINATE CONTAINED IN RED SLUDGE - Google Patents

Abstract

PROCESS FOR RECOVERING IN THE FORM OF CRYOLITHE AND CHIOLITHE, BY ATTACK WITH HYDROFLUORIC ACID OF THE SODIUM ALUMINATE CONTAINED IN THE LIQUEUR OF IMPREGNATION OF THE RED SLUDGE BEFORE DISCHARGE, RESULTING FROM THE ATTACK OF THE BAUXITE PROCESS ACCORDING TO BAYON. QUANTITATIVE PRECIPITATION BY HYDROFLUORIC ACID OF ALUMINA IN SOLUTION, EVEN AT VERY LOW CONCENTRATION, IS POSSIBLE IN THE FORM OF CRYOLITH ACCORDING TO CERTAIN OPERATING CONDITIONS WHICH ARE STUDIED.

Description

PROCESS FOR RECOVERING SODIUM ALUMINATE CONTAINED IN

RED SLUDES

  TBOMNICAL DOMAIN The invention relates to a method for recovering sodium aluminate

  present in the impregnation liquor of red sludge from the Bayer process.

  BXPOeEk43U, PROB [LiB It is well known in general that red sludge, the insoluble residue of bauxite attack by the Bayer process, is washed by resuspension in water, in order to extract the impregnating liquor constituted by concentrated sodium aluminate solution. The sludge after washing is rejected while the wash water enriched with impregnating liquor is reintroduced into the Bayer circuit, generally at the dilution stage of the

  sodium aluminate liquor from the attack of bauxite.

  The effectiveness of the washing of red sludge, often carried out by decanting washing against the current, depends in particular on the number of washes as well as the volumes of water involved. To the extent that one wants to limit the loss of sodium aluminate in the sludge before discharge, large volumes of wash water should therefore be used, which can not be recycled directly to the Bayer circuit, even as dilution liquor after attack, without prior concentration . This concentration is carried out by evaporation, a large energy-consuming operation which can in this case also lead to a prolonged immobilization of certain installations and consequently limit the overall capacity of the alumina production line. For the skilled person, it is therefore necessary to find the best compromise, in economic terms, between a gain in material yield and operating surcharges.

that it generates in return.

  With a view to improving the recovery of alumina manufacturing waste according to the Bayer process, the applicant has studied a process for preparing cryolite and chiolite from sodium aluminate solutions to be diluted.

  originating from washing waters with red mud prior to discharge.

ART ANTMRIUR

  It is well known to produce double fluorides of sodium and aluminum in the form of cryolite, AlF33NaF, and chiolite, AIF3, 5/3 NaF by the action of hydrofluoric acid on sodium aluminate solutions whose concentration in alumina is at least 100 grams per liter. These solutions are generally obtained by redissolution of alumina with trihydrate, gibbsite or hydrargillite, in a concentrated sodium liquor and at a temperature close to 100C. This process is costly in terms of the nature of the raw materials used and the energy consumption, which is why several methods of producing synthetic cryolite advocate the use of materials.

  raw materials such as fluorine and hydrofluoric acid.

  Thus, French Pat. No. 1,202,322 describes a method for recovering fluorine and hydrofluoric acid vapors in residual gases, in particular from aluminum production electrolysis cells. The entrapment of the fluorine is carried out by washing the residual gases with water. The absorbed fluorine forms a dilute solution of hydrofluoric acid which is recycled as washing water and progressively enriches to a weight content of HF of 3 to 10%. The diluted hydrofluoric acid thus recovered is then brought into hot contact, either directly with a concentrated sodium aluminate solution to directly precipitate a cryolite or in a first step with a suspension of aluminum trihydroxide in water to forming a soluble aluminum trifluoride in the hydrofluoric acid solution which is in a second step neutralized with a concentrated alkali liquor to precipitate the cryolite. Cryolite precipitation yields reach 90% and can be improved by addition of solid or highly concentrated sodium chloride to decrease the solubility of sodium double fluorides and

  of aluminum (French patent 2,103,484.

  A similar reaction process is described in French Pat. No. 2,075,063, which is distinguished by the fact that the trapping of fluorine and hydrofluoric acid vapors in the residual gases is carried out by the dry route, by the implementation of

  direct contact of the gases with sodium aluminate granules.

  These processes have in common to use as raw materials for combination with fluorine or hydrofluoric acid recovery, compounds in the solid state or in concentrated solution taken from the group of aluminum aluminate.

  sodium, sodium hydroxide and sodium carbonate, aluminum trihydroxide.

  Under these conditions, the economic interest of such processes is linked to the sole possibility of recovering fluorine, which therefore implies being on a site

  production or use of fluorinated products.

OBJECT OF THE INVENTION

  The process according to the invention consists in carrying out the precipitation of cryolite and of chiolite from a very dilute sodium aluminate solution, the alumina concentration of which can be less than 1 g / liter with a concentration ratio R.sub.p. - Al 2 O 3 / Na 2 O between 0.35 and 0.6 in which the respective concentrations of Al 2 O 3 in solution and Na 2 O in solution in the free or combined state are expressed in grams per liter. Said dilute sodium aluminate solution is contacted with hydrofluoric acid to precipitate the cryolite and the chiolite at controlled pH between 3.5 and 9.5. Under these conditions, the precipitation of aluminum (in the alumina state) in cryolite form becomes quantitative and can reach a yield of 94%, the loss corresponding substantially to the solubility of the cryolite under the conditions of the experimentation. After separation of the solid and liquid phases by decantation or centrifugation, the solid phase consisting of cryolite and a little chiolite in the measure where Rp is between 0.54 and 0.6, is dried then

  calcined by progressively increasing the temperature from 100 ° to 500 ° C.

  During her research, the Applicant first made many samples of red sludge from an industrial chain of alumina production, just before their rejection. This red mud leaving the last counter-current scrubber was filtered cold to extract the impregnation water from the last wash. In the filtrate, the alumina content varied within a concentration range of between 3 and 6 g / liter whereas the Rp-Al 2 O 3 g / l / Na 2 O 'remained between 0.45 and 0.6 and this for

  standard washing conditions in the industrial chain.

  For the skilled person, a first observation is the significant reduction in the Rp of the Bayer liquor, which after attack, dilution and decantation, is between 1 and 1.2 and drops to about 0.5 in the impregnation zones. sludge from the same liquor but containing a sodium aluminate strongly diluted by successive washings. This reduction of Rp is undoubtedly due to a retrogradation of a portion of the alumina in solution in the form of alumina trihydrate precipitated in red sludge as a result of the pH change caused by the successive dilutions. The fact of keeping a ratio of concentration Rp always close to 0.5 is interesting to precipitate directly by simply adding hydrofluoric acid cryolite since the theoretical ratio AI / Na for this compound is 0.39 and 0.54 expressed under the

Rp - Al 2 O 3 / Na 2 O form.

  In the present case, the fact of treating with HF solutions containing only 3 g / liter of alumina could constitute an impossibility to the quantitative precipitation of the cryolite. In a new series of tests still carried out from samples taken on the industrial chain, the applicant has endeavored to determine under which conditions it is possible to precipitate quantitatively in the form of cryolite and chiolite

  alumina in solution, at concentrations below 10 g / liter.

  Thus, after filtration of a suspension of red sludge after washing, the liquor constituting the filtrate had a concentration of 3.6 g / liter of Al 2 O 3 and 7.3 g / l of Na 2 O, thus corresponding to a R 2 0.49. On different fractions of 1 liter of said liquor, heated between 80 and 90 C and agitated, additions of hydrofluoric acid in varying proportions to vary the pH, allowed to obtain different precipitates which after filtration and drying at 100-C have been analyzed. X-ray examination showed that it was in all

  case of cryolite AIF3 3NaF well crystallized.

  The table below shows the influence of pH on the performance of

  precipitation of alumina in the form of cryolite.

Table I

  At pH> 8, a large proportion of alumina remains in solution.

  At pH <5, the solubility of cryolite in acidic medium increases

  precipitation yield decreases.

  At <pH <7, the precipitation of Al 2 O 3 as a cryolite is almost quantitative, being only limited by the solubility of the cryolite in an aqueous medium, ie 0.58 g / l representing 0.25 g / l of alumina and loss of

  precipitation yield of nearly 7%.

  The transformation yields of Na 2 O and HF into cryolite are a little lower than that of alumina at pH 5-6 because of the slight overconsumption of

  HF to neutralize the small excess of uncombined free soda.

  Similarly, the acceptable lower limit of alumina concentration in red sludge wash water was investigated within the most favorable pH range, ie between pH 5 and 7 to obtain a

  minimum yield of cryolite precipitation of 90%.

  By successive dilutions of a liquor from the filtration of red sludge and whose initial concentration of alumina was 5.8 g / liter and Rp = 0.53, it was precipitated hot by HF at a pH of about 6%. cryolite with the following results: Tests on 1-liter fractions solution A1203-3.6 g / l Rp -0.49 HFg pH Rendt% cryolite Rendt% Na2O Rendt% HF

6.38 9.5 65 58 86

7.66 8.3 89 81 98

8.13 7 89 79 92

9.3 5.5 94 84 85

, 4 3.5 91 82 75

, 4- ---

and the

Table 2

  The transformation yield of alumina into insoluble cryolite significantly decreases at concentrations of less than 3 g / liter of Al 2 O 3 to tend towards O when one approaches, for very low concentrations, the solubility limit of the cryolite in the conditions of the experiment. As an indication, this solubility at 20 ° C is 0.60 g / liter for AIF3 3NaF equivalent

at 0.25 g / liter of Al 2 O 3.

  Finally, note that the hydrofluoric acid used for these. tests was an industrial solution of HF at 40% by weight. It is quite possible to use more dilute hydrofluoric acid solutions, especially from recovery, but the weight content of HF should be not less than 5% in order not to create an additional dilution effect which can reduce the precipitation yield when the AI203 concentration does not exceed 3

  g / liter in the dilute solution of sodium aluminate.

  A final series of cold cryolite precipitation tests were carried out in order to overcome the costly reheat operation at 80/90 ° C of the diluted sodium aluminate solution upon addition of hydrofluoric acid

  to accelerate the precipitation of cryolite and facilitate its filtration.

  From the same liquor (Al 2 O 3 - 5.8 g / l; Rp - 0.53) added with 40 wt% hydrofluoric acid to pH = 6, a precipitating cryolite was precipitated without heating. slowly. By centrifugation the clear juices could be easily separated from the solid phase constituted by a sludge containing 42% of cryolite which was dried at 100 ° C. and then calcined. Concentration yield A1203 g / liter Yield% cryolite

, 8 94

3.6 94

0.72 75

0.48 51

0.36 9

  transformation of alumina into insoluble cryolite was as before very

close to 94%.

  From these different tests, it has therefore proved possible to precipitate cold and quantitatively in the form of cryolite, by addition of hydrofluoric acid at controlled pH (5-7), the sodium aluminate contained in the water of impregnation of the red sludge before rejection and that at concentrations in

  alumina less than 10 g / liter and may even reach less than 3 g / liter.

  The description in successive stages of the recovery process according to

  the invention, sodium aluminate contained in the red sludge before discharge,

  will be better understood from a general diagram represented by FIG.

  According to this figure, the sludge suspension L1 from the last scrubber

  a Bayer circuit is filtered at station A. The solid phase S1 constituted by the.

  red sludge cake containing 20 to 30% moisture and discarded while the liquid phase L2 after having undergone safety filtration at station B to remove the fine solid particles 82, gives a clear liquor L3 which is then neutralized at controlled pH between 5 and 6 by a solution of industrial hydrofluoric acid L4 to precipitate cryolite C. The resulting suspension L5 is then centrifuged at station D to separate the liquid phase L6 which is rejected, the solid phase S3 constituted by the wet cake

  cryolite which is dried and calcined at the station E to give cryolite S4.

  These conditions of implementation have been verified in the following example of industrial application

EXAMPLE OF APPLICATION

  117.5 m3 of red sludge taken at the outlet of the last washer of an industrial cat, and containing 450 g / liter of dry matter in suspension were filtered. While the wet cake representing about 52.9 tonnes of solids was discarded, the filtrate after a second juice clarification filtration represented a volume of 72.5 m3 of a dilute 3.6% sodium aluminate solution. g / liter of Al 2 O 3 and 7.3 g / liter of Na 2 O, which is an Rp of 0.49. The quantity of alumina in solution was therefore 261 kg representing 138.17 kg of aluminum which was precipitated in the form of a cryolite at room temperature and at a pH controlled between 5 and 6 by slow addition of 1.46 m 3 of solution. HF & 40% by weight, ie 1686 kg of solution containing

  674.4 kg of HF (equivalent to 640.7 kg of fluorine).

  The precipitation reaction was carried out by continuous mixing of the metered components arriving at the base of a first stirred reactor communicating by overflow with a second stirred reactor where the pH control was carried out and possibly the adjustment of the flow rate. hydrofluoric acid in the

head reactor.

  At the outlet, always by overflow of the second reactor, the suspension was sent to a storage tank of large capacity, slightly agitated and to feed a centrifuge. Indeed, because of the difficulties of separation of the solid phase of cryolite, by simple decantation, the suspension

  was centrifuged and a clear juice filtrate of about 72 m3 containing A1203 -

  0.25 g / l ,. Na2O - 1.17 g / l and HF - 1.40 g / l was removed. The insoluble cake containing more than 50% moisture was dried at 100 ° C and then calcined at 500 ° C. The weight of cryolite thus recovered was 1002 kg. Examination, by diffraction

  X-rays, showed that it was well crystallized cryolite.

  Material balance 1. Conversion efficiency A1203 into cryolite theoretical weight of cryolite AIF3 3NaF precipitated From 138.17 kg

of aluminum: 1074.65 kg.

  Actual weight obtained: 1002 kg - Loss 72.65 kg.

Yield: 93.2%.

  This result confirms the laboratory tests, and is corroborated by the controlled loss from the discharges 72000 liters x 0.25 g - 18 kg of alumina

representing 74 kg of cryolite.

  2. Yield of transformation of Na 2 O into cryolite Amount of Na 2 O put into play: 72500 x 7.3 - 529.25 kg representing 392.67 kg of Na in solution. The quantity theoretically

  combined in 1074.65 kg of cryolite is 353.13 kg or 93%.

  The actual amount combined in 1002 kg of cryolite is 329.25 kg or 83.8%. The loss of 392.67 - 329.25 - 63.42 kg of Na representing 85.47 Na 2 O is well transformed by the control of the losses in the discharges ie 72000 x

1.17 - 84.24 kg of Na2O.

  3. Yield of transformation of HF into cryolite The actual amount of HF involved is 674.4 kg or 640.7 of fluorine. The amount of fluorine actually combined in 1002 kg of cryolite is

543.94 kg.

The yield is therefore 84.9%.

  The excess fluorine of 640.7 - 543.94 - 96.76 kg or 101.85 kg of HF corresponds to

  well there also to the quantity lost in the discharges namely 72000 x 1.40 -

, 8 kg of HF.

the

Claims (8)

  1. Process for recovering in the form of cryolite and chiolite, by etching hydrofluoric acid of the sodium aluminate contained in the impregnation liquor of the red mud before rejection, resulting from the bauxite attack according to the process Bayer, characterized by the following steps: a) The red mud impregnating liquor containing sodium aluminate at a minimum concentration of 0.7 g / l of alumina is removed by known separation means and with a ratio of of concentration Rp - A1203   soluble / Na2O between 0.35 and 0.6.   b) said liquor is filtered to remove fine particles in suspension and clarify the juices.   c) is introduced into the filtered liquor at a temperature below 100-C and stirring a fluorbydric acid solution until a pH is reached stabilized between 3.5 and 9.5.   d) The solid phase consisting of cryolite and a little chiolite is then separated by known means when the Rp of the sludge impregnation liquor   red is between 0.54 and 0.6, the liquid phase that is rejected.   e) drying at 100-C and then calcining at a temperature <500 * C to constant weight said solid phase consisting of cryolite and optionally chiolite.   2. Method according to claim 1, characterized in that the minimum concentration of alumina in the diluted sodium aluminate liquor, after   separation of red mud, is> 3 g / liter.   3. Method according to claims 1 and 2 characterized in that the ratio Rp of   concentrations of Al 2 O 3 in g / liter and Na 2 O in g / liter in the diluted liquor   sodium aluminate is between 0.45 and 0.6.   4. Process according to Claims 1 to 3, characterized in that the temperature of   contacting the diluted sodium aluminate liquor with the solution   of hydrofluoric acid is less than 40-C. l1   5. Process according to Claims 1 to 4, characterized in that the pH is maintained   between 5 and 7 when the diluted aluminate liquor is brought into contact with   sodium with the hydrofluoric acid solution.   6. Process according to claims 1 and 5, characterized in that the acid solution   hydrofluoric fluid used to precipitate cryolite and adjust pH to   desired is a solution whose HF content is at least 5%.   7. Process according to claim 1, characterized in that the separation of the phase   Cryolite solid suspension is carried out by centrifugation.   8. Process according to claim 1, characterized in that the separation of red sludge from their impregnation liquor consisting of dilute sodium aluminate is,   performed by filtration or centrifugation.