CA2048520A1 - Method for working up fluorine-containing residues - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A method for working up fluorine-containing residues with recovery of aluminum fluoride according to which a mixture of finely particulate fluorine- and carbon-containing residues and SiO2 and SiO2-containing materials is heated in the presence of gaseous chlorine to temperatures between 400° and 900°C with about 98% of the fluoride content of the mixture being converted to gaseous silicon tetrafluoride, and, thereafter, the gaseous silicon tetrafluoride is converted either directly or after a transformation to hydrofluosilicic acid into aluminum fluoride by reaction with aluminum hydroxides or aluminum oxides.

(AZ.app.1)

Description

2 ~

M}~TI~OD.FOR WORRING UP FLUORINI~--CONTAINING R13SI~U13S

BAC~GROUND OF TE~ NTION
The present invention relates to a method for working up fluorine-containing residues, particularly from the area of the primary production of aluminum in electrolysis cells, with recovery of aluminum fluoride.
In the primary production of aluminum, fluorine-containing re~idues occur primarily in the consumed lining material of the electrolysis cells. The vat lining of an electrolysis cell for production of aluminum comprises e~sentially two regions. One region consists of carbon or graphite and is formed of a thick layer built up from blocks.
Thl~ layer serves as the cathode of the electrolysis cell. The lower half of the cathode region is provided with grooves in which current-carrying steel rails are embedded. In another region, below the carboD or cathode bottom, a refractory heat lnsulation is placed. Refractory material~, such a~ firebrick, lightwelght refractory brick, aluminum oxide, calcium silicate panels, etc., can be used as the refractory heat insulation.
The upper side walls of the electrolysis vat are lined predominantly with carbon or graphite panels; however, silicon carbide panel~ or other resistant, refractory material~ are al~o used in different combinations as lining material~.
The service life of such a linlng normally is 4 to 8 years. After that, the consumed lining material must usually 2 2 ~
be chipped from the electrolysi~ vat walls and di~posed of.
The electrolysi~ cell i~ then lined completely with new carbon and refractory materials.
During the period that the aluminum electroly~is cell is being operated, the components of the electrolyte melt penetrate into the carbon cathode bottom and partially also into the refractory in~ulation below. The main constituent~ of the molten electrolyte are cryolite ~Na3AlF3), sodium fluoride (NaF), aluminum fluoride (AlF3) and calcium fluoride (CaF2).
Aside from aluminum, sodium is also deposited into the cathode.
This sodium diffuse~ primarily into the carbon bottom and into the insulating materials. As a consequence of this sodium infiltration, the greater part of the consumed cell lining reacts as a strong alkali. In addition, metallic sodium in contact with carbon and nitrogen forms sodium cyanide, which is then encountered in small concentrations in the material chipped out from the bottom. Moreover, compounds of aluminum, such as aluminum carbide and aluminum nitride, are found in the dismantled carbon material from the cathode region. These compounde readily react with water or moisture to form aluminum hydroxide and methane or ammonia. The composition of the consumed lining materiale naturally fluctuates greatly. Some reference valuee for the composition of the carbon-rich material from the cathode zone are given below in ~ by weight:
C: 25 - 50 Ft 10 - 18 Alt 5 - 15 Na: 10 - 15 Ca: 1 - 2 Fe: 0.5 -Si: 0.2 - 2 CN: 0.1 - 0.3 AlN: 0.3 - 3 (difficult to determine) Al4C3, 0.2 - 2 ~difficult to determine) 3 2 ~ 2 ~
Directly below the carbon bottom there are layer~ of precipitated material of varying thickness which contain a large amount of cryolite. In the refractory insulation, the fluoride content decreaseH steeply from top to bottom. If firebrick or lightweight refractory brick was used for the heat insulation, an SiO2 content of about 50% by weight can be expected in the refractory region.
Various methods have been proposed for working up the consumed material chipped out of the cells in order to recover the fluorine content of the chipped material. An exsmple of a known method is leaching of the comminuted bottom material with a sodium hydroxide ~olution, as a result of which the fluoride is di~solved in the solution as NaF. Essentially only the consumed carbon material from the cathode region can be recycled economically according to thi~ method. Moreover, the fluorine content is leached out very incompletely; namely, only up to the extent of 60 to 80%. A sludge-like, carbon-contain-ing leaching residue, which i~ difficult to dispo~e of, remains behind. From the NaF-containing solution, only cryolite can be obtained by neutralization of the solution with hydrofluoric acid and addition of AlF3. However, in order to maintain the u~ual AlF3-rich melt bath compo~ition, the electrolysis plants requlre predominantly aluminum fluoride a~ a make-up salt.
When an insulation material containing SiO2 (fire, clay brick~, ln~ulating brlcks) and/or Al203 from the cathode region is ueed, the components are dissolved as sodium sillaate or ~odium aluminate. In 80 doing, considerable amount~ of NaOH are consumed u~ele~ly.
Pyrohydroly~i~ is a further known recycling method according to which the con~umed lining and the carbon portion from the cathode region react with water vapor at temperature~
above 1000C. AlF3 and NaF react with water vapor to form HF
and the metal oxides Al203 and Na20. The reaction between AlF3 and H20 proceeds relatively easily. On the other hand, the pyrohydrolytic reaction between NaF and H20 is succes~ful only 4 2 ~ 2 ~
at very high temperatures above 1100C and with a large exces~
of water vapor. The ~F concentration in the waste gas is, consequently, extremely low and mu~t be increa~ed by costly mean~, or HF mu~t be adsorbed on a large amount of aluminum oxide. Moreover, the HF-containing wa6te gas i~ contaminated by hydrofluosilicic acid, which is formed from HF and SiO2. It is a further disadvantage of this method that calcium fluoride is barely reacted and that the pyrohydrolysis residue is highly sintered becau~e of the high operating temperature. Re~idues with a higher SiO2 content, for example, from the rofractory region, cannot be recycled by this method, since the ~intering temperatures vary inversely with the SiO2 content, and the fluorine content accordingly remains high in the final residue.
A ~ilicon pyrohydroly~is method for recycling residues with a high SiO2 content is described in Au-A-16787/88, corresponding to ~P-A-0 294 300. According to this method, the consumed lining re~idueA are heated in the presence of an oxidizing gas and water vapor, optionally with the addition of SiO2, to temperatures clearly in excess of lOOO-C.
The fluorides are converted with water vapor into HF, and the metal oxides are converted into a silicate phase. At the noaeesarily high operating temperatures, the silicate phase is at lea~t partly liquld, which 1B pa~ty or molten. The fluoride~ are al BO dissolved in this pa~ty or molten pha~e, 80 that here aleo the fluorine aan be extracted only lnaompletely.
The final residue, therefore, alway~ has a relatively high re~idual fluorine content, and the disposal of this material would, therefore, appear to be risky, at the very least. As with pyrohydrolysis, a large exces~ of water vapor i~
necessary. The ga~eous ~F product ie, therefore, obtained in a correspondingly highly diluted form, BO that, before it react~
to form AlF3, it must be concentrated and separated from the ~ilicon that has been carried over.

-. , SUMMARY OF TEIE INVENTION
It is an object of the present invention to provide a method of recycling fluoride-contalning residues which permit~
at least 98% of the fluorine content of the fluorine-containing re~idue~, part ~ularly from the primary production of aluminum, to be removed at temperatures well below 100~-C.
A further object of the invention is to provide a method of working up fluorine-containing residues which produce a harmless final residue with a fluorine content of not more than 0.5% by weight, while providing a sLmple recovery of pure AlF3, for example, for return to the aluminum melt electrolysis.
The objects of tbe invention are achieved by providing a method of working up the fluorine-containing residues in which finely particulate fluorine-containing and carbon-containing residues are heated together with Si02 and Si02-containing materials in the presence of ga~eous chlorine to temperatures between 400-C and 900-C. As a result, the fluorides are converted to gaseous SiF4 and corresponding metal chlorldes. Carbon is necessary for the reduction and it can, at the ~ame time, be utilized a~ a ~30urce of heat by oxidation to CO or CO2. The SiF~ obtainet i~ transformed either direatly or after conversion to H2SiF6, into an aluminum fluoride. In this way, it is possible to produce a harmless final residue and, at the ~ame time, a pure AlF3 which can be returned, for example, directly to the aluminum melt electrolysis. The reaction~ which take place according to the inventive method, are described in greater detail below:

PRIFF D~SCRIPTION OF TH~ DRAWINGS
Fig. 1 ~chematically show~ an aluminum electrolysis cell.

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DBSCRIPTION OF T~E P~EF~RR~D ~MBODIM~NTS
Fig. 1 ~how~ an aluminum electrolysis cell where C i8 a carbon region with ~teel current-carrying rail~ R extending through the lower half thereof. F designates the refractory heat insulation region of the cell lining. ~~,-After the re~idue material ha6 been chipped off the lining, a mixture of the residue material with SiO2 or SiO2-containing materials i~ heated in the presence of gaseou~
chlorine.
The ~odium fluoride contained in the re~idues is con~erted as followss 4NaF + SiO2 ~ xC + 2Cl2 - 4NaCl + SiF~ + (2-x)CO2 ~ (2x-2)CO (1) Reaction (1) proceeds optimally at temperatures between 600-and 800-C.
The conversion of AlF3 to SiF~ proceeds particularly eaeily with AlCl3 being formed primarily by the following reaction:
4AlF3 + 3SiO2 ~ 6Cl2 + 3C - 3SiF4 + 4AlCl3 + 3CO2 (2) Advantageously, AlCl~ aleo acte ae a chlorinating agent and reaate further with SiO2 and NaF according to the following reaction:

4AlCl~ + 3SiO2 + 12NaF ~ 3SiF4 + 12NaCl + 2Al20~ ~3 Equations ~2) and (3) show that AlCl3, which is formed by reaction of Cl2 with AlF3 in the pre~ence of sio~ and C, is consumed by its reaction with SiO2 and NaF to form NaCl and SiF4. The net result of reactions (2) and (3) is that no chlorine i~ consumed for the conversion o AlF~ into SiF~ 80 that, in principle, the following reaction takes places 7 2~
4AlF3 + 3SiO2 - 3SiF4 + 2Al203 (4) Cl2 or AlCl3 i~ required only for transforming NaF
into SiF4 and NaCl, according to the reaction:

12NaF + 3SiO2 + 6Cl2 + 3C ~ 3SiF4 + 12NaCl + 3CO2 (4a) If the amount of available chlorine i8 limited, the formation of AlCl3 can be avoided. The same i~ also true for the conceivable carbochlorination of Al203 according to the reaction:

2Al203 ~ 6Cl2 + 3C - 4AlCl~ + 3CO2 (5) The carbochlorination of Al20, to AlCl3 i8 intensified as the temperature is increased.
As has already been mentioned, other aluminum compound~, ~uch a~ aluminum nitride and aluminum carbide, may also be contained in the consumed carbon material from region C. These compound~ are converted with chlorine to AlCl3, which is consumed again in accordance with Reaction (3).
Calaium fluoride is the most stable fluorine compound in the re~idue. The inventive method also permits fluorine to be extracted efficiently, as SiF~, from the CaF2 in the pre~ence o SiO2, Cl2 and Cs 2CaF2 + sio2 + 2Cl2 + 2C - SiF4 + 2CaCl2 + 2CO (6) The wa~te gas from the first step of the method, which contain~ SiF4, obtained as a result of heating fluorine and aarbon-containing residues, together with SiOz or sio2-containing materials, in the presence of gaseous chlorine to 400-900C, can be passed, optionally after an intermediate purification step, directly into the Al~OH)3 ~uspension to obtain aluminum fluoride as follows:

2~8~2~

3SiF4 + 4Al(OH)3 + x~320 -- 4AlF3 + 3Sio2.yH20 (7) It is furthermore po~sible to ab~orb the SiF4 from the waste gas ~tream into water.- When ab~orbed into water, the SiF4 decomposes into hydrofluo~ilicic acid and silica gel:

3SiF4 + 2H20 = 2H2SiF6 ~ SiO2.aq (8) From the resulting solution, the desired aluminum fluoride can be obtained by reaction with aluminum hydroxide employing known method~, which are de~cribed, for example, in a ~urvey by J.
Massone (Chemiker-Zeitung, February 1972, ~ol. 96, pages 65-76):

H2SiF6 + 2Al(OH)3 + aq -- 2AlF3 + 4H20 + SiO2.aq (9) The water-soluble AlF3, formed according to reaction ~7) or reaction (9), is separated from the precipitated ~ilica gel. It i~ crystallized as AlF3. 3H20 from the ~olution and then freed from water by heating. A pure aluminum fluoride is formed, which can be returned to the aluminum melt electroly-8i~. The ~ilica gel formed i8 a valuable material, which can be ~old, or example, as colloidal or highly disperse silica.
In accordance with the pre~ent invention, consumed cell linings from the aluminum electrolysi~ vat which contain ~iliaa, can be employed. In contrast to pyrohydroly~is, in which the pre~ence of SiO2 impede~ the deci~ive reactions, SiO2 is a neces~ary reactant for the formation of SiF4 in the present method. Thi6 means that not only the fluorine-lmpregnated carbon material from region C can be u~ed in the recovery process, but also the fluorine-impregnated, SiO2-rich refractory material, such as firebrick, from region F. Since the fluorine, which i~ bound as CaF2, can also be extracted effectively, even calcium fluoride-containing ~lags or ores can be processed with the consumed cell lining material and 9 2 ~ 2 ~
converted into aluminum fluoride. The method, furthermore, is suitable for the refining of fluoride-containing du~ts from the purification of water gases from aluminum electrolysis cells.
These dusts are heavily contaminated by carbon particles and extraneou~ metals and can, therefore, be returned only conditionally to the cell~. If, for example, the production of aluminum of the highest purity is required, such dust~ must be removed and depo~ited on special dumps.
The charging mixture of fluorine- and carbon-containing residues and SiO2 ~r SiO2-containing material~ must be 80 composed that the sio2 content corre~ponds at least to the quantity which is required for converting the whole of the fluorine content stoichiometrically to SiF~. In principle, any exce~ of sio2 content can be selected. The condltions with respect to the reaction kinetics are optimum when the sio2 is pre~ent in an amount from a 1.5-fold to a 2-fold ~toichiometric exces~. When the proportion of consumed lining material from region F in the charging mixture i~ high, it is entirely po~ible that a SiO2 exce~s of, for example, 5 times the ~toichiometric amount may be present. Because of the low reaction temperatures, there is no danger of melting or ~intering the charging mixture even in the case of ~uch SiO2 contente. The SiO2 content in the charging mixture can be adju~ted, for example, by the addition of SiO2 or an appropriate amount of cell material chipped out of region F.
The amount of ga~eous chlorine supplied mu~t correspond at lea~t to that required to convert the cations of the fluoride~ contained in the charging mixture, stoichiometri-cally, into the corre~ponding chloride~. ~owever, chlorine lo~ses with re~pect to the fluorine extraction may result, for example, from the presence of cyanides in the consumed cathode layer. Cyanides are present predominantly as NaCN and in small amounte as Ca(CN)2. These hazardou~ materials are rapidly and efficiently de~troyed by the present method, for example, in ~5 accordance with the following reactions ~ . , , , . - .

lO 20~85~
2NaCN + Cl2 = 2NaC1 ~ 2C + N2 (10) or, even more easily, under oxidizing conditions (for example, at the outlet end of the reactor) according to reaction (11):

2NaCN + 2 ~ Cl2 = 2NaCl + 2C0 + N2 (11) A further increa~ed consumption of chlorine occur~, for example, owing to the fact that Na, which is bound as sodium carbonate, i~ converted under reducing conditions to NaCl. To compensate for the aforementioned chlorine lo~e~, it has proven to be advantageous to exceed the amount of chlorine required for the ~toichiometric extraction of fluorine by up to 5%. A further increase in the amount of chlorine supplied doe~
IS not lead to a noticeable improvement in yield with re~pect to the fluorine extraction.
The carbon content of the charging mixture i~
advantageou~ly adjusted 80 that the combu~tion of the carbon, which i~ not used for reduction, suffices to cover all the heat required by the proce~s.
The temperature range which is to be adhered to for the reaction~ lie~ between about 400- and 900-C. Below 400C, the reaction~ proceed incompletely at best. Above 900C, the danger exl~t~ that there will be sintering proce~es in the reaction mixture, a~ a re~ult of which the fluorine extraction is impeded or ~uppre~ed altogether. In order to ensure that the fluorine i~ extracted a~ completely as po~ible, the operating temperature ~hould be eelected ~o that the reaction mixture pa~e~ through the fir~t or heating step of the J
propo~ed method as long as po~ible in a solid form.
Fxperimentally, an optimum yield of SiF4 was obtained at temperature~ ranging from 600 to 800C.
Ga~e~ and solid~ are advantageou~ly conducted in oppo~ite direction~. The reaction~ then take place over the entire temperature range between the charging temperature at 11 2~ 2~
the inlet end and the temperature of the re~idue at the outlet end of the reactor. Air for combustion and chlorine are supplied at the outlet end, where the temperature~ could be of the order of 900~C. The reaction gases that are produced in the procese leave the reactor with a temperature of 100 to 300-C.
It is an advantage of this procedure that the AlCl3, which is temporarily formed in the hot zone, i8 decomposed again countercurrently to the reaction of the ~olid mixture and in accordance with Reaction ~3). This step of the inventive method is advantageou~ly carried out in a rotary kiln or in a multi-deck furnace.
Under the process conditions described, it is po~sible to remove at leaet 98~ of the fluorine content from the re~idues used at temperature~ clearly below lOOO-C. The harmless final residue has only an extremely low residual fluorine content of less than 0.3% by weight. Fluorine contents of about 0.1% by weight were achieved experimentally.
If necessary, water-soluble component~ of the final residue, euch as eodium chloride and calcium chloride, can be leaahed out. After that, the reeidue consists es~entiall~ of aluminum and eilicon oxidee. Slight amount~ of accompanying oxides, euch ae iron oxide and titanium oxide, are aleo present. Thie reeldue can be dumped abeolutely eafely. Pure AlF3, which can be returned directly to the aluminum melt electroly6is, ie obtained in a s~mple manner from the SiF4 produced upon heating o the charging mixture. Colloidal or highly dieperse eilica ie produced as a further valuable material.

Claims (9)

1. A method for working up fluorine-containing residues with recovery of aluminum fluoride comprising the steps of:
a. preparing a charging mixture comprising:
(1) particulate fluorine- and carbon-containing residues; and (2) at least one of SiO2 and SiO2-containing materials in amounts at least sufficient to stoichiometri-cally convert all of the fluorine to SiF4;
b. heating the charging mixture, in the presence of gaseous chlorine, to a temperature between 400°C
and 900°C, to react the fluorine in said charging mixture, whereby about 98% of the fluoride content of the charging mixture is converted to gaseous SiF4; and e. converting the gaseous SiF4 into aluminum fluoride by reaction with a member selected from the group consisting of aluminum hydroxide and aluminum oxide.

2. The method of claim 1 wherein, prior to reaction of the SiF4 to form aluminum fluoride, the SiF4 is converted to H2SiF6 by absorption of the SiF4 into water.

3. The method of claim 1 wherein the heating in the presence of chloride is conducted at a temperature between 600°
and 800°C.

4. The method of claim 1, wherein the charging mixture and gaseous chlorine are flowed countercurrently during reaction.

5. The method of claim 1 wherein the heating is carried out in one of a rotary kiln and a double-deck furnace.

6. The method of claim 1 wherein the gaseous chlorine is supplied in an amount which corresponds to 1 to 1.5 times the stoichiometrically-required amount for tying up cations of the fluorides contained in the charging mixture.

7. The method of claim 1 wherein the member selected from the group consisting of SiO2 and SiO2-containing materials is added in such an amount that the total content of SiO2 in the charging mixture corresponds to 1 to 5 times the stoichiometrically-required amount for conversion of the total fluorine content to SiF4.

8. The method of claim 1 wherein the fluorine- and carbon-containing residues are at least one member selected from the group consisting of consumed lining materials from aluminum electrolysis furnaces, dusts from the waste gas purification systems of aluminum electrolysis furnaces, fluorine-containing slags and ores.

9. The method of claim 1 wherein the residues remain solid during heating.