US3106448A

US3106448A - Recovery of cryolite

Info

Publication number: US3106448A
Application number: US100268A
Authority: US
Inventors: Whicher Charles Harry; Nickle Alexander Gordon; Dolega-Kowalewski An Krzysztof
Original assignee: Aluminium Laboratories Ltd
Current assignee: Alcan Research and Development Ltd
Priority date: 1961-04-03
Filing date: 1961-04-03
Publication date: 1963-10-08
Anticipated expiration: 1980-10-08
Also published as: GB981243A; ES276075A1; SE310658B; CH409417A; DE1207362B

Description

Oct. 8, 1963 Filed April :5, 1961 C. H. WHICHER ETAL RECOVERY OF CRYOLITE 2 Sheets-Sheet 1 ALEXANDER G. NICKLE To ALL M/u ,4A/ m BY EJ o GA- owALEwsKi 5 0mm? ANDRZ K u LE K Oct. 8, 1963 c. H. wHlcHER ETAL 3,106,448

REcovERYoF CRYOLITE Filed April s, 1961 2 sheets-sheet 2 Fick E.

ALEXANDER e. meme BYANDRZEJ .voLEaA-KowALswsm Ro-ui.- 3, Maw

y ATTORNEY- i Quebec, Canada, assignors to Aluminium Laboratories Limited, Montreal, Quebec, Canada, a corporation of Canada Filed Apr. 3, 1961, Ser. No. 100,268 9 Claims. (Cl. 23-88) rIhisinvention relates to the recovery ofY cryolite from the lining of electrolytic cells used for the production of aluminum wherein an electric current is passed through a molten bath of cryolite (NaAlFG) containing alumina (A1203) dissolved therein.

The electrolytic cell is made up of a sturdy steel outer shell and an inner lining of a heavy impervious layer of baked electrically conductive carbon which forms the cathode of the cell; to control heat losses the cell is usually insulated by a layer of insulating powder or brick` placed between the carbon lining and the external steel shell, Eventually, however, the carbon lining of the cell deteriorates with use due to the penetration of the electroly-tic bath and products of electrolysis resulting ultimately in warping and even the formation of fissures andcracks at which time the cell must be taken out of service and the lining replaced.

It has been found to be economical to recover any cryolite in the carbon cell lining particularly when it contains a substantial amount of cryolite admixed therein, such as an amount of cryolite in the range of Y10% to 30% by weight more or less.

Various methods and techniques have been suggested and employed for the recovery of cryolite values or the admixture of sodium fluoride (NaF) and aluminum fluoride (AlF'a) from the used carbonaceous lining. For instance, it has been proposed to recover fluoride values such as cryolite from the used carbonaceous cell lining material by treating nely divided cell material with an aqueous caustic solution to effect reaction between the iluoride values or cryolite therein with sodium hydroxide (NaOH) to yield water soluble sodium fluoride and water soluble sodium aluminate (NaAlO2) and then to treat the resulting-,solution to precipitate cryolite therefrom. This methodV of recovering cryolite from cell lining material suffers from the disadvantage of requiring a considerable amount of excess sodium hydroxide which is costly while the time necessary to effect the reaction is somewhat prolonged with the further disadvantage that it is accompanied by much foaming due to gas formation and lastly the product is contaminated with alumina.

Accordingly, it is an-object of this invention to pro- United States Patent O vide an improved process for the recovery of fluoride l values, such as cryolite, from materials containing the same.

It is another object of this invention to provide an improved process for 4the recovery of cryolite from used carbonaceous cell lining material containing cryolite admixed therewith.

Still another object of this invention is to provide a process for the recovery of cryolite from carbonaceous Vcell lining material containing cryolite admixed therewith wherein improved yields of recovered cryolite are obtainable. f

. ICC

How these and other objects of this invention are achieved will become apparent in the light of the accompanying disclosure made with reference to the accompanying drawings wherein:

FIG. l is a How diagram or tlow chart illustrating various steps in the process of this invention; and

FIG. 2 graphically illustrates the influence of lalreactant, sodium carbonate, employed in the process of this invention upon the solubility of fluoride-containing material and alumina in the cell lining material when treated in accordance with the practice of this invention.

In accordance with this invention we have provided an improved method for the recovery of luoride or tluorine values,` such as cryolite, from material containing the same, such as used carbonaceous cell lining material employed in the electrolytic reduction or rening of aluminum, by contacting said material at an elevated temperaturewith an amount of a water-soluble metal carbonate, such as sodium carbonate, sodium bicarbonate Y or an admixture thereof, under conditions, particularly of time and temperature, to effect reaction between cryolite and the added carbonate in accordance with the following chemical equation:

'In the accompanying disclosure carbonate is meant to include carbonate, bicarbonate and carbonate and bi- `carbonate-containing mixtures. Y p `As a result of the foregoing reaction, fluoride material,

The precipitated cryolite is then separately recovered and may then be re-used in the electrolytic cell.

The reaction between cryolite-and the water-soluble carbonate, such as sodium carbonate, to yield Watersoluble sodium fluoride and sodium aluminate is carried out atan elevated temperature, such as a-temperature of about 500 C. or lower. Faster reaction rates are possible at higher temperatures; a satisfactory reaction rate is attainable at a temperature of about 800"V C. or even higher, such as a temperature of about 900 C. more or less. At about 900 C., however, the cell lining material being treated tends to become sticky and more difficult to handle. Accordingly, it is preferred to carry out the roasting or high temperature conversion operation at a temperature not greater than about 875 C., however, if there is not enough `soda present to react with all the `cryolite the mixture will get sticky at a temperature lower than normal'and, furthermore, the presence of soda permits much faster heating rates.

rial is converted to sodium fluoride and sodium aluminate.

These materials can then be readily leached out of the remaining solid gangue materials by water, preferably hot water, or by a dilute caustic solution, such as an aqueous caustic solution containing less than about 1% by weight sodium hydroxide, eg. an amount of sodium hydroxide of about 2 grams per liter. The resulting aqueous solution is then treated by contact with carbon dioxide, preferably gaseous carbon dioxide, to precipitate cryolite therefrom together with the formation of sodium car- -bonate which remains dissolved in the aqueous solution. The resulting aqueous solution or spent liquor can then be treated for the recovery of sodium carbonate therefrom or employed directly to treat additional cell lining material for the recovery of cryolite therefrom.

Reference is now made to the drawings, particularly FIG. 1 which illustrates by way of a fiow chart, an embodiment of this invention directed to the recovery of cryolite from used carbonaceous Cryolite-containing cell lining material and from the solids which result from cleaning the gases escaping from the electrolytic cells. The used carbonaceous cell lining material to be treated in accordance with this invention may have the chemical analysis set forth in the accompanying Table I.

TABLE I Chemical component: Percent by weight Cryolite 16.6 Sodium fluoride 11.0 Sodium hydroxide 3.0 Sodium carbonate 7.0 Caustic soluble alumina1 11.0 Inert alumina 26.6 Carbon or similar carbonaceous material 20.0 Calcium uoride 3.3 Other materials (by difference) 1.5

1Caustic soluble aluminum compounds expressed as A1203 and corrected for aluminum in Cryolite.

The used cell lining material is supplied from a suitable source, not shown, by means of suitable solids handling equipment, not shown, via line to a crushing operation or crusher 11 wherein relatively large size or relatively massive pieces of the cell lining material are reduced to a smaller, more conveniently handled size. From crushing operation 11 the resulting crushed cell lining material is supplied via line 12 to su-itable fine grinding equipment such as ball mill 14 wherein the already crushed cell lining material is reduced to a relatively fine size, such as a size below about minus 8 mesh. It is preferred, however, to carry out the grinding operation within ball mill 14 such that the resulting reduced cell lining material has a mesh size below about minus 35 mesh. The smaller the size of the finely reduced cell lining material, the faster the cell lining material (i.e., cryolite therein) reacts with the sodium carbonate in the subsequent roasting or high temperature conversion operation in accordance with this invention.

Desirably, the fine grinding of the cell lining material in ball mill 14 is carried out inthe presence of aqueous sodium carbonate-containing solution, such as aqueous sodium carbonate-containing spent liquor, supplied from a suitable source, not shown, via line 15. By carrying out the fine grinding operation in the presence of an aqueous sod-fum carbonate-containing solution the finely reduced cell lining particles are wetted with a film of aqueous sodium carbonate-containing solution which, upon drying, leaves a residue of sodium carbonate on these particles. This coating or residue of sodium carbonate on the finely-divided cell lining particles serves to supply at least part of the process sodium carbonate requirements in the subsequent high temperature conversion or roasting operation.

Following the fine grinding operation within ball mill 14 the resulting reduced cell lining material is supplied via line 16 to filter 18 which may be a plate and frame filter or a continuous rotary vacuum filter. The aqueous i sodium carbonate-containing filtrate recovered from filter 1S is recycled via line 17 to contact and to wet additional cell lining material undergoing size reduction in ball mill 14.

The filtered finely divided solids are removed from filter 18 and supplied Via line 19 to solids mixer 20 wherein these solids are mixed with a water soluble carbonate, such as a water soluble alkali metal carbonate, e.g. sodium carbonate or sodium bicarbonate or mixtures thereof. Sodium carbonate is preferred in the practice of this invention, `desirably dry, anhydrous sodium carbonate, which is supplied to mixer 20 from a suitable source, not shown, via line 23. It is mentioned, however, that instead of dry sodium carbonate, an aqueous slurry or a concentrated solution of sodium carbonate can be employed to supply all or a part of the sodium carbonate requirements of the high temperature conversion operation.

Within mixer 20 the finely divided cell lining material is thoroughly admixed with the sodium carbonate added thereto. Following Vthe solids mixing operation the resulting, substantially homogeneous admixture of cell lining material and sodium carbonate is supplied via line 21 to high temperature conversion or roasting zone or roaster 22.

Roaster 22 may comprise any suitable means or apparatus for effecting high temperature contact between a gas, such as an oxygen-containing gas, e.g. air, and the solids admixture supplied via line 21. Roaster 22 may be a batch type roaster or continuously operating type roaster, `such as a rotating kiln or multihearth rabble arm roaster of the Herreshof type. Preferably roaster 22 is a Herreshof type roaster, the solids rabbling contact afforded therein serving to promote good solid-solid contact between the soda carbonate and the cell lining material and also between the `oxygen-containing gas, air, supplied `fro-m a suitable source, not shown, via line 24 and the carbon or carbonaceous material in the cell lining material. Further, by use of a Herreshof type furnace or roaster, temperature control during the high temperature conversion or roasting operating can readily be effected by bleeding air into the furnace or by by-passing the furnace gases around any hearth. Temperature control during the roasting operation is desirable since cell lining material varies greatly as to carbon or carbonaceous material content. It is obvious, however, as indicated that many other types of roasters or kilns can be employed provided these are effective to provide a high temperature environment for the sodium carbonate and cryolite undergoing reaction.

Because of the relatively high carbon content of the cell lining material and also because the high temperature conversion or roasting operation involving the reaction between Cryolite and the sodium carbonate yields carbon dioxide, the roaster off gases removed from roaster 22 via line 25 contain a substantial amount of gaseous carbon dioxide, about 20% by volume, more or less. The amount of carbon dioxide therein can be varied to some extent depending upon the amount of air supplied via line 24 to roaster 22 and the carbon content of the cell lining material undergoing treatment.

During the roasting operation, by controlling the amount of sodium carbonate added via line 21 to roaster 212 in admixture with the cell lining material, the amount of alumina, derivable from the Cryolite in the cell lining material, rendered soluble can be regulated so as to control the resulting recovered cryolite product purity. In the roasting operation if Ithere is a deficiency of sodium carbonate the `free energy change favors the formation of sodium fluoride rather than sodium aluminate in accordance with the following chemical equation:

The resulting formed alumina, however, because it has been heated to a high Itemperature during the roasting operation, is essentially water-insoluble and insoluble in dilute caustic yand therefore cannot contaminate the cryolite recovered as product in the subsequent cryolite precipitation operation. Normally, alumina surlicient to provide a weight ratio of A1203:F of about 0.45 in the cryolite precipitator or in the solution from which the cryolite is precipitated, is desired. Therefore, the amount of soda or sodium carbonate in the process will generally be in excess of that needed for the fluoride only and the reaction in roaster 22 will proceed in accordance with the chemical equation: j

Na3AlF6-i-2Na2CO3- NaF-i-NaAlOZ-l- 2G02 An excess of alumina can easily be added when processing ordinary cell lining material by adding more sodium carbonate and this may be desirable if auxiliary sources of lluoride or fluorine-containing materials, `such as scrubber solutions vfro-zn hydrogen iiuoride'or aluminum iluoride plants or aluminum smelters, are available and employed for the recovery of fluoride values therefrom, or sodium ll-uoride obtained from lluosilicate is available.

The elect of the amount of added sodium carbonate during the roasting operation upon the solubility of fluoride-containing materials and alumina is graphically illustrated in laccompanying FIG. 2. Accordingly, as indicated in FIG. 2 the weight ratio of alumina to fluorine in the precipitator solution can be varied in accordance Awithtthe amount of sodium carbonate added in the solids admixture supplied. via line 21 to or kpresent within roaster 22.

Scrubber solids are obtainable during the electrolytic 'refininglof aluminum by collecting the material volatilized from the cell, particularly at the anode, by water scrubbing. Y

The scrubber solutions may be utilized in the process of this invention and are conveniently utilized therein by being supplied `from a suitable source, not shown, via lline 28 to iilter 29 for the removal of the solids therefrom. The resulting iiltered solids are withdrawn lfrom iilter 29 via line 30 and supplied via line 30 into mixer 20 for adm-ixture with the crushed and nely ground cell lining material therein.

`Cell lining material and scrubber solids normally Vcontain about 12-30% by Weight carbon. When these materials are roasted in adrnixture with sodium carbonate in roaster 22 to convert the cryolite therein to water-soluble sodium aluminate and `sodium lluoride at an elevated temperature, such as at least above about 500 C., the carbon therein is burned. This result is highlyxdesirable because the heat so =liberated, about 1600L4000 B.t.u. per pound of cell liningmaterial, helps .to lmaintain the elevated temperature within roaster 22 needed to efr'ect the sodium carbonate-cryolite reaction and to evaporate any spent liquor which may be supplied or recycled to roaster in the conventional aqueous caustic cryolite recovery process are destroyed by reaction with air, sodium carbonate and/or water within roaster 22. Water-soluble cyanides which sometimes present a problem in the ,conventional caustic cryolite recovery process are also destroyed within roaster 22.

`Within `leaching circuit or yleaching tank 32 the watersoluble materials in the'roaster product are dissolved, the

sodium fluoride and sodium aluminate in roaster product dissolving in the aqueous solution supplied to leaching tank- 32 via line 34. Although sodium fluoride and sodium aluminate are water-soluble, they tend -to react to form cryolite in the resulting solution Within leaching tank 32 in accordance with the following chemical equation:

NaF-i-NaAlOg-l-2H2O2Na3AlF6-1-4Na0l-I The resulting formed cryolite tends .to precipitate, particularly when the leaching solution approaches saturation with respect to the sodium fluoride and/ or the temperature of the leaching solution falls below about 50 C. Accordingly, it is desirable to maintain fthe aqueous leaching solution within leaching circuit or leaching tank 32 relatively hot, e.g. above about 50 C. It has been found, however, that the addition of sodium hydroxide, which may be added to leaching tank 32 in the form of a concentrated (eg. 50% by wt.) aqueous solution via line 35 in an amount of about 2 grams per liter based on the leaching solution, ile. below about 1% by weight based on the leaching solution, elfectively prevents the precipitation of cryolite during the leaching operation. Generally, the higher the concentration of alumina in the resulting leaching solution, thegreater the need for the added sodium hydroxide therein.

Following the leaching operation within leaching tank 32 the 'leached solids are supplied via -line 36 to washing circuit or Washing tank 38 wherein the leached solids are washed with fresh wash water supplied to washing tank 38 from a suitable source, not shown, via line 39. The resulting wash Water or overow from washing tank 38 is supplied via line 34 to leaching tank 32 to dissolve the water-soluble uor-ides and sodi-urn aluminate from the roaster product solids supplied thereto via line 26.

Because most or all of the carbon has been removed from the roaster product, thereby reducing the amount of solids being handled and vleached within leaching tank 32, and because of the excellent settling and ltering characteristics exhibited by the insoluble solids of the roaster produce being leached within leaching ltank 32, it has been found that the recovery of water-soluble fluoride from the roaster product is improved. Specilically, it has Y been observed that in the cryolite recovery operation in 22, such as with the sodium carbonate slurry supplied to `grinding before leaching and it can be readily cooled by being supplied or added directly via line 26 tothe leaching circuit or leaching tank 32 `for the ydissolution of the water-soluble sodium fluoride and sodium aluminate therefrom. l

' It has been found that the roasted product from roaster 22 isvery easily and rapidly leached free of soluble linoride and that the insoluble solids therein readily and rapidly settle and are easily filtered.` There is no gas formation or foaming problem presented during `the leaching operation. The aluminum metal, aluminum carbide and aluminum nitrides and otherV relatively surface active materials which tend to make a stable foam 'accordance with this invention the water-soluble uorine in tlrefftailings, recovered from washing tank 38 via line 40, ydoes not exceed 3% of the available Water-soluble fluorine in .the lcell lining feed 'material whereas in the conventional caustic cryolite recovery process, the `amount of soluble uoride in thetailingsamounts to about 45% of the total in the feed. The tailings recovered trom washing tank 38 via line V40 contain up to '80%'V by 'weight alumina and if a red mud lime-soda sinter plant is operated nearby the alumina in these tailings can readily be utilized to commercial advantage.

, `The leaching circuit 32l and/or the washing circuit `38 may comprise` a number of thickeners, filters or centrifuges orequivalent solids-liquids concentrating and/or separating devices. Concurrent or countercurrent operation of the leaching and/or Washing circuit may be employe'd but the countercurrent operation for both leaching and washingis preferred in the practice of this invention toreduce dilution. Further, it is preferred to leach and wash at temperatures close to the boiling point of the leaching and |washing solutions butv this is not essential. In actual practice, it has been found possible to reach a concentration of 15 g.p.l. (grams per liter) water- 2? soluble fluoride in the leaching circuit overflow in line 41 while at the same time effecting at least 97% solution of the available caustic-soluble fluoride in the cell lining feed material, such as may be supplied to the process via line 10.

The resulting solution or overflow from leaching circuit 32 is withdrawn therefrom via line 41 and supplied to surge tank 42 which is also, if desired, supplied with the filtrate of the scrubber slurries obtained from filter 29 via line 44. Because of the uctuations in the uoride and sodium carbonate contents of the cell lining material being processed the retention time of the liquids Within surge tank 42 should be about three times the roaster retention time with good mixing. Desirably, the liquid within surge tank 42 is sampled and analyzed frequently. -If the alumina to iluorine weight ratio therein is too low the sodium carbonate content in the feed supplied to roaster 22 via line 21, or present within roaster 22, should be increased. If the ratio is too high the sodium carbonate should be decreased. This control is desirable so that a reasonably high grade f recovered cryolite will be produced at all times.

The liquid in surge tank 42 is supplied via line 45 to rst stage cryolite precipitation or precipitator 46 wherein the liquid is contacted with the gaseous carbon dioxide in the roaster olf gases recovered from roaster 22 via line 25 after having passed through evaporator 48 and then to first stage precipitator 46 via

lines

49 and 50. `It is thus seen that the carbon dioxide generated within roaster 22 by the oxidation of the carbonaceous material therein and generated by the reaction between the sodium carbonate and the cryolite in the cell lining material therein is advantageously recovered and employed to effect the precipitation and regeneration of cryolite from the water-soluble sodium fluoride and sodium aluminate supplied to precipitator 46 in the liquid overflow via line 41.

Within precipitator 46 cryolite is formed by reaction between carbon dioxide and sodium fluoride and sodium aluminate in accordance with the following chemical equation:

The material from first stage precipitator 46 is then supplied via line 51 to thickener 52 wherein a portion of the aqueous sodium carbonate-containing solution or spent liquor recovered via line 54 is supplied, if desired, via line 55 to leaching circuit 32. ln actual practice the spent liquor thus recycled via line 55 to leaching circuit 32 contains about 3 grams per liter fluorine. The reason for the recycle of the partially spent liquor in line 55 to leaching circuit 32 is to reduce the water intake in the overall process and to reduce the evaporator requirements. This recycle also serves to raise the sodium carbonate content in the spent liquor returned to the roaster such as via line 31 from evaporator 48.

The cryolite-containing slurry is removed from thickener 52 via line 56 and passed to filter 5S for the separation of the cryolite therefrom. The resulting separated cryolite is then passed via line 59y to dryer 60 and the resulting dried product recovered therefrom via line 61.

The liltrate from filter 58 is supplied via line 62, together with the remaining portion of the partially spent liquor removed from thickener 52 via

lines

54 and 64, to second stage precipitator 65 wherein it is contacted with gaseous carbon dioxide supplied via line 49. Second stage cryolite precipitation or second stage precipitator 65 is employed to elect substantially complete recovery of all the available cryolite from the raw material supplied to the process.

The slurry from second stage precipitator 65 is passed via line `66 to thickener 68 for the removal of solids therefrom which are returned via line 69 to first stage precipitator 46. The resulting spent liquor is recovered from thickener 68 via line 70 and is advantageously recycled to the process as the spent liquor supplied to ball mill 14 via line 15 or as the liquid employed as the scrubbing liquid in the production of scrubber slurries supplied to filter 29 via line 28 or the spent liquor may be supplied directly via line 71 to evaporator 48 for concentration of sodium carbonate therein and the production of the sodium carbonate slurry supplied via line 31 to mixer 20, or the spent liquor may be supplied directly, without concentration, to mixer 20 or to roaster 22 for temperature control therein, all as indicated in FIG. 1 of the drawings.

As indicated hereinabove the cell lining material may contain as much as about 3% by weight sodium hydroxide and as much as about 10% by weight sodium carbonate. In the conventional caustic cryolite recovery process the sodium carbonate present in the cell lining material is inert. However, in the cryolite recovery process in accordance with this invention involving roasting the cryolite-containing cell lining material in the presence of sodium carbonate, both the sodium hydroxide and the sodium carbonate react substantially quantitatively with the cryolite to form water-soluble sodium fluoride and water-soluble sodium aluminate. Because of this the sodium ion intake of the process of this invention is substantially less than that of the conventional caustic cryolite recovery process and therefore the spent liquor bleed is substantially less and uorine losses in the spent liquor are, accordingly, substantially reduced. Further, by evaporating part of the spent liquor all the sodium carbonate necessary for continuous operation of the subject process can be provided by the cell lining material itself, with sufficient surplus for scrubber operations if desired. Further, if another need for spent liquor presents itself it may even be more desirable or more economical to purchase dry soda ash (sodium carbonate) rather than to evaporate the spent liquor for the concentration and recovery of sodium carbonate therein.

It is seen, therefore, that by following the practices of this invention a superior and improved process is provided for the recovery of cryolite from cryolite-containing used carbonaceous cell lining material and solids collected from the gases escaping from the cell, particularly as compared with the conventional cryolite recovery process employing aqueous caustic. More specifically, the subject sodium carbonate high temperature sintering or roasting process possesses the following advantages over the conventional caustic cryolite recovery process:

(l) Fluorine recovery is increased from about 87% to about 93% of the available fluorine in the cell lining material.

(2) The amount of process sodium hydroxide is reduced to about 10% of that required in the conventional caustic cryolite recovery process.

(3) By-product sodium carbonate produced in the precipitators can be employed directly to treat the cell lining material without an intermediate causticizing, and make-up sodium carbonate is provided by the sodium carbonate present in the cell lining material itself.

(4) The sodium ion intake in the subject process is reduced by as much as 40%, thereby affording a correspondingly, smaller spent liquor bleed.

(5 Good control of the uorine to alumina ratio in the overflow liquor supplied to the cryolite precipitator is available, thereby making possible the production of a consistent, higher grade recovered cryolite product.

(6) The tailings to be disposed of are reduced in weight by as much as 30%.

(7) The leaching operation for the recovery of the water-soluble sodium fluoride and water-soluble sodium aluminate from the roasted product is easily carried out, is substantially trouble-free and requires smaller tankage. The insoluble portion of roasted product settles and TABLE u Comparison Between Conventional Caustic Cryolite Recovery Process and Subject Sodium Carbonate Roast Process Caustic Soda ProcessV Roast;

Process Potlim'ng to process lbs.. 100.0 100.0 Total available fluorine.-.. -.1bs.. 16. 3 16. 3 N aOH in potlim'ng lbs.. 3. 0 3. 0 NaOH added -..lbs 22.8 2. NaaGOa in potlining lbs-. 7. 0 7.0 NazCOa added... lbs 8.2 NaiCOa in spent liquor... .lbs.- 46. 2 28. 2 Leach residue lbs.- 60. 0 40. 6 C O2 required lbs-- 31. 3 20. 7 100% cryolite produced... lbs-. 22.4 24.0 Grade oi cryolite percent-- 85 90 As will be apparent to those skilled in the art in the light of the foregoing disclosure, many modifications, alterations or substitutions are possible in the practice of this invention without departing from the spirit or scope thereof.

We claim:

l. A method of recovering cryolite values from the carbonaceous cell lining of an electnolytic cell employed in the electrolytic refining of `alumiminr which comprises roasting an admixture consisting essentially of cryolitecontaining cell lining materialin the presence of added Water-soluble carbonate selected =from the group consisting of sodium carbonate and sodium bicarbonate, said roasting operation being carried out in the presence of a stoichiometric excess of said water-'soluble carbonate and at an elevated temperature and for a period of time suicient to react substantially all of the cryolite in said c-arbonaceous cell lining materi-al with said carbonate with the resulting 'formation of sodium fluoride and sodium aluminate in accordance with the chemical equation contacting `the resul-ting reaction admixture containing Sodium iluoride and sodium aluminate with .an aqueous solution to dissolve said sodium iluoride and said sodium aluminatev therefrom, reacting the resulting aqueous solution with car-bon dioxide yto precipitate cryolite therefrom and recovering the resulting precipitated cryolite.

2. A method of recovering cryolite values from cryolitecontaining carbonaceous cell lining material which comprises roasting an admixture consisting essentially of said carbonaceous cryolite-containing material with added soda carbonate, s-aidnoasting operation being carried out in the presence of a stoichiometric excess of sodium carbonate and at an elevated temperature and for a period of time sufficient to react ysubstantially all of the cryolite therein with sodium carbonate with the resulting formation of sodium flouride and sodium aluminate in accordance with the chemical equation y f Na3AlF6+2Na2CO3e GNaF-l-NaArlOg-l-ZCOz together with the formation of` sodium carbonate, separately recovering the precipitated cryolite and employing the sodium carbonate containedin the aforesaid aqueous solucion -to react with additional carbonaceous cryolitecontaining material for the recovery of cryolite therefrom.

3. A method in accordance with claim.- 2 wherein said aqueous solution employed to dissolve the sodium fluoride and sodium aluminate cfrorn lthe reaction adrnixture contains a minor amount of sodium hydroxide dissolved therein Isuflicient to inhibit the precipitation of cryolite therefrom duri-ng the aforesaid dissolution operation.

4. A method of recovering cryolite from carbonaceous cryolite-containing cell lining material employed in the electrolytic refining of aluminum which comprises reducing said cryolite-containing cell lining material to panticle iorm, roasting an admixture consisting essentially of the resulting particle from' cell lining material in the presence of an added water-soluble carbonate selected from the group consisting :of sodium bicarbonate and sodium carbonate, said roasting operation being carried out in the presence of a stoichiometric excess of said water-soluble C. and for .a period of time suilicient to form sodium fluoride `and sodium `aluminate with substantially all of the cryolite in said cell lining material in accordance with the equal-i011 Na3A1F5+2Na2CO3 leaching the resulting reaction mixture with an aqueous solution to dissolve sodium fluoride and sodium -aluminate therefrom, subjecting .the resulting leached reaction mixture to Washing with water to yield the aforesaid aqueous solution, contacting the resulting aqueous solution recovered frorn the aforesaid leaching ope-ration, now containing sodium fluoride and sodium aluminate dissolved therein, with gaseous carbon dioxide to precipitate cryolite therefrom and separating the resulting precipitated cryolite.

5. A method in accordance with claim 4 wherein said leaching operation is carried out in the presence of dissolved caustic in an amount of about 2 grams NaOH per liter of aqueous solution. Y

6. A method in accordance with claim- 4 wherein at least a part of said gaseous carbon dioxide employed to precipitate cryolite is obtained by the oxidation of the carbonaceous material in said carbonaceous cell lining material during said roasting operation.

7. A method in accordance with claim 4 wherein said water-soluble carbonate is sodium bicarbonate.

8. A method Iin accordance with claim 4 wherein said Water-soluble carbonate is sodium carbonate.

9. A method of recovering cryolite values from carbonaceous, cryolite-containing cell lining material of an eleotrolytic cell employed in the -electrolytic refining of aluminum'which comprises, reducing said carbonaceous cryolite-containing cell lining material to particle form in the presence of aqueous spent liquor containing sodium carbonate dissolved therein; separating the resulting reduced cell lining material from said spent liquor, admixing added sodium carbonate with theV resulting reduced cell lining material to yield an admixture containing a stoichiometric excess of sodium carbonate, subjecting the resulting -admixture consisting essentially of reduced cell lining material and sodium carbonate to roasting at an elevated temperature in the range 500-900 C. for a period of time snicient to eiect reaction between the sodium carbonate in said -admixture and substantially all of the cryolite contained in said cell lining material in said admixture in accordance with the equation i NaaAlF-l-ZNazCOla NaF-l-NaAlOg-l-ZCOg subjecting the resulting reaction admixture now containing sodium fluoride and sodium aluminarte to contact with an aqueous solution to dissolve said sodium iluoride and said sodium aluminate therefrom, contacting the resulting solution containing sodium fluoride and sodium aluminate dissolved therein with carbon dioxide to precipitate cryolite thereform and to yield a resulting spent liquor containing sodium carbonate dissolved therein, separating the resulting precipitated cryolite and employing said resulting spent liquor as the yaforesaid first mentioned spent liquor to Contact additional said cell lining material.

References Cited in the le of this patent UNITED STATES PATENTS Morrow Aug. 16, 1932 Morrow et `al Apr. 2, 1940 Brodal et -al Sept. 11, 1951 Albert et al July 26, 1955 Clukey Ian. 24, 1956

Claims

1. A METHOD OF RECOVERING CRYOLITE VALUES FROM THE CARBONACEOUS CELL LINING OF AN ELECTROLYTIC CELL EMPLOYED IN THE ELECTROLYTIC REFINING OF ALUMINUM WHICH COMPRISES ROASTING AN ADMIXTURE CONSISTING ESSENTIALLY OF CRYOLITECONTAINING CELL LINING MATERIAL IN THE PRESENCE OF ADDED WATER-SOLUBLE CARBONATE SELECTED FROM THE GROUP CONSISTING OF SODIUM CARBONATE AND SODIUM BICARBONATE, SAID ROASTING OPERATION BEING CARRIED OUT IN THE PRESENCE OF A STOICHIOMETRIC EXCESS OF SAID WATER-SOLUBLE CARBONATE AND AT AN ELEVATED TEMPERATURE AND FOR A PERIOD OF TIME SUFFICIENT TO REACT SUBSTANTIALLY ALL OF THE CRYOLITE IN SAID CARBONACEOUS CELL LINING MATERIAL WITH SAID CARBONATE WITH