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RECOVERY of CRYOLITE Filed April 3, 196L

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RECOVERY of CRYOLITE Filed April 3, 196L Oct. 8, 1963 C. H. WHICHER ETAL 3,106,448 RECOVERY OF CRYOLITE Filed April 3, 196l. 2 Sheets-Sheet 2 Fig. E. 7A/A AAAAC7 OA /WCaAAS/WaS2A4 (WWASø//A///7/ oa AZ/oe/da Awa 4, 4, /w/sea cazz Z/W6 4.7zfe easy/wa 40 20 0 A. /0 Af 20 225 31 66%. 44, ca. 1242/43 Czz/ Z///6 cates EGEEE ALEXANDER G, NICKLE BYANDRZE. K. DOLEGA-KoWALEWSK Roets, sulfa-nu ATTORAWA/ 3,106,448 United States Patent Office Patented Oct. 8, 1963 1 2 How these and other objects of this invention are 3,106,448 achieved will become apparent in the light of the ac RECOVERY OF CRYOLTE companying disclosure made with reference to the ac Charles Harry Whicher, Alexander Gordon Nickle, and Andrzej Krzysztof Dolega-Kowalewski, all of Arvida, companying drawings wherein: Quebec, Canada, assignors to Aluminium Laboratories FIG. 1 is a flow diagram or flow chart illustrating Limited, Montreal, Quebec, Canada, a corporation of various steps in the process of this invention; and Canada FIG. 2 graphically illustrates the influence of a re Filed Apr. 3, 1961, Ser. No. 100,268 actant, sodium carbonate, employed in the process of 9 Claims. (CI. 23-88) this invention upon the solubility of fluoride-containing O material and alumina in the cell lining material when This invention relates to the recovery of cryolite from treated in accordance with the practice of this invention. the lining of electrolytic cells used for the production of In accordance with this invention we have provided aluminum wherein an electric current is passed through a an improved method for the recovery of fluoride or molten bath of cryolite (Na3AIFs) containing alumina fluorine values, such as cryolite, from material containing (Al2O3) dissolved therein. 5 the same, such as used carbonaceous cell lining material The electrolytic cell is made up of a sturdy steel employed in the electrolytic reduction or refining of outer shell and an inner lining of a heavy impervious aluminum, by contacting said material at an elevated layer of baked electrically conductive carbon which forms temperature with an amount of a water-soluble metal the cathode of the cell; to control heat losses the cell carbonate, such as sodium carbonate, sodium bicarbonate - is usually insulated by a layer of insulating powder or 20 or an admixture thereof, under conditions, particularly brick, placed between the carbon lining and the external of time and temperature, to effect reaction between cryo steel shell. Eventually, however, the carbon lining of lite and the added carbonate in accordance with the the cell deteriorates with use due to the penetration of the following chemical equation: electrolytic bath and products of electrolysis resulting ultimately in warping and even the formation of fissures 25 NasAIFs--2Na,CO. 96NaF+NaAlOa+2CO, and cracks at which time the cell must be taken out of In the accompanying disclosure carbonate is meant to service and the lining replaced. include carbonate, bicarbonate and carbonate and bi It has been found to be economical to recover any carbonate-containing mixtures. cryolite in the carbon cell lining particularly when it As a result of the foregoing reaction, fluoride material, contains a substantial amount of cryolite admixed there 30 such as cryolite, is converted to water-soluble materials, in, such as an amount of cryolite in the range of 10% such as sodium fluoride and sodium aluminate. Follow to 30% by weight more or less. - ing the above-described conversion operation, the result Various methods and techniques have been suggested ing reaction admixture containing sodium fluoride and and employed for the recovery of cryolite values or the sodium aluminate is contacted with an aqueous solution admixture of sodium fluoride (NaF) and aluminum 35 to extract or dissolve the water soluble sodium fluoride fluoride (AlF) from the used carbonaceous lining. For and sodium aluminate therefrom. The resulting aqueous instance, it has been proposed to recover fluoride values solution, now containing sodium fluoride and sodium such as cryolite from the used carbonaceous cell lining aluminate dissolved therein, is then contacted with carbon material by treating finely divided cell material with an dioxide (CO) to precipitate cryolite therefrom in ac aqueous caustic solution to effect reaction between the cordance with the following chemical equation: fluoride values or cryolite therein with sodium hydroxide (NaOH) to yield water soluble sodium fluoride and water soluble sodium aluminate (NaAlO2) and then to The precipitated cryolite is then separately recovered and treat the resulting solution to precipitate cryolite there may then be re-used in the electrolytic cell. from. This method of recovering cryolite from cell 45 The reaction between cryolite and the water-soluble lining material suffers from the disadvantage of requiring carbonate, such as sodium carbonate, to yield Water a considerable amount of excess sodium hydroxide which soluble sodium fluoride and sodium aluminate is carried is costly while the time necessary to effect the reaction out at an elevated temperature, such as a temperature of is somewhat prolonged with the further disadvantage that about 500 C, or lower. Faster reaction rates are pos it is accompanied by much foaming due to gas formation sible at higher temperatures; a satisfactory reaction rate and lastly the product is contaminated with alumina. is attainable at a temperature of about 800 C. or even Accordingly, it is an object of this invention to pro higher, such as a temperature of about 900 C. more or vide an improved process for the recovery of fluoride less. At about 900. C., however, the cell lining material being treated tends to become sticky and more difficult values, such as cryolite, from materials containing the 55 Sae. to handle. Accordingly, it is preferred to carry out the It is another object of this invention to provide an roasting or high temperature conversion operation at a improved process for the recovery of cryolite from used temperature not greater than about 875 C., however, carbonaceous cell lining material containing cryolite ad if there is not enough soda present to react with all the mixed therewith. cryolite the mixture will get sticky at a temperature Still another object of this invention is to provide a 60 lower than normal and, furthermore, the presence of soda process for the recovery of cryolite from carbonaceous permits much faster heating rates. cell lining material containing cryolite admixed therewith During the high temperature roasting operation Sub wherein improved yields of recovered cryolite are stantially all the cryolite contained in the cell lining mate obtainable. rial is converted to sodium fluoride and sodium aluminate. 3,106,448 3. A. These materials can then be readily leached out of the sodium carbonate-containing filtrate recovered from filter remaining solid gangue materials by water, preferably hot 18 is recycled via line 17 to contact and to wet addi water, or by a dilute caustic solution, Such as an aqueous tional cell lining material undergoing size reduction in caustic solution containing less than about 1% by weight ball mill 4. sodium hydroxide, e.g. an amount of sodium hydroxide The filtered finely divided solids are removed from of about 2 grams per liter. The resulting aqueous solu filter 18 and supplied via line 19 to solids mixer 20 tion is then treated by contact with carbon dioxide, pref wherein these solids are mixed with a water soluble erably gaseous carbon dioxide, to precipitate cryolite carbonate, such as a water soluble alkali metal carbonate, therefrom together with the formation of sodium car e.g. sodium carbonate or sodium bicarbonate or mixtures bonate which remains dissolved in the aqueous solution. 10 thereof. Sodium carbonate is preferred in the practice The resulting aqueous solution or spent liquor can then of this invention, desirably dry, anhydrous sodium car be treated for the recovery of sodium carbonate there bonate, which is supplied to mixer 20 from a suitable from or employed directly to treat additional cell lining source, not shown, via line 23. It is mentioned, how material for the recovery of cryolite therefrom. ever, that instead of dry sodium carbonate, an aqueous Reference is now made to the drawings, particularly 5 slurry or a concentrated solution of sodium carbonate can FIG. 1 which illustrates by way of a flow chart, an be employed to supply all or a part of the sodium car embodiment of this invention directed to the recovery bonate requirements of the high temperature conversion of cryolite from used carbonaceous cryolite-containing operation. cell lining material and from the solids which result from Within mixer 20 the finely divided cell lining material is cleaning the gases escaping from the electrolytic cells. 20 thoroughly admixed with the sodium carbonate added The used carbonaceous cell lining material to be treated thereto. Following the solids mixing operation the re in accordance with this invention may have the chemical sulting, substantially homogeneous admixture of cell lin analysis set forth in the accompanying Table I. ing material and sodium carbonate is supplied via line TABLE I 21 to high temperature conversion or roasting Zone or Chemical component: Percent by weight 25 roaster 22. Cryolite -------------------------------- 16.6 Roaster 22 may comprise any suitable means or ap Sodium fluoride-------------------------- 1.0 paratus for effecting high temperature contact between a Sodium hydroxide------------------------ 3.0 gas, such as an oxygen-containing gas, e.g. air, and the Sodium carbonate------------------------- 7.0 solids admixture supplied via line 21. Roaster 22 may Caustic soluble alumina ------------------ 11.0 30 be a batch type roaster or continuously operating type Inert alumina---------------------------- 26.6 roaster, such as a rotating kiln or multihearth rabble arm Carbon or similar carbonaceous material.----. 20.0 roaster of the Herreshof type.
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