Oct. 9, 1962 D. C. GERNES ETAL 3,057,681 PRODUCING ALUMINIUM FLUORIDE

Oct. 9, 1962 D. C. GERNES ETAL 3,057,681 PRODUCING ALUMINIUM FLUORIDE Filed Jan. 13, 1960 3 Sheets-Sheet 3

FUEL GAS STACK SPENT HEAT AR GAS LIQUOR EXCHANGER ABSORBER 2ND STAGE CRUDE DISTILLATION MIST -- -- RECYCLED UNIT TRAP ACID IST STAGE

BOTTOMS COOLER EXCHANGER ACID FORTIFIED SPENT LIQUOR ALUMINA ?ATOH RECYCLED MATERIAL RIHYDRATE REACTORS REACTOR RECYCLEDSILICA K FÀTRATE PUMP OFF FIAL TRATE CAKE SPENT WATER SILICA ??? S??gA? %AE SILICA FILTER MASH CAKE "I WASHI" WASTE F?L TRATE CRYSTALLIZERS RECYCLED SEED

DLUTION LOUOR OVERFLOW

F_TRATE

THKCKENER ?????? FILTER LQUOR SEED

FILTER

DRYER IOFF-GAS SCRUBBER FILTER CAKE CALCINER

AIF3 RECYCLED SPENT LIQUOR PRODUCT INVENTORS

SCRUBBER LIQUOR DONALD C. GERNES WILLIAM R. KING ge E. 4. BY??????? ( ORNEY 3,057,681 United States Patent Office Patented Oct. 9, 1962 1. 2 3,057,681 fluoride is produced by adding solid, finely divided PRODUCING ALUMNUM FLUORIDE alumina hydrate to a fluosilicic acid solution. The tem Donald C. Gernes, Los Gatos, and William R. King, perature of the reaction is within the range of about 60° Sunnyyale, Calif., assignors to Kaiser Aluminum & 70° C. and preferably about 65° C. The fluosilicic acid Chemical Corporation, Oakland, Calif., a corporation 5 should have a fluorine to silicon mole ratio of less than 6, of Delaware i.e., it should be a silica-rich acid. The finely divided Filed Jan. 13, 1960, Ser. No. 2,275 alumina hydrate is continuously added to an excess of 16 Cains. (CI. 23-88) acid for a period of about cine-half hour or more. After This invention relates to an improved process for the the addition of all the alumina hydrate to the acid, there production of aluminum fluoride from fluosilicic acid 10 is an additional digestion period of at least about two and alumina hydrate. hours to complete the reaction. After the digestion Aluminum fluoride finds use in many industrial proc period, the reaction liquor is filtered hot and the insoluble esses. It is one of the minor constituents added to the silica is easily separated therefrom. Solid aluminum electrolytic cells during the production of aluminum. It fluoride trihydrate is crystallized from the hot filtrate is also used in the preparation of white enamels, as an liquor by the addition of aluminum fluoride seed. The anti-reflection coating in complex optical systems, as a crystals of aluminum fluoride trihydrate obtained are constituent in welding fluxes, in the preparation of fluor dehydrated to produce the final product of anhydrous rine-containing glasses, etc. aluminum fluoride. The spent liquor from the crystalliza Aluminum fluoride does not occur in any natural de tion step is processed and returned to the reaction step posits except for the very rare mineral fuellite 20 for further production of aluminum fluoride. Also, in (AIF.H2O) accordance with the invention, there is included a novel Therefore, for industrial purposes, it must be manufac process for obtaining a refined fluosilicic acid liquor, tured. Generally, it is well known that aluminum fluo suitable for use in the reaction step, from crude fluosilicic ride may be produced by stoichiometrically reacting hy acid by distillation whereby the fluosilicic acid is vapor drated alumina or an alumina-containing material with 25 ized and the vapors then absorbed by spent liquor from fluosilic acid in an aqueous solution at elevated temper the crystallization step. atures. Aluminum fluoride is formed in the solution. THE REACTION STEP Insoluble silica is also formed as one of the reac The reaction between fluosilicic acid and alumina tion products and remains suspended in the solution. hydrate in an aqueous medium produces aluminum The removal of the suspended silica from the solution is 30 fluoride in solution and a silica precipitate as follows: very difficult, since the silica may be present in a gelat inous form. The difficulty may be overcome by using dilute (1-3 percent) solutions; however the process be In order to commercially produce the aluminum fluo comes uneconomical commercially thereby . Another ride economically, it is necessary to obtain a higher per problem to contend with is the retention of AlF (hy 35 centage conversion, a supersaturated aluminum fluoride drated) by the silica precipitate. Various procedures solution having a concentration of about 9%, and a have been advanced for the preparation of aluminum readily, easily filterable silica. Furthermore, loss of fluoride which allegedly overcome the above mentioned aluminum fluoride to the silica filter cake should be kept difficulties and at the same time achieve a high yield of at a minimum. These requirements are readily achieved product. 40 by this invention by conducting the reaction under cer U.S. Patent No. 2,842,426 issued July 8, 1958, to E. M. tain critical operating conditions. Glocker describes a process whereby substantially pure The order of combining the reactants materially affects aluminum fluoride is prepared by reacting bauxite with the reaction. The continuous addition of solid alumina not more than its stoichiometric equivalent of hydro hydrate to the acid in solution produces a faster filtering fluoric or hydrofluosilicic acid at temperatures in the 45 silica cake and a higher percentage of conversion than range of 100-190° F. (37.8°-88° C.). It is preferred the gradual addition of the acid to a slurry of the alumina to operate the process with an excess of 5-15% alumina or the rapid mixing of acid and alumina. in the reaction mixture. Critical conditions are recited Table I shows the effect of the order of combining for conducting the reaction in order to separate the re alumina trihydrate and H2SiF6 on the filterability of the sulting aluminum fluoride solution from the unreacted 50 silica precipitate produced in the reaction. In each test solids and silica at the proper time, i.e., the point of maxi one reactant was continuously added to the other reac rnum solubility of the aluminum fluoride. tant over a period of thirty minutes and the reaction mix British Patent No. 782,423 to Fisons Ltd. discloses a ture stirred for an additional two hours; the temperature process for the preparation of aluminum fluoride which was maintained at 65° C. In tests 1-3, alumina trihydrate involves adding fluosilicic acid of a concentration in the 55 was added to approximately 900 grams of 8.5% H2SiF6. range 5-15% by weight to an aqueous slurry of alumi In tests 4 and 5 the H2SiF6 was added as 30% acid to a num hydroxide, whereby the silica is obtained in an easily slurry of alumina trihydrate in about 700 grams of filterable condition. The reaction is carried out desirably WateT. Table I with stoichiometrical amounts at preferred temperatures 60 of 80°-100° C., or at 60-75° C. At the preferred tem EFFECT OF ORDER OF ADDITION OF REACTANTS perature range, the use of excess aluminum hydroxide Per does not appear to possess any advantage. At 60-75 C. Test Reactant in Filterability cent the use of excess aluminum hydroxide appears to assist No. Order of Addition eXC0SS of Silica pre- con the filtration of the silica. cipitate Ver The present invention provides for an improved process 65 | Sion 1 for the production of aluminum fluoride from alumina 1----- Al(OH)3 to HSife---- 10%. Al(OH)3. Fast.----... 00 hydrate and fluosilicic acid whereby there are obtained a ------do------Neither------do- 86 - 10% HSiF6- ? supersaturated aluminum fluoride solution, an easily fil t terable silica precipitate, high yields, and a minimum loss 84 of fluoride to the silica precipitate. In accordance with the present invention, aluminum 1 Based on Al content of AFs solution made. 3,057,681 3 4 The rate of addition (feeding time) of the alumina in excess of stoichiometric amount. The reactions were hydrate to the acid solution also affects the filterability all conducted at 65° C. The Al(OH)3 was fed to the of the silica cake produced and the loss of fluorine to heated acid at different rates so that feed times of 30 the cake. Table II shows the effect that different rates minutes, 1 hour and 2 hours for the 2.530 kg. of Al(OH)3 of addition of alumina hydrate have on the filterability 5 were required. Total reaction time in each case was 4 of the silica cake produced and on the loss of fluorine hours, after which the reaction mixtures were filtered to the cake. In each test, 125.5 grams of Al(OH)3 was hot (65 C.). In the filtering operation the reaction fed to 8.48% H2SiF (in 5% excess over stoichiometric mixtures were agitated at 65° C. and a 0.1 sq. ft. filter amounts) at varying rates. The temperature was main leaf was immersed therein for 15 seconds, a cake form tained at 65° C. and the total reaction time was 2.5 0 ing time to give at least a 4 inch cake. In some of hours, except as noted. The reaction mixtures were fil the tests, as noted in Table III, the cake forming time tered through a No. 42 Whatman filter paper on an 11 was 30 seconds.

Table III FILTRATION COMPARISON TEST FOR DIFFERENT TOTALALUMNA FEED TIME FILTRATION DATE OF REACTION AT 65° C. 0.1 ft.2 filter leaf, Dynel cloth, absolute pressure 250 mm. Hg for cake form, wash, and dry-test made four hours after the beginning offeed to the reaction 30 minutes feed Cake weight, Percent Wolume Filtration time, sec. Volume, ml. grains dry Cake ratio Wash F (per-Percent Test Temp. solids of thick. wash rate cent of solids ? ?. wet inches HaO, ml.fsec. dry in IForm Wash Dry Filtrate Wash Wet Dry cake liquid cake) slurry ill cake

65 15 O O 844 266 122. 37.8 30.9 0.50 3.16 65 5 15 0 869.5 490 149.0 39.5 26.5 0.56 4.47 65 15 20 10 855 530 28.6 38.6 30.0 0.56 5.90 65 15.------i0 771 ? ------!?? ------??????---?????? 0.50 ? ? ? - - - - - ??------?? 65 15 ------10 700 -?------? ------0?.0 ? ------

0.53 3.49 22.0

65 10 840 65 10 765 65 10 1,240 65 10 1,220 65 10 1,271.3 57.7 54.0

cm. Buechner funnel. The pressure difference across Table IV gives further data on filtration tests for 30 the filter was 27 inches of mercury. The cakes varied minute and 60 minute time periods for addition of alu in thickness from 18 mm. for fast filtering to 35 mm. mina. The tests of Table IV were under the same con for slow filtering precipitates. ditions as those of the tests of Table III, except that no Table II EFFECT OF RATE OF ADDITION OF Al(OH)

Time Of | Filtra- Dry WS F loss, addition tion Weight of Weight Solids Wash Ratio: HO} F in dry (percent Percent Exp. No. of 125.5 g. time wet cake, of dry (percent rate, Usedi- cake of total reaction Al(OH)3 (min.) (g.) cake, (g.) of wet (cc.imin.) liquor (percent) fluorine) (min.) cake) hangup

5.0 25,0 403.2 74.6 18.5 K4. 0.61 3.5 10.10 100 5.0 8.0 383.0 78.2 20.4 50 0.66 12.37 10.05 100 15.0 6.0 380.1 69.0 18.2 104 1.03 8.59 6.16 100 5.0 3.0 367.0 80. 4 21.9 195 1.57 10.91 9.3 96.7 30.0 7.0 344.8 74.7 2.6 100 0.74 3.84 10.5 100 60,0 3.5 344. 71.3 21, 4 100 0. 7.34 5. 44 94.4 60.0 2.5 299.7 63.0 21.0 300 1.90 2.97 1.93 98.5 90.0 1.5 245.2 59.0 24.0 900 2.00 3.57 2.16 98.3

13.7 hours reaction time, 4.0 hours reaction time. Table III shows comparative results of filtration tests excess acid was used. The fluosilicic acid used. con (which were of a substantially larger scale than the tests 70 tained 20.8% F and 5.91% Si. It was diluted to 8.1% of Table II), for 30 minutes, 1 hour and 2 hour time equivalent H2SiF6. The reactions were conducted at periods for addition of alumina. The tests were car 65 C., and the total reaction time was 4 hours. The ried out in a 30 liter tank, and the charge to the tank filtration tests made on the reaction mixtures were car consisted of 10.45 kg. of 23.5% H2SiF6, 18.52 kg. of ried on in the same manner as those for the tests shown tap water and 2.530 kg. of Al(OH)3, the acid being 5% 75 in Table III. 3,057,681

Table IV FILTRATION TEST RESULTS AFTER 240 MINUTES’ REACTION 60 minutes feed time

Filtration time, sec. Cake Volume, ml. Cake weight, g. Volumeratio Percentdry Percent Wash Percent Test thick, wash solids solids rate, F in in. EO, in wet in Inl/sec. dry Form Wash Dry Filtrate Wash Wet Dry liquid cake slurry cake in cake

20 0 O 0.56 897 333 7. 45.3 2, 66 26.6 4?28 33.3 II. 45 20 20 20 0.56 87. 617 168 43.8 4.99 26.2 4, 28 30.9 1.44 20 30 30 0.56 829 934 62 43.3 7.85 26.6 4.28 31. 1,47

30 ninutes feed time

20 10 10 0.56 67 27 176 35, 1 I, 54 20.0 4.18 21.7 2.06 20 20 20 0.56 653 A07 170 34.5 3.00 20.3 4.8 20.4 1. 48 20 30 30 0.56 634 580 165 33.4 4.39 20, 2 4.18 19.3 1.43

The results of Tables III, III and IV show that the tests were performed using a 0.1 sq. ft. filter leaf immersed fastest filtering cakes were obtained with the slowest addi in the slurry for 25 seconds, an arbitrary cake forming tions of alumina hydrate. Additions made continuously time to give at least a /2 'cake. for periods of about one hour to about two hours and preferably about one hour produced the fastest filtering 25 Table V silica cakes containing the lowest fluorine contents. The alumina hydrate should be added continuously during the FILTRATION COMPARISC)N TESTS ON SILICA PRECIP addition period and the rate of addition does not neces ITATE MADE FROMI HE"-RICHI AND SiO2-RICH H2Si'a sarily have to be uniform. The additions should not be Filtration time Cake intermittent. Feed times of as low as one-half hour have 30 (sec.) Filtrate thick- FiltrationPercent Acid volume ness rate (ml.f solids been found suitable and produced a readily filterable (ml.) (in.) sec.) in cake silica with low fluorine loss. Form 1 Dry 2 A better reaction rate is obtained if the alumina hydrate added to the acid is very fine in particle sizes. The use HF-Rich SiR- 25 33 175

(SEAL) Attest: ERNEST W. SWIDER DAWID L. LADD Attesting Officer Commissioner of Patents