fluoride recovery from phosphorus production

Figure 1. Diagram of fluorine scrubber.

POLLUTION CONTROL:

Fluoride Recovery From Phosphorus Production t

The trend toward large plant capacities and increasing costs for pollution abatement are mak ing the recovery of valuable fluorine compounds from waste gases more attractive.

J. C. Barber, and T. D. Farr, Tennessee Valley Authority, Muscle Shoals, Ala.

W H E N PHOSPHATE ROCK IS PROCESSED TO MAKE VARI- OUS phosphate chemicals, fluorine in the rock is evolved as gaseous hydrogen fluoride, silicon tetrafluoride, or a mixture of these two gases. It is estimated that 200,- 000 tons of fluorine was evolved in processing about 22 million tons of phosphate rock in the United States during 1968. The percentages of fluorine compounds in the effluent gases are generally quite small; usually the effluent gases contain compounds of silicon, and they may contain phosphorus compounds, carbon dioxide, oxides of sulfur, water vapor, and entrained solid and liquid particles. The development of processes for the recovery of useful fluorine compounds from such effluents has been handicapped by the low

56 November 1970

concentration of fluorine and by the asso contaminants.

Substantially all the fluorine in the exhaust from phosphate processing must be removed in to comply with State and local pollution abate

spread pollution State water quali sult of the Water of soluble fluoride

narily limited to a few parts per million, depending the use of the receiving stream.

CHEMICAL ENGINEERING PROGRESS (Vol. 66, No. 11

ENGINEERING PROGRESS (Vol 66, No. 1 1 ) 57 November 1970

Figures 3. Pond for treatment of water containing fluorides.

F to P,On weight ratio in the mixture is 0.106, as com- pared with an F to P,O;, ratio as high as 0.14 for some other ores.

The phosphate mixture a t TVA is dried, agglom- erated by compacting, and calcined in a traveling grate-rotary kiln combination a t temperatures of 2200- 2300°F. About 8% of the fluorine in the phosphate volatilizes as a mixture of H F and SiF, during calcination a t a temperature of about 2,200"F. In addition to fluorine compounds, exhaust gases from the caicirier coritairi phosphate dust a d F2G5 , the hitei- results from burning by-product carbon monoxide as fuel, since this gas contains a small amount of phosphorus vapor as an impurity. The gases also contain a small amount of sulfur compounds. Gases from the calcining operation go through dust collectors and are then scrubbed with water to remove the fluorine. Figure shows the scrubbing equipment used a t TVA.

Indurated phosphate is put into electric furnaces and smelted with coke and silica to produce phosphorous. About 8.2 tons of by-product slag result from the production of each ton of phosphorous. More than 90%, of the fluorine put in the furnace comes out in the slag. A small amount of fluorine is given off when the slag is tapped, and some of i t is discharged when slag is waterquenched or expanded ( 2 ) .

Phosphorous furnace condensing system Fluorine volatilizes from phosphorus furnaces and

comes off in the gas offtake, together with the phosphorus vapor and by-product carbon monoxide gas. Figure 2 is a diagram of the phosphorus furnace condensing system. The furnace gas mixture is treated in an electrostatic precipitator to remove dust, and some fluoride present as particulate matter is removed from the gas. The gases are then scrubbed with water to con- dense the phosphorus vapor ; this operation removes

the gaseous fluorine compounds and the remain of the particulate fluoride. The scrubber water recycled and neutralized to maintain 5.5 to 6.0. Fluosilicates and phosphate the scrubber solution. Some sodium compounds volatilize from the furnace bine with fluosilicic acid to form so sium fluosilicates which dissolve in the scrubber The scrubber water also contains some suspe phosphorus particles. Particulate matter fromi tile gas wile11 piiusphoi*us is coii lect in part of the phosphorus and for like material called phosphorus sludge contain some of the fluorine given furnace.

Since water containing suspended phosphorus highly toxic to marine animals, i t must be collect and treated to prevent pollution. At TVA the phos phorus-contaminated water is collected, clarified, and reused (3) . Fluosilicates and phosphates present as dissolved salts accumulate in the water, and part of the clarified water must be bled off t o control the salt concentration. The water taken from the recycled stream normally contains about 10 g. of F, 17 g. of P205, and 10 g. of NH, per liter of solution.

The solid wastes containing fluorine (preci dust and phosphorus sludge) are returned charge preparation process and smelted in t nace. The precipitator dust contains 4.5% flu0 is slurried, the slurry is clarified by settling, a wet solids are mixed with the raw phorus sludge is treated to remove residue is incinerated, and the ash i raw materials. The fluorine in the amounts to about 12 lb./ton of phos It is estimated about half of the flu0 terials volatilizes when the materials are ca

58 November 1970 CHEMICAL ENGINEERING PROGRESS (Vol 66, No 1 1 )

dioxide is limited, and the solution is not highly cor- rosive, Moreover, the partial pressure of ammonia over such solutions is low and little ammonia is lost in the scrubbing operation (7).

Phosphorus compounds in the gases dissolve in the ber solution, and the weight ratio F:P20B in the

liquors may vary widely. For example, recycled liquors obtained at the phosphorus furnaces will have weight ratios F:P203 ranging from 0.6 to 2. The liquor formed by scrubbing the gas evolved from phosphate rock calcination has ratios ranging from about 10 to

ited for

orine in ber liquors by adding ammonia to raise the pH to the range 8 to 9; the fluosilicate decomposes and precipitates of hydrated silica and iron hydroxide are sepa- rated from the ammonium fluoride solution. The silicon and iron contents of the filtrate will correspond to weight ratios F :Si02 and F:Fe20, well above the

m values specified for cryolite and aluminum

In laboratory-scale tests of the silica removal step, kiln gas scrubber liquor (F, 31; SO,, 11; P205, 2; Fe203, 1 ; and S, 17 g./liter) was ammoniated t o pH 9, the mixture was filtered and the precipitates washed

, the ob- preparing

,

adding am- ' d reacts with

mmonium ueous am-

for cryolite and aluminum fluoride. The filtration rate was rapid. A subsequent pilot-plant test of the silica- removal step with liquor obtained by scrubbing superphosphate den gas showed a filtration rate of about 100 gal./sq. ft./hr. (8).

NadIF, from ammoniated superphosphate den gas er liquor by the alkaline method.'

Cryolite - Composition, yo Recovery, yo

pH 8.5) contained, g./liter, F, 37; P205, 0.3;

+Basis: stoichiometric requirements in reaction 1.

NGINELRING PROGRESS (Vol 66, No 1 1 ) 59 November 1970

Table 2. Preparation of Na,AIF, from ammoniated kiln gas scrubber liquor by the acid method.*

Precipitation Cryolite

Wt. Ratio Reagents, yo** Compositio.n, yo F P,OS AI Na PH 40 110 95 5.8

100 100 5.8 95 95 4.0

30 95 95 5.0 10 95 95 5.0 5 95 95 4.0

bo5 F AI Na

0.03 51.5 0.06 52.7 13.2 29.3 0.04 52.5 12.5 28.8 0.05 - - - 0.08 53.0 13.3 28.3 0.05 52.9 13.1 30.1

- -

Recovery, yo F AI Na 86 - 92 90 86 87 92 83

85 95 80 '

80 92 80

- - -

"Stock solution of ammoniated liquor (pH 8.5) contained, g./liter, F, 52; P20s, 1.3; SO2, 0.6. **Basis: stoichiometric requirements in reaction 2.

Cryolite preparation The ammonium fluoride solution from the silica-

removal step will contain most of the P,05 initially present in the scrubber liquors, and the weight ratio F:P,O, may range from less than 1 to more than 100. Three typical ammonium fluoride solutions obtained from plant operations were used to evaluate, on a laboratory scale, several methods for preparing specification-grade (minimum wt. ratio F :P,O, of 460) sodium cryolite, ammonium cryolite, and aluminum fluoride. Stock solution I from superphosphate den gas scrubber liquor contained F , 37; PnOr,, 0.3; and SiO,, 0.55 g./liter. Stock solution I1 from kiln gas scrubber liquor, after concentration, contained F, 52 ; P,O,, 1.3; and SiO,, 0.6 g./liter. Stock solution I11 from phosphorus condenser liquor contained F, 55; P,O,, 33.2; and SO2, 0.6 g./liter.

The cryolite methods evaluated are summarized below :

6NH,F + NaAIO, + 2NaOH -+ NA3A1F, + 6NH3 + 4H,O (1)

6 (NH4) ,SO4 ( 2 )

3 (NH4) ,SO4 (3)

12NH,F + 3Na,SO, + Al, (SO,) + 2Na,AlF, + 12NH4F + Al, (SO,) :< 4 2 (NH,) ,AlF, +

In the alkaline process for preparing sodium cryolite, reaction 1, which is similar to that used by the aluminum industry, solutions of the reactants are heated to boiling and mixed, and the hot alkaline solution is acidified to the phenolphthalein end-point by the slow addition of carbon dioxide. The mixture is cooled to room temperature and, if necessary, further acidified with carbon dioxide. The precipitated cryolite is filtered off, washed, and dried. The ammonia may be recycled. The alkaline method was evaluated with test solutions whose weight ratio F:P,O, ranged from 25 to above 150 as prepared from stock, solution I, with or without additions of ammonium phosphate solutions.

The results, summarized in Table 1, show that specification-grade sodium cryolite was prepared from superphosphate den gas scrubber liquor (1) with weight ratios F:P,O, of 25 or higher by using 90% of the stoichiometric quantities of aluminum and sodium, (2) with weight ratios F:P,O, of 50 or higher by using 95% of the stoichiometric quantities of aluminum and sodium, and (3) with weight ratios F :P,O,

60 November 1970

of 125 or higher by using the stoichiometric quam tities of aluminum and sodium. The cryolite precipi. tates contained fluorine equivalent to 94- to 97% Na,AlF,, and 83- to 95% of the fluorine in the test solution was recovered.

In the acid method for preparing sodium cryolite, reaction 2, a slightly acidic solution of ammonium fluoride (pH 4 to 6) is mixed with solutions of sodium and aluminum salts, preferably the sulfates, without heating. The precipitated cryolite is filtered off, washed, and dried. Ammonia may be recovered from the filtrate by adding lime and heating. The acid method was evaluated with test solutions whose weight ratios F:P20,, ranged from 5 to 40 as prepared from stock solution I1 with or without additions of ammonium phosphate solution. The amopnt of phosphate precipitated with the cryolite from the solutions was determined as functions of the proportions of sodium and aluminum added, and of the pH of the precipitation mixtures. The terminal pH of the precipitation mixtures, which ranged from 4 to 6, was contm!led by adding sulfuric acid to stock so!utior? II before adding sodium and aluminum sulfates.

Conditions for preparing specification-grade sodium cryolite by the acid method are summarized in Table 2. When the weight ratio F:P20, is about 5 and the pH of the mixture is about 4,95% of the sodium and aluminum salts stoichiometrically required to form Na,AlF, with all the fluorine present in the test solution may be used. When the F:P20, ratio is 40 or higher and the pH is in the range 5 to 6, specification- grade sodium cryolite can be made with an excess of the precipitating agents. The products contained fluorine equivalent to\94- to 98% Na,AlF,, and 80- to 92% of the fluorine in the test solutions was recovered. Similar results were obtained when sodium chloride was used instead of sodium sulfate. In general, recovery of fluorine as cryolite from a given ammonium fluoride solution increased when the pH of the precipitation mixture increased, and when the proportions of sodium and aluminum salts added increased.

In the method for preparing ammonium c reaction 3, a soluble aluminum salt such as alu sulfate is added to a neutral or slightly acidic s of ammonium fluoride (pH 4 to 7), the unh mixture is stirred 1 to 5 minutes, and the ammonium cryolite is allowed to settle for minutes. The cryolite is filtered off, washed,

CHEMICAL ENGINEERING PROGRESS (V

Table 3. Preparation of (NH,)JIF, from ammoniated phosphorus condenser liquor or kiln gas scrubber liquor."

Cryolite

sit,ion of kiln gas scrubber liquor given in Table 2. hiometric requirements in reaction 3.

rom the Conversion methods A study was made of the conversion of ammonium

dium cryolite, as represented by the

4)3AIFS + 3NaC1 = Na,AlF, + 3NH,Cl sts, ammonium cryolite was added to a satu-

rated solution of sodium chloride, and the slurry either heated to boiling and then filtered, or stirred for 30 minutes a t room temperature before filtration. The results showed that ammonium cryolite (F , 56.7 to

0.09%) was converted to specification-grade sodium cryolite when an excess (50- to 100% ) of sodium chloride was used. The range of compositions of the sodium cryolite products was : F, 51.9 to 52.5; Na, 27.8 to

to 0.09%. More than 97% of the fluorine was recovered.

Ammonium cryolite is converted to aluminum fluoride by thermal decomposition a t about 550" C (7). By this method, half of the fluorine goes to aluminum fluoride, and the other half which may be recycled,

The preferred method involves heating ammonium i th aluminum hydroxide or with alumina, as

F, + Al(OH), -+ ZAIF, + 3NH3 + 3Hz0

57.5; AI, 13.8 to 14.0; NH,, 25.8 to 25.9; PzO5, 0.03 to 3, show that very

from with the

32.3, AI, 12.5 to 12.8; NH,, <0.4; C1, <0.4; P205, 0.03 e Obtained when 95% Of the thee- aluminum was added to a liquor

io F :P205 of 10. Also, specification- prepared when F:p2Gs

ranged between 4.6 and 5, and 95% Of Was used- Moreover, SPecifica-

volatilizes as ammonium bifluoride.

by the following reactions :

minutes a t 525" C., and that recovery of fluorine in the calcined product increased from 92- to 98% as the mole ratio A1 (OH) : (NH,) ,AlF, increased from 1 to 2. Activated alumina, AI20,*0.5H20, reacted with ammonium cryolite about as rapidly as did aluminum hydroxide, but calcined alumina reacted more slowly

ammonium cryolite was removed from the slurry by filtration. The

A1 ENGINEERING PROGRESS (Vol 66, No 11) November 1970 61

PHOSPHATE EFFLUENT GAS

(SiFq AND/OR HF) I L ' SCRUBBER

NH4F + (NH4)z S I Fg SOLN pH 5 T 0 6

I NH3 I I

DH 4 TO 7 L . . . . - J

LIME I I

>IOO"C -- WASTE -+- - FILTRATE - --

PpO,, 0. I % MAX

5 5 0 " TO 6OO0C

A I F3 PREFERRED PRODUCT

Figure 4. Ammonium cryolite and aluminum fluoride recovered from waste liquors.

duction cells instead of using aluminum fluoride, pro- vided the cryolite is put in the cell below the bed of calcined alumina.

This conversion method was studied further in a small rotary kiln. The kiln, 8 in. i.d. and 6 ft. long, was made of 10-gauge A.I.S.I. Type 316 stainless steel, and heated externally by electrical equipment. It contained flights to impart a rolling motion to the bed of solids and had 2 in. retention rings. Air was drawn through the kiln cocurrent with the solids, and the gaseous reaction products were collected in two scrubbing towers in series.

In the pilot-plant tests, the temperature ranged from 400"- to.640" C, the retention time ranged from 10 to 45 minutes, and the mole ratio AI (OH) : ( NH4) AIF, ranged from 1.0 to 1.25. The best products were obtained when the calcination temperature was in the range of 540"- to 600" C, the mole ratio AI(OH),: (NH,),AIF, was in the range 1.0 to 1.1, and the mixture was calcined for a minimum of 20 minutes. Under these conditions the products contained a t least 95% of the input fluorine, more than 90% of which was in the form of aluminum fluoride. About 95% of the input ammonia was volatilized and could be recycled to the fluorine scrubbers. A flow diagram of the ammonium cryolite-aluminum fluoride process is given in Fig- ure 4.

Evaluation Commercial fluorine compounds (cryolite and alu-

minum fluoride) can be recovered from phosphorus plant fluorine wastes. Several processes have been

62 November 1970

described in which fluorine compounds are recovered pure enough to meet stringent specifications required in the production of aluminum. In the recovery processes, waste gases are scrubbed with ammonia solution to form ammonium fluoride and ammonium fluosilicate. Then the scrubber solutions are ammoniated further to precipitate silicon and iron compounds, and these impurities are removed by filtration. The subsequent steps used to recover specification-grade products from the purified solution depend upon its P206 content. Alternate steps are described for using ammonium fluoride solution with high P205 content.

At TVA it has not been economical to recover commercial fluorine compounds from the scrubber solutions. The amount of fluorine available for valuable product recovery is small. Too, the cost of treating the waste water is low because granulated slag used to neutralize the waste water and remove the fluorine is itself a waste material. At other plants, substantially larger amounts of fluorine may be available for recovery and the economics of valuable product recovery may be more favorable. With other phosphates and the use of higher phosphate calcination temperatures, as much as 30- to 40% of the fluorine in the phosphate may be driven off instead of 8% volatilization obtained a t TVA. The fluorine available for product recovery may amount to 200 Ib. F/ ton of phosphorus, or higher, instead of 60 Ib./ton as at TVA. Also, other plants produce considerably larger amounts of phosphorus than is produced a t TVA, and this will improve the economics of fluorine recovery.

Costs for fluoride disposal are increasing because stringent pollution abatement standards are being put into effect. Double liming of waste water is required a t some places because the fluoride concentration of the waste water must be reduced to low values. The increases in disposal costs also make valuable product recovery more attractive. #

Literature cited 1. Lehr J. R.. G. H. McClellan. J. P. Smith, and A. W. Frazier,

Colldque International Sur le8 Phosphatee Mineraux Solidea, 2 , pp. 29-44, Toulouse, France (May 16-20, 1967).

2. Barber, J. C.. Chem. Eng. Progr., No. 9, 64, 78 (1968). 3. , Chem. Eng. Progr., No. 6 , 65, i 0 iiS69j. 4. Gartrell, F. E., and J. C. Barber, Chem. Eng. Progr.. No. 10, 62,

A A 11Qfifib _ _ ~-""",. 5. Grant. H. 0.. Chem. Eng. Progr., No. 1, 60, 53 (1964). 6. Sanders, M. D., and W. C. Weber, Proc.: ISMA Technical Confer-

ence, Helsinki, Finland (September 3-5. 1963). 7. Tarbutton, Grady, Thad D. Farr, Thomas M. Jones, and Harry T.

Lewis, Jr., Znd and Eng. Chem., No. 10, 50, 1,526 (1958). 8. Heil. F. G., R. D. Young, and J . J. Stumpe. Ag. and Food Chem.,

No. 6, 9, 457 (1961). 9. Simons. J H., Fluorine Chemistry". 1, p. 41, ,Academic Press,

New York (1960).

J. C. Barber is a member of the M ager's staff at TVA's National Fertili Development Center. He has bee N A since graduation from the G Institute of Technology. Barber author of several papers dealin fertilizer development and phos production technology.

T. D. Farr is a research physica at N A ' s National Fertilizer Dev Center. He received his master'

CHEMICAL ENGINEERING PROGRESS (Vol. 66, No 1

fluoride recovery from phosphorus production

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