250
Ind. Eng. Chem. Prod. Res. Dev.
Musaev, M. R., Zizin, V. G., Zh. Prikl. Khim., 29, 803 (1956). Navruzov, K., et. al., U.S.S.R. Patent 287911 (1970). Ocon, E. A., U S . Patent 2299844 (1942). Olah, G. A., Kaspi, J., Bukaia, J., J . Org. Chem., 42, 4187 (1977). O'Sullivan, D. A., Chem. Eng. News, 57 (17),1 1 (1979). Pearson, D. E., J. Chem. SOC. Chem. Commun., 397 (1974). Pearson, D. E., U.S. Patent 4 133 838 (1979). Schriesheirn, A., in G. A. Olah, "Friedel-Crafts and Related Reactions", Interscience, New York, 1964. Sean, J. W., U.S. Patent 2373475 (1945). Tongberg, C. O.,Pickers, J. D., Fenske, M. R., Whitmore, F. C., J. Am. Chem. SOC.,54, 3136 (1932). Tongberg. C. O., Pickers, J. D., Fenske, M. R., Whitmore, F. C., J. Am. Chem. SOC., 54, 3706 (1932).
1980, 19, 250-255
Tongberg, C. O., Pickers, J. D., Fenske, M. R., Whitmore, F. C., J. Am. Chem. Soc.. 54. 3710 (1932). Tsukervanik, J. P., 2. Gen. Chem. U.S.S.R., 15, 699 (1945). Van Wazer, J. R., "Phosphorus and Its Compounds", Vol. I , Interscience, New York, 1958. Venkaturarnu, S. D., Cleveland, J. H., Pearson, D. E., J. Org. Chem., 44, 3082
(1979). Wornbles, R. H., M.S. Thesis, Vanderbik Univeristy, 1975. Worthy, W., Chem. Eng. News, 57 (35),20 (1979). Zubritskii, K. V., Tr. Voronezh. Med. Inst., 87, 79 (1972).
Received for review October 18, 1979 Accepted February 21, 1980
Calcium Fluoride Production in a Phosphoric Acid Plant P. S. O'Neill Agrico Chemical Company, Faustina Works, Donaldsonville, Louisiana 70346
This paper describes a new process developed by Agrico Chemical Co. for the manufacture of a metallurgical type fluorspar for use in the steel industry from phosphoric acid plant pond water. In the Agrico process, pond water is treated with finely ground calcium carbonate and the resulting raw precipitate is separated and washed. The dried product contains a minimum 70% effective CaF, and a maximum 0.3% S. It may be in the form of a wet cake or dry briquets. The substantially defluorinated water is processed without further lime consumption to precipitate and settle co-product silica which is consolidated with the gypsum byproduct of phosphoric acid manufacture. The clarified liquor which retains 50-90% of the phosphate in the original water is returned to the pond system. The Agrico process can be used in conjunction with a conventional lime treatment process for water control and disposal or may be operated as an independent plant. This process provides a way to produce fluorspar from US. domestic sources to replace a large part of the imports which supply 80% of present US. consumption.
Fluorine in Phosphoric Acid Processing A typical Florida phosphate rock contains 3.7% F which is equivalent to 250 lb of fluorine per ton of product Pz06 in phosphoric acid. Table I shows the distribution of fluorine in the manufacture of wet process phosphoric acid in a large modern plant. As there is no evidence (Liner0 and Baker, 1978) for significant environmental losses in a properly operated plant, the streams shown account for 100% of the fluorine input. The fluorine contained in the product acid and byproduct gypsum is not recoverable in current practice. The vaporized fluorine is recovered in recirculated scrubber water mostly in the operation of the evaporators. The scrubber water also serves the purpose of cooling the barometric condenser. Because of the relatively enormous volumes needed for cooling, scrubbing efficiency is usually very high. The contaminated water is reused after cooling in a storage pond. Its fluorine content reaches a steady level of about 1% , due mainly to precipitation phenomena (Frazier et al., 1977). Many phosphoric acid plants feature a fluorine recovery unit on the evaporator train, usually of the Swift/Swensen type (Sanders and Weber, 1963; Lihou, 1964; Bidder and Hallsworth, 1974). Using recirculated fluosilicic acid as scrubbing liquor, up to about 15% of the fluorine introduced with the rock is recovered as a clean almost pure solution of fluosilicic acid containing 20-2570 H,SiF6. Of the 52% of fluorine input potentially recoverable, usually 10-15% is collected as fluosilicic acid and the remainder (about 40% or 100 lb/ton of PzO,) is trapped in the recirculated pond water. The fluorine in pond water has essentially zero value and is not at present recovered for conversion into a commercial product. 0196-4321/80/1219-0250$01,00/0
Table I. Fluorine Distribution Phosphoric Acid Plant
t
reactor vapors 52% by wt evaporator vapors 22 product acid (54%P 2 0 , ) gypsum byproduct 26 basis: 1000 tpd Prayon plant source: Agrico Chemical Co. data
Calcium Fluoride Manufacture from Fluosilicic Acid Calcium fluoride is attractive as a byproduct to a phosphoric acid manufacturer. It is a stable solid usually handled in bulk. It may be stored outdoors and in most respects resembles ground phosphate rock. In the form of a wet filter cake it shows minimal dusting problems. It is known in its ore form as fluorspar. The basic reaction of calcium carbonate with fluosilicic acid to produce calcium fluoride is 3CaC0, + HzSiF6+ ( n - l)HzO 3CaF, + SiOz.nH,O + 3c02 (1)
-
Calcium carbonate is the most economical source of calcium. Published literature (Hellberg et al., 1976; Sprecklemeyer, 1974; Schneider and Niederprun, 1977; Becker and Massonne, 1977; Nash and Blake, 1977; Butt, 1957; Gloss, 1957; Zaitsev et al., 1978) shows methods for conducting the reaction which involve dilution of the feed acid or use of a limestone slurry. Short reaction times are desirable to minimize contamination of the product with co-precipitated silica. The separation of the calcium fluoride can present problems. Beneficial effects are reported in the presence of aluminum and sulfate ions (Becker and Massonne, 1977). 0 1980 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980
A
I I
-
251
I
I
I
I
I
I I
I
STORAGE
I I I
I
I
I
I
SETTLER (2)
TREATMENT
SPENT WASH
* WASH WASTE
Ca SO, .ZH@
CdOH), NaOH,NH-
ALKALI WASH
:.
FILTER
1I
DRIER
1
I
Figure 1. Fluorspar process flowsheet. Table 11. Fluorspar Gradesa grade
CaF, content
physical form
(1)acid-spar ( 2 ) met-spar
97% min ( a ) 60-85% ( b ) 70-80%
flotation filter cake gravel briquets
__-
Small quantities of a ceramic grade filter cake con. taining 88-9270 CaF, are also marketed. a
Pure fluosilicic acid, while yielding a high grade product, is an expensive raw material. Some attention has been directed in the past to the conversion of the fluorine values in the zero-value recirculated pond water into calcium fluoride (Mills and Hirko, 1975; Legal, 1973; Baumann and Bird, 1970; Hillyer and Wilson, 1959). The methods published require co, production of calcium phosphate containing silica and feature a total cleanup of pond water. No method describes the production of fluorspar which meets current market specifications. Present Calcium Fluoride Usage Calcium fluoride is the basic raw material for the fluorine chemicals industry and is the predominant fluxing agent used in steel furnace operation. These two outlets constitute more than 85% of consumption in the U S . which was reported in 1978 in the region 1.2-1.4 mm tons. Approximately 80% of this is imported coming chiefly from Mexico and South Africa (Singleton, 1979). The industry is based on two grades as shown in Table 11; each represents about one half of total consumption. The chemical specifications published for fluorspar are of great relevance to the manufacture of synthetic spar and are shown in Table 111 (Wood, 1975). As will be shown later, calcium sulfate and phosphate are major contaminants of fluorspar recovered from phosphate operations. The specification quoted for sulfur is usually understood to refer to sulfides or elemental sulfur. However, contacts with steel manufacturers have indicated a reluctance to handle sulfur in any form. Phosphorus limits are not usually quoted. Operators in the steel industry prefer to avoid phosphorus because of its embrittling effect on steel. A level of about 1% P seems to be the maximum acceptable at present for general steelmaking purposes. Details of the Agrico Process Agrico Chemical Co. has undertaken the development of a process which would allow conversion of the fluorine
Table 111. Chemical SDecifications of Fluorsuar component acid-spar met-spar CaF, min. eff.a SiO, max CaCO, max S max heavy metal oxide max a % CaF, min. eff. by wt.
-5
97 1.0 1.25 0.03 0.4
(70CaF, actual
-
2.5
70 0.3 0.4 X %
SO,) %
in pond water into an acceptable grade of fluorspar subject to the considerations that (a) there be no interference with phosphoric acid plant operation or output, (2) there will be only one product and no significant loss of phosphate values already solubilized in the water, and (c) the process has economic attractions independent of any presumed environmental or other operational benefit. An outline flowsheet of the Agrico process is shown in Figure 1. Some of the stages shown may not be necessary depending on pond water quality and whether a briquetted product is desired or not. Pond water is fed directly to the main reactor along with ground calcium carbonate. The resulting slurry is conveyed to a settler. The solids-rich underflow is pumped to the wash stages and the overflow is sent to the silica precipitation section. If the pond water feed is high in sulfate content, Le., more than 0.4% SO4,pretreatment with calcium carbonate is required to precipitate the excess sulfate as gypsum. This can be done using a small mix tank feeding a 24-h holding area which could be part of the pond system. Careful control of the carbonate addition is necessary to minimize fluoride loss. In the acid wash step, the calcium fluoride rich sludge is resuspended with fresh pond water or pond water containing a small quantity of fluosilicic acid or an acidic fluosilicate-containing stream such as produced in the operation of a fluosilicate salt plant. The resulting slurry is thickened in a settler or hydroclone and the solids-rich fraction is conveyed to the sulfate wash unit. Here the sludge is treated with a recycled alkaline solution. The sulfate is dissolved giving a final solid product having a maximum of 0.3% S, dry basis. Small quantities of an alkali such as caustic soda or ammonia are consumed. Regeneration of the spent washings with hydrated lime keeps this cost to a minimum.
252
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980
Table IV. Phosphoric Acid Plant-Pond
Water Analyses"
_______
gypsum pond
Looling pond
F PZO, %SO, SiO, Na, 0 CaO
0.923% 0.968 0.541 0.468 0.228
1.031% 0.815 0.436 0.515 0.135 0.129 0.090 0.034 0.010 0.031
4
a
-
component
0
0.180
0.064 0.037 0.040 0.019
3
GROUND OYSTER SHELL I C a C 0 3 ) F NENESS
100%- 100 U S , STD MESH
-x-
4 . 70%--200 4-
99%-325
' "
'
'
"
Source: Agrico, June-Dec 1977
0 9
I
P2 \
4
SO,
oo! 05
I
I
10
15
I
20
T I M E , HOURS
a
t
\
Figure 3, Fluorspar precipitation, effect of CaC03 fineness batch experiments, 30 "C. Table V . Acid Wash-Phosphate wash conditions _
_
_
Reduction ~
solid (dry basis) % P,O,
g'g wet solid unwashed washed ~
liquor 0
20
IO
CaC03 1-325 MESH) g / I
30
40
ADDED
Figure 2. Precipitation of fluoride, phosphate and sulfate salts from pond water, batch experiments, 30 "C.
The washed product slurry is filtered using a rotary vacuum filter, and the cake obtained contains between 30 and 40% moisture. This is then dried in conventional drying equipment to a final moisture content of 10%. The defluorinated silica-laden water from the first part of the process is mixed with raw pond water in the proportion of about 40% pond water to 60% treated water giving a liquor having a pH in the range 1.6 to 1.8. Silica separation is slow under these conditions and a settle-able precipitate is obtained. A small lagoon is needed to provide the 7-day settling time. In excess of 80% of the liquor may be overflowed as clarified water and returned to the pond system. The remainder may be centrifuged to yield a sludge containing 10% silica which can be mixed with the gypsum byproduct from the phosphoric acid plant. A chemical analysis of two pond waters obtained a t one of our plants is shown in Table IV. This plant has a two-pond system. However, for various reasons a lot of cross-mixing occurs. The gypsum pond shows high P and S levels whereas the F level is higher in the cooling water pond. The basic chemical reactions occurring during the treatment of pond water with calcium carbonate are shown in eq 1 to 4, CaC03 + H3P04+ HzO CaHPO4.2HZ0+ COz ( 2 ) CaC03 + H2S04+ HzO CaSO4-2HZO+COz (3) H4Si04+ ( n - 2)H20 Si02.nHz0 (4) -+
--
pond water pond water + 25% H,SiF, ( 9 : l j fluosilicate plant waste pH 1.4, 1%F
20 7
8.09 10.97
6.04 3.54
10
8.06
4.20
Figure 2 illustrates the behavior of the principal elements F, P, and S with increasing additions of finely ground calcium carbonate. The pH change is not a reliable guide to the extent of the reaction. Phosphate precipitation and redissolution results in rapid advance of the p H to values in the range 2.5-3.0 followed by slow adjustment to 2.4-2.6. As the diagram shows, about 80% of the fluoride can be removed before substantial phosphate precipitation occurs. A very significant factor in the outcome of these experiments is the particle size distribution of the calcium carbonate used. Experiments using ground oyster shell of three commercial size ranges is shown in Figure 3. The coarser the carbonate is, the less fluoride is precipitated. The sulfate in the solution seems to coat the particles so that even under prolonged agitation as much as 10% of the dried solid phase is unreacted carbonate, using a -200 mesh feed. The use of a more thoroughly ground calcium carbonate results in more rapid reaction, better fluoride recovery, less contamination with phosphate and sulfate, and a minimum of unreacted calcium carbonate. The raw precipitate settles rapidly. The acid wash for the purpose of phosphate reduction is based on well-known chemistry. Table V shows the outcome of some typical washing experiments. Pond water is an appropriate wash liquor. Pond water containing small additions of fluosilicic acid is very effective. The phosphate in the product is probably dicalcium phosphate CaHP04.2H20.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 2, 1980
Table VI. Calcium Flluoride-Dry Solid Analyses" component dried ppt F p, os
washed product
35.3% 8.8% 6.6%
so4
37.0% = 76.0% CaF, 4 . 0 % = 1.7% P 0 . 9 % - 0.3% S
Other elements, ranges %: CaO, 50-55; A1,0,, 2.54.0; SiO,, 1.0-2.0; Na,O, 0.8-1.1; MgO, 0.4-0.6; F e 2 0 3 , 0.4-0.6;K20,0.1-0.2;Sb, 152ppm;Pb,71;Ba, 34;As,4. a
Table VII. Re-Treatment of Pond Water for Sulfate Reduction CaCO, added, time, gIL h 12 72 24 24 1
10 10 10 10 32
Table VIII.
pond water % F %SO,
supernatant
%F
%SO,
1.00 0.99 0.96 1.03 0.98
0.85 0.79 0.84 1.01 0.07
0.38 0.30 0.40 0.42 0.48
0.80 0.40 0.54 0.81 0.72
Silica Separation Data" 4 days
filtrate, 33 "C pond water :filtrate 1 . O : l 25 "C 0 . 5 : l 25 " C
1 4 days
A
B
4.86
26
3.52 3.31
57 62
B
A
41 5.15 5.23
5 days
39 53
7 days
0 . 4 : l 33 "C 3.65 A, F:Si mole ratio in solution B, % clarification
78
5.28
80
a The first three entries are the results of 4-L beaker tests. The last item is, t h e outcome of a 55-gal drum test, and shows better clarification because of t h e greater depth of liquid available for settling.
The sludge from this step containing 50-60% moisture is then washed to reinove sulfate ion. This is performed preferably in a two-shge countercurrent arrangement. The wash used is a dilutt: alkali solution. The solid product after filtration and drying contains no more than 0.3% S. This is probably in the form of gypsum CaSO4.2H2O. The filter cake normally contains 3C-40% moisture and is dried to 10% moisture. This is the form in which most acid spar is sold currently. Pressure filtration tests have
shown promise of direct production of a cake containing 15-2070 moisture, which would require a minimum of drying. Table VI shows an analysis of both the raw precipitate and the washed product. Two other areas requiring further explanation are the pretreatment step and the silica separation stage. Pretreatment is necessary if the pond water contains sulfate levels in excess of 0.4%. It has been demonstrated in the patent literature (Mills and Hirko, 1975) that substantial quantities of calcium carbonate may be dissolved in pond waters without immediate precipitation. We have found with pond waters containing 0.5-1.0% SO4 that supersaturation with respect to gypsum occurs under these conditions. A precipitate is obtained after a period of 24 h and the liquor shows a sulfate level of 0.4% wit,h little loss of fluoride. Table VI1 shows some typical data. Silica separation presented many problems. The aging of the defluorinated liquor from the calcium fluoride precipitation step results in the formation of a gel. The return of this liquor directly to the pond system would therefore be unacceptable. Literature data shows (Iler, 1955) that the rate at which monomeric dissolved silica polymerizes and forms either gels or precipitates is strongly pH dependent. It is slowest in the range 1.5-2.0. We have been able to form settle-able precipitates in mixtures of treated and untreated pond water in proportions such that the pH is in this range. Table VI11 shows the results of some relevant experiments. The rate of formation is slow, however, so that large volumes must be retained. A small lagoon is probably appropriate. The main step of the process has been operated continuously for periods of 8-12 h over 5-day periods in a small pilot plant using a 20-gal reactor. Flow rates up to 0.5 gpm of pond water were used. Over 500 lb of product has been produced. Table IX shows average results for three runs. The wash stages have been performed on beaker scale and in batches of 40 gal a t a time. The partially dried solid from the precipitation stage has been tested in a commercial briquetting press and conditions under which it may be converted to a satisfactory briquet have been demonstrated. Besides elemental analysis, the dry solid has been characterized using X-ray powder diffraction studies. See Table X. The predominant crystalline species is fluorite,
Table IX. PrecipitatorSemicontinuous ODeration" CaCO pond water 0.986 F 0.689 P,O, 0.420 SO4 l0.954 F 0.691 P,O, 0.400 SO, 0.948 F 0.692 P,O,
253
____product
gl L
fineness
residence time, h
25.3
-325
1.0
25.2
71.1
8.57
6.37
25.5
-325
1.2
25.0
70.2
8.97
5.76
25.6
--200
3.5
24.0
58.7
6.50
g/L
% CaF,
% P,O,
%
so,
I
{
i
% CaCO,
13.8
Analysis of the solid was carried o u t on product dried to constant weight a t 100 "C. Table X. X-ray Powder Diffraction Data (Intensity % of Maximum Peak) d, a
__
sample dried ppt washed dried ppt CaF, (fluorite)a
4.02 2
3.360
3.153
5.8
100 99 94
2.13
1.931
1.647
1.366
1
90 100 100
23 32 35
12
1.115 _______ 16
CaF, (fluorite)taken from "Crystallographic Properties of Fertilizer Compounds". Chemical Engineer Bulletin No. 6. TVA (May, 1967), J. 13. Lehr, L.S. H. Brown, A. W. Frazier, J. P. Smith, and R. D. Thresher. a
254
Ind. Eng. Chem. Prod. Res Dev., Vol. 19, No. 2, 1980
CaF,. There was no evidence for significant quantities of a crystal obchukhrovite Ca4S04(AIF6)(SiF6)F.10Hz0, served in a number of phosphoric acid plant streams (Fairweather, 1937; Lehr et al., 1966; Frazier et al., 197’7). Raw Material Requirements While the process may be operated on many different ponds, some general criteria may be established for optimum operation. A high F/P205ratio in the water is desirable. A sulfate level in the range 0.3 to 0.4% SO4 is necessary to ensure easily settled and filtered fluorspar. A high F content is attractive in order to minimize the volumes of liquid to be handled. However, as levels of 2.0% F are approached, the silica byproduct becomes less stable in solution and tends to form a gel over a period of 2-3 h. This could interfere with calcium fluoride separation. A number of the commercially available ground limestone or oyster shell materials would be suitable for use in the process. The fineness required would demand 99% passing a 325 mesh standard sieve. However, up to 50% could be coarser material (90% passing 200 mesh) in a two-reactor setup where the finer carbonate was added to the second tank. The requirements in regard to elemental composition would place limits on silica and sulfur contents. Silica would have to be no higher than 1.0% SiO, and sulfur preferably zero. Hydrated lime could be used instead of finely ground calcium carbonate. It is advantageous, however, to have part of the feed as calcium carbonate as it seems to have a beneficial effect on the separation properties of the precipitate. Production Rates The operation of this process will undoubtedly have a significant impact on the pond system. This will depend on the detailed operation of the system. Basically, two types of pond are in use as described previously, a single multi-purpose pond and a dual system where cooling water is segregated to a greater or lesser extent. A computer modeling technique has been published (Berry and Busot, 1976) which is based on the use of a water and elemental mass balance to predict fluoride and phosphate steady-state levels. Application of a simplified version of this model to data for one of Agrico’s plants which features a dual system but which has been operated with considerable cross-mixing predicts higher levels than actually observed, suggesting some direct precipitation losses. An analysis of pond chemistry published by Lehr and his co-workers at the TVA (Frazier et al., 1977; Lehr, 1978) suggests initial precipitation of fluorides as alkali metal fluosilicates. These salts are congruently soluble and in the pond system could redissolve. Typical water analyses show concentrations expected for saturation levels of these salts as shown in Table V. Such pond water is in contact with group I1 and I11 metal salts over long periods in a pond system. The establishment of new equilibria dominated by much less soluble calcium, aluminum, and magnesium fluorides and fluosilicates will result in permanent immobilization by precipitation principally as fluorite and chukhrovite. The operation of a fluorspar process as described might prevent such largely irreversible precipitation by lowering the F and A1203 concentrations. The fluorine already precipitated as sodium and potassium fluosilicates or retained by the gypsum moisture might be partially recovered during the time such a process was operating. However, while these factors would have a positive effect on the amount of fluorine recoverable they are not quan-
Table XI. Chemical Analyses of BO F Slags and Metalsa ~
~~~
slag expt mar
S
BOF-1 mineral BOF-2 Agrico
0.096 0.100
F
~
metal
P
CaO
1 . 5 0.26 42.1 1 . 2 0.51 53.2
S
P
0.011
