CA1045258A

CA1045258A - Recovery of fluorine and phosphate values from waste water

Info

Publication number: CA1045258A
Application number: CA221,300A
Authority: CA
Inventors: Ronald J. Hirko; Harold E. Mills
Original assignee: Occidental Petroleum Corp
Current assignee: Occidental Petroleum Corp
Priority date: 1974-04-03
Filing date: 1975-03-05
Publication date: 1978-12-26
Also published as: FR2266662B1; NL7503991A; JPS50142496A; GB1505146A; FR2266662A1; BE827305A

Abstract

ABSTRACT
Calcium fluoride is produced from pond waters resulting from phosphoric acid processing by treating the pond waters with calcium carbonate in two stages to precipitate out at least 80% of the fluorine values from the waters as calcium fluoride. After removal of the calcium fluoride the filtrate is treated with calcium oxide to remove a substantial portion of the remaining fluorine values as calcium fluoride.
After removal of these calcium fluoride solids, the filtrate is treated with another charge of calcium oxide to produce dicalcium phosphate (dical) which is separated from the aqueous phase. The aqueous phase is treated with an additional charge of calcium oxide to remove a substantial portion of the solids from the aqueous phase leaving waters that can be discharged as waste or recycled as process waters.

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Description

~0~5Z58 The present invention is concerned with recovering fluorine and phosphate values from the waste waters ("pond waters") resulting from the production of wet process phosphoric acid, having as an objective the recovery of such values in the form . of calcium fluoride and calcium orthophosphate while neutralizing the waste waters so they can be discharged into rivers and streams ; without fear of pollution, or be recycled as process waters in the production of phosphoric acidO
A wet process phosphoric acid plant using the dihydrate technique uses large amounts of water, typically at a rate of about 50 gallons per minute for each unit of plant output " capacity as measured in product P205 tons per day: this is equivalent to about 210 litres per minute per unit of plant ^. output capacity expressed in tonnes per day~ Much of this water is repeatedly recycled but a substantial amount is continually directed to a waste pond to restrict accumulation - ~ -of contaminants in the water. The pond water contains from about 0.1 to about 5% fluorine, from about 0.1 to about 5%
P205, from about Ool to about 2.5% SiO2, from about 0.1 to `~ 20 about 0.5% dissolved calcium and from about 0.1 to about 0.5%
soluble sulphate saltsO The fluorine content of such pond water is of major concern because it can present an ecological hazard, while the P205 content both represents a loss of a ~ ~ valuable product and an ecological hazardO
i Traditionally pond waters have been passed through settling basins prior to their discharge into rivers and streams.
At.times, pond waters are treated with limestone and lime to precipitate out fluorine and other values before discharge to < meet pollution control laws and regulationsO In the settling basins, the amounts of the various chemical values, such as i - 2 -r~

t 1045'~58 ~
fluorine, P205, calcium, etc. in the water decrease so that the pond water when discharged into the streams contains lesser, but appreciable, amounts of these materials. Not only does this discharge of pond waters add chemical values to streams, but it also causes a decrease in the pH of the streams. Pond water typically is acidic and has a pH from about 1 to about 3.
Workers in the art have recognized the economic loss and ecological problem of pond waters and have developed methods of treating pond watersO However, it appears that none of these methods have been economically attractive or feasible since none of the methods are in commercial use in the United StatesO For example, DoRo Randolph developed a method which is disclosed in U.S. Patent 3,625,6480 The Randolph method comprises treating pond water with milk of lime to adjust the pH of the resulting ::
slurry to between about 3.2 and 3.5 whereby 99% of the available fluorine is precipitated out as calcium fluorideO The calcium fluoride is separated from the aqueous phase and treated with sulphuric acid, or other strong acid, to liberate the hydrogen ;~
fluoride (HF) gas and yield a slurry of gypsum, sulphuric acid ~ 2a and phosphoric acid. The latter slurry can be recycled back ; into a conventional wet acid phosphoric acid process to recover -~
the P205 values. The HF gas can be upgraded by conventional -~
methods. The aqueous phase, after removal of the calcium ~ -fluoride, is treated with an additional 10% milk of lime to adjust the pH to between 4.7 and 5 to precipitate out dicalcium phosphate. Dicalcium phosphate is separated from the aqueous phase and is upgraded in a conventional dicalcium phosphate plant ~` or cycled to a conventional wet process phosphoric acid plant to ;
recover the phosphate values~ The aqueous slurry is then treated ` 30 with additional milk of lime to adjust the pH to between 6 and 7 ;~

P: , .~, , .

10~5258 wherein further solids precipitate out, such as gypsum. The solids are sepa-rated from the now almost neutral aqueous phase and passed to waste. The aqueous phase is then recycled as process water to the phosphoric acid plant or discharged into streams or rivers.
The object of the present invention is to provide an economic method of treating phosphate waste waters so as to recover many of the valu-able chemical values therein which are ecologically undesirable products in streams and lakes. The present invention also aims to provide a method of rendering the pond waters neutral so that they can be discharged into streams or rivers or recycled as process water.
Accordingly the present invention provides for the method of treating waste water containing fluorine and P205 values that comprises:
in a first stage, adding with agitation to the waste water a suitable basic inorganic calcium compound in an amount to provide between 0.3 and 0.8 equivalents of calcium per equivalent of fluorine ~F) values in the water;
thereafter, in a second stage, agitating the calcium treated waste water effluent of said first stage with additional said calcium compound, in an amount to form calcium fluoride solids and such that the total calcium com-pound addition in the two stages provides not less than 0.8 equivalents of calcium per equivalent of F values in the waste water; and separating the solids from the aqueous phase following said second stage.
According to a preferred embodiment of the present invention there is provided a method of treating waste water containing fluorine and P205 values, comprising forming an aqueous calcium carbonate slurry containing 5% to 50~ calcium carbonate; then in a first stage, agitating said waste water with an amount of said calcium carbonate slurry to provide between 0.3 and 0.8 equivalents of calcium per equivalent of fluorine (F) values in the water; thereafter, in a second stage, agitating the calcium carbonate-treated waste water with additional said calcium carbonate slurry in an amount to form calcium fluoride solids and such that the total slurry addition in the two stages provides not less than 0.8 equivalents of calcium per equivalent of F values in the waste water; and separating the solids from the aqueous - 4 _ ,~'' : , ~045Z~i8 phase following said second stage.
In typical practice of the method, pond water is treated with cal-cium carbonate, such as ground limestone or an aqueous slurry of ground lime-stone, in a first stage to form calcium salts containing phosphate and fluorine values. Between about 30 and about 80% of the stoichiometric or equivalent amount and preferably about 40% of such amount of calcium per equivalent of fluorine in the pond water is added in the first reaction stage.
The reaction is normally carried out at ambient or equilibrium temperatures;
however, the reaction can be carried out at any temperature between the freezing temperature of the pond water and its boiling point. The residence time for this reaction stage is from about one-half minute to about 60 minutes, being preferably about 5 minutesO The pond water and the calcium carbonate are agitated to ensure maximum reaction between the calcium carbon-ate and the pond water valuesO
The treated pond water is then passed to a second stage wherein additional calcium carbonate is added so that no less than about 80% equiva-lent of calcium carbonate total per equivalent of fluorine is added in the two steps to the pond water. The reaction in the second stage is also normally carried out at atmospheric or ambient temperatures; however, the reaction can be carried out at any temperature between the freezing point of ~
the treated pond water and its boiling point. The residence time in the ;
second stage is from about one-half minute to about 60 minutes, being prefer-ably about 30 minutes.
In the second stage a slurry is formed containing solid calcium fluoride and some solid calcium phosphate values. This slurry is passed to a separation stage wherein the solids are separated from the aqueous phase.
The resulting solid cake is washed in an acid wash stage with a dilute solution of a strong mineral acid, such as sulphuric acid, to remove phosphate ` and other values from the solid calcium fluoride phase to raise the F/P2O5 weight ratio thereof. The solid phase is subsequently washed with water in a water wash stage to substantially remove most of soluble values, leaving a solid product containing up to about 45% by weight fluorine as calcium , ,: :

~045258 fluoride.
The wash acid and the wash water are combined with the aqueous phase in a mixer wherein they are mixed together to form a first stream which is passed to a third stage wherein it is treated with calcium oxide, such as ground lime OT aqueous lime slurry, to raise the pH of the first stream to between about 3 and about 4 preferably between 3.6 and 3.8, the pH of the first stream being initially between about 108 and about 206 because of the addition of wash acidO In the third stage, additional calcium fluoride is formed which is subsequently separated from the stream in a separator stage and recycled to the acid wash stage where it is subsequently treated to the acid washing and water washingO
The aqueous phase from the first separator stage is passed as a second stream to a fourth reactor stage wherein it is mixed with a calcium oxide slurry to adjust the pH of the resulting mixture to between about 5 and about 8, preferably from about 6~5 to about 7020 In the fourth stage, calcium orthophosphate ~"dical" or dicalcium phosphate) precipitates out and is subsequently separated from the stream in a second separator stage to yield dical solid. The aqueous phase from the second separator is passed as a third stream to a fifth reactor stage wherein the third stream is combined with slurried calcium oxide to adjust the pH to between about 8 and about llo The calcium oxide is normally added to the third, fourth and fifth stages as a 5 to 50% by weight aqueous slurry, preferably about 10 to about 25% slurry.
The calcium oxide slurry is prepared from fresh water, not pond waters or process waters containing F and P205 valuesO The mixture from the fifth stage is passed through a third separator stage wherein solids, mainly calcium values and silicon dioxide, are separated from the aqueous phase and passed `; to waste. The remaining aqueous phase is water which can be recycled back into a wet acid phosphoric acid.
The accompanying drawing is a schematic illustration of the method of the present inventionO
R~erring to the drawing, pond water is passed to the first reactor stage 10. Simultaneously, a stream of slurried calcium carbonate 12 . .

. , .

~O~S'~S8 is also passed to the reactor 10~ l`he stream 12 originates in a mixer 14 wherein a source of ground calcium carbonate, such as limestone, is mixed with fresh water, not pond water or process water containing F or P2O5 values, to form the calcium carbonate slurry. The calcium carbonate source is ground to at least a -10 mesh (U.S~ Standard), preferably to a 90% -200 mesh. The calcium carbonate slurry is formulated with a solids content of between about 5 and about 50%, preferably about 10 to about 25%. The calcium carbonate slurry and pond water are agitated in the reactor 10 for about 5 minutes, although shorter or longer reaction times can be employed. The amount of calcium carbonate added to reactor 10 is designated as Z1D Z1 represents the equivalents of calcium added to the pond water per equivalent of fluorine in the pond water. Zl typically has a value of from about 0.3 to about 008, preferably having a value of about 0~4O The reaction that occurs in reactor ~ -10 normally produces soluble calcium values such as CaSiF6, although in some instances insoluble values will be formed. Preferably the reaction is so ~`
managed that no solids will be formed in reactor lOo The reaction mixture from reactor 10 passed as a stream 16 to ~ ~ -. a second reactor stage 18 wherein the stream 16 is mixed with additional slurried calcium carbonate via stream 200 The stream 20 also originates from the mixer 14 The reaction mixture in reactor 18 is agitated for a period of about 30 minutes, although shorter or longer residence times as described ~ above can be employed. The amount of calcium carbonate added to reactor 18 is i designated as Z2- Z2 is equal to at least ZR minus Zl' where ZR designates the minimum amount, in stoichiometric equivalents, of calcium that can be added to reactors 10 and 18 to remove from about 85 to I00% of the fluorine values from the feed pond water. ZR' on a mineral acid-free basis, has a value equal to 1 + 0.19 Rw, where Rw is the weight ratio of P2O5 to fluorine in the pond waterO ZR usually has a value of from about l-to about 20 Z2 has a value ranging from about Ool to about 2, preferably being from about 0.4 to about 1.4; and an especially preferred range of Z2 values is from about 1 to about 1.4. In theory, the ZR valu; should provide sufficient calcium :,, .

to remove substantially all the fluorine from the pond water. However, as explained above, the pond water contains other values besides fluorine, such as P2O5 and sulphate, which can complex with the calcium and limit the amount of calcium available for reaction with the fluorine values. The ZR value takes account of those other values. Preferably more calcium is added, such as 5% excess, than calculated from the ZR value. However, excessive amounts of calcium are not necessary and interfere with the acid wash stage described belowO
The reaction mixture from reactor 18 is passed as a stream 24 to a first separator stage 260 At least 85% of the fluorine values in stream 24 are solid calcium fluoride, preferably at least 95% of the fluorine values being solid calcium fluoride. In addition, substantially all the iron and aluminium values in the stream 24 are present as solids, and about 50% of the sulphate valùes and about 25 to about 50% of the P2O5 values are present as solids.
Ths separator 26 is a conventional liquid solid separator, such as a filter, a centrifuge or a decanting tankO The calcium fluoride solids are separated from the liquid phase of stream 24 in separator 26 and passed as a stream 28 to an acid washer 30. The solids are washed with a stream 31 of a dilute aqueous solution of a strong mineral acid in the washer 30, the solids being washed with from about a 1% to about a 50% (w/w) mineral acid solution, preferably from about a 5% to about a 25% (w/w) acid solution. The calcium fluoride solids are preferably washed with an amount of mineral acid about equivalent to the P2O5 and CO2 values contained in the solids, plus or minus 10%. The acid concentration and wash temperature are so controlled that reaction between the fluoride values and the acid is minimized to prevent formation of hydrogen fluoride. The maximum wash temperature is about 180Fo (82C.). Although any strong mineral acid, such as hydrochloric, perchloric, nitric, sulphuric, hydrofluoric or other strong mineral acid, can be used in the wash step, sulphuric acid is preferred. In this acid wash step, a sub-stantial portion of the P2O5 values are removed from the solid product. In addition, other values, such as CaC03, MgO, Fe2O3, Al2O3 are also removed :
. , .

from the solid in varying amounts by the acicl wash.
The washed slurry stream 32 leaving the washer 30 is passed to separator 34 wherein the solids are separated from the aqueous phase.
Separator 34 is a conventional liquid-solid separator. The solids from separator 34 are passed as stream 36 to a water washer 38 wherein the solids are washed with fresh water, not process waters or pond watersO The solids are washed with at least an equal weight amount of water, preferably about

2 weights of water, although larger amounts of water can be used in the wash~
The water wash removes many of the soluble P205 values from the solidsO
The acid wash and water wash up-grade the calcium fluoride solids with respect to the percentage of contained fluorine and the F/P205 weight ratio.
The washed solids product of the washer 38, as a slurry, is passed to a third separator stage 42 as a stream 44O Separator 42 is a conventional liquid-solid separator such as the separator 26 described aboveO Solids are ~`
separated from stream 44 in separator 42 to yield a cake containing up to about 45% fluorine as calcium fluoride. The cake can be processed by conven-~ tional means Cnot shown) into hydrogen fluoride or it can be dried by conven-i tional means (not shown) at a temperature between about 90 and 150C. and stored or transported as a dry solid. The calcium fluoride cake is mostly calcium fluoride; however, the cake could also contain a small amount of P205 values (usually less than about 5%), silicon dioxide values ~usually less than about 3%) and sulphate valuesO
Thus the calcium fluoride solids of the cake are normally of suf-ficient purity to prepare hydrogen fluoride directly therefrom by heating the cake solids with sulphuric acid by conventional methods. If the P205 concen-tration is greater than about 5 to 10% or if the silicon dioxide concentration is greater than 5% in the cake solids, it is difficult to prepare good quality hydrogen fluoride from the calcium fluorlde solids by treatment with sulphuric acid, since at high temperature in an acid environment, phosphorus and silicon will react with fluorine to produce gaseous fluorine values such as PF30, and SiF4 which contaminate the hydrogen fluoride produced and are _ g _ lO~SZ58 difficult to remove. Accordingly, it is one of the achievements of the present invention that a calcium fluoride solid product can be obtained which can be directly converted into hydrogen fluoride by conventional means.
The aqueous phases from the separator 26, 34 and 42 are passed to a mixer 48 as streams 50, 52 and 54 respectivelyO In the mixer 48, the various streams are thoroughly mixed together and passed to a third reactor stage 56 as stream 580 In the mixer 48 some solids, mainly gypsum, may settle out and are removed as stream 60 as waste solids.
In reactor 56, stream 58 is reacted with a slurry of calcium oxide from stream 620 The origin of stream 62 is a mixer 64 wherein a ground calcium oxide feed source, such as lime, and water are mixed to form a calcium oxide slurry. The calcium oxide feed source is ground to at least -10 mesh (UOSO
Standard), preferably at least 90% -200 mesh. The calcium oxide slurry pre-pared in the mixer 64 contains from about 5 to about 50% solids, preferably from about 10 to about 25% solids. The calcium oxide slurry is prepared from fresh water and not pond waters or process waters.
The stream 58 has a pH between about 1.8 and about 206. Sufficient slurried calcium oxide is added to reactor 56 to raise the pH of the resulting reaction mixture from about 3 to about 4, preferably from about 3.6 to about 2Q 3.8. The retention time of stream 58 in reactor 56 is from about 30 seconds to about 60 minutes, preferably being about 15 minutes. The reaction temperature within reactor 56 may have any value between the freezing and boiling points of the reaction mixture but is preferably about ambient temper-ature. The reaction mixture contains about 0.01 to about 0.2% fluorine, a substantial portion of which is precipitated out as calcium fluoride upon the addition of the calcium oxideO
The reaction mixture from the reactor 56 is passed to a fourth separator stage 68 as stream 70O The separator stage 68 is a conventional liquid-solid separator such as the separator 26 described above. The solids separated out in the separator 68 consist of calcium fluoride and phosphate values and are recycled as stream 72 to stream 28 wherein the solids undergo the acid wash and the water wash as described ahove as part of stream 28.

, The aqueous phase separated in separator 68 is passed to a fourth reactor stage 74 as stream 760 In reactor 74, the aqueous phase is treated with slurried calcium oxide from stream 75 which has its origin from mixer 640 The reaction temperature and reaction residence time in the reactor 74 are similar to those in reactor 56. The pH of the stream 76 is from about 3 to about 4. Sufficient calcium oxide is added to reactor 74 to raise the pH
of the reaction mixture to between about 5 and about 7 to cause the precipita-tion of dical (calcium orthophosphate or dicalcium phosphate). The reaction slurry from reactor 74 is passed to a fifth separator stage 78 as stream 800 The separator 78 is a conventional liquid-solid separator such as the separator 26 descrlbed above. The solids separated in separator 78 consist principally of dical. The dical produced by this method is of sufficient purity to employ in animal feed. That is, the P/F weight ratio in the dical is greater than lOOo ' ' The aqueous phase from the separator 78 is passed to a fifth reactor stage 82 as stream 84. Stream 84 is combined with a calcium oxide slurry from stream 86 in the reactor 82. Stream 86 has its origin from mixer 64. Sufficient calclum oxide is added to the reaction mixture in the reactor 82 to raise the pH to a value of between about 8 and about 11 thereby to cause ` 20 precipitation of a substantial portion of the remaining soluble minerals as ` insoluble values, such as silicon dioxide solids and other solids from the reaction mixture.
The reaction mixture in the reactor 82 is passed to a sixth separator stage 88 as stream 900 The separator 88 is a conventional liquid-solid separator such as the separator 26 as described aboveO Separated solids from the separator 88, consisting chiefly of silicon dioxide solids, are sent to waste as stream 920 The aqueous phase from separator 88 consists of a nonacidic water which is substantially free of calcium, fluorine, P205 and sulphate valuesO This water is discharged as stream 94 and can be used as a fresh water for industry or it can be discharged into streams or lakesO
In another embodiment of the present invention, ammonia is added to the reaction stream before the stream 80, preferably as the indicated stream .. :. - .
,., , :

8 to the reactor 18 or to the reactor 10. The ammonia is maintained at a concentration between about 10 and about 5000 parts per million, preferably a concentration of about 600 to around 800 parts per million active ammonia in the reaction stream. The presence of ~mmonia in the reaction stream optimizes the yield of calcium fluoride and the P/F ratio of the dical produced in reactor 74. The reason for this is not thoroughly understood. However, it is believed that the ammonia complexes with the fluorine values, furnishing soluble fluorine values that can readily react with soluble calcium values to form the insoluble calcium fluoride. Thus the presence of ammonia not only enhances the quality of the produced dical but also increases the removal of fluorine from the pond waters and the overall yield of calcium fluoride.
The following Examples are included to further illustrate the present invention. In these examples, all percentages are weight percentages.

To 3834 grams of pond water at room temperature containing on a weight basis 0.94% F (predominantly H2SiF6), 1.70% P2O5, 0.41% SO3, 0O59% Si02, and other soluble metallic compounds, at 110F. (43C.), were added 34.4 grams of ground commercial limestone (95% effective CaC03) as a 44D7% aqueous slurry (Zl = 0 34) The mixture was agitated for five minutes while the pH in-creased from 1039 to 1.850 No fluoride values precipitated. Ground commer-~ cial limestone (10609 grams) wetted with water was added to the reaction ; mixture and agitated (Z2 = 1.07). ZR (theory) = 1 + 0.1927 x 1O70/0.94 =
1.35; ZR (used) = 1 + 141~3 x 0.95/94.93 = 1041 (4% excess Ca )O The pH of ' the reaction mixture after 37 minutes of agitation was 3Ø Calcium fluoride precipitated from the mixture and was separated therefrom by decantation and filtration, and dried at 105C. About 141 grams of calcium fluoride solids were recovered having the following analysis: 25.0% F, 15.3% P2O5, 3.6% SO3, 49.8% CaO, 2.0% Si02, 2.0% CO2 plus other metallics and 12.8% hydration waterO The filtrate contained 00017% F, 1018% P2O5 and 0.62% SiO2o The dried calcium fluoride solids were washed with 10% H2SO4 and water to reduce the ; P2O5 content of the solids to about 3~5%O

.
: , To 2000 grams of pond water at room temperature containing 0084% F
and 1.52% P205 was added 15.7 grams of ground commercial limestone slurried with water (Zl = 0 34) The resulting mixture was agitated for about 5 minutes. Additional ground commercial limestone ~17~9 grams) was added to the reaction mixture and the resulting mixture was agitated for an additional 200 minutes to ensure equilibrium conditions ~Z2 = 0.38; ZR = 1.35). The mixture was filtered. The filtrate contained 0.64% F and 1030% P2O5 represent-ing a 24% fluoride precipitation as calcium fluoride~
; 10 EXAMPLE 3 In an experiment similar to that discussed in Example 1, the lime-stone was totally added at the start of the reaction rather than in two incrementsO The filtrate contained about 25% of the fluoride following fil-i tration after 60 minutes of agitation.

Pond water similar to that used in the previous Examples was heated to 200F. (93C.); and to this solution, 104 equivalents of limestone per equivalent of contained fluoride were added in two stages with agitation to precipitate over 85% of the fluorine values as calcium fluoride. No adverse ~ 2Q effects due to the temperature were observed.
,, EXAMPLE 5 ` Pond water, at room temperature, was diluted ten-fold with water to contain 00087% F and 0.153% P2O5. A quantity of pure limestone equivalent to a 129% stoichiometric amount based on initial total fluoride was added incrementally as in Example 1 ~Zl = 0-4; Z2 = 0.89; ZR = 1.34). Ninety-five percent of the fluoride precipitated was calcium fluorideO

One thousand grams of pond water concentrated to contain 5.0% F
and 8.5% P2O5 was chilled to 40Fo ~4C~) and equilibrated with 52.7 grams of a commercial limestone slurried with water ~Zl = 0~38). After 5 minutes of agitation an additional 131.7 grams of limestone were added and agitation continued for five more minutes ~Z2 = 0 95; ZR = 1033)o An 80% fluoride :
., ~

.. . . . . .

10~5Z58 recovery as calcium fluoride solids was realized.
; EXAMPLE 7 A pond water containing 1% F as a 1:1 mole ratio of HF and H2SiF6 and 1% P205 was treated with a ground limestone slurry (10.5 parts CaC03 per 1000 parts pond water by weight). The resulting solution was agitated for 5 minutes (Zl = 0~38)~ About 20% of the fluoride precipitated at this time. Twenty-one grams of additional slurried limestone were added to the solution ~Z2 = 0.76; ZR = lol9); and after 40 minutes of equilibration, the solids were flocculated, settled and separated. The calcium fluoride solids ! 10 were dried at 105C~ and contained about 30% F and about 12% P205O EXAMPLE 8 Pond water containing 0~45% F and 0.77% P205 was agitated and equilibrated with commercial limestone, which amounted to 40% of the stoichio-metric quantity required for total fluoride precipitation at room temperature ~ -for 5 minutes (Zl = 0-40) An additional 70% limestone stoichiometry was added; and at 20 minutes of total reaction time (Z2 = 0-7; ZR = 1~33)~ 0.12 equivalents of aqueous MH3 were added per equivalent of initial fluoride and agitation continued for an additional 30 minutes. The resulting mixture was separated into a solid cake and filtrateO The soluble fluoride concentration in the filtrate was 0~56% F. This amounts to a 87% fluoride conversion to calcium fluoride, more than would be anticipated from the amount of calcium added ~Zl ~ Z2 = 82% ZR)~ The addition of ammonia apparently made some of the in situ calcium available for reaction with the fluorineO

One thousand grams of Example 1 pond water, at room temperature, was equilibrated with 1. 25 equivalents of commercial limestone in two steps ` per equivalent of fluoride (Zl =0 4; Z2 = 0085; ZR = 1~36)o After 19 minutes of reaction, the aqueous phase contained 0016% F. At this time, 0~069 equiva~
lents of aqueous ammonia per equivalent of initial fluoride were introduced into the reaction mixture. The aqueous phase contained 0014% F after ten minutes of reaction timeO Subsequently, an additional 0~17 equivalents of ~:
ammonia per fluoride equivalent were added to the reaction mixture, and after .~ .
~` ' ' . ' -~045Z58 60 minutes of reaction time, the fluoride concentration in the filtrate was 0.0049% F. A 99.4% recovery of fluoride as calcium fluoride was realised This shows that most of the fluorine in pond water containing a very low F
concentration can be recovered as calcium fluoride without excessive CaC03 usage.

One hundred grams of dry, raw calcium fluoride solids such as pro-duced in the preceding examples containing 25.3% F, 1409% P205, 2.11% A1203, 52.6% CaO, 0.44% MgO, 1030% Fe203 and 3048% C02 was slurried with 100 grams of water in a Teflon beaker. To this slurry were added 125 grams of 38.3% -HN03 which corresponded to 0.955 equivalents of HN03 per contained equivalent of P205 (equivalent weight taken as 23.66) plus loOO equivalent of HN03 per equivalent of containing C02. The resulting slurry was agitated at room temperature for about 15 minutes and filtered. The collected solids were further washed with three 500 gram portions of water. The leached calcium fluoride solids after drying at 105C. contained 36.8% F and 6004% P205. The ~ acid washing and subsequent water washes removed 7104 and 802% of the initial- -c ly contained phosphorus and fluorine respectivelyO The quantity of phosphorus ~ not leached from the raw calcium fluoride was in correlation with the quanti- ;
t 2Q ties of metallics such as iron, aluminium and magnesium which were not removed The acid washing can also be conducted with other mineral acids, such as hydrochloric acid and sulphuric acid, with similar resultsO

The same calcium fluoride solids that were treated in Example 10 were leached at room temperature with 9.8% aqueous H2S04 using the same pro-cedure as described in Example lOo However, 76.6 and 8.7 percent of the phos-phorus and fluorine were leached from the raw calcium fluorideO The washed calcium fluoride solids when dried at 105-110Co contained 2204% F, 3.3% P205 and 27.2% S03 resulting in the removal of 12% of the F and 78% of the P205.

The same dry, raw calcium fluoride solids as was used in Example 10 were slurried to 50% solids with waterO The calcium fluoride solids were ~,~, , . . :

, leached with 60~ 4% aqueous H2S04 such that there existed 1.00 and lo 26 equivalents of sulphuric acid per contained equivalent of C02 and P205 res-pectively. The temperature ranged from 150 to 200Fo (65 to 93C)o After filtration and subsequent water washing, the dry washed calcium fluoride contained 2006% F and 2~11% P205. About 87~6 and 12~4 percent of the phos-phorus and fluorine were contained in the combined leach and wash solutions respectively. At this temperature, approximately 602% of the initial fluorine was volatilized from the systemO

No loss in phosphorus leach-efficiency is observed when 4% H2S04 is used and the system is maintained at 40Fo (4C)~ However, final P205 concentration in combined wash liquors is not very suitable for economic reasons for subsequent dical recovery because of the dilute state of combined liquors.

A solution was prepared by combining the separated limestone treated pond water, such as was produced in Example 1, and the acid leach and water wash mixture, such as were produced in Example llo The solution contained 0~22% F~ 1009% P20s~ 0~4% SiO2~ and other soluble impurities. To 300 grams of this solution were added a total of 2089 grams of pure calcium oxide slurried with water. The resulting mixture was agitated for 35 minutes.
The pH increased from an initial 2~2 value to 9.7. The filtrate contained 6~4 ppm F, less than 3 ppm P205 and 252 ppm Si02 and was suitable for release into public waters~

To 300 grams of the same initial solution as was used in Example 14 were added 1044 grams of pure calcium oxide slurried with waterO The resulting mixture was agitated for about 20 minutes at room temperature to yield calcium fluoride solids which were removed by filtration. The filtrate had a pH of 3.4 and contained 220 ppm F and 9170 ppm P205O The weight ratio of P/F in the aqueous phase was increased from 2016 to 1802 after the addi-tion of the limeO

,` ,. ' ~ ~ .

To 300 grams of the same initial solutions as was used in Example 14 were added 0~72 gram of pure calcium oxide slurried with water and 1028 grams of a 240 2% NH3 solution at room temperatureO The resulting solution was equilibrated for 20 minutes to a pH of 303 to yield calcium fluoride solids which were removed by filtration. The filtrate contained about 32~2 ppm F
and 9200 ppm P205. In the presence of ammonia, less lime was required to obtain a weight ratio of P/F of about 122 than was required in Example 15 to obtain a P/F weight ratio of 18020 ~- 10 EXAMPLE 17 Pond water was enriched with ammonia to contain about 0008% N and treated with a slurry of commercial limestone in two steps as described in Example lo After separation of the calcium fluoride solids, the defluorinated waste water was combined with acid leach and wash solutions to render a composite solution having a pH of 2~0 and containing 0~112% F, lolO % P205, -0~28% SiO2 plus other valuesO To 900 grams of this final composite solution ~ at room temperature were added 2~94 grams of a commercial lime; assessed at `, 85% active CaO, and the resulting mixture was agitated for 35 minutes to a final pH of 3~5 to 3~60 The solid material formed was flocculated, settled O and separated from the aqueous phase which contained 0~0027% F and 0~905% P205 and a weight ratio of P/F of 146~ The solid material precipitated after dry-ing at 105-110C contained 19~5% F~ 2303% P205~ 44~0% CaO, 1~47% SiO2~ 0~21%
NH3 plus other minor values. This solid material is suitable for acid leaching `~ and washing to obtain a further quantity of calcium fluoride.

To 200 grams of a composite solution similar to that used in Example 17 but containing 4~ 69% P205 and 0. 87% F were added 5013 grams of pure lime slurried with water and 6047 grams of a 29~0% NH3 solution. This , mixture was agitated for about 15 minutes to a pH of 4~0 while the temperature i 30 decreased from 180F~ to about 150F~ (82 to 65Co)~ The filtrate contained 3018% P205 and 0~0078% F yielding a P/F weight ratio of 178 - 17 ~

. .~ , . .

The procedure of Example 18 was conducted at 40F (4Co) with no adverse effects on the resulting P/F ratio in the final filtrate, 646 ~ 4 grams of the final aqueous solution, having a P/F weight ratio of 146~ produced in Example 17 were treated with 3~89 grams of a commer-cial lime slurried with waterO The resulting mixture was agitated at room temperature for 15 minutes. The pH increased from 306 to 607~ The solid precipitate was recovered by decantation and filtration, dried at 105 to 110C.,and found to contain 0~08% F, 39~2% P205, 31~9% CaO, 204% SiO2~ 21~8% weight loss at 800C~ plus other minor values. This solid having the stoichiometry ~:
of dicalcium phosphate, CaHPO4, contained a P/F weight ratio of 2140 This same solid contained 44~7% P2O5 when dried at 164Co EXAMPLE 21 ~ `
To a solution containing 29~5 ppm F and 1~23% P2O5 was added slurried commercial lime to a final pH of about 805~ The mixture was agitated at room temperature for 90 minutes and the solids were recovered by filtration and were dried at 105 to 110C. The solid material analyzed 0~14% F~ 39~7%
P2O5, 37~4% CaO, 6-3% SiO2 (P/F = 124)-An initial solution similar to that used in Example 21 was treated with commercial lime to a pH of 6020 The solids dried at 105 to 110Co con-tained 0~12% F~ 40~6% P2O5 and 30~7% CaO ~P/F = 148)o A reaction similar to that of Example 22 was conducted at 180Fo (82C~)o No adverse effects on the final product were observedO

A reaction similar to that of Example 22 but performed at 40F~
~4C~) showed no adverse effects on the solid materialO

A solution containing 000010% F~ 0~10 % P2O5 and 0~3% SiO2 at a pH of 7.0 was treated with slurried lime to a pH of 9O0 and agitated to 30 ~ 18 ~

~ ,. .. : .
. . . ~: ` - -' . . .

lO~S258 minutes. The water decanted from the settled solution contained minute amounts of F, P205 and SiO2.

A pond water containing 0~85% F, 1049% P205, plus other impurities was heated to 203F~ ~95C~) and was then equilibrated with a total of 1~35 equivalents of commercial limestone per contained equivalent of fluoride in two stages as described in Example lo In the first stage Zl calcium equival-ents added equalled 0~4 and the Z2 calcium equi~alents equalled 0~95 in the second stage (ZR = 1~34)~ After 20 minutes of reaction time and subsequent separation of the solid material, the recovered solids at 105C. drying, contained 25~6% F~ 14~2% P205 ~ 50~3% CaO, 1~8% SiO2~ plus other impurities and hydrated water. This material was of the same nature as found in Example 1 and shows that the precipitation of fluoride is not extremely dependent on temperature. The filtrate obtained was heated to 203F~ (95C~) and was com-bined with slurried commercial lime to an equilibrated pH of 306 where calcium fluoride solids formedO The solids were separated from the liquid by decantation and filtration and analyzed 9.0% F, 31~7% P20s~ 48~6% CaO, 104%
SiO2, plus other impurities when dried at 105C~ The aqueous phase, which had a P205/F weight ratio of 333~ from this reaction was heated to 194F~
~90C~) and equilibrated to pH 702 with slurried commercial lime to form dical solids. After about 15 minutes of reaction, the dical solids were separated by decantation and filtration and were dried at 105C~ This material contain-ed 0~21% F~ 21~5% P20s~ 4604% CaO, 901% SiO2~ plus other impurities and is suitable for granulation with phosphoric acidO

Pond water containing 1~01% Fo and 1~84% P205 was first treated with 560 ppm of NH3 and then reacted with commercial limestone, the solid material isolated and the filtrate combined with sulphuric acid wash solutions such as prepared in Example 12~ In the first stage, Zl = 0038 calcium equivalents were added and in the second stage, Z2 = 0.81 calcium equivalents were added (ZR = 1035)o This final mixture containing 0~27% F and 1070% P205 was equilibrated at pH 3O5 to 306 for 20 minutes to render a P/F weight ratio ... . . .
:

104525~
of 153 in the aqueous phase after recovery of the solidsO This liquid phase was further equilibrated with commercial lime to a pH of 10.0 for about 15 minutesO The recovered solids contained dicalcium phosphate and nearly all of the silica which would otherwise remain in solution at pH values of about 7.0 to 705 or less.

A pond water containing 0.3% F and 0.05% P205 was treated with one equivalent of calcium as limestone per equivalent of contained fluoride with the mixture agitated for fifteen minutes (Zl = 1.00). An additional OoOl equivalents of calcium per equivalent of contained fluoride were further added -~
~Z2 = 0.01; ZR = 1.03) and equilibrated by agitation for forty-five additional minutes. The calcium fluoride precipitate contained 40% F and 1% P2050 ~` A pond water containing 003% F and 200% P205 was treated with limestone such that 1.1 equivalents of calcium were added per equivalent of . :.
contained fluoride ~Zl = lol)~ The mixture was equilibrated by agitation for five minutes after which time 1.19 equivalents of calcium per equivalent of contained fluoride were added (Z2 = lol9; ZR = 2029)~ and the solution equilibrated for an additional thirty minutes. The precipitated calcium ~ 20 fluoride contained 14% F and 28% P205.
; EXAMPLE 30 One thousand gra~.s of pond water containing 0.7% F and 103% P205 along with varied levels of other substances normally found in pond water was treated at 95F. ~35C.) with 7076 grams of 95% effective limestone slurried as a 40% mixture with water and agitated for five minutes (Zl = 004)o To this mixture was added an additional 18063 grams of limestone as in the ^l previous manner and the agitation was continued for an additional thirty - minutes (Z2 = 0.96; ZR = 1036). A raw calcium fluoride slurry containing 40%
solids was collected by filtration of the reaction mixture and contained 23080 grams of solids that, dried at 105C. were found to contain 25.0% F, 1500% P205 and 2.0% C02. The filtrate or treated pond water contained 1059 '!, grams of fluorine and analysed 0012% F and 1011% P205. A sulphuric acid wash :~' - . .
*

1~)45Z58 was applied to the raw calcium fluoride slurry whereby it was combined with 7049 grams of sulphuric acid as ten percent aqueous solution and agitated for fifteen minutes at 95F. (35C.). Subsequent to filtration of the solids the filter cake was washed with two 119 gram portions of waterO The 23080 grams of dry calcium fluoride contained 22.7% F and 3.74% P205 with the diluent being primarily calcium sulphateO The combined acid and water wash filtrates were combined with the treated pond water to give 951013 grams of a : solution containing 0.17% F and 1.82% P205. This solution was made to contain 500 ppm NH3 by the addition of a 25% NH3 aqueous solution. Further treatment at 95F. (35C.) with 4.10 grams of 95% effective lime as a 20% water solution to a pH of from 3.7 to 3.8 and with 15 minutes of agitation resulted in the precipitation of a second calcium fluoride solid having a dry-base analysis . of 2000% F and 2000% P205 and amounting to 7077 gramsO This material in slurry form was taken back to the sulphuric acid leach stage and recycled through the systemO The resulting filtrate contained 0.0031% F, 1011% P205 and amounted to 954.12 gramsO A further lime treatment comprised the addition of 7.48 grams of lime as in the previous manner in order that a pH of 609 to ;'! 701 was maintained at 95F. ~35C.) with fifteen minutes of agitationO The resulting 25043 grams of dry calcium phosphate contained 0012% F and 39.5%
P205, a P/F weight ratio of 144, and was suitable for granulation to produce a feed grade material. The filtrate contained 000011% F, 00056% P205 and all of the soluble silica initially present in ~he pond waterO By this scheme of pond water treatment, 99.6% of the fluorine was converted into a synthetic calcium fluoride and 77~1% of the phosphate was transformed into a material which confor s to the P/F of 1~ ratio necessary for a feed grade phosphate.

.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. The method of treating waste water containing fluorine and P2O5 values that comprises: in a first stage, adding with agitation to the waste water a suitable basic inorganic calcium compound in an amount to provide between 0.3 and 0.8 equivalents of calcium per equivalent of fluorine (F) values in the water; thereafter, in a second stage, agitating the calcium treated waste water effluent of said first stage with additional said calcium compound, in an amount to form calcium fluoride solids and such that the total calcium compound addition in the two stages provides not less than 0.8 equivalents of calcium per equivalent of F values in the waste water; and separating the solids from the aqueous phase following said second stage.

2. The method of claim 1 wherein the calcium compound is calcium carbonate.

3. The method of claim 2 which includes the step of prewetting the calcium carbonate to form an aqueous slurry containing 5 to 50% calcium carbonate prior to the addition of the calcium carbonate in the first and second stage.

4. The method of claim 1, 2 or 3 including the step of purifying the calcium fluoride in the solids separated from the aqueous phase following the second stage.

5. The method of claim 3 including the additional step of treating the waste water with ammonia to establish an ammonia concentration of from about 10 to about 5000 ppm in said water.

6. The method of claim 5 in which the ammonia addition is such as to establish an ammonia concentration within the range 600 to 800 ppm in the water.

7. The method of claim 5 or 6 in which the ammonia is added to the calcium carbonate-treated waste water effluent of said first stage.

8. The method of claim 3 in which the slurry addition in the second stage is such as to provide from about 1 to about 2 equivalents of calcium per equivalent of fluorine in the waste water.

9. The method of claim 3 in which the separated solids are washed with an aqueous solution of a mineral acid to remove acid soluble values;
and thereafter washed with water to remove the mineral acid and aqueous soluble values to leave solids enriched in calcium fluoride.

10. The method of claim 9 in which said separated solids are washed with an amount of mineral acid about equivalent to the P2O5 and CO2 values in said solids.

11. The method according to claim 10 wherein the effluent of said second stage after separation from said solids is combined with both the mineral acid washings and the water washings to form an aqueous mixture that is agitated with calcium oxide in an amount to raise the pH of the aqueous mixture to a value between about 3 and about 4 to convert a substantial por-tion of the remaining fluorine values in the aqueous mixture to insoluble calcium fluoride solids.

12. The method of claim 11 in which the aqueous mixture is treated with ammonia to establish a concentration between about 10 and about 5000 ppm of ammonia in the aqueous mixture prior to its said agitation with calcium oxide.

13. The method of claim 12 in which the calcium fluoride solids are separated from the treated aqueous mixture to yield a second crop of calcium fluoride solids.

14. The method of claim 13 in which the second crop of calcium fluoride solids are washed with an aqueous mineral acid to remove the acid soluble values therefrom and then washed with water to remove acid and water soluble values therefrom to give solids enriched in calcium fluoride.

15. The method of claim 13 in which the second crop of calcium fluoride solids are combined with the solids separated from the second stage effluent.

16. The method of claim 13 in which the treated aqueous mixture, after separation of calcium fluoride solids therefrom, is treated with additional calcium oxide to raise the pH of the aqueous mixture to a value of from be-tween about 5 and about 7 to form insoluble dicalcium phosphate solids.

17. The method of claim 16 in which said insoluble dicalcium phosphate solids are separated from the treated aqueous mixture.

18. The method of claim 17 in which the treated aqueous mixture after separation of said dicalcium phosphate solids, is treated with further calcium oxide to raise the pH of the aqueous mixture to between about 8 and about 11 thereby to remove a substantial portion of the soluble mineral values from the aqueous phase by the formation of insoluble mineral solids.

19. The method of claim 18 in which said insoluble mineral solids are separated from the treated aqueous mixture, yielding an aqueous phase sub-stantially free of fluorine, P2O5, calcium and silicon dioxide values.

20. The method of claim 19 in which the waste water is treated with ammonia to maintain a concentration of between about 10 and about 5000 parts per million of ammonia in the waste water during said first and second stages.

21. The method of claim 20 in which the ammonia treatment is such as to maintain a concentration of between about 600 and 800 parts per million of ammonia in the waste water during said first and second stages.