GB1601669A - Recovery of fluorine and phosphate values from waste water - Google Patents

Description

(54) RECOVERY OF FLUORINE AND PHOSPHATE VALUES FROM WASTE WATER (71) We, OCCIDENTAL PETROLEUM CORPORATION, a Corporation organised and existing under the laws of the State of California, United States of America, of 10889 Wilshire Boulevard, Los Angeles, California 90024, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- THIS 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 acid.

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 P2O5 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% P2O5 , from about 0.1 to about 2.5% SiO2, from about 0.1 to about 0.5% dissolved calcium and from about 0.1 to about 0.5% soluble sulphate salts.

(Unless otherwise indicated, all percentages herein are weight percentages). 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 hazard.

Traditionallyond 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 regulations. In the settling basins, the amounts of the various chemical values, such as fluorine, P2O5, 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 waters. However, it appears that none of these methods has been economically attractive or feasible since none of the methods is in commerical use in the United States. For example, D. R. Randolph developed a method which is disclosed in U.S. Patent 3,625,648. 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 fluoride. The calcium fluoride is separated from the aqueous phase and treated with sulphuric acld, or other strong acid, to liberate the hydrogen fluoride (HF) gas and yield a slurry of gypsum, sulphuric acid and phosphoric acid. e 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 with additional milk of lime to adjust the pH to bet ween 6 and 7 whereupon further solids, such as gypsum, precipitate out. These solids are separated 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.

In the Specification of our Patent No.

1 505 146 we have disclosed what we have discovered to be an economic method of treating phosphate waste waters so as to recover many of the valuable chemical values therein which are ecologically undesirable products in streams and lakes, our said Specification disclosing a treatment method that also renders the pond waters neutral so that they can be discharged into streams or rivers or recycled as process waters.

Thus, our aforesaid Specification discloses and claims a method of treating waste water resulting from the production of wet process phosphoric acid and containing fluorine and P2O5 values, that comprises, in a first stage, agitating said waste water with an amount of calcium carbonate 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 calcium carbonate in an amount to form calcium fluoride solids, and such that the total calcium carbonate 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.

In typical practice of that method, the waste or pond water is treated with calcium carbonate in the form of ground limestone or in the form of an aqueous slurry of ground limestone. In the first stage, this calcium carbonate addition results in the formation of calcium salts containing phosphoate and fluorine values. Preferably about 0.4 equivalents of calcium per equivalent of fluorine in the pond water is added in the first stage, which is normally carried out at the temperature of the pond water but which may, however, be carried out at any temperature between the freezing temperature of the pond water and its boiling point.

The residence time for this first stage is usually from one-half minute to 60 minutes, being preferably about 5 minutes. The pond water and the calcium carbonate are agitated to ensure maximum reaction between the calcium carbonate and the pond water values.

The treated pond water is then passed to a second stage wherein additional calcium carbonate is added so that the total addition of calcium carbonate in the two stages is not less than 0.8 equivalents of calcium per equivalent of fluorine initially present in the pond water, the second stage addition preferably being such as to result in a total addition of 1 to 2 equivalents of calcium per equivalent of fluorine. The second stage treatment is also normally carried out at the pond water temperature although, like the first stage treatment, the second stage treatment may 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 usually from one-half minute to 60 minutes, being preferably about 30 minutes.

In the second stage a slurry is formed containing solid calcium fluoride and some solid calicum phosphate values. This slurry is passed to a separation stage wherein the solids are separated from the aqueous phase. The resulting solid cake is preferably washed in an acid wash stage with an aqueous solution of a strong mineral acid, such as sulphuric acid, to remove phosphate and other values from the solid calcium fluoride phase thereby to raise the F/P20s weight ratio thereof. Preferably the solids are washed with an amount of mineral acid about equivalent to the total of the P2O5 and CO2 values contained in the solids. The solid phase is subsequently washed with water in a water wash stage to substatianlly remove most of the aqueous soluble values and mineral acid, leaving a solid product that may contain up to about 45% by weight fluorine as calicum fluoride.

The aqueous mineral acid washings and the wash water from the solids are preferably combined with the treated waste water effluent of the second stage in a mixer wherein they are mixed together to form a first stream which is passed to a third stage wherein it is agitated with calcium oxide, such as ground lime or aqueous lime slurry, in an amount sufficient to raise the pH of the first stream to a value ranging from 3 to 4, preferably between 3.6 and 3.8, the pH of the first stream usually being initially between 1.8 and 2.6 because of the addition of wash acid. In this third stage, a substantial proportion of the remaining fluorine values m the aqueous mixture is converted to additional calcium fluoride which is conveniently separated from the stream in a separate stage to yield a second crop of calcium fluoride solids which is recycled to the acid wash stage where it is subjected again to the acid washing and water washing to produce solids enriched in calcium fluoride. This second crop of solids can be combined with the solids separated from the second stage effluent.

The aqueous phase from the said separator stage is conveniently passed as a second stream to a fourth stage wherein it is treated with further calcium oxide to raise the pH of the aqueous mixture to a value ranging from 5 to 8, preferably from 6.5 to 7.2. In this 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 stage may then be passed as a third stream to a fifth treatment stage wherein the third stream is further treated with calcium oxide to raise the pH of the aqueous mixture to a value ranging from 8 to 11 to remove a substantial proportion of the soluble mineral values of the aqueous mixture by formation of insoluble mineral solids. The calcium oxide is normally added to the third, fourth and fifth stages in the form of an aqueous slurry having a solids content of 5 to 50%, preferably from 10 to 25%. The calcium oxide slurry is prepared from fresh water, not pond water or process waters containing F and P2Os values. 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 is substantially free of fluorine, P2Os, calcium and SiO2 values and can be recycled back into a wet process phosphoric acid plant.

Our said Specification also discloses the addition of ammonia to the waste water, prior to its first stage treament with calcium carbonate, in an amount to establish an ammonia concentration of up to 5000 parts per million and preferably in an amount to establish an ammonia concentration within the range 600 to 800 ppm. The presence of ammonia in the process stream appears to optimise the yield of calcium fluoride and the P/F ratio of the "dical" solids produced.

The reason for this effect is not known but it is postulated that the ammonia complexes with the fluorine values, forming soluble fluorine complexes that can readilv react with soluble calcium values to form the insoluble calcium fluoride. Thus the presence of ammonia in the process stream not only enhances the quality of the "dical" solids but also increases the removal of fluorine from the waste water, and the overall yield of calcium fluoride.

In the method disclosed in our said Specification, the calcium values added to the waste water in the said first and second stages are in the form of calcium carbonate, a reactant that has the merit of being inexpensive and usually readily available, and the further advantage of facilitating control of pH in the reaction stages by reason of the buffering effect of the carbonate ion: however, we have now discovered that the added calcium values in either or both of the said first and second reaction stages may be partly or wholly in the form of calcium oxide that may be added to the water either in the form of a dry powder or as an aqueous slurry. The reaction conditions are unchanged by this substitution or part substitution of calcium oxide for the carbonate in either reaction stage.

Accordingly, the present invention provides a method of treating waste water as disclosed and claimed in our said Patent but modified in that at least some of the calcium carbonate specified for agitation with the water in at least one of said first and second stages is substituted by calcium oxide.

Such substitution may be adopted in all embodiments of the method as disclosed in our said Patent.

WHAT WE CLAIM IS: 1. A method of treating waste water resulting from the production of wet process phosphoric acid and containing fluorine and PzOs values, in accordance with any one of the claims of Patent No. 1505146, mod ified in that at least some of the calcium carbonate specified for agitation with the water in at least one of said first and second stages of the claimed method is substituted by calcium oxide.

2. A method according to claim 1, wherein said calcium oxide is added to the water as an aqueous slurry.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. ranging from 5 to 8, preferably from 6.5 to 7.2. In this 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 stage may then be passed as a third stream to a fifth treatment stage wherein the third stream is further treated with calcium oxide to raise the pH of the aqueous mixture to a value ranging from 8 to 11 to remove a substantial proportion of the soluble mineral values of the aqueous mixture by formation of insoluble mineral solids. The calcium oxide is normally added to the third, fourth and fifth stages in the form of an aqueous slurry having a solids content of 5 to 50%, preferably from 10 to 25%. The calcium oxide slurry is prepared from fresh water, not pond water or process waters containing F and P2Os values. 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 is substantially free of fluorine, P2Os, calcium and SiO2 values and can be recycled back into a wet process phosphoric acid plant. Our said Specification also discloses the addition of ammonia to the waste water, prior to its first stage treament with calcium carbonate, in an amount to establish an ammonia concentration of up to 5000 parts per million and preferably in an amount to establish an ammonia concentration within the range 600 to 800 ppm. The presence of ammonia in the process stream appears to optimise the yield of calcium fluoride and the P/F ratio of the "dical" solids produced. The reason for this effect is not known but it is postulated that the ammonia complexes with the fluorine values, forming soluble fluorine complexes that can readilv react with soluble calcium values to form the insoluble calcium fluoride. Thus the presence of ammonia in the process stream not only enhances the quality of the "dical" solids but also increases the removal of fluorine from the waste water, and the overall yield of calcium fluoride. In the method disclosed in our said Specification, the calcium values added to the waste water in the said first and second stages are in the form of calcium carbonate, a reactant that has the merit of being inexpensive and usually readily available, and the further advantage of facilitating control of pH in the reaction stages by reason of the buffering effect of the carbonate ion: however, we have now discovered that the added calcium values in either or both of the said first and second reaction stages may be partly or wholly in the form of calcium oxide that may be added to the water either in the form of a dry powder or as an aqueous slurry. The reaction conditions are unchanged by this substitution or part substitution of calcium oxide for the carbonate in either reaction stage. Accordingly, the present invention provides a method of treating waste water as disclosed and claimed in our said Patent but modified in that at least some of the calcium carbonate specified for agitation with the water in at least one of said first and second stages is substituted by calcium oxide. Such substitution may be adopted in all embodiments of the method as disclosed in our said Patent. WHAT WE CLAIM IS:

1. A method of treating waste water resulting from the production of wet process phosphoric acid and containing fluorine and PzOs values, in accordance with any one of the claims of Patent No. 1505146, mod ified in that at least some of the calcium carbonate specified for agitation with the water in at least one of said first and second stages of the claimed method is substituted by calcium oxide.

2. A method according to claim 1, wherein said calcium oxide is added to the water as an aqueous slurry.