CN1608034A - Purification of phosphoric acid plant pond water - Google Patents

Abstract

A process for the partial purification of contaminated phosphoric acid plant pond water is described, in which the pond water is treated sequentially, with two basic compounds, clarified, aged, clarified again and re-acidified. The thus treated pond water, still containing the majority of the phosphate originally present, can then be concentrated via the removal of essentially pure water, using any conventional means of concentration, without the formation of solid precipitates. In addition, a simplified process for the partial purification of contaminated phosphoric acid plant pond water having a molar calcium plus magnesium to fluorine ratio greater than or equal to about 0.60, is described, in which the pond water is treated with a basic compound, clarified, aged, clarified again and re-acidified. The thus treated pond water, still containing the majority of the phosphate originally present, can then be concentrated via the removal of essentially pure water, using any conventional means of concentration, without the formation of solid precipitates.

Description

Purification of pond water in phosphoric acid plant

This application claims the benefit of: US Provisional Application Serial No. 60/353,359 (filed October 25, 2001), entitled "Purification of Phosphoric Acid Plant Pool Water"; US Provisional Application entitled "Purification of Phosphoric Acid Plant Pool Water." These applications are fully incorporated herein by reference.

Background of the invention

Phosphoric acid is produced by what is known as the "wet process" which involves reacting finely ground phosphate rock with sulfuric acid. The result of the various reactions is a slurry comprising phosphoric acid, calcium sulfate and various impurities derived from the phosphate rock. The slurry is typically filtered to separate the phosphoric acid product from the by-product calcium sulfate. The phosphoric acid thus obtained is then used to produce various phosphate fertilizers used in agriculture. The calcium sulfate filter cake is typically washed with water to thereby increase the recovery of phosphoric acid product. Most of the wash water is sent back to the phosphoric acid production process as make-up water. However, part of the wash water remains in the calcium sulfate filter cake and is discharged from the filter together with the filter cake. This residual water contains small amounts of phosphoric acid and small amounts of all other impurities present in the phosphoric acid product. The calcium sulfate filter cake is typically washed from the filter with additional water and pumped as a slurry to storage or disposal.

Calcium sulfate will settle in storage or disposal areas, releasing excess water. This released water is typically collected in trench and sump systems and recycled to the phosphoric acid production plant for reuse (ie for washing the calcium sulfate filter cake). These ditches and pools are also used as facilities for collecting other water in and around the phosphoric acid plant (such as cleaning water and washing water, fresh water fume scrubber), and as a collection facility for phosphoric acid runoff and leakage in the plant. At the same time, since these ditches and pools are located outdoors, they also collect rainwater.

Since all water in these ditches and pools contains small amounts of phosphoric acid and other impurities normally found in phosphoric acid, we consider it contaminated water. It must therefore be treated or purified to remove phosphoric acid and other impurities before discharge to the environment. In some cases, barring inclement weather conditions, in an efficiently operating phosphoric acid plant there is a balance between water entering the sump system and water evaporating such that virtually all of this contaminated water can be contained within the plant. Get recycled and reused. In this case, there is no need for treatment and discharge of the contaminated water (commonly referred to as "pool water").

However, there are still various situations that require treatment and discharge of such contaminated pool water. One of these is the occurrence of heavy rains of unusual duration. The other is a situation where a phosphoric acid plant is shut down for a sustained period of time or permanently.

Many factors affect the specific components and their concentrations within the contaminated pool water. Therefore, there is no typical composition for pool water other than some phosphate. However, there are some chemical components that can be found in pool water, examples of their concentration ranges are as follows:

chemical components concentration range P 1,700-12,000ppm SO ₄ 4,300-9,600ppm F 200-15,000ppm Si 100-4,100ppm   (ammonia) N 40-1,500ppm Na 1,200-2,500ppm Mg 160-510ppm Ca 450-3,500ppm K 80-370ppm Fe 5-350ppm Al 10-30ppm Cl 10-300ppm

A method well known in the art to treat or purify the pool water is double liming. The method involves dosing calcium compounds (such as CaCO ₃ , Ca(OH) ₂ or CaO) to pool water in two stages so that phosphate and other impurities form a solid sediment and are separated from the water thus purified. This method is described in Manual of Fertilizer Processing, edited by Francis T. Nielsson, Marcel Dekker, Inc. (1987), pp. 480-482; GA Mooney et al., Removal of Fluoride and Phosphorus from Phosphoric Acid Waste with Two Stage Line Treatment, pp. Proceedings of the 33rd Industrial Waste Symposium, Purdue University (1978); GA Mooney et al., Laboratory and Pilot Treatment of Phosphoric Acid Wastewater, presented at a joint meeting of the Central Florida Peninsula, American Institution of Chemical Engineers, Florida (1977); and U.S. Patent No. 5,112,499 ; 4,698,163; 4,320,012; 4,171,342; 3,725,265 and 3,551,332. However, there are several problems with this method. One of the problems is the generation of large amounts of slurry. A slurry (ie a mixture of precipitated impurities, unreacted calcium compounds and water) is produced in both the first and second stages of the process. These slurry materials are usually deposited in settling ponds requiring large land areas. While it is possible to regenerate and recover some slurry from the first stage of the process, the second stage slurry is quite bulky, extremely difficult to dewater and has little economic value. Large reservoirs are therefore required for permanent storage of such slurries. Another problem associated with this method of pond water treatment is that due to the large amount of water present in the slurry, only about 50-60% of the pond water entering the process can be discharged. Significantly larger process equipment is therefore required compared to other methods. A third problem with this approach is that without significant reprocessing at the added cost, essentially all of the economically valuable phosphate contained in the pool water will be converted into a form unsuitable for use as a fertilizer. Finally, a fourth problem with this treatment method is that the purified pool water only meets discharge standards and cannot replace the fresh water normally required by phosphoric acid plants (eg for steam generation).

Another common method of water purification is reverse osmosis. The method is based on applying an external pressure to a saline solution in contact with a semipermeable membrane such that the applied pressure exceeds the osmotic pressure of the aqueous component of the solution in contact with the membrane. Such a portion of the water is forced to pass through the membrane in the opposite direction while the other components of the solution (ie various soluble salts) do not pass through the membrane. The result is a purified water stream (permeate) and a stream with increased salt content (waste or concentrate). Reverse osmosis is well known in the art and is described in Douglas M. Ruthven, ed., Encyclopedia of Separation Technology, Vol. 2, pp. 1398-1430, John Wiley & Sons, Inc. (1997); S. Sourirajan and T. Matsusuura, Reverse Osmosis/Ultrafiltration Principles, National Research Council of Canada, Ottawa, Canada (1985); B. Parekh, ed., Reverse Osmosis Technology, Marcel Dekker, Inc., New York (1988); R. Rautenbach and R. Albrecht, Membrane Processes, John Wiley & Sons, Inc., New York (1989) and other publications. Reverse osmosis is also described in various US patents, such as US Patent Nos. 4,110,219, 4,574,049, 4,876,002, 5,006,234, 5,133,958, and 6,190,558.

Some attempts have been made to use reverse osmosis to purify pond water from contaminated phosphoric acid plants. However, these attempts have generally failed since pool water is a saturated solution. So as soon as the water is removed from the pool, the solution becomes supersaturated and various salts precipitate, quickly clogging the membranes used in reverse osmosis and preventing more pure water from flowing through them.

But if reverse osmosis can be made to work in the treatment of polluted phosphoric acid plant pond water, many economic and environmental benefits can be obtained. One of the benefits is the ability to recover the phosphate contained in the pool water in an economically useful manner (ie, a concentrated liquid phosphate solution). The second advantage is that it can greatly reduce the huge land area required for the settlement tank in the secondary ash leaching method, and can save the large water storage tank required for permanent storage of slurry. A third benefit is that the pure water obtained from the reverse osmosis system is pure enough to be used where fresh water is required (ie certain fume scrubbers, steam boiler feed systems, etc.). Additional benefits will be apparent to those skilled in the art.

Summary of the invention

One of the objects of the present invention is to provide a method for the pretreatment of polluted phosphoric acid plant pond water (hereinafter referred to as "pond water") which allows conventional reverse osmosis techniques to be used to purify the pretreated pond water.

Another object of the present invention is to provide a simplified method for the pretreatment of polluted phosphoric acid plant pool water (hereinafter referred to as "pool water") when the molar ratio of calcium + magnesium to fluorine in the pool water is greater than or equal to about 0.60 , which makes it possible to purify pretreated pool water using conventional reverse osmosis technology.

A third object of the present invention is to provide a method of pretreatment or partial purification of pool water which allows the use of any suitable means to concentrate said pretreated pool water by separating out pure water without A solid precipitate formed in the so concentrated solution.

The fourth object of the present invention is to provide a method for partial purification of pool water which allows continuous treatment of partially purified pool water by a conventional reverse osmosis system without precipitation of various compounds in the reverse osmosis system A phenomenon that clogs the membranes used in the system and renders them ineffective.

A fifth object of the present invention is to provide a method for selectively removing various specific ions and compounds from pool water to such an extent that the treated pool water can be further treated by a conventional reverse osmosis system to obtain essentially The permeate stream consists of pure water and the reject or concentrate stream consists of phosphate and other impurities contained in the treated pond water fed to the reverse osmosis system.

A sixth object of the present invention is to provide a method of purifying pond water which greatly reduces the generation of undesirable sludge.

The above and other objects will be achieved by the first embodiment of the present invention, which provides a method for partially purifying pre-treated pool water, comprising the following steps: adding a first compound that can react with fluoride in the pool water and A compound that forms a substantially insoluble fluoride salt; adding a second basic compound or a base-forming compound, the phosphate formed by the cationic moiety of said second compound should be soluble in the presence of water; thereby Settling of the precipitate formed; decanting or separating off the clear liquid part of the mixture; maintaining the liquid part of the mixture for a time sufficient to decompose the silicic acid present into hydrated silica; The acid is added to the liquid solution to increase the solubility of ions remaining in solution to a level greater than or equal to their concentration in the solution after concentration treatment by removal of substantially pure water.

A second embodiment of the present invention provides a simplified method of partially purifying pre-treated pool water comprising the steps of: adding a first alkaline compound to precipitate substantially insoluble salts in the pool water that comprise a majority of calcium and fluorine set while most of the phosphate remains in solution (i.e., the phosphate of the base is soluble); settle the precipitate thus formed; decant or separate off the clear liquid portion of the mixture; keep the liquid portion of the mixture for a period of time a time sufficient to decompose any silicic acid present to form hydrated silica; the separation of the hydrated silica and the addition of acid to the liquid solution so obtained increases the solubility of the ions remaining in solution to a level greater than or equal to that which would be removed by Concentration of substantially pure water in the treated solution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A first embodiment of the present invention provides a method of partially purifying polluted phosphoric acid plant pond water which allows the partially purified or pretreated pond water to be concentrated by removing pure water without creating obstructions to concentrating means or equipment or Precipitated solids or accretions which fail. Thus, for example, pretreated pool water may be processed through a conventional reverse osmosis system comprising one or more stages, resulting in a pure water stream and a concentrated pool water stream. The term "pool water" as used herein is defined as a dilute solution of phosphoric acid, also containing sulfuric acid, silicic acid, fluoride ions, calcium ions, sodium ions normally present in and out of phosphate fertilizer plants using the so-called "wet process" to produce phosphoric acid , ammonia ions and other ionic and nonionic substances.

In the method of this embodiment of the invention, a first compound that reacts with fluoride in the pool water and forms a substantially insoluble fluoride salt is added to the pool water. Several compounds of this type include magnesium-, calcium-, strontium-, and barium-containing compounds. Among these, which are particularly economical because of their low cost, are calcium-containing compounds including calcium carbonate, calcium hydroxide and calcium oxide. The amount of the compound added to the pool water depends on the concentration of fluoride in the pool water, the molar ratio of the cationic moiety of the compound to the fluorine in the pool water is in the range of 0.45-0.80, or more preferably in the range of 0.50-0.70, or Still more preferably in the range of 0.55-0.65. Therefore, if the amount of fluorine to be treated in the pool water is 19 pounds and the compound added is calcium oxide, the amounts of calcium oxide corresponding to the above three ranges are about 25.2-44.9 pounds, 28.0-39.2 pounds, and 30.8-30 pounds, respectively. 36.4 lbs. The first compound may be added to the pool water in dry form (paste) or as a liquid slurry, both within the scope of the present invention. In addition, the first compound may be added to the pool water in a batch or continuous manner, and these are within the scope of the present invention. The first compound is mixed with the pool water for a time sufficient for the compound to react with the fluorine in the pool water. Typically this time is from 1 minute to 30 minutes, depending on the form of compound added to the pool water and the configuration of the mixing equipment.

After the first compound has mixed with the pool water for a sufficient time, the mixture can be clarified to remove solids formed from the reaction of the first compound with the pool water. Somewhat better results are obtained in practice if the solids formed by the reaction of the first compound with the pool water are left in the pool water. However, it is still within the scope of the invention to remove these solids here.

Now add the second compound to the pool water. This second compound may be added to the same vessel if the pool water is being treated in a batch manner and solids formed by the reaction of the first compound with the pool water are not removed. If the pool water is being treated in a continuous manner, it must be transferred to a second storage tank or container, whether or not with the reaction product formed by the addition of the first compound. This second compound must be a strong base or a compound capable of forming a strong base in the presence of water. In addition, the cationic component of this second compound must be such that the phosphate formed therefrom remains soluble in the pool water. Examples of strongly basic compounds that form soluble phosphates include sodium hydroxide and potassium hydroxide. An example of a compound which, in the presence of water, forms a strong base and renders phosphate soluble is ammonia. Other compounds meeting the above two criteria may also be used and remain within the scope of the present invention. The second compound, either in pure form or in solution, is added and mixed with the pool water in sufficient amount to raise the pH of the resulting pool water to a range of 4.2-8.0, or more preferably to a range of 5.0-6.5, or still More preferably it is raised to the range of 5.5-6.0. The term "pure form" as used above refers to the physical state of the compound (ie sodium hydroxide or potassium hydroxide as a solid, or anhydrous ammonia as a gas or liquid under pressure) rather than the chemical purity of the compound. In terms of chemical purity, conventional technical grades of purity are acceptable for both the first and second compounds.

As used above and elsewhere, pH is defined as the molar concentration of hydrogen ions in a solution expressed in negative powers of ten.

The addition of the first and second compounds to the pool water and the ensuing chemical reaction resulted in the formation of a solid precipitate in solution. The solid precipitate may be removed from the solution by settling the solid and decanting the liquid, centrifuging the liquid, filtering the liquid, or otherwise separating the clear liquid from the solid precipitate. Furthermore, it is not necessary to separate all liquids from solids. Thus, the solids can be isolated as a slurry containing, for example, 50% by weight solids and 50% by weight liquid. In addition, it is not critical that all solids be separated from the liquid, although it is preferred that the concentration of suspended solids in the liquid does not exceed 0.5% by weight.

If a chemical analysis is carried out on the clear liquid thus obtained, it can be found that the concentration of calcium and fluorine has been significantly reduced compared to the original pool water, while the concentration of phosphate has only been reduced by a small amount.

The clear liquid is allowed to age. The purpose of aging is to break down the silicic acid present in the liquid into hydrated silica. The aging time should be at least 2 hours, preferably at least 16 hours. While there is no upper limit to the aging time, and we have found that longer aging times would be beneficial, practicality and economics dictate that the maximum aging time is generally limited to about 10 days or less.

After aging is complete, the hydrated silica formed during aging must be removed from the liquor. This can be accomplished by any conventional solid-liquid separation technique, including centrifugation, filtration or settling, among others. It is particularly practical to employ a flocculant, especially a cationic flocculant, followed by settling the flocculated silica and decanting the resulting liquid. The amount of flocculant required and the method of dosing will depend, among other factors, on the concentration of hydrated silica in the liquid and the specific flocculant type used. Therefore, it is necessary to conduct laboratory experiments to determine the dosing parameters of the flocculant through well-known techniques in the art. The use of flocculants to remove hydrated silica is not part of the present invention and is used here only to illustrate one efficient way of accomplishing the components of the present invention which include removal of silica from partially pretreated pool water.

The process has thus far removed most of the calcium, fluorine and silica that were originally present in the pool water. Thus, the pool water has been partially purified.

After removal of the hydrated silica, a clear liquid substantially saturated with various ions and their salts is obtained. In order to separate pure water from the liquid solution without causing the precipitation of these salts, it is necessary to adjust the solubility relationship of these salts in the solution. This is accomplished by lowering the pH by adding an acid or a compound that forms an acid in the presence of water to the solution. One method of lowering pH by adding acid is described in US Patent No. 5,338,456. But in the process described in US Pat. No. 5,338,456, the addition of acid converts carbonates such as calcium carbonate to carbon dioxide, which is removed from the water in forced air and vacuum degassers. Thus the net effect of adding acid in U.S. Patent No. 5,338,456 is to reduce the solubility of carbon compounds in solution, rather than the purpose of adding acid to the pool water as in this embodiment of the present invention is to increase the solubility of various ions and salts present . Examples of various acids that can be used include sulfuric acid, sulfurous acid, phosphoric acid, hydrochloric acid and nitric acid. Examples of various useful compounds that will form acids in the presence of water include sulfur trioxide, sulfur dioxide, hydrogen chloride and nitrogen dioxide. Other acids or compounds capable of forming acids in the presence of water may be used and remain within the scope of the invention. Acids that should not be used to adjust pH include hydrofluoric acid and fluorosilicic acid. The amount of acid, or compound capable of forming an acid when water is present, added to the liquid should be sufficient to give the resulting solution a pH in the range of 2.0-4.0, or more preferably 2.5-3.5, or still more preferably in the range of 2.9-3.1 Inside.

The liquid solution thus obtained is essentially clear and stable to subsequent precipitation. Additionally, pure water can be separated from liquid solutions without the formation of solid precipitates by any of several methods, including reverse osmosis, evaporation, or other means.

The process of this embodiment of the invention will be better understood with reference to Examples 2, 3 and 4 below. But it should be clear that these examples are only used for illustration rather than limiting the present invention. Unless otherwise stated, the percentages used in the following examples refer to weight percentages.

A second embodiment of the present invention provides a simplified method for partial purification of polluted phosphoric acid plant pond water when the molar ratio of calcium+magnesium to fluorine in the pond water is greater than or equal to about 0.60. Treated pool water can be concentrated by removing pure water without the formation of precipitated solids or deposits that would interfere with or render ineffective the concentration device or equipment. Thus, for example, pretreated pool water may be processed through a conventional reverse osmosis system comprising one or more stages, resulting in a pure water stream and a concentrated pool water stream. The term "pool water" as used herein is defined as a dilute solution of phosphoric acid, also containing sulfuric acid, silicic acid, fluoride ions, calcium ions, sodium ions normally present in and out of phosphate fertilizer plants using the so-called "wet process" to produce phosphoric acid , ammonia ions and other ionic and nonionic substances.

In the method of the second embodiment of the invention, a strong base or a first compound which will form a strong base when pure in water is added to the pool water. The cationic portion of the first compound must be such that the phosphate formed remains soluble in the pool water. Examples of strongly basic compounds that form soluble phosphates include sodium hydroxide and potassium hydroxide. An example of a compound that forms a strong base in the presence of water and the phosphate formed is soluble is ammonia. Other compounds meeting the above two criteria may also be used and remain within the scope of the present invention. The first compound, either pure or in solution, is added and mixed with pool water in sufficient amount to raise the pH of the resulting pool water to the range of 6.0-8.0, or more preferably to the range of 6.5-7.5. The term "pure form" as used above refers to the physical state of the compound (ie, sodium hydroxide or potassium hydroxide as a solid, or anhydrous ammonia as a gas or liquid under pressure), rather than the chemical purity of the compound. In terms of chemical purity, conventional technical grades of purity are acceptable for both the first and second compounds.

As used above and elsewhere, pH is defined as the molar concentration of hydrogen ions in a solution expressed in negative powers of ten.

The addition of the first compound to the pool water and the ensuing chemical reaction resulted in the formation of a solid precipitate in solution. These solid precipitates can be removed from solution by settling the solids and decanting the liquid, centrifuging the liquid, filtering the liquid, or by other means of separating the clear liquid from the solid precipitate. Furthermore, it is not necessary to separate all liquids from solids. Thus, solids may be isolated as a slurry containing, for example, 50% by weight solids and 50% by weight liquid. In addition, it is not critical that all solids be separated from the liquid, although it is preferred that the concentration of suspended solids in the liquid does not exceed 0.5% by weight.

If a chemical analysis is carried out on the clear liquid thus obtained, it can be found that the concentration of calcium and fluorine has been significantly reduced compared to the original pool water, while the concentration of phosphate has only been reduced by a small amount.

The clear liquid is allowed to age. The purpose of aging is to break down the silicic acid present in the liquid into hydrated silica. The aging time should be at least 2 hours, preferably at least 16 hours. While there is no upper limit to the aging time, and we have found that longer aging times would be beneficial, practicality and economics dictate that the maximum aging time is generally limited to about 10 days or less.

If the initial concentration of silicon in the pool water is less than about 120 ppm, no hydrated silica precipitates will form and the steps of aging and separating the hydrated silica described above will not be required.

After aging is complete, the hydrated silica formed during aging must be removed from the liquor. This can be accomplished by any conventional solid-liquid separation technique, including centrifugation, filtration or settling, among others. It is particularly practical to employ a flocculant, especially a cationic flocculant, followed by settling the flocculated silica and decanting the resulting liquid. The amount of flocculant required and the method of dosing will depend, among other factors, on the concentration of hydrated silica in the liquid and the specific flocculant type used. Therefore, it is necessary to conduct laboratory experiments to determine the dosing parameters of the flocculant through well-known techniques in the art. The use of flocculants in the removal of hydrated silica is not part of the present invention and is used here only to illustrate one efficient way of accomplishing the components of the present invention which include removal of silica from partially pretreated pool water. Methods for removing silica using flocculants are described in US Patent Nos. 5,595,717, 5,453,206, 5,409,614, and 5,200,165.

The process has thus far removed most of the calcium and fluorine that were originally present in the pool water. Thus, the pool water has been partially purified.

After removal of the precipitated solids formed by the dosing of the first compound and, if desired, of the hydrated silica, a clear liquid substantially saturated with the various ions and their salts is obtained. In order to separate pure water from the liquid solution without causing these salts to precipitate, it is necessary to adjust the solubility relationship of these salts in the solution. This can be accomplished by adding an acid or a compound capable of forming an acid when present to the solution, thereby lowering the pH.

One method of lowering pH by adding acid is described in US Patent No. 5,338,456. But in the process described in US Pat. No. 5,338,456, the addition of acid converts carbonates such as calcium carbonate to carbon dioxide, which is removed from the water in forced air and vacuum degassers. Thus the net effect of adding acid in U.S. Patent No. 5,338,456 is to reduce the solubility of carbon compounds in solution, rather than the purpose of adding acid to the pool water as in this embodiment of the present invention is to increase the solubility of various ions and salts present . Examples of various acids that can be used include sulfuric acid, sulfurous acid, phosphoric acid, hydrochloric acid and nitric acid. Examples of various useful compounds that will form acids in the presence of water include sulfur trioxide, sulfur dioxide, hydrogen chloride and nitrogen dioxide. Other acids or compounds capable of forming acids in the presence of water may be used and remain within the scope of the invention. Acids that should not be used to adjust pH include hydrofluoric acid and fluorosilicic acid. The amount of acid, or compound capable of forming an acid when water is present, added to the liquid should be sufficient to give the resulting solution a pH in the range of 2.0-4.0, or more preferably 2.5-3.5, or still more preferably in the range of 2.9-3.1 Inside.

The liquid solution thus obtained is essentially clear and stable to subsequent precipitation. Additionally, pure water can be separated from liquid solutions without the formation of solid precipitates by any of several methods, including reverse osmosis, evaporation, or other means.

The process of the second embodiment of the present invention will be better understood with reference to Example 1 below. But it should be clear that this embodiment is only used to illustrate the present invention, not to limit the present invention. Unless otherwise stated, the percentages used in the following examples refer to percentages by weight.

Example 1

A 1000.3 g sample of contaminated phosphoric acid plant pool water containing 0.4092% _P2O5 , 0.0582% Ca, _0.0445 % F, 0.0098% Si and 0.5307% _SO4 was obtained from an industrial wet process phosphoric acid plant reservoir system . To this sample was added 5.52 grams of 50% sodium hydroxide solution and mixed for 7 minutes. The pH of the above solution after mixing was 7.28. The solution is then allowed to stand and the precipitated solids from the chemical reaction between the sodium hydroxide and the pool water settle to form a slurry at the bottom of the vessel. After 16 hours, 916 grams of clear liquid were decanted from the vessel, leaving a slurry of 8.40% by weight of the initial pond water and sodium hydroxide solution. Since the initial concentration of silicon in the pool water samples was lower than that at which hydrated silica was formed (ie, about 0.0120%), no aging step was required for the hydrolysis of silicic acid to hydrated silica. The pH of the resulting clear liquid was then adjusted to 3.01 by adding 1.41 grams of 96% sulfuric acid. Laboratory analysis of _the solution sample at this time indicated that the solution sample contained 0.238% _P2O5 , 0.0026% Ca and 0.0089% F.

A 791.51 gram sample of this solution was then placed in a beaker on a laboratory stirrer-hot plate and heating was applied to evaporate the water while the solution was stirring. This process was continued until the final weight of the solution was 59.23 grams, which indicated that 732.28 grams of water had evaporated. At this point, the residual solution was still very clear with no precipitated solids. Laboratory analysis of a sample of the evaporated solution showed it to contain 3.38% _P2O5 , 0.0705% Ca and _0.1506 % F.

Example 2

A 3780 g sample of contaminated phosphoric acid plant pool water containing 1.85% _P2O5 , _0.121 % Ca, 0.360% F, 0.074% Si and 0.425% _SO4 was obtained from an industrial wet process phosphoric acid plant reservoir system . To this sample was added 17.3 grams of calcium oxide and the solution was mixed for about 20 minutes. Next, 50% sodium hydroxide solution was added and mixed with the above solution in an amount of 42.16 grams, sufficient to increase the pH to 5.0. The solution is then allowed to stand and the precipitated solids from the chemical reaction between the calcium oxide, sodium hydroxide and pool water settle to form a slurry at the bottom of the vessel. After 16 hours, 3524 grams of clear liquid were decanted from the above vessel, leaving a slurry of 8.22% by weight of the initial pond water, calcium oxide and sodium hydroxide solution. The clear solution was aged for an additional 32 hours. At this time, the appearance of the liquid appears cloudy to some extent due to the decomposition of silicic acid present in the hydrated silica. 0.41 grams of flocculant was added and mixed with 1750 grams of the above aged solution. The flocculant used was manufactured by Arr-Maz Products, LP under the designation 1046C. The solution was then allowed to stand for 5 hours, and the flocculated silica settled to form a slurry at the bottom of the vessel. After 5 hours, 1350 ml of clear liquid was decanted from the above container, leaving a slurry of approximately 23% by volume of the original 1750 g sample. The pH of the resulting clear liquid was adjusted to 3.01 by adding 1.99 grams of 96% sulfuric acid. Laboratory analysis of a sample of _the solution at this time indicated that it contained 1.50% _P2O5 , 0.0064% Ca, 0.0202% F and 0.0027% Si.

A 900 gram sample of this solution was then placed in a beaker on a laboratory stirrer-hot plate and heating was applied to evaporate the water while the solution was stirring. This process was continued until the final weight of the solution was 103 grams, which indicated that 797 grams of water had evaporated. At this point, the residual solution was still very clear with no precipitated solids. Laboratory analysis of a sample of _the evaporated solution showed it to contain 7.41% _P2O5 , 0.0392% Ca, 0.121% F and 0.023% Si.

Example 3

A 226 _gallon pool water sample (1,917 lbs) containing 1.61% _P2O5 , 0.119% Ca, 0.470% F, 0.084% Si, 0.440% _SO4 and 0.230% Na was used for this test . To this sample was added 17.64 lbs of calcium oxide and the solution was mixed for about 30 minutes. A 50% sodium hydroxide solution was then added and mixed with the above solution in an amount of 21.34 lbs, sufficient to increase the pH to 5.6. The solution is then allowed to stand and the precipitated solids from the chemical reaction between the calcium oxide, sodium hydroxide and pool water settle to form a slurry at the bottom of the vessel. After about 4 hours, about 206 gallons of clear liquid were decanted from the reaction tank to a second tank. After correcting for the volume of clarified liquid not diverted from the reaction tank, the slurry left was 3.54% of the original 226 gallons of pond water volume. The clear solution was aged for an additional 36 hours. The two flocculants were then sequentially added to the 206 gallons of liquid in the second tank. The first flocculant, designated 1018C, was added in an amount equal to 150 ppm by weight and mixed for about 3 minutes. A second flocculant labeled 811E was added in an amount equal to 8 ppm by weight and mixed for about 30 seconds. Both flocculants are manufactured by Arr-Maz Products, LP. The solution was allowed to stand for about 1 hour, then about 170 gallons of clear liquid was decanted from the second tank to the third tank. After correcting for the volume of clarified liquid not diverted from the second tank, the silica slurry left was 7.28% of the original 226 gallons of pond water volume. The pH of the resulting 170 gallons of clear liquid was adjusted to 3.0 by adding 2.44 pounds of 97.35% sulfuric acid. Laboratory analysis of a sample of the solution at this time showed it to contain 1.7% _P2O5 , _0.0102 % Ca, 0.0114% F, 0.0070% Si, 0.482% Na, and 0.530% _SO4 . At this point, the total amount of slurry produced by the partial purification process of this embodiment of the invention was 10.82% of the initial pond water volume, or 24.45 gallons.

The aforementioned 170 gallons of pretreated, partially purified pool water prepared according to the method of this embodiment of the invention were treated through a conventional two-stage reverse osmosis system. The first-stage reverse osmosis module consists of a conventional sand filter, a 5-micron cartridge filter, a feed tank, a 5-HP positive displacement high-pressure pump, and a single-membrane unit. The membrane unit used in the first section of the module is a seawater unit with the US filter number SW2530. The operating conditions of the first-stage reverse osmosis module are as follows: the inlet pressure is 850 psi (pounds per square inch), the temperature is 102°F, and the permeate recovery rate is about 80.6% (volume). Thus, the first stage reverse osmosis module produced about 137 gallons of purified permeate stream and about 33 gallons of waste or concentrate stream. Laboratory analysis of the permeate stream indicated it contained 0.0036% _P2O5 , _0.0017 % Ca, 0.0045% F, 0.0005% Si, 0.0572% Na and 0.0115% _SO4 . Laboratory analysis of the waste stream indicated it contained 5.65% _P2O5 , _0.0429 % Ca, 0.0379% F, 0.0369% Si, 2.144% Na and 2.830% _SO4 .

The 137 gallons of permeate stream obtained from the first stage reverse osmosis module was mixed with about 0.55 lbs. of 50% sodium hydroxide to increase its pH to about 9. This solution is then fed to the second stage reverse osmosis module. Except for the sand filter, the second-stage reverse osmosis module contains basically the same components as the first-stage reverse osmosis module. The membrane unit used in the second stage assembly is a brackish water unit with the US filter number BW2530. The operating conditions of the second-stage reverse osmosis module were as follows: inlet pressure 195 psi (pounds per square inch), temperature 85°F, permeate recovery rate about 94.5% (volume). The second stage reverse osmosis module produced a permeate stream of about 130 gallons and a waste or concentrate stream of about 6.8 gallons. Laboratory analysis of the permeate stream from the second stage _module showed it to contain 0.00005% _P2O5 , <0.00001% Ca, 0.00008% F, 0.0001% Si, 0.0008% Na and <0.0001% of SO ₄ . Laboratory analysis of the waste stream indicated it contained 0.0680% _P2O5 , _0.00002 % Ca, 0.00515% F, 0.0028% Si, 0.1370% Na, and 0.0300% _SO4 .

Thus, the pretreatment method of this embodiment of the invention makes it possible to Approximately 72.1% by volume of raw pond water is recovered as substantially pure water in the second reverse osmosis permeate stream and as an economically valuable solids-free solution in the first reverse osmosis waste stream About 68.5% P ₂ O ₅ in the original pond water.

Example 4

A 206 _gallon pool water sample (1,718 lbs) containing 1.57% _P2O5 , 0.111% Ca, 0.500% F, 0.094% Si, 0.460% _SO4 and 0.230% Na was used for this test . To this sample was added 19.82 lbs of calcium oxide and the solution was mixed for about 30 minutes. Anhydrous ammonia was then injected under the surface of the liquid and mixed with the above solution in an amount of 3.19 lbs, sufficient to increase the pH to 5.6. The solution is then allowed to stand and precipitated solids from the chemical reaction between the calcium oxide, anhydrous ammonia and pool water settle to form a slurry at the bottom of the vessel. After about 4 hours, about 180 gallons of clear liquid were decanted from the reaction tank to a second tank. After correcting for the volume of clarified liquid not diverted from the reaction tank, the slurry left was 3.40% of the original 206 gallons of pond water volume. The clear solution was aged for an additional 120 hours. The two flocculants were then sequentially added to the 180 gallons of liquid in the second tank. The first flocculant, designated 1018C, was added in an amount equal to 100 ppm by weight and mixed for about 3 minutes. A second flocculant labeled 811E was added in an amount equal to 8 ppm by weight and mixed for about 30 seconds. Both flocculants are manufactured by Arr-Maz Products, LP. The solution was allowed to stand for about 1 hour, then about 120 gallons of clear liquid was decanted from the second tank to the third tank. After correcting for the volume of clarified liquid not diverted from the second tank, the silica slurry left was 13.3% of the original 206 gallons of pond water volume. The pH of the resulting 120 gallons of clear liquid was adjusted to 3.0 by adding 1.62 pounds of 97.35% sulfuric acid. Laboratory analysis of a sample of the solution at this time showed it to contain 1.01% _P2O5 , _0.0102 % Ca, 0.0139% F, 0.0042% Si, 0.150% _NH3 , and 0.540% _SO4 . At this point, the total amount of slurry produced by the partial purification process of this embodiment of the invention was 16.7% of the initial pond water volume, or 34.4 gallons.

The aforementioned 120 gallons of pretreated, partially purified pool water prepared according to the method of this embodiment of the invention were treated through a conventional two-stage reverse osmosis system. The system components and membrane units are the same as those used in Example 2 above. The operating conditions of the first-stage reverse osmosis module are as follows: the inlet pressure is 390-675psi (varied during the test), the temperature is about 103°F, and the permeation recovery rate is 71%-80.5% (volume, which varies with the inlet pressure) . The first stage reverse osmosis module produced about 96.6 gallons of purified permeate stream and about 23.4 gallons of waste or concentrate stream under all test conditions. Laboratory analysis of the permeate stream indicated it contained 0.0047% _P2O5 , < _0.0001 % Ca, 0.0039% F, 0.0001% Si, 0.0014% _NH3 and 0.0027% _SO4 . Laboratory analysis of the waste stream indicated it contained 4.56% _P2O5 , _0.0429 % Ca, 0.0452% F, 0.0318% Si, 0.650% _NH3 and 2.847% _SO4 .

The 96.6 gallons of permeate stream obtained from the first stage reverse osmosis module was mixed with about 0.39 lbs of 50% sodium hydroxide to increase its pH to about 9. This solution is then fed to the second stage reverse osmosis module. The operating conditions of the second-stage reverse osmosis module were as follows: inlet pressure 150 psi (pounds per square inch), temperature 90°F, permeate recovery rate about 85.5% (volume). The second stage reverse osmosis module produced a permeate stream of about 82.7 gallons and a waste or concentrate stream of about 13.9 gallons. Laboratory analysis of the permeate stream from the second stage _assembly showed it to contain 0.00016% _P2O5 , <0.00001% Ca, 0.00016% F, 0.00002% Si, 0.00058% _NH3 and 0.0001% of SO ₄ . Laboratory analysis of the waste stream indicated it contained 0.0317% _P2O5 , _0.0005 % Ca, 0.0259% F, 0.0004% Si, 0.0061% _NH3 and 0.0184% _SO4 .

Thus, the pretreatment _method of this embodiment of the invention allows Approximately 64.4% (by volume) of raw pond water can be recovered as substantially pure water in the second RO permeate stream and as an economically valuable solids-free solution in the first RO waste stream Recover about 70.8% of P ₂ O ₅ in the original pond water.

Embodiment 4 (comparative example)

A laboratory test of the prior art double leaching method was carried out. The results were used as a basis for comparison with the method of this embodiment of the invention. The method involves adding CaO or Ca(OH) ₂ to the pool water until the pH reaches 5.0-5.5. Settle the slurry formed from the reaction between the pool water and CaO or Ca(OH) ₂ and decant the clear liquid. Then add CaO or Ca(OH) ₂ to the clear liquid until the pH value reaches 11.5-12.0. Additional slurry developed which was allowed to settle and the clear liquid decanted. The clarified liquid is usually analyzed and discharged if legal discharge standards are met.

A 3000 gram pool water sample (2950 ml) containing 1.84% _P2O5 , _0.1072 % Ca, 0.310% F and 0.460% _SO4 was used for this test. To this sample was added 48.2 grams of Ca(OH) ₂ to bring the pH to 5.31. The solids produced by the reaction of the pool water with the Ca(OH) ₂ were then allowed to settle for 16 hours. 2529 g (2555 mL) of clear liquid was decanted leaving a first stage slurry with a volume of 395 mL (13.4% by volume). An additional 30.0 g of Ca(OH) ₂ was added to the clear liquid to bring the pH to 11.8. The solution was allowed to stand for 3 days, then 1715 mL of clear liquid was decanted, leaving 840 mL of an additional slurry. Laboratory analysis of _the clarified liquid showed that it contained 0.00045% _P2O5 and 0.0007% F, which is believed to meet the statutory emission standards.

Therefore, as illustrated in the above-mentioned Examples 2 and 3, the pretreatment method of this embodiment of the present invention makes 72.1% and 644% of the original pond water reach the discharge standard respectively, and adopts the prior art treatment method to make the amount of the original pond water up to the standard Only 58.1%. In addition, the pretreatment method of this embodiment of the present invention can recover 68.5% (Example 2) and 70.8% (Example 3) of the P ₂ O ₅ contained in the original pond water in a cost-effective manner, while using the prior art The secondary liming process loses all of _{the P2O5} in the slurry waste stream. Finally, the total amount of waste slurry produced by the pretreatment method of this embodiment of the present invention is only 10.82% (Example 2) and 16.7% (Example 3) of the original pool water volume, while the prior art method produces The total slurry volume is 41.9% of the original pool water volume.

Although the invention has been described in terms of specific embodiments, it should be clear that various modifications are possible and the invention is only limited by the scope of the appended claims.

Claims (9)

1. a part treating pond water makes described pond water be able to further purification by isolating pure basically water, and the method for solid precipitation does not take place, and described method comprises following each step:

First kind of compound joined in the water of a certain amount of pond to improve the pH value of gained solution, wherein said first kind of compound is the maybe alkaligenous compound of energy shape when having water of alkali, and the cationic moiety of described first kind of compound can make its phosphoric acid salt still keep solvable in described solution;

Make the throw out sedimentation that forms thus;

Isolate the clarified liq part of mixture, keep the liquid portion of described mixture to be enough to for one section to make the silicic acid of existence to be decomposed into the time of hydrated SiO 2, isolate the hydrated SiO 2 slurry; With

Add second kind of compound to reduce the pH value of solution, second kind of compound of described adding is acid or becomes acid compound, and the various ionic solubleness that are retained in the solution are increased.

2. the method for claim 1, described method adds the third compound before also being included in and adding first kind of compound, described the third compound joins in the water of a certain amount of pond, by with described pond water in fluorochemical react, and form insoluble basically fluoride salt and precipitate.

3. the method for claim 2, wherein said the third compound is a calcium containing compound, is selected from lime carbonate, calcium hydroxide or calcium oxide, magnesiumcarbonate, magnesium hydroxide or magnesium oxide.

4. each method in the aforementioned claim, wherein said the third compound the amount in the water of described pond of joining makes the mol ratio of the cationic moiety of described compound and the described fluorine in the water of described pond in the scope of 0.55-0.65.

5. each method in the aforementioned claim, wherein said first kind of compound is selected from sodium hydroxide, potassium hydroxide and ammonia.

6. the amount that each method in the aforementioned claim, wherein said first kind of compound join in the water of described pond is enough to make the pH value of gained solution to be increased in the scope of 6.5-7.5.

7. each method in the aforementioned claim is wherein isolated after the clarifying basically liquid from add described first kind of formed slurry of compound, makes the clarified liq ageing 36-72 hour.

8. each method in the aforementioned claim is wherein isolated after the described hydrated SiO 2 slurry, and the amount that is reduced to the 2.9-3.1 scope with the pH value that is enough to make described solution joins described second kind of compound in the described clarified liq.

9. each method in the aforementioned claim wherein joins described second kind of compound of isolating in the clarified liq that obtains behind the described hydrated SiO 2 slurry and is selected from sulfuric acid, sulfurous acid, phosphoric acid, hydrochloric acid, nitric acid, sulphur trioxide, sulfurous gas, hydrogenchloride and nitrogen peroxide.