KR101153739B1 - Recovering Method of Phosphate Compound from Waste Water - Google Patents

Abstract

A method for recovering phosphate components from wastewater is disclosed that reduces the amount of waste generated and enables the recycling of resources. In the method for recovering the phosphate component from the wastewater according to the present invention, hydrated lime is added to wastewater containing phosphoric acid, sulfuric acid and hydrofluoric acid to precipitate sulfuric acid as gypsum, and hydrofluoric acid to calcium fluoride, and phosphoric acid dissolved in water as phosphate. A first step of maintaining the state, a second step of precipitating calcium phosphate salt by adding lime to the supernatant after removing the precipitate, and recovering the precipitated calcium phosphate salt. Here, the slaked lime used in the second step is preferably fine particles pulverized to pass 99% or more of the standard mesh of 200 mesh or more, or fine particles having an average particle diameter of 20 ㎛ or less, the first step is pH 3.0 to 4.5 It is preferably carried out in the range, the second step is preferably performed in the range of pH 7.5 ~ 10.0. The calcium phosphate salt thus recovered has a high P / Ca molar ratio, especially P / Ca molar ratio in the range of 0.6-1.0.

Phosphate Wastewater, Selective Sedimentation, Recycling, Waste, Limestone, Calcium Phosphate, Phosphite

Description

Recovering Method of Phosphate Compound from Waste Water

The present invention relates to a method for recovering phosphate components from wastewater, and more particularly, to wastewater generated from a phosphate manufacturing process in which phosphoric acid is treated with sulfuric acid to obtain phosphoric acid, that is, wastewater containing phosphoric acid, sulfuric acid, and hydrofluoric acid in a calcination cycle. The present invention relates to a method for recovering phosphate components from wastewater, which can greatly reduce the generation of waste by treating calcium phosphate salts by treatment and also recycle resources.

Phosphoric acid is an essential component of plant growth and is one of the three major components of fertilizers. Various phosphoric acid compounds are also used for industrial purposes. The starting point of these phosphate compounds starts from the production of phosphoric acid by reacting phosphate ore with acid, and Korea has imported and used an average of 1.3 million tons of phosphate ore for the past 20 years. However, phosphorus is also classified as a substance that causes eutrophication in rivers and lakes and must be removed below a certain concentration in wastewater treatment plants.

Phosphorite is composed of compounds such as Ca ₃ (PO ₄ ) ₂ , CaF ₂ , SiO ₂ , Fe, Al, etc., mostly 32% of P ₂ O ₅ , around 45% of CaO, about 10% of SiO ₂ , about 3% It is composed of F (CaF ₂ form), Fe ₂ O ₃ , Al ₂ O ₃ , and the like. Phosphoric acid can be produced in various ways, but the most inexpensive method is to pulverize phosphate ore and react with sulfuric acid to extract phosphoric acid. The schematic chemical equation is as follows.

Ca ₃ (PO ₄ ) ₂ + CaF ₂ + H ₂ SO ₄ → H ₃ PO ₄ + CaSO ₄ ↓ + HF

As shown in the chemical equation, the calcium component in the phosphate forms gypsum and the fluorine component forms hydrofluoric acid. Since gypsum is a crystalline material, the more gypsum is produced, the lower the yield of phosphoric acid by containing phosphoric acid formed, and the lower the quality of phosphoric acid if hydrofluoric acid is contained in phosphoric acid. Phosphoric acid is extracted by using hot steam to separate phosphoric acid from gypsum, and the primary crude crude phosphoric acid content is about 53% and fluorine concentration is about 1.4%. This crude crude acid is used as a raw material for various phosphate compounds after various purification processes.

The gypsum produced during the reaction is strongly acidic due to the remaining acid and is stored in the field. During the transfer to the yard and the leachate generated by rain, various acids such as phosphoric acid, sulfuric acid, and hydrofluoric acid are present at high concentrations. Conventionally, the wastewater is neutralized using slaked lime, and the waste generated is used as a low-cost recycling material such as landfill or gypsum for gypsum board production.

More specifically, the leachate generated in the phosphoric acid manufacturing plant is a high concentration of wastewater, and the approximate composition is phosphoric acid (H ₃ PO ₄ ) 10,000-15,000 mg / l, sulfuric acid 5,000-15,000 mg / l, hydrofluoric acid 3,000-6,000 mg / l It consists of degree, other components, and suspended solids (SS). The wastewater is neutralized and discharged by removing phosphorus and fluorine within the wastewater within the regulated values. The chemical used for neutralization is slaked lime and most of the harmful substances can be removed by the following reaction.

H ₂ SO ₄ + Ca (OH) ₂ → CaSO ₄ + 2 H ₂ O

CaSO ₄ + 2 H ₂ O → CaSO ₄ ? 2H ₂ O ↓

H ₃ PO ₄ + Ca (OH) ₂ → CaHPO ₄ + 2 H ₂ O (pH 8 or less)

CaHPO ₄ + 2 H ₂ O → CaHPO ₄ ? 2H ₂ O ↓

2 H ₃ PO ₄ + 3 Ca (OH) ₂ → Ca ₃ (PO ₄ ) ₂ ↓ + 6 H ₂ O (pH 9 and above)

2 HF + Ca (OH) ₂ → CaF ₂ ↓ + 2 H ₂ O

The chemical reaction formula can simply remove fluorine or P components, but because of the high concentration of substances in the wastewater, it does not precipitate or remove all substances in one step. It will be discharged within the regulation level. In the wastewater treatment step, the pH is adjusted to 4.5 to 5.5 to precipitate the fluorine at least 95% and the P component at 40 to 80%. Precipitation in this way provides a precipitate with a large particle size and easy sedimentation and dehydration. Thereafter, the supernatant can be removed by adjusting the pH to 9-10 using calcined lime again to remove P and F contents in the supernatant within several tens of ppm. The supernatant of the secondary precipitated wastewater can be reduced to 1 to 3 mg / l in the supernatant when the flocculation and sedimentation are performed by finally adding an inorganic coagulant.

However, the former method is fluoride ion (F ^-) harmful substances in the waste water and a phosphate ion (PO ₄ ^{3 -)} was not there was only interested in the removal of, recycling of the precipitated sludge is not considered. In addition, in the previous method, the content of impurities such as Ca, gypsum, and fluorine was too high compared to the content of P, which is an active ingredient in the precipitated sludge, and it was impossible to reuse it.

In recent years, as the price of various raw materials increases and the need for environmental protection increases, the amount of sludge to be disposed of is reduced and resources for recycling are needed. Therefore, the present inventors have found a method of subdividing the treatment process of wastewater containing phosphoric acid, separating only the effective phosphoric acid component in the wastewater, and separating and obtaining it in a reusable state.

In order to separate only recyclable phosphoric acid components from wastewater generated in the process of producing phosphoric acid, it is necessary to first remove impurities such as sulfate ions, fluorine ions, and other suspended substances in the wastewater, and to selectively separate only phosphate ions. . When the wastewater containing phosphoric acid is neutralized by calcination, most of the fluorine and sulfate ions are removed (F: 50 mg / l) in the wastewater after the first neutralization at pH 5, and the main component of the wastewater is phosphate ion (2,700 mg). / l). However, when the pH was raised to 9 or more in the sintered circuit to remove phosphate from the wastewater by secondary treatment, the Ca / P ratio in the sludge was higher than that of phosphate, which made it impossible to recycle.

Therefore, the present inventors have effectively tried to obtain a form that can be reused by effectively separating only the phosphoric acid component which is an active ingredient contained in wastewater generated in the process of producing phosphoric acid. That is, the present inventors have found a reaction condition capable of maximally removing fluoride ions as impurities in primary neutralization and maximizing reusable phosphate ions in the process of treating phosphate wastewater. In addition, when slaked lime and Phosphate ions react to form calcium phosphate precipitates, it is known that poorly soluble calcium phosphate is deposited on the surface of slaked lime particles to lower the P / Ca ratio of the slaked lime. By adjusting the reaction pH it is possible to obtain a useful calcium phosphate precipitate to complete the present invention.

Accordingly, it is an object of the present invention to provide a method for recovering phosphate components from wastewater which allows to reduce the amount of waste generated and to recycle resources.

It is also an object of the present invention to provide a method for recovering phosphate components from wastewater which raises the phosphate component available in the wastewater as much as possible and recovers the phosphoric acid component at a high P / Ca ratio during the precipitation of the phosphoric acid component.

In order to achieve the above object, the method for recovering the phosphate component from the wastewater according to the present invention is added to the wastewater containing phosphoric acid, sulfuric acid and hydrofluoric acid to precipitate sulfuric acid as gypsum, precipitated hydrofluoric acid as calcium fluoride, Phosphoric acid is a first step of maintaining the dissolved state in water with phosphate, a second step of precipitating calcium phosphate salt by adding lime to the supernatant after removing the precipitate, and recovering the precipitated calcium phosphate salt Include.

Here, the slaked lime used in the second step is a fine powder pulverized to pass 99% or more of the standard mesh of 200 mesh or more, or a fine powder having an average particle diameter of 20 ㎛ or less, the first step is pH 3.0 ~ 4.5 It is preferably carried out in the range, the second step is preferably performed in the range of pH 7.5 ~ 10.0, P / Ca molar ratio of the calcium phosphate salt is recovered is preferably in the range of 0.6 ~ 1.0. The wastewater is preferably generated from a phosphate production process in which phosphate is treated with sulfuric acid to obtain phosphoric acid.

The present invention has the effect of reducing the amount of waste generated, the use of raw material phosphate ore by recovering the effective phosphate component in the phosphate wastewater discarded as waste. Recovering 9,500 mg / l of phosphate ions from a phosphate wastewater treatment plant that treats 8,000 tonnes of wastewater a day can reduce the amount of waste generated by 50,000 tonnes per year on a 300-day basis, and replace phosphate with about 47,000 tonnes. It is expected to be possible.

The present invention will be described in detail with reference to the drawings. 1 shows a schematic diagram of a process for recovering phosphate components from wastewater according to the present invention, and FIG. 2 shows a slaked lime crushing apparatus for making finely ground hydrated lime used in the present invention.

And sulfate ion (SO ₄ ^{2 -) -} a hydrofluoric acid ion (F) contains a large amount in order to recover the phosphate ions present in the waste water from the phosphoric acid production process by the phosphate compound which can be used first effluent phosphate ions (PO ₄ ^{3 -} be removed by separation from) is required. To this end, as shown in FIG. 1, the wastewater collected by the sump is neutralized by calcination in the primary neutralization tank. At this time, the present inventors found that when neutralizing the wastewater to the calcination cycle, when neutralizing the pH to 3 or more, most fluoride ions and sulfate ions form precipitates, and only phosphate ions may be present in the wastewater. If the pH is too low, it is difficult to completely remove fluorine, but the concentration of fluorine ions in the wastewater at pH 3 or higher, preferably at pH 3.5 or higher, has no correlation with pH. When the pH is increased to 5 or more, a considerable amount of phosphate ions are precipitated, thereby lowering the recovery rate of phosphate ions. Therefore, in order to firstly neutralize the wastewater containing phosphoric acid so that only a large amount of phosphate ions are present, it was concluded that the optimal method was to adjust the pH to 3.0 to 4.5 and more preferably to 3.5 to 4.0. Figure 3 and Figure 4 is a phosphoric acid ion ^{_{(PO 4 3 -) 13,000 ㎎}} / ℓ, using a fluoride ion to calcium hydroxide 3,900 ㎎ / ℓ of the waste water that contains a graph showing the ion configuration of the supernatant was neutralized primary. According to the figure, 95% or more of fluorine ions were removed, and it was found that about 80% of the phosphate ions were dissolved in the vicinity of pH 3.7.

In the first reaction, sulfate and fluorine ions are removed by precipitation with gypsum (CaSO ₄ ) and CaF ₂ , and then the supernatant is recovered by dehydration and drying after forming calcium phosphate precipitate using slaked lime in a secondary neutralization tank. The calcium phosphate precipitate thus recovered can be used again as a raw material in the phosphoric acid manufacturing process. Reusable calcium phosphate species may include calcium hydrogen phosphate (DiCalcium Phosphate; DCP, CaHPO ₄ -2H ₂ O) or calcium phosphate (Ca ₃ (PO ₄ ) ₂ ). DCP is a calcium phosphate compound used for feed. The calcium phosphate compound thus recovered can be reused as a raw material in the phosphoric acid manufacturing process, that is, replacing the phosphor ore.

The existing wastewater neutralization method is not easy to reuse because the molar ratio of P / Ca is less than 0.56, the molar ratio of phosphate P / Ca, which is reusable due to precipitation formation of insoluble calcium phosphate compounds on the surface of calcium particles. It was. For reference, the P / Ca molar ratio of DCP is 1.07 and the P / Ca molar ratio of calcium phosphate is 0.67. When the material is reused in the phosphate manufacturing process with a high molar ratio, it is possible to minimize the formation of gypsum. The fluorine ion contained is also significantly low, there is an advantage that can be produced in excellent quality phosphoric acid.

These recyclable calcium phosphate compounds can be obtained by adjusting the quality of the slaked lime, the particle size, the reaction pH, the reaction time, etc. when the phosphate ions are precipitated using the slaked lime. Among them, the most important is to use finely pulverized particles or fine particles having an average particle diameter of 20 μm or less through 99% or more of standard meshes of 200 mesh or more through pulverization to the particle size of slaked lime. If the particle size of the slaked lime is too large, it is not preferable because poorly soluble calcium phosphate salt is deposited on the surface of the slaked lime particles to lower the P / Ca ratio of the final product. However, even if the particle size of the lime is very small, it does not cause any particular problem. However, too small slaked lime may be determined to be of an appropriate size because its manufacturing cost may increase and economic efficiency may not be good. For example, if the average particle diameter is 1 μm or more, or fine particles pulverized to pass 99% or more of the standard mesh of 500 mesh or less, it may be referred to as a suitable size. As shown in FIG. 2, the slaked lime is mixed with the process water before being used in the neutralization tank and then pulverized by a pulverizer, and then the slaked lime of the above conditions is separated and introduced into the neutralization tank by the separator. The large particle sized lime separated by the separator is circulated and pulverized again.

Next, this step of forming a precipitate of calcium phosphate salt is preferably performed in the range of pH 7.5 ~ 10.0. In this pH range, the precipitate of calcium phosphate salt may be formed in about 10 minutes to 1 hour. The P / Ca molar ratio of the calcium phosphate salt thus formed may be higher than that of phosphate ore, in particular, in the range of 0.6 to 1.0.

On the other hand, in the embodiment of the present invention it was confirmed that the neutralization of the calcination circuit prepared to pass through the 325 mesh (mesh) network can obtain a precipitate having a P / Ca ratio of 0.6 or more. The obtained calcium phosphate precipitate can be recycled as a raw material to replace the phosphate ore through the drying process.

Hereinafter, the present invention will be illustrated in more detail through examples. However, the reaction conditions mentioned in the examples are not limited to the phrase, and may be carried out even if a slight change in conditions in the general common sense range.

Comparative Example 1 (existing neutralization method)

2 l of wastewater containing phosphoric acid (H ₃ PO ₄ ) 13,500 mg / l, hydrofluoric acid (HF) 4,000 mg / l, sulfuric acid (H ₂ SO ₄ ) 8,000 mg / l in a beaker, with a 1/8 horsepower motor It is mounted on a mechanical stirrer capable of adjusting the rotation speed with a stirring rod having a rotor length of 10 cm. In the case of raw wastewater, it is difficult to measure pH accurately with a general laboratory pH meter due to the influence of hydrofluoric acid. A slurry of slaked lime prepared at a CaO 17.5% concentration is added while properly adjusting the rotation speed of the stirrer. The slaked lime slurry is weighed before and after loading and the dose recorded. When CaSO ₄ and CaF ₂ precipitates are formed, the pH meter is installed and slaked lime is continuously added as the pH changes. The reaction is continued for about 15 minutes to complete the first reaction at pH 4.8 and allow the reaction to stand. The slaked lime input was 148 g. The product of the first reaction included heavy crystalline gypsum and calcium fluoride, which was good in settling, and after 10 minutes of precipitation, the sludge volume was 360 ml (2,120 ml of the total solution). The precipitated sludge was filtered and dried to give 68.7 g of precipitate. The CaO content in the precipitate was 39.8%, PO ₄ ^{3 -} content was 25.3% (P ₂ O ₅ in terms of concentration 18.9%), F ^- content was 11.3%. This sludge is usually phosphate rock components (CaO 45% or less, P ₂ O ₅ perch least 32%, F ^- content of less than 3%) it is not possible to recycle a much lower quality compared to.

The composition of the supernatant was PO ₄ ^{3 -} content 4,800 mg / l, and the F ^- content was 16 mg / l. Take 1 liter of this supernatant, place it in the above stirrer, and inject the above-mentioned slaked lime in view of pH. When the reaction is 15 minutes to pH 9.5, the stirring is stopped and precipitated. The amount of slaked lime injected was 52.5 g. After 10 minutes, the precipitate settles down and the layers separate. The volume of the precipitate was 250 ml (1,050 ml total solution). The PO ₄ ^{3 −} content in the supernatant was 88 mg / l and the F ⁻ content was 10 mg / l. The precipitate was filtered off and dried to yield 17.8 g of product. The water content of this product was 60%. CaO content in the dried precipitate was 59.8%, P ₂ O ₅ concentration was 19.8%.

The supernatant of the secondary reactor is discharged by treating the Al-based inorganic coagulant in the tertiary reactor with a concentration of PO ₄ ^3- ions of 8 mg / l and a concentration of F ⁻ ions of 10 mg / l or less.

Comparative example 2 (use of lime with large particle size)

The reaction was continued for about 15 minutes by the method of Comparative Example 1 to complete the first reaction at pH 3.7, and the reaction was allowed to stand. The slaked lime input was 106 g.

The composition of the supernatant was PO ₄ ^{3 −} content 11,600 mg / l and the F ⁻ content 190 mg / l. Take 1 liter of this supernatant, place it in the above stirrer, and inject the above-mentioned slaked lime in view of pH. After reaction for 15 minutes to reach pH 9.8, the stirring is stopped and precipitated. The amount of slaked lime injected was 70.3 g. After 10 minutes, the precipitate settles down and the layers separate. The volume of the precipitate was 400 ml (total volume 1,060 ml). The PO ₄ ^{3 −} content in the supernatant was 93 mg / l and the F ⁻ content was 6 mg / l. The precipitate was filtered off and dried to give 29 g of product. The water content of this product was 73%. CaO content in the dried precipitate was 48.2%, P ₂ O ₅ concentration was 24.4%. The molar ratio of P / Ca was 0.4, which was significantly lower than that of the phosphorescent ore of 0.56, making it difficult to reuse as a raw material to replace phosphorescent ore.

Example 1 (with crushed lime)

Reaction conditions are set by the method of Comparative Example 1. 500 g of the slaked lime slurry used in Comparative Example 1 was added to an Attrition mill with an average bead size of 2 mm in a pot volume of 1 L, and ground at a speed of 2,000 rpm for 5 minutes, followed by 325 Mesh standard mesh. To separate the pulverized lime. The CaO concentration of the slurry thus separated was 17.1%. As a result of measuring the particle size of this slaked lime slurry using an alcohol solvent with a laser scattering particle size meter, the average particle diameter was 10 micrometers and the maximum particle diameter was 30 micrometers. This slaked lime is added and the reaction is continued for about 15 minutes to complete the first reaction at pH 3.7, and the reactant is left still. The slaked lime input was 106.2 g. The product of the primary reaction had a sludge volume of 180 ml (volume of 2,080 ml of the total solution) after 10 minutes of precipitation.

The composition of the supernatant was PO ₄ ^{3 −} content 10,400 mg / l and the F ⁻ content 72 mg / l. Take 1 liter of this supernatant, place it in the above stirrer, and add pulverized lime with pH. After reaction for 15 minutes to pH 9.7, the stirring is stopped and precipitated. The amount of slaked lime injected was 43.7 g. After 10 minutes, the precipitate settles down and the layers separate. The volume of the precipitate was 340 ml (1040 ml total solution). The PO ₄ ^{3 −} content in the supernatant was 110 mg / l and the F ⁻ content was 6 mg / l. The precipitate was filtered off and dried to give 24.1 g of product. The water content of this product was 68%. CaO content in the dried precipitate was 35.05%, P ₂ O ₅ concentration was 31.9%, F concentration 0.38%. The component of this product has a higher P / Ca mole ratio (phosphorite 0.56, product 0.71 of the example) than the component of phosphorite (less than 45% CaO, more than 32% P ₂ O ₅ concentration, less than 3% F ^- content), The low fluorine content reduces gypsum production when used in the production of phosphoric acid, and produces high quality phosphoric acid in high yield because of less hydrofluoric acid.

The supernatant of the secondary reactor is discharged by treating the Al-based inorganic coagulant in the tertiary reactor with a concentration of PO ₄ ^3- ions of 8 mg / l and a concentration of F ⁻ ions of 10 mg / l or less.

Example 2 (using crushed lime, adjusting reaction time)

Primary neutralization was adjusted to pH 3.6 by the method of Example 1. The composition of the supernatant PO ₄ ^3- content of 10,700 ㎎ / ℓ, F ^- content was 101 ㎎ / ℓ. 1 L of this supernatant is taken and placed in a stirrer, and pulverized lime is added while seeing pH. The reaction time was adjusted to 35 minutes and the reaction pH was adjusted to 8.2. The amount of slaked lime injected was 32.6 g. After 10 minutes, the precipitate settles down and the layers separate. The volume of the precipitate was 180 mL (1,030 mL total solution). The PO ₄ ^{3 −} content in the supernatant was 520 mg / l and the F ⁻ content was 21 mg / l. The precipitate was filtered off and dried to give 20.7 g of product. The water content of this product was 59%. CaO content in the dried precipitate was 35.4%, P ₂ O ₅ concentration was 36.3%, F concentration 0.35%.

The table below compares the data for the products obtained for each reaction condition.

When the wastewater is treated by the method of the embodiment, it is possible to save more than 30% of the chemicals used for the second neutralization, and to obtain recyclable phosphate compounds of better quality than phosphate ore, which can reduce waste and recycle resources. .

1 shows a schematic diagram of a process for recovering phosphate components from wastewater in accordance with the present invention.

Figure 2 shows a hydrated lime crushing apparatus for making fine pulverized lime used in the present invention.

3 is a graph showing the concentration of fluorine ions remaining in the supernatant according to pH when the wastewater containing phosphate ions and fluorine ions is treated with calcination.

4 is a graph showing the concentration of phosphate ions remaining in the supernatant according to pH when the wastewater containing phosphate ions and fluorine ions is treated with calcination.

Claims (5)

A first step in which hydrated lime is added to wastewater containing phosphoric acid, sulfuric acid and hydrofluoric acid to precipitate sulfuric acid as gypsum, hydrofluoric acid as calcium fluoride, and phosphoric acid as phosphate to be dissolved in water; A second step of precipitating calcium phosphate salt by removing lime and calcium fluoride, and then adding lime to the wastewater; and Recovering the precipitated calcium phosphate salt from the wastewater. The method of claim 1, The slaked lime used in the second step is fine particles ground to pass 99% or more of the standard mesh of 200 mesh or more, or fine particles having an average particle diameter of 20 μm or less. The method according to claim 1 or 2, The first step is carried out in the range of pH 3.0 ~ 4.5, the second step is a method for recovering the phosphate component from the waste water, characterized in that carried out in the range of pH 7.5 ~ 10.0. The method according to claim 1 or 2, P / Ca molar ratio of the recovered calcium phosphate salt is a method for recovering the phosphate component from the wastewater, characterized in that the range of 0.6 ~ 1.0. The method according to claim 1 or 2, The wastewater is a method for recovering the phosphate component from the wastewater, characterized in that from the phosphate manufacturing process of treating the ore with sulfuric acid to obtain phosphoric acid.

Images