JPS6141278B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a dephosphorizing agent and a dephosphorizing method suitable for treating water containing phosphates. In recent years, the progress of eutrophication in closed water bodies such as lakes, marshes, and inner bays has become a serious problem.
Phosphate present in water has been highlighted as a cause of eutrophication, and its removal has been raised as an urgent issue. Phosphates, which cause eutrophication, are contained in tap water, sewage, industrial water, factory wastewater, boiler water, etc.; It exists in the form of phosphate. As a method to remove such phosphates, water containing phosphates is treated in the presence of calcium ions,
A method has been proposed in which contact with crystal species containing calcium phosphate, such as phosphate rock (Dissertation
Abstracts International, Vol.33, No.12, Part
I, pages 5878-B, etc.). This method removes phosphate ions contained in water by converting them into calcium phosphate, such as hydroxyapatite, and crystallizing them into crystal seeds. , has been attracting particular attention in recent years because it has significantly improved processing efficiency. By the way, natural phosphate rock is mainly used as the crystal seed, but the supply of phosphate rock is limited and stable supply is difficult, and the chemical composition and physical properties differ depending on the production area. However, there were problems such as the difficulty of stable treatment due to differences in dephosphorization effects. The present invention is intended to solve these conventional problems, and provides a dephosphorization agent and a dephosphorization agent that can be easily produced and can perform efficient dephosphorization by supporting calcium phosphate crystals using zeolite or lime as a carrier. The purpose of this invention is to provide a dephosphorization method using this method. This invention includes the following two inventions. (1) Contains a substance in which a carrier made of zeolite or limestone is brought into contact with an aqueous solution containing phosphoric acid or its salt, separated from the solution, and then brought into contact with a solution containing limestone to form calcium phosphate crystals. Dephosphorizing agent. (2) A packed bed filled with a dephosphorizing agent in which a carrier made of zeolite or limestone is brought into contact with an aqueous solution containing phosphoric acid or its salt, separated from the solution, and then brought into contact with a lime solution to produce calcium phosphate. A dephosphorization method characterized by passing water containing phosphate in the presence of calcium ions. Types of zeolite and limestone as carriers,
The origin is not particularly limited, and both natural and artificial ones can be used. Zeolites with high silica content relative to alumina, such as clinoptilolite and mordenite, are preferred; these can withstand use up to pH 2, and have a lower usable pH range than other zeolites and limestone, which have a pH of only 4. Marble, coral stone, calcite, etc. are preferable as limestone that has a wide range of properties. Both zeolite and limestone have excellent chemical adsorption ability for phosphorus and are suitable as a carrier, and either one or a mixture of both can be used as a carrier. Although these may be used as a carrier in powder form, granular materials are preferable because they can form a packed layer during the dephosphorization treatment. In the case of granular materials, both zeolite and limestone require a greater powdering strength as a carrier than conventional phosphate rock. The carrier as described above is first brought into contact with a solution containing phosphoric acid or a salt thereof to support phosphorus. Both zeolite and limestone have the property of adsorbing phosphorus from a phosphoric acid (salt) solution, and by bringing them into contact by an impregnation method or the like, they can easily adsorb and support phosphorus. As the phosphate, various orthophosphates such as sodium phosphate and potassium phosphate can be used. The concentration of the phosphoric acid (salt) solution that is brought into contact with the carrier is
It can be arbitrarily selected from 100mg/ to several tens of weight%. The contact operation may be either a batch operation or a continuous water flow method. The pH of the aqueous phosphoric acid (salt) solution during contact is not particularly limited, but acidic conditions are preferred in order to efficiently support phosphorus, especially pH 6.
Conditions below are desirable. This is thought to be because the state of dissociation of phosphate differs depending on the pH, and the tendency for adsorption to the carrier surface is higher at low pH.At pH 6 or higher, it takes time to obtain stable treated water quality. Contact time depends on carrier, type of phosphate, concentration, and pH.
It may take about 3 to 50 hours, although it varies depending on the situation. The carrier that has undergone the phosphorus loading treatment is separated from the phosphorus-containing solution. Separation may be carried out by ordinary gravity separation, and if necessary, washing with water may be performed. The separated solution can be used for the next loading operation. At this time, by adding phosphate as necessary, a stable supporting treatment effect can be maintained. If the next operation is carried out without separating the solution, the phosphate ions in the solution will react with the slaked lime and a large amount of calcium phosphate precipitate will be produced, complicating the treatment process. Moreover, at this time, since slaked lime is consumed, the required amount of slaked lime increases and processing cost increases. Furthermore, fine calcium phosphate crystal seeds having a weak bonding force to the carrier surface adhere to the carrier surface and are detached during use, which is undesirable because the treatment effect is reduced. The carrier from which the solution has been separated is brought into contact with a solution containing lime, and the phosphorus supported on the carrier surface reacts with calcium hydroxide to produce calcium phosphate crystals with high dephosphorizing properties. As the solution containing lime, a slaked lime aqueous solution can be used, but other substances may be mixed therein. In order to produce high-quality calcium phosphate crystals, the amount of calcium hydroxide must be greater than a certain amount relative to the amount of supported phosphorus, and if this is insufficient, the performance of the resulting dephosphorizing agent will be poor. In order to obtain a dephosphorizing agent with a high removal rate under standard dephosphorizing conditions, it is desirable to contact the dephosphorizing agent with a solution containing calcium hydroxide in an amount of 4 times or more by weight relative to the amount of supported phosphorus. The concentration of calcium hydroxide in the solution is 0.1 to 5% by weight, and the contact time is 1 to 7%.
A few days is enough. The product obtained by the above operation is as it is,
Or it can be used as a dephosphorizing agent in a mixed state with other substances. The dephosphorization method is the same as in the case of conventional crystal seeds such as phosphate rock, and if the dephosphorization agent is obtained in powder form, it is brought into contact with raw water containing phosphate in the form of a slurry, and if it is obtained in granular form, it is brought into contact with raw water containing phosphate. A packed bed is formed and water is passed through it. When the activity of the dephosphorizing agent decreases due to continued dephosphorization, it can be reactivated by performing the above-described calcium phosphate crystal production operation again. In addition, in the above operation, it is also possible to add other chemicals, or to use or add other treatments in order to promote the reaction or make it more efficient. The second invention of the present invention is to remove phosphate ions in the form of hydroxyapatite by passing water containing phosphate in the presence of calcium ions through the packed bed filled with the dephosphorizing agent obtained above. This is a method of dephosphorization by crystallization on a phosphorizing agent, and the usage of the dephosphorizing agent will be explained below regarding this method. The raw water to be treated in the present invention is water containing phosphates and organic substances, and includes secondary treated water such as sewage, human waste, and industrial wastewater. Crystallization is performed by bringing such raw water into contact with a dephosphorizing agent in the presence of calcium ions. The reaction that occurs at this time varies depending on the reaction conditions, but is usually expressed by the following formula. 5Ca ²⁺ +7OH ^- +3H ₂ PO ₄ ^- → Ca ₅ (OH) (PO ₄ ) +6H ₂ O ...(1) In order to efficiently remove phosphate from water containing phosphate, use equation (1). It is necessary to allow the reaction to proceed to the right, and for this purpose, it is necessary to add a calcium agent or an alkaline agent as necessary to make calcium ions and hydroxide ions present. If the amount of these ions becomes too large, fine precipitates may be formed in places other than the dephosphorizing agent, or calcium carbonate precipitates may be formed, so it should be within a range where these do not occur. That is, the amounts of calcium ions and hydroxyl ions are higher than the solubility of hydroxyapatite produced in equation (1),
This is a condition in which hydroxyapatite is produced at a concentration lower than supersolubility, that is, at a concentration in the metastable range. Here, supersolubility is the concentration at which crystals begin to precipitate when no crystal seeds are present in the reaction system. In order to keep the amounts of calcium ions and hydroxide ions within the above ranges, a calcium agent and/or an alkaline agent is added to the phosphate-containing water as necessary. Suitable amounts of calcium and alkaline agents to be added can be determined in advance through simple experiments, but if the phosphate content in the raw water is 50mg/or less, the calcium ion content is 10 to 200mg/, and the pH is approximately 6 to 12. . Calcium agents used in this invention include calcium hydroxide and calcium chloride; alkaline agents include sodium hydroxide, potassium hydroxide,
Examples include calcium hydroxide. The contact between the water containing phosphate and the dephosphorizing agent depends on the packed bed water flow system, but in the case of a fixed bed, it is 9 to 35 mesh
In the case of a fluidized bed, it is filled with a dephosphorizing agent having a particle size of 36 to 300 mesh, and water is passed in an upward or downward direction at a flow rate of SV1 to 20 hr ^-1 to precipitate hydroxyapatite crystals. In the case of upward flow, suspended matter can be trapped in the large particle size portion of the lower layer, and crystallization can be performed in the small particle size portion of the upper layer with high activity. Similarly, when water is passed in a downward flow, a filter medium with a smaller specific gravity and larger particle size than the dephosphorizing agent is stacked on top of the dephosphorizing agent packed layer to avoid adhesion of suspended matter to the surface of the dephosphorizing agent. However, it is desirable to remove suspended solids using this filter medium. If the surface of the dephosphorizing agent becomes contaminated or clogged during water flow, periodically perform backwashing using upward flow to spread out the dephosphorizing agent and remove impurities attached to the surface. is desirable. Water flow conditions during backwashing include a flow rate of about 20 to 80 m/hr, and a backwashing time of about 5 to 60 minutes. When crystallization is performed as described above, hydroxyapatite with low solubility is mainly produced according to equation (1), which crystallizes on the surface of the dephosphorizing agent, and the phosphate concentration in the treated water becomes low. In addition, in the above treatment, the raw water is pretreated prior to the dephosphorization operation, the treated water is subjected to post treatment, or other treatments are used in conjunction with the dephosphorization treatment, or chemicals are added. It is also possible. As described above, according to the present invention, it is possible to efficiently and stably produce a dephosphorizing agent with excellent dephosphorizing performance with simple operations, and also to perform efficient dephosphorizing treatment using the same. can. A similar technique is to obtain a dephosphorizing agent by contacting granular materials such as sand, anthracite, and activated carbon with a solution containing phosphates adjusted to pH 6 or higher in the presence of calcium salts. In this method, calcium phosphate is difficult to crystallize, so the dephosphorization performance cannot be said to be good. In contrast, in the present invention, phosphorus is strongly adsorbed chemically or physicochemically in advance on zeolite or limestone, which has good phosphorus adsorption properties, and then crystallized, so uniform crystals are formed and excellent. It shows dephosphorizing activity. Next, examples and comparative examples of the present invention will be described. Example 1 200ml (195g) of clinoptilolite natural zeolite with particle size of 16-32 meshes (1-0.5m/mφ)
was placed in an Erlenmeyer flask and washed 5 times by decantation using 500 ml of tap water each time. Next, the carrier was transferred to 500 ml of a monosodium phosphate (NaH ₂ PO ₄ ) aqueous solution (PH 4.3) adjusted to a phosphorus concentration of 8000 mg/day, and after being impregnated for one day, the reaction residue and the zeolite carrier were separated. The amount of phosphorus supported on the zeolite carrier in the above impregnating and supporting treatment operation was calculated from the decrease in phosphorus concentration in the phosphoric acid solution, and was 4.4 mg-P/g-support. Subsequently, in 500 ml of 3.2 wt% slaked limestone emulsion,
The above carrier was subjected to contact treatment for 3 days. The weight ratio between the amount of slaked lime used and the amount of supported phosphorus is approximately 15. After 3 days of contact treatment, the reaction residue was separated and thoroughly washed with tap water. 150 ml of the dephosphorizing agent prepared in this way was added to the inner diameter
Packed into a 30mm acrylic column, with a phosphorus concentration of 2.
At the column inlet, calcium chloride aqueous solution and sodium hydroxide aqueous solution were added to synthetic water whose total alkalinity was adjusted to about 100 mg/, and the calcium ion concentration was about 45 mg/, and the pH was 8.5 to 8.8.
Continuous liquid flow treatment was carried out in an upward flow for about two weeks at a flow rate of SV2hr ^-1 . After the water flow started, the average phosphorus concentration of the treated water remained almost stable at 0.18 mg/. Example 2 Dephosphorization was carried out under the same conditions as in Example 1, except that monosodium phosphate aqueous solutions (PH4.4 to 4.7) with different phosphorus concentrations were brought into contact with zeolite, and the amount of phosphorus supported was changed. A drug was prepared. In addition, a dephosphorizing agent was prepared in the same manner as above, using limestone (coralstone) having a particle size of 16 to 32 mesh instead of zeolite. The obtained dephosphorizing agent was subjected to a liquid passage treatment under the same conditions as in Example 1, and the results regarding the supported amount of phosphorus and the phosphorus concentration of the treated water are shown in the graph of the drawing together with the results of Example 1. Comparative Example 1 A dephosphorizing agent was prepared in the same manner as in Example 2 using activated carbon having a mesh particle size of 16 to 32, and the results of the liquid flow treatment are also shown in the graph of the drawing. Compared to Examples 1 and 2, the dephosphorization performance is lower, and the phosphorus removal rate is about 40%. Comparative Example 2 The zeolite, limestone, and activated carbon used in Examples 1, 2, and Comparative Example 1 were filled without treatment, and the dephosphorization ability of the carrier itself was tested by passing liquid under the same conditions as Example 1. In all cases, the phosphorus removal rate was less than 10%. Example 3 In Example 1, a dephosphorizing agent was prepared and tested under the same conditions as in Example 1, except that the weight ratio of the amount of slaked lime during slaked lime treatment and the amount of supported phosphorus was changed. The results are shown in Table 1.

[Table] Example 4 Example 1 except that sodium hydroxide was added to the phosphate solution to change the pH.
Table 2 shows the results of preparation and testing under the same conditions as .

[Table] Comparative Example 3 The treatment was carried out in the same manner as in Example 1, except that the sample was brought into direct contact with the slaked lime emulsion without being brought into contact with the phosphate aqueous solution. As a result, 4
Until the 1st day, the same level of treated water as in the present invention was obtained, but after the 5th day, the quality of the treated water deteriorated rapidly: 0.64 mg/on the 7th day, 0.86 mg/on the 10th day, and 0.86 mg/on the 15th day. in
It was 1.18mg/. After 15 days had elapsed, the same lime treatment as above was performed again, and continuous operation was performed again. As a result, treated water of good quality was obtained until the 7th day, but
After that, it got extremely bad. The following results can be found from the above results. The dephosphorizing agent of the present invention has much better dephosphorizing performance than those in which the carrier is not specially treated, only lime is treated, or activated carbon is treated in the same manner as in the present invention. ing. From the results in the drawings, the dephosphorization performance tends to improve as the amount of phosphorus carrier increases, especially at 0.7 mg.
-P/g-carrier or more, more preferably 1 mg-
It is noticeable above P/g carrier. From the results in Table 1, even if the amount of supported phosphorus is large, if the amount of calcium hydroxide is insufficient,
Since excellent dephosphorization performance cannot be obtained, it is desirable to increase the amount by at least 4 times the amount of supported phosphorus. From the results in Table 2, it is desirable that the aqueous solution containing phosphoric acid or its salt that comes into contact with the carrier has a pH of less than 6.

[Brief explanation of the drawing]

The drawings are graphs showing the results in Examples.

Claims (1)

[Claims] 1. After bringing a carrier made of zeolite or limestone into contact with an aqueous solution containing phosphoric acid or its salt,
A dephosphorizing agent containing a substance that is separated from the solution and then brought into contact with a solution containing lime to form calcium phosphate crystals. 2 The amount of phosphorus supported on the carrier by contacting with an aqueous solution containing phosphoric acid or its salt is 0.7 mg-P/g-
The dephosphorizing agent according to claim 1, which is more than a carrier. 3. The dephosphorizing agent according to claim 1 or 2, wherein the aqueous solution containing phosphoric acid or its salt that contacts the carrier has a pH of less than 6. 4. The dephosphorizing agent according to any one of claims 1 to 3, wherein the amount of calcium hydroxide relative to the amount of phosphorus supported in the lime-containing solution is 4 times or more by weight. 5. After contacting a carrier made of zeolite or limestone with an aqueous solution containing phosphoric acid or its salt,
The treatment is performed by passing water containing phosphate in the presence of calcium ions through a packed bed filled with a dephosphorizing agent that is separated from the solution and then brought into contact with a lime solution to produce calcium phosphate. This is a dephosphorization method. 6 The amount of phosphorus supported on the carrier by contacting with an aqueous solution containing phosphoric acid or its salt is 0.7 mg-P/g-
The dephosphorization method according to claim 5, wherein the amount is more than a carrier. 7. The dephosphorization method according to claim 5 or 6, wherein the aqueous solution containing phosphoric acid or its salt that contacts the carrier has a pH of 6 or less. 8. The dephosphorization method according to any one of claims 5 to 7, wherein the amount of calcium hydroxide relative to the amount of phosphorus supported in the lime-containing solution is 4 times or more by weight.