JP2004089760A - Highly selective dephosphorizing agent and method for producing the same - Google Patents

Abstract

An object of the present invention is to provide a dephosphorizing agent which enhances the selectivity between phosphorus dissolved in water and other anions and selectively removes phosphorus even in the presence of other anions.
A general formula ^{_{M II 1-x M III x}} (OH) 2 A n- y · mH 2 O
^{(M II} is at least one selected from divalent ^{metals, M III} is at least one selected from among trivalent ^{metal, A n-n-valent} anion, x, y and m are 0 <x ≦ 0.67, 0 <y ≦ 1, 0 ≦ m ≦ 2)
The heat-treated product of the crystal of the composite metal hydroxide represented by the formula (1) is used as an active ingredient, and a highly selective dephosphorizing agent having a selectivity of 5 or more for phosphorus ions dissolved in water.
[Selection diagram] None

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a dephosphorizing agent for efficiently removing and purifying phosphorus, which is contained in rivers, lakes, marshes, seawater, and water and sewage, and is a pollution source, and a method for producing the same.
[0002]
[Prior art]
Phosphorus in wastewater discharged from factories and households causes eutrophication of rivers, lakes and marshes and seawater, causing environmental destruction, and its removal and recovery is a major social problem.
[0003]
Therefore, a variety of underwater dephosphorization agents and underwater dephosphorization methods have been proposed. That is, as the underwater dephosphorizing agent, for example, a dephosphorizing material composed of a reaction product of a calcareous raw material, a siliceous raw material and a zeolite (Japanese Patent Application Laid-Open No. 2001-9470), and a substance mainly composed of allophane are molded. A slag obtained by melting treatment of industrial waste and a slag obtained by melting industrial waste are pulverized, and calcium oxide in the slag is removed by alkali treatment to remove porous material. Phosphorus-removed inorganic adsorbent (JP-A-63-39632), ash discharged from a fluidized-bed boiler as a main component (JP-A-5-261378), hydrotalcites A dephosphorizing agent containing as an active ingredient (JP-A-2000-24658) and the like are known, and examples of the dephosphorizing method in water include aluminum oxide and sodium oxide. Of dewatering seawater by bringing seawater into contact with porous alumina (Japanese Patent Application Laid-Open No. 6-328067), adding magnesium ions to phosphate wastewater containing ammonium ions, adjusting the pH value to 8 or more, and then adding phosphorus A method in which phosphorus in wastewater is precipitated as magnesium ammonium phosphate particles on the surface layer of the above-mentioned granular material by passing through a packed layer of the granular material containing magnesium ammonium phosphate (Japanese Patent Application Laid-Open No. 63-200888). A method of adding a salt solution to the aqueous solution to adjust the electric conductivity of the solution to 2000 micromhos / cm or more when contacting with a granular material having a phosphorus removing ability in the presence of ions to perform contact dephosphorization. (Japanese Unexamined Patent Publication (Kokai) No. 3-207489), a phosphorous-containing wastewater is brought into contact with a composite metal hydroxide containing a divalent metal and a trivalent metal, and the Adsorbing components, then a method of reproducing the phosphorus adsorbent which has adsorbed the phosphorus component is treated with an aqueous solution of an alkali metal salt or alkaline earth metal salts (JP-A-11-57695) are known.
[0004]
However, these dephosphorizing agents and dephosphorization methods reduce the phosphorus removal rate when other anions, particularly sulfate ions, coexist, and cannot actually completely prevent eutrophication in seawater and the like. Was.
[0005]
[Problems to be solved by the invention]
Under such circumstances, the present invention enhances the selectivity between phosphorus dissolved in water and other anions, and even in the presence of other anions, the removal rate of phosphorus is selectively increased. The purpose is to provide a high dephosphorizing agent.
[0006]
[Means for Solving the Problems]
The present inventors have intensively studied to develop a dephosphorizing agent capable of selectively removing phosphorus with a high selectivity even when coexisting with other anions. In the composite metal hydroxide, that is, in the heat-treated product of the crystal of the composite metal hydroxide having a predetermined composition, the object can be achieved by adjusting the selectivity coefficient of phosphorus to sulfate ions to be 5 or more. The present invention has been made based on this finding.
[0007]
That is, the present invention has the general formula ^{_{M II 1-x M III x}} (OH) 2 A n- y · mH 2 O (I)
(At least one is M ^II in formula selected from the divalent metal, M ^III is at least one selected from among trivalent metal, A ^n-represents an n-valent anion, x, y and m are numbers satisfying 0 <x ≦ 0.67, 0 <y ≦ 1, 0 ≦ m ≦ 2)
A highly selective dephosphorizing agent, characterized in that a heat-treated product of a crystal of a composite metal hydroxide represented by the following formula is used as an active ingredient, and the selectivity of phosphorus dissolved in water to sulfate ions is 5 or more, And a mixed aqueous solution of a water-soluble salt of an inorganic acid or an organic acid and an alkali hydroxide is dropped into an aqueous solution containing a water-soluble compound of a divalent metal and a water-soluble compound of a trivalent metal, and then kept at a temperature of 0 to 90 ° C. reacted Te, formula ^{_{M II 1-x M III x}} (OH) 2 A n- y · mH 2 O (I)
(At least one is M ^II in formula selected from the divalent metal, M ^III is at least one selected from among trivalent metal, A ^n-represents an n-valent anion, x, y and m are numbers satisfying 0 <x ≦ 0.67, 0 <y ≦ 1, 0 ≦ m ≦ 2)
A precipitate of crystals of the composite metal hydroxide represented by or a water-soluble salt of an inorganic or organic acid and an alkali hydroxide are added to an aqueous solution containing a water-soluble compound of a divalent metal and a water-soluble compound of divalent manganese. After the dropwise addition of a mixed aqueous solution of the following formula, the reaction is carried out under oxidizing conditions while maintaining the temperature at 0 to 90 ° C., and the general formula M ^II _1-x Mn (III) _x (OH) _{2 An} ^- _y · mH ₂ O (II )
^{(M II, A n-, x} , y and m in the formula have the same meanings as defined above)
Wherein a precipitate of crystals of the composite metal hydroxide represented by the formula (1) is formed, and then the precipitate is separated and heat-treated at 200 to 500 ° C. to provide a method for producing a dephosphorizing agent.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Examples of the divalent metal M ^II in the general formula (I) of the present invention include Mg, Zn, Cu (II), Co (II), and Ni (II). Mg, Zn and Co (II). Examples of the trivalent metal M ^III include Al, Mn (III), Fe (III), and Co (III), and Mn and Al are particularly preferred. Each of M ^II and M ^III may be a single metal or a composite of two or more metals.
[0009]
Next, the anion A ^n- is in formula (I), it is necessary to have an ion-exchange, for example carbonate ions, sulfate ion, a halogen ion, such as a hydroxide ion. Of these, hydroxide ions are particularly preferred, and carbonate ions and hydrogen carbonate ions are preferred in that they are easily converted to hydroxide ions by heat treatment.
[0010]
The dephosphorizing agent of the present invention is clearly different from the conventional dephosphorizing agent in that it is based on the reaction between phosphoric acid and a metal, whereas it is based on ion exchange. In addition, this ion exchange is selectively performed only with the ionizer of phosphoric acid even when carbonate ions, halogen ions, nitrate ions, and sulfate ions coexist.
[0011]
Next, x in the general formula (I) is a number in a range of more than 0 and 0.67 or less, and m needs to be a number in a range between 0 and 2.
Then, the anion ^{A n-} quantities y are, Mn, when containing a transition metal such as Co, for valency is not single, the y is the theoretical value with respect to (x / n), the allowable range of about 30% have. Particularly preferred are those in which x is around 0.33 and m is in the range of 0 ≦ m ≦ 2.
[0012]
The dephosphorizing agent of the present invention contains a heat-treated product of a crystal of a composite metal hydroxide having a composition represented by the general formula (I) as an active ingredient, and reacts with sulfuric acid ions of phosphorus dissolved in water. It is necessary that the selection coefficient is 5 or more. This heat-treated product has a characteristic that the crystal structure cannot be maintained and the layer structure is broken, but the layer structure is recovered when phosphorus is adsorbed. This high selectivity for phosphorus is considered to be exhibited by voids between layers of the dephosphorizing agent. That is, it is considered that the layer structure is disordered in the dephosphorizing agent of the present invention, and the size of the void (4 to 6 °) matches the size of the phosphate ion.
Unlike the interlayer gap d determined by X-ray diffraction, the size of the void cannot be determined directly, but is determined indirectly due to selective adsorption and the like. The size of the substance having the same d value varies depending on the type of the constituent element and the heat treatment conditions.
[0013]
Therefore, in the present invention, in order to heat-treat a crystal of the composite metal hydroxide having the composition in the general formula (I) to obtain a crystal having a predetermined selectivity, first, a composite metal hydroxide having a suitable composition It is necessary to prepare crystals of Examples of such a composition include those in which M ^III in the general formula (I) is Mn, Al, or a composite of Al and Co, and M ^II is Mg, Ni, Zn, or a composite of Co and Zn. .
[0014]
And, examples of the crystal of the composite metal hydroxide having such a composition, general formula _{Mg 1-x Mn x (OH} ) 2 (HCO 3) y · mH 2 O (III)
Zn _1-x Al _x (OH) ₂ (HCO ₃ ) _y · mH ₂ O (IV)
(Co, Zn) _1-x (Co, Al) _x (OH) ₂ (HCO ₃ ) _y · mH ₂ O (V)
And _{Ni 1-x Mn x (OH} ) 2 (HCO 3) y · mH 2 O (VI)
(However, x, y and m in each formula have the same meaning as described above.)
Can be mentioned.
The above (Co, Zn) and (Co, Al) mean that the divalent metal Zn and the trivalent metal Al are partially substituted by Co. When a transition metal such as Mn or Co is included, the value of y does not always match the theoretical value (x / n) because the valence is not single. Therefore, the value of y is allowed in the range of 0-1.
[0015]
By heating the bicarbonate of the crystal of the composite metal hydroxide having such a composition at a temperature in the range of 200 to 500 ° C., preferably 250 to 350 ° C., the decarboxylation reaction proceeds, and the compound becomes a hydroxide type. Change. When Mn, Co, and Fe are contained as metal components, these have a plurality of valences, so that the temperature, the atmosphere, the reaction time, and the like are appropriately controlled to have a necessary configuration and an effective layer void. It is necessary to.
[0016]
In the method of the present invention, the water-soluble compound of a divalent metal and the water-soluble compound of a trivalent metal used as a raw material include halides, nitrates, and sulfates of metals such as Mn, Mg, Zn, Co, Ni, Fe, and Al. , Carbonate and the like. In addition, hydroxides such as magnesium hydroxide, aluminum hydroxide and iron hydroxide can be used if desired.
[0017]
Then, as the water-soluble salts of inorganic or organic acids the anions A ^n- sources in the general formula (I), carbonate, sulfate, hydrochloric, inorganic acid or oxalic acid such as nitric acid, acetic acid , Citric acid, alkali metal salts of organic acids such as lactic acid, alkaline earth metal salts, ammonium salts and the like are used. Particularly preferred are alkali metal carbonates and alkali metal hydrogencarbonates, for example, sodium carbonate, potassium carbonate, Sodium bicarbonate and potassium bicarbonate. Examples of the alkali hydroxide used in combination with the above include alkali metal and ammonium hydroxides such as sodium hydroxide, potassium hydroxide, and ammonium hydroxide.
This mixed aqueous solution of a water-soluble salt of an inorganic acid or an organic acid and an alkali hydroxide is used in an amount of about 1 to 40 moles per mole of the divalent metal and trivalent metal compounds.
[0018]
When a water-soluble compound of divalent manganese is used in place of the water-soluble compound of a trivalent metal, a mixed aqueous solution of a water-soluble salt of an inorganic acid or an organic acid and an alkali hydroxide is added dropwise, and then the oxidizing conditions are applied. By reacting slowly at a temperature of 0 to 90 ° C. under, for example, an atmosphere in which the atmosphere is not particularly controlled, the general formula M ^II _1-x Mn (III) _x (OH) _{2 An} ^- _y · mH ₂ O (II )
(A ⁿ⁻ in the formula represents an n-valent anion, and M ^II , x, y and m have the same meaning as described above.)
The precipitate of the crystal of the composite metal hydroxide represented by
[0019]
A mixed aqueous solution of a water-soluble salt of an inorganic acid or an organic acid and an alkali hydroxide is added dropwise to the water-soluble compound of a divalent metal and the water-soluble compound of a trivalent metal prepared as described above, and then 0 to 90 ° C. At a temperature of This reaction is usually carried out for 1 to 5 hours with stirring. By this reaction, crystals of the composite metal hydroxide represented by the general formula (I) precipitate. The crystals are separated by filtration or centrifugation, washed with water until neutral, and air-dried. Thereby, crystals of the composite metal hydroxide are obtained as a powder.
[0020]
This powder is found to be hydrotalcite having a composition represented by the general formula (I) from X-ray structure analysis, composition analysis, valence analysis of each metal, and mass reduction by TG-TDA analysis.
[0021]
Next, the crystal of this composite metal oxide is placed in an electric furnace and subjected to a heat treatment at a temperature of 200 to 500 ° C., preferably 250 to 350 ° C. for 2 to 10 hours. By this heat treatment, anions in the composition are eliminated and the layer structure is rearranged, so that a highly selective dephosphorizing agent having a selectivity of 5 or more for phosphorus dissolved in water is obtained.
[0022]
The thus obtained dephosphorizing agent of the present invention coexists in a phosphorus-dissolved aqueous solution with anions such as chloride ions, nitrate ions, sulfate ions, and carbonate ions, which are generally considered to inhibit the adsorption of phosphorus. However, phosphorus can be selectively and efficiently adsorbed.
[0023]
【Example】
Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.
The selectivity of the phosphorus desorbing agent in each of the examples for the sulfate ion of phosphorus dissolved in water was determined as follows.
[0024]
A mixed anion adsorption experiment was performed using an equimolar (0.02 mmol) mixed solution of carbonate ion, chloride ion, nitrate ion, dihydrogen phosphate ion, and sulfate ion (all used as sodium salts). Calculate the adsorption capacity of each anion from the amount of reduction of the ion species, and take the ratio of the adsorption capacity of dihydrogen phosphate ion as the numerator and the adsorption capacity of other anions as the denominator. The selectivity of dissolved phosphorus for the ions was taken as the coefficient.
[0025]
Example 1
An aqueous solution containing 80 parts by mass of sodium hydroxide and 40 parts by mass of sodium carbonate was dropped into an aqueous solution containing 1 part by mass of manganese (II) chloride and 3 parts by mass of magnesium chloride while stirring, and reacted at 60 ° C. for 2 hours, A precipitate was obtained. Then, the precipitate was centrifuged, washed with water until neutral, and air-dried. As a result of powder X-ray structure analysis, composition analysis, and manganese valence analysis, the product was identified as Mg _0.75 Mn _0.25 (OH) ₂ (HCO ₃ ) _0.25 · mH ₂ O type hydrotalcite. Was done. Further, the value of m was found to be between 1 and 2 from the mass reduction by TG-TDA analysis.
[0026]
This sample was heated in an electric furnace at 300 ° C. for 4 hours to produce a dephosphorizing agent, and the anion adsorption performance was measured by the following method. That is, 0.02 mmol of carbonate ion, chloride ion, nitrate ion, dihydrogen phosphate ion and sulfate ion (all of them are sodium salts) were added to 0.1 g of this sample, and mixed anion adsorption experiment was performed at 27 ° C. After 3 days, the solution was analyzed by anion chromatography. As a result, 99% of the added dihydrogen phosphate ion was adsorbed, whereas 9% of the sulfate ion was adsorbed. That is, the selectivity coefficient of the dissolved phosphorus with respect to the sulfate ion in the water of the dephosphorizing agent was 11. In addition, when the saturated adsorption capacity of phosphoric acid alone was measured, it was 1.4 mmol / g.
[0027]
Example 2
An aqueous solution containing 80 parts by mass of sodium hydroxide and 40 parts by mass of sodium carbonate was added dropwise to an aqueous solution containing 1 part by mass of aluminum chloride and 3 parts by mass of zinc chloride while stirring, and the mixture was reacted at 60 ° C. for 2 hours. Obtained. The precipitate was centrifuged, washed with water until neutral, and air-dried. The product was subjected to powder X-ray structure analysis and composition analysis, and as a result, was identified as Zn _0.75 Al _0.25 (OH) ₂ (HCO ₃ ) _0.25 · mH ₂ O-type hydrotalcite. In addition, the value of m was found to be between 1 and 2 from the mass decrease by TG-TDA analysis.
[0028]
Next, this product was heated in an electric furnace at 300 ° C. for 4 hours to produce a dephosphorizing agent, and its anion adsorption performance was measured by the following method. That is, 0.02 mmol of carbonate ion, chloride ion, nitrate ion, dihydrogen phosphate ion, and sulfate ion (all of them are sodium salts) were added to 0.1 g of the dephosphorizing agent, and the mixed anion adsorption experiment was performed for 27 days. After 3 days at C, the solution was analyzed by anion chromatography. 73% of dihydrogen phosphate ions were adsorbed, while 6% of sulfate ions were adsorbed. Therefore, the selectivity of dissolved phosphorus for sulfate ions in water of this dephosphorizing agent was 13. In addition, when the saturated adsorption capacity of phosphoric acid alone was measured, it was 1.2 mmol / g.
[0029]
Example 3
An aqueous solution containing 40 parts by mass of sodium carbonate is added dropwise to an aqueous solution containing 1 part by mass of aluminum chloride, 1.5 parts by mass of cobalt (II) chloride, and 1.5 parts by mass of zinc chloride while stirring, and then an aqueous solution of sodium hydroxide is used. The pH was adjusted to 9 with. This mixture was reacted at 80 ° C. for 4 hours to obtain a precipitate. Next, the precipitate was centrifuged, washed with water until neutral, and dried at 40 ° C. for 1 day. As a result of powder X-ray structure analysis, the product was identified as hydrotalcite. Also, the value of m was found to be between 1 and 2 from the decrease in mass by TG-TDA analysis.
[0030]
Next, this product was heated in an electric furnace at 300 ° C. for 4 hours to produce a dephosphorizing agent, and the anion adsorption performance was measured by the following method. That is, 0.02 mmol of carbonate ion, chloride ion, nitrate ion, dihydrogen phosphate ion, and sulfate ion (all of them are sodium salts) were added to 0.1 g of the dephosphorizing agent, and the mixed anion adsorption experiment was performed for 27 days. After 3 days at ℃, the solution was analyzed by anion chromatography. As a result, the adsorption of dihydrogen phosphate ions was 91%, while the adsorption of sulfate ions was 2%. Therefore, the selectivity of dissolved phosphorus for sulfate ions in water of this dephosphorizing agent was 55. In addition, when the saturated adsorption capacity of phosphoric acid alone was measured, it was 0.6 mmol / g.
[0031]
Example 4
An aqueous solution containing 80 parts by mass of sodium hydroxide and 40 parts by mass of sodium carbonate was added dropwise to an aqueous solution containing 1 part by mass of manganese (II) chloride and 3 parts by mass of nickel chloride while stirring, and reacted at 60 ° C. for 2 hours, A precipitate was obtained. The precipitate was centrifuged, washed with water until neutral, and air-dried. The product thus obtained was subjected to powder X-ray structure analysis, composition analysis, and valence analysis of manganese, and as a result, Ni _0.75 Mn _0.25 (OH) ₂ (HCO ₃ ) _0.25 · mH ₂ O type was obtained. Was identified as hydrotalcite. In addition, the value of m was found to be between 1 and 2 from the mass decrease by TG-TDA analysis.
[0032]
This product was heated in an electric furnace at 300 ° C. for 4 hours to produce a dephosphorizing agent, and the anion adsorption performance was measured by the following method. That is, 0.02 mmol of carbonate ion, chloride ion, nitrate ion, dihydrogen phosphate ion, and sulfate ion (all of them are sodium salts) were added to 0.1 g of the dephosphorizing agent, and the mixed anion adsorption experiment was performed for 27 days. After 3 days at ℃, the solution was analyzed by anion chromatography. As a result, 24% of dihydrogen phosphate ion was adsorbed and 4% of sulfate ion was adsorbed. Therefore, the selectivity of dissolved phosphorus for sulfate ions in water of this dephosphorizing agent was 6. In addition, when the saturated adsorption capacity of phosphoric acid alone was measured, it was 0.16 mmol / g.
[0033]
Comparative Example A commercially available Mg _0.75 Al _0.25 (OH) ₂ (HCO ₃ ) _0.25 type hydrotalcite heated in an electric furnace at 300 ° C. for 4 hours was used as a dephosphorizing agent. The adsorption performance was measured by the following method. To 0.1 g of the dephosphorizing agent, 0.02 mmol of carbonate ion, chloride ion, nitrate ion, dihydrogen phosphate ion and sulfate ion (all sodium salts) were added, and the mixed anion adsorption experiment was performed at 27 ° C. After 3 days, the solution was analyzed by anion chromatography. As a result, 80% of the added dihydrogen phosphate was adsorbed, whereas 36% of sulfate, 13% of chloride and 10% of nitrate were added. Adsorbed, these were mainly ion exchanged with carbonate ions. In addition, when the saturated adsorption capacity of phosphoric acid alone was measured, it was 1.1 mmol / g. Further, the selectivity of dissolved phosphorus for sulfate ions in water of this dephosphorizing agent was about 2.
[0034]
From the above results, it can be seen that the highly selective dephosphorizing agent of the present invention has higher phosphorus selectivity than the conventional one.
[0035]
【The invention's effect】
According to the present invention, a highly selective dephosphorizing agent capable of selectively adsorbing phosphorus can be obtained even when an anion such as a halogen ion, a nitrate ion, a sulfate ion, or a carbonate ion is present in an aqueous solution containing phosphorus. Therefore, it is useful for removing phosphorus from seawater.

Claims (9)

Formula ^{_{M II 1-x M III x}} (OH) 2 A n- y · mH 2 O
(At least one is M ^II in formula selected from the divalent metal, M ^III is at least one selected from among trivalent metal, A ^n-represents an n-valent anion, x, y and m are numbers satisfying 0 <x ≦ 0.67, 0 <y ≦ 1, 0 ≦ m ≦ 2)
A highly selective dephosphorizing agent characterized in that a heat-treated product of a crystal of a composite metal hydroxide represented by the formula (1) is used as an active ingredient, and that the selectivity of phosphorus dissolved in water to sulfate ions is 5 or more. The highly selective dephosphorizing agent according to claim 1, wherein M ^III in the general formula is Mn. The highly selective dephosphorizing agent according to claim 1, wherein M ^III in the general formula is Al or a combination of Al and Co. Highly selective dephosphorization agent according to claim 2, wherein a is ^{- M II} is Mg or ^{Ni, A n-the} HCO ₃ in the general formula. Highly selective dephosphorization agent according to claim 3, wherein a is ^{- M II} is ^{Zn, A n-the} HCO ₃ in the general formula. Highly selective dephosphorization agent according to claim 3, wherein a is ^{- M II} is Co and ^{Zn, A n-the} HCO ₃ in the general formula. A mixed aqueous solution of a water-soluble salt of an inorganic acid or an organic acid and an alkali hydroxide is dropped into an aqueous solution containing a water-soluble compound of a divalent metal and a water-soluble compound of a trivalent metal, and the temperature is maintained at 0 to 90 ° C. reacted, the general formula ^{_{M II 1-x M III x}} (OH) 2 A n- y · mH 2 O
(At least one is M ^II in formula selected from the divalent metal, M ^III is at least one selected from among trivalent metal, A ^n-represents an n-valent anion, x, y and m are numbers satisfying 0 <x ≦ 0.67, 0 <y ≦ 1, 0 ≦ m ≦ 2)
A method for producing a dephosphorizing agent, comprising: forming a precipitate of a crystal of a composite metal hydroxide represented by the formula: and then separating the precipitate and performing a heat treatment at 200 to 500 ° C. An aqueous solution containing a water-soluble compound of a divalent metal and a water-soluble compound of divalent manganese is dropped into a mixed aqueous solution of a water-soluble salt of an inorganic acid or an organic acid and an alkali hydroxide, and then oxidized at 0 to 90 ° C. reacted keeping the temperature, the general formula ^{_{M II 1-x Mn (III}} ) x (OH) 2 a n- y · mH 2 O
(At least one is ^{M II} in formula selected from among the divalent ^{metal, A n-represents} an n-valent anion, x, y and m are, 0 <x ≦ 0.67,0 <y ≦ 1, 0 ≦ m ≦ 2)
A method for producing a dephosphorizing agent, comprising: forming a precipitate of a crystal of a composite metal hydroxide represented by the formula: and then separating the precipitate and performing a heat treatment at 200 to 500 ° C. 9. The method for producing a dephosphorizing agent according to claim 7, wherein the water-soluble salt of an inorganic acid or an organic acid is an alkali carbonate.