Processing method

Abstract

To provide a processing method capable of efficiently separating phosphor and heavy metals from an object to be processed at a low cost and suppressing an amount of a waste liquid and an amount of chemical substances to be used.SOLUTION: The processing method repeatedly performs a series of steps including: a first dissolution step of blending an object to be processed with an acidic liquid to obtain a first blending product; a step of performing solid-liquid separation of the first blending product; a step of blending a first liquid separated from the first blending product with a first precipitation agent and elevating its pH to obtain a second blending product; a step of performing solid-liquid separation of the second blending product; a second dissolution step of processing a second solid separated from the second blending product with an alkaline liquid to obtain a third blending product; a step of performing solid-liquid separation of the third blending product; a step of blending a third liquid separated from the third blending product with a second precipitation agent and controlling its pH to 12 or over to obtain a fourth blending product; and a step of performing solid-liquid separation of the fourth blending product, wherein a second liquid is subjected to the first dissolution step, and a fourth liquid is subjected to the second dissolution step for recycling, and an alkaline-earth metal in the second liquid after recycling is recovered as a resource.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a processing method.

Background Art

[0002] Sludge ash generally contains a large amount of phosphorus as a highly useful resource, and there have been attempts to separate, recover, and effectively utilize phosphorus (see, for example, Patent Document 1).

[0003] Thus, although sludge ash generally contains a large amount of phosphorus as a highly useful resource, on the other hand, it also contains heavy metals at a relatively high content rate. When it contains heavy metals together with phosphorus, it is necessary to remove the heavy metals in advance in order to effectively utilize the phosphorus.

[0004] However, in the conventional methods, the recovery rate of phosphorus is low, and the heavy metal content rate of the residue after phosphorus recovery cannot be sufficiently reduced. In order to sufficiently reduce the heavy metal content rate of the residue, there have been problems such as a long processing time and a large processing cost. Therefore, although it contains phosphorus as a useful component, it has been landfilled as industrial waste, or even when phosphorus is recovered, the residue contains a large amount of phosphorus and heavy metals, so the residue cannot be effectively utilized and has been landfilled as industrial waste, which has been a major problem from the viewpoints of effective utilization of resources and environmental protection.

[0005] On the other hand, the present inventor has succeeded in efficiently separating heavy metals at low cost from an object to be treated containing phosphorus and heavy metals by performing each step such as acid treatment and alkali treatment in a predetermined order, and recovering phosphorus at a very high recovery rate (for example, 80% or more) (see Patent Document 2).

[0006] However, in the method described in Patent Document 2, the amount of waste liquid after phosphorus recovery is large, and chemical substances such as NaOH and HCl used for dissolving phosphorus cannot be effectively utilized. Also, the usage amount is relatively large, and further cost reduction is required.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] An object of the present invention is to provide a treatment method capable of efficiently separating phosphorus and heavy metals from a material to be treated containing phosphorus and heavy metals at low cost, and suppressing the amount of waste liquid and the amount of chemical substances used as a whole.

Means for Solving the Problems

[0009] Such an object is achieved by the following present invention. The treatment method of the present invention comprises: mixing an object to be treated containing phosphorus and heavy metals with an acidic liquid to dissolve the phosphorus and heavy metals contained in the object to be treated, and obtaining a first mixture containing a first liquid and a first solid in a first dissolution step; separating the first liquid in which phosphorus and heavy metals are dissolved from the first solid in a first solid-liquid separation step; mixing the first liquid with a first precipitant and increasing the pH to precipitate a second solid containing phosphorus and heavy metals, and obtaining a second mixture containing a second liquid and the second solid in a first precipitation step; separating the second solid containing phosphorus and heavy metals from the acidic second liquid in a second solid-liquid separation step; dissolving the phosphorus contained in the second solid with an alkaline liquid to obtain a third mixture containing a third liquid and a third solid in a second dissolution step; separating the third liquid in which phosphorus is dissolved from the third solid containing heavy metals and phosphorus in a third solid-liquid separation step; mixing the third liquid with a second precipitant and setting the pH to 12 or more to precipitate a fourth solid containing phosphorus, and obtaining a fourth mixture containing a fourth liquid and the fourth solid in a second precipitation step; and separating the fourth solid containing phosphorus from the alkaline fourth liquid in a fourth solid-liquid separation step, and repeating a series of steps including these to obtain a composition containing phosphorus at a high content rate and having a low heavy metal content rate. The second liquid obtained in the second solid-liquid separation step Of which 10 mass% or more is recycled by being fed to the first dissolution step, and the fourth liquid obtained in the fourth solid-liquid separation step Of which 10 mass% or more is recycled by being fed to the second dissolution step, and the second liquid after recycling Alkaline earth metal in is recovered as an alkaline earth metal resource containing at least one of a magnesium salt and a calcium salt.

Advantages of the Invention

[0010] According to the present invention, there is provided a treatment method capable of efficiently separating phosphorus and heavy metals from a material to be treated containing phosphorus and heavy metals at low cost, and suppressing the amount of waste liquid and the amount of chemical substances used as a whole. In particular, an alkali metal salt can be recovered from the waste liquid generated in the acid elution-precipitation process, and the drainage generated during the recovery of the alkali metal salt can be reused as an acid eluent, significantly reducing the generation of waste water, and can be substantially reduced to zero. In addition, the NaOH solution can be circulated and used from the liquid alkali elution-precipitation, and the generation of waste water can be reduced to zero.

Brief Description of the Drawings

[0011]

Figure 1

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Embodiments for Carrying Out the Invention

[0012] Hereinafter, preferred embodiments of the present invention will be described. Figure 1 is a flowchart showing an example of the treatment method of the present invention.

[0013] The treatment method of the present invention comprises a series of steps repeated as follows: a first dissolution step of mixing a material to be treated containing phosphorus and heavy metals with an acidic liquid to dissolve the phosphorus and heavy metals contained in the material to be treated, and obtaining a first mixture containing a first liquid and a first solid; a first solid-liquid separation step of separating the first liquid in which phosphorus and heavy metals are dissolved from the first solid; a first precipitation step of mixing the first liquid with a first precipitant and raising the pH to precipitate a second solid containing phosphorus and heavy metals, and obtaining a second mixture containing a second liquid and a second solid; a second solid-liquid separation step of separating the second solid containing phosphorus and heavy metals from the acidic second liquid; a second dissolution step of dissolving the phosphorus contained in the second solid with an alkaline liquid to obtain a third mixture containing a third liquid and a third solid; a third solid-liquid separation step of separating the third liquid in which phosphorus is dissolved from the third solid containing heavy metals and phosphorus; a second precipitation step of mixing the third liquid with a second precipitant and setting the pH to 12 or higher to precipitate a fourth solid containing phosphorus, and obtaining a fourth mixture containing a fourth liquid and a fourth solid; and a fourth solid-liquid separation step of separating the fourth solid containing phosphorus from the alkaline fourth liquid, thereby obtaining a composition containing phosphorus at a high content rate and having a low heavy metal content rate. Then, the second liquid obtained in the second solid-liquid separation step is recycled by feeding it to the first dissolution step, and the fourth liquid obtained in the fourth solid-liquid separation step is recycled by feeding it to the second dissolution step. After recycling, the second liquid is recovered as an alkaline earth metal resource containing at least one of a magnesium salt and a calcium salt.

[0014] As a result, phosphorus and heavy metals can be efficiently separated from the material to be treated containing phosphorus and heavy metals at low cost. In addition, the amount of industrial waste can be significantly reduced, and the amount of waste liquid as general waste generated from the phosphorus recovery process can also be significantly reduced, and can be substantially reduced to zero, thereby significantly reducing the total amount of waste. Further, by performing recycling as described above in the treatment method, the amount of chemical substances used in the entire treatment method can be significantly suppressed. For example, since the entire amount of NaOH can be recycled and used, and the entire amount of HCl can be recovered as an alkaline earth metal salt, the entire amount of chemical substances used can be recovered as a product.

[0015] In addition, by performing post-treatment after removing heavy metals, phosphorus can be recovered in a high-grade state with an extremely low heavy metal content rate. As a result, phosphorus as a highly useful resource can be suitably recovered from the material to be treated and suitably reused. Further, phosphorus can be recovered from the material to be treated at a very high recovery rate (for example, 90% or more. More specifically, almost 100%).

[0016] In addition, in the treatment method of the present invention, not only phosphorus and heavy metals are separated, but also an alkaline earth metal resource containing at least one of a magnesium salt and a calcium salt can be suitably recovered. From such an alkaline earth metal resource, magnesium salts and calcium salts can be easily isolated in a highly pure state, respectively. In other words, high-grade magnesium salts and calcium salts can be obtained.

[0017] In the present invention, the heavy metal means a metal element in which the corresponding single metal has a specific gravity greater than that of iron at 25°C. Further, at least a part of each step included in a series of steps may be treated under different conditions in each cycle.

[0018] <First Dissolution Step> In the first dissolution step, the material to be treated containing phosphorus and heavy metals is mixed with an acidic liquid to obtain a first mixed liquid. This dissolves phosphorus and heavy metals contained in the object to be treated.

[0019] In the object to be treated, phosphorus is usually contained in the form of oxides (such as P2O5), phosphoric acid, phosphates, etc. In this specification, a compound containing phosphorus as an atom (including ionic substances) including these forms and a phosphorus atom contained in the compound may be simply referred to as phosphorus.

[0020] In the object to be treated, heavy metals are contained in the form of metal oxides (including complex oxides), elemental metals, alloys, metal salts, etc. In this specification, a compound containing heavy metals as an atom (including ionic substances) including these forms and a heavy metal atom contained in the compound may be simply referred to as heavy metals. The same applies to components indicated by other element names or element symbols.

[0021] The object to be treated used in this step may be any object as long as it contains phosphorus and heavy metals, but it is preferably contains impurities such as Fe, Al, and Mg in addition to phosphorus and heavy metals.

[0022] As the object to be treated, for example, sludge ash can be preferably used. Sludge ash generally contains phosphorus, which is a valuable resource, together with heavy metals, and is generated in large quantities around the world. Therefore, by using sludge ash as the object to be treated, the effect of reducing the amount of industrial waste is particularly large, and there is a possibility of recovering a large amount of phosphorus, which is a valuable resource. In addition, sludge ash generally contains impurities such as Fe, Al, and Mg in a more appropriate ratio together with phosphorus and heavy metals. Therefore, the crystal grain size of phosphate can be more preferably controlled, and the solid-liquid separation efficiency and the phosphorus recovery efficiency can be further improved. In addition, since calcium phosphate compounds adsorb or react with heavy metals, heavy metals in the solution can be efficiently separated and removed. In other words, by using sludge ash as the object to be treated, the effects of the present invention are more significantly exhibited.

[0023] The acidic liquid used in this process is not particularly limited, but is preferably a strong acid with a pH (hydrogen ion exponent) of -1.5 or more and 1.5 or less, more preferably -1.0 or more and 1.0 or less, and even more preferably -0.7 or more and 0.3 or less.

[0024] As the acidic liquid, for example, sulfuric acid, nitric acid, acetic acid, hydrochloric acid, or a liquid containing two or more of these can be used, but hydrochloric acid is preferably used.

[0025] Thereby, chloride ions can be suitably supplied into the system, the second liquid obtained in the second solid-liquid separation step can contain chloride ions at a relatively high concentration, the second recycling can be suitably carried out, and the separation and isolation of alkaline earth metal salts from the second liquid after recycling can be carried out more suitably. In addition, the handling of the alkaline earth metal salts obtained as alkaline earth metal resources becomes easier.

[0026] The pH of the liquid phase (i.e., the first liquid in which phosphorus and heavy metals are dissolved) at the end of this process is preferably -0.5 or more and 7.0 or less, more preferably -0.3 or more and 5.0 or less, and even more preferably -0.2 or more and 2.0 or less.

[0027] The dissolution rate of phosphorus into the liquid phase at the end of this process is preferably 70% or more, more preferably 90% or more, and most preferably almost 100%.

[0028] In addition, this process is preferably carried out while stirring a mixture of the object to be treated and the acidic liquid. For stirring the mixture of the object to be treated and the acidic liquid, various stirring devices and various mixing devices can be used. In addition, this process may be carried out batchwise or continuously.

[0029] In addition, in a series of steps after the second cycle, there may be a first melting step of newly adding the object to be processed, or there may be a first melting step of not newly adding the object to be processed.

[0030] <First solid-liquid separation step> In the first solid-liquid separation step, the first liquid in which phosphorus and heavy metals are dissolved is separated from the first solid.

[0031] Thereby, the first solid, which is a solid substantially free of phosphorus and heavy metals, can be obtained. The first solid can be discarded, for example, as general waste that is not industrial waste. Further, for example, it can be suitably used as constituent materials such as marine fertilizers, soil conditioners, bricks, and concrete. Also, since the first solid has a low phosphorus content, it is also desirable to be effectively used as a cement raw material and a construction material.

[0032] The method of solid-liquid separation is not particularly limited, and examples include decantation, filtration, centrifugation, etc., and a plurality of methods may be combined. In this step, even if a relatively large amount of solid (first solid) is contained in the separated liquid phase (first liquid), by recycling it as the solid phase (third solid) separated in the third solid-liquid separation step described in detail later, the solid (first solid) can be suitably separated from the liquid phase (first liquid) in the first solid-liquid separation step after the second cycle. Therefore, this step can be suitably performed by a simple method such as decantation or filtration using a filter medium with a coarse mesh, which is advantageous for further reducing the cost required for the implementation of the treatment method.

[0033] Also, in this step, if necessary, the once-separated first solid may be washed with a liquid such as water (first washing step). The liquid A used for washing the first solid may be, for example, fed to subsequent steps together with the first liquid separated by solid-liquid separation, or recycled together with the second liquid.

[0034] The phosphorus content in the first solid separated by solid-liquid separation is preferably 10.0% by mass or less, more preferably 1.0% by mass or less.

[0035] The heavy metal content (however, when containing a plurality of heavy metal elements, the total amount thereof; the same shall apply hereinafter) in the first solid separated by solid-liquid separation is preferably 5% by mass or less, more preferably 0.1% by mass or less.

[0036] <First precipitation step> In the first precipitation step, the first liquid separated in the first solid-liquid separation step is mixed with a first precipitant and the pH is increased to precipitate a second solid containing phosphorus and heavy metals, thereby obtaining a second mixture containing a second liquid and the second solid. In particular, phosphorus is precipitated as a phosphate (for example, calcium hydrogen phosphate dihydrate, calcium phosphate, magnesium hydrogen phosphate heptahydrate, magnesium hydrogen phosphate trihydrate, etc.). This facilitates the handling of substances other than phosphorus and heavy metals in subsequent steps. In addition, the content of components other than phosphorus and heavy metals in substances containing components other than phosphorus and heavy metals can be reduced, and the selectivity in the separation of heavy metals and the amount of impurities mixed in when recovering phosphorus can be reduced.

[0037] In addition, by precipitating a phosphate under such conditions, the nucleation and growth of the phosphate can be suitably controlled, and the phosphate can be precipitated as particles composed of microcrystals. As a result, in the subsequent second dissolution step, the phosphate can be easily dissolved, and phosphorus (dissolved phosphorus) can be suitably separated from heavy metals (solid heavy metals). Furthermore, since the particles of the phosphate become larger, solid-liquid separation becomes easier.

[0038] In addition, when the object to be treated contains impurities such as Fe, Al, and Mg together with phosphorus and heavy metals, in this step, the impurities can more effectively prevent the coarsening of the crystals of phosphates (especially calcium salts of phosphoric acid).

[0039] In this step, any substance or composition may be used as long as the first liquid can be mixed with the first precipitant and the pH can be increased. As the first precipitant, for example, Ca-based substances such as CaCl2, Ca(OH)2, and CaCO3, Al-based substances such as Al salts, Fe-based substances such as Fe salts, Mg-based substances such as Mg salts, etc. can be used, but it is preferable to use dolomites.

[0040] Thereby, in this step, phosphorus can be precipitated as calcium salts of phosphoric acid (for example, calcium hydrogen phosphate dihydrate, calcium phosphate, magnesium phosphate, etc.), and the subsequent steps can be carried out more preferably. In addition, by using dolomites containing Mg and Ca, which are alkaline earth metals, as the first precipitant, the alkaline earth metal resources obtained by the treatment method of the present invention will contain highly useful Mg and Ca. In addition, dolomites are generally inexpensive and are particularly advantageous in further reducing the cost required for the implementation of the treatment method.

[0041] Examples of the dolomites used in this step include dolomite, hydrated dolomite (slaked dolomite, including dolomite plaster), lightly burned dolomite, calcined dolomite (dolomite clinker), etc., and one or more selected from these can be used in combination.

[0042] In addition, in this step, it is preferable to add an Fe-based substance. Thereby, in this step, phosphorus can be more preferably precipitated as calcium salts of phosphoric acid (for example, calcium hydrogen phosphate dihydrate, calcium phosphate, etc.) and iron phosphate, and the subsequent steps can be carried out more preferably. Iron phosphate can be precipitated at a lower pH.

[0043] As the Fe-based substance, any substance that can be dissolved in an acid may be used. For example, iron chloride, iron sulfate, etc. can be mentioned, and one or more selected from these can be used in combination.

[0044] Also, in this step, an Al-based substance may be added. Thereby, the same effects as adding an Fe-based substance can be obtained.

[0045] In addition, in this step, an Fe-based substance and an Al-based substance may be used in combination. The Al-based substance may be any substance that can be dissolved in an acid. For example, aluminum chloride, aluminum sulfate, etc. may be mentioned, and one or more selected from these can be used in combination.

[0046] At the end of this step, the pH of the liquid phase is preferably 0.0 or more and 7.0 or less, more preferably 1.0 or more and 6.0 or less, and even more preferably 1.5 or more and 5.0 or less.

[0047] Thereby, calcium hydrogen phosphate dihydrate, magnesium hydrogen phosphate heptahydrate, magnesium hydrogen phosphate trihydrate, etc. as phosphates can be more suitably produced. By producing the phosphate as such a compound, the phosphate is suitably precipitated in this step (the first precipitation step), and has higher solubility under alkaline conditions, and can be more suitably dissolved in the subsequent second dissolution step. Further, in the second precipitation step after the second dissolution step, it is suitably precipitated as Ca5(PO4)3OH having low solubility even at a high alkalinity of pH 12 or more, and phosphorus can be more suitably recovered. Such an effect is more remarkably exhibited when the pH of the liquid phase at the end of this step is 5.0 or less.

[0048] <Second solid-liquid separation step> In the second solid-liquid separation step, a second solid containing phosphorus and heavy metals is separated from an acidic second liquid.

[0049] Thereby, it can be separated into a second solid which is a solid containing high-concentration phosphorus and heavy metals, and a second liquid which is a liquid phase substantially free of heavy metals.

[0050] The method of solid-liquid separation is not particularly limited, and examples include decantation, filtration, centrifugation, etc., and a plurality of methods may be combined. In this step, even when the separated liquid phase (second liquid) contains a relatively large amount of solid (second solid), since the separated liquid phase (second liquid) is recycled to the first dissolution step, the solid (second solid) can be preferably separated from the liquid phase (second liquid) in the second solid-liquid separation step after the second cycle. Therefore, this step can be preferably carried out by a simple method such as decantation or filtration using a filter medium with a coarse mesh, which is advantageous for further reducing the cost required for the implementation of the treatment method.

[0051] Also, in this step, if necessary, the once-separated second solid may be washed with a liquid such as water (second washing step). In this case, for example, washing may be performed by spraying water on the second solid after solid-liquid separation in the second solid-liquid separation step. The liquid B used for washing the second solid is recycled together with the separated second liquid to the first dissolution step. At this time, an acid (for example, HCl) may be added to adjust the pH to be approximately the same as that of the acidic liquid used in the first dissolution step.

[0052] The phosphorus content in the solid-liquid separated second liquid is preferably 0.01% by mass or more and 20% by mass or less, and more preferably 0.05% by mass or more and 5% by mass or less.

[0053] The heavy metal content in the solid-liquid separated second liquid is preferably 5% by mass or less, and more preferably 0.1% by mass or less.

[0054] <Second Dissolution Step> In the second dissolution step, the phosphorus contained in the second solid is dissolved with an alkaline liquid to obtain a third mixture containing a third liquid and a third solid.

[0055] By using such an alkaline liquid, phosphorus can be selectively dissolved while preventing the dissolution of heavy metals contained in the second solid. In particular, as described above, in the first precipitation step, since phosphates are precipitated under predetermined conditions, the nucleation and growth of the phosphates are preferably controlled, and the phosphates are in a state where they are easily dissolved in alkali. Furthermore, the iron phosphate obtained in the first precipitation step quickly dissolves in an alkaline solution to become iron hydroxide and precipitate, so the recovery rate of phosphorus can be increased. On the other hand, heavy metals are generally difficult to dissolve in an alkaline liquid, and even if they dissolve, they become hydroxides and exist as solids. As a result, phosphorus as a useful substance that can be used in fertilizers, etc., and heavy metals can be preferably separated. Also, the final solid waste (industrial waste) can be reduced.

[0056] Also, in the above-described process (particularly, the first solid-liquid separation step), since the object to be treated has already been significantly reduced in weight, in this step, a small-sized device (for example, a device having a volume about one-fifth of the treatment device used in the conventional method) can be used.

[0057] The pH of the alkaline liquid used in this step is not particularly limited, but is preferably 10 or more, more preferably 12 or more and 15.3 or less, and even more preferably 13 or more and 15.0 or less.

[0058] The alkaline liquid only needs to exhibit alkalinity as a whole liquid. Examples of the alkaline substance contained in the alkaline liquid include NaOH, KOH, etc.

[0059] Among them, the alkaline liquid used in this step preferably contains a metal hydroxide as the alkaline substance, more preferably contains a hydroxide of an alkali metal, and even more preferably contains NaOH.

[0060] As a result, while more effectively preventing the dissolution of heavy metals, phosphorus contained in the second solid can be more efficiently dissolved. Further, such an alkaline substance is relatively expensive and is required to be recycled and used. Since the above alkaline substance can be recycled and used, it is preferable also from the viewpoints of cost reduction and stable treatment, etc.

[0061] The pH of the liquid phase at the end of this step is not particularly limited, but is preferably 10 or more, more preferably 12 or more and 15 or less.

[0062] <Third solid-liquid separation step> In the third solid-liquid separation step, the third liquid in which phosphorus is dissolved is separated from the third solid containing heavy metals. As a result, most of the phosphorus contained in the object to be treated is separated from the heavy metals.

[0063] Note that most of the phosphorus contained in the third mixture is present in the third liquid, but a part of it is also contained in the third solid. That is, the third solid separated in this step contains heavy metals, metals other than heavy metals, and also phosphorus. In the present invention, by subjecting such a third solid to recycling as described in detail later, the phosphorus contained in the third solid can also be separated from the heavy metals and recovered. Therefore, one of the features is that the recovery rate of phosphorus from sludge ash is particularly excellent.

[0064] The method of solid-liquid separation is not particularly limited, and examples thereof include decantation, filtration, centrifugation, etc., and a plurality of methods may be combined and carried out.

[0065] Further, in this step, if necessary, the once-separated third solid may be washed with a liquid such as water (third washing step). In this case, for example, washing may be performed by spraying water on the third solid after solid-liquid separation in the third solid-liquid separation step. Note that the liquid C used for washing the third solid may be supplied to subsequent steps together with, for example, the third liquid subjected to solid-liquid separation, or may be recycled together with the fourth liquid described in detail later.

[0066] The phosphorus content in the third solid separated into solid and liquid is preferably 50% by mass or less, more preferably 30% by mass or less, in terms of phosphoric acid (P2O5).

[0067] The third solid may be reused by subjecting it to the first dissolution step. Further, the third solid may be subjected to the first precipitation step together with the first liquid after being dissolved in an acidic liquid such as hydrochloric acid.

[0068] The total heavy metal content in the third liquid separated into solid and liquid is preferably 5000 ppm or less, more preferably 500 ppm or less.

[0069] <Second precipitation step> In the second precipitation step, the third liquid is mixed with a second precipitant and the pH is adjusted to 12 or higher to precipitate a fourth solid containing phosphorus, thereby obtaining a fourth mixture containing a fourth liquid and the fourth solid.

[0070] Thereby, phosphorus can be treated as a phosphate (e.g., hydroxyapatite, calcium phosphate, etc.) which is a solid substance, and storage, transportation, etc. can be carried out more suitably. In particular, in this step, a high-purity phosphate substantially free of heavy metals can be obtained as the fourth solid.

[0071] Further, by carrying out this step (second precipitation step) under alkaline conditions with a pH of 12 or higher, hydroxyapatite (Ca5(PO4)3OH) can be preferably generated and precipitated, and can be preferably recovered. Further, even if magnesium phosphates are generated, the magnesium phosphates can be easily separated because they dissolve at a pH of 12 or higher.

[0072] Further, by adjusting the pH of the liquid phase of the fourth mixture to 12 or higher, the fourth liquid separated in the subsequent fourth solid-liquid separation step can be suitably recycled to the second dissolution step. Further, it may be subjected to subsequent steps together with the third liquid.

[0073] The pH of the liquid phase of the fourth mixture at the end of this step may be 12 or higher, preferably 12 or higher and 15.3 or lower, and more preferably 13 or higher and 15 or lower.

[0074] The second precipitant used in this step only needs to have a function of promoting precipitation such as phosphates. For example, Ca-based substances such as Ca(OH)2 and CaCO3, Mg-based substances such as Mg(OH)2 and MgCO3, etc. can be mentioned. Among them, at least one of Ca(OH)2 and CaCO3 is preferred, and Ca(OH)2 is more preferred.

[0075] Thereby, while efficiently supplying the calcium component that becomes a part of the calcium salt of phosphoric acid into the system, the pH of the mixture can be suitably adjusted. As a result, in this step, the usage amount of the substance mixed with the third liquid can be suppressed, and this step can proceed efficiently. In addition, the balance between the calcium content and the pH in the mixture in this step can be suitably adjusted, the precipitation efficiency of phosphorus can be improved, and the impurity content in the fourth solid can be made lower. Further, it is possible to more reliably prevent phosphorus from being inadvertently redissolved before the completion of the subsequent fourth solid-liquid separation step.

[0076] In this step, it is preferable to add calcium so as to satisfy the following conditions. That is, when the amount of substance of phosphorus in the system at the end of this step is X P [mol] and the amount of substance of calcium is X Ca [mol], it is preferable to satisfy the relationship of 0.1 ≦ X Ca / X P ≦ 3.0, and more preferably to satisfy the relationship of 0.5 ≦ X Ca / X P ≦ 2.0.

[0077] <Fourth solid-liquid separation step> In the fourth solid-liquid separation step, the fourth solid containing phosphorus is separated from the alkaline fourth liquid.

[0078] As a result, the phosphorus-containing material can be handled as a solid, and its handling becomes easy. Further, the separated fourth solid contains phosphate with high purity and has an extremely low heavy metal content, so it can be suitably used for fertilizers and the like. In particular, it can be suitably used for fertilizers and the like with simple treatment, whether or not post-treatment is carried out. Further, the separated fourth liquid is recycled by being subjected to the second dissolution step. However, even when finally discarded, since it substantially does not contain heavy metals, it is not necessary to treat it as industrial waste liquid by neutralizing with an acid.

[0079] The method of solid-liquid separation is not particularly limited. For example, decantation, filtration, centrifugation, etc. may be mentioned, and a plurality of methods may be combined. In this step, even when the separated liquid phase (fourth liquid) contains a relatively large amount of solid (fourth solid), since the separated liquid phase (fourth liquid) is recycled to the second dissolution step, the solid (fourth solid) can be suitably separated from the liquid phase (fourth liquid) in the fourth solid-liquid separation step after the second cycle. Therefore, this step can be suitably carried out by a simple method such as decantation or filtration using a filter medium with a coarse mesh, which is advantageous for further reducing the cost required for the implementation of the treatment method.

[0080] In this step, if necessary, the once-separated fourth solid may be washed with a liquid such as water (fourth washing step).

[0081] As a result, for example, the content of impurities such as Ca(OH)2 contained in the fourth solid and the content of the fourth liquid remaining in the fourth solid can be made lower. The liquid D used for washing the fourth solid may be recycled, for example, together with the fourth liquid separated by solid-liquid separation to the second dissolution step, or may be supplied to subsequent steps together with the third liquid.

[0082] The total content rate of heavy metals in the fourth separated solid-liquid separated solid is preferably 5000 ppm or less, more preferably 1200 ppm or less, and even more preferably 500 ppm or less.

[0083] <Recycling of the second liquid> In the treatment method of the present invention, an acid such as HCl is added to the second liquid obtained in the second solid-liquid separation step so that the pH becomes approximately the same as the pH of the acidic liquid used in the first dissolution step, and then it is recycled by subjecting it to the first dissolution step.

[0084] The second liquid obtained in the second solid-liquid separation step generally exhibits acidity. Therefore, by subjecting the second liquid to the first dissolution step, acidic components (for example, hydrochloric acid, etc.) contained in the second liquid can be suitably reused, and effects such as suppressing the amount of chemical substances used in the entire treatment method and reducing the cost required for implementing the treatment method can be obtained.

[0085] The pH of the second liquid obtained in the second solid-liquid separation step to be recycled is preferably 8 or less, more preferably 6 or less, and even more preferably 2 or more and 5 or less.

[0086] At least a part of the second liquid obtained in the second solid-liquid separation step may be recycled, but the ratio of the second liquid obtained in the second solid-liquid separation step that is subjected to and recycled in the first dissolution step is preferably 10% by mass or more, more preferably 50% by mass or more, and even more preferably 80% by mass or more and 100% by mass or less. Thereby, the above-described effects are more significantly exhibited.

[0087] Also, the second liquid obtained in the second solid-liquid separation step only needs to be recycled at least once or more, and although there is no limit to the number of times the second liquid is recycled in a series of steps performed repeatedly, it is preferably 3 times or more, and more preferably 5 times or more and 14 times or less. As a result, the above-described effects are more significantly exhibited.

[0088] In addition, when the content rate of the calcium salt and the content rate of the magnesium salt in the second liquid obtained in the second solid-liquid separation step are equal to or lower than the solubility at 25°C, it is preferable to recycle the second liquid by subjecting it to the first dissolution step.

[0089] As a result, the above-described effects are more significantly exhibited. In addition, the recovery efficiency of the alkaline earth metal resources can be made more excellent.

[0090] When the concentration of the alkaline earth metal in the second liquid is 10% by mass or more, it is preferable to recover it to produce an alkaline earth metal salt, and when it is 30% by mass or more, it is more preferable to recover it to produce an alkaline earth metal salt.

[0091] As a result, the above-described effects are more significantly exhibited, and the recovery efficiency of the alkaline earth metal resources can be made more excellent.

[0092] <Recycling of the Fourth Liquid> In the treatment method of the present invention, the fourth liquid obtained in the fourth solid-liquid separation step is recycled by subjecting it to the second dissolution step.

[0093] The fourth liquid obtained in the fourth solid-liquid separation step generally exhibits alkalinity. Therefore, by subjecting the fourth liquid to the second dissolution step, the alkaline components (for example, sodium hydroxide, etc.) contained in the fourth liquid can be suitably reused, and effects such as suppressing the amount of chemical substances used in the entire treatment method and reducing the cost required for implementing the treatment method can be obtained.

[0094] The pH of the recycled fourth liquid is preferably 10 or more, and more preferably 13 or more.

[0095] At least a part of the fourth liquid obtained in the fourth solid-liquid separation step may be recycled. The ratio of the fourth liquid obtained in the fourth solid-liquid separation step that is supplied to and recycled in the second dissolution step is preferably 10% by mass or more, more preferably 50% by mass or more, and even more preferably 80% by mass or more and 100% by mass or less. Thereby, the above-described effects are more significantly exhibited.

[0096] Also, the fourth liquid obtained in the fourth solid-liquid separation step only needs to be recycled at least once. The number of times the fourth liquid is recycled in a series of steps performed repeatedly is preferably 3 times or more, more preferably 5 times or more and 100 times or less. Thereby, the above-described effects are more significantly exhibited.

[0097] <Recovery as an alkaline earth metal resource> In the treatment method of the present invention, the second liquid after recycling is recovered as an alkaline earth metal resource containing at least one of a magnesium salt and a calcium salt.

[0098] Thus, in the present invention, since phosphorus is recovered and, separately from this, an alkaline earth metal which is a useful substance is also recovered, it is preferable from the viewpoint of effective resource recovery. Further, by selling the obtained alkaline earth metal resource or the like, the cost required for the entire treatment method can be further reduced.

[0099] The second liquid after recycling may be used as an alkaline earth metal resource in a liquid state, or after removing the solvent, the solvent may be returned to the second liquid and the solid may be used as an alkaline earth metal resource in a solid state.

[0100] Also, when the second liquid after recycling contains a plurality of types of alkaline earth metal ions, these may be separated by a physical method or a chemical method and used as an alkaline earth metal resource.

[0101] More specifically, for example, when the second liquid after recycling contains a magnesium salt such as MgCl2 and a calcium salt such as CaCl2, by mixing the second liquid with a substance containing sulfate ions, MgSO4 and CaSO4 are generated in the system. On the other hand, the solubility of MgSO4 is sufficiently higher than that of CaSO4. As a result, the calcium ions and sulfate ions reacted as described above are excluded from the liquid phase which is the reaction system, while the magnesium ions remain in the liquid phase which is the reaction system. Therefore, for example, while precipitating CaSO4 as a solid phase, the magnesium ions can be left in the liquid phase in the form of MgCl2, and by solid-liquid separating the composition after mixing with the substance containing sulfate ions, a liquid containing a high content of magnesium salt and a solid containing a high content of calcium salt can be separated.

[0102] By drying the liquid containing a high content of magnesium salt, highly pure MgCl2 can be obtained. Also, when obtaining MgCl2, for example, treatments such as recrystallization may be performed. Since the MgCl2 thus obtained has a high purity, it can also be sold as a reagent. Furthermore, the solvent generated during drying can be returned to the second liquid and reused.

[0103] The solid containing a high content of calcium salt is highly pure CaSO4 and can also be sold as a reagent or a flame retardant.

[0104] When generating CaSO4 as described above, the pH of the composition after mixing the second liquid with the substance containing sulfate ions is preferably 3 or more and 12 or less, and more preferably 4 or more and 8 or less.

[0105] <Recycling of the third solid> In the treatment method of the present invention, the third solid obtained in the third solid-liquid separation step may be recycled by subjecting it to the first dissolution step. Also, the third solid may be recycled by dissolving it in an acidic liquid such as hydrochloric acid and then subjecting it to the first precipitation step together with the first liquid.

[0106] When the third solid-liquid separation step is completed, most of the phosphorus contained in the object to be treated subjected to the first dissolution step is contained in the third liquid, but is also contained in the third solid at a relatively high content rate. More specifically, the third solid obtained in the third solid-liquid separation step of each cycle contains phosphorus at a rate comparable to the phosphorus content rate in natural phosphate ore. Therefore, by subjecting this third solid to the first dissolution step or the first precipitation step, phosphorus resources can be recovered more efficiently.

[0107] When recycling the third solid obtained in the third solid-liquid separation step, at least a part of the third solid obtained in the third solid-liquid separation step may be recycled, but the ratio of the third solid obtained in the third solid-liquid separation step that is subjected to and recycled in the first dissolution step or the first precipitation step is preferably 10% by mass or more, more preferably 50% by mass or more, and even more preferably 80% by mass or more and 100% by mass or less. Thereby, the above-described effects are more significantly exhibited.

[0108] However, by repeating the recycling, heavy metals are concentrated in the third solid, so when the heavy metals exceed a certain concentration, it is necessary to discharge them without reuse. Here, the "certain concentration" affects the heavy metal concentration of the fourth solid and indicates a concentration exceeding the fertilizer standard value.

[0109] Also, the number of recycling times of the third solid in the series of steps performed repeatedly is preferably 3 times or more, and more preferably 5 times or more and 100 times or less. Thereby, the above-described effects are more significantly exhibited.

[0110] Also, the third solid that is subjected to and recycled in the first dissolution step or the first precipitation step preferably has a heavy metal content rate of 10% by mass or less, and more preferably 1% by mass or less.

[0111] As described above, the preferred embodiments of the present invention have been explained, but the present invention is not limited thereto.

[0112] For example, the processing method of the present invention may have steps other than the steps described above (for example, a pretreatment step, an intermediate processing step, a post-treatment step, etc.).

Example

[0113] Hereinafter, the present invention will be described in detail based on specific examples, but the present invention is not limited thereto.

[0114] (Example 1) The following is how a treatment method using sludge ash as the object to be treated was carried out.

[0115] First, sludge ash was prepared and subjected to a drying treatment at 110 °C for 2 hours to make the water content 0%. This sludge ash contained Fe, Al, Mg, Ca, etc. in addition to phosphorus and heavy metals.

[0116] Next, 3000 mL of hydrochloric acid with a concentration of 2.0 M / L (2 molar concentration) was placed in an Erlenmeyer flask and heated at 80 °C. Then, 600 g of sludge ash was added into this Erlenmeyer flask and stirred for 2 hours using a magnetic stirrer. As a result, the phosphorus oxide in the sludge was eluted as phosphate ions (the first dissolution step).

[0117] After that, a filter paper was set on a filter to perform solid-liquid separation, and a first liquid and a first solid were obtained (the first solid-liquid separation step).

[0118] Next, while adding calcined dolomite to 500 mL of the first liquid, the mixture was stirred, and the pH was measured using a pH meter. When the pH of the solution reached 5.0, the addition of calcined dolomite was stopped, and stirring was continued for an additional 30 minutes (the first precipitation step). As a result, a second solid as a solid containing phosphorus and heavy metals was precipitated, and a second mixture containing the second solid and the second liquid was obtained. At this time, phosphorus was mainly precipitated as phosphate. As the calcined dolomite, one with a molar ratio of Ca to Mg of 6:4 was used. Also, the amount of added dolomite (total of Ca and Mg) relative to the phosphorus in the first liquid was 4:3 in molar ratio.

[0119] Thereafter, a filter paper was set in a filter, and solid-liquid separation was performed using a vacuum pump (the second solid-liquid separation step).

[0120] After drying the second solid, which is the solid phase obtained in the second solid-liquid separation step, 100 g was taken, and this was put into an Erlenmeyer flask containing 500 mL of an aqueous NaOH solution with a concentration of 2.0 M / L (reference: 2 molar concentration), and stirred at 80 °C for 2 hours. As a result, phosphorus was redissolved (the second dissolution step). The pH of the reaction solution at the end of this step was 13.5.

[0121] The third liquid, which is the liquid phase in which phosphorus was dissolved, was separated by solid-liquid separation using a filter paper, and separated from the third solid, which is the solid phase containing heavy metals (the third solid-liquid separation step).

[0122] Next, while stirring, calcium hydroxide was added to the solid-liquid separated third liquid so that the ratio (molar ratio) of the amount of substance of phosphorus in the third liquid to the amount of substance of added calcium became 1:1.5, and stirring was continued for an additional 90 minutes (the second precipitation step). As a result, calcium phosphate salt was precipitated. This step was carried out at room temperature. Also, the pH of the reaction solution at the end of this step was 13.5 or higher.

[0123] Thereafter, a filter paper was set in a filter, and solid-liquid separation was performed to obtain a fourth liquid containing NaOH and having a pH of 13.5 or higher, and a fourth solid mainly composed of calcium phosphate salt (the fourth solid-liquid separation step).

[0124] The content rates of the respective heavy metals in the obtained fourth solid were all below the reference values for the respective heavy metals. For example, Pb was 12 ppm or less with respect to the reference value of 100 ppm, Cd was 0.22 ppm with respect to the reference value of 5 ppm, As was 0.8 ppm with respect to the reference value of 50 ppm, Ni was 5.52 ppm with respect to the reference value of 300 ppm, and Cr was 5.77 ppm with respect to the reference value of 500 ppm.

[0125] The third solid obtained in the third solid-liquid separation step was recycled by being fed to the first dissolution step together with a new object to be treated, the second liquid obtained in the second solid-liquid separation step was recycled by being fed to the first dissolution step, and the fourth liquid obtained in the fourth solid-liquid separation step was recycled by being fed to the second dissolution step, whereby a series of steps from the first dissolution step to the fourth solid-liquid separation step were repeated. The number of recycling times of the third solid, the number of recycling times of the second liquid, and the number of recycling times of the fourth liquid were all set to 3 times. However, from the second recycling, the treated sludge ash was 100 g, and the solid-liquid ratio and the concentration of HCl were carried out under the same conditions as in the first time.

[0126] As a result, in the first dissolution step after the second time, it was necessary to add hydrochloric acid to a concentration of 2.0 M / L, but the residual hydrochloric acid could be utilized and the amount of hydrochloric acid used could be reduced. Also, it was not necessary to add and use an aqueous NaOH solution with a concentration of 2.0 M / L in the second dissolution step after the second time. That is, by recycling, the amount of NaOH used as a whole could be significantly reduced to 1 / 100 or less compared to the methods described in Patent Documents 1 and 2, and the amount of HCl used could also be reduced.

[0127] Also, regarding the fourth solid obtained in the second and third fourth solid-liquid separation steps, similar to the fourth solid obtained in the first fourth solid-liquid separation step, the content rate of each heavy metal was below the reference value for each heavy metal. For example, Pb was 15 ppm or less with respect to the reference value of 100 ppm, Cd was 0.3 ppm with respect to the reference value of 5 ppm, As was 1.0 ppm with respect to the reference value of 50 ppm, Ni was 10 ppm with respect to the reference value of 300 ppm, and Cr was 10 ppm with respect to the reference value of 500 ppm.

[0128] The fourth solids obtained in the second and third fourth solid-liquid separation steps were combined, washed with water, and then dried. As a result, phosphates with a purity of 95% were obtained. The total recovery rate of phosphorus as a fertilizer was 71%.

[0129] Also, the second liquid obtained in the third second solid-liquid separation step contained 8.5% by mass of MgCl2 and 6.6% by mass of CaCl2. When 100 mL of this second liquid was mixed with 0.076 M sulfuric acid and the pH was adjusted to 5 with Mg(OH)2, CaSO4 precipitated. Also, when the reaction liquid in which CaSO4 had precipitated was subjected to solid-liquid separation and the separated solid was washed with water and dried, CaSO4 with a purity of 90% or more was obtained. When the liquid phase after solid-liquid separation was dried, MgCl2 with a purity of 90% or more was obtained.

[0130] (Example 2) The treatment method was carried out in the same manner as in the first cycle of Example 1, except that the pH in the first precipitation step was set to 3.0.

[0131] (Example 3) The treatment method was carried out in the same manner as in the first cycle of Example 1, except that the pH in the first precipitation step was set to 4.0.

[0132] (Example 4) In the second precipitation step, the treatment method was carried out in the same manner as in Example 1, except that the ratio (molar ratio) of the amount of substance of phosphorus in the second liquid to the amount of substance of calcium added was set to 1:1.

[0133] (Example 5) In the second precipitation step, the treatment method was carried out in the same manner as in Example 1 except that the ratio (molar ratio) of the amount of phosphorus substance in the second liquid to the amount of calcium substance added was 1:1.25.

[0134] (Example 6) In the second precipitation step, the treatment method was carried out in the same manner as in Example 1 except that the ratio (molar ratio) of the amount of phosphorus substance in the second liquid to the amount of calcium substance added was 1:1.75.

[0135] (Example 7) In the first precipitation step, the treatment method was carried out in the same manner as in Example 1 except that a mixture of calcium hydroxide and iron chloride with a molar ratio of 8:2 was used as the first precipitant.

[0136] Also in Examples 2 to 7, similar to Example 1, the content rate of each heavy metal in the fourth solid obtained in the fourth solid-liquid separation step was below the standard value of the fertilizer for each heavy metal. The contents of Cd, As, Ni, and Pb in the sludge ash as the starting material were 1.5 to 6 times the fertilizer standard.

[0137] In addition, also in Examples 2 to 7, similar to Example 1, when the fourth solids were combined and washed with water and then dried, phosphates with a purity of 95% or more could be obtained. The transfer rate of the phosphorus component contained in the sludge ash to the phosphate (fourth solid) was about 72% in Example 2, about 70% in Example 3, 45% in Example 4, about 60% in Example 5, and about 72% in Example 6. In Example 7 where an iron component was added to the first precipitant used in the first precipitation step, the transfer rate of the phosphorus component contained in the sludge ash to the phosphate (fourth solid) was about 80%.

[0138] For Examples 1 to 3 above, in order to evaluate the degree of phosphorus transfer in the first dissolution step, the first precipitation step, the second dissolution step, and the second precipitation step, the phosphorus contents of the first liquid, the second solid, the third liquid, and the fourth solid obtained in the first cycle were determined respectively, and from these results, the phosphorus transfer rate was determined. The results are shown in FIG. 3. From these results, it can be seen that phosphorus has migrated at a high rate in all cases. In Example 2 where the pH in the first precipitation step is 3.0, the phosphorus precipitation rate in the first precipitation step is low at 85%, but a slightly higher result was obtained for the phosphorus transfer rate obtained as the fourth solid.

[0139] Also, for Examples 1, 4 to 6 above, in order to evaluate the influence of calcium hydroxide (the second precipitating agent) on the precipitation amount of the fourth solid in the second precipitation step, the ratio of calcium to phosphorus was changed, and the phosphorus transfer rate (precipitation rate) from the third liquid obtained in the first cycle to the fourth solid was determined. The results are shown in FIG. 4. From these results, it was found that the phosphorus transfer rate increased as the ratio of calcium to phosphorus increased. When the ratio (molar ratio) of the amount of phosphorus in the second liquid to the amount of calcium added was 1:1.5 or more, a phosphorus transfer rate of almost 100% of the phosphorus in the solution was obtained. When it was 1:1.75, it increased by about 2% compared to 1:1.5, but it can be seen that there is no significant difference.

[0140] Also, for Example 1, in order to evaluate the degree of phosphorus transfer in the first dissolution step to the second precipitation step in the first to third cycles, the phosphorus contents of the first liquid, the second solid, the third liquid, and the fourth solid obtained in the first to third cycles were determined respectively, and from these results, the phosphorus transfer rate was determined. The results are shown in FIG. 2. From these results, it can be seen that phosphorus has migrated at a high rate in all cases. In particular, in the second and third cycles, when based on the amount of the object to be treated newly added, the phosphorus transfer rate in the first dissolution step and the first precipitation step exceeds 100%, and it is clear that phosphorus is effectively recovered from the third solid supplied by recycling.

[0141] Also, for Example 1, the concentrations of magnesium ions and calcium ions contained in the second liquid obtained in the 1st to 3rd cycles were determined, and the results are shown in FIG. 5. From these results, it can be seen that each time a series of steps is repeated in each cycle, the concentrations of magnesium ions and calcium ions in the fourth liquid increase, and it is possible to suitably recover them as an alkali metal salt.

[0142] Also, for Example 1, in order to evaluate the degree of transfer of each heavy metal in the first dissolution step, the first precipitation step, and the second precipitation step, for the first liquid, the second solid, and the fourth solid obtained in the first cycle, the contents of heavy metals Pd, Cd, As, Ni, and Cr were determined respectively, and from these results, the transfer rate of each heavy metal was determined. The results are shown in FIG. 6. From these results, it can be seen that the content rate of each heavy metal in the fourth solid is very low.

[0143] Also, for Example 1, in order to evaluate the phosphorus dissolution output in the second dissolution step, a test was conducted to compare the phosphorus elution rate when using a 2.0 M / L concentration NaOH aqueous solution during the first cycle and the phosphorus elution rate when using the fourth liquid obtained in the second cycle. The results are shown in FIG. 7. From these results, it can be seen that even the fourth liquid mainly composed of NaOH generated during phosphate formation in the second precipitation step has sufficient phosphorus dissolution output and can be suitably recycled.

[0144] Also, the third solid obtained in the third solid-liquid separation step was dissolved in hydrochloric acid instead of being subjected to the first dissolution step, and then, except for being subjected to the first precipitation step together with the first liquid, the treatment method was carried out in the same manner as in each of the above examples. As a result, excellent results similar to the above were obtained.

[0145] A graph showing the influence on the phosphorus recovery rate when an Fe component is added to the first calcium-based precipitant in the first precipitation step and when an Fe component is added to dolomite is shown in FIG. 8 in comparison between Example 1 and Example 7.

[0146] For Example 1, the XRD patterns of the second solid deposited in the first precipitation step of the first cycle and the XRD pattern of the fourth solid deposited in the second precipitation step are shown in FIG. 9. From each XRD pattern, it can be seen that the second solid deposited in the first precipitation step is CaHPO4·2H2O, and the fourth solid deposited in the second precipitation step is Ca5(PO4)3OH.

Industrial Applicability

[0147] The treatment method of the present invention comprises: a first dissolution step of mixing a material to be treated containing phosphorus and heavy metals with an acidic liquid to dissolve the phosphorus and heavy metals contained in the material to be treated, thereby obtaining a first mixture containing a first liquid and a first solid; a first solid-liquid separation step of separating the first liquid in which phosphorus and heavy metals are dissolved from the first solid; a first precipitation step of mixing the first liquid with a first precipitant and raising the pH to precipitate a second solid containing phosphorus and heavy metals, thereby obtaining a second mixture containing a second liquid and the second solid; a second solid-liquid separation step of separating the second solid containing phosphorus and heavy metals from the acidic second liquid; a second dissolution step of dissolving the phosphorus contained in the second solid with an alkaline liquid to obtain a third mixture containing a third liquid and a third solid; a third solid-liquid separation step of separating the third liquid in which phosphorus is dissolved from the third solid containing heavy metals and phosphorus; a second precipitation step of mixing the third liquid with a second precipitant and setting the pH to 12 or higher to precipitate a fourth solid containing phosphorus, thereby obtaining a fourth mixture containing a fourth liquid and the fourth solid; and a fourth solid-liquid separation step of separating the fourth solid containing phosphorus from the alkaline fourth liquid. By repeating this series of steps, a composition with a high phosphorus content and a low heavy metal content can be obtained. The second liquid obtained in the second solid-liquid separation step is recycled by being fed to the first dissolution step, and the fourth liquid obtained in the fourth solid-liquid separation step is recycled by being fed to the second dissolution step. After recycling, the second liquid is recovered as an alkaline earth metal resource containing at least one of a magnesium salt and a calcium salt. Therefore, it is possible to provide a treatment method capable of efficiently separating phosphorus and heavy metals from a material to be treated containing phosphorus and heavy metals at low cost, and suppressing the amount of waste liquid and the amount of chemicals used as a whole. Accordingly, the treatment method of the present invention has industrial applicability.

Claims

1. A first dissolution step of mixing a material to be treated containing phosphorus and heavy metals with an acidic liquid to dissolve the phosphorus and heavy metals contained in the material to be treated, and obtaining a first mixture containing a first liquid and a first solid; A first solid-liquid separation step of separating the first liquid in which phosphorus and heavy metals are dissolved from the first solid; A first precipitation step of mixing the first liquid with a first precipitant and raising the pH to precipitate a second solid containing phosphorus and heavy metals, and obtaining a second mixture containing a second liquid and the second solid; A second solid-liquid separation step of separating the second solid containing phosphorus and heavy metals from the acidic second liquid; A second dissolution step of dissolving the phosphorus contained in the second solid with an alkaline liquid, and obtaining a third mixture containing a third liquid and a third solid; A third solid-liquid separation step of separating the third liquid in which phosphorus is dissolved from the third solid containing heavy metals and containing phosphorus; A second precipitation step of mixing the third liquid with a second precipitant and setting the pH to 12 or higher to precipitate a fourth solid containing phosphorus, and obtaining a fourth mixture containing a fourth liquid and the fourth solid; A treatment method for obtaining a composition containing phosphorus at a high content rate and having a low heavy metal content rate by repeatedly performing a series of steps including a fourth solid-liquid separation step of separating the fourth solid containing phosphorus from the alkaline fourth liquid, Recycling by feeding 10% by mass or more of the second liquid obtained in the second solid-liquid separation step to the first dissolution step, Recycling by feeding 10% by mass or more of the fourth liquid obtained in the fourth solid-liquid separation step to the second dissolution step, The treatment method is characterized in that alkaline earth metals in the second liquid after recycling are recovered as an alkaline earth metal resource containing at least one of magnesium salts and calcium salts.

2. The treatment method according to Claim 1, wherein when the content rate of calcium salts and the content rate of magnesium salts in the second liquid obtained in the second solid-liquid separation step are equal to or lower than the solubility at 25°C, the second liquid is recycled by feeding it to the first dissolution step.

3. The treatment method according to Claim 1 or 2, wherein 10% by mass or more of the third solid obtained in the third solid-liquid separation step is recycled by feeding it to the first dissolution step.

4. The treatment method according to claim 3, wherein when the content rate of heavy metals in the third solid obtained in the third solid-liquid separation step is 10% by mass or less, the third solid is recycled by subjecting it to the first dissolution step.

5. The treatment method according to claim 3 or 4, wherein in the series of steps performed repeatedly, the number of recycling times of any one of the second liquid, the fourth liquid, and the third solid is 3 times or more.

6. The treatment method according to any one of claims 1 to 5, wherein calcium phosphate or magnesium phosphate is precipitated in the first precipitation step, and this is separated as a solid in the second solid-liquid separation step.

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