JP3391263B2 - Method for recovering phosphorus from incinerated ash containing phosphorus - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for recovering phosphorus from incineration ash of waste containing phosphorus such as sewage sludge.

[0002]

2. Description of the Related Art Sludge generated at a sewage treatment plant is dehydrated and then incinerated, and most of it is dumped at a final disposal site. However, year by year, it becomes difficult to secure the final disposal site, and as the shortage becomes serious, effective use of incineration ash has been demanded.

By the way, the general composition of ash obtained by incinerating ordinary sewage sludge is as shown in Table 1. As shown in this table, it is noted that sludge incineration ash contains a large amount of phosphorus. This phosphorus is the phosphorus contained in the sewage sludge, which remains in a concentrated state by incineration. Therefore, the phosphorus content in the incineration ash differs depending on the type of flocculant added when the sludge was concentrated, and a higher polymer flocculant was added than that containing lime (lime-based sludge incinerator ash). The substance (polymer sludge incineration ash) contains a larger amount of phosphorus (P ₂ O ₅ ). Further, among the polymer-based sludge incineration ash, as shown in Table 2, the sludge incineration ash generated when the sewage is dephosphorized contains a larger amount of phosphorus. Some of the polymer-based dephosphorization sludge incineration ash contains phosphorus in an amount comparable to that of low-grade phosphate rock.

Under such circumstances, a method for recovering phosphorus in sludge incineration ash has been proposed. JP 7-2
Japanese Patent Application No. 51141 and Japanese Patent Application Laid-Open No. 9-77056 describe a phosphorus recovery technique by an acid decomposition-solvent extraction method. In this method, an acid is added to sludge incineration ash to form sludge, and phosphorus compounds in the incinerator ash are decomposed to convert phosphorus into water-soluble phosphoric acid. Next, the sludge is subjected to solid-liquid separation to obtain a liquid containing phosphoric acid, and then phosphoric acid in the liquid is extracted with an organic solvent.

Further, Japanese Unexamined Patent Publication No. 9-145038 discloses that
A phosphorus recovery technique by the melting method is described. In this method, a carbon source is added to the incinerated ash to heat and melt it, the phosphorus compound in the incinerated ash is reduced to volatilize phosphorus, and this phosphorus is recovered.

[0006]

[Table 1]

[0007]

[Table 2]

[0008]

However, each of the above conventional techniques has the following problems.

The techniques described in JP-A-7-251141 and JP-A-9-77056 are based on a method that combines complicated operations such as acid decomposition and solvent extraction. Besides the problem that complicated processing must be performed in multiple stages,
There is also a problem that a large amount of residue is generated when the sludge incineration ash is acid-decomposed and solid-liquid separated, and these residues must be disposed of as waste.

Further, the technique described in Japanese Patent Laid-Open No. 9-145038 is based on a method of treating sludge incineration ash under severe conditions in which it is heated to a high-temperature molten substance. There is a problem of consuming energy.

It is an object of the present invention to provide a method capable of recovering phosphorus in sludge incineration ash with a simple treatment.

[0012]

In order to achieve the above object, in the first invention, incineration ash containing phosphorus (hereinafter,
(Hereinafter simply referred to as incineration ash) is mixed with a carbon source, and the mixture is charged into a closed heating furnace to heat the incineration ash without melting it , and the phosphorus in the incinerator ash is heated as an element. It is made into the form of phosphorus and is volatilized, and the exhaust gas containing the volatilized elemental phosphorus is brought into contact with water to condense and collect the elemental phosphorus. With a closed heating furnace
Then, it is preferable to use a kiln furnace .

In the second aspect of the invention, the incinerator ash containing phosphorus is mixed with a carbon source, and the mixture is charged into a closed heating furnace to heat the incinerator ash without melting the ash to incinerate. Phosphorus in the ash is volatilized in the form of elemental phosphorus, air is mixed with the exhaust gas containing the vaporized elemental phosphorus, the elemental phosphorus is oxidized to form phosphorus pentoxide, and then phosphorus pentoxide is contained. The exhaust gas is brought into contact with water to absorb phosphorus pentoxide into the water, and the phosphorus in the exhaust gas is collected in the form of phosphoric acid. As a closed heating furnace
For this, it is preferable to use a kiln furnace .

[0014]

The present inventors have investigated a method for taking out phosphorus from incinerated ash without using harsh conditions such as the melting method and without complicated operations such as acid decomposition and solvent extraction. As a result of various experiments, it was found that the phosphorus compounds in the incinerated ash are reduced and the phosphorus is volatilized simply by adding a carbon source to the incinerated ash and heating it.

FIG. 3 is a diagram showing an example of the experimental results.
, Heating temperature and phosphorus in treated ash (P₂O _Five) Content of
It shows the relationship. In this experiment,
Mix graphite powder as a carbon source to the incineration ash of the composition shown,
It was placed in a heating furnace and heated. The heating furnace used at this time is a tube
Although it was a circular electric furnace, it is the same as when using a closed heating furnace.
The nitrogen gas so that the furnace does not become an oxidizing atmosphere.
Heated while circulating. The heating was performed for 30 minutes. Addition
The heat temperature was variously changed between 700 ° C and 1250 ° C. That
P in the treated ash taken out from the electric furnace₂O_FiveAnalyze
It was

[0017]

[Table 3]

According to FIG. 3, when heated to about 900 ° C. or higher, volatilization of phosphorus begins, and when heated to 1000 ° C., about 1/3 of phosphorus (P ₂ O ₅ ) in the treated ash volatilizes.

As the heating temperature rises,
P₂O_FiveVolatilization rate increases, but exceeds 1100 ° C
And, a part of the incineration ash begins to melt and the temperature exceeds 1250 ° C.
When heated to the temperature range, a relatively large amount of incinerated ash melts
As it adheres to the furnace wall of the heating furnace,
The limit must be about 1250 ° C. And P₂O
_FiveThe volatilization rate is further increased and the adhesion of the melt does not occur.
When trying to carry out regular operations, the heating temperature is 1
It is desirable to set the temperature in the range of 100 ° C to 1200 ° C.
Yes.

The furnace for heating the incinerated ash can be a batch type as long as it can prevent the flow of the outside air from becoming an oxidizing atmosphere and the furnace temperature can be maintained at 1000 to 1250 ° C. It may be a continuous type. As the continuous heating furnace, for example, an external heating kiln furnace can be used.

As the carbon source to be mixed with the incineration ash, powder of a substance containing carbon as a main component such as coke, graphite, used activated carbon (waste activated carbon), etc. is used. In addition to this, those containing organic substances such as sewage sludge,
Alternatively, an organic substance such as polyvinyl alcohol used as a granulating aid can also be used as a carbon source. The above organic substances act as a reducing agent after being carbonized in a heating furnace.

The incineration ash of sewage sludge is shown in Table 1 and Table 2.
As shown in Fig. 2, the composition of the sludge varies greatly depending on the sludge treatment method and also varies depending on the area where the sludge is generated. Therefore, in the heat reduction treatment of incinerated ash, it is necessary to adjust the components as necessary.

Phosphorus in the incinerated ash is said to exist as compounds such as calcium phosphate and aluminum phosphate. When these compounds are heated in the presence of a carbon source, the formula (1) and ( It is considered that the reaction of the formula (2) proceeds.

2Ca ₃ (PO 4) ₂ + 6SiO ₂ + 10C = 6CaO · SiO ₂ + 10CO + P ₄ ··· (1) 4AlPO ₄ + 10C = 2Al ₂ O ₃ + 10CO + P ₄ ··· (2) However, calcium phosphate is ( As shown in equation (1), S
Since it cannot be reduced only by adding a carbon source unless in the presence of iO ₂ , incineration with a large amount of CaO, such as lime-based sludge incineration ash treated with sludge by adding slaked lime and ferric chloride. When treating ash, it is necessary to add a SiO ₂ source to adjust the composition. As the SiO ₂ source, a substance containing a large amount of SiO ₂ , such as silica stone or coal ash, is used and added to the incineration ash.

The phosphorus volatilized by the above reaction is contained in the exhaust gas in the form of elemental phosphorus and is discharged. Therefore, if the exhaust gas is brought into contact with water and cooled, the elemental phosphorus condenses and is collected as molten yellow phosphorus. Gathered.

When air is mixed with the exhaust gas containing elemental phosphorus, elemental phosphorus is oxidized to form phosphorus pentoxide, as shown in the equation (3). Next, when the exhaust gas containing phosphorus pentoxide is brought into contact with water, phosphorus pentoxide is absorbed by water and phosphoric acid is generated as shown in formula (4).

Thus, the phosphorus removed from the incinerated ash is
It is recovered as yellow phosphorus or phosphoric acid by any of the above methods.

P4 + 5O ₂ = 2P ₂ O ₅ (3) P ₂ O ₅ + 3H ₂ O = 2H ₃ PO ₄ (4)

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory diagram of an example according to an embodiment of the present invention. In this embodiment, incinerated ash generated when sewage sludge is incinerated and carbon source powder such as coke, graphite and waste activated carbon are put in a mixer 10 and mixed, and the mixture is sent to a granulator 11. , Add some granulation aid and water to make granules. After drying the granular material, the granular material is charged into a closed heating furnace 12 such as an external heating type kiln furnace which is maintained so as not to have an oxidizing atmosphere in the furnace.
Heat to 00 ° C-1250 ° C. By this heating, the phosphorus compound in the incineration ash is reduced to produce elemental phosphorus, and this elemental phosphorus becomes vapor of phosphorus and is vaporized. Further, the incinerator ash (treated ash) from which the phosphorus is volatilized and removed is discharged from the heating furnace 12.

Then, the vaporized elemental phosphorus vapor is contained in the exhaust gas and extracted from the heating furnace 12. The exhaust gas extracted from the heating furnace 12 is sent to the dust remover 20 while being maintained at a temperature of 300 ° C. or higher so that phosphorus vapor is not condensed.
Dust is removed. The exhaust gas from which dust has been removed by the dust remover 20 is sent to the condenser 22. In the condenser 22, water is sprayed to cool the exhaust gas, and the vapor of elemental phosphorus contained in the exhaust gas is condensed. The condensed elemental phosphorus becomes yellow phosphorus in a molten state and drops with the spray water.
It is extracted to 3 and collected. Since the exhaust gas in which phosphorus is collected contains a combustible gas such as CO, it can be used for fuel.

FIG. 2 is an explanatory diagram of another example according to the embodiment of the present invention. In the description of FIG. 2, description of the same processing steps as in FIG. 1 will be omitted. In this embodiment, the process of volatilizing phosphorus from the incinerated ash is performed in the same manner as in FIG.

In this embodiment, the exhaust gas from which dust has been removed by the dust remover 20 is sent to the combustion chamber 21. In the combustion chamber 21, air is introduced and CO in the exhaust gas
At the same time as the process of burning the gas is performed, the vapor of elemental phosphorus is oxidized into phosphorus pentoxide by the reaction of the equation (2).
At this time, since the reaction heat of the equation (2) is very large, when treating the exhaust gas containing a large amount of elemental phosphorus, it is necessary to perform the oxidation treatment while cooling the exhaust gas. The exhaust gas containing phosphorus pentoxide is introduced into the absorption tower 30. In the absorption tower 30, a solution of dilute phosphoric acid is circulated and sprayed, and phosphorus pentoxide is absorbed by water in the dilute phosphoric acid by the reaction of the formula (3) to become phosphoric acid. Further, the absorption tower 30 is replenished with water for reacting with phosphorus pentoxide, and the concentration of the phosphoric acid solution in the tower is maintained at a predetermined value. Then, the phosphoric acid solution in an amount corresponding to the produced amount is extracted and stored in the phosphoric acid storage tank 31. The exhaust gas discharged from the absorption tower 30 is subjected to removal of phosphoric acid mist by the mist collector 32, and then sprayed with water in the gas cleaning tower 33 to be cleaned and emitted to the atmosphere.

Next, the properties of the treated ash discharged from the heating furnace 12 will be described. As mentioned above, most of the incineration ash generated when incinerating sewage sludge is not effectively used and is discarded, but one of the reasons is that phosphorus is contained in the incineration ash. Can be mentioned. However, in the treated ash discharged in the embodiment shown in FIGS. 1 and 2, phosphorus is volatilized and removed, and the phosphorus content is significantly reduced. Therefore, according to the above-described embodiment, it is possible to effectively use the treated ash that is the residue obtained by recovering phosphorus from the incinerated ash.

The cement raw material is one of the applications in which the phosphorus content is a problem when the incinerated ash is effectively used, but the treated ash discharged in the above-mentioned embodiment is the phosphorus content. It has a low viscosity and can be used as a cement raw material. When used as a cement raw material, the allowable amount of phosphorus is said to be about 10% as P ₂ O ₅ , but the phosphorus content rate of the treated ash discharged in the above embodiment is as follows. It is well below its tolerance, as described in the Examples section.

Example 1 100 g of incinerated ash of sewage sludge having the composition shown in Table 4 was mixed with 10 g of graphite powder, and the mixture was heated to 1150 ° C.
It was charged into a tubular electric furnace (heating furnace) held by, and heated in a nitrogen stream for 30 minutes while circulating a small amount of nitrogen gas. The exhaust gas discharged from the furnace was passed through a conduit that was heated and kept warm by winding a sheath heater, and was cooled through an absorption bottle containing hot water. The condensed and collected phosphorus accumulated at the bottom of the absorption bottle.

Further, the heat-treated treated ash was taken out and its composition was analyzed. The analysis results of this treated ash are shown in Table 4. According to this table, the phosphorus content of the treated ash was significantly lower than the value of the incinerated ash before treatment, and was a value acceptable as a cement raw material.

Example 2 100 kg of incinerated ash of sewage sludge having the composition shown in Table 4 was mixed with 10 kg of coke powder crushed to a size of 0.2 mm or less, and 10 liter of a 20% solution of polyvinyl alcohol was added to the mixture. Granulate by adding 10
Granules with a size of about mm were made. Then, after drying this granular material, it was charged into an external heating type kiln furnace (heating furnace) whose internal temperature was maintained at 1150 ° C. at a rate of 20 kg / hour and heated.

The exhaust gas discharged from the kiln furnace was introduced into a combustion chamber and burned, and then sent to a spray tower to spray water as an absorbing liquid to absorb the phosphorus content in the exhaust gas. driving,
Water was replenished to prevent the absorption liquid from decreasing. After the operation of the heating furnace was completed, the total amount of the absorption liquid in the absorption tower was extracted and analyzed, and 100 liters of phosphoric acid liquid was obtained.
The concentration of P ₂ O ₅ was 17%, and the total amount of treated ash discharged from the kiln furnace was taken out every one hour, and its composition was analyzed. Regarding this analysis value, the values during the test period are summarized in an average value and shown in Table 4. As shown in this table, the phosphorus content of the treated ash is significantly lower than that of the incinerated ash before treatment,
The value was acceptable as a cement raw material.

[0039]

[Table 4]

[0040]

According to the present invention, phosphorus in the incinerated ash is volatilized in the form of elemental phosphorus simply by mixing the carbon source with the incinerated ash, charging the mixture into a closed heating furnace and heating. The elemental phosphorus can be collected and recovered in its original form by treating the exhaust gas containing the volatilized elemental phosphorus, or the elemental phosphorus can be converted into the form of phosphoric acid. It can also be collected and collected.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an example according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of another example according to the embodiment of the present invention.

FIG. 3 is a diagram showing a relationship between a heating temperature and a phosphorus content of treated ash.

[Explanation of symbols]

10 mixer 11 granulator 12 Closed heating furnace 20 dust remover 21 Combustion chamber 22 condenser 30 absorption tower 32 Mist collector 33 gas cleaning tower

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Takuya Shinagawa Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-9-145038 (JP, A) JP-A-10 -279301 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B09B 3/00-5/00 F23G 7/00

Claims (3)

(57) [Claims] 1. An incineration ash containing phosphorus is mixed with a carbon source, and the mixture is charged into a closed heating furnace to heat the incineration ash without melting it, so that the phosphorus in the incineration ash is in the form of elemental phosphorus. A method for recovering phosphorus from incineration ash containing phosphorus, characterized in that the exhaust gas containing the vaporized elemental phosphorus is contacted with water to condense and collect the elemental phosphorus. 2. An incinerator ash containing phosphorus is mixed with a carbon source, and the mixture is charged into a closed heating furnace to heat the incinerator ash without melting it, so that the phosphorus in the incinerator ash is in the form of elemental phosphorus. And volatilize it, mix air with the exhaust gas containing the volatilized elemental phosphorus, oxidize the elemental phosphorus to generate phosphorus pentoxide, and then contact the exhaust gas containing phosphorus pentoxide with water to pentoxide A method for recovering phosphorus from incineration ash containing phosphorus, which comprises absorbing phosphorus in water and collecting phosphorus in exhaust gas in the form of phosphoric acid. 3. The method for recovering phosphorus from incinerated ash containing phosphorus according to claim 1, wherein the closed heating furnace is a kiln furnace.