JP2001079516A - Treatment of fly ash - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely prevent the elution of heavy metals such as lead in fly ash over a long period by adding reducing-period slag by-produced from electric- furnace slag containing CaO, SiO2 and Al2O3 as the main components to the fly ash at the time of insolubilizing the heavy metals in the fly ash, adding phosphoric acid and kneading the admixture. SOLUTION: When the fly ash generated from the incinerator and melting furnace for urban refuse, industrial waste, etc., is treated, the reducing-period slag by-produced from electric-furnace slag consisting essentially of CaO, SiO2 and Al2O3 is added to the fly ash, phosphoric acid is added, and the admixture is kneaded. Consequently, the heavy metals such as lead and cadmium contained in the strongly alkaline fly ash generated from a dry waste gas treating equipment are insolubilzed, the elution is surely prevented, and the fly ash is solidified like cement. Meanwhile, ferric polysulfate and water are added, as required, and kneaded, and the amt. of phosphoric acid to be added and the amts. of the ferric polysulfate and water to be added, as required, are determined by the state and pH of the kneaded material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating fly ash generated from incinerators and melting furnaces for municipal waste, industrial waste, and the like. In particular, the present invention relates to a method for insolubilizing heavy metals such as lead and cadmium contained in a strongly alkaline fly ash generated from a dry exhaust gas treatment facility, and reliably preventing the elution of the heavy metals to solidify like cement.

[0002]

2. Description of the Related Art When municipal waste and industrial waste are incinerated or melted, fly ash is generated together with combustion exhaust gas. In order to capture this fly ash, it is usually collected using a bag filter or an electric dust collector, etc., and recycled or landfilled.

Incidentally, the fly ash thus obtained contains concentrated volatile heavy metals such as lead and cadmium. Therefore, when performing landfill disposal, it is necessary to perform a treatment for sufficiently preventing elution. . In particular, fly ash generated from dry exhaust gas treatment equipment that neutralizes and removes HCl and SOx in combustion exhaust gas with slaked lime often uses excess slaked lime. And the eluate shows strong alkalinity. In this strongly alkaline environment, a large amount of lead, which is an amphoteric metal contained in fly ash, tends to elute, so it must be subjected to more sufficient preventive treatment before it is landfilled.

[0004] Conventionally, when landfilling fly ash, landfill disposal is not approved unless the landfill criterion value for sludge and the like specified by the Environment Agency (Notification No. 13 of the Environment Agency) is satisfied. Therefore, it is necessary to treat the fly ash so that the elution amount of heavy metals (particularly lead and cadmium) contained in the fly ash falls below the reference value. The method of preventing the elution of heavy metals contained in fly ash is determined by the Minister of Health and Welfare, and the methods include a cement solidification method, a chemical treatment method, an extraction method using an acid or other solvent, and a melt solidification method. But the equipment,
From the viewpoint of treatment cost and the like, at present, the cement solidification method, the chemical treatment method, or the combination of the cement treatment method and the chemical treatment method is mainly used.

[0005] As a method of solidifying cement, a method is generally used in which water is added to fly ash and portland cement is added to solidify the cement. As a chemical treatment method, an organic chelating agent is used as a chemical, and a heavy metal such as lead is chelated and insolubilized, or phosphoric acid or a salt thereof is added to convert a heavy metal such as lead into a poorly soluble phosphate compound. To insolubilize (Japanese Patent Publication No. 4-617)
No. 10) has been proposed.

[0006]

However, the former method of solidifying cement mainly involves physical containment of heavy metals by a cement hydrate and insolubilization of heavy metals by the alkalinity of the cement. For this reason, there is a concern that heavy metals after landfill treatment may be eluted, such as the collapse of cement solids due to aging and the re-elution of amphoteric heavy metals such as lead in an alkaline environment.

[0007] In the latter chemical treatment method, the treatment with an organic chelating agent may require as much as several tens of percent of the fly ash, which contains very large amounts of heavy metals such as lead, resulting in cost reduction. Has the disadvantage of being expensive. In addition, there is a concern that heavy metals such as insolubilized lead and the like may be re-eluted due to the activity of microorganisms in the soil and the degradation of chelate bonds due to ultraviolet rays after being landfilled. Further, in recent years, it has been reported that heavy metals such as lead elute in a large amount in an elution test simulating acid rain.

Further, in the method of adding phosphoric acid or a salt thereof to fly ash described in Japanese Patent Publication No. 4-61710, a large amount of fly ash containing a large amount of unreacted slaked lime is used in order to lower the pH. Need phosphoric acid, and lower the pH,
As the date passes, the pH continues to decrease, and heavy metals such as lead are eluted. Above pH
This is because, since lead is an amphoteric metal and is eluted under acidic or alkaline conditions, the pH is preferably adjusted to about 9 to 11 in order to prevent this.

Further, fly ash containing a large amount of calcium chloride, which is a reaction product of HCl and slaked lime in flue gas,
Due to deliquescence, it absorbs atmospheric moisture,
After the treatment, in an extreme case, the slurry may be in the form of a slurry, which may make it difficult to handle the treated material.

[0010] By the way, among the four methods specified by the Minister of Health and Welfare, cement cement, chemical treatment, or a combination of cement treatment and chemical treatment, which are currently the mainstream methods of fly ash treatment, provide cement, Landfilling using resources such as drugs is not commercially desirable. Furthermore, in recent years, resource saving and recycling have been called for, and from that point of view, there is a demand for a method for treating resources typified by cement, chemicals, and the like while using as little as possible.

Therefore, the present invention solves the above-mentioned problems of the prior art, has good handleability of the processed material, and can always safely and surely prevent the elution of heavy metals such as lead even with the lapse of time, Moreover, it is an object of the present invention to provide a method for treating fly ash which is inexpensive and contributes to resource saving and recycling.

[0012]

SUMMARY OF THE INVENTION The method for treating fly ash according to the present invention is a method for insolubilizing heavy metals contained in fly ash generated by an incinerator or melting furnace.
It is characterized by adding a reducing phase slag by-produced from electric furnace slag mainly composed of SiO2 and Al2O3, and adding and kneading phosphoric acid.

According to the present invention, heavy metals such as lead contained in fly ash react with phosphoric acid to form a hardly soluble compound, and further, by adding slag for reducing steelmaking, steelmaking reduction is achieved. The slag has the same hydraulic properties as cement, which maintains the shape of the treated material and also has the ability to maintain the pH, so it is possible to prevent the elution of heavy metals such as lead contained in fly ash for a long time. can do.

Further, by adding ferric polysulfate, even for fly ash containing a large amount of unreacted slaked lime, pH can be adjusted without requiring a large amount of phosphoric acid, and fly ash can be efficiently and reliably produced. It can prevent elution of heavy metals such as lead contained therein. The general formula of ferric polysulfate is [Fe ₂ (OH) _n (SO ₄ )
_{3 - n / 2} ] _m and is mainly used as a flocculant in the field of water treatment. Ferric chloride, PAC, sulfate bands, etc., which are used as flocculants like ferric polysulfate, can also be used in fly ash treatment, but metal corrosion is more severe than ferric polysulfate It is necessary to consider the corrosion resistance of the drug injection equipment, and it is necessary to increase the amount of addition,
It can be costly.

It is preferable to add water to make the kneading easier. Then, it is preferable that the obtained processed material is subjected to molding, granulation, etc., from the viewpoint of good handling and workability in transporting and discharging.

The phosphoric acid to be added, the ferric polysulfate and water to be added as required, depend on the state of the kneaded product and p.
The addition amount is determined by the value of H. This is because the appropriate value differs depending on the equipment used and the properties of fly ash. When the processed material is to be molded using a mold, the processed material may be formed into a slurry so as to be easily put into the mold.

[0017]

Embodiments of the present invention will be described below in detail. In the present invention, phosphoric acid and slag for reducing steelmaking are added to fly ash and, if necessary, ferric polysulfate and water are added and kneaded to fix heavy metals in the fly ash.

In the present invention, the phosphoric acid to be added to the fly ash may be, for example, a monophosphate such as monosodium dihydrogen phosphate (NaH ₂ PO ₄ ) as long as it originally has a phosphate group. Is possible, but in view of the cost, orthophosphoric acid (H ₃ PO ₄ ), which is generally distributed, is the cheapest, and is therefore suitable for this fly ash treatment method.

In the present invention, the steelmaking reduction slag added to the fly ash is one of the slag produced as a by-product from the steelmaking electric furnace, and is currently recycled into temporary roadbed materials and cement raw materials. There is also a demand for recycling for other uses. In terms of chemical and physical properties, the aqueous solution has an alkaline pH of 11 to 12, and has a cement-like hydraulic property in itself. It is preferable that the proportion of the fly ash be 15 to 30% by weight. This is because if the addition amount is too small, the ability as a solidifying agent to maintain the shape of the processed product and the ability as a buffering agent to maintain pH are not exhibited. In addition, even if the amount is too large, it is not preferable to increase the amount of industrial waste due to the current social situation such as shortage or tightness of landfill sites. In the present invention, the water and ferric polysulfate added to the fly ash vary depending on the properties of the fly ash and the desired shape of the processed material, and are appropriately adjusted through a desk test or the like.

(Examples) Prior to the examples, Table 1 shows the chemical composition of the fly ash used in the present invention, and Table 2 shows the results of dissolution tests performed in accordance with Notification No. 13 of the Environment Agency.

[0021]

[Table 1]

[0022]

[Table 2]

From Tables 1 and 2, this fly ash is a strongly alkaline fly ash from a dry exhaust gas treatment facility containing a large amount of unreacted slaked lime. In immobilizing the heavy metal of this fly ash, what must be particularly noted is lead and cadmium, and the results of the dissolution test (Notification No. 13 of the Environment Agency) below describe lead and cadmium.

Table 3 shows the chemical composition of the slag in the steelmaking reduction period added in the present embodiment.

[0025]

[Table 3]

In order to observe the effect of immobilizing heavy metal on the fly ash of phosphoric acid and the steelmaking reduction period slag, 85% phosphoric acid, steelmaking reduction period slag and water are added to the fly ash and kneaded, and a curing period is left for one week. A dissolution test (Notification No. 13 of the Environment Agency) was performed, and the dissolution amounts of lead and cadmium were measured. In addition, as a comparative example, a method in which only phosphoric acid was added to fly ash in a similar manner (Comparative Example 1),
What used Portland cement (Comparative Examples 2 to 5), alumina cement (Comparative Examples 6 to 9), activated clay (Comparative Examples 10 to 13), and bentonite (Comparative Examples 14 to 17) instead of the slag for the steelmaking reduction period Done. Table 4 shows the results.

[0027]

[Table 4]

According to Table 4, except for the steelmaking reduction period slag of 15% (Example 1) and the one to which Portland cement was added (Comparative Examples 2 to 5), the reference value for landfill related to sludge (determined by the Secretary of the Environment Agency) (Reference value), which is a good result. This is largely dependent on the pH of the solidifying agent. Generally, Portland cement has a pH of about 12 or more, alumina cement has a pH of about 11 to 12, activated clay has a pH of about 6, and bentonite has a pH of about 10.
The dust in the steelmaking reduction period of this example had a pH of about 11 to 12. Therefore, those to which Portland cement is added (Comparative Examples 2 to 5) cannot fall below the landfill determination reference value (reference value determined by the Secretary of the Environment Agency) unless sludge acid is added in a larger amount. .

Next, in order to check the long-term stability of the processed material,
Dissolution test after 1 month of curing (Notification 1 by the Environment Agency)
No. 3) was performed, and the elution amounts of lead and cadmium were measured.
Table 5 shows the results.

[0030]

[Table 5]

As shown in Table 5, those having excellent long-term stability of the treated products are those to which slag for reducing steelmaking is added and those to which alumina cement is added (Comparative Examples 24-26). In the case where activated clay was added (Comparative Examples 27 to 30) and the case where bentonite was added (Comparative Examples 31 to 34), the pH was too low as compared with the dissolution test one week after curing (Notification No. 13 of the Environment Agency). Lead and cadmium have eluted. In addition, for those to which Portland cement was added (Comparative Examples 19 to 22), since the treated material increased in strength due to the hydration reaction of the cement and physically contained lead and cadmium in the treated material, the dissolution test ( Environment Agency Notification 13
No.) results are better than those after one week of curing, however, they cannot stably fall below the landfill criteria for sludge (standards set by the Secretary of the Environment Agency).

By the way, when Tables 4 and 5 are viewed comprehensively,
From the viewpoint of preventing heavy metal elution of fly ash, those added with phosphoric acid and slag during the steelmaking reduction period and those added with phosphoric acid and alumina cement are superior. However, from the viewpoint of cost, when the alumina cement is set to 1, the slag in the steelmaking reduction period is also an industrial waste, and therefore is 1/20 to 1/60.
And it is quite low cost. Therefore, it is clear that the use of the slag in the steelmaking reduction period can greatly contribute to resource saving and recycling, and it can be seen that the reduction period slag is effective as a solidifying agent.

Next, part of the phosphoric acid was replaced with ferric polysulfate in order to further reduce the fly ash treatment cost of the slag of the phosphoric acid and the steelmaking reduction period. The cost at this time is 0.3 for polyiron, assuming that phosphoric acid is 1. The amount of slag added in the steelmaking reduction period was 25% by weight based on 100 parts by weight of fly ash.
Table 6 shows the results.

[0034]

[Table 6]

From Table 6, it can be seen that even if phosphoric acid is reduced and ferric polysulfate is added, the effect of preventing fly ash from eluting heavy metals can be sufficiently obtained. Furthermore, since the addition amount of ferric polysulfate has an effect of preventing heavy metal elution in a wide addition amount range of 1 to 10% by weight with respect to fly ash, the control of the chemical injection amount at the actual fly ash treatment site can be achieved. It also has the advantage of being easy to do.

Next, the dissolution test described above (notification 13 by the Environment Agency)
No.), the curing period was one week and one month. However, in actual incineration or melting facilities of municipal garbage and industrial waste, physical problems (when it is difficult to secure a place) make it unrealistic. Since there is a possibility that the curing period may become too long, the same test was conducted for shorter curing days. Table 7 shows the results.

[0037]

[Table 7]

Examples 17 to 20 in Table 7 (curing 1
Days) and Examples 25 to 28 (three days of curing)
1-4 (curing for one week) and Examples 5-8 (curing for one month)
The conditions are the same except that the curing days are different. According to Table 7, the curing period cannot be less than the landfill judgment standard value (standard value set by the Secretary of the Environment Agency) for sludge etc. for one day, but if the curing period is three days, some curing days are not enough. Although there is a compounding ratio that can be judged as, it was found that almost all compounding ratios can sufficiently fall below the reference value for landfill for sludge (standard value set by the Secretary of the Environment Agency). With this number of days, it should be practically sufficient.

[0039]

As described above in detail, the fly ash treatment method of the present invention has the following effects. First, heavy metal such as lead contained in fly ash reacts with phosphoric acid to form a hardly soluble compound. In addition, slag of steelmaking reduction period is added by adding steelmaking reduction period slag. Since it has the ability to maintain the shape of the treated product and maintain the pH with cement-like hydraulic properties, it is possible to prevent the elution of heavy metals contained in fly ash for a long period of time.

Second, the present invention is to effectively utilize the steelmaking reduction period slag, which is also industrial waste in a steelmaking company, and to use low-cost ferric polysulfate instead of phosphoric acid which is expensive in terms of cost. Since it can be used, it is the most cost-effective method of solidifying and insolubilizing fly ash like cement.

Thirdly, unlike the method of solidifying fly ash using resources such as portland cement as described above, the cement-like solidification capacity of the slag in the steelmaking reduction period, which is also industrial waste, is effectively used. Therefore, industrial waste is recycled, which can greatly contribute to resource saving.

Fourthly, in the treatment of fly ash, when the amount of phosphoric acid is reduced and ferric polysulfate is added, the addition amount of ferric polysulfate is 1 to 10% by weight based on fly ash. The effect of preventing heavy metal elution of fly ash can be obtained in the amount range, so that the injection amount of the drug at the site can be easily controlled.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Fuyuki Shochiku 3-chome, Sakuracho, Kariya-shi, Aichi Pref. (72) Inventor Yoshiya Kiriyama 3-chome, Sakuracho, Kariya-shi, Aichi F-term (Reference) 4D004 AA37 AA43 AB03 CA34 CA45 CC03 CC11 CC12

Claims (3)

[Claims] 1. A reducing slag by-produced from an electric furnace slag mainly composed of CaO, SiO2 and Al2O3 for insolubilizing heavy metals of fly ash generated by an incinerator or melting furnace. A fly ash treatment method comprising adding phosphoric acid and kneading the mixture. 2. The method for treating fly ash according to claim 1, further comprising adding ferric polysulfate. 3. The method for treating fly ash according to claim 1, wherein water is further added.