JPS6036826B2 - Method for removing unstable heavy metals from incinerated fly ash - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for rendering harmless and stabilizing incinerated fly ash containing heavy metals generated in garbage incineration plants, etc., and particularly relates to a method for stabilizing unstable heavy metals contained in incinerated fly ash. .

The dust generated when general garbage, municipal garbage, industrial waste, animal and plant remains, various sludge, etc. are incinerated at incinerators is collected by electrostatic precipitators, multi-cyclones, etc.)
It has been dumped.

However, such incinerated fly ash contains toxic heavy metals at high concentrations in an unstable form, and in recent years, heavy metals have been leached from abandoned fly ash and polluted the local environment, which has been raised as a major social problem. Now you can. That is, in general, toxic heavy metals exist in the form of stable oxides in incinerated fly ash, as well as in the form of soluble salts such as chlorides and sulfates. For example, fly ash collected from an electrostatic precipitator at a municipal waste incineration plant contains high concentrations of Zn, Pb, and AI, although the type and concentration vary depending on the season and region.
In addition to heavy metals such as , Fe, and Sn, Cu, Cd, Cr, and M
Heavy metals such as n, Hg, Ni, Bi, Sr, Ti, and Sb and metal ions such as Na, K, Ca, and Mg are mostly contained in the form of water-soluble salts, such as chlorides and sulfates. ing. Therefore, if incinerated fly ash with this composition is directly dumped in a landfill, it is inevitable that harmful heavy metals will be released into the natural environment. As a solution to this problem, a method of mixing cement with fly ash, kneading it, solidifying it, and then dumping it is being considered.
However, this method consumes a large amount of cement to process fly ash, which not only wastes resources but also increases the amount of waste. Furthermore, due to the alkaline state caused by cement, there is a risk that amphoteric metals such as Cr, Pb, and Sn may be redissolved and cause secondary pollution. Furthermore, since incinerated fly ash generally contains many types and large amounts of salts, it is often difficult to harden it. An object of the present invention is to provide a method for removing and stabilizing unstable heavy metals in incinerated fly ash.

The method of the present invention involves dispersing incinerated fly ash in water or an acidic aqueous solution to transfer substantially all of the soluble heavy metals contained in the incinerated fly ash to the liquid phase, separating and removing the remaining fly ash, and removing the extracted heavy metals. It is characterized in that a ferrite production reaction is carried out in the contained liquid phase to incorporate heavy metals into the ferrite precipitate crystals, the ferrite precipitates are separated and removed, and the liquid from which the heavy metals have been removed is treated for discharge or reuse. According to the present invention, the unstable toxic heavy metals in the incinerated fly ash are converted into stable ferrite compounds, and the treated fly ash does not contain any harmful heavy metals that pose a risk of deterioration. Therefore, it is possible to directly dispose of it in a landfill, and the recovered stable ferrite compound can be reused as an industrial raw material in various fields.

Furthermore, if it is not to be reused, it is of course possible to mix it with treated fly ash and dispose of it in a landfill. In the present invention, fly ash is first dispersed in water or an acidic aqueous solution to transfer substantially all of the soluble heavy metals to the liquid phase. To make this complete, fly ash equivalent to 5 to 3% by weight is dispersed, and the pH of the liquid is
4.0 to 7.0 by adding mineral acid as necessary.
It is preferable to fly while adjusting the range. This is 5
% or more is better and more economical, and 30% or less prevents the risk of saturation of soluble salts and insufficient elution of heavy metals. If the fly ash concentration exceeds 30%, the elution operation may be repeated two or more times, or the washing described later may be repeated two or more times.

In addition, a pH of 4.0 or higher is more economical as only unstable heavy metals can be eluted and stable components are not eluted, while a pH of 7.0 or lower prevents the danger of unstable heavy metals not being eluted. This is because it can be done. Furthermore, after separating the residual fly ash from the heavy metal-dissolved liquid using a vacuum furnace filtration machine or the like, it is preferable to wash the separated water-containing residual fly ash at least once to remove the eluted heavy metals adhering to the water-containing fly ash. For example, it is best to wash the hydrated fly ash at least once with an amount of water of 1 to 5 pip per volume of the hydrated fly ash. Determined by fly ash concentration, dehydrator performance, etc. The separated residual fly ash from which substantially all unstable heavy metals have been released through such treatment can be disposed of without fear of secondary pollution. When the liquid containing dissolved heavy metals extracted from fly ash is washed, it is subjected to ferrite generation treatment along with the washing liquid.

That is, a known ferrite production reaction is carried out in the liquid, dissolved heavy metals are incorporated into the produced ferrite precipitate crystals, and the ferrite precipitates are separated using, for example, a magnetic separator, thereby completely removing harmful heavy metals from the liquid phase. . The harmful metals incorporated into the generated ferrite are stable and there is almost no risk of re-elution. Conventionally, the method used to remove harmful heavy metals from the furnace fluid is to add alkali to the furnace fluid to form a precipitate with low solubility (neutralization pseudo-method). It is very difficult to process fly ash treatment liquids that contain many different types of heavy metals at once, and the precipitated particles are so fine that it takes a long time to pass through the furnace.
The hydroxide sludge produced by this method is extremely unstable, and if it is disposed of in a landfill along with the treated fly ash, there is a risk that harmful heavy metals will be re-leached.
On the other hand, ferrite sludge is stable and can be mixed with treated fly ash and disposed of in a landfill, or the ferrite sludge itself can be used as a radio interference prevention material and as a raw material for various industries as a resource. It is also possible to reuse. As the ferrite generation treatment, a known ferrite generation treatment may be used, but in particular, considering the heavy metals eluted from the fly ash and the characteristics of the treatment process, ferrous salt should be used in an amount equal to or more than twice the total weight concentration in the furnace fluid. is dissolved, alkali is added to make the solution alkaline (preferably pHIO or higher), the temperature is raised to 5000 or higher by flooding, and then air or other oxidizing gas is blown in to react under a saturated amount of dissolved oxygen. It is preferable to use a method of producing a ferrite compound. After the ferrite has been separated and removed, the treatment solution that does not contain harmful heavy metals is neutralized by adding an acid such as sulfuric acid and discharged, or it is reused for dispersing fly ash, cleaning fly ash and waste smoke, and other uses. . However, since incinerated fly ash generally contains nearly 50% of soluble salts such as common salt, trinitrate, and potassium chloride, there is a risk of salt damage if the treatment solution is discharged as is. In such cases, safety can be further improved by treating the neutralized treatment solution using diaphragm electrolysis to generate and collect caustic soda, sulfuric acid, and chlorine gas, and then discharging the recovered gas.
This is also preferable from the standpoint of effective use of resources. In other words, electricity for diaphragm electrolysis can be generated using the heat of incineration at the incinerator, and the recovered caustic soda can be used as at least part of the alkali added for ferrite generation treatment, and the recovered sulfuric acid can be used as at least a part of the alkali added for ferrite generation treatment. It can be used as at least part of the added acid for neutralization. In addition, we use recovered sulfuric acid to dissolve iron waste such as empty cans accumulated at incinerators to create ferrous salts such as ferrous sulfate, which are then added to ferrous salts for ferrite production. It can also be used as at least a part of. Of course, a ferrous salt may be similarly produced using a recovered acid other than sulfuric acid, and this may be used for producing ferrite. As described above, according to the present invention, it is possible to bring about preferable resource reuse and closed processing and circuitization in garbage incineration plants and the like. Example 1 45k9 of incinerated fly ash collected by an electrostatic precipitator was mixed with 200k of water.
After dispersing the saw and stirring for 5 minutes, add sulfuric acid to -6.0
After adjusting the temperature, the mixture was heated for another 1 minute to elute soluble salts and harmful heavy metals.

This was subjected to solid-liquid separation using a Brecoat type vacuum furnace filtration machine, the filtrate cake was again dispersed in 100% water to wash heavy metals in the adhering water, and solid-liquid separation was performed again using a precoat type vacuum furnace filtration machine. The first furnace liquid and the second cleaning furnace liquid were mixed, and the total salt concentration was measured using TDSJISKOI02, and the heavy metal concentration was measured using atomic absorption spectrometry. The results are shown in Table 1. Table 1 (
Units ppm) Total soluble salt concentration (TDS) 11,000 Total mercury
0.02 Cd 86Cr o
．． 03 Fe2Pb 18.3
Ni 04Mn 11.5
The moisture content of the Zn 5000Cu 15 dehydrated cake (treated fly ash) was 49.0%, and the weight loss of the fly ash due to elution of soluble components was 50%.

After adding 15.3k9 of ferrous sulfate (heptahydrate) to the furnace filter and thoroughly mixing the solution, add a caustic soda solution (4.9k
g) was added to neutralize the solution, the pH was set to 10 or more, and the solution temperature was raised to 6000, at which time the oxidation reaction of the ferrous compound was continued while blowing air. After 2 hours, the reaction was stopped and the produced black magnetic substance was separated.

The heavy metal concentrations in the furnace fluid were measured by atomic absorption spectrometry, and as shown in Table 2, all were below the wastewater standard value. In addition, we conducted an elution test on the generated black magnetic substance (ferrite sludge) based on Environment Agency Notification No. 22, and the result was 3.
As shown in the table, all values were below the standard values Table 2 Heavy metal concentration in the treatment solution (unit: ppm) Total mercury 0.005 or less Total chromium 0.5 or less Pb o. 5 or less Cd o. 05 Below Cu l. 0 or less Fe l. 0 or less Mn l. 0 or less Zn l. 0 or less Ni l. 0 Below Table 3 Ferrite sludge port test results (ppm) Alkyl mercury Not detected Mercury Not detected but 0.5 or less Cadmium 0.1 or less Hexavalent chromium 0.1 or less Arsenic 0.05 or less When a dissolution test was conducted on the dehydrated cake (treated fly ash) using the same method, the results shown in Table 4 were obtained.

Table 4 Elution test results for treated fly ash Alkyl mercury Not detected Mercury Not detected Lead 1 ppm or less Cadmium 0.1 ppm or less Hexavalent chromium 1.0 ppm or less Arsenic Based on the results of 0.05 ppm or less, no harmful metals were detected. It is confirmed that the incinerated fly ash contained in a stable form was able to be transferred to a stable and harmless ferrite compound.

When the mixture of treated fly ash and ferrite sludge is disposed of in a landfill, it fully satisfies the values regulated by the Marine Pollution Prevention Law, the Waste Disposal Law, etc., and does not require complicated processes such as concrete molding. Example 2 45k9 of fly ash was dispersed in 300ml of water, coagulated for 10 minutes, then hydrochloric acid was added to adjust the pH to 5.0, and the mixture was stirred for another 10 minutes to elute soluble salts and harmful heavy metals. Ta.

After passing through the oven through a diaphragm press filter, the cake is
00 Washed with that water. Table 5 shows the results of analyzing the total salt concentration and heavy metal concentration of the mixed solution of furnace liquid and cleaning waste liquid. The moisture content of the dehydrated cake (treated fly ash) was measured using an infrared spectrometer and was found to be 46.7%. Table 5 Total salt concentration and heavy metal concentration in furnace fluid and washing waste fluid (unit: ppm) Total soluble salt concentration (TDS) 50,000 total Water Silver
0.008 Total Chromium 0.01 Pd ll. 0Mn
6.2 Cu
22.9Cd 4
0.6Fe l. 0Ni
o. 8 Zn 2,900Sn
20.0Sb
l. 4 (TDS is by JIS KOIOI, heavy metal concentration is by atomic absorption method) After dissolving 34.2k9 of ferrous sulfate (heptahydrate) in the above solution containing harmful metals, a caustic soda solution (5.1k9NaOH) is added. Add to neutralize, set the pH to 10 or higher, and lower the solution temperature to 70.
When the temperature rose to qo, 4 N/day of air was blown into the reactor to continue the oxidation reaction of the ferrous compound under saturated dissolved oxygen, thereby incorporating harmful heavy metals into the produced black magnetic material (ferrite).

The reaction was terminated after 1 hour, and the produced black magnetic material (ferrite sludge) was separated by a magnetic method.

Table 6 shows the results of measuring the heavy metal concentration in the furnace liquid by atomic absorption spectrometry. Next, the ferrite sludge and dehydrated cake (treated fly ash) separated by the magnetic method were subjected to an elution test in accordance with Environment Agency Notification No. 22, and the results shown in Table 7 were obtained. Table 6 Heavy metal concentration in treatment furnace liquid Total mercury 0.005 ppm or less Total chromium 0.5 Pb o. 5 Cd o. 05 〃 Cu 1.0 〃 Fe l. 0 〃 Mh l. 0 〃 ・Zn 20 〃 Nj o. 1 Sn o. 1 〃 Sb o. 1 Table 7 Elution test results for ferrite sludge and treated fly ash Treated fly ash and ferrite sludge fully satisfy the Marine Pollution Prevention Law, Waste Disposal Law, etc. when disposed of in a landfill, and furnace sludge after ferrite treatment Just by neutralizing it, it satisfies the Water Pollution Prevention Law and can be discharged as is.

Claims (1)

[Claims] 1 Disperse incinerated fly ash in water or acidic aqueous solution to a concentration of 5
~30% by weight, and the pH of this water or acidic aqueous solution is 4~30% by weight.
7 and stir it, the remaining incinerated fly ash is separated from the water or acidic aqueous solution, the separated incinerated fly ash is further dispersed in water and washed one or more times, and then the remaining incinerated fly ash is removed from the washing water. The incinerated fly ash is separated, and ferrous ions and alkali are added to the soluble heavy metal-containing liquid, which is a mixture of the washing water and the water or acidic aqueous solution from which the remaining incinerated fly ash was separated. After making the liquid alkaline by adding
Air or oxidizing gas is blown into the liquid to generate ferrite precipitate, ferrite is separated by magnetic separation, and alkali and acid are recovered from the separated liquid by diaphragm electrolysis. A method for removing unstable heavy metals from incinerated fly ash, characterized in that: is used for the washing, and an alkali is used for the ferrite precipitation step.