JP2006255501A - Method of removing heavy metal in flying ash - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable high-efficiency removal of lead in flying ash discharged from waste treating equipment. <P>SOLUTION: A method of removing heavy metals in flying ash comprises an alkali extraction process 25 mixing flying ash discharged from waste treating equipment with an aqueous alkali solution, the first solid-liquid separation process 27 of separating the mixed solution formed in the process 25 into precipitate and filtrate, a precipitation process 29 of adding a water-soluble sulfide to the filtrate separated in the process 27 and the second solid-liquid separation process 31 of separating the mixed solution formed in the process 29 into precipitate and filtrate. The method is characterized by carrying out the alkali extraction process 25 through heating to remove lead with high efficiency. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for removing heavy metals from fly ash discharged from a waste treatment apparatus.

  Since fly ash discharged from waste treatment equipment contains heavy metals, it is desirable to remove and recover heavy metals from fly ash from the viewpoint of environmental pollution prevention and effective utilization of heavy metal resources. ing.

  Therefore, conventionally, it has been proposed to remove and recover heavy metals from fly ash by a wet processing method using acid or alkali (Patent Document 1, Non-Patent Document 1).

JP 7-109533 A "Development of next-generation waste incineration facilities 1996-2010 general report", Waste Research Foundation, March 1999

  However, according to the conventional heavy metal removal technology, various heavy metals can be removed, but there is a problem that it is not possible to meet the demand for removing lead in fly ash with high efficiency.

  An object of the present invention is to make it possible to remove lead from fly ash with high efficiency.

  As a result of research including experiments to solve the above problems, when mixing alkaline aqueous solution with fly ash discharged from waste treatment equipment to elute or extract lead, the mixture is heated. It has been found that the elution rate of lead can be increased.

  Based on this knowledge, the method for removing heavy metals from fly ash according to the present invention includes a first step of mixing an alkaline aqueous solution with the fly ash discharged from the waste treatment apparatus, and a mixture mixed in the first step. A second step of separating the liquid into a precipitate and a filtrate, a third step of adding a water-soluble sulfide to the filtrate separated in the second step, and a mixed solution produced in the third step In the method for removing heavy metals from fly ash, wherein the first step is performed by heating. .

  That is, in the first step, some heavy metals mainly containing lead in fly ash are eluted in the alkaline aqueous solution, and the remaining heavy metals are precipitated as hydroxides. This precipitate is separated and removed in the second step. On the other hand, by adding a water-soluble sulfide in the third step to the filtrate in the second step, heavy metals mainly containing lead in the filtrate are precipitated as sulfides. By separating the precipitate in the next fourth step, heavy metals mainly containing lead can be separated and recovered.

  In the above case, the heating temperature of the mixed solution in the first step can be 90 ° C. or higher, preferably 100 ° C. or higher, and the lead elution rate can be further improved. For example, the amount of lead that can be finally separated can be approximately 60% to 85% by weight with respect to the amount of lead in the fly ash. The alkaline aqueous solution preferably has a hydroxide ion concentration of 0.5 mol / liter or more and 4 mol / liter or less.

  As described above, according to the present invention, lead can be removed from fly ash with high efficiency.

  Hereinafter, the present invention will be described based on embodiments.

  FIG. 1 shows a process diagram of an embodiment of a method for removing heavy metals from fly ash according to the present invention, and FIG. 2 shows an embodiment of a waste treatment system to which the method for removing heavy metals from fly ash in FIG. 1 is applied. The whole block diagram is shown.

  In FIG. 2, the waste 1 is pulverized to an appropriate size and charged into the pyrolysis reactor 2. The pyrolysis reactor 2 heats the waste 1 input from one end in a low-oxygen atmosphere and transfers it to the other end while pyrolyzing, and discharges the pyrolysis gas 3 and the pyrolysis residue 4 from the other end. The pyrolysis gas 3 is supplied to the combustion melting furnace 5 and burned. The pyrolysis residue 4 contains combustible materials such as pyrolytic carbon generated by pyrolysis and non-combustible materials such as metal pieces, glass, ceramics, and concrete pieces contained in the waste 1. The apparatus 6 separates the pyrolytic carbon 7 into other incombustibles. The separated pyrolytic carbon 7 can be supplied to the combustion melting furnace 5 for combustion treatment. In the combustion melting furnace 5, the pyrolysis gas 3 and the pyrolysis carbon 7 are subjected to combustion treatment. Due to this combustion heat, the ash contained in the pyrolysis gas 3 and pyrolysis carbon 7 is usually melted to form slag, which is discharged from the bottom of the furnace through the furnace wall into, for example, a solidified slag. The The combustion exhaust gas 8 from the combustion melting furnace 5 is guided to the heat recovery device 9 and recovered. The heat recovery device 9 includes, for example, a heat exchanger that heats a gas (for example, air) used as a thermal decomposition heat source of the thermal decomposition reactor 2, a waste heat boiler that generates steam used for power generation, and the like. can do. The exhaust gas 10 discharged from the heat recovery device 9 is guided to a dust collector 11 including a bag filter and the fly ash 18 contained in the exhaust gas 10 is collected. The exhaust gas 12 discharged from the dust collector 11 is guided to a desalting device 14 to which a desalting agent 13 is added and configured by a bag filter. In the desalting apparatus 14, a layer of the desalting agent 13 is formed on the filter cloth surface of the bag filter, and chlorine and sulfur in the exhaust gas 13 passing through the desalting agent layer react with the desalting agent 13. Separated from the exhaust gas 12. The exhaust gas 15 that has passed through the desalting apparatus 14 is discharged into the atmosphere from the chimney 17 via the induction fan 16.

  On the other hand, the fly ash 18 gravity-settled in the heat recovery device 9 and the fly ash 18 collected by the dust collector 11 are guided to the heavy metal removal device 20 and processed.

  Next, a heavy metal removal device 20 that is a feature of the present embodiment will be described with reference to FIG. As shown in FIG. 1, an alkali extraction step 25 in which fly ash 18 is mixed with an alkaline aqueous solution to extract heavy metals in the fly ash, and a mixed solution containing the heavy metals extracted in the alkali extraction step 25 is mixed with a precipitate and a filtrate. A first solid-liquid separation step 27 to be separated into a solid, a precipitation step 29 in which a water-soluble sulfide is added to the filtrate separated in the solid-liquid separation step 27, and a liquid mixture produced in the precipitation step 29 It is comprised from the 2nd solid-liquid separation process 31 isolate | separated into.

  In the alkali extraction step 25, an alkaline aqueous solution such as caustic soda is added to the fly ash 18 and the mixture is stirred and heated. Here, the reason for heating is to increase the elution rate of lead. By this step, lead and some heavy metals in the fly ash 18 are eluted into the alkaline aqueous solution. Fly ash precipitates with the remaining heavy metal hydroxides.

  The precipitate in the alkali extraction step 25 is separated and removed in the first solid-liquid separation step 27. For example, the separated precipitate 33 can be returned to the combustion melting furnace 5 via the drying device 21 to be melted into slag. In addition, you may return to a furnace, without drying.

In the next precipitation step 29, water-soluble sulfides such as sodium hydrosulfide (NaHS) and sodium sulfide (Na ₂ S) are added to the filtrate discharged from the solid-liquid separation step 27. This addition amount may be a water-soluble sulfide of 2 equivalents or more with respect to the amount of lead in the filtrate. As a result, heavy metals mainly containing lead are precipitated as sulfides, and almost 100% of lead in the filtrate is precipitated. In the precipitation step 29, there is no need for pH adjustment, and lead is easily precipitated by adding a water-soluble sulfide to the filtrate as it is.

  The precipitate containing lead produced in the precipitation step 29 is separated and removed in the second solid-liquid separation step 31. Thereby, heavy metals mainly containing lead can be separated and removed. Since the precipitate 35 separated in the precipitation step 29 contains lead at a high content rate, it can be used as a resource after being reduced at the base, for example. Alternatively, it can be disposed of after appropriate processing. On the other hand, since the filtrate separated in the solid-liquid separation step 31 contains salts in the fly ash 18, a desalination treatment or the like is performed by a treatment step (not shown) so as to satisfy drainage standards. Drain outside.

  As described above, according to the heavy metal removal method of the present embodiment, lead in fly ash is extracted at a high elution rate by heating a mixed solution with an alkaline aqueous solution for extracting heavy metals in fly ash. Since lead is precipitated and separated by adding a water-soluble sulfide to the lead-containing extract, lead can be separated and recovered with high efficiency.

  Here, one example of the processing conditions in the alkali extraction step 25 is shown below.

Heating temperature: 90 ° C. or higher, preferably 100 ° C. or higher Alkali (NaOH) concentration: 0.5 mol / liter Solid-liquid ratio: Fly ash / alkaline aqueous solution = 1/10
-Stirring time: 5 minutes This processing condition is an example, for example, alkali concentration, solid-liquid ratio, and stirring time can be changed as needed.

  Next, experimental data indicating that the elution rate of lead is increased by heating the mixed solution in the alkali extraction step 25 will be described with reference to FIGS. 3 and 4. Here, FIG. 3 is an example of the composition analysis data of fly ash used in the experiment, and the contents of lead and other elements are shown. FIG. 4 is a graph showing the relationship between the heating temperature of the alkaline extract and the lead elution amount under the condition where the alkali concentration is 0.5 mol / liter.

  As a result of extracting the fly ash having the composition shown in FIG. 3 with an alkaline aqueous solution under the conditions of the above example, as shown in FIG. 4, the ratio of lead elution with respect to the lead content in the fly ash is The temperature was about 60% by weight at 90 ° C. and about 85% by weight at 100 ° C. Thus, lead can be eluted with high efficiency by heating the mixed liquid of fly ash and aqueous alkali solution. In addition, since the elution rate of lead at the time of mixing fly ash with the alkaline aqueous solution of normal temperature (25 degreeC) is about 50%, according to a present Example, an elution rate can be improved significantly. For example, the lead content in the precipitate 35 was 82.5% as shown in the analysis result of FIG. From this, according to this embodiment, lead can be selectively removed from the fly ash 18.

  In the above examples, the concentration of NaOH was 0.5 mol / liter, but it is appropriate to carry out under the conditions of 0.5 mol / liter to 4 mol / liter from the experimental data shown in FIG. It is. That is, FIG. 6 shows the relationship between the NaOH concentration and the lead elution amount. When the concentration is less than 0.5 mol / liter, the elution rate is not so high, but the elution rate increases as the NaOH concentration increases. Recognize. However, when the NaOH concentration exceeds 1 mol / liter, the elution rate of lead tends to be saturated. On the other hand, when the NaOH concentration is higher than about 4 mol / liter, the viscosity of the NaOH aqueous solution increases, and solid-liquid separation of the precipitate and the filtrate takes time. Therefore, the upper limit of the NaOH concentration is 4 mol / liter. Is preferred.

  As described above, according to the present embodiment, lead can be separated and recovered from fly ash with high efficiency.

It is a processing-process figure of one Embodiment of the heavy metal removal method of fly ash of this invention. It is a whole lineblock diagram of one embodiment of a waste disposal system to which a heavy metal removal method of fly ash of the present invention is applied. It is a figure which shows an example of the composition analysis data of fly ash. It is a figure showing the relationship between the heating temperature of an alkaline extract, and the amount of lead elution. It is a figure which shows an example of lead content in the deposit finally obtained. It is a diagram showing the relationship which NaOH concentration has on the amount of lead elution.

Explanation of symbols

25 Alkali extraction process 27 Solid-liquid separation process 29 Precipitation process 31 Solid-liquid separation process

Claims (3)

A first step of mixing an alkaline aqueous solution with fly ash discharged from a waste treatment apparatus, a second step of separating the mixed solution mixed in the first step into a precipitate and a filtrate, A third step of adding a water-soluble sulfide to the filtrate separated in step 2, and a fourth step of separating the mixed solution produced in the third step into a precipitate and a filtrate, In the heavy metal removal method of fly ash that removes heavy metals from fly ash,
A method for removing heavy metals from fly ash, wherein the first step is performed by heating.   The method for removing heavy metals from fly ash according to claim 1, wherein the heating temperature in the first step is 90 ° C or higher, preferably 100 ° C or higher.   3. The fly ash heavy metals according to claim 1, wherein the alkaline aqueous solution in the first step has a hydroxide ion concentration of 0.5 mol / liter or more and 4 mol / liter or less. Removal method.

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