JP5359955B2 - How to prevent elution of heavy metals - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for preventing the elution of heavy metals which can reduce the elution of lead as compared with the case when phosphoric acid alone is added to fly ash when a combustion exhaust gas is sprayed with an alkali metal compound containing carbonic acid such as sodium hydrogen carbonate. <P>SOLUTION: The combustion exhaust gas is added with the alkali metal compound containing carbonic acid by spraying. The fly ash is added with a calcium compound, the pH of an elute indicated by Environment Agency Notification No.13 test (L/S=10) is adjusted to be 8-11, and the calcium ion concentration of the elute is adjusted to be at least 50 mg/L. As the calcium compound, specifically, slaked lime, calcium chloride, unslaked lime, calcium nitrate, calcium nitrite, calcium silicon, calcium acetate, etc., and organic acid calcium salts are exemplified. Among them, slaked lime and calcium chloride are good in calcium solubility and suitable. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention relates to a method for preventing elution of heavy metals from fly ash collected in combustion facilities such as municipal waste waste incinerators, industrial waste incinerators, power generation boilers, carbonization furnaces, and private factories.

  As described in JP-A-9-99215, after spraying slaked lime on exhaust gas containing hydrogen chloride or sulfur oxide, it is widely performed to remove dust with a dust collector. The collected fly ash insolubilizes heavy metals such as Pb with a heavy metal fixing agent such as cement, a chelating agent, and phosphoric acid, and disposed of in landfill. Among these, heavy metal fixation with a phosphoric acid compound is effective in that it can form chloropyromorphite and stably fix heavy metal fixation in the form of minerals for a long period of time.

  In order to increase the efficiency of removing sulfur oxides from flue gas, sodium hydrogen carbonate, which is more reactive with sulfur oxides than calcium compounds such as slaked lime, is sometimes sprayed (JP 2002-263442, JP 2006-110423). However, when sodium hydrogen carbonate is sprayed in this way, the elution amount of heavy metal may be larger than that when calcium compound is sprayed. Therefore, in JP-A-2006-110423, phosphoric acid and phosphoric acid are used for fly ash. The addition of salts or iron (II) salts is described to reduce elution of heavy metals.

JP-A-9-99215 JP 2002-263442 A JP 2006-110423

As described above, when the amount of carbonic acid-containing alkali compound such as sodium hydrogen carbonate added to the combustion exhaust gas is increased, the elution of lead contained in the collected fly ash may increase. This is presumed to be because carbonic acid in the eluate is combined with lead to form a compound such as Pb ₂ CO ₃ or PbCO ₃ · PbO having higher solubility than normal Pb (OH) ₂ .

  The present invention reduces the elution of lead when spraying a carbonate-containing alkali metal compound such as sodium bicarbonate to combustion exhaust gas, compared to when adding a heavy metal fixing agent such as phosphoric acid to fly ash alone. An object of the present invention is to provide a method for preventing elution of heavy metals.

The method for preventing elution of heavy metal according to the present invention (Claim 1) is a method for preventing elution of heavy metal from fly ash collected from combustion exhaust gas sprayed with a carbonate-containing alkaline compound. Carbon dioxide content 90,000 mg-CO ₃ PH of the eluate shown in the Environmental Agency Notification No. 13 test (L / S = 10) while adding calcium-eluting calcium compound and phosphate compound and / or organic chelate compound to fly ash of / kg or more Is adjusted to 8.0-11, and the calcium ion concentration in the eluate is adjusted to 50 mg / L or more, and the heavy metal elution prevention method comprises sodium carbonate, sodium carbonate, , Potassium hydrogen carbonate, potassium carbonate, sesquisodium carbonate, and natural soda, and the calcium compound is slaked lime Calcium chloride, quicklime, calcium nitrate, calcium nitrite, calcium silicon, calcium acetate, is characterized in that the member selected from calcium salts of organic acids.

Elution prevention method of heavy metals according to claim 2 is characterized in that Oite to claim 1, the calcium compound is slaked lime and / or calcium chloride.

The method for preventing elution of heavy metal according to claim 3 is characterized in that, in claim 1 or 2 , the calcium ion concentration in the eluate is adjusted to 50 to 3000 mg / L.

The method for preventing elution of heavy metal according to claim 4 is characterized in that, in any one of claims 1 to 3 , the total carbonic acid concentration in the eluate is 900 mg-CO ₃ / L or less. It is what.

The heavy metal elution prevention method according to claim 5 is the method according to any one of claims 1 to 4 , wherein the calcium compound is added to the fly ash so that the pH and calcium ion concentration of the eluate are within the above ranges. It is characterized by this.

It has been found that lead elution can be prevented by adding calcium compounds such as calcium hydroxide and calcium chloride to carbon dioxide-containing fly ash collected from combustion exhaust gas sprayed with a carbonate-containing alkali metal compound. This is presumably because lead is converted into a form of Pb (OH) _{2 having} low solubility by fixing carbonic acid as calcium carbonate by adding a calcium compound. In addition, the pH in the eluate in the Environmental Agency Notification No. 13 test (L / S = 10) is adjusted to 8 to 11, more preferably 9 to 10.5, and the calcium concentration is adjusted to 50 mg / L or more. Thus, a stable heavy metal treatment effect can be obtained even in difficult-to-treat fly ash containing carbonic acid.

  In the present invention, by adding a phosphoric acid compound and / or an organic chelate compound to the eluate, a stable treatment effect for heavy metals can be obtained.

Incidentally, carbonate and carbonate concentrations bicarbonate were combined in the eluate is preferably adjusted to below 900mg-CO 3 / _L.

  Hereinafter, the present invention will be described in more detail.

In the present invention, a carbonic acid-containing alkali metal compound is added by spraying to combustion exhaust gas generated in combustion facilities such as municipal waste waste incinerators, industrial waste incinerators, power generation boilers, carbonization furnaces, and private factories. The spray position of the carbonic acid-containing alkali metal compound is upstream of a dust collector such as a bag filter or an electric dust collector. The amount of carbonic acid-containing alkali metal compound added is controlled according to the measured value by measuring the sulfur oxide and / or hydrogen chloride concentration in the combustion exhaust gas before spraying the carbonic acid-containing alkali metal compound or in the gas downstream of the dust collector. While it is preferably the case of sodium bicarbonate, HCl is usually the major acid gas generated by the combustion facility, to the combined content of SO _x, 0.5 to 3 equivalents, particularly 0.7 About 1.5 equivalents are preferably added.

The carbonated alkaline compound, sodium hydrogen carbonate, sodium carbonate, potassium hydrogen carbonate, potassium carbonate, sodium sesquicarbonate, natural soda can be exemplified.

  The carbonic acid-containing alkali compound is preferably a fine powder having a particle size of 30 μm or less, particularly 5 to 20 μm, in order to increase the reactivity with acidic gases such as sulfur oxides and hydrogen chloride. As the carbonic acid-containing alkali compound, an agent having a adjusted particle diameter may be applied, or a pulverizing facility may be provided, and the carbonic acid-containing alkali compound having a coarse particle diameter may be added while being pulverized on site.

In the present invention, not only the carbonate-containing alkali compound but also a calcium compound such as slaked lime may be added to the combustion exhaust gas by spraying. Thereby, the sulfur oxide, hydrogen chloride, etc. in combustion exhaust gas can further be reduced. As the calcium compound, slaked lime is suitable, and among them, those having a high specific surface area such as a BET specific surface area of 30 m ² / g or more, particularly 35 m ² / g or more are preferred. Slaked lime with a high specific surface area has high reactivity with acid gas, and enables more efficient and stable treatment. However, the specific surface area of slaked lime may be other than the above. Moreover, the addition position of slaked lime may be added to the exhaust gas before the dust collector, or may be added to the temperature-reduced water of the gas cooling tower and sprayed as a slurry. When used as a slurry, it is preferably used as a 1-20% slurry.

  In the present invention, the calcium compound is added to the fly ash, the pH of the eluate shown in the Environmental Agency Notification No. 13 test (L / S = 10) is adjusted to 8.0 to 11, and Adjust the calcium ion concentration to 50 mg / L or more.

By adding a calcium-eluting calcium compound to the fly ash, as described above, the carbonate in the fly ash or the eluate is fixed as calcium carbonate, and soluble lead compounds such as Pb ₂ CO ₃ , PbCO ₃ .PbO, etc. Is suppressed, and lead elution is prevented.

The present invention is highly effective when applied to fly ash having a carbonic acid content of 90,000 mg-CO ₃ / kg or more, although it depends on the calcium content of fly ash. This is because carbonic acid increases in the eluate when the carbonic acid content of fly ash is 0.8 equivalents or more, particularly 1 equivalent or more, relative to the calcium content.

As the calcium compound, a compound having high solubility in water is preferable. Calcium compounds, specifically, slaked lime, calcium chloride, lime, calcium nitrate, calcium nitrite, calcium silicon, acetic calcium, calcium salts of organic acids are exemplified. Among them, slaked lime and calcium chloride are preferable because of good solubility of calcium. In addition, calcium carbonate is unsuitable for the reaction mechanism.

  There is no restriction | limiting in particular in the addition place of these calcium compounds, What is necessary is just to add in fly ash. It may be added before the kneader to which the heavy metal fixing agent is added, may be added to a conveyor transported from a dust collector such as a bag filter to the kneader, or may be sprayed on combustion exhaust gas. Further, a calcium compound and at least one or more of a heavy metal fixing agent, a neutralizing agent, an iron compound, and a solidifying agent described later may be mixed and added. Thus, when adding a calcium compound mixed with various chemical | medical agents, the number of chemical | medical agent addition facilities can be decreased, and it is suitable also in terms of space and cost.

The amount of calcium compound added depends on the calcium content in the fly ash, but is 0.1 to 2 equivalents, particularly 0.2 to 0.2 times the total amount of CO ₃ ²⁻ and HCO ₃ ⁻ in the fly ash. It is preferably about 1 equivalent times.

  In addition, when slaked lime is sprayed into the combustion exhaust gas and the spray amount of slaked lime falls within the range of this calcium compound addition amount, it is not necessary to add a calcium compound to the collected fly ash. Good.

  In the present invention, the pH of the eluate shown by the Environmental Agency Notification No. 13 test (L / S = 10) of the collected fly ash is 8.0 to 11, preferably 8.5 to 11, particularly preferably 9 to 9. Adjust to 10.5. In order to adjust the pH within this range, calcium hydroxide (slaked lime) is preferably used as the calcium compound, but an alkaline aqueous solution such as sodium hydroxide may be added.

  In the present invention, the calcium concentration in the eluate is adjusted to 50 mg / L or more, preferably 50 to 3000 mg / L, particularly preferably 100 to 500 mg / L. Thereby, elution of heavy metals, particularly lead, from the fly ash can be reduced. In order to set the calcium concentration in the eluate to 50 mg / L or more, the amount of the calcium compound added may be adjusted.

  In the present invention, the elution amount of heavy metal can be further reduced by adding a heavy metal fixing agent such as a phosphate compound and / or an organic chelate to the collected fly ash. When a phosphoric acid compound is added to fly ash, chloropyromorphite is formed, and heavy metals are fixed in the form of minerals for a long time. As the phosphoric acid compound, water-soluble phosphoric acid or phosphate is preferable. The form may be a powder or an aqueous solution. Examples of phosphoric acid include orthophosphoric acid (orthophosphoric acid), polyphosphoric acid, metaphosphoric acid, hypophosphoric acid, phosphorous acid, hypophosphorous acid, pyrophosphoric acid, and superphosphoric acid. As phosphates, primary sodium phosphate, secondary sodium phosphate, tertiary sodium phosphate, primary potassium phosphate, secondary potassium phosphate, tertiary potassium phosphate, primary calcium phosphate, secondary calcium phosphate, Primary magnesium phosphate, secondary magnesium phosphate, primary ammonium phosphate, secondary ammonium phosphate, lime perphosphate, sodium tripolyphosphate, potassium tripolyphosphate, sodium hexametaphosphate, potassium hexametaphosphate, sodium pyrophosphate, pyrophosphate Examples include potassium, sodium phosphite, potassium phosphite, sodium hypophosphite, potassium hypophosphite and the like. Among these, orthophosphoric acid, primary phosphate, secondary phosphate, tertiary phosphate, tripolyphosphate, hexametaphosphate, and pyrophosphate exhibit a good heavy metal fixing effect. In addition, since orthophosphoric acid has a high acidity and there is a concern of corrosion to piping, an alkaline agent such as an aqueous solution of phosphate or sodium hydroxide is mixed, and the pH is adjusted to 3 or more. Also good.

  Examples of the organic chelate compound include piperazine dithiocarbamic acid or a salt thereof, diethyldithiocarbamic acid or a salt thereof, dimethyldithiocarbamic acid or a salt thereof, dibutyldithiocarbamic acid or a salt thereof, and the like. The salt of the organic chelate compound is preferably a sodium salt or a potassium salt.

  When there is a large amount of alkali residue in the fly ash, an inexpensive neutralizer such as aluminum chloride, polyaluminum chloride, hydrochloric acid, or sulfuric acid band may be added to the fly ash. Further, when hexavalent chromium, arsenic, selenium, mercury or the like is eluted, it is effective to add an iron-based compound in order to suppress the elution. Examples of the iron-based compound include ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, polyiron sulfate, and iron powder, and ferrous chloride is most preferable.

  When the fly ash is solidified, cements such as calcined gypsum, Portland cement, early strength cement, jet cement, blast furnace cement, and alumina cement may be added as a solidifying agent.

  EXAMPLES The present invention will be described more specifically with reference to the following examples, but the present invention is not limited thereto.

[Sample fly ash]
In an industrial waste incineration facility, fly ash (hereinafter referred to as fly ash A) sprayed with sodium hydrogen carbonate with an average particle diameter of 8 μm at 30 kg / h on an exhaust gas flue (180 ° C., gas flow rate 12,000 N-m ³ / h). As the acid gas concentration increased with the change in garbage quality, the fly ash (hereinafter referred to as fly ash B) was collected by spraying this sodium bicarbonate at 60 kg / h on the same flue. .

Pb of the fly ash, Cd, Ca content and _{CO ³} 2- + HCO ^{3 -} the total value of the content of the _(mg-CO 3 / kg) are shown in Table 1.

[Preliminary tests 1 and 2]
First, in order to evaluate the elution behavior of heavy metals such as Pb from the fly ash A and B, 500 g of pure water (L / S = 10) was added to 50 g of each fly ash A and B, and the mixture was continuously stirred with a stirrer for 6 hours. During continuous stirring, the pH was measured with a pH electrode, hydrochloric acid (6N) or sodium hydroxide (5N) was added dropwise as a pH adjuster, and stirring was performed while adjusting the pH to about 10.5. After continuous stirring for 6 hours, the mixture was filtered with a filter paper (Advantech GS-25, 1 μm), and the pH of the filtrate and Pb, Cd, Ca, CO ₃ ²⁻ , HCO ₃ ⁻ concentration in the filtrate were measured. Indicated. The preliminary tests 1 and 2 were conducted in order to grasp the difference in the elution behavior of Pb depending on the fly ash composition when the Pb elution amount largely changes depending on the pH, and when adjusted to almost the same pH. In this test, stirring was performed while dropping the pH adjusting agent. However, after adding the same amount of the pH adjusting agent in advance, the eluate shown in the Environmental Agency Notification No. 13 test (L / S = 10) was used. Even if obtained, the same result is obtained.

As shown in Table 1, the content of carbonic acid (CO ₃ ²⁻ + HCO ₃ ⁻ ) in fly ash B is significantly increased as compared with fly ash A.

  Further, as shown in Table 2, the fly ash B has a significantly increased Pb elution amount compared to the fly ash A. In the case of fly ash B, the amount of elution of Pb exceeds the landfill standard (Pb <0.3 mg / L, Cd <0.3 mg / L) in the usual addition treatment of heavy metal fixing agent (phosphoric acid, organic chelate). It has become.

  In addition, compared with the fly ash A, the fly ash B has a significantly increased carbonic acid concentration in the eluate and a significantly reduced Ca ion concentration in the eluate. This is because a large amount of carbonic acid is eluted from the fly ash B in which the spray amount of sodium hydrogen carbonate is increased, and calcium ions react with carbonic acid to form calcium carbonate.

[Test No. 1-7 (no calcium compound added, pH 7.6-11.6)]
The pH of the eluate of fly ash B was varied between 7.6 and 11.6 with hydrochloric acid (6N) and sodium hydroxide (5N), and the same conditions (L / S = 10. Pb, Cd, and carbonic acid concentrations were measured when heavy metals were eluted by 6 hours continuous stirring. The results are shown in Table 3.

  As shown in Table 3, this test No. In Nos. 1 to 7, the calcium compound is not added, and the Pb and Cd concentrations in the eluate are high.

[Test No. 8-14 (10% calcium hydroxide added, pH 7.6-11.6)]
Test No. 1 except that 10% by weight of calcium hydroxide was added to Fly Ash B. In the same manner as in 1 to 7, the Pb, Cd, and carbonic acid concentrations when heavy metals were eluted were measured. The results are shown in Table 3.

  As shown in Table 3, the test No. was adjusted to pH = 8.5 to 11.0. In Nos. 10 to 13, the concentrations of Pb and Cd in the eluate were low, but the pH was 7.6 and 7.9. In Nos. 8 and 9, the Pb and Cd concentrations were slightly high, and the pH was 11.6. In 14, the Pb concentration is high.

[Test No. 15 to 19 (10% calcium hydroxide, 5% phosphoric acid aqueous solution (75% concentration) added, pH 7.6 to 11.7)]
Test No. 1 except that 10% by weight of calcium hydroxide and 5% of 75% by weight phosphoric acid aqueous solution were added to Fly Ash B. In the same manner as in 1 to 7, the Pb, Cd, and carbonic acid concentrations when heavy metals were eluted were measured. The results are shown in Table 3.

  As shown in Table 3, the test Nos. Were adjusted to pH = 8.1 to 11.0. In Nos. 16 to 18, the Pb and Cd concentrations in the eluate were low, but the pH was 7.6. In No. 15, the Cd concentration was slightly high, and the pH was 11.7. In 19, the Pb concentration is high.

[Test No. 20-24 (10% calcium hydroxide and organic chelating agent added, pH 7.5-11.5)]
Test No. 1 except that 10% by weight of calcium hydroxide and 1% of organic chelating agent (40% by weight aqueous piperazine dithiocarbamate aqueous solution) were added to Fly Ash B. In the same manner as in 1 to 7, the Pb, Cd, and carbonic acid concentrations when heavy metals were eluted were measured. The results are shown in Table 3.

  As shown in Table 3, the test Nos. Were adjusted to pH = 8.1 to 11.0. In Nos. 21 to 23, Pb and Cd concentrations in the eluate were low, but No. In No. 20, the Cd concentration was slightly high and No. 1 with pH = 11.5. In 24, the Pb concentration is high.

[Test No. 25-28 (test with different Ca compounds)]
<Test No. 25>
Test No. In 12 Ca a _(OH) 2 10 wt% CaCO _3, instead of adding the addition 10% by weight, except that the pH was adjusted to 10.2 were dissolution test of heavy metals in the same manner. The results are shown in Table 4.

<Test No. 26>
Test No. 12 except that 10% by weight of CaCl ₂ was added instead of 10% by weight of Ca (OH) ₂ , and the elution test for heavy metals was performed in the same manner except that the pH was adjusted to 10.1. The results are shown in Table 4.

<Test No. 27>
Test No. 12, 10% by weight of CaCO ₃ was added instead of 10% by weight of Ca (OH) ₂ . A heavy metal elution test was conducted in the same manner except that phosphoric acid similar to 17 was added and the pH was adjusted to 10.1. The results are shown in Table 4.

<Test No. 28>
Test No. 12, 10% by weight of CaCl ₂ was added instead of 10% by weight of Ca (OH) ₂ . A heavy metal elution test was conducted in the same manner except that phosphoric acid similar to 17 was added and the pH was adjusted to 10.2. The results are shown in Table 4.

Table 4 shows, when using the CaCl ₂ instead of Ca _{(OH) 2,} as in the case of Ca _{(OH) 2,} Pb, can be reduced the elution of Cd, with CaCO ₃ In this case, the elution amount of Pb increases. This is because when CaCO ₃ is used, the concentration of carbonic acid in the eluate increases, and a soluble Pb compound is produced.

[Test No. 29, 30 (no calcium compound added, only phosphoric acid or chelating agent added)]
Tests were conducted for the effect of not adding Ca (OH) ₂ .

That is, Test No. _{2 in} which Ca (OH) ₂ and phosphoric acid are added. 17 when no Ca (OH) ₂ was added (however, pH = 10.3). 29. In addition, Test No. _{2 in} which Ca (OH) ₂ and a chelating agent are added. No. 22 was the case where no Ca (OH) ₂ was added. 30. The results are shown in Table 5.

  As shown in Table 5, if the Ca compound is not added, the Pb concentration in the eluate increases.

  For reference, the fly ash A No. The same tests as in Nos. 29 and 30 were performed, and the results are shown in Table 6.

  As shown in Table 6, in the case of fly ash A with a small spray amount of sodium bicarbonate, elution of Pb and Cd is sufficiently reduced by phosphoric acid and a chelating agent.

  From the above Examples and Comparative Examples, it is clear that the amount of Pb elution can be significantly reduced by adding a calcium-eluting calcium compound to fly ash collected by spraying sodium bicarbonate. Further, by adjusting the pH when adding the calcium compound to 8.0 to 11, preferably 8.5 to 11 and particularly preferably 9 to 10.5, heavy metals such as Pb and Cd can be stably treated. can do. Furthermore, it was confirmed that elution of heavy metals such as Pb and Cd can be stably prevented when a phosphoric acid compound and an organic chelating agent are used in combination.

Claims (5)

In a method for preventing heavy metal elution from fly ash collected from combustion exhaust gas sprayed with a carbonate-containing alkaline compound,
While adding calcium-eluting calcium compound , phosphoric acid compound and / or organic chelate compound to fly ash having a carbonic acid content of 90,000 mg-CO ₃ / kg or more ,
Elution of heavy metals by adjusting pH of eluent shown in Environmental Agency Notification No. 13 test (L / S = 10) to 8.0-11 and adjusting calcium ion concentration in eluate to 50 mg / L or more A prevention method,
The carbonate-containing alkali metal compound is at least one of sodium bicarbonate, sodium carbonate, potassium bicarbonate, potassium carbonate, sodium sesquicarbonate, and natural soda;
The method for preventing elution of heavy metals, wherein the calcium compound is selected from slaked lime, calcium chloride, quicklime, calcium nitrate, calcium nitrite, calcium silicon, calcium acetate, and calcium organic acid salt.   The method for preventing elution of heavy metal according to claim 1, wherein the calcium compound is slaked lime and / or calcium chloride. According to claim 1 or 2, elution prevention method of heavy metals, characterized in that adjusting the calcium ion concentration in the eluate to 50~3000mg / L. In any one of claims 1 to 3, the elution prevention method of heavy metals, characterized in that carbon dioxide concentrations obtained by summing the carbonate and bicarbonate of the eluate is below 900mg-CO 3 / _L. In any one of claims 1 to 4, elution prevention method of heavy metals, characterized in that the pH and calcium ion concentration of the eluate is added to the calcium compound with respect to fly ash so that said range.