JP5176839B2 - Soil or slag treatment method - Google Patents

Description

  The present invention relates to a method for treating soil or slag, and more particularly, to a method for effectively insolubilizing hexavalent chromium contained in highly alkaline soil or slag containing hexavalent chromium to prevent its elution.

  Chromium generally exists stably in a zero-valent, trivalent, or hexavalent form. Among these, hexavalent chromium is a harmful substance.

  Conventionally, as a method for treating hexavalent chromium, there is generally a method in which a reducing agent is added and hexavalent chromium is reduced to trivalent chromium to render it harmless (for example, Patent Documents 1 and 2).

  However, hexavalent chromium contained in highly alkaline soils and slags is treated to the environmental standard with conventional reducing agents that exhibit reducing power in the acidic region, which is commonly used in the past, due to the influence of high alkalinity. It is impossible or a large amount of reducing agent is required for processing up to environmental standards.

For example, conventionally used reducing agents such as sodium sulfite, sodium thiosulfate, sodium bisulfite, and hydrogen peroxide exhibit a reducing power in the acidic region and are poor in reducing power under highly alkaline conditions. That is, when a conventional reducing agent is used, the reduction reaction proceeds in an acidic solution according to the following reaction formula, so that the reducing ability under alkaline conditions is poor.
4H ₂ CrO ₄ + 6NaHSO ₃ + 3H ₂ SO ₄ →
2Cr ₂ (SO ₄ ) ₃ + 3Na ₂ SO ₄ + 10H ₂ O

In addition, these reducing agents such as sodium sulfite lose their reducing power by easily reacting with oxygen in the atmosphere (oxidant) by the following reaction formula and decomposing.
2SO ₃ ²⁻ + O ₂ → 2SO ₄ ²⁻

  In addition, even divalent iron compounds such as ferrous sulfate and ferrous chloride used as inexpensive reducing agents form hydroxides under alkaline conditions and are easily oxidized to oxygen and lose their reducing power.

In addition, even when hexavalent chromium is reduced to trivalent chromium with a reducing agent, hexavalent chromium is more stable than trivalent chromium under high alkaline conditions, so it can be easily treated in the presence of an oxidizing agent such as oxygen. It will be oxidized to hexavalent.
That is, in the high alkali region, chromium is stable in the form of hexavalent chromium even at a low redox potential. For this reason, in the high alkali region, hexavalent chromium exists in the form of chromate ions (CrO ₄ ²⁻ ). However, chromate ions are reduced by the following reaction formula by a reducing agent, and trivalent chromium (hydroxylated) Chrome).
CrO ₄ ²⁻ + 4H ₂ O + 3e ⁻ → Cr (OH) ₃ + 5OH ⁻
The produced chromium hydroxide reacts with oxygen (oxidant) in the atmosphere as follows under alkaline conditions to produce hexavalent chromium.
4Cr (OH) ₃ + 3O ₂ + 8OH ⁻ → 4CrO ₄ ²⁻ + 10H ₂ O
Therefore, even if the reduction treatment of hexavalent chromium is carried out and once reduced to trivalent chromium, trivalent chromium is easily oxidized by contact with an oxidizing agent such as oxygen. A stable reduction and insolubilization treatment cannot be expected.
JP 2001-121133 A JP-A-11-106243

  The present invention solves the above-mentioned conventional problems, easily reduces hexavalent chromium in a highly alkaline hexavalent chromium-containing soil or slag to trivalent, and stably reduces and insolubilizes as trivalent chromium in the long term. It aims to provide a way to do.

The soil or slag treatment method of the present invention (Claim 1) contains hexavalent chromium and has a pH of 10 or more in the dissolution test solution in the Ministry of the Environment Notification No. 18 dissolution test on March 6, 2003. or slag, erythorbic acid you and / or a salt thereof (hereinafter, they referred to as "organic acid (salt)".) was in the iron powder and the soil or slag processing method for insoluble hexavalent chromium is added The ratio of the organic acid (salt) to the iron powder is organic acid (salt): iron powder = 1: 0.5 to 5 (mass ratio) .

Processing method of soil or slag 請 Motomeko 2 resides in that in Claim 1, the average particle size of the iron powder is characterized in that it is a 20Nm～2mm.

Processing method of soil or slag according to claim 3, in claim 1 or 2, characterized in that solidified by adding solidification agent to the organic acid (salt) and soil or slag was added iron powder.

The method for treating soil or slag according to claim 4 is the method according to claim 3 , wherein the solidifying material is one or more selected from the group consisting of water glass, gypsum, magnesium oxide, quicklime, slaked lime, and cement. It is characterized by.

  According to the method for treating soil or slag of the present invention using a specific organic acid (salt) as a reducing agent, the pH of the dissolution test solution in the Ministry of the Environment Notification No. 18 dissolution test on March 6, 2003 is 10 or more. Hexavalent chromium in some highly alkaline hexavalent chromium-containing soil or slag can be easily reduced to trivalent with a small amount of chemical addition, and stably reduced and insolubilized as trivalent chromium over the long term. Elution can be reliably prevented.

  That is, the organic acid (salt) used as a reducing agent in the present invention effectively exhibits a reducing action in a wide pH range from acidic to alkaline. In addition, it has low reactivity with oxygen, is stable for a long time, and has a certain degree of reduction persistence. Therefore, even if hexavalent chromium is produced from trivalent chromium once reduced, it exhibits a long-term reducing action that reduces it to trivalent chromium again.

However, the organic acid (salt) is consumed over time and disappears. Therefore, a treatment having further long-term stability can be performed by adding iron powder in addition to the organic acid (salt) (Claim 1 ).
The iron powder preferably has an average particle size of 20 nm to 2 mm (claim 2 ).

In the present invention, organic acid that solidified by adding solidification agent to the soil or slag was added (salt) and iron powder, it is possible to enhance the long-term stability of more insolubilization treatment (claim 3 ) In this case, examples of the solidifying material to be used include one or more selected from the group consisting of water glass, gypsum, magnesium oxide, quicklime, slaked lime, and cement (Claim 4 ).

  Hereinafter, the method for treating soil or slag of the present invention will be described in detail.

  In the present invention, the soil or slag to be treated contains hexavalent chromium, and the pH of the dissolution test solution in the Ministry of the Environment Notification No. 18 dissolution test on March 6, 2003 (hereinafter referred to as “the Ministry of the Environment notification dissolution pH value”). Is soil or slag that is 10 or more. Even if the elution pH value of the soil or slag to be treated in the present invention is less than 10, there is no problem in the treatment effect, but the conventional general reducing agent has no effect or is weak, or is added in a large amount For the purpose of the present invention of treating soil or slag under high alkaline conditions that require water, the Ministry of the Environment notification elution pH value is 10 or more, for example, 10-13. The content of hexavalent chromium in soil or slag is not particularly limited, but usually the amount of hexavalent chromium dissolved in the dissolution test solution in the dissolution test No. 18 of the Ministry of the Environment Notification No. 18 on March 6, 2003 (six (Valent chromium concentration) is about 0.05 to 10 mg / L.

  In the present invention, the organic acid (salt) added to the soil or slag as a reducing agent is erythorbic acid, tannic acid, formic acid, oxalic acid, cysteine or an alkali metal such as sodium salt or potassium salt of these organic acids. These are salts such as salts, and these may be used alone or in combination of two or more.

  Of these, erythorbic acid and / or a salt thereof are particularly preferred among these in view of the insolubilizing effect of hexavalent chromium, and sodium erythorbate is particularly preferred.

  The amount of organic acid (salt) added to the soil or slag to be treated is the content of hexavalent chromium in the soil or slag to be treated, the storage status of treated products, the presence or absence of iron powder, the elution amount of hexavalent chromium, and elution Depending on the pH of the test solution, the processed product is dissolved in the standard of elution amount (0.05 mg / L or less in hexavalent chromium concentration in the elution test in the Ministry of the Environment Notification No. 18 dissolution test on March 6, 2003) The amount of addition of the organic acid (salt) when not used in combination with iron powder is usually 0.05 to the dry mass of the soil or slag, although it is appropriately determined as the amount that can be processed to the level to be applied. It is preferable to set it as 20 mass%, especially 0.1-5 mass%.

In the present invention, by further adding iron powder to the soil or slag together with the organic acid (salt), a long-term stable treatment can be performed.
The reason why iron powder is used in combination with an organic acid (salt) and not iron powder alone is that iron powder, like the above-mentioned general reducing agents, has relatively low reactivity under high alkaline conditions and insolubilizes hexavalent chromium. This is because a large amount of addition is necessary. On the other hand, according to the present invention, in order to treat a slight amount of hexavalent chromium that is used together with an organic acid (salt) and the organic acid (salt) disappears in the long term, A small amount is sufficient, and an effective insolubilizing effect is exhibited by a synergistic effect with an organic acid (salt).

Reduction of hexavalent chromium with iron powder is performed by reduction on the surface of zero-valent iron and reaction with dissolved divalent iron as follows.
CrO ₄ ²⁻ + Fe + 4H ₂ O → Cr (OH) ₃ + Fe (OH) ₃ + 2OH ⁻
CrO ₄ ²⁻ + 3Fe ²⁺ + 8H ₂ O → Cr (OH) ₃ + 3Fe (OH) ₃ + 4H ⁺

In addition, the addition amount of an organic acid (salt), the presence or absence of combined use with iron powder, and the addition amount of iron powder are determined by the storage condition of the processed material after reduction insolubilization processing.
In particular, when the treated product is stored in a state where it is not in contact with oxygen at all, the reduced chromium is not easily oxidized, so that a sufficient effect can be obtained with only an organic acid (salt).

When the organic acid (salt) and iron powder are used in combination, the amount of the organic acid (salt) added to the treated soil or slag is 0.05 to 20% by mass, particularly 0.1%, based on the dry weight of the soil or slag. -5 mass%, the addition amount of iron powder is 0.05-10 mass%, especially 0.1-5 mass%, and the use ratio of organic acid (salt) and iron powder is organic acid (salt): iron powder = 1: you 0.5 to 5 (mass ratio).

  The iron powder added to the soil or slag preferably has an average particle size of D50 of 20 nm to 2 mm. When the average particle diameter of the iron powder is too large, the specific surface area becomes small and a sufficient insolubilizing effect cannot be obtained.

In the present invention, by solidified organic acid to be treated soil or slag (salt) after addition to reduction treatment of iron powder, by adding treated solidified material, more stable insolubilization effect Can be obtained.

  In this case, examples of the solidifying material to be used include water glass, gypsum, magnesium oxide, quicklime, slaked lime, cement, and the like. These may be used alone or in combination of two or more. .

  The addition amount of these solidifying materials may be such that the processed product can be solidified to the intended hardness, and varies depending on the type of solidifying material used, the moisture content of the treated soil or slag, etc. It is preferable to set it as 1-50 mass% with respect to the dry weight of slag, especially about 1-30 mass%.

  In the present invention, the organic acid (salt), iron powder, and solidified material added to the treated soil or slag may be in a state where it can be sufficiently uniformly mixed with the object to be treated, and remains in powder form. Alternatively, it may be added in the form of an aqueous solution dissolved in water or in the form of a dispersed slurry.

  In addition, the organic acid (salt) and the iron powder may be mixed and added in advance, or may be added separately.

  Addition and mixing of organic acid (salt), iron powder, and solidifying material to the soil or slag to be treated can be performed using a kneader to sufficiently disperse the additive agent and obtain a good treatment effect. This is preferable. In this case, when the moisture content of the treated soil or slag is low and the kneadability is poor, water is appropriately added so that the moisture content is 0.5 to 40% by mass, preferably 5 to 30% by mass. Is preferred.

  Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.

[Processing object]
In the following, the properties of the treated soil or slag are as follows.
<Soil>
The dissolution amount of hexavalent chromium in the elution test solution described below according to [Elution Test] was 0.32 mg / L, pH was 7.6, and the water content was 15% by mass.
<Slag (steel (electric furnace) slag)>
The dissolution amount of hexavalent chromium in the elution test solution described below according to [Elution Test] is 0.2 mg / L, pH is 12.1, and water content is 17% by mass.

[Dissolution test]
The dissolution test of the object to be treated or the treated object was performed by the following method.
500 mL of pure water was added to 50 g of an air-dried sample, and the mixture was shaken for 6 hours with a reciprocating shaker having a shaking width of 4 cm and a shaking speed of 200 times / min.
After standing still for a while, the supernatant was filtered through a 0.45 μm membrane filter to obtain an elution test solution, and the hexavalent chromium elution amount and pH were measured.
The hexavalent chromium in the obtained elution test solution was quantified according to JIS K0102 65.2 (lower limit of quantification 0.04 mg / L).

[Ratio Comparative Examples 1 to 18, Reference Example 1 to 30]
The chemicals shown in Tables 1 and 2 were added to the treatment objects shown in Tables 1 and 2 and mixed, and the treatments were cured overnight and then air-dried and subjected to a dissolution test. Note that iron powder having an average particle diameter of 70 nm was used. The same applies to Example 1 and Comparative Examples 20 and 21 described later.
The results are shown in Tables 1 and 2.

From Tables 1 and 2, the following is clear.
In Reference Examples 1 to 24 where neutral soil with an elution pH value of 7.6 was treated, there was almost no difference depending on the type of the additive, and the same treatment results were obtained.

When the highly alkaline slag having an elution pH value of 12.1 by the Ministry of the Environment was treated, in Comparative Examples 1 to 18 to which a chemical other than sodium erythorbate was added, the soil elution standard for hexavalent chromium (0.05 mg / L It is necessary to add a large amount of drug until it can meet the following). On the other hand, in Reference Examples 25 to 30 using sodium erythorbate, the elution amount of hexavalent chromium can be reduced with a small addition amount.

[Example 1 , Comparative Examples 19-21 , Reference Example 31 ]
In order to evaluate the long-term stability of the additive, we confirmed the long-term elution behavior of hexavalent chromium.
In the test, a predetermined amount of drug was added to and mixed with slag (but no drug was added in Comparative Example 19) and stored in a polypropylene container. After a predetermined period of time, the polypropylene container A part of the sample inside was taken out and air-dried, and an elution test was performed on the air-dried sample. The results are shown in Table 3 and FIG.

In addition, the chemical | medical agent added in each case is as follows.
Reference Example 31 : Addition of 0.5% by mass of sodium erythorbate (vs. dry mass) Example 1 : 0.5% by mass of sodium erythorbate (vs. dry mass) and
Addition of 0.3% by mass of iron powder (vs. dry mass) Comparative Example 19: Addition of no reducing agent Comparative example 20: Addition of 1% by mass of iron powder (vs. dry mass) Comparative example 21: 5% by mass of iron powder (vs. dry mass) ) Addition

Table 3 shows the following.
In the steelmaking slag with no chemical added in Comparative Example 19, the elution amount of hexavalent chromium increases with time.

  In the system in which only the iron powder of Comparative Example 20 and Comparative Example 21 was added, the amount of elution was in accordance with the amount added, and the hexavalent chromium was sufficiently eluted if a large amount of iron powder was not added under highly alkaline conditions. It turns out that it cannot suppress.

In Reference Example 31 , only sodium erythorbate was added, but the elution of hexavalent chromium was suppressed for several hundred days and is stable for a relatively long period.
In Example 1 , iron powder was further used in combination, but the elution of hexavalent chromium was suppressed for a longer time than the addition of sodium erythorbate alone by using a small amount of iron powder.

Thus, as a reducing agent, by using a combination of sodium erythorbate and iron powder, even at high alkaline conditions, a hexavalent chromium irreducible insolubilization, it is possible to more long-term stabilized.

It is a graph which shows the result of Example 1, the reference example 31, and Comparative Examples 19-21.

Claims (4)

Containing hexavalent chromium, the soil or slag pH of the elution test solution is 10 or more in 2003 March 6 Ministry Notification No. 18 elution test, erythorbic acid Contact and / or salts thereof (hereinafter, these Is called "organic acid (salt)" and iron powder to insolubilize hexavalent chromium, and the treatment method of soil or slag is used, the proportion of organic acid (salt) and iron powder used is organic acid (Salt): Iron powder = 1: 0.5-5 (mass ratio) In Claim 1 , The average particle diameter of the said iron powder is 20 nm-2 mm, The processing method of the soil or slag characterized by the above-mentioned. According to claim 1 or 2, the processing method of soil or slag, characterized in that solidified by adding solidification agent to the organic acid (salt) and soil or slag was added iron powder. 4. The method for treating soil or slag according to claim 3 , wherein the solidifying material is one or more selected from the group consisting of water glass, gypsum, magnesium oxide, quicklime, slaked lime, and cement.

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