JP5099533B2 - Soil improvement material - Google Patents

Description

  The present invention relates to a soil conditioner that has a high moisture content and requires a solidification treatment and is useful for improving soil containing heavy metal ions.

  In applications where it is necessary to solidify the soil whose ground is soft due to construction sludge or groundwater, the use of inexpensive cement-based materials is the mainstream. On the other hand, there is an insolubilization method as one of methods for purifying soil contaminated with heavy metals. In this method, the amount of elution of heavy metals into the water in the soil can be reduced. For example, a method of injecting potassium sulfate, sodium sulfate, calcium sulfide, sodium phosphate or the like as a heavy metal insolubilizing material is performed. In the conventional insolubilization method, the excavated soil is often treated as contaminated soil as it is. Recently, a method for insolubilizing in situ without excavating soil has been disclosed and practiced (see, for example, Non-Patent Document 1).

  By the way, when trying to reinforce the soil with a ground improvement material such as cement for the purpose of preventing the solidification of the soil in a soft state or the strength of the soil from decreasing, this reinforcing portion exhibits alkalinity by using cement. It will be. Thus, when arsenic is contained in the soil, the arsenic is eluted into the soil when the soil becomes alkaline. As a result, there arises a problem that, for example, the upper limit of the amount of elution into the soil defined in the environmental standards is exceeded, and the treated soil becomes contaminated soil. On the other hand, when an insolubilization treatment that produces a poorly soluble compound such as Pb or As is performed, the compound becomes soluble by using an alkaline treatment agent in combination, and the effect of the insolubilization treatment Causes a problem that it will be reduced.

Conventional soil solidifying materials and heavy metal insolubilizing materials have been used separately and independently, in other words, have had their respective effects in their respective uses. For example, the method of adding to soil separately, respectively, and the method of mixing both and adding were used. However, in such a method, when the effects of both solidification of the soil and insolubilization of heavy metal are achieved, the number of steps increases, resulting in inefficiency, and at the same time, there is a problem that it is not economically suitable. Until now, there has been no material that combines the effects of both soil solidifying material and heavy metal insolubilizing material.
Kenichi Kudo and Kazuo Takahashi, Journal of Japan Society of Civil Engineers, Vol. 87, 2002, p. 48-50

  The present invention has been made in view of such problems, and solidifies the soil by utilizing the water-absorbing effect of neutral gypsum as a soil-solidifying material, and at the same time prevents heavy soil from increasing the powder pH of the soil. Is intended to suppress the harmful effects of re-elution and to maximize the insolubilization effect of heavy metal adsorbents, and even in soil containing moisture, insolubilization treatment and soil solidification treatment are simultaneously performed without drilling It aims at providing the novel soil improvement material which has the effect that it can be performed. If the soil improvement material of this invention is used, in the processing method which needs to perform both soil solidification and heavy metal insolubilization, the objective of both soil solidification and heavy metal insolubilization can be achieved efficiently.

Means for Solving the Problem and Embodiment of the Invention

The present invention is a powder containing gypsum and iron oxide, having a specific surface area of 10 to 100 m ² / g and a powder pH of 6.0 to 9.0. Providing materials.
The soil improvement material of the present invention is a powder containing gypsum and iron oxide.

  In the present invention, gypsum is mainly composed of hemihydrate gypsum and / or anhydrous gypsum. Using gypsum that is mainly composed of hemihydrate gypsum and / or anhydrous gypsum with excellent water absorption, for example, to efficiently absorb and solidify soil in a soft ground due to construction sludge and groundwater. Can do.

  In the present invention, the iron oxide may contain gypsum, for example, by the production method described below with an example. As the iron oxide, those obtained by the production method described below are preferable. Those skilled in the art will recognize that the iron oxide obtained by this production method is amorphous hydrated iron oxide and / or ferric hydroxide, although no clear peak is observed by X-ray diffraction analysis. Can be reasonably estimated.

Iron oxide in the soil conditioner of the present invention, a weight ratio with respect to the gypsum was converted the iron oxide Fe ₂ O ₃ 0.05~0.5: it is preferred to include in the 1 range. The soil improvement material in this range can be used as it is for both soil solidification and heavy metal insolubilization. When the weight ratio of the iron oxide is high, the gypsum component is reduced, so that the effect when used as a solidifying material is reduced. On the other hand, when the weight ratio of iron oxide is low, the effect as a heavy metal insolubilizing material is reduced.

The preferable specific surface area of the soil improvement material of this invention is 10-100 m < ² > / g.
When the specific surface area of the powder of the soil improving material is less than 10 m ² / g, the adsorption of heavy metals in the soil and the water absorption effect are insufficient. Moreover, when the specific surface area of the powder of a soil improvement material exceeds 100 m < ² > / g, the adsorption | suction and water absorption effect of heavy metal in soil will be saturated.

The powder pH of the soil improvement material of the present invention is 6.0 to 9.0.
When the powder pH of the soil improving material is less than 6.0, the heavy metal becomes soluble in water. Moreover, if the powder pH of the soil conditioner exceeds 9.0, the heavy metal once adsorbed may be in a soluble state. Furthermore, considering that, for example, the pH of the environmental standard value for drainage is 5.8 to 8.6, the powder pH of the soil improvement material of the present invention is more preferably in the range of 6.0 to 9.0. I understand.

  The form of the soil improvement material of the present invention is preferably a fine particle, that is, a powder from the viewpoint that the effect of heavy metal ion adsorption is high. From this viewpoint, the average particle size of the soil improvement material of the present invention is preferably 20 μm or less. That is, the average particle size is set to 20 μm or less from the viewpoint that the effect of the soil improvement material as a water-absorbing material is proportional to the content of hemihydrate gypsum or anhydrous gypsum and increases the solidification effect per unit gypsum content. The thickness is preferably 10 μm or less.

  Regarding the range of the particle size of the soil improvement material of the present invention, the soil improvement material of the present invention having a hemihydrate gypsum content of 63% and a particle size of 10 μm, and the competitor's hemihydrate gypsum content of 86% and a particle size of 37 μm. The soil amendment material was compared. As a result, as shown in Examples and Comparative Examples, the optimum addition ratios of the two solidification materials were the same. From this, it was found and confirmed that fine particles have a greater effect of solidifying the soil improvement material. The optimum addition rate of the solidifying material was determined by visual observation whether the soil to be tested was packed in a glass tube and the soil left when the glass tube was pulled up was left as it was.

In soil conditioner of the present invention, in a weight ratio in terms of the iron oxide to said gypsum _Fe 2 _{O 3 0.05~0.5:} preferably contains at 1. If it is in this range, the soil improvement material of this invention can be used as it is for both the use of soil solidification and heavy metal insolubilization. When the weight ratio of the iron oxide is high, the gypsum component is reduced, so that the effect when used as a solidifying material is reduced. On the other hand, when the weight ratio of iron oxide is low, the effect as a heavy metal insolubilizing material is reduced.

The soil improvement material of this invention may further contain 0.1 to 4 weight% by making alkaline-earth metal content other than Ca into an oxide. Examples of suitable alkaline earth metals other than Ca include Mg, Sr and Ba. More preferably, it is Mg. By further containing the alkaline earth metal, the production of magnetite is suppressed and fine hydrated iron oxide and / or ferric hydroxide is produced. The soil improvement material of the present invention containing this fine hydrated iron oxide and / or ferric hydroxide exhibits particularly excellent adsorption effect on heavy metals. In the case of a soil improvement material further containing less than 0.1% by weight with an alkaline earth metal content other than Ca as an oxide, a magnetite with a small specific surface area is generated, so the specific surface area of the soil improvement material is 10 m ² / It becomes less than g, and the effect of heavy metal insolubilization falls. In addition, in the case of a soil improvement material further containing more than 4% by weight with an alkaline earth metal content other than Ca as an oxide, the effect as an impurity will come out, relatively gypsum in the soil improvement material And the content ratio of iron oxides decreases, and it becomes impossible to exhibit the effect of adsorption and solidification as a soil improvement material.

  The soil improvement material of this invention has the effect excellent in insolubilization of a heavy metal. The reason for this is thought to be that heavy metal ions are adsorbed and insolubilized by the hydroxyl group of the iron oxide, particularly hydrated iron oxide and / or ferric hydroxide. Examples of heavy metal ions that are adsorbed and insolubilized by the soil improvement material of the present invention include As, Se, Cd, Pb, Cr (hexavalent), Hg, and the like. Therefore, the contaminated soil can be improved, for example, in accordance with environmental standards based on the Basic Law for Soil Countermeasures.

An example of a manufacturing method The soil improvement material of this invention can be typically manufactured with the following method.
Waste sulfuric acid containing iron, for example, waste sulfuric acid generated in the manufacturing process of sulfuric acid-type titanium oxide, contains high-concentration sulfuric acid and Fe, Mn, Ti, Si, Al, etc. after use in the digestion reaction of ilmenite ore Although it is a solution, the sulfuric acid content etc. of this are neutralized under the conditions of pH 2.0-5.0 using the alkali containing calcium carbonate and magnesium hydroxide. This produces gypsum.

  After removing the gypsum generated in the neutralization treatment step, a gypsum separation liquid is obtained. This gypsum separation liquid contains iron sulfate and magnesium sulfate. The gypsum separation liquid is neutralized with calcium hydroxide under conditions of 35 to 45 ° C. and pH 7.0 to 9.0. A gypsum and iron oxide precipitate is formed in a slurry state. This slurry-like precipitate contains about 50 to 70% as a gypsum content, and the average particle size when it is made into a powder is 20 μm or less. Thereafter, the slurry is subsequently oxidized with air stirring to obtain a slurry containing gypsum and atomized iron oxide. The slurry containing the gypsum and iron oxide obtained is filtered and washed, dried and fired, whereby the soil improving material of the present invention can be obtained. The main component of the atomized iron oxide was estimated to be hydrated iron oxide and / or ferric hydroxide.

  When the waste sulfuric acid has a limitation on the unit price of the soil improvement material and it is necessary to use a corresponding inexpensive raw material, it is economically advantageous to use the waste sulfuric acid generated in the production process of sulfuric acid method titanium oxide. preferable.

  Since iron sulfate in the gypsum separation liquid obtained in the above treatment step is neutralized with an equivalent amount of alkali, such as calcium hydroxide, magnetite particles having a small specific surface area, which is usually unfavorable for heavy metal adsorption, are mixed into the product. However, since magnesium ions are present in the neutralization reaction solution, the formation of magnetite is not recognized. In addition, the oxidation after neutralization with alkali strongly generates aeration to form fine iron oxides mainly composed of hydrated iron oxide and / or ferric hydroxide, for example, the oxidation rate of Fe as described later. Is preferably aerated so as to be 0.01 to 0.40 g / L / min.

  The gypsum separation liquid has an Fe concentration of 9 to 11 g / L, the calcium hydroxide slurry required for neutralization is 1.0 to 2.2 times equivalent to Fe, and the temperature during neutralization is 35 to 45 ° C. By controlling the pH within the range of 7.0 to 9.0, gypsum and ferrous hydroxide precipitate are generated. The powder pH of the soil improvement material is determined by the equivalent ratio with respect to Fe of the calcium hydroxide slurry, and the powder pH can be controlled within the range of 6.0 to 9.0 when set in the above range.

The stirring oxidation performed after the neutralization can be performed by stirring and oxidizing by blowing air at such a rate that the oxidation rate of Fe becomes 0.01 to 0.40 g / L / min.
The composition of the iron oxide obtained by separating the gypsum component from the slurry containing gypsum and iron oxide had very low crystallinity, and no diffraction peak was found in the analysis by X-ray diffraction. Even in transmission electron micrograph observation, no crystalline material was observed, and it was confirmed to be an aggregate of amorphous ultrafine particles having a BET specific surface area of 150 to ²⁰⁰ m ² / g.

  The particle size of the gypsum produced is determined by the temperature and stirring strength when neutralizing the initial gypsum separation liquid with calcium hydroxide. Neutralization at 40 ° C., the highest solubility of gypsum, produces the finest gypsum. The gypsum obtained at this time is preferred. Further, if the stirring air is increased to some extent, fine particles are more easily generated.

  The resulting slurry containing gypsum and iron oxide is filtered and washed, dried at a temperature of 110 ° C., and subsequently fired at a temperature of 250 to 300 ° C. This firing may be performed in the air or in the presence of an inert gas. The gypsum obtained after drying is a dihydrate, and can be changed to hemihydrate gypsum or anhydrous gypsum having a greater water absorption effect by subsequent baking at 250 ° C. or higher.

Below, the measuring method of each characteristic is demonstrated.
(Powder pH)
In accordance with JIS K 5101-17-1: 2004, 5.0 g of a sample is weighed and transferred to a 200 ml Erlenmeyer flask with a screw stopper. Add 100 ml of distilled water and heat, boil gently for 5 minutes after boiling. After cooling to room temperature, water corresponding to the weight loss is added to make up for mixing, and the pH is immediately measured with a pH meter equipped with a glass electrode.
(Specific surface area)
About 0.3 g of dry sample is weighed and transferred to the measuring cell. After deaeration treatment at 150 ° C. for 20 minutes, B.F. E. T.A. The specific surface area is measured by a one-point method (low pressure method).
(Gypsum average particle size)
Using an optical microscope (Digital HD microscope WH-7000 manufactured by KEYENCE), the shape of the gypsum particles in the sample was observed at a magnification of 1000 times, and the major axis diameter was measured for 300 particles, and the average particle size was calculated. .
(Iron oxide Fe ₂ O ₃ content)
According to JIS K 5109: 1999, 0.3 g of a sample is weighed and transferred to a 300 ml conical beaker. Add 15 ml of concentrated hydrochloric acid and dissolve by heating. After adding the stannous chloride solution, it is rapidly cooled, and after adding 10 ml of a saturated mercuric chloride solution and a mixed acid, it is titrated with a 0.1 N potassium dichromate solution.
(Measurement of Pb elution amount and As elution amount)
This was carried out in accordance with JISK0102 54 and JISK0102 61 stipulated in the Soil Contamination Countermeasures Law.

Hereinafter, examples showing the effects of the present invention will be described. However, the following examples are provided for illustrative purposes only, and the scope of the invention is not limited by these examples.
(Example 1)
[Manufacture of soil amendments]
Waste sulfuric acid generated in the manufacturing process of sulfuric acid method titanium oxide is neutralized at pH 2.5-4.0 using HT slurry made by Ube Materials as alkali containing calcium carbonate and magnesium hydroxide, and further gypsum component The solution containing Fe and Mg after removing the iron is heated to 40 ° C. and neutralized with calcium hydroxide at pH 7-9 to produce ferrous hydroxide and gypsum (dihydrate). Aerated for a period of time to produce hydrated iron oxide or ferric hydroxide. This slurry containing gypsum and ferric hydroxide is filtered and washed, then the cake is molded into a cylindrical shape and sent to a band dryer set at 270 ° C., followed by drying at low temperature, and the gypsum is semi-watered. Changed to gypsum. The generated soil improvement material has a specific surface area of 72 m ^{2 /} g, pH 8.0, and the average particle size of the hemihydrate gypsum contained is 10 μm, and the weight ratio of iron oxide converted to Fe ₂ O ₃ with respect to gypsum. It was included at a ratio of 0.33: 1. Further, it contained 3.2 wt% of MgO as an alkaline earth metal oxide.
[Evaluation of soil conditioner]
As shown in Table 1, the Pb content is 2200 mg / kg, the As content is 250 mg / kg, the Pb elution amount is 0.044 mg / l, the As elution amount is 0.08 mg / l, and the heavy metal content is As a result of adding 50 g / kg of the soil improvement material of the present invention to the contaminated soil whose elution amount exceeded the environmental standard, and measuring the elution amount, the Pb elution amount was less than 0.005 mg / l, and the As elution amount was It decreased to 0.008 mg / l, and cleared the environmental standard of 0.01 mg / l of soil elution. Moreover, as a result of adding and mixing 50 g / kg, which was the optimum addition rate of the solidifying material, the soil improvement material of the present invention was added to the contaminated soil that became sludge due to the high water content, handling properties were greatly improved, It was possible to carry it off-site for subsequent soil purification treatment.
(Comparative Example 1)
In place of heavy metal-contaminated soil shown in Example 1, 10 ml / kg of polyiron and 50 g / kg of calcium carbonate added to the soil as conventionally used heavy metal insolubilizers, the As elution amount decreased to 0.006 mg / l. However, the Pb elution amount was 0.013 mg / l, and environmental standards could not be satisfied.
(Comparative Example 2)
In place of heavy metal-contaminated soil shown in Example 1, 10 ml / kg of polyiron and 50 g / kg of cement as a heavy metal insolubilized material were added to the soil, and the Pb elution amount was 0.001 mg / l. Since the elution pH was raised to 10.4 by cement and became alkaline, the As elution amount increased to 0.28 mg / l, significantly exceeding the standard.

[Effect of the invention]
As described above, the soil improvement material of the present invention contains gypsum and iron oxide, has a specific surface area of 10 to 100 m ² / g, and a powder pH of 6 to 9, It is a material. By using the soil improving material of the present invention, the effects of both the soil solidification treatment and the heavy metal insolubilization treatment can be achieved. Moreover, when it is necessary to perform such soil solidification treatment and heavy metal insolubilization treatment, the use of the soil improvement material of the present invention eliminates the need for the mixing step of both treatment materials, and simplifies the overall process. can do. As a result, there is an advantage that the cost required for soil treatment can be reduced.

Claims (6)

A soil conditioner comprising gypsum and iron oxide, having a specific surface area of 10 to 100 m ² / g and a powder pH of 6 to 9, comprising an alkaline earth metal other than Ca The soil conditioner further comprises 0.1 to 4% by weight in the form of an oxide, with the total weight of the soil conditioner being 100% by weight . The soil improvement material according to claim 1, wherein the gypsum is mainly composed of hemihydrate gypsum and / or anhydrous gypsum. The ratio of iron oxides and gypsum, iron oxides the iron oxide in a weight ratio in terms of _Fe 2 _{O 3:} Gypsum = 0.05-0.5: claim 1 or characterized in that it is a 1 2. The soil improvement material according to 2. The soil improvement material according to any one of claims 1 to 3, wherein the average particle size of gypsum is 20 µm or less. The soil improvement material according to any one of claims 1 to 4 , wherein waste sulfuric acid discharged from a sulfuric acid method titanium oxide production process is used as a raw material. Iron oxide has a BET specific surface area of 150 to 200 m ² It is / g, The soil improvement material in any one of Claims 1-5 characterized by the above-mentioned.