JP2002188086A - Method for alkali soil neutralization treatment by phosphate buffer solution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a soil improvement method for alkali soil neutralization treatment by recognizing the importance of pH and high buffer action of phosphate buffer solution. SOLUTION: As a result of a neutralization test (laboratory test) of coat ash and phosphate buffer solution, reclaimed soil of coal ash is neutralized and pH is made neutral. No change in pH is confirmed even with the lapse of time. Consequently neutralization treatment of reclaimed land is made possible by addition of phosphate buffer solution.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
And a neutralization ground improvement method for alkaline soil focusing on a high buffering action.

[0002]

2. Description of the Related Art Coal ash discharged from thermal power plants is industrial waste, and the discharged coal ash is landfilled and managed as a managed final disposal site. The area of landfilled with coal ash is increasing year by year, and it is necessary to reuse and effectively use coal ash and its land. One of the effective uses of this land is as a lot for sale.

[0003] Coal ash is classified as industrial waste, and coal ash landfill is considered as a waste disposal site. Therefore, studies on the neutralization of landfill soil for the purpose of effective utilization have not been made much.

In the field where coal ash is used as a neutralizing agent,
Example of adding coal ash to acid mine drainage (plan to inject coal by-products into Omega coal mine to reduce acid mine drainage, GRAY TA et, MORAN TC, Procannu Int pittsb
urgh coal conf, vol.15th, No.S-1-1 / S-8-5, 1998)
Also, as a flue gas desulfurization device, a method of neutralizing and desulfurizing by mixing sulfurous acid gas and coal ash (Characteristics of CT-121 in coal-fired ash mixing; Chiyoda technical report, vol 16, No. 2,
p.34-36, 1995).

[0005] As fields for neutralizing coal ash itself, leachate is blown with carbon dioxide gas, neutralization of coal ash with hydrochloric acid and sulfuric acid (research on leach processing of coal ash, Yoshihisa Hamada et al. Report of current research institute, No.42, p.81, 1987)
There is. Also, as a heavy metal insolubilizing agent for fly ash, there is a fly ash treating agent Ashnite from Kurita Water Industries Ltd.

[0006] As described above, there are few methods of neutralizing coal ash landfill itself, and there have been reports of neutralization by mixing acidic soil as a research stage. In the method of mixing the acid soil, the amount of the acid soil is required to be larger than expected in order to perform the neutralization treatment, which increases the cost, and the pH rises with time. Therefore, there is a need for a neutralization technique that can be applied relatively easily to alkaline soil, especially coal ash landfill, and that does not change pH after neutralization.

[0007]

Hitherto, there has been little research and development on neutralization treatment for effective utilization of coal ash landfill. At present, there is a neutralization treatment by mixing with acidic soil, but a large amount of soil is required for neutralization, which is difficult in terms of construction and cost. Also, since the basic principle is neutralization with sulfuric acid, a component of acid soil,
The pH may fluctuate after the neutralization treatment.

Accordingly, when neutralizing alkaline soil (coal ash landfill), it can be easily constructed (a large amount of soil material is not used or the amount can be reduced), and the pH of the neutralized soil varies. Is required.

Coal ash contains harmful substances such as lead in its components, and may be eluted in large amounts when a strongly acidic solution such as sulfuric acid is used. Considering that the solution penetrates underground, a strongly acidic solution cannot be used as a neutralizer for coal ash. Therefore, the solution used as the neutralizing agent is desirably a solution having a low concentration at a weakly acidic to acidic pH.

[0010]

SUMMARY OF THE INVENTION According to the present invention, a solution is added (or mixed) by using an acidic phosphate solution which forms a phosphate buffer with its alkali as a neutralizing agent for neutralizing coal ash. This is advantageous because it is easy to construct and the soil volume after construction does not increase.

Further, the phosphate buffer produced has a high buffering action. After neutralization, when the alkaline component is released from the coal ash, protons are released or combined with the component, thereby increasing the pH.
The effect of suppressing an increase in the temperature is produced. Therefore, the problematic fluctuation in pH after the neutralization treatment is reduced.

Since the optimum amount is determined by a test for determining the amount of neutralization of the coal ash with the phosphate buffer before construction, the amount of excess phosphate buffer discharged as leachate can be minimized.

[0013]

When considering the effective use of coal ash landfill, the pH of the neutralized coal ash needs to be maintained neutral without being changed by external factors such as precipitation. Therefore, a high buffering capacity is required for the neutralizing agent. A strongly acidic solution such as sulfuric acid does not have this buffering ability, and the pH immediately rises when neutral components such as calcium are dissolved from coal ash over time after neutralization.

Dihydrogen phosphate monosodium or phosphoric acid 2
A solution of monopotassium hydrogen or the like satisfies this condition (buffering capacity), but for industrial purposes, the use of a monosodium dihydrogen phosphate solution is preferred. In addition to the above, formation of a hardly soluble salt by phosphoric acid and calcium is also expected.

As a result of a coal ash neutralization test (indoor test) with an acidic phosphate solution forming a phosphate buffer, the coal ash landfill was neutralized and the pH became neutral. Further, it was confirmed that the pH did not change even after the passage of time. This has enabled the neutralization of the landfill by the generation of a phosphate buffer.

[0016]

EXAMPLE The results of a laboratory test (a neutralization test of coal ash using a phosphate buffer) are shown. As the acidic phosphate solution for forming the phosphate buffer, a monosodium dihydrogen phosphate solution was used. The concentration and pH are as shown in Table 1. Table 1 shows the phosphate buffer (sodium dihydrogen phosphate) used in the laboratory test.
And the pH of the solution.

[0017]

[Table 1]

The test includes a coal ash neutralization test for examining the possibility of neutralization of coal ash with the generated phosphate buffer, and a pH change with time for a neutralized soil subjected to a neutralization treatment using a phosphate buffer. A flood test was conducted to examine the changes. In addition, pure water was continuously passed through the neutralized soil, and a water passing test was conducted to examine the pH change of the leachate.

FIG. 1 shows the results of the coal ash neutralization test. This shows the relationship between the mixing ratio of the acidic phosphate solution (ml) for forming a phosphate buffer solution to coal ash (g) and soil pH. As a _{result, 1mol / L-NaH 2 PO} 4, 0.1mol / L-NaH 2 P
It was found that both O _{4 and} coal ash can neutralize coal ash. However, looking at the change in pH after 24 hours, it was recognized that it was effective to neutralize 1 g of coal ash with 0.5 ml of 1 mol / L-NaH ₂ PO ₄ .

FIG. 2 shows the results of the soil flooding test with the phosphate buffer added.
It was shown to. Using the sample was neutralized with _{1mol / L-NaH 2 PO 4} 0.5ml against coal ash 1g. 1g of coal ash: distilled water
It was measured for coal ash treated to 2.5 ml. as a result,
It was found that the pH of the neutralized soil remained neutral over time.

FIG. 3 shows the results of the water passage test of the neutralized soil.
As a result, it was clarified that the pH remained neutral even in the leachate.

[0022]

As a result of conducting a neutralization test (indoor test) with coal ash and an acidic phosphate solution for forming a phosphate buffer, a phosphate buffer (1 mol / L-NaH ₂ PO ₄ ) was produced (coal ash). 1g
0.5ml), the coal ash became neutral (pH 7.4). Also,
It was confirmed that the pH did not change (pH 7.5) even after the elapse of time under the flooded condition (up to 36 days). The leachate was also neutralized (pH 7.
It was confirmed that 5) was maintained and the effect on pH of groundwater was reduced.

[Brief description of the drawings]

FIG. 1 shows a coal ash neutralization test in which a pH of a mixed soil with respect to a mixing ratio of a coal ash and a phosphate buffer solution in a laboratory test.

FIG. 2 shows a time-dependent change in pH of neutralized coal ash soil in a laboratory test.

FIG. 3 shows the pH of leachate when pure water is continuously passed through the neutralized coal ash soil in the laboratory test.

Claims (3)

[Claims] 1. A target soil is neutralized by adding and mixing an acidic phosphate solution which forms a buffer with the soil alkali to an alkaline soil, and neutralized by a buffering action of a generated phosphate buffer. A soil improvement method characterized by maintaining the pH of the treated soil. 2. The soil improvement method according to claim 1, wherein the alkaline soil is a highly alkaline coal ash landfill. 3. The soil improvement according to claim 1, wherein the optimum value of the acidity and / or concentration of the acidic to weakly acidic acid phosphate solution is determined by a preliminary test on alkaline soil to be treated. Construction method.