JP2006095463A - Purification method for soil contaminated with lead - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a soil purification method which enables the volatilization and removal of lead from soil contaminated with the lead at an extremely high removal rate by optimizing the condition of heating treatment. <P>SOLUTION: The soil contaminated with lead is subjected to heating treatment at a temperature of ≥950°C and ≤1,300°C for ≥10 minutes under flowing gas. A period of time required for increasing the temperature of the soil from 500°C to 950°C by heating is within 20 minutes. As the flowing gas is required only to flow during the heating treatment of the soil, there is no necessity to specify its linear velocity and flow rate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for purifying soil contaminated with lead, and more particularly to a method for purifying soil by subjecting soil contaminated with lead to heat treatment to volatilize and remove lead.

  In recent years, there have been many cases where soil is contaminated with harmful substances, and there are concerns about the health effects of contaminated soil. Therefore, social demands for establishing such countermeasures have been strengthened, and the “Soil Contamination Countermeasures Law” has been enforced, and legal regulations for soil contamination are being developed.

  Under such circumstances, various techniques for purifying soil have been reported. Among them, heat treatment with a rotary kiln is one of the common soil purification technologies, and as described in Non-Patent Documents 1, 2, 3, and 4, research has already been promoted at home and abroad, and at a commercial level. It is a technology that is being adopted.

  Thus, although the heat processing by a rotary kiln is an existing technique, it is not applied very much regarding lead. This is because lead is considered to be difficult to volatilize and remove by heat treatment, and the purification of soil has advanced in the direction of insolubilization.

However, there is a problem with the reliability of the insolubilization treatment, such as the fact that the pH of the insolubilized soil changes abruptly under the influence of acid rain or alkalinization with an insolubilizing agent, leading to the re-elution of lead. Is emerging. Furthermore, with the recent enactment of the “Soil Contamination Countermeasures Law”, the idea of using the content as a standard has occurred. Therefore, there is a need for a technique for reducing lead content in soil itself, that is, removing lead from soil, not simply insolubilization. Under such circumstances, heat treatment has been reviewed as one of the technologies for removing lead in soil.
Global Environment Center Foundation, "Heating Technology for Heavy Metal Contaminated Soil", GEC Environmental Technology Information Database (NETT21), [online], July 2002, Global Environment Center, [July 30, 2004 search ], Internet <URL: http://nett21.gec.jp/SGC#DATA/JP/html/sgcj-052.html> Kozo Ueda and two others, “Thermal Desorption Behavior of Soil Contaminants”, Hitachi Zosen Technical Report, Hitachi Zosen Corporation, April 1998, Vol. 59, No. 1, p. 74-80 Robert C. Thurnau, “Low-temperature desorption treatment of co-contaminated solids: TCLP as an evaluation technique”, Journal of Journal of Hazardous Materials, (Netherlands), Elsevier Science, 1996, Vol. 48, p. 149-169 Dennis A. Clifford, two others, “Volatilizing Toxic Metals from Soil”, Waste Management, (USA), Pergamon Press (Pergamon) Press), 1993, Vol. 13, p. 467-479

  However, the conditions for the heat treatment are not so much studied at present. For example, Non-Patent Document 1 describes that “a rotary kiln preheated to 1000 to 1100 ° C. is charged with contaminated soil by a feeder, heated while passing air, volatilizes volatile heavy metals, and removed from the contaminated soil. Heavy metals that remain without volatilizing also change to oxides that are difficult to dissolve in water by heat treatment ”(see“ Technical Overview ”on page 1). It was found that it was hardly desorbed by heating at ˜900 ° C. ”(see“ 4.2 Demonstration Test Results ”on the right column of page 78). In Non-Patent Document 3, lead is at 650 ° C. for 60 minutes. It is shown that it does not evaporate even after heat treatment (see FIG. 6), and the amount of soil input to the kiln (˜12.5%), moisture content (˜20%), kiln rotation speed (˜190 inch / Etc.) also affects the evaporation of lead (See FIGS. 7 to 9), Non-Patent Document 4 states that “at 750 ° C., a lead removal rate of about 50% was achieved. Gave 93% lead removal in hydrogen or nitrogen, but only 73% lead removal in air ”(see“ Summary ”on page 467). In fact, the optimum condition for heat treatment for removing lead from the soil has not been established.

  The present invention has been made in view of such circumstances, and the object thereof is to volatilize lead at a very high removal rate from soil contaminated with lead by optimizing the heat treatment conditions. An object of the present invention is to provide a soil purification method that can be removed.

  When volatilizing and removing lead in soil by heat treatment, the mechanism is very complex and details are not clear. The change in state of lead and volatilization / evaporation due to heat treatment is a complex phenomenon in which reaction and diffusion transfer are mixed. Considering both reaction and mass transfer, the rate-determining step of the process of dissociating, moving, and evaporating lead supported on soil particles is as follows: (1) The process of dissociating some bound or adsorbed lead from the soil; (2) A process in which dissociated lead diffuses and moves to the evaporation surface, and (3) a process in which lead that reaches the evaporation surface moves through the film in the gas phase.

  Moreover, considering from the material aspect of the soil, the soil is made of aluminosilicate (a complex oxide of aluminum and silicon) and takes various crystal forms depending on the Si / Al atomic ratio, and other metal oxides coexist. Further, it is known to take another crystal structure. By itself, volatile lead is expected to be trapped in these crystal structures and not volatilize. Furthermore, it is expected that the reaction in which lead and aluminosilicate form a composite oxide will compete with the volatilization of lead.

  Therefore, when considering the operating conditions of the actual machine, the present inventors have optimized the controllable factors such as heating temperature, holding time, heating time, linear velocity of flow gas, flow rate, etc. It was possible to control the rate-determining step of the process of dissociation, migration and evaporation, trapping of lead in the aluminosilicate crystal structure, and competition between lead volatilization and complex oxide formation. As a result, the present inventors have found a heat treatment condition capable of volatilizing and removing lead from soil contaminated with lead at an extremely high removal rate, and have completed the present invention.

  That is, the present invention is a method for remediating soil contaminated with lead, wherein the soil contaminated with lead is subjected to heat treatment at a temperature of 950 ° C. or higher and 1300 ° C. or lower for 10 minutes or more under flowing gas. The time from heating to 500 ° C. to 950 ° C. is within 20 minutes.

  In the purification method of this invention, it is preferable that the time which heat-processes the said soil at the temperature of 950 degreeC or more and 1300 degrees C or less is 10 minutes or more and less than 30 minutes. Moreover, it is preferable that the temperature which heat-processes the said soil is 1000 degreeC or more and 1100 degrees C or less.

  The purification method of the present invention can volatilize and remove lead from soil contaminated with lead with a very high removal rate. In addition, since only heat treatment is performed under predetermined conditions, excluding exhaust gas treatment, no soil post-treatment is required, and the soil can be purified efficiently, simply, and at low cost.

  In the purification method of the present invention, when the soil contaminated with lead is subjected to heat treatment at a temperature of 950 ° C. or higher and 1300 ° C. or lower for 10 minutes or more under flowing gas, the soil is heated to 500 ° C. to 950 ° C. It takes 20 minutes or less. In the present invention, unless otherwise specified, “lead” refers to all forms of lead contained in the soil, such as metal lead and lead oxide, as well as alkali metal ions and alkaline earth present in the soil. It means a salt formed with a metal ion, a chloride ion, a sulfate ion, a nitrate ion, a carbonate ion, or a salt bound to a hydroxyl group of a soil component. In the present invention, the temperature of the heat treatment basically means the temperature of the soil unless otherwise specified, but since the thermal conductivity of the soil is high, the ambient temperature in the vicinity of the heat-treated soil May be regarded as the temperature of the heat treatment.

  The heat treatment of the soil is performed at a temperature of 950 ° C. or higher and 1300 ° C. or lower, preferably 1000 ° C. or higher and 1100 ° C. or lower. If the heating temperature is less than 950 ° C., lead is not sufficiently volatilized and removed from the soil. As for the upper limit of the heating temperature, for example, lead chloride (II) has a boiling point of 950 ° C., lead oxide (II) has a boiling point of 1470 ° C., and metal lead has a boiling point of 1740 ° C. There is no meaning. In general, it is sufficient to heat to about 1300 ° C. If the temperature exceeds 1300 ° C., the soil sinters, which causes a problem in reusing the soil after purification. In addition, if heat treatment is performed at an unnecessarily high temperature, the amount of energy used increases and a special heating facility is required, which increases the processing cost.

  If the time for heat-treating the soil at a temperature of 950 ° C. or higher and 1300 ° C. or lower, that is, the holding time is at least 10 minutes, lead can be sufficiently volatilized and removed from the soil. If the retention time is less than 10 minutes, lead is not sufficiently volatilized and removed from the soil. The upper limit of the holding time is not particularly limited, but is preferably 30 minutes. Even if it heat-processes over 30 minutes, it will only consume useless time and energy, and cannot be recommended. The upper limit of the holding time is preferably 20 minutes.

  In the heat treatment of the soil, it is important that the time during which the temperature of the soil is raised to 500 ° C. to 950 ° C., that is, the temperature rise time is within 20 minutes. When the temperature rising time exceeds 20 minutes, the reaction in which lead and soil components form a composite oxide becomes dominant, and the volatilization removal of lead is suppressed. The temperature raising time is preferably within 10 minutes, more preferably within 5 minutes. The lower limit of the temperature raising time is the time required for the soil to rise from the normal temperature to the predetermined heating temperature when the normal temperature soil is immediately exposed to the atmosphere of the predetermined heating temperature. However, it is about 2 minutes.

The heat treatment of the contaminated soil is performed under circulation gas. If the temperature of the heat treatment is sufficiently high, the lead once volatilized from the soil will not form complex oxides with the soil components again, but the lead immediately after volatilizing from the soil does not use the circulating gas or is distributed When the linear velocity or flow rate of the gas is low, there is a possibility of proceeding in the direction of forming complex oxides with soil components. Therefore, it is necessary to circulate the gas during the heat treatment of the soil. However, as shown in the following experimental examples, the circulation gas only needs to be circulated during the heat treatment of the soil. There is no need to specify linear velocity or flow rate. As typical values, for example, the linear velocity is 0.28 cm / sec or more and 2.8 cm / sec or less, or the flow rate per unit mass of the soil is 0.002 m ³ / gh or more and 0.040 m ³ / gh or less. What is necessary is just to heat-treat soil under the circulation gas of. The flowable gas that can be used may be appropriately selected depending on the heat treatment apparatus to be used. For example, air, nitrogen, argon, or a mixture thereof may be used, and air, nitrogen, or a mixture thereof is used in consideration of processing costs. It is preferable to do.

  In order to carry out the purification method of the present invention, a conventionally known heat treatment apparatus such as an internal heat or external heat type rotary kiln, a rotary stoker furnace or the like may be used. When using a rotary kiln, a cylindrical kiln with a fluff inside is preheated to a specified temperature using a fuel such as natural gas or heavy oil, and the kiln is rotated by charging the soil with a feeder. The lead is volatilized and removed from the soil by heating and supplying gas. In the case of an internal heat type rotary kiln, since a burner is cooked and heated in the kiln, it is necessary to blow air as a circulation gas. Oxygen is consumed by the combustion, and as a result, the oxygen concentration in the furnace atmosphere becomes about 5 to 10%. In the case of an external heat type rotary kiln, since the kiln is heated from the outside, the atmosphere in the furnace can be operated, and an inert gas such as nitrogen or argon can be blown as a circulation gas. In any case, in the heat treatment, the soil input speed, the kiln rotation speed, the residence time of the soil in the furnace, the linear velocity or flow rate of the circulating gas, etc. are adjusted as appropriate so as to satisfy the above conditions. That's fine. It should be noted that the time from heating to 500 ° C. to 950 ° C. within 20 minutes is that the temperature is measured in advance at each point in the furnace, and the soil corresponds to 500 ° C. and 950 ° C. It means that the time to pass through two points is within 20 minutes.

  The gas circulated when lead is volatilized and removed from the soil may be cooled and then released into the atmosphere after the lead is removed by ordinary exhaust gas treatment. As the exhaust gas treatment, for example, after wet cleaning with a solution such as water, caustic soda, slaked lime, sodium sulfide, etc., dust is removed by a bag filter or a wet or dry electric dust collector. The waste liquid discharged by wet cleaning may be recycled as water used for wet cleaning after the solid content is removed by normal waste water treatment. The solid content can be dehydrated and then solidified with cement to be reclaimed or used as a raw material for lead smelting.

  According to the purification method of the present invention, it is possible to purify the soil by volatilizing and removing lead from the soil at a very high removal rate only by heat-treating the soil contaminated with lead under predetermined conditions. When the purification method of the present invention is used, the lead removal rate reaches 99% or more as shown in the following experimental example. Therefore, the purification method of the present invention surpasses the conventionally known purification method of soil contaminated with lead.

The present invention will be described in more detail by experimental examples, but the present invention is not limited to these experimental examples.
First, the heating furnace used in the experimental example, the heat treatment procedure, and the lead content analysis method will be described.

(heating furnace)
The heating furnace used for the heat treatment is shown in FIG. The heating furnace 1 includes a horizontal reaction tube 2 having a diameter of 50 mm and a length of 1000 mm, and the sample 5 filled in the sample stage 4 can be heated to a desired temperature by an electric heater 3 wound around the horizontal reaction tube 2. The sample stage 4 is a molded product of silica alumina fibers having a rectangular parallelepiped shape with a width of 30 mm, a length of 500 mm, and a thickness of 50 mm. Detachable flanges 6 and 7 are attached to both ends of the reaction tube 2 so that the sample 5 filled in the sample stage 4 can be removed when the sample tube 4 is inserted into or extracted from the reaction tube 2. A gas introduction port is provided at the center of the flange 6 attached to one end of the reaction tube 2, and nitrogen gas is introduced into the reaction tube 2 from a nitrogen gas supply source 8 including a nitrogen gas cylinder and a flow meter and distributed. In addition, a gas discharge port is provided at the center of the flange 7 attached to the other end of the reaction tube 2, and the nitrogen gas circulated inside the reaction tube 2 can be discharged. .

(Procedure for heat treatment)
The heat treatment of the soil was performed as follows. First, nitrogen gas was introduced into the reaction tube 2 from the nitrogen supply source 8 and circulated, and the atmosphere in the reaction tube 2 was replaced with nitrogen gas. The reaction tube 2 was heated with the electric heater 3 to adjust the atmospheric temperature in the reaction tube 2 to a predetermined heating temperature. While flowing nitrogen gas, the flange 7 attached to one end of the reaction tube 2 was removed, and the sample stage 4 filled with the soil sample 5 was inserted into the reaction tube 2. The end of the reaction tube 2 was at room temperature. The sample 5 was thinly spread and filled in the sample filling portion of the sample stage 4. The thickness of the sample 5 was, for example, 1 to 2 mm when the mass was 5.0 g. The flange 7 was attached to the reaction tube 2, and the sample stage 4 filled with the sample 5 was moved to the center of the reaction tube 2 while flowing nitrogen gas. In addition, the temperature of each point in the reaction tube 2 is measured in advance, and the time during which the sample 5 passes through two points corresponding to 500 ° C. and 950 ° C., that is, the soil is heated to increase the temperature from 500 ° C. to 950 ° C. The time taken was defined as the temperature raising time. Thereafter, after holding for a predetermined heating time, the sample stage 4 filled with the sample 5 was moved to one end of the reaction tube 2 and naturally cooled. The flange 7 was removed, the sample stage 4 filled with the sample 5 was extracted, and the lead content was analyzed.

(Lead content analysis method)
The lead content of the soil was analyzed according to Kansui No. 127 II7.1. Briefly, the analysis was performed as follows. The soil is heated in a mixed solution of nitric acid and hydrochloric acid to dissolve lead and filtered, and then the filtrate is heated until immediately before evaporation to dryness to remove residual nitric acid and hydrochloric acid as much as possible, and the residue is 10 mL of hydrochloric acid (1 + 1). The test solution was dissolved in water and made up to 100 mL with water. This test solution was analyzed by atomic absorption spectrometry to determine lead.

Experimental Examples 1-13
Using actual contaminated soil (lead content 120 mg / kg, particle size less than 1 mm), the temperature at which the soil is heated (hereinafter referred to as “heating temperature”), and the time at which the soil is heated at a predetermined temperature (hereinafter referred to as “holding time”) ), The time for the soil to pass through the point of 500 ° C. to 950 ° C. in the reaction tube (hereinafter referred to as “temperature rise time”) and the linear velocity of the gas that is circulated during the heat treatment (hereinafter referred to as “circulation gas linear velocity”) Therefore, heat treatment was performed under the conditions shown in Table 1, and the lead content of the soil before and after the heat treatment was analyzed. The obtained results are shown in Table 1. In addition, the lead removal rate was calculated | required by following Formula.

1. Examination of heating temperature Table 2 shows the experimental results of Table 1 examined for the heating temperature. In Experimental Examples 7 and 11, the lead removal rate was 99.9% or higher at 1000 ° C. and 1100 ° C., respectively, whereas in Experimental Example 10, the lead removal rate was 94.4% at a heating temperature of 900 ° C. Met. Therefore, in order to achieve a lead removal rate of 99.9% or higher, a heating temperature of 1000 ° C. or higher is required.

2. Examination of holding time Table 3 shows the experimental results of Table 1 examined for the holding time. In Experimental Examples 3, 2, and 1, the lead removal rate was 99.9% or more at 10 minutes, 20 minutes, and 30 minutes, respectively, whereas in Experimental Example 13, lead removal was performed at 5 minutes. The rate was 87.5%. For this reason, a retention time of 10 minutes or more is sufficient to achieve a lead removal rate of 99.9% or more.

3. Examination of temperature rise time Table 4 shows the experimental results in Table 1 examined for the temperature rise time. In Experimental Examples 7 and 12, the lead removal rate was 99.9% or more and 99.0% at 2 minutes and 20 minutes, respectively, whereas in Experimental Example 6, the heating time was 40 minutes. The lead removal rate was 58.5%. For this reason, in order to achieve a lead removal rate of 99.0% or higher, it is necessary that the temperature rising time is within 20 minutes.

4). Examination of circulating gas linear velocity Table 5 shows the experimental results in Table 1 examined for circulating gas linear velocity. In Experimental Examples 5, 9, 8, and 1, the flow gas linear velocity was 0.28 cm / s, 0.7 cm / s, 1.4 cm / s, and 2.8 cm / s, respectively, and the lead removal rate was 99.9% or more. there were. From this, a lead removal rate of 99.9% or more is achieved at a flow gas linear velocity of 0.28 cm / s or more.

5. Examination of flow gas flow rate We examined flow gas flow rate per unit mass of soil (hereinafter referred to as “flow gas flow rate”) instead of flow gas linear velocity. The flow rate of the circulating gas was determined by the following formula.

Table 6 shows the experimental results of Table 1 examined with respect to the flow rate of the circulating gas. In Example 5,4,1 was respectively circulation gas flow rate 0.002m ³ /gh,0.020m ³ /gh,0.040m ³ / lead removal ratio of 99.9% or more by gh. For this reason, a lead removal rate of 99.9% or higher is achieved at a flow rate of 0.002 m ³ / gh or higher.

From the above examination results, in order to volatilize and remove lead from soil contaminated with lead with a very high removal rate (99.0% or more), the heating temperature is 950 ° C. or more, the heating time is 10 minutes or more, and the heating time is Must be within 20 minutes. With respect to the circulating gas, it is only necessary that the gas is circulating during the heat treatment of the soil. As typical values, for example, the linear velocity is 0.28 cm / sec or more and 2.8 cm / sec or less, or the flow rate per unit mass of the soil is 0.002 m ³ / gh or more and 0.040 m ³ / gh or less. What is necessary is just to heat-treat soil under the circulation gas of. In the above experimental example, actual contaminated soil with a lead content of 120 mg / kg was used as a sample, but the same result was obtained even in soil with a very high degree of contamination with a lead content of several thousand mg / kg. Can be obtained.

  Since the purification method of the present invention can volatilize and remove lead from soil contaminated with lead at a very high removal rate, it is a very effective technique for solving problems related to soil lead contamination.

It is sectional drawing which showed typically the heating furnace used by the experiment example.

Explanation of symbols

1 Heating furnace 2 Reaction tube 3 Electric heater 4 Sample stage 5 Sample 6, 7 Flange 8 Nitrogen gas supply source

Claims (3)

  When heat-treating the soil contaminated with lead at a temperature of 950 ° C. or higher and 1300 ° C. or lower for 10 minutes or more under a circulating gas, the time from heating to 500 ° C. to 950 ° C. is within 20 minutes. A method for remediating soil contaminated with lead, characterized in that:   The method for purifying soil contaminated with lead according to claim 1, wherein the time for heat-treating the soil at a temperature of from 950 ° C to 1300 ° C is from 10 minutes to 30 minutes.   The method for purifying soil contaminated with lead according to claim 1 or 2, wherein a temperature for heat-treating the soil is 1000 ° C or higher and 1100 ° C or lower.

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