KR20230052638A - Remediation method for uranium(U) contaminated soil - Google Patents

Abstract

The present invention relates to a method for restoring uranium-contaminated soil, which can effectively remove uranium from soil. According to the present invention, the method for restoring uranium-contaminated soil comprises the steps of: (a) mixing the uranium-contaminated soil with an alkali solution and dispersing soil particles; (b) dividing the uranium-contaminated soil into three groups of coarse particles, intermediate particles, and fine particles; (c) supplying and washing a carbonate solution to the coarse particles and the intermediate particles to elute and remove uranium from the soil particles; (d) separating magnetic particles, including iron oxide to which uranium is bonded, from the fine particles through magnetic separation with respect to the fine particles; (e) dividing the group of fine particles, from which the magnetic particles are separated, into high-specific-weight particles and low-specific-weight particles by performing specific-weight sorting; and (f) washing the low-specific-weight particles with carbonate.

Description

Remediation method for uranium(U) contaminated soil}

The present invention relates to a soil restoration technique, and more particularly to a technique for purifying soil contaminated with radioactive nuclides such as uranium and cesium.

Radionuclides in soil can be classified into geogenic, cosmogenic, and anthropogenic. Radionuclides introduced into the soil by human activities result from nuclear power plant accidents, nuclear tests, and scientific and medical applications.

Among the radionuclides, uranium (U, uranium) has an atomic number of 92, ²³⁴ U (0.0054%, half-life 2.45*10 ⁵ years), ²³⁵ U (0.7204%, half-life 7*10 ⁸ years), ²³⁸ U (99.2742%, half-life 4.46 *10 ⁹ years) is a representative isotope. It is a radioactive heavy metal (19.05 g/cm ³ ) that emits α particles and is found in the earth's crust, rocks and soil at concentrations of 2-4 mg/kg. The U concentration of soil is determined by the U concentration of the parent rock, but in some areas, the concentration of uranium of natural origin (background level) appears higher than the background level due to mining activities, nuclear fuel production, nuclear accidents, nuclear tests, and inflow of waste. Uranium-contaminated soil can threaten humans and ecosystems due to radiation toxicity and chemical toxicity.

The present invention is to solve the above problems, and an object of the present invention is to provide a method for effectively restoring soil contaminated with uranium based on research on the binding state of soil particles and uranium and the behavior of uranium in soil. .

Meanwhile, other unspecified objects of the present invention will be additionally considered within the scope that can be easily inferred from the following detailed description and effects thereof.

A method for restoring uranium-contaminated soil according to the present invention for achieving the above object includes: (a) dispersing soil particles by mixing uranium-contaminated soil with an alkali solution; (b) sizing the uranium-contaminated soil into three groups of coarse particles, medium particles, and fine particles; (c) removing uranium from the soil particles by supplying an aqueous carbonate solution to the coarse and medium particle groups to wash them; (d) separating magnetic particles including iron oxide to which uranium is bound from the groups of thin particles through magnetic separation of the groups of thin particles; and (e) performing specific gravity separation on the fine particle groups from which the magnetic particles are separated to separate them into high specific gravity particles and low specific gravity particles.

According to the present invention, the step of irradiating ultrasonic waves to the contaminated soil in a suspension state in which the contaminated soil is mixed with an aqueous alkali solution, and generating a vortex in the suspension in which the contaminated soil is mixed with an aqueous alkali solution so that the soil particles are mutually It is preferable to further include a rubbing step.

In one example of the present invention, it is preferable to further include the step of eluting and removing uranium from the low specific gravity soil particles by supplying an aqueous carbonate solution to the low specific gravity particles separated in the specific gravity separation and washing them.

In one example of the present invention, it is preferable to further include a group separation step of mutually separating heterogeneous minerals constituting the coarse particles by pulverizing the coarse particles.

In one example of the present invention, the coarse particles may be set to more than 250 μm, the medium particles to 50 to 250 μm, and the thin particles to be less than 50 μm.

In one example of the present invention, the alkaline solution is a 1-5M NaOH aqueous solution, and the contaminated soil and the NaOH aqueous solution may be mixed in a weight ratio of 1: 3-10.

The present invention provides a method for effectively removing uranium from soil based on analysis of the type, existence and behavior of uranium in soil.

That is, uranyl ions with high solubility are dissolved and removed through carbonate washing. For forms containing uranium but with low solubility, the concentration of uranium in contaminated soil can be reduced by removing soil particles containing uranium through particle size screening, magnetic separation, and specific gravity separation.

On the other hand, even if the effects are not explicitly mentioned here, it is added that the effects described in the following specification expected by the technical features of the present invention and their provisional effects are treated as described in the specification of the present invention.

1 is a table for explaining the types, existence forms, and behaviors of uranium in soil.
2 is a schematic flowchart of a method for restoring uranium-contaminated soil according to an example of the present invention.
※ It is revealed that the accompanying drawings are illustrated as references for understanding the technical idea of the present invention, and thereby the scope of the present invention is not limited.

In describing the present invention, a detailed description thereof will be omitted if it is determined that a related known function may unnecessarily obscure the subject matter of the present invention as it is obvious to those skilled in the art.

The present invention sought a method for removing uranium from soil particles based on precise research on the binding form and behavior of uranium in soil. Therefore, the bond form and behavior of uranium in soil will be explained first.

The species and forms of uranium according to the oxidation state of uranium in soil are summarized in the table of FIG. 1.

Referring to the table of FIG. 1, uranium in soil exists in the form of free ions, adsorption, precipitates, and complexes, and mobility and toxicity are determined according to the form of existence. The presence of uranium in soil is determined by pH, redox potential, ligand type and concentration. There are +2, +3, +4, +5, and +6 oxidation states of uranium, and +4 and +6 forms are stable in the surface environment.

Tetravalent uranium is stable in reducing environments. In reducing environments, tetravalent uranium precipitates as oxides, hydroxides, fluorides, phosphates, silicates, arsenates, and vanadates. Raninite (UO ₂ ), coffinite (USiO ₄ ·nH ₂ O), and uranium tetrafluoride (UF ₄ ) are known as tetravalent uranium minerals commonly found in uranium-contaminated soil. They have a specific gravity of 5 to 11 g/cm ³ , which is very high compared to general silicate minerals, and is characterized by very low solubility and very low mobility. It is also known that tetravalent uranium minerals exist as microparticles included in the surface of silicate minerals or soil aggregates. Also, in a reducing environment rich in organic matter, tetravalent uranium forms a complex with organic matter and is precipitated.

Hexavalent uranium is stable in an oxidizing environment. Hexavalent uranium exists as uranyl (UO ₂ ²⁺ ), uranyl-complex, less soluble oxy-hydroxide, silicate, phosphate, and arsenate minerals (precipitates). In an acidic condition of pH≤2.5, uranyl (UO ₂ ²⁺ ), which is a free ion state, is stable, and forms a stable complex with phosphate and carbonate in neutral and alkaline environments.

F ^- , Cl ^- , NO ₃ ^- , SO ₄ ^2- , CO ₃ ^2- , PO ₄ ^3- , It has a strong tendency to form complexes with organic matter. The uranyl-organic complex is adsorbed on the surface of silicate minerals and iron oxide, and the uranyl- (F ^- , Cl ^- , NO ₃ ^- , SO ₄ ^2- , CO ₃ ^2- ) complex is adsorbed on the surface of iron oxide or co-precipitated. . For example, uranyl, cation uranyl-complex (UO ₂ OH ⁺ , UO ₂ F ⁺ , UO ₂ Cl ^{+ ,} etc.) is adsorbed on negatively charged sites of clay minerals and organic matter, and anion uranyl-complex [UO ₂ (CO ₃ ) ₃ ^{4 -} , UO ₂ (OH) ₃ ^-, etc.] are known to be adsorbed on the positively charged sites of iron oxide. The uranyl-complex adsorbed on iron oxide also appears as co-precipitated form with iron oxide. U-oxide, phosphate, silicate, and U-containing iron oxide are produced in the form of fine particles included in the surface coating or granulation of silicate minerals. Organic matter and clay minerals that have absorbed U are produced as cementing materials for aggregates that bind silt and sand particles to form aggregates.

In addition, hexavalent uranium minerals, uranyl oxy-hydroxide, silicates, phosphates, and arsenates have very low solubility and exist in abundance in the form of surface coatings of silicate minerals. Among uranyl minerals, UO ₃ , U ₃ O ₈ , oxy-hydroxide, silicate, phosphate, and arsenate are known to have very low solubility. On the other hand, uranyl carbonate and uranyl sulfate are easily soluble in water. The specific gravity of uranyl minerals with low solubility is known to be 3~5 g/cm ³ . It has a lower specific gravity than tetravalent uranium minerals, but a higher specific gravity than common silicate minerals.

Hereinafter, a method for restoring uranium-contaminated soil according to an embodiment of the present invention will be described in more detail with reference to the accompanying drawings. 2 is a schematic flowchart of a method for restoring uranium-contaminated soil according to an example of the present invention.

Referring to FIG. 2, in the present invention, first, uranium-contaminated soil is reacted with an alkali solution. In this example, the alkaline solution uses NaOH aqueous solution.

When the contaminated soil reacts with NaOH aqueous solution, the soil pH rises. When the soil pH rises, the surface charge of minerals with variable charge (pH dependent charge) among soil particles changes to negative charge, and the entire soil particle becomes negatively charged. Therefore, soil particles are dispersed by mutual repulsive force, and anion U-complex is detached from soil particles by electrical repulsive force. In addition, when a strong alkaline environment is created by the aqueous sodium hydroxide solution, the surface of silicate minerals, which are the main constituent minerals of the soil, is dissolved, and uranium minerals coated on the surface of the silicate minerals and iron oxide containing uranium are separated from the silicate minerals. In addition, as the organic matter in the soil is dissolved in an alkaline environment, uranium adsorbed to the organic matter or uranium formed in a complex with the organic matter is separated.

In other words, when uranium-contaminated soil is treated with NaOH aqueous solution, soil particles are dispersed, anion U-complex is desorbed from the soil, uranium-containing iron oxide coated on the surface of silicate minerals and uranium minerals are separated, organic matter is melted, and adsorbed uranium is separated. Separation takes place from the agglomerates of fine mineral grains containing uranium.

The NaOH aqueous solution treatment method reacts the contaminated soil with the NaOH aqueous solution at a weight ratio of 1: 3-10. The concentration of NaOH in the aqueous solution is appropriately 1 - 5M. The higher the content of clay in the soil, the higher the proportion of the aqueous solution, and the higher the content of iron oxide and organic matter, the higher the NaOH concentration.

After reacting NaOH aqueous solution and contaminated soil as above, ultrasonic waves are irradiated on the NaOH-contaminated soil suspension and attrition scrubbing is performed. When ultrasonic waves are injected into a soil suspension, cavitation-collapse occurs, which causes liquid jet impact on soil particles. This causes desorption of soil organic matter and desorption of fine particles coated on the surface of large particles. The amount of ultrasonic injection is 200 - 1500 J/ml, and the higher the content of clay, iron oxide and organic matter in the soil, the higher the ultrasonic scanning energy.

Attrition scrubbing refers to the process of generating vortices in the soil suspension so that the soil particles come into contact and rub against each other. Attrition scrubbing causes disintegration of soil aggregates, dispersal of soil particles, and particle fragmentation. A vortex may be generated in the suspension using a known vortex generator. Ultrasonic irradiation and attrition scrubbing may proceed simultaneously as in this example, but may proceed sequentially in another example.

As described above, when the reaction between the contaminated soil and the NaOH aqueous solution, ultrasonication of the soil suspension, and attrition scrubbing are performed, the aggregated soil aggregates are disintegrated and separated into individual soil particles, and the soil particles are dispersed. In addition, as the surface of silicate minerals melts, minerals including uranium and iron oxide are separated. As the organic matter is dissolved, uranium adsorbed or forming a complex with the organic matter is separated. In other words, uranium is not removed from the soil through the above process, but among minerals including uranium, forms that are weakly bound to other minerals or organic matter are separated individually.

Now, through the solid-liquid separation, the contaminated soil and aqueous solution are separated from each other, and particle size separation is performed. For particle size separation, a sieve or a cyclone may be used. Through particle size separation, contaminated soil is separated into three groups: coarse particles, medium particles, and fine particles. The particle size standards may vary depending on the conditions of the contaminated soil. In this embodiment, the standards are set so that fine particles are less than 50 μm, medium particles are in the range of 50 to 250 μm, and coarse particles are greater than 250 μm. That is, fine particles are 'silt & clay', medium particles are 'fine sand', and coarse particles are 'medium sand'. Minerals containing uranium, such as iron oxide and clay minerals, are mainly concentrated in thin particles and relatively less in coarse particles. Accordingly, subsequent processing is performed for each group through particle size separation.

For coarse particles, liberation is performed. Coarse particles are highly likely to contain two or more minerals combined. When uranium minerals are included in soil particles composed of heterogeneous minerals, efficiency is reduced in selectively separating only minerals including uranium by physical and chemical methods. Through group separation, multi-mineral particles are pulverized and separated into single mineral particles. Of course, if uranium is not included in the coarse particles through sample investigation of the coarse particles, the coarse particles can be immediately recycled without group separation. In addition, even if the coarse particles are not multi-minerals, separation of groups is not required. In other words, group separation is selectively applied.

After group separation, carbonate washing is performed on coarse and medium particles. Carbonate washing removes uranyl ions (UO ₂ ²⁺ ) from the soil. Uranyl (UO ₂ ²⁺ ) ions tend to form stable complexes with CO ₃ ^2- in neutral-alkaline environments. That is, when a mineral (solid) containing uranium and a carbonate are reacted as shown in the following reaction equations (1) and (2), the uranyl ion dissolves in an aqueous solution while forming a uranyl-carbonate complex with carbonic acid. The concentration of tetravalent uranium in contaminated soil can be reduced by making uranyl ions in a complex form and eluting them into an aqueous carbonate solution.

Ca(UO ₂ ) ₂ (PO ₄ ) _2(s) + 6CO ₃ ^2- --> 2UO ₂ (CO ₃ ) ₃ ^4- + Ca ²⁺ + 2PO ₄ ^3- ... (1)

Ca(UO ₂ ) ₂ (PO ₄ ) _2(s) + 6HCO ₃ ^2- --> 2UO ₂ (CO ₃ ) ₃ ^4- + Ca ²⁺ + 2PO ₄ ^3- + 6H ⁺ ...(2)

The carbonate used for washing is Na ₂ CO ₃ or NaHCO ₃ , and the carbonate concentration of the carbonate aqueous solution is 0.3 - 1.5M, and the solid-liquid weight ratio of 1: 3 to 10 (soil: aqueous solution) can be reacted for 6 to 24 hours.

After the carbonate washing is completed, the coarse and medium particles can be returned to the soil after analyzing the uranium concentration through sampling and meeting the standard value.

For the group of fine particles of 50 μm or less separated through particle size separation, magnetic separation and specific gravity separation are sequentially performed. First, magnetic separation is performed. As described above, uanyl-complex anions are often adsorbed or co-precipitated on the iron oxide surface. Iron oxide in the contaminated soil is separated from the silicate mineral surface through preceding processes (NaOH treatment, ultrasonic irradiation, and attrition scrubbing). In this way, in a state where iron oxide exists independently from other minerals in the contaminated soil, iron oxide having paramagnetic properties can be separately separated from the contaminated soil through magnetic separation. The concentration of uranium in the soil can be reduced by removing iron oxide containing uranium from the soil. The magnetic separator may use a wet high gradient magnetic separation.

After magnetic separation, gravity separation is carried out. Because uranium has a high specific gravity, minerals containing uranium have a higher specific gravity than normal silicate minerals. For example, the specific gravity of tetravalent uranium minerals is in the range of 5-11g/cm ³ , and silicate, phosphate, oxy-hydroxide, vanadate, and arsenate minerals containing hexavalent uranium have specific gravity in the range of 3-5g/cm ³ . The specific gravity of silicate minerals is relatively low in the range of 2.6 g/cm ³ . Therefore, when the specific gravity separation is performed based on the specific gravity of 3-4 mg/cm ³ , minerals containing uranium and general silicate minerals not containing uranium can be separated from each other. For the specific gravity separation, a device such as a cyclone or a Mozley MGS (Multi-Gravity Separator) may be used. By separating soil particles containing uranium from a group of fine particles through specific gravity separation, the concentration of uranium in soil can be reduced.

Iron oxides separated in magnetic separation and high specific gravity products separated through particle size screening all contain uranium, so uranium is removed through a subsequent process or disposed of as radioactive waste.

In particle size separation, it is preferred to carry out a carbonate wash for low specific gravity products. Although iron oxides and high-density products have relatively high uranium concentrations, low-density products, which consist mainly of fine particles, also contain high levels of uranium. The carbonate washing may be performed in the same manner as previously performed for the coarse and medium particles. However, it is preferable to increase the concentration of the carbonate solution because fine particles have a relatively high contamination concentration compared to coarse particles. Among the fine particles, uranium can be removed below the standard value for the remaining low specific gravity products except for iron oxide and high specific gravity products by carbonate washing alone.

Finally, the NaOH aqueous solution and carbonate solution used in the treatment of contaminated soil contain uranium, so wastewater treatment is performed by using drying and adsorption methods among wastewater treatment methods.

As described above, in the present invention, the soil in the aggregate state is dismantled into individual particles using reaction with a basic solution for uranium-contaminated soil, ultrasonic irradiation, and ATTRITION SCRUBBING, and uranium-containing minerals weakly bound to other minerals are mutually separated. separated out Thereafter, fine particles having a high contamination concentration and coarse particles and medium particles having a low contamination concentration are separately separated through particle size separation. For coarse and medium particles, uranium is removed by eluting uranyl ions in the contaminated soil with a carbonate solution through carbonate washing, a chemical method. For medium and coarse particles, the contamination concentration can be reduced below the standard value through carbonate washing. However, for fine particles with a high concentration of contamination, particles containing uranium are separately separated through physical methods such as magnetic separation and specific gravity separation prior to chemical methods. Magnetic and specific gravity separation physically separate soil particles bound to uranium. By removing particles containing uranium, the concentration of uranium in the entire contaminated soil can be reduced. After removing iron oxide and high specific gravity products, uranium is removed from the soil by eluting uranyl ions into an aqueous solution through carbonate washing for low specific gravity products among fine particles.

That is, in the present invention, uranium is melted and removed through carbonate washing, and fine soil particles containing uranium are separately separated through magnetic separation and specific gravity separation. Through the above process, the concentration of uranium-contaminated soil can be reduced below the standard value.

The protection scope of the present invention is not limited to the description and expression of the embodiments explicitly described above. In addition, it is added once again that the scope of protection of the present invention cannot be limited due to obvious changes or substitutions in the technical field to which the present invention belongs.

Claims (8)

(a) dispersing soil particles by mixing uranium-contaminated soil with an alkaline solution;
(b) sizing the uranium-contaminated soil into three groups: coarse particles, medium particles, and fine particles;
(c) removing uranium from the soil particles by supplying an aqueous carbonate solution to the coarse and medium particle groups to wash them;
(d) separating magnetic particles including iron oxide to which uranium is bound from the groups of thin particles through magnetic separation of the groups of thin particles; and
(e) separating the magnetic particles into high specific gravity particles and low specific gravity particles by performing specific gravity screening on the group of thin particles from which the magnetic particles are separated; According to claim 1,
The uranium-contaminated soil restoration method further comprising the step of irradiating ultrasonic waves to the contaminated soil in a suspension state in which the contaminated soil is mixed with an aqueous alkali solution. According to claim 1,
The method of restoring uranium-contaminated soil, characterized in that it further comprises the step of generating a vortex in the suspension in which the contaminated soil is mixed with an alkaline aqueous solution so that the soil particles rub against each other. According to claim 1,
The method of restoring uranium-contaminated soil, characterized in that it further comprises the step of eluting and removing uranium from the low specific gravity soil particles by supplying and washing the carbonate solution to the low specific gravity particles separated in the specific gravity screening. According to claim 1,
The method of restoring uranium-contaminated soil, characterized in that it further comprises a group separation step of pulverizing the coarse particles to separate the different types of minerals constituting the coarse particles from each other. According to claim 1,
The coarse particles are greater than 250 μm, the medium particles are 50 to 250 μm, and the fine particles are uranium-contaminated soil restoration method, characterized in that less than 50 μm. According to claim 1,
The alkaline solution is a 1-5M NaOH aqueous solution,
The uranium-contaminated soil restoration method, characterized in that the contaminated soil and the NaOH aqueous solution are mixed in a weight ratio in the range of 1: 3 to 10. According to claim 1,
A method for restoring uranium-contaminated soil, characterized in that for separating high specific gravity particles and low specific gravity particles based on specific gravity 3-4mg / cm ³ during the specific gravity screening.

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