KR100304731B1 - Coagulation Methods and Coagulants for Boric Acid and / or Borate Solutions - Google Patents

Abstract

A unique method of solidifying a solution comprising boric acid and / or borate salts is disclosed herein. Boron species are polymerized in the solution to form polyborate, and then the solution is solidified by mixing a coagulant prepared from a completely inorganic material with the solution. Therefore, the solid form prepared by the method of the present invention is free from aging problems. Boron species in solution are not just encapsulated or embedded waste, but participate in the coagulation reaction and become a major part of the total reactant. Thus, the total volume of the solid form produced in the present invention is 1/10 or less of the volume produced by conventional cementation.

Description

Coagulation Method and Coagulant for Boric Acid and / or Borate Solution

Solutions containing boric acid and / or borates are mainly produced during operation of a pressurized water nuclear power plant. Because these solutions are radioactive, they need to be converted into chemically and physically stable coagulants by coagulation to ensure nuclear safety. For the coagulation treatment of such radioactive solutions, three main methods recently used are cement solidification, plastic solidification and bitumen solidification. As cement coagulation has the lowest coagulation volume efficiency among the three methods, the cost for the final treatment of coagulants of radioactive wastes is consequently, although operation is the simplest and cement coagulations are generally considered to have long term safety. As it is calculated by volume, cement coagulation will gradually be replaced in countries where the cost for final treatment is increasing. On the other hand, both plastic coagulation and bitumen coagulation use organic materials as coagulants. Although higher volumetric efficiency can be obtained by the above two methods, bitumen coagulants are burnable and have a low strength, so in some cases they have been burned in foreign countries during the processing of bitumen coagulation. Many countries in Europe have already banned the use of bitumen coagulation laws and, with the exception of bitumen coagulation systems for export to relatively underdeveloped countries and bitumen coagulation systems that were initially installed and still in use in many other countries. The system is rarely retrofitted. It is almost certain that bitumen coagulation is increasingly excluded. As for the plastic coagulation method, it remains a subject of debate about its use; While new systems are being introduced continuously, the negative view is that plastics are susceptible to ageing, and humans have only been using plastic for about 50 years, so waste plastic The quality of the coagulants remains stable for more than 300 years and it cannot be confirmed that they will not change materially. Therefore, in many European countries, plastic coagulation is no longer used. In general, the use of plastic coagulation in the future is mainly related to whether volumetric efficiency can be increased in solidification by inorganic coagulants and the final treatment cost can be reduced to an acceptable level. Otherwise, due to the burden of the final processing cost, it can be expected that plastic coagulation methods with excellent volume efficiency in solidification will continue to be employed. Judging from the current state, based on the known research that the long-term stability can be ensured by the properties of the inorganic coagulant, the inorganic coagulation method can also be advantageous in terms of volume efficiency by the volume reduction of the coagulant. Research on increasing the coagulation volume efficiency of inorganic coagulants is currently the main direction of research on coagulation of low radioactive waste.

Conventional cement solidification techniques are also an inorganic solidification method. When this method is used for coagulation of borate wastes, boric acid is generally adjusted to basic with sodium hydroxide, and boric acid is concentrated to a solution containing 21,000 ppm, after which lime and cement are added thereto and the solution Mix thoroughly to let it solidify. Since there is an interfering effect on cement hydration hardening by boric acid, the amount of borate waste added to the cement slurry should not be excessive. In addition, the content of boric acid in the borate waste coagulum produced by the conventional non-improved coagulation method is generally suitable not to exceed 5% by weight, otherwise it may be a problem on the grade. An improvement on the conventional cement coagulation method is the addition of lime, which helps boric acid to form insoluble calcium borate crystals, thereby avoiding obstruction of the hydration hardening action of the cement, thereby helping to improve the bulk efficiency of solidification. This concept is used precisely in the so-called advanced cement coagulation method developed by the Japanese company JGC Cooperation, where lime is first added to liquid borate waste and the solution is stirred at 40-60 ° C. for about 10 hours. It is aged with calcium borate and grown into crystals. The solution is then filtered to obtain calcium borate crystals and finally the calcium borate crystals are coagulated with cement. By this method, 190 gallons of liquid borate waste comprising 21,000 ppm of boron can be solidified into 55 gallons of coagulation barrel. While the volumetric efficiency of solidification has been shown to be significantly improved compared to conventional methods, the cost of facility investment is also relatively high, while the operation is slow and the process is somewhat complex.

In addition, there are many methods for the coagulation of borate wastes, which are carried out as inorganic coagulants. For example, in US Pat. No. 4,293,437 or French Patent FR-A-2,423,035, the borate solution is an alkaliizer barite having a precipitation effect. baryta) to make a concentrated suspension slurry comprising barium borate precipitate. After further addition of the alkaline silicate, which acts as a suspending agent, the cement and bitumen emulsion are finally added to the suspension slurry to solidify the slurry. In this process, the boron content is increased by the production of a suspension of barium borate precipitate and finally solidified with cement and bitumen emulsion. The final coagulation product of this process includes 233 g / l borate and the coagulation volume efficiency is higher than that of conventional cement coagulation.

In the method disclosed in US Pat. No. 4,210,619, lime is added to a solution comprising 11% boric acid, and boric acid is converted to insoluble calcium borate, then cement is added to the resulting slurry and mixed for coagulation. In US Pat. No. 4,800,042, lime is also added to the borate solution to convert boric acid to calcium borate, and as a further step, to obtain a higher coagulation volume efficiency than in US Pat. No. 4,210,619 in the next step after filtration separation of the calcium borate. So that it solidifies with cement. The principle of this process is exactly the same as the advanced cement solidification method of Japan JGC.

Next, in US Pat. No. 4,620,947, magnesium oxide powder or magnesium hydroxide powder is first added to a borate solution to make magnesium borate, and then cement is added thereto to shake the mixture. Before the final colloid is produced, calcium oxide or calcium hydroxide is added for coagulation. According to the conditions used in this patent, the concentration of boric acid in the liquid waste is about 10% by weight, and the weight of added lime, cement, magnesium hydroxide and calcium oxide is several times the weight of boric acid. Therefore, the volumetric efficiency is very low, and the compressive strength of the resulting coagulation bodies is also very low, with a maximum value of only 22.5 kg / cm ² .

In contrast, US Pat. No. 4,664,895 discloses the solidification of liquid borate waste by adding sodium metasilicate to a highly concentrated borate solution. The boric acid concentration used in this process is high enough to exceed 30% by weight of the liquid waste, so that the process can generally achieve high volumetric efficiency. However, the compressive strength of the coagulation bodies is only between 500 psi and 700 psi (35-49 kg / cm ² ), which is not high enough. Most importantly, the coagulum produced in this process is in the state of silicic acid and its water resistance is not satisfactory.

U.S. Pat. No. 4,906,408 discloses a method of solidifying liquid borate wastes comprising boric acid and waste resins, and according to this method the focus of the invention is between borate and cement or water causing expansion and cracking in the coagulum. In order to avoid any unsatisfactory reactions that occur, it is given to converting boric acid to calcium boroettringite and calcium monoboroaluminate. According to this method a very low concentration of borate solution is used, and also 1.75 times the volume of cement and silicon additive per unit volume of the borate solution. It is therefore easy to see that the solidification volume efficiency according to this method is also very low.

In the above prior art, most have adopted a technique of adding an alkaline precipitant to convert the borate into an insoluble boride, and then adding a coagulant cement or bitumen for solidification. For example, US Pat. No. 4,293,437 discloses the addition of alkaline barite to produce a suspension of barium borate precipitation; US Pat. Nos. 4,210,619, 4,800,042 and 4,906,408 disclose the addition of lime to convert borate into insoluble calcium borate; US Pat. No. 4,620,947 discloses the addition of magnesium oxide or magnesium hydroxide to make magnesium borate. However, from the point of view of the present invention, although there is an improvement in the solidification volume efficiency of liquid borate waste in these methods, such solidification methods cannot adequately coagulate the volumetric efficiency of boric acid for the following reasons. ; The weight percent of borate in the coagulant is liable to be significantly limited because (1) the added alkaline precipitant essentially increases the amount of waste, and (2) the borate salts are still regarded as waste that needs to be embedded. Therefore, the solidification volume efficiency cannot be greatly improved.

It is therefore an object of the present invention to provide a useful coagulation mechanism that is completely different from the methods described above.

In the present invention, borate itself is no longer just a waste to be embedded, but a solidification reactant. In order for boric acid to participate effectively in the coagulation, the boric acid must be in the dissolved state, and thus, in view of the quality requirements for the coagulant, boric acid in the solution can be present as an insoluble borides solution. If any, they must be maintained above a certain level. Thus, the borides are preferably in the salt form of a high solubility, and the most suitable form is sodium borate and other highly water soluble borate salts such as potassium borate, lithium borate and ammonium borate may also be used. Therefore, the aim for the solidification according to the invention is not limited to the form of sodium borate. Also, considering the use of additives, every effort should be made to avoid boride precipitation.

Boric acid is a neutral water soluble crystal, and liquid borate wastes produced at nuclear power plants are generally alkaline controlled by sodium hydroxide. Sodium hydroxide and boric acid from the solution may be, for example, Na ₂ B ₂ O ₄ .4H ₂ O (sodium metaborate); Na ₂ B ₄ O ₇ .4H ₂ O, Na ₂ B ₄ O ₇ .5H ₂ O, and Na ₂ B ₄ O ₇ .10H ₂ O (disodium tetraborate); NaB ₅ O ₈ . 5H ₂ O (sodium pentaborate); And Na ₂ B ₈ O ₁₃ .4H ₂ O (disodium octaborate). Sodium borate in aqueous solution varies considerably in chemical form, so for convenience, use the ppm concentration of boron in the indicator solution. In water, the solubility of sodium borate varies considerably with changes in chemical form and is affected by the manipulation and control of the pH value of the solution. In fact, the pH value is an important factor affecting the chemical form of sodium borate in solution, which can be adjusted by the addition of an acid solution or an alkali solution. Basically speaking with respect to sodium borate solution, the level of pH value represents the level of the molar ratio of sodium: boron in the solution. The higher the molar ratio of sodium to boron, the higher the pH value will be. Experimental results show that the solubility of sodium borate is high when the pH is between 7 and 9, and the boron content in the dissolved state can reach higher than 135,000 ppm in solution at 40 ° C when the pH is between 7 and 8. . This very high level of solubility is mainly obtained as the borate forms a very stable transient supersaturated solution. If the molar ratio of sodium to boron is too high, the concentration of dissolved boron drops significantly. It was also found by the present invention that the concentration of dissolved boron can be effectively increased by adjusting the pH value with phosphoric acid when the molar ratio of sodium: boron is high. Even so, the molar ratio of sodium to boron is preferably 1.2 or less.

In addition, the concentration of dissolved boron can be significantly increased by increasing the temperature of the solution. ; However, the higher the temperature, the faster the rate of the curing reaction, which can lead to drawbacks such as insufficient mixing time or too high a reaction temperature. Even so, if the solution can be mixed first and then cooled properly, the starting temperature of the solution can be higher, but even then the starting temperature of this solution is still most preferably 100 ° C. or lower when the curing agent is added.

In light of the findings of the present invention, high concentrations of borate solutions tend to polymerize, and as the concentration increases, the polymerization also increases. Experimental results show that in a sodium borate solution with a molar ratio of sodium to boron of 0.3028, the density of the solution and the sodium borate concentration in the solution maintain a linear direct proportional relationship from beginning to end. Viscosity of the solution shows a linear direct proportionality only at low concentrations, and when the boron concentration reaches 80,000 ppm, the viscosity starts to increase rapidly and distinctly, and after reaching about 100,000 ppm, the viscosity becomes much more rapidly, thus increasing the concentration. The higher is, the stronger the tendency to polymerize. Experiments of the present invention demonstrate that this polymerization action has a very important effect on the quality of the cured product of sodium borate. It was found that when the boron concentration of the borate solution is high, borate salts with higher degree of polymerization are produced, and when the borate salt with higher degree of polymerization reacts with the coagulant of the present invention, the strength of the coagulation product is also found to be higher. This is a feature of the present invention, which constitutes a very useful and excellent recycling process that allows the method of the present invention to achieve very high volumetric efficiency and quality of the coagulum. A method of solidifying waste with a curable slurry prepared by uniformly mixing a high concentration of borate solution with cementitious material, volcanic ash material and several additives is disclosed in US Pat. No. 5,457,262. More suitable coagulant materials will be disclosed in the present invention, which further improves the quality of the coagulant by the method of the present invention.

Based on the experiments of the present invention, materials suitable as coagulants for the high concentrations of borates described above include, in addition to the cementitious materials, volcanic ash materials, and designated additives disclosed in the patent, insoluble or poorly soluble solids by reaction with boric acid or borate salts. It found that all other materials that could form All of these can be used as coagulants.

However, considering the quality of the coagulation product, the coagulant material can provide the coagulant with excellent compressive strength, water resistance, and durability. The coagulum structure is fine and compact, so that the pore size is small and the number is small. It is desirable to have a substance that prevents the release of moisture. As a result of the experiment, among those materials, salts, phosphates and carbonates or mixed salts of metallic silicates, as well as oxides and hydroxides of divalent or more metals, have been found to be most suitable. In selecting the materials, the structural stability of the coagulation products formed by these materials together with boric acid or borate salts and the heating effect during solidification must be taken into account. Ideal coagulation products should have minimal expansion or contractility; On the other hand, the less heat released, the better the coagulation reaction will be.

Although the material of the coagulant is used alone, there is also a coagulation effect, but in general, it is relatively suitable to use a composite coagulant composed of a composition composed of different materials so that the coagulation products all have good quality. For example, a reaction between magnesium oxide and boric acid produces a coagulum with significant water resistance. However, when excessive magnesium oxide is used, the shrinkage of the coagulum becomes considerably larger and the coagulum becomes brittle. This is a disadvantage regarding stability in the structure of the coagulation body. Therefore, the amount of magnesium used should not be excessive and the coagulum is prone to cracking when used excessively. Again, for example, when silica is used as a coagulant material, relatively little heat is released in the coagulation reaction, but the compressive strength of the coagulant is low and the water resistance is also not satisfactory. Therefore, the amount of silica used should not be excessive. The materials used are not limited to those which can cause a coagulation reaction directly with boric acid or its salts, and certain materials used are other coagulants that enhance the quality or contribute to the quality of the components other than boric acid in the liquid waste. It is intended to compensate for the lack of ingredients. For example, when the liquid waste is in the sodium salt of boric acid, the sodium salt of the coagulant generally dissolves relatively easily after coagulation so that it has an unsatisfactory water resistance in the coagulum, thus improving measures to overcome this problem. It is necessary to take A viable method is to add an appropriate amount of silicic acid to bring sodium into the state of sodium silicate for reaction with other metallic oxides, hydroxides or salts and to form an insoluble salt of sodium silicate to prevent the sodium salt from dissolving. Oxides, hydroxides or salts of barium, zirconium and titanium are good coagulant components and can be used as fillers to enhance the stability of the active material or structure for the coagulant.

Experiments according to the invention demonstrate that the higher the amount of coagulant used, the higher the viscosity of the mixed slurry and the higher the heating temperature. If mixed well under these conditions, the coagulant is also excellent in quality. However, if the amount of coagulant used is excessive, problems in the mixing process will occur, and a homogeneous mixing effect will not be achieved, and heterogeneity will occur within the structure of the coagulum, which will be undesirable in terms of quality. Generally speaking, it is appropriate to use 0.7 kg or less coagulant per kg of solution, with 0.3 to 0.5 kg being most preferred.

Hereinafter, the coagulation method and the preparation of the coagulant according to the present invention will be described through Examples, which are examples of a part of the present invention and do not represent the full scope of the present invention. It is not limited.

Example 1

288 parts by weight of 95% sodium hydroxide and 1,400 parts by weight of 99% boric acid were each divided into two equal parts. The two equal parts were each divided twice again and each part was added sequentially to 600 parts by weight of distilled water with stirring. The order of addition is as follows: sodium hydroxide-boric acid-sodium hydroxide-boric acid. The mixed solution was heated slightly until the sodium hydroxide was completely dissolved so that the boric acid was completely dissolved. The dissolved boron concentration of the solution obtained as above was 105,943 ppm, and the molar ratio of sodium / boron was 0.3. After the boric acid had dissolved, the solution was kept stirring and cooled to 40 ° C. The solution was kept at this temperature for immediate use. Before adding the coagulant, the solution was reweighed and replenished with water at the same temperature to determine the weight lost by moisture evaporation during the preparation process.

16 parts by weight of Portland Type II cement manufactured by Taiwan Cement, 13 parts by weight of tribasic magnesium phosphate powder, and 0.4 parts by weight of stranded carbon fiber were mixed, homogenized and ground to form a coagulant powder. Then, the coagulant powder was gradually added to the boric acid solution prepared for use, and stirred vigorously so that the coagulant powder was mixed with the solution to form a homogeneous slurry. The weight ratio of coagulant to waste fluid was 0.4. Agitation was stopped 10 minutes after complete addition of the coagulant, and the slurry was immediately poured into a cylindrical polyethylene plastic model with an inner diameter of 5 cm and a height of 11 cm and left at room temperature. Thirty days after coagulation, the samples were demolded and cut into 10 cm long cylindrical specimens. The specimens were again tested for compressive strength by the ASTM C39 method according to the US Nuclear Regulatory Commission quality certification. From the experimental results, the average compressive strength of the five samples was 189 kg / cm ² .

Example 2

The borate solution and the coagulant were prepared in the same steps as in Example 1. The concentration of boron dissolved in the solution and the molar ratio of sodium: boron were also the same as in Example 1; However, the component of the coagulant was changed to 4 parts by weight of type 2A mud coagulant (for composition, see U.S. Patent 5,457,262) together with 1 part by weight of magnesium oxide, 1 part by weight of tribasic magnesium phosphate and 0.09 part by weight of outer strands of carbon fiber. The weight ratio of coagulant to liquid waste used was 0.3328. Demolded 7 days after coagulation and experiments were run similarly on the 5 samples. As a result, the compressive strength was 130 kg / cm ² .

Example 3

The borate solution and the coagulant were prepared in the same steps as in Example 1. The concentration of dissolved boron and the molar ratio of sodium: boron in the solution were the same as in Example 1; However, the coagulant component was changed to 15 parts by weight of Portland cement together with 3 parts by weight of fume silica, 7 parts by weight of silicon phosphate, and 0.4 parts by weight of carbon fiber. The weight ratio of coagulant to liquid waste used in coagulation was lower than 0.289. From the results, the compressive strength after the coagulation body was preserved for 8 months was 105 kg / cm ² and the domestic compressive strength was 93 kg / cm ² .

Example 4

The borate solution was prepared in the same steps as in Example 1, the concentration of boron in the solution was 120,000 ppm and the molar ratio of sodium: boron was 0.32. Thus, a fine powder of BaSiO ₃ was used as the coagulant, and coagulation was carried out in a ratio of 0.37 parts by weight of coagulant per each part by weight of borate solution. It was demolded 7 days after coagulation and the experiment was performed similarly on 5 samples. From the results, the compressive strength was 61 kg / cm ² .

Example 5

The borate solution was prepared in the same steps as in Example 1, but the molar ratio of sodium to boron was increased and the pH of the solution was adjusted to 85% phosphoric acid. The simulated liquid borate wastes produced were determined to contain 77,728 ppm of boron, a molar ratio of sodium to boron of 0.7, and 25,909 ppm of phosphoric acid (H ₃ PO ₄ ). The manufacturing process of the coagulant was also the same as in Example 1, and the composition was 6 parts by weight of magnesium oxide, 0.3 parts by weight of carbon fiber in the outer strand, and 13 parts by weight of Type IIA mud coagulant from Taiwan Cement. In coagulation, the weight ratio of coagulant to liquid waste was 0.2383. It demolded 30 days after coagulation and the experiment was performed similarly on 5 samples. From the results, the compressive strength was 193 kg / cm ² and the domestic compressive strength was 172 kg / cm ² .

The coagulation product produced by the method of the present invention has high strength, no problem of aging, and the coagulation volume efficiency is greatly improved as compared with the conventional method.

Claims (10)

A method of solidifying a solution containing boric acid and / or borate, comprising the following steps: 1) adjusting the pH of the solution to 7-10 by adding an acid solution or an alkaline solution; 2) concentrating the solution by evaporating water so that the water content of the solution is 30% or less, and all boron species remain soluble to promote the formation of high polymerization rates of polyborate; And 3) using a mixed powder of one or several kinds of oxides, hydroxides, salts or complexes of divalent or more metals as a coagulant, and mixing them homogeneously with the solution to produce a curable slurry. The method of claim 1 wherein the metal salt of the coagulant component is a barium salt, magnesium salt, silicate, phosphate, or carbonate. The method of claim 1 wherein the metal oxide, hydroxide or salt thereof among the coagulant components is an oxide, hydroxide or salt of calcium, silicon, barium, magnesium, aluminum, iron, titanium and zirconium. 4. The method of any one of claims 1 to 3, wherein the complex of metal oxides among the coagulant components is a cement-base material, a cementitious amorphous or volcanic ash material. The method according to claim 4, wherein the weight ratio of the coagulant and the borate solution is 0.7 or less. The method according to claim 4, wherein the starting temperature of the borate solution mixed with the coagulant powder is 100 ° C or less. The method according to claim 4, wherein the borate in the solution is sodium borate and the molar ratio of sodium: boron is 1.2 or less. 5. The method of claim 4 wherein sodium hydroxide or phosphoric acid is used to adjust the pH value of the solution. 3. The method of claim 2 wherein the phosphate or silicate in the coagulant component is silicic acid phosphoric acid. The method of claim 2 wherein the barium salt of the coagulant is barium silicate.