KR19990050832A - Coagulation Method and Coagulant for Boric Acid and / or Borate Solution - Google Patents

Abstract

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

Description

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

Solutions containing boric acid and / or borates are mainly produced during the operation of a pressurized water nuclear power plant. Because these solutions are radioactive, coagulation is required to convert them into chemically and physically stable solids 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. Among the three methods, cement coagulation has the lowest coagulation volume efficiency, and consequently its operation is the simplest and cement coagulants are generally considered to have long term safety, but the cost for the final treatment of coagulants of radioactive waste is As it is calculated by volume, cement coagulation will gradually be replaced as the cost for final treatment increases daily. On the other hand, both plastic coagulation and bitumen coagulation use organic materials as coagulants. Both methods can achieve higher volumetric efficiency, but bitumen coagulants can be burned and of low strength, so foreign burned during bituminous coagulation. Many countries in Europe have already banned the use of bitumen coagulation laws, and in many other countries, bitumen coagulation systems were initially installed and are still in use, and are generally intended to be exported to developing countries. Few countries want to rebuild. It is almost certain that bitumen coagulation is increasingly excluded. As for the plastic coagulation method, its use remains a subject of debate; While newly installed systems are continually joining, the negative view is that plastics are waste-prone, because plastics are susceptible to ageing and the history of plastic use by humans has only lasted about 50 years. It is thought that it is impossible to confirm that the quality of the coagulation bodies is still stable for more than 300 years and does 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 the volume efficiency of solidification with inorganic coagulants, which can reduce the final processing cost to an appropriate level. Otherwise, due to the burden of the final processing cost, it can be expected that plastic coagulation methods with excellent volumetric efficiency at solidification will continue to be adopted. At present, the solidification volume efficiency of the inorganic coagulant is based on the fact that the quality of the inorganic coagulant can ensure long-term stability and that the inorganic coagulant method has advantages in terms of volume efficiency if the volume of the coagulant is reduced. Research to improve is the main direction of recent studies on solidification of low radioactive wastes.

Conventional cement solidification techniques are also an inorganic solidification method. When this method is used for the coagulation of borate wastes, boric acid is generally adjusted to basic with sodium hydroxide, and after boric acid is concentrated to a solution containing 21,000 ppm, lime and cement are added thereto and the solution mixed well. Later, they move to rest and solidify. Since there is an interfering effect on the cement hydration hardening by boric acid, the content 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 solidification method, which is not improved, is generally suitable not to exceed 5% by weight, otherwise the grade may be a problem. The addition of lime is an improvement associated with conventional cement coagulation, which helps to improve the volumetric efficiency of coagulation by allowing boric acid to make insoluble calcium borate crystals, thereby avoiding disturbing the hydration hardening action of the cement. 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 wastes, and solutions at 40 to 60 ° C are shaken for about 10 hours. It is aged with calcium borate and grown into crystals. Next, the solution is filtered to obtain calcium borate crystals, and finally the calcium borate crystals are coagulated with cement. By this method, 190 gallons (gal) of liquid borate waste containing 21,000 ppm of boron can be solidified into 55 gallon barrels of coagulant. Although the volumetric efficiency of solidification has been shown to be significantly improved compared to conventional methods, the equipment investment is also generally high, slow to operate and the process is slightly complex.

In addition, there are many methods for the solidification 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 used to make a concentrated suspension slurry comprising barium borate precipitate. It is neutralized with alkalizer barite with precipitation effect. 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 formation of a suspension of barium borate precipitate and finally solidified into cement and bitumen emulsions. 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 process disclosed in US Pat. No. 4,210,619, lime is added to a solution comprising 11% boric acid, and after the boric acid is converted to insoluble calcium borate, 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 in the next step after filtration separation of the calcium borate into cement to obtain a higher solidification volume efficiency than in US Pat. No. 4,210,619. Solidifies. 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 that 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 high concentration of 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 coagulants is 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. Patent No. 4,906, 408 discloses a method of solidifying liquid borate wastes and waste resins comprising boric acid, which according to this method is not satisfactory which occurs between borate and cement or water causing expansion and cracking in the coagulum. It is emphasized that boric acid is converted to calcium boroettringite and calcium monoboroaluminate to avoid unsuccessful reactions. 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. Therefore it can be imagined 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. ; (1) The added alkaline precipitant essentially increases the amount of waste, and (2) the borate salts are still considered waste that needs to be embedded, so the weight percent of borate in the coagulation body is subject to significant limitations. 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, the borate salt itself is no longer a waste to be embedded, but a reactant upon solidification. Since boric acid can effectively participate in coagulation, boric acid must be in a dissolved state, and thus, in view of the need for the quality of the coagulants, boric acid may be present in a solution of insoluble boride, but in the solution, boric acid dissolved in a solution has a certain concentration. You must keep it. Thus, the borides are preferably in the salt form of high solubility, in which the most suitable form is sodium borate and other highly water soluble borate salts. For example 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 precipitation in the boride.

Boric acid is a neutral water soluble crystal, and liquid borate wastes produced at nuclear power plants are generally alkaline controlled with sodium hydroxide. From the solution, sodium hydroxide and boric acid were added to xNa ₂ O. yB ₂ O ₃ . It can be produced from various compounds of the zH ₂ O family, for example Na ₂ O. B ₂ O ₃ . 4H ₂ O (sodium metaborate); Na ₂ O. 2B ₂ O ₃ . 4H ₂ O. Na ₂ O. 2B ₂ O ₃ . 5H ₂ O and Na ₂ O. 2B ₂ O ₃ . 10H ₂ O (disodium tetraborate); NaB ₅ O ₈ . 5H ₂ O (sodium pentaborate); And NA ₂ O. 4B ₂ O ₃ . 4H ₂ O (disodium octaborate). Sodium borate in aqueous solutions varies considerably in chemical form, therefore, for convenience, the ppm concentration of boron in the indicator solution is used. 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. Basically, as for the sodium borate solution, the level of the pH value represents the level of the molar ratio of sodium to boron in the solution; The higher the molar ratio of sodium to boron, the higher the pH value will be. Experimental results show that sodium borate has high solubility when the pH is between 7-9, and the dissolved boron content reaches higher than 135,000 ppm in solution at 40 ℃ when the pH is between 7-8. Show that you can. This superhigh level of solubility is mainly obtained as the borate forms a very stable transient supersaturated solution. When the molar ratio of sodium to boron is very high, the concentration of dissolved boron drops significantly. It has been found by the present invention that when the molar ratio of sodium to boron is high, the concentration of dissolved boron is effectively increased by controlling the pH value to fall with phosphoric acid.

In addition, it is also possible to significantly increase the concentration of dissolved boron by raising the temperature of the solution; However, the higher the temperature, the faster the rate of the curing reaction may result, for example, inadequate mixing time or excessive high temperature progression. However, after mixing, if the solution is adequately cooled, the temperature can be raised further, but when the curing agent is added, it is most preferred that the temperature of this solution is 100 ° C or lower.

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. The experimental results show that in 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. The viscosity of the solution shows a linear direct proportionality only at low concentrations, and when the boron concentration reaches 80,000 ppm, the viscosity begins to increase rapidly, and after reaching about 100,000 ppm, the viscosity increases with higher concentrations. The higher the faster you can show that the tendency to polymerize is stronger. Experiments of the present invention demonstrate that this polymerization has a very important effect on the quality of the cured product of sodium borate. It has been found that when the borate solution has a higher concentration, borate salts with higher degree of polymerization are produced, and the strength of the coagulation product is also higher when the borate salt with higher degree of polymerization is reacted with the coagulant of the present invention. It consists of a very useful and excellent recycling which provides a method according to the invention which can achieve both ultrahigh volumetric efficiency and ultrahigh quality of coagulation body, which is a feature of the invention. A method of solidifying wastes into curable slurries prepared by uniformly mixing several additives with cementitious materials, volcanic ash materials and high concentrations of borate solutions is disclosed in US Pat. No. 5,457,262. In the present invention, more suitable materials are disclosed as coagulants, which further improves the quality of the coagulants of the process.

Based on the experiments of the present invention, materials suitable as coagulants for the high concentrations of borates described above, in addition to the cementitious materials, volcanic ash materials and indicator additives disclosed in this patent, can be used to make boric acid or borate to make insoluble or almost insoluble solids. It was found to contain all other substances that could react with it. All of these can be used as a coagulant.

However, given the quality that the coagulation product must have, the material for the coagulant can provide the coagulant with significant compressive strength, water resistance and durability, and can provide a fine and dense coagulum structure with few small pores. And a substance capable of preventing the release of moisture is preferable. As a result of the experiment, among such materials, salts, phosphates and carbonates or mixed salts of metallic silicates, as well as oxides and hydroxides of divalent or higher metals, have been found to be most suitable. In the choice of materials, the structural stability of the coagulation products formed by these materials together with boric acid or borate salts and the thermal effects during solidification must be taken into account. Ideal coagulation products should have minimal expansion or contractility; On the other hand, the lower the release of heat, the more favorable the solid reaction will be.

When the substance of the coagulant is used alone, it also has a coagulation effect. Generally speaking, however, it is generally suitable to use complex coagulants produced in compositions from other materials, in which case the coagulation products are all of good quality. For example, the 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 large, 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. If used excessively, the coagulum is prone to cracking. Again, for example, when silica is used as the coagulant material, the release of heat in the coagulation reaction is relatively low, but the compressive strength of the coagulum 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 directly coagulate with boric acid or its salts, and the use of certain materials enhances the solidification or contributes to the quality of other components than boric acid in solution wastes. It is intended to compensate for the lack of ingredients. For example, when the liquid waste is in the sodium salt state of boric acid, the sodium salt of the coagulant generally dissolves relatively easily after coagulation, resulting in unsatisfactory water resistance in the coagulum, thus improving to overcome this problem. It is necessary to take the usual means. 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 insoluble salts of sodium silicate to avoid dissolution of the sodium salt. 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 as the amount of coagulant used increases, the viscosity of the mixed slurry increases and the temperature of the heat also increases. Under these conditions, good mixing results in good coagulation. 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.

Next, the coagulation method and preparation of the coagulant according to the present invention are described by way of examples, which are not intended to limit the scope of the present invention, as the examples which are a part of the present invention do not represent the full scope of the present invention. It doesn't happen.

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 amounts. The two equal amounts were each separated twice again and each part was added in sequence to 600 parts by weight of shaken deionized water. The order of addition is as follows: sodium hydroxide-boric acid-sodium hydroxide-boric acid. The mixed solution was slightly heated to allow complete dissolution of boric acid until the sodium hydroxide was completely dissolved. The dissolved boron concentration of the obtained solution 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 use. Before adding the coagulant, the solution was reweighed and replenished with water at the same temperature in order to determine the weight lost by evaporation of water in the preparation method.

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 outer strand carbon fiber were mixed, homogenized and ground to a coagulant powder. This coagulant powder was then added gradually to the boric acid solution prepared for use and vigorously stirred to coagulate the coagulant powder with the solution to form a homogeneous slurry. The weight ratio of coagulant to waste solution was 0.4. Ten minutes after stopping the stirring, the coagulant was completely added and the slurry was poured into a cylindrical polyethylene plastic model having an inner diameter of 5 cm and a height of 11 cm and left at room temperature. After 30 days of coagulation, the mold was demolded to obtain five samples and cut into 10 cm long cylindrical specimens. The specimens were again tested for compressive strength under the ASTM C39 method according to the US Nuclear Regulatory Commission's 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 and the molar ratio of sodium: boron dissolved in the solution 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 storing the coagulated body for 8 months was 105 kg / cm ² , and the compressive strength of domestic water 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 purified powder of BaSiO ₃ was used as a coagulant, and coagulation was carried out at 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 low to 85% phosphoric acid. Similar liquid borate wastes produced were determined to contain 77,728 ppm 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; 2) concentrating the solution such 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 it homogeneously with the solution to prepare 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. A method according to any one of claims 1, 2 or 3, wherein the complex of metal oxides in the coagulant component is a cement-base material, portland cement, furnace slack or fly ash. 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.