WO2012010917A1

WO2012010917A1 - Additive-containing aluminoborosilicate and process for producing the same

Info

Publication number: WO2012010917A1
Application number: PCT/HU2011/000075
Authority: WO
Inventors: Istvan Schremmer; Bernadett Ivett KOVÁCS; Zsolt Szitkai
Original assignee: Gic Ipari Szolgaltato Es Kereskedelmi Kft; G I C Ipari Szolgaltato es Kereskedelmi kft
Current assignee: Gic Ipari Szolgaltato Es Kereskedelmi Kft; G I C Ipari Szolgaltato es Kereskedelmi kft
Priority date: 2010-07-19
Filing date: 2011-07-15
Publication date: 2012-01-26
Anticipated expiration: 2013-01-19
Also published as: HU1000378D0; KR20170089042A; KR20130042569A; EP2596503A1; RU2013104040A

Abstract

The long-term (terminal) storage of low- and medium-activity nuclear waste is an extremely important environmental issue. A significant amount of these types of waste are constituted by thick liquid or crystalline materials containing boric acid or sodium borate. Our invention provides a simple solution to the problem of solidifying waste material. The process according to the invention is relatively cheap, involves low volume increase, and the resulting solidified form has advantageous characteristics. The invention relates to producing an additive-containing aluminoborosilicate at low temperature, preferably at room temperature from the following components: a liquid alkali silicate, liquid and/or solid boric acid or borate, an aluminium-containing inorganic component, and at least one additive. The process for producing the material comprises the steps of producing a homogeneous liquid from the liquid components, producing a solid powder from the solid components, and gradually admixing the powder component to the liquid component.

Description

Additive-containing aluminoborosilicate and process for producing the same

Technical field of the invention, fields of application

The invention relates to the production of additive-containing aluminoborosilicates at low temperature by mixing powder and liquid components.

The additive-containing aluminoborosilicate thus produced have exceedingly advantageous characteristics as structural materials and may be utilized in fields where high water and heat resistance, refractoriness, strength, impact resistance, and light weight are required. The fields of application of the invention can be divided into nuclear and non- nuclear applications.

Non-nuclear applications

• Producing fire- and heat-resistant coatings that are initially soft and harden after being applied to the surface to be protected, as well as producing refractory and heat resistant structural elements. Before setting (hardening) the material may be applied to combustible surfaces (such as wood, paper, and various plastics) by spreading or spraying. The method can be applied for producing fire safety doors, non-combustible, plastics-based heat insulation elements (e.g. polystyrene heat insulation blocks coated with additive-containing aluminoborosilicate), as well as fire- and heat-resistant wood and paper parts. These materials can be primarily utilized in the construction industry and in interior architecture.

• The materials may be utilized for producing fire resistant structural parts in a manner that during the setting process the composition that is transforming into an additive- containing aluminoborosilicate is mixed with small particles of suitable materials. Such materials are for instance polystyrene beads, crushed wood and paper, as well as sawdust. This method may be applied for producing fire- and heat-resistant construction elements.

Nuclear applications

• Due to their high boron content, additive-containing aluminoborosilicates are capable of capturing neutrons. The proposed aluminoborosilicate material is suitable for making neutron-capturing walls and structural elements.

• By adding either lead oxide or oxides of other high mass-number elements to the additive-containing aluminoborosilicate a gamma ray absorbing material can be obtained. • Highly radioactive, worn-out ion exchange resin, produced during the normal operation of nuclear power plants, may, also be embedded in additive-containing aluminoborosilicate polymers. Thereby, long-term protection against the release into the environment of radioactive isotopes bonded with the ion exchange resin can be achieved. · Low and medium-activity, boron-containing liquid and solid radioactive waste is produced in high amounts during the regular operation of pressurized water nuclear reactors, and also as a result of accidents. The current regime involves collecting the waste, and after optional chemical treatment evaporating it and storing it for a limited time (depending on the degree of evaporation) either in water-soluble crystalline form or as a thick liquid in facilities located on the premises of the power plant. Nuclear power plants have finite interim storage capacity, and therefore it is important to solve the problem of the long-term, safe storage of this type of waste. Nuclear waste must be stored for several hundreds of years as it contains isotopes that, though appear in low concentration, represent significant activity having a half-life of typically 30 years ( Cs). Radioactive waste can only be stored for a prolonged period of time in solid, water-insoluble form. The present invention may be applied for solidifying the above mentioned nuclear waste at room temperature with no or only minimal increase of the volume of the waste material relative to its initial state. The solidified waste form produced with the application of the present invention is heat and fire-resistant, with the radioactive isotopes being bonded in such a way that they are not released to a significant extent even when leached in water. The solidified waste produced with the application of the present invention can be safely stored for a prolonged period of time at underground radioactive waste disposal facilities, primarily in barrel containers. Of the above listed nuclear applications the most important one is the storage of radioactive waste.

Description of prior art

Non-nuclear applications

In general usage, as well as in scientific discourse, the term "borosilicate" is associated with borosilicate glasses. Borosilicate glasses are boron oxide-containing low thermal expansion glasses having a softening temperature of about 820 degrees Celsius. These glass types are produced at temperatures of 800-1000 degrees Celsius, and usually have technical applications, e.g. are applied for making laboratory equipment etc. To our knowledge, to date no one has produced an additive-containing aluminoborosilicate at low temperature.

Nuclear applications

Solidification of radioactive waste containing water-soluble boron compounds (boric acid or borates)

Cementation

One of the earliest solutions applied for solidifying radioactive waste is cementation. Cementation involves mixing the waste with cement, and, if necessary, water and additives, and storing the cement-embedded waste in barrel containers after setting. A major problem with the cementation of boron-containing wastes is that boron compounds inhibit the setting of the cement. To circumvent that, the waste must either be chemically pre-treated, large amounts of additives must be used, or special cement compositions must be applied. US patent application No. 20090156878 discloses a method applying a sulphoaluminate cement comprising gypsum, lime, and sand as additives. In case conventional cement is applied, the waste material containing boric acid becomes cementable only due to the application of special organic dispersants and set retardation agents (US PAT 4,504,317).

A common disadvantage of all methods involving cementation is that the cement and its additives should be applied in great quantities, which results in the 2.5-3 times volume increase of the solidified waste with respect to the initial waste volume. This is a huge disadvantage, as two-thirds of the capacity of the highly expensive nuclear waste storage facilities is used up for storing ballast materials. Another disadvantages of processes involving cementation are that isotopes may dissolve under the effect of water relatively easily from the cement-embedded waste, and also that above 800 degrees Celsius the cement breaks into pieces due to the volatilization of the water bonded therein. This latter characteristic of the cement also poses a problem for construction applications.

Embedding in plastic

Another conceivable solution for storing radioactive waste is embedding the waste in plastic. An obvious advantage of this solution is that the waste in its form embedded in the plastic cannot be dissolved by water at room temperature from the solidified material. This solution involves either mixing the waste with molten plastic or mixing the plastic monomers with the radioactive waste in containers, and producing the plastic later by stirring and adding a catalyst. The newly formed plastic embeds the waste as it sets. US patent No. 4,582,638 discloses such a method, which may generally be carried out utilizing many different sorts of plastic, additives, and catalyst.

However, solutions that involve embedding the waste in plastic have numerous disadvantages. On the one hand, usually only dewatered waste my be embedded in plastic, and thus concentrates containing boric acid have to be evaporated and crystallised in a first step, which is energy-demanding and therefore costly. On the other hand, the monomers of the applied plastics are usually toxic, and severe foaming may occur during the production process. The volume of the waste form obtained at the end of the process is usually double the volume of the original, crystalline radioactive waste. The product is also not fire-resistant and may melt when subjected to heat. As organic polymers, plastic are prone to degradation when subjected to nuclear radiation (similarly to the effect of UV radiation). It is therefore questionable whether the solidified waste can remain stable in the course of a few hundred years.

Embedding in paraffin

Probably one of the simplest solutions for making radioactive waste water-resistant is embedding it in paraffin. This is brought about by mixing the crystallized radioactive waste with molten paraffin (wax), and letting it cool and solidify. Such a method is disclosed in the document US 5,879,110.

Although it provides sufficient water-resistance, embedding in paraffin has a number of disadvantages. First, the volume of the waste increases to approx. 1.5 times the original waste volume during the process. Second, due to the characteristics of paraffin the solidified waste form is obviously not heat-resistant at all, which poses high risks in case of long-term storage. Similarly to the above solution, only dewatered waste may be embedded in paraffin, and therefore liquid waste solutions containing boric acid have to be crystallised first, which is energy-consuming and expensive.

Embedding in asphalt/bitumen

Embedding the waste in asphalt or bitumen is in many respects similar to embedding in paraffin. Asphalt and bitumen are more resistant than paraffin. Both materials are heat resistant up to about 180 degrees Celsius. For embedding radioactive waste in asphalt, several additives, among others olefinic hydrocarbons have to be applied to achieve sufficient moulding characteristics and to provide that the crystalline waste can be mixed well with the basically apolar, hydrocarbon-based asphalt (US patent No. 4,832,874). A similar solution is disclosed in the document US 4,252,667, with the notable difference of utilizing bitumen instead of asphalt.

Both solutions have the common drawback of high volume increase of the waste, with the volume of the product being usually double the volume of the original radioactive waste. The solidified waste form is heat resistant up to 180 degrees Celsius, which is not a particularly high temperature, and the application of organic additives (olefins) poses fire risks. In case boron-containing liquid waste solutions are solidified, their water content has to be removed previously to the application of these methods, which increases the costs of the process in a way similar to paraffin-embedding methods.

Vitrification producing borosilicate glass

Vitrification is usually applied for the ultimate disposal of high-activity solid radioactive waste, typically spent nuclear fuel material. Since conventional glasses are not particularly suitable for this application, heat resistant, high boron-content borosilicate glasses are usually utilized, which are also more resistant chemically. The process involves mixing the water-free, solid waste with molten, liquid glass at a temperature of 700-1000 degrees Celsius, and letting the mixture solidify, or mixing the waste to the components of the glass and heating the mixture to the temperature where glass is produced from the components. With the application of vitrification, highly resistant waste forms may be produced. Vitrified nuclear waste can be stored for a prolonged time without any significant risk.

Though initially vitrification was applied only for the disposal of small quantities of high-activity nuclear waste, some recent patents propose new methods for vitrifying borate- containing waste. United States patent No. 4,710,266 discloses a method where the glass is heated applying microwave energy. The document US 4,424,149 teaches that the waste to glass ratio may be 1 to 3 at best. In case of boric acid solutions, first the solution has to be neutralized, and vitrification may follow only after the solution is evaporated and crystallized Such a method is disclosed in the document US 4,595,528. Characteristics of the vitrified material thus produced may of course be controlled by additives. US patent No. 4,725,383 describes the effect of oxides utilized as additives, as well as the technology applied for vitrification.

Although a very stable form of radioactive waste may be produced through vitrification, in our opinion it cannot be regarded a satisfying solution for solidifying large amounts of boric acid-containing waste produced during the normal operation of nuclear power plants. All solutions involving vitrification have the following common disadvantages: high temperature is required (700-1000 degrees Celsius), consequently they are very costly, the radioactive waste needs to be dewatered completely before processing, and the volume of the waste increases significantly during the solidification process. The volume of vitrified waste is approximately three times the original volume of the radioactive waste.

Mixing the waste with sodium water glass

The prior art solution closest to our invention is disclosed in US patent No. 4,664,895 that describes a process for solidifying waste solutions containing boric acid or borate simply by adding sodium metasilicate (sodium water glass). The process according to the invention is applicable only in case the waste solution is in liquid state and has a boric acid concentration of at least 30 percent by weight. The solidification process comprises the steps of neutralizing the boric acid solution by adding sodium hydroxide, and adding sodium water glass and sulphuric acid to the solution thus prepared. No additives are applied during the process. According to the patent specification the solidified waste form has a compressive strength of 100-700 PSI (0,7-4,8 MPa), but even this compressive strength may only be achieved by applying sodium water glass. In case potassium silicate (potassium water glass) is applied, the compressive strength of the solidified waste is significantly lower.

It is easily seen that the process disclosed in US 4,664,895 is based on coagulating the silica from the water glass by an acid, primarily sulphuric acid. The waste material is embedded in, or solidified by the silica polymer coagulating under the effect of the acid. It is due to this chemical composition and material structure that, according to the specification, the compressive strength of the solidified waste form is only 0,7-4,8 MPa, a very low value compared to the compressive strength of stone-like materials. The solidified waste obviously also contains a high amount of water.

The compressive strength of the material according to the present invention is, however, always higher than 5 MPa, and may be as high as 20-30 MPa. Applying our method it is possible to make solid blocks not only from liquid solutions but also from crystalline borax. According to our method no acid is utilized, only aluminium-containing inorganic components and additives are applied during the solidification process. In our case the application of potassium water glass increases (rather than decreases) the compressive strength of the solidified waste. The end product of our process has low water content, and therefore the solidified waste is heat resistant up to 600-800 degrees Celsius.

These differences between the two processes result from the obvious fact that the chemical reactions occurring during the processes are completely different, which of course results in the solidified waste having different chemical composition and structure. According to the method described in US patent No. 4,664,895 the boron contained in the waste is embedded/encapsulated in a silica polymer produced from coagulated silica. Our invention, however, does not contain steps for coagulating silica. Our process, as it is described in detail below, concerns the production of a homogeneous, additive-containing aluminoborosilicate. As it is clearly seen from the above description, our solidification method is fundamentally different from the method disclosed in the patent specification US 4,664,895.

Definition of the task of the invention

The aim of the invention is to develop a process enabling the production of additive- containing aluminoborosilicates at low temperature, preferably at room temperature. The phrase "low temperature" is hereinafter used to refer to a temperature range of 0-120 degrees Celsius, while the phrase "room temperature" designates a temperature range of 20-25 degrees Celsius. The additive-containing aluminoborosilicates produced according to the present invention have the following advantageous characteristics:

• High mechanical stability, high compressive and bending strength

• High hardness and impact resistance

• Good fire and heat resistance

• Low density compared to stone, ceramics and metals

• Easy production. The material may be prepared by mixing powders and liquids, by casting or moulding, either in a batch or continuous process. The material is pliable immediately after mixing, but sets in a short time.

• Characteristics of the aluminoborosilicates produced at room temperature may be adjusted easily by the application of additives.

• In case radioactive waste is solidified, bonded radionuclides are not released to a significant extent even if the material is leached in water.

Objective of the invention

The objective of the invention is to provide an additive-containing aluminoborosilicate for nuclear and non-nuclear applications and a process for the production thereof that can be produced relatively simply and cheaply.

From the aspect of nuclear applications the main objective of the invention is to provide that radioactive waste solutions containing boric acid or borates, as well as thicker radioactive sludges, slurries or powders, can be solidified such that the solidified form is suitable for long-term (terminal) storage, and fulfils the following requirements:

• The materials added during solidification should not only encapsulate the boric acid- containing waste, but should react with it, thereby forming a homogeneous, resisting material.

• The volume of the solidified waste should be higher than the original volume of the boric acid-containing waste by maximum 0-15%.

• The solidified waste should not contain added organic materials and should not be inflammable.

• The added inorganic material content of the solidified waste form should be lower than 70% by mass, and more preferably around 50% by mass.

• The boron concentration of the solidified waste, expressed in boron trioxide, should be higher than 3% by mass, and preferably 6.5% by mass or higher.

• The aluminium concentration of the solidified waste, expressed in aluminium oxide, should be lower than 25% by mass, preferably around 7% by mass.

• The boron to aluminium molar ratio of the solidified waste should be higher than 0.2, and preferably around 2.

• The solidified waste should be heat resistant, not significantly losing its characteristic properties after a 600-degree Celsius heat treatment.

• The compressive strength of the solidified waste should be higher than 5 MPa on the 28th day after preparation.

• The hardness of the solidified waste, measured on the Mohs hardness scale, should be higher than 7 (the solidified waste should be able to scratch glass).

• The leachability index of the waste as determined utilizing the leachability test defined in US standard ANSI/ANS- 16.1-2003 should be higher than 6.

There is no point in producing high-quality solidified waste forms if the applied production technology is way too complicated or very energy-consuming (hence expensive). Only those processes for solidifying radioactive waste may become widespread in practice that are neither too complex nor too costly. The process to be applied for producing the solidified waste form should therefore fulfil the following requirements: The process should be able to be implemented at low temperature, preferably also at room temperature (at 20-25 degrees Celsius), basically implying that no external heating should be necessary.

It should not be necessary to crystallize the borate or boric acid-containing waste before solidification. The solidification process should be applicable to concentrates and boric acid solutions discharged from nuclear power plants that contain 330-400 g of dry solids (sodium borate) per litre.

The process material should not foam during preparation and mixing to an extent that would hinder the technology process.

The solidified waste form should be preparable by simple mixing, even in the barrel containers applied for its long-term (or terminal) storage.

Description of the inventive idea

Our invention is based on the recognition that alkali silicates, as alkaline substances, form aluminoborosilicates when they are reacted with borates and certain aluminium- containing inorganic components at low temperature. During the process, liquid alkali silicates and liquid or solid borates should be mixed with aluminium-containing inorganic components, and with additives applied in smaller quantities. The properties of the aluminoborosilicates thus synthesised may be adjusted beneficially by adding different amounts of inorganic materials to achieve the desired characteristics.

The alkali silicates may be produced from silicon dioxide and alkali hydroxides either before or during the mixing of the components. In most cases kaolin or kaolin pre-treated at high temperature may be applied as aluminium source. Applicable additives are Ca-silicate and other silicates, zeolite, diatomite, clay minerals and metal oxides (e.g. ZnO, CaO, MgO, Ti0₂, A1₂0₃, lead oxides, chromium oxides, manganese oxides, cobalt oxides, iron oxides, boron oxides). Before setting finishes, the additive-containing aluminoborosilicates thus produced may of course be mixed with different filler materials.

During the chemical reaction according to the invention, inorganic polymer-structure aluminoborosilicates and additive-containing aluminoborosilicates are produced. Compared to the various natural and synthetic aluminosilicates the main difference in the inorganic polymer structure is that in our invention the aluminium atoms are largely replaced by chemically similar boron atoms. The most important tenet of the process for producing a waste form suitable for safe long-term storage from a concentrate containing radioactive boron or from radioactive contaminated crystalline boric acid or borate is that the boron-containing waste is transformed to additive-containing aluminoborosilicate. Thereby, in the solidified waste the material added to the radioactive waste not only encapsulates the waste but chemically reacts with it and forms a relatively chemically resistant, homogeneous, pure or additive-containing aluminoborosilicate polymer. The newly formed material consists of inorganic, rather than organic, components. The chemical reaction producing the aluminoborosilicate material occurs in aqueous and aqueous gel phases. Our solution for the safe long-term storage of borate or boric acid-containing radioactive waste therefore consists in chemically transforming the waste at low temperature to stone-like additive-containing aluminoborosilicate blocks.

The generic solution fulfilling the objective set before the present invention is described in the independent claims. Preferred ways of carrying out the method according to the invention are described in the dependent claims.

The aluminoborosilicates according to the invention may be produced at low temperature, preferably at room temperature such that alkaline alkali silicates are reacted with boric acid and/or borates and with certain aluminium-containing inorganic components. To reach an optimum approximating the desired material characteristics, the properties of the aluminoborosilicates thus synthesised may preferably be adjusted by adding low quantities of inorganic additives. The material may be produced in a fibre-reinforced form, in which case solid fibre materials should be admixed to the prepared composition when it is still plastic.

The process for producing the material involves first producing a liquid component and a solid component in fine powder form, and then admixing at low temperature the powder component to the liquid component in small charges. Depending on the type and amount of the applied additives, the thick composition thereby obtained solidifies at room temperature in a time between 10 minutes and 1 hour, and continues to set slowly, with its compressive strength reaching its final value in 3 weeks. Setting time is shorter at temperatures higher than room temperature.

In case the process is applied for solidifying borate-containing nuclear waste, the boron content of the material comes from the nuclear waste. In the solidified waste form the radioactive waste is not only encapsulated by and embedded in the added material, but the added material chemically reacts with it and forms a new resistant, homogeneous material (additive-containing aluminoborosilicate) that contains the radioactive isotopes in chemically bonded form. The nuclear waste solidified in such a manner may be stored for a prolonged period of time at underground radioactive waste disposal facilities without releasing radioactive isotopes.

The material may be produced at low temperature, applying either manual or machine stirring. In case of industrial manufacturing either batch-type or continuous technologies may be applied. In case radioactive waste is processed, it is necessary to provide means for radiation shielding.

Additive-containing aluminoborosilicates may be produced at low temperature from the following basic components:

In case of all powder-form components, but especially in case of components containing silicon and aluminium, the grain size distribution of the powder is extremely important. As a general rule, it can be said that the smaller the grain size of the applied material, the better the mechanical properties of the additive-containing aluminoborosilicate will become.

In case of the aluminium source compound a grain size range of 1-1000 micrometres is suitable, but preferably materials with a grain size less than 90 micrometres should be applied. For silicon sources (e.g. silicon dioxide) the suitable grain size range is 0.1-10 micrometres, but preferably materials having a grain size of 0.1-5 micrometres should be applied to ensure that an additive-containing aluminoborosilicate with sufficiently good characteristics is obtained. In case of silicon dioxide the grain size should be so small as to enable the powder to be dissolved in 30-40% potassium- or sodium hydroxide solution at room temperature.

The properties of the aluminoborosilicate material may be adjusted by applying different additives. For additives a grain size range of 1-500 micrometres is suitable, but it is preferable to apply materials with a grain size of 10-90 micrometres. Based on their effective behaviour, additives may be divided into the following groups:

Additives for improving mechanical properties and water resistance

Zinc oxide (ZnO)

Magnesium oxide (MgO)

Manganese oxides, such as manganese dioxide (Mn0₂)

Calcium silicate minerals, calcium hydroxide

Cement

Additives for achieving special properties

Lead oxide: Provides shielding against gamma radiation, to be applied for making radiation shielding elements.

Barium sulphate: Provides shielding against gamma rays, to be applied for making radiation shielding elements.

Magnetite: Borosilicates containing added magnetite are attracted by magnet.

From the aspect of practical application those materials are also important which, if applied in low concentration (below 1% by mass) do not affect the properties of the additive- containing aluminoborosilicate but become chemically bonded in the additive-containing aluminoborosilicate polymer. Such elements are for instance cobalt and cesium. These elements have radioactive isotopes appearing in nuclear waste.

In many cases, especially in case of non-nuclear applications, it is expedient to mix the additive-containing aluminoborosilicate with filler materials before setting. Depending on the field of application, the applied amount of filler materials may be 0.01-5 times the amount of the additive-containing aluminoborosilicate. The applied filler material may be a mineral containing aluminium and silicon, including but not limited to:

Corundum

Perlite

Diatomite

Sand

Zeolite

Grog

Volcanic tuffs

Flue ash from power stations

It is usually expedient to apply materials with a grain size of 1-2000 micrometres as filler material.

Adjusting the setting time of the additive-containing aluminoborosilicate:

· The setting time may be most efficiently controlled by adjusting the ratio of the potassium and sodium concentration of the additive-containing

aluminoborosilicate. In case of high sodium to potassium molar ratio the material sets slowly, while in case of high potassium to sodium molar ratio it sets quickly. In the latter case the fully set additive-containing aluminoborosilicate material has better mechanical characteristics and improved water resistance.

• Setting time may also be controlled by additives, for instance adding titanium

dioxide or in certain cases calcium hydroxide increases setting time.

• The temperature at which the material is prepared by mixing may also affect

setting time: the material sets more quickly if a higher temperature is chosen.

The composition (expressed in oxides, given in percents by mass) of the additive-containing aluminoborosilicate preparable at low temperature can have the following concentration ranges per element: Elementary component Concentration Preferable range concentration (mass %) range

(mass %)

Boron (B₂0₃) 3-18 4-12

Aluminium (A1₂0₃) 5-30 6-21

Silicon (Si0₂) 20-40 26-37

Potassium (K₂0) 4-15 5-13

Sodium (Na₂0) 2-6 4-5

Hydrogen (H₂0) 15-45 25-40

Calcium (CaO), Zinc (ZnO), Magnesium (MgO),

Titanium (Ti0₂), Lead (PD3O4), 0-10 0-3 Manganese (Mn0₂), Iron (Fe₂0₃), Barium (BaO),

Chromium (Cr₂0₃), Cobalt (CoO), Cesium (Cs₂0),

The molar ratio of major components of the additive-containing aluminoborosilicate prepared at room temperature may be within the following ranges:

The elementary components oxygen (O) and hydrogen (H) occur in high proportion in the basic compounds utilized to produce the additive-containing aluminoborosilicate. However, since their quantity or proportion is not one of the best characterizing properties of the aluminoborosilicate, their concentration ratios are not specified.

The process for producing the additive-containing aluminoborosilicate involves first producing both a liquid component and a solid component in fine powder form, and then admixing at low temperature the powder component to the liquid component in small charges. For industrial-scale manufacturing, necessitated by the characteristics of the applied materials and the chemical reaction, the following special technological considerations have to be taken into account:

• Before mixing both the liquid and powder components should be separately homogenized.

In case of industrial manufacturing, machine mixing is necessary. For mixing the liquid and powder components a mixer suitable for mixing viscous substances, such as ribbon mixers, or kneading machines used in the food industry, should be applied

• The powder component should always be added to the liquid component in small charges.

The liquid component, however, cannot be added in small charges to the powder component.

• In case borate or boric acid-containing radioactive waste is solidified, the liquid component may be partially or entirely constituted by a thick liquid obtained by partially evaporating the aqueous radioactive waste solution. In this case it is not necessary to crystallize the waste material. The radioactive waste material may therefore constitute a portion of both the liquid and powder components.

• Setting of the material may be retarded by cooling the container in which the material is being mixed.

• The borosilicate material may be produced by casting or moulding. For moulding a granular, earth-damp material should be produced by the mixing process, with the final shape of the material being determined by the shape of the mould.

• In case the process is applied for solidifying radioactive waste, manufacturing equipment must possess radiation shielding.

• The manufacturing process can be batch-type or continuous. In a continuous-type process the material may be transported utilizing a transport screw. The manufacturing process can be automated.

Major process parameters of both the manual and the machine process are as follows:

Manufacturing parameter Parameter range

Temperature at which the additive- 0-120 degrees Celsius, but preferably

containing aluminoborosilicate is room temperature (20-25 deg Celsius)

produced

Pressure Normal atmospheric pressure

Mean mixing time 1-30 minutes Examples

Example 1

Manufacturing an additive-containing aluminoborosilicate material for non-nuclear applications

In Example 1 a process for making an additive-containing aluminoborosilicate at room temperature is described. The process involves first preparing a liquid component and a solid, fine powder component, and then admixing the powder to the liquid component in small charges.

Steps of producing the liquid component:

First, a Na-borate solution is prepared. 26 g of solid NaOH is added to 153.6 g of water, and then 38 g of crystalline boric acid (H3BO3) is dissolved in the solution. Thereby a solution with a mass of 220 g and volume of 200 cm³ is obtained. 43 g of KOH and a further 12 g of NaOH is dissolved in this solution to complete the production of the liquid component.

Producing the fine powder component:

256 g of Si0₂, 150 g of A1₂0₃, 26 g of B₂0₃, 3 g of Ti0₂, and 4 g of Ca(OH)₂ are mixed together.

The powder component is admixed to the liquid component in small charges, and the composition thus obtained is cast in mould and left to solidify at room temperature. The thick composition thereby obtained solidifies in under half an hour, and continues to set slowly, with its compressive strength reaching its final value in 3 weeks. The solidified material is extremely hard, to the effect that a sharp piece of the material can scratch glass.

Before it sets, further additives may be added to the material. To improve mechanical characteristics it may be beneficial to apply fibre reinforcement, involving the addition of fibres to the still plastic composition. The fibre reinforcement material may for instance be carbon fibre, basalt fibre, glass fibre, Kevlar fibre, or other fibres.

Example 2

Transforming a radioactive boric acid-containing concentrate into additive-containing aluminoborosilicate.

Example 2 illustrates the process of transforming a radioactive boric acid-containing concentrate into an additive-containing aluminoborosilicate polymer. Disregarding organic contaminants and isotopes present in small quantities (e.g. ⁶⁰Co, ¹³⁷Cs etc.) the composition of the radioactive boric acid-containing concentrate is 70% by mass water, 18,3% by mass boric acid and 11.7% by mass sodium hydroxide. The density of the concentrate is about 1.2 kg/litre. For solidifying 100 litres of boric acid-containing concentrates of similar composition in a manner that the solidified waste form fulfils the above described requirements, a fine powder material of the following composition is admixed to the liquid concentrate:

28 kg solid potassium hydroxide, 63 kg silicon dioxide, 23 kg kaolin, 4 kg zeolite, 4 kg Ca- silicate, 8 kg aluminium oxide, 2 kg magnesium oxide.

The thick composition thereby obtained solidifies in under half an hour, and continues to set slowly, with its compressive strength reaching its final value in 3 weeks. The solidified material is extremely hard, to the effect that a sharp piece of the material can scratch glass.

The volume of the solidified waste thus produced is only 5% higher than the volume of the original liquid radioactive waste solution.

For industrial application it is expedient to increase the quantity of the components such that the material may be mixed and later stored in 200-litre barrel containers usually applied for waste storage.

Example 3

Encapsulating radioactive contaminated crystalline borax in an additive-containing aluminoborosilicate.

Example 3 illustrates the application of the process for encapsulating radioactive contaminated crystalline borax in an additive-containing aluminoborosilicate polymer.

For the solidification of 1 kg of radioactive crystalline borax (Na₂B₄0₇ · 10H₂O) in a manner complying with the above detailed requirements the following process is applied: 0.44 kg of solid potassium hydroxide is dissolved in 0.7 kg of water. Then, 1 kg of radioactive contaminated crystalline borax is dissolved in this solution, and the powder component having a composition of 0,67 kg Si0₂, 0,52 kg kaolin and 0,03 kg zeolite is added in small charges with continuous stirring.

The thick composition thereby obtained solidifies in under half an hour, and continues to set slowly, with its compressive strength reaching its final value in 3 weeks. The solidified material is extremely hard, to the effect that a sharp piece of the material can scratch glass. The volume of the waste solidified in such a manner is only 1.05 times the volume of the 1 kg of crystalline borax that was the input of the process.

Advantageous effects related to the invention

The long-term (terminal) storage of low- and medium-activity nuclear waste is an extremely important environmental issue. A significant amount of these types of waste are constituted by thick liquid or crystalline materials containing boric acid or sodium borate. Under normal operating conditions each PWR block may produce hundreds of cubic metres of these types of waste per year. Nuclear waste must be stored for several hundreds of years as it contains isotopes that, though appear in low concentration, represent significant activity having a half-life of typically 30 years (¹³⁷Cs) or even much longer. Since it is not considered safe to store such types of waste in liquid or water-soluble crystalline form, the application of solidification technologies is a must. Currently the most widespread of such technologies is cementation. Since cementation causes a 2.5-3 times volume increase of the waste, the total cost of waste storage or disposal raises significantly due to the high construction costs of underground disposal facilities. Although a number of new methods for solidifying the above mentioned waste types have become known recently, all of these have one or more of the following drawbacks: volume increase during solidification, high costs, relatively easy release of the isotopes from the solidified waste form. Our invention provides a simple solution for waste solidification that is relatively low-cost, involves low volume increase, and provides a solidified waste form having excellent characteristics. Based on the above, it is esteemed that the present invention is of great significance for the protection of the environment, and may contribute significantly to the long-term sustainability of safe nuclear power generation.

Since additive-containing aluminoborosilicates have extremely advantageous mechanical characteristics, and may be produced at low temperatures relatively cheaply, they may be applied in many non-nuclear fields, for instance to replace porcelain and ceramic materials. Due to their advantageous characteristics, aluminoborosilicates may also be applied as fire- or heat-resistant coating or as fire-resistant structural elements.

Claims

1. Additive-containing aluminoborosilicate, characterised by that the components thereof are mixed at low temperature, preferably at room temperature, the components being a liquid alkali silicate, liquid and/or solid boric acid or borate, an alumim^'um-containing inorganic component, and at least one additive.

2. The aluminoborosilicate according to Claim 1, characterised by that the composition thereof, expressed in oxides, is the following: boron 3-18 percent by mass, aluminium 5-30 percent by mass, silicon 20-40 percent by mass, potassium 4-15 percent by mass, sodium 2-6 percent by mass, hydrogen 15-45 percent by mass.

3. The aluminoborosilicate according to one of the preceding Claims, characterised by that the potassium to sodium molar ratio is 0.2-4.0, preferably 0.5-2.0.

4. The aluminoborosilicate according to one of the preceding Claims, characterised by that the alkali silicate is prepared from silicon dioxide and an alkali hydroxide.

5. The aluminoborosilicate according to one of the preceding Claims, characterised by that the aluminium-containing inorganic component is kaolin or kaolin treated at high temperature.

6. The aluminoborosilicate according to one of the preceding Claims, characterised by that the additive is a silicate, for instance Ca-silicate, a clay mineral, or a metal oxide.

7. The aluminoborosilicate according to one of the preceding Claims, characterised by that a filler material is admixed to the components.

8. The aluminoborosilicate according to Claim 7, characterised by that the filler material is an aluminium or silicon-containing mineral material, such as corundum, zeolite, or flue ash from power stations.

9. The aluminoborosilicate adapted for embedding radioactive waste according to any one of the preceding Claims, characterised by that the liquid and/or solid boric acid or borate comes from boron-containing low or medium-activity radioactive waste.

10. Process for producing an additive-containing aluminoborosilicate at low temperature, preferably at room temperature from the following components:

liquid alkali silicate,

liquid and/or solid boric acid or borate,

an aluminium-containing inorganic component, and

at least one additive,

the process comprising the steps of:

- producing a homogeneous liquid from the liquid components,

- producing a solid powder from the solid components,

- gradually admixing the powder component to the liquid component.

11. The process according to Claim 10, characterised by that the grain size of the solid powder component is 0-1000 micrometres.

12. The process according to Claim 11 characterised by that the aluminium-containing solid powder has a grain size lower than 90 micrometres.

13. The process according to Claim 11, characterised by that the grain size of the silicon- containing solid powder is between 0.1 and 10 micrometres, and preferably between 0.1 and 5 micrometres.

14. The process according to Claim 11, characterised by that the grain size of the additive is between 1 and 500 micrometres, and preferably between 10 and 90 micrometres.

15. The process according to any one of Claims 10-14, characterised by that a filler material is admixed to the mixture after mixing the liquid and powder components but before the solidification of the mixture.

16. Application of the aluminoborosilicate produced as a result of the process according to Claims 10-15 as a fire- and heat-resistant coating, characterised by that the mixture is applied to a combustible surface before it solidifies, the mixture being applied to the surface by spreading or spraying.

17. Application of the aluminoborosilicate produced as a result of the process according to Claims 10-15 as a fire- or heat-resistant structural element, characterised by that the mixture is cast or moulded into a mould before it solidifies.

18. Application of the aluminoborosilicate produced as a result of the process according to Claims 10-15 for solidifying radioactive waste, characterised by that a solidified, water- insoluble waste form is produced by admixing the solid powder component to the radioactive waste stored in liquid state in barrel containers.