RU2320389C2 - Method for stabilizing of wastes in dehydrated ceramics with chemically bound phosphates (versions) - Google Patents

Abstract

FIELD: utilization of waste forms in ceramics with chemically bound phosphates for immobilization of wastes.

SUBSTANCE: method involves preparing suspension containing wastes, water, binder such as metal oxide, and phosphate binder; holding suspension for hardening so that hard hydrated matrix is formed; removing bound water from hard hydrated matrix, said process being commonly carried out by supplying heat to hardened matrix preferably in an amount sufficient for releasing of water bound within hydrated matrix but insufficient for evaporation of other water-free matrix components such as metals and radioactive components, with temperatures ranging between 100 C and 200 C being commonly considered as sufficient temperature range. According to another version, wastes and water are mixed before suspension preparing procedure, so that selected amount of water may evaporate from wastes and water mixture before procedure of preparing said suspension. Another subject-matter of invention is direct method for producing of dehydrated wastes, involving removing water by heating and mixing of suspension as it hardens. Also described in Specification are forms of wastes with ceramic matrix produced by mentioned above methods.

EFFECT: increased efficiency in producing of dehydrated forms of wastes having increased resistance to radiolysis and available in optimized volume.

47 cl, 4 tbl, 2 dwg

Description

FIELD OF THE INVENTION

The present invention relates to the use of waste forms in ceramics with chemically bound phosphates (CBRS) for immobilizing waste, in particular to the use of dehydrated CBRS to produce waste forms with increased resistance to radiolysis in the process of stabilization of radioactive waste and with an optimized volume.

State of the art

Streams of liquid, semi-liquid, semi-solid and solid high-level waste are generated in primary production, in reprocessing or in the extraction of nuclear materials. Historically, these wastes accumulated in anticipation of financing and reliable technologies for their treatment and disposal. There are numerous places around the world to store these wastes, mainly in countries that have or have had nuclear weapons programs or production facilities for producing nuclear energy. Many of these waste streams contain typically high concentrations of well-known non-radioactive inorganic ions, such as chlorides, sulfates and nitrates of potassium, sodium and calcium, together with minor concentrations of highly radioactive components, such as plutonium, cesium, technetium, strontium and many other transuranic elements. Some waste streams may also contain non-radioactive, but harmful components, such as mercury, lead, organic substances, fluorides, other salts, acids and bases, aqueous and anhydrous substances, or other waste and not waste in liquid, solid or sludge form.

The stabilization of this waste requires that pollutant products, including soluble heavy metal ions, be effectively immobilized. Traditional high-temperature waste treatment methods (e.g., incineration, vitrification) are for the most part not suitable for the treatment of the waste streams described above, since the fact that they are based on the use of high temperatures creates the risk of the release of volatile pollutants. In addition to this, high temperature processes generate unwanted secondary waste streams. The well-known low-temperature approach is aimed at stabilizing harmful waste using inorganic (e.g. pozzolanic) binders such as cement, lime, dust from kilns and / or fly ash. The disadvantages of this approach include high sensitivity to the presence of impurities, high porosity of solid waste forms, low waste loading and the resulting large volume of final waste forms.

Organic binders (for example, thermosetting polymers) are rarely used due to their high cost and increased complexity in use. Organic binders are not compatible with water-based wastes unless these wastes are pretreated and converted into emulsion or solid form, and organic binders are subject to deterioration due to environmental factors, including biological and ultraviolet radiation. In addition, organic binders also contribute to the radiolytic formation of H ₂ within the waste form.

More recently, an alternative low-temperature approach to the stabilization and solidification of low-level mixed wastes was developed at the Argonne National Laboratory in the United States by incorporating or loading these wastes into chemically bound phosphate ceramics (CBPC). This method immobilizes the waste by curing so that the waste is physically microencapsulated into a dense ceramic matrix. Microencapsulation systems are quite attractive in that the bonds formed in these systems are either ionic or covalent, or both, and therefore more durable than hydration bonds in cement systems. In addition, the process with ceramic compositions is exothermic and economical. Phosphates, in particular, are excellent candidates for use in the stabilization of radioactive and hazardous wastes, since phosphates of radionuclides and harmful metals are mainly insoluble in groundwater.

US patent No. 5645518 (Wagh et al.) Describes in detail the stages of the process of solidification of liquid or solid waste in some CBPC products using acid-base reactions. The CBPC products disclosed in this patent have a complex structure including a basic crystalline phase, for example, non-verite (MgHPO ₄ · H ₂ O), and an insoluble, stable phase. Most waste components are evenly distributed within the matrix of phosphate ceramics.

US patent No. 5830815 (also Wagh et al.) Describes an improvement in the manufacturing method of CBRS through the use of two temperature control processes. This patent also discloses an excellent CBPC product, magnesium potassium phosphate hexahydrate (MKP) (MgKPO ₄ · 6H ₂ O).

MKP is formed bypassing the use of acid by mixing the oxide powder with potassium dihydrophosphates to obtain ceramics at elevated pH, CBPC similar to MKP can be obtained from dihydrophosphates or other monovalent metals. MKC is formed in accordance with equation (1) below:

SVRS waste form, such as FIBC, is a dense, solid material with excellent durability and high resistance to fires, chemicals, humidity; FIBC ceramic products have been intensively studied by the US Department of Energy in waste processing projects.

SIRS, as described in the above patents, has proven to be somewhat problematic for stabilizing radioactive waste, in particular high-level waste. Each of the well-known CBPCs is a hydrated ceramic product with water bound inside the ceramic matrix. For example, MKP (MgKPO ₄ · 6H ₂ O) contains six molecules of bound water for each ceramic molecule, while other MKP compositions can contain all 22 H ₂ O molecules for each ceramic molecule. Radioactive waste usually emits α-, β-, n-particles and γ-rays, which can split the bound water in hydrated CBRS in the process related to radiolysis, which leads to the formation of hydrogen. Hydrogen can increase the pressure in storage containers or other forms of waste, which can cause destruction of containers or forms of waste and the ingress of moisture (for example, from air or groundwater or other elements). Water also shows a tendency to reflect nuclear radiation, increasing the chance that high-level waste can reach a critical state if the low load of the waste is not artificially maintained.

There is a need for dehydrated CBPC waste forms that are highly resistant to radiolysis.

Disclosure of invention

One aspect of the present invention is a method for stabilizing ceramic wastes with chemically bound phosphates (CBPC). The method consists in preparing a suspension including waste, water, an oxide binder and a phosphate binder, as well as a selected amount of one of the reducing agents and oxidizing agents. The resulting suspension is cured into a solid hydrated CBPC matrix. In the next step, water bound inside the solid hydrated CBPC matrix is removed. Typically, bound water is removed by supplying heat to the cured CBPC matrix. Preferably, the amount of heat supplied to the cured CBPC matrix is selected so that it is high enough to remove the water bound inside the hydrated CBPC matrix, and at the same time low enough to avoid the volatilization of other water-free matrix components, such as metals and radioactive components. Typically, a heating temperature selected in the range of 100 ° C. to 200 ° C. is sufficient to evaporate bound water without removing other components from the matrix. Removing bound water increases the resistance of the final waste form to radiolysis if this form is used to encapsulate radioactive waste. In addition, the removal of bound water from the final waste form can reduce the mass of the final dehydrated waste form.

In another embodiment of the invention, in which the waste and water are pre-mixed before the suspension is prepared, a selective amount of water can evaporate from the waste and its mixture with water even before the suspension is prepared. Moreover, the amount of water removed from the mixture of waste with water is selected so that the solids content in the combination of waste with water after the removal stage is equal to or less than 90 wt.%. This step may result in a smaller and lighter mass form of waste containing a high percentage of waste inside, compared to a form obtained by conventional methods. Subsequent dehydration is also possible due to the heat treatment of the solidified waste form, as described above.

Another aspect of the invention is a direct method for producing anhydrous CBRS, in which water is removed from the suspension, usually by heating and stirring the suspension, in the process of curing the suspension. This method allows to obtain fully or partially dehydrated CBPC waste form. If necessary, the partially dehydrated form of the waste can be completely dehydrated by additional heating after the matrix has been cured.

Further aspects of the invention are ceramic matrix waste forms obtained by the methods described above.

Brief Description of the Figures

Figure 1 schematically shows a system for processing waste.

Figure 2 graphically presents the results of differential thermal analysis of dehydrated CBPC waste forms.

Detailed Description of the Preferred Embodiment

The dehydrated or anhydrous CBRS described herein can be obtained from conventional hydrated CBRS waste forms. Thus, in the present description, there is first a discussion of obtaining hydrated CBPC waste forms, and then a discussion of dewatering techniques.

I. Synthesis of hydrated CBRS

Ceramics with chemically bound phosphates (CBRS) of various types have proven to be effective in encapsulating and stabilizing solid and liquid wastes. Magnesium Potassium Phosphate Hydrate (MKP) is a particularly effective CBRS for stabilizing harmful and radioactive waste. The present invention is disclosed in relation to MKP. However, the invention is applicable to any hydrated CBPC. MKR is obtained by the reaction between magnesium oxide (MgO) and monopotassium phosphate (KH ₂ PO ₄ ) in solution. The formation of MKP is controlled by the reaction:

This reaction gives a solid, dense ceramic MKC (MgKPO ₄ · 6H ₂ O), which acts as a crystalline matrix-core for waste. During the reaction, harmful and radioactive contaminants also react with KH ₂ PO ₄ to form practically insoluble phosphates. Then the bulk of the ceramics microencapsulates reactive contaminants into a dense crystalline matrix.

FIBC waste forms are obtained by slowly mixing the mixture of waste, MgO and KH ₂ PO ₄ in water. Due to the dissolution of KH ₂ PO _{4, the} solution contains potassium ions, dissociable potassium, phosphates and protons (H ⁺ ) and is therefore acidic. The acidity of the solution increases the solubility of MgO, oxides of harmful metals and, to some extent, radioactive contaminants and leads to the dissolution of MgO and pollutants. This, in turn, leads to the release of Mg ²⁺ and metal ions of pollutants. These cations react successively with hydrated phosphate ions to form phosphates.

To obtain MKP ceramics, it is necessary that Mg ²⁺ slowly react with phosphate ions. On the other hand, it is necessary that the harmful metals and radioactive components react quickly so that they can be encapsulated as phosphates in the MCR matrix. If metals and radioactive components react at a rate that is lower than the rate of formation of the MCR matrix, then in this case phosphate anions will not be enough to react with the waste components, and the waste components will not be fixed in the matrix. Thus, in order to achieve effective stabilization, it is necessary that the pollutants turn into phosphates almost spontaneously.

The curing reaction defined by equation (2) can be quantified by studying the thermodynamics of dissolution and the general reactions of oxides in a solution of KH ₂ PO ₄ · KH ₂ PO ₄ has high solubility and its dissolution can be represented as

(a certain amount of K is also ionized, as indicated by the formation of a certain amount of KNO ₃ ), where the superscripts indicate the ionic charge. This reaction is endothermic and occurs even before the dissolution of other components. If alkaline metal oxide, for example MgO, is added to the acidic phosphate solution, then the pH of the solution will slowly increase, as the acid is neutralized. The initial pH of the KH ₂ PO ₄ ≈ 4 solution, but dissolution of the oxide neutralizes the acidic solution and subsequently leads to the formation of ceramics at pH ≈ 8. It can be seen that the addition of metal oxide to the suspension serves to increase the pH of the suspension,

In the presence of 2H ⁺ ions released by KH ₂ PO ₄ , MgO dissolves as a result of the reaction:

where (aq) means hydrated ion. The equations of reactions 2-4 underlie the action of the binder in the MKP and are typical for the SVRS process. Ions from equations 3 and 4 together with five additional moles of water react with the formation of the matrix material MgKPP ₄ · 6H ₂ O, as shown in reaction equation 2.

US Pat. No. 5,830,815 (Wagh et al.) Teaches that both liquid and solid wastes can be stabilized and encapsulated in MKP. In the case of solid waste, this waste is first pulverized and then a phosphate binder is added to them to form a reaction slurry.

The stabilization method using MKP or many other CBRS can be used both for high pH waste streams and for low pH waste streams. The stabilization method using MKP is functional in the pH range from about 2 to 10. Typically, high pH waste can be neutralized to the pH range by adding phosphoric acid (H ₃ PO ₄ ). Likewise, low pH waste can be neutralized to the operating pH range by the addition of additional magnesium oxide (MgO) or other similar oxide base. Neutralization is necessary in order to bring the pH of the suspension to a pH range within which the dissolution of the constituents of the binders and constituents of the waste, as well as the necessary acid-base reactions, can occur. An additional advantage that can make the FIBC form of waste or other CWS waste forms more suitable for storing radioactive waste can be obtained if magnesium oxide (MgO) or other similar oxide base is added in an amount in excess of the stoichiometric amount required for the formation of CWS matrices. In this case, an excess of magnesium oxide (MgO) or other oxide base can function as a getter that binds free hydrogen as it forms and thereby reduces the effect of radiolysis. Other materials that are not involved in the formation of the ceramic matrix can also be added, if required, to act as getters.

The use of magnesium oxide (MgO) or other metal oxides is effective, in particular, for stabilizing waste such as nitric acid. Metal oxide not only neutralizes the acidity of waste, but also metal oxide, for example, magnesium oxide (MgO), will capture nitrates and keep them in solution, thereby preventing the escape of nitric acid when heat is supplied and facilitating the conversion of nitrates to more stable phases in ceramics.

US Pat. No. 6,133,498 (Singh et al.), Incorporated into the list of references to this application, indicates that it may be useful to introduce an oxidation or reduction step in a method for producing MKP in order to control the rate of an acid-base reaction during the formation of a phosphate-ceramic system. . The addition of certain reducing agents also facilitates the stabilization of radioactive and / or harmful waste materials containing metal anions. Suitable reducing agents for this purpose are preferably selected from the group including, but not limited to, sodium monosulfide (Na ₂ S), potassium monosulfide (K ₂ S), calcium sulfide (CaS), iron sulfide (FeS), iron sulfate ( FeSO ₄ · 7H ₂ O), sodium thiosulfate (Na ₂ S ₂ O ₅ ), sulfur dioxide (SO ₂ ), sodium borohydride (NaBH ₄ ), hydrazine, sodium bisulfite (NaHSO ₃ ), calcium hydroxide (Ca (OH) ₂ ) , sodium hydroxide (NaOH), sodium carbonate (Na ₂ CO ₃ ), sulfuric acid (H ₂ SO ₄ ) and formic acid (HCOOH), among others. A preferred reducing agent is a tin salt, such as tin chloride (SnCl ₂ ).

It is important to note that each of the above CBPC materials is a hydrated ceramic. Hydrated ceramic is a ceramic that contains water molecules bound inside a ceramic matrix. For example, the stoichiometrically obtained MKP, as discussed in detail above, contains six water molecules bound within the crystalline matrix for each molecule of potassium magnesium phosphate (MgKPO ₄ ). As discussed in detail below, water bound within the CBPC matrix can regulate the stability and effectiveness of the CBPC to stabilize and encapsulate certain types of waste. If water is present in excess, then some CBPC materials can have up to 22 moles of H ₂ O bonded inside the ceramic matrix for every mole of ceramic molecules.

CBRS methods are useful for stabilizing and encapsulating a wide range of wastes, including but not limited to ash, cement, silica, sludge, boiler jacket sludge, pyrophoric (self-igniting) substances, salt mixtures, volatile substances such as mercury , lead, cadmium, chromium and nickel. An important advantage of the invention is that using these methods, it is possible to stabilize radioactive materials. Such materials include uranium, plutonium, thorium, americium, nuclear fission products, high level waste, low level waste derived from high level waste, transuranium elements and other types of radioactive waste. In addition, CBRS methods are suitable for stabilizing both waste and non-waste materials in liquid, solid or slurry forms, as well as other nuclear materials such as by-products of nuclear weapons and nuclear energy production processes.

The CBPC methods described above are also effective in stabilizing liquid, solid and sludge wastes with a high level of leached salt ions, such as sodium (Na), nitrate (NO ₃ ), nitrite (NO ₂ ). Sulphates and chlorides can also stabilize within the CBPC matrix. It is assumed that CBRS obtained with the salt component of the waste will contain a ceramic matrix MgNaPO ₄ · nH ₂ O, the mineral part of which is likely to stabilize NaKNO ₃ , partially stabilize NO ₃ and may consist of various mineral forms having the following chemical forms formulas:

It is also assumed that using CBPC methods in which magnesium (Mg) is replaced by calcium (Ca) or an ion of some other divalent metal, a similar and related class of minerals can be obtained.

However, it has been proven that the use of hydrated CBRS to stabilize radioactive waste, in particular high-level waste, is somewhat problematic due to the content of water bound inside the ceramic matrix. As indicated above, radioactive waste typically emits α-, β-, n-particles and γ-rays, which can cause radiolysis and the generation of hydrogen gas. In addition to this, the large volume and mass of solid waste forms are also a known disadvantage of waste storage using hydrated CBRS. As discussed in detail below, the present invention provides methods for dehydrating CBPC waste forms, thereby reducing the potential risk of radiolysis, reducing the mass of the final waste form, and in some cases the volume of the final waste form. In the case of highly active waste and materials, the transportation of the system and containers can be controlled by a remote control system.

II. Waste treatment machine

Embodiments of the present invention and methods for producing fully or partially dehydrated CBRS are discussed below in the context of an apparatus for producing CBRS waste forms schematically shown in FIG. It should be noted that the apparatus of FIG. 1 is only one example from a large number of apparatuses suitable for use in the present invention.

As shown in FIG. 1, the waste treatment system 10 includes a waste treatment tank 20 with walls 22 defining an interior space 24 of the tank. The reservoir 20 may be an open type, as shown, or a closed type. The waste treatment tank 20 may be a conventional storage tank of the type currently used to store some liquid waste, such as liquid radioactive waste. Depending on the nature of the waste being processed, it can be beneficial right up to the efficient placement of the waste treatment tank 20 inside the glove box or similar closed device in order to limit the spread of radioactive material or other harmful components of the waste.

A phosphate binder, such as potassium phosphate (KH ₂ PO ₄ ), may be supplied to the interior of the reservoir 24 from the phosphate supply system 40 through a phosphate supply line 42. An oxide binder, such as magnesium oxide (MgO), may be supplied to the interior of the reservoir 24. from the oxide supply system 46 through the oxide supply pipe 48. Suitable CBRS can be prepared using oxides of divalent metals, for example calcium (Ca), aluminum (Al), or any other divalent metal oxide as binders. Waste from the waste collection system 50 may be delivered via a waste feed pipe 52 to the interior of the tank 24. If necessary, water can be supplied to the interior of the tank 24 from the water supply system 56 through a water supply pipe 58.

The mixing system 30 can be used to mix materials entering the interior of the tank 24. The mixing system 30 in FIG. 1 includes an electric motor 32 (which can be placed outside the glove box 26 to limit contamination) connected to the mixer 34 using a detachable clutch 36. The mixer in figure 1 is schematically shown as a series of vanes diverging to the sides, but this is shown for illustrative purposes only, as agitators of any shape can be used. The clutch 36 can be fixed with the possibility of selective engagement with the shaft of the mixer 34 for rotation by the electric motor 32 and at the same time with the ability to quickly disconnect the mixer 34 from the electric motor 32. For example, the clutch 36 can provide a key connection between the mixer 34 and the electric motor 32, which allows selective connection or disconnect the agitator 34 from the motor 32 by axial movement. In other embodiments, the mixing system 30 shown in FIG. 1 may be replaced by any of a large number of systems that are capable of efficiently mixing materials entering the interior of the tank 24.

The waste treatment system 10 may also include a temperature controller 60 operatively connected to the glove box 26 and / or the waste treatment tank 20 to control the temperature of the material in the interior of the tank 24. The temperature controller 60 may, for example, include a water jacket to circulate hot or cold liquid around the tank 20. Alternatively, the temperature controller 60 may include a microwave source or a series of infrared heating panels mounted so that the radiation is directed directly at and / or into the tank 20 In other embodiments, the temperature controller 60 is not used. It can be useful if the reaction in the tank is exothermic enough to heat the contents to the desired temperature.

As explained in more detail below, during processing, water can evaporate from the contents of the interior space 24 of the tank. If this is required by the nature of the waste in the waste disposal system 50, water vapor and any other gas in the glove box 26 can be sent to the scrubber 64 through the gas line 66. After cleaning in the scrubber 64 to remove harmful volatile materials, gas can be vented to the atmosphere via a ventilation pipe 68 .

The treated waste may be cured in the tank 20. Alternatively, it may be useful to remove the mixed components from the tank 20 before they are cured, for example, in continuous mode instead of batch mode. In such an embodiment, the contents of the reservoir 20 may be supplied to the storage reservoir 70 through the outlet 72.

The controller 80 can be used to monitor the operation of the waste treatment system 10. The controller 80 can be operatively connected to one (or more) mixing system 30, to a thermostat 60, to a phosphate supply system 40 or a phosphate supply pipe 42, to an oxide supply system 46 or a pipe oxide supply 48, with a waste supply system 50 or a waste supply pipe 52 and with a water supply system or a water supply pipe 58. The controller 80 may include at least one computer with a software processor programmed to control operations with the specified components for waste treatment in the waste system 50.

III. Obtaining dehydrated CBRS waste forms from solidified hydrated CBRS waste forms

As noted above, the cured hydrated MKP waste form contains at least six water molecules bound within the ceramic matrix for each molecule of potassium magnesium phosphate (MgKPO ₄ ). This bound water can be released from the waste after it has cured. Other typical CBRS are also somewhat hydrated. Although the invention described here is considered in relation to FIBC, it also applies to any hydrated CBRS.

In typical cases, the bound water inside the FIBC matrix evaporates by supplying heat to the solidified waste form. Preferably, the amount of heat supplied to the solidified form of the waste is selected that promotes the evaporation of the bound water, but at the same time, the amount of heat is limited to prevent other components from escaping from the waste. In most cases, the heating temperature of the waste is preferably above 100 ° C. to allow evaporation of the bound water. Preferably, the waste should not be heated to a temperature above 200 ° C in order to avoid volatilization of the bound heavy metals and radioactive contaminants. The above temperature range for the effective dehydration of hydrated CBPC waste forms without volatilization of other waste components may vary depending on the nature of the ceramic or the waste fixed in it and the contaminants contained therein. For each type of CBRS, it is preferable that the selected heating temperature be higher than the minimum temperature necessary for the evaporation of water, but lower than the temperature at which the volatilization of pollutants can occur. The use of heating for a sufficient period of time leads to the transition of all or part of the ceramic form of waste into anhydrous FIBC ceramics (MgKPO ₄ ).

Although the use of heating is the most typical method of removing bound water from a hydrated CBPC waste form, other suitable methods are also possible. For example, reducing pressure on the form of waste in a vacuum chamber can lead to partial dehydration of the waste. The use of ultrasonic or light energy can speed up the process.

Partial or complete dehydration of the hydrated CBPC waste form after its curing will not reduce the final size of the cured form. However, the mass of the form will decrease, and the effect of radiolysis can be minimized if the form is used to encapsulate radioactive waste. The author of this application has established experimentally that partial or complete dehydration of the MKP waste form can be carried out at temperatures below 120 ° C-130 ° C. Dehydration was confirmed by differential thermal analysis (DTA) of waste forms encapsulating two different waste streams. The first form of waste was obtained by the methods described in detail above, providing for the encapsulation of waste in the MCR with the addition of a liquid waste stream simulating a supernatant carrying cesium (Cs), technetium (Tc), strontium (Sr) and transuranic elements (TRU). The second form of waste, obtained in a similar way, but prepared from a simulated waste stream of sludge containing Cs, Tc, Sr and TRU extract from other streams and combined with the sludge in a tank, is discussed below. The physical properties of the two forms of waste are shown in table I

Table I.
Physical properties of waste forms Property First form: supernatant-based waste stream Second form: sludge-based waste stream + extract Density (g / cm ³ ) 1.88 1.72 The approximate value of open porosity 7.9 4,5 Compression Force (psi) 1800-3500

Figure 2 shows the results of differential thermal analysis of the first FIBC waste form encapsulating the supernatant waste stream. In Fig. 2, the y axis shows the change in microvolts of the microbalances, which indicates the level of mass loss of the material during heating. With increasing temperature, the mass continues to decrease, indicating the loss of some component of the waste. These losses were identified using differential scanning calorimetry (DSC) as the loss of bound matrix water. Equation 6 describes this separation of water.

In the study graphically presented in FIG. 2, upon reaching a temperature of 128 ° C., bound matrix water evaporated from the waste form. In addition, there was no mass loss, except for a temperature of ≈260 ° С, at which insignificant losses of other components of the waste form were again noted.

The amount of water that evaporates from waste forms depends on how strongly this water is bound in the mineral matrix of each of the waste forms. To assess the loss of bound water in this study, samples of waste forms encapsulating both types of waste were heated for three hours, cooled, and weight loss was measured. Weight loss data are given in table II.

Table II.
Mass losses associated with dehydration of waste Sample Identity Water before heating (% wt.) Water loss during heating (% wt.) Supernatant-based waste stream 46.6 First form of waste 18.5 * 15.3 Sludge / extract based waste stream 74.8 - Second form of waste 32.25 * 33.75 * Bound water from the binder component MgKPO ₄ -6H ₂ O in the form of waste

The heating results presented in column 3 of table II imply that in the case of a second (sludge / extract) form of waste, all bound matrix water evaporated. Actual water loss after heating is slightly higher than the estimated amount of water in the form of waste. This can be explained by the fact that, possibly, some of the bound water from minerals also evaporated. However, in the case of the first form of waste, only 75% wt. water. At the same time, the reason for the retention of some of the water in this form of waste is not known reliably. In all likelihood, all bound matrix water evaporated, but some of it was reabsorbed during the cooling of the samples. Water retention may also be due to the fact that the supernatant-based waste stream contained a significantly higher amount of nitrates than the sludge + extract stream. The establishment of partial water retention mechanisms will require additional research, but one thing is clear that most or all of the bound water from both forms of waste evaporates from the matrix in the temperature range from 120 ° C to 150 ° C.

IV. Minimization of bound water during the formation of MKP

The amount of bound water inside the CBPC matrix for a given amount of waste, along with the volume and mass of the final waste form, can be minimized by more precisely controlling the amount of water present in the formation reactions. By partially dehydrating the liquid waste or slurry before mixing the waste stream with binders suitable for preparing CBRS, the amount of water present can be reduced to the minimum amount required for reactions, which can lead to a corresponding reduction in the amount of binder used. The forms of waste obtained in this way will be smaller and lighter than the forms of waste obtained in the traditional way at a specific level of waste loading and require less energy for subsequent dewatering.

Tables III and IV below show the stages of the process for obtaining the MCR forms of waste after removing a selected amount of water from liquid waste or suspension by evaporation by heating, centrifugation, filtration or other separation methods before mixing the waste stream with oxide or phosphate binders. The described process is applicable to other compositions of CBRS. For each sample shown in table III or table IV, the initial amount of waste is constant. Evaporation of some of the water present in the waste stream entering the treatment allows the use of a correspondingly smaller amount of oxide and phosphate binders. Thus, the final total mass and volume of the ceramic form of the waste is significantly reduced, which entails a corresponding increase in the final load of the waste. A small amount of sodium sulfide reducing agent (Na ₂ S) was added to the first (cesium-bearing supernatant) waste stream to stabilize hexavalent chromium, and phosphoric acid (H ₃ PO ₄ ) was used to neutralize the second waste stream (sludge / extract). These stages accelerate the stabilization chemistry of general waste and, in all likelihood, do not prevent the reduction in the size and mass of the final waste form, achieved through partial dehydration of the incoming flows.

Table III.
Dehydration of waste forms based on cesium supernatant (all weight values are in grams) Act Parameters of each series of samples The initial amount of waste (g) 300 300 300 300 The initial volume of waste (ml) 210 210 210 210 Evaporate this amount of water (g) 63 78 93 108 Water that remains in the waste after drying (g) 75 60 45 thirty Add this amount of Na ₂ S and mix well (g) 1,5 1,2 0.9 0.6 Add binder and mix (g) MgO 37.5 KH ₃ PO ₄ 112.5 MgO 30 KH ₂ PO ₄ 90 MgO 22.5 KH ₂ PO ₄ 67.5 MgO 15 KH ₂ PO ₄ 45 Final total ceramic mass (g) 387 342 327 252 Final total ceramic volume (ml) 215 190 182 140 Volume reduction (%) -3 9 13 33 The final load of waste (% wt.) 61 65 72 76

Table IV.
Sludge / extract waste form dehydration (all weight values are in grams) Act Parameters of each series of samples Other waste The initial amount of waste (g) 300 300 300 300 800 The initial volume of waste (ml) 227.8 227.8 227.8 227.8 606 waste pH 13.8 Neutralize waste 14% wt. concentrated solution of H ₃ PO ₄ Add 82 grams of solution until pH <9. The amount of water in this solution will be 70.5 grams, the rest is H ₃ PO ₄ Evaporate this amount of water (g) 220 235 250 265 Water that remains in the waste after drying (g) 75 60 45 thirty Binder (g) MgO 37.5
KH ₂ PO ₄ 112.5 MgO 30
KH ₂ PO ₄ 90 MgO 22.5
KH ₂ PO ₄ 67.5 MgO 15
KH ₂ PO ₄ 45 The final mass of ceramics 300 255 210 175 The final volume of ceramics (ml) 174 148 122 96 Volume reduction (%) 23 35 46.3 58 The final load of waste (% wt.) fifty 53 57 63.6

Although the partial dehydration of the incoming waste streams allows a final waste form to be smaller and lighter in mass, the MKP matrix still contains water bound within the matrix. If it is necessary to reduce the risk of radiolysis, the forms obtained with partial dehydration of the incoming streams can be dehydrated after curing by removing bound water from the matrix, as described above.

V. A direct method for producing anhydrous CBRS waste forms

Both methods discussed above include two-stage cooking processes in the event that an anhydrous final waste form is desired. At the first stage, a hydrated CBRS form of waste is obtained, the mass of which can be reduced due to partial dehydration of the incoming waste streams. In the second stage, the cured form of the waste is partially or completely dehydrated by the addition of heat or using another dehydration method to form a cured matrix. But a third, direct, method for producing anhydrous SIRS waste forms is also possible.

In the following discussion of methods for preparing anhydrous CBRS, reference is made to the waste treatment system 10, schematically depicted in FIG. 1. And, as before, the invention is disclosed in relation to the preparation of MKP. It should be remembered that this was done only to illustrate the invention and that the following methods are not limited to the use of specific structures, systems or types of CBPCs shown in the figures or discussed above.

As discussed above, KH ₂ PO ₄ from the phosphate supply system 40, MgO from the oxide supply system 46, waste from the waste supply system 50 and, optionally, water from the water supply system 56 may be supplied to the waste treatment tank 20 of FIG. 1. These materials can be mixed using a mixing system 30 to obtain a suspension.

The relative amount of phosphate binder, oxide binder and waste added to the waste treatment tank 20 will depend, at least in part, on the nature of the waste materials. For example, you can add more liquid waste with a higher water content than when the waste is dry or has a lower water content. It is estimated that the loading of the waste (i.e., the relative amount of waste in the final solid waste form) is about 85% by weight. (in terms of dry weight) can work with many types of waste. In the case of wastes that are supposed to contain harmful materials that need to be leached (e.g., heavy metals), a lower level of waste loading may be more acceptable. For example, it is assumed that waste streams carrying some heavy metals may be limited by the level of loading of about 70 wt% into the final solid form of waste.

The temperature of the reacting components in the waste treatment tank 20 can be controlled by a temperature controller 60 to evaporate excess water in the measured mode and to control the nature of the final reaction product. If the mixing of the suspension described above is carried out at a temperature of approximately above room temperature, as is traditionally done in most storage systems of CBPC waste, the reaction of the oxide binder and phosphate binder can give MKP (MgKPO ₄ · 6H ₂ O). In order to avoid the likelihood of radiolysis in the solidified form of the waste due to the preparation of anhydrous MKP, the oxide and KH ₂ PO ₄ suspension can be involved in the reaction at an elevated temperature sufficient to remove water as the ceramic matrix forms. As noted above, some components of hazardous waste streams, such as HgCl, begin to disappear at elevated temperatures. To limit their volatilization, it may be desirable to carry out the reaction of the suspension oxide and KH ₂ PO ₄ at a temperature above the temperature necessary to remove water as the matrix forms, but below the temperature at which other components of the ceramic matrix or contaminants may volatilize. Usually this temperature is in the range from 100 ° C to 200 ° C.

In one exemplary embodiment, when treating waste materials including a heavy metal or a radioactive component, the suspension may be mixed at a temperature below 100 ° C, as described above in the description of the main method for producing hydrated CBRS. During this initial period of relatively low temperature mixing, heavy metals or radioactive components of the waste stream can form practically insoluble phosphate components, which will later be encapsulated in a ceramic matrix. It is estimated that a time of the order of about 10-15 minutes will be sufficient for the formation of insoluble phosphates by contaminants in multiple waste streams. Subsequently, the temperature controller 60 can be used for a gradual jump in temperature during the mixing of the suspension to such a level that part of the water, which by that time will be bound inside the ceramic matrix, could evaporate during curing of the ceramic matrix. The use of a stepwise heating cycle can limit the volatilization of radioactive heavy metals or other waste contaminants before the formation of insoluble phosphates.

The mixing system 30 may continue mixing the suspension as water evaporates from the mixture of reacting components in the reservoir 20. In addition to maintaining good mixing of the components, continued mixing will help release water vapor from the suspension when heat is applied. This, in turn, will reduce the number of voids in the solid form of waste and, thereby, increase its strength and reduce its volume. The pore volume in the final solid form of waste can ideally reach about less than 5% of the volume of the solid form of waste, but at the same time it is not known whether such porosity can be achieved. Several MKR compositions with a porosity of less than about 1% were obtained, although without attempting to simultaneously minimize the volume of the waste form.

As the reaction proceeds and the water evaporates, it becomes increasingly difficult to operate the agitator 34. The mixing system 30 may stop stirring the suspension when the suspension reaches a final consistency. If necessary, salts, including nitrates, can be added to the slurry to slow down the rate of curing of the ceramics, to facilitate control of the encapsulation process and to ensure a complete matrix formation cycle before all water has evaporated or otherwise been removed. The final consistency can be determined by a number of methods. It can be determined by adjusting the force required to drive the agitator 34 by an electric motor 32; as soon as the required driving force reaches a predetermined limit, the controller 80 can turn off the motor 32, which will turn off the mixer 34. If necessary, the mixer 34 can then be dismantled from the reactor vessel 20 and reused in the next reactor vessel. Alternatively: the mixer 34 may remain in suspension until it is solidified into the final solid waste form. A detachable clutch 36 between the mixer 34 and the motor 32 will facilitate the removal of the solid waste form, including the mixer 34, from the electric motor 32. After that, a new mixer 34 can be connected to the electric motor 32 to handle the next batch of waste.

At the end of mixing, the suspension can be maintained to complete the solidification of the reacting components in the suspension into solid waste. To achieve uniformity of the solid form of the waste, the suspension at the time of the final consistency must be sufficiently viscous to avoid unwanted precipitation of the components of the suspension. Applying a sufficient amount of heat during mixing and curing can provide the direct formation of anhydrous CBPC product, such as anhydrous MKP (MgKPO ₄ ). The forms of waste obtained by the direct dehydration method are characterized by reduced volume and mass for a given amount of encapsulated waste compared with the forms of waste obtained by traditional methods of preparing hydrated CBRS. If the waste form obtained by the anhydrous method is only partially dehydrated, then subsequent dehydration can be undertaken by heating the solidified waste form, as described above.

Other aspects, objects, and advantages of the present invention will become apparent from a study of the description, drawings, and the following claims.

Claims (47)

1. A method for stabilizing wastes in ceramics with chemically bound phosphates, comprising the following steps: preparation of a suspension comprising waste, water, a binder in the form of a metal oxide and a phosphate binder; exposure of the suspension to cure it in a solid hydrated ceramic matrix with chemically bound phosphates and removal of bound water from a solid hydrated ceramic matrix with chemically bound phosphates. 2. The method according to claim 1, in which the bound water is removed from the hydrated ceramic matrix by heating. 3. The method according to claim 1, in which the waste is pre-mixed with water before preparing the suspension and the selected amount of water is removed from the waste mixture with water before preparing the suspension. 4. The method according to claim 1, in which water is also removed from the suspension during curing by stirring. 5. The method according to claim 1, in which the binder in the form of a metal oxide is an oxide of a divalent metal, and the phosphate binder is KH ₂ PO ₄ . 6. The method according to claim 1, in which add the selected amount of one of the oxidizing agents or reducing agents to the waste or suspension before curing. 7. The method according to claim 1, in which add H ₃ PO ₄ to the suspension in order to lower the pH of the suspension. 8. The method according to claim 1, in which additional metal oxide is added to the suspension in order to increase the pH of the suspension. 9. The method according to claim 1, in which neutralizing material is added to the waste having a first pH level before the suspension is cured to neutralize the waste to such an extent that the second pH level is different from the first pH level. 10. The method according to claim 1, wherein the waste is dehydrated prior to mixing with a binder in the form of metal oxide and a phosphate binder. 11. The method according to claim 1, in which neutralizing material is added to the waste in order to neutralize the waste before mixing the waste stream with a metal oxide binder and a phosphate binder. 12. The method according to claim 1, which contains at least partially dehydrated waste before the suspension is cured. 13. The method of claim 1, wherein the hydrogen absorbing agent is added to the waste or to the suspension to reduce hydrogen generation caused by radiolysis. 14. The method according to claim 1, in which the waste is basic waste with a pH level that has been reduced by the addition of a neutralizing agent. 15. The method according to claim 1, in which salt is added to the suspension to control the speed of the curing reaction during the mixing of the suspension. 16. The method according to claim 1, in which at least one of the oxidizing agents or reducing agents is added to the waste or to the suspension. 17. The method according to claim 1, in which the hydrated ceramic matrix is in the vacuum chamber, and the bound water is removed from the matrix by reducing the pressure in the vacuum chamber. 18. The method according to claim 2, in which the matrix is heated to a temperature chosen between a lower first temperature, at which bound water begins to evaporate from the hydrated ceramic matrix, and a higher second temperature, at which the anhydrous components of the hydrated ceramic matrix evaporate. 19. The method according to claim 3, in which the selected amount of water is removed from the mixture of waste with water by evaporation by heat. 20. The method according to claim 3, in which the amount of water removed from the mixture of waste with water, is chosen so that the solids content in the combination of waste with water after the stage of removal was equal to or less than 90 wt.%. 21. The method according to claim 4, in which the water is removed from the suspension by evaporation by heating and the suspension is heated to a curing temperature selected between the first curing temperature at which water is removed from the suspension as it cures and the second curing temperature at which volatilize anhydrous components of the suspension. 22. The method according to claim 5, in which the binder in the form of a metal oxide is MgO. 23. The method according to claim 9, in which the waste is acidic waste containing neutralized waste with at least one metal oxide. 24. The method according to p, in which the heating temperature of the matrix is selected in the range from 100 to 200 ° C. 25. The method according to item 21, in which the selected curing temperature is from 100 to 200 ° C. 26. A method for stabilizing waste in ceramics with chemically bound phosphates, comprising the following steps: mixing waste with water; removing the selected amount of water from the mixture of waste with water and obtaining a residual mixture of waste with water; the preparation of a suspension comprising a residual mixture of waste with water, a binder in the form of a metal oxide and a phosphate binder for reaction with the residual mixture of waste with water and curing the suspension to a solid ceramic matrix with chemically bonded phosphates having a minimum amount of water bound therein. 27. The method according to p, in which the selected amount of water is removed from the mixture of waste with water by evaporation by heat. 28. The method according to p, in which the amount of water removed from the mixture of waste with water, is chosen so that the solids content in the mixture of waste with water after the stage of removal was equal to or less than 90 wt.%. 29. The method according to p, in which water is removed from the suspension in the process of mixing and curing. 30. The method according to p, in which the binder in the form of a metal oxide is an oxide of a divalent metal, and the phosphate binder is KH ₂ PO ₄ . 31. The method according to p, in which add the selected amount of one of the oxidizing agents or reducing agents to the waste or to the suspension before it is cured. 32. The method according to p, in which H ₃ PO _{4 is} added to the suspension in order to lower the pH of the suspension. 33. The method according to p, in which additionally add metal oxide to the suspension in order to increase the pH of the suspension. 34. The method according to clause 29, wherein the water is removed from the suspension by heat evaporation and the suspension is heated to a curing temperature selected between a first curing temperature at which water is removed from the suspension as it cures and a second curing temperature at which anhydrous evaporates suspension components. 35. The method according to clause 30, in which the binder in the form of a metal oxide is MgO. 36. The method according to clause 34, in which the curing temperature is selected between 100 and 200 ° C. 37. A method for stabilizing waste in ceramics with chemically bound phosphates, comprising the following steps: the preparation of a suspension comprising waste, water, a binder in the form of a metal oxide and a phosphate binder, and removal of water from the suspension in the process of mixing and curing with the formation of a solid ceramic matrix with chemically bound phosphates with a minimum amount of water bound in it. 38. The method according to clause 37, in which water is removed from the suspension by evaporation by heating and the suspension is heated to a curing temperature selected between the first curing temperature, at which water is removed from the suspension as it cures, and the second curing temperature, at which anhydrous evaporates suspension components. 39. The method according to clause 37, in which the binder in the form of a metal oxide is an oxide of a divalent metal, and the phosphate binder is KH ₂ PO ₄ . 40. The method according to clause 37, in which add the selected amount of one of the oxidizing agents or reducing agents to the waste or suspension before curing. 41. The method according to clause 37, in which H ₃ PO _{4 is} added to the suspension in order to lower the pH of the suspension. 42. The method according to clause 37, in which additionally add metal oxide to the suspension in order to increase the pH of the suspension. 43. The method according to § 38, in which the curing temperature is selected between 100 and 200 ° C. 44. The method according to § 39, in which the binder in the form of a metal oxide is MgO. 45. The ceramic form of waste obtained by the method according to claim 1. 46. The ceramic form of waste obtained by the method according to p. 26. 47. The ceramic form of waste obtained by the method according to clause 37.

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