JPS5975196A - Method of solidifying water soluble radioactive waste havingnitrate - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for solidifying nitrate-containing aqueous radioactive waste by evaporating and concentrating the waste solution in the presence of phosphoric acid and converting the resulting phosphate into a dissolvable solid.

In nuclear engineering, large (HAW), medium (MAW)
and radioactive liquid waste with low (LAW) specific activity is produced. Most of this waste is aqueous nitric acid, which is produced in the deprotection center during reprocessing of spent nuclear fuel.

For safe final storage, this liquid waste must be converted into a final storage product that is as stable as possible to leaching. For this purpose, it is necessary to neutralize nitric acid,
This is often done with soda or caustic soda. However, this results in high sodium nitrate contents in the radioactive waste solution, which either has a negative effect on the properties of the final storage product or complicates the process for disposing of this waste.

In order to solidify HAW-liquid waste, many methods are known for transferring the radionuclide into a leaching-stable matrix, such as a phosphate glass or a borosilicate glass. In these methods, nitrates are decomposed, often under reducing conditions, introducing residual alkali metals into the matrix.

In this way, for example, the vitrification process (W, T (eimerl, Behan
cllunghochrarH, oaktj, v8r
Abfalle, Chemie in un
sererZeit Volume 12, A3 (1978), 8
2-88), a HAW-solution of nitric acid is concentrated by evaporation,
Converted to phosphate glass by denitrification, drying, firing and smelting in the presence of phosphoric acid. According to DE 22 40 929 A1, the evaporative concentration is carried out in a reducing atmosphere using formaldehyde as reducing agent. The disadvantage of this method is that (1) baked goods are relatively easily dissolved in water;

For this reason, it can be melted by smelting, especially at temperatures of about 1000°C (
For example, 1 Itsu Patent Publication No. 22451.49) to about 12
At 00°C, it becomes insoluble in water.

This smelting requires large industrial outlays, which is actually only appropriate in the case of processing T-TAW solutions.

According to DE 2807324, inter alia sodium dihydrogen phosphate (NaH2P) is used instead of phosphoric acid.
An alternative to the PAMELA process using O4) requires smelting in the same way to improve properties, e.g. to reduce solubility.

According to Japanese Published Patent No. 2,125,915, highly active liquid waste is evaporated and concentrated and powdered.

The production of baked goods by denitrification with the addition of red phosphorus also requires subsequent embedding of the decomposition products into the glass melt.

Another method of burning radioactive waste containing sthorium nitrate is described in German Published Patent Application No. 2603157.
The iron removal shavings or f powder is added to prevent adhesion of particles in the fluidized bed through the formation of NaFeO2.

However, this product is also unsuitable for final storage storage due to its water solubility.

Furthermore, according to European Patent Publication No. 43397, HA
Methods of embedding W-waste in glass blocks made of polymeric aluminum phosphate are known. Monophosphate AQ (H2PO4
) The preparation of 3 must be carried out carefully so as not to form other undesired aluminum phosphates which do not polymerize into the phosphate glass. Furthermore, disadvantages are the high consumption of H3PO4 (molar ratio of H3PO4,An at least 6:l) and the high melting temperature of more than 10 oo'c. Furthermore, NaN
Methods involving the fixation of HAW-g ceramics without an O3 component cannot be easily transferred to the treatment of MAW/LAW-solutions with a high NaN*3 content.

The so-called MINERVA method was developed by EUROCHEMIC.
C, Som-bret, Present 5tatu
s of Techniques Used for H
igh -Level Liquid -Waste
5olidifi-cation, Radioac
tive Waste Management 2 (
1) (1981) pp. 58-60), the matrix for embedding the radioactive waste is present as granules in a solid phase moved by a stirrer, followed by the dosing of a decomposition product solution, phosphoric acid and aluminum hydroxide. using freshly formed aluminum phosphate. Reaction temperature 300-55
At 0°C, ``Mineralization'' ■Pottery is formed, the main component of which is tri-aluminum phosphate (M, PO4). Phosphate glasses are obtained by processing by smelting.A particular disadvantage of this process is the production of large amounts of radioactive waste due to the large aluminum phosphate component.

In addition to the known HAW-conditioning method,
MA, W- or T-AW = In the liquid waste fixation method, the dissolved sodium nitrate is often co-embedded in the matrix. The high salt content can lead to significant incompatibility with the matrix under certain disturbance case conditions, for example in the case of cement as a binder and water ingress into the final storage product or in the case of organic binders and fire as a disturbance case. tolerance may occur. This incompatibility significantly limits the content of radioactive waste in the IJ Fuchs and does not result in a decrease in waste volume due to increased concentration.

The purpose of the present invention is to remove nitrate-containing aqueous radioactive waste,
The objective is to obtain a method of evaporating and concentrating a waste solution in the presence of phosphoric acid, converting the resulting phosphate into a soluble solid and solidifying it, which increases the amount of waste to embed and increases the volume of the final stored product. can be reduced, resulting in a product with good compatibility with known binders, allowing the production of anhydrous final storage products.

This purpose according to the invention is to remove the alkali metal phosphate residues remaining after evaporative concentration from alkaline earth metal oxides and/or alkaline earth metal oxides.
or mixed with alkaline earth metal hydroxide followed by temperature 5
This problem can be solved by converting it into a sparingly soluble alkali metal/alkaline earth metal phosphate through Fl treatment at 00 to 950°C.

The radionuclides and other impurities are preferably NaCa (Mg
) PO4 compounds are fixed in renanitic phosphate mixed crystals.

Phosphoric acid, NaNO3300,! i'／／! is dissolved and added to the containing waste solution and the solution is evaporated to dryness to remove the nitric acid, producing a residue consisting primarily of water-soluble sodium phosphate containing radionuclides and other impurities.

Surprisingly, the water-soluble components of this residue can be obtained by mixing the residue with powders of alkaline earth metal oxides and/or alkaline earth metal hydroxides, in particular the elements magnesium and calcium, at temperatures of 500 to 950°C. It was found that heating to ℃ markedly reduced it. In contrast to this time-baked mixture, the water solubility of the alkali metal phosphate residues is only slightly reduced by heat treatment without addition of alkaline earth metals. In this way, the insoluble components of the waste converted to phosphates increase by approximately 10% as the calcination temperature increases by approximately 10%.
At ℃, it only increases by 2 to 10%, but at 700
'C was found to increase to 12-20%. On the other hand, waste products converted to alkali metal phosphates are converted into CaO
By mixing with powder and heating to 700°C, the insoluble components increased to 80%.

For this purpose, an amount of CaO corresponding to a molar ratio of -Na:Ca=1:1 was sufficient in time to form -NaCaPO4. ■ In the aqueous extract of the calcined powder mixture, only up to 4 g of nitrate per liter was found, compared to at least 21.99/l in the untreated aqueous waste containing 3300 jJ/' of NaNO3.

It has proven particularly advantageous to adjust the molar ratio of nitrate to phosphoric acid used to between 2 and 3 before evaporating the solution and removing the nitric acid. As a result, Na as an intermediate product,
Favorable formation of 2HPO4 is promoted.

Furthermore, a heat treatment of the alkali metal phosphate residue at a temperature of 300 DEG to 500 DEG C. before mixing with the alkaline earth metal oxide and/or alkaline earth metal hydroxide has proved particularly advantageous. This also results in Na2 as an intermediate product.
1 (formation of PO4 is promoted.

Due to their low molar mass and low cost, MgO and/or CaO or their hydroxides are preferably used. This is because, in particular, industrial magnesia, dolomite or quicklime are particularly advantageous in price. The addition of MgO has been found to be particularly preferred. The insoluble components of the waste converted to alkali metal/alkaline earth metal phosphates increased by up to 95% after mixing with a stoichiometric amount of M170 and calcining at 700°C.

X-ray examination revealed that the poorly soluble compound Na4Mg (PO
4) 2 was detected. The sodium content of this compound is
Rather, it is present at about 30% by weight, which is even higher than the sodium content of readily soluble sodium nitrate in the waste (27% by weight). ■ Nitrate could no longer be detected in the aqueous extract of the calcined powder mixture. The content of soluble Cs or Sr was up to 0.25% or 0.05% and was therefore a factor of 400 or 2000 lower than in the aqueous waste.

A further reduction in water solubility was preferably obtained by total addition of calcium hydrogen phosphate (CaHPO4) to a mixture consisting of waste material converted to alkali metal phosphate and CaO powder, followed by calcination at 700°C. Na2I(
Obtained for PO4. When sufficient stoichiometric amounts of Ca0 and CaHPO4 were used to form NaCaPO4, the insoluble components increased to 97%. In this case, in order to make the waste almost completely free of nitrates, the waste converted to alkali metal phosphates must be pretreated with 500
It has been found preferable to carry out a heat treatment at .degree.

X-ray analysis revealed that the insoluble compound NaCaPO4 [Buch-w
a], dit)] was found to be formed. When approximately % by weight of silicon dioxide is added, a renanitic mixed crystal (3NaCaP) having an apatite-like structure is similarly produced.
O4・Ca2Si,04) is obtained (,H,R
emy.

Lehrbuch der Anorganische
n Chemie, I 0th edition, 1960, 1 volume,
752 pages). In this insoluble mineral compound, a small amount of waste radionuclides and other impurities are practically completely embedded together. In that case, for example, a surface area of 50 to 200 m
Surface active S:LO2 with /g has proven to be particularly advantageous.

Particularly large capacity reductions are obtained if the powder mixture of alkali metal phosphate residue and additives is compressed into shaped bodies, for example cylinders or cuboids, before heat treatment. Owing to the poorly soluble nature of the alkali metal/alkaline earth metal phosphates formed, their inclusion in a leaching-stable matrix can optionally be completely abandoned.

Due to the good compatibility of the sparingly soluble and almost chemically inert alkali metal/alkaline earth metal phosphates produced in this way with known leaching-stable matrices, such as cement or bitumen, The matrix can be loaded with alkali metal/alkaline earth metal phosphates in a particularly advantageous manner to obtain a large reduction in capacity.

Next, the present invention will be explained in detail with reference to examples.

Example 1 The starting material is a MAW/deprotection center with the following composition:
LAW-liquid waste mimic (EZ-waste mimic): NaN0-, 300 &/lA
lO,23,9/lCa
1.50g/lCr
0.08. ! 9/lCu
O, 1.5g/lFe
O, 38,! 9/VK
0.08g/1Mg
O,'75. ! 9/IMn
O,08,! 9/1Mo 0.
38g/lNi, 0.08.
! 9/lZn 0.15. ! 7
/lZr 0.08 g/
lCs 10 g/l! Sr.
10 9/lNa2HPO
45g/1 TBP 0.2 g/1DB
P 0.2 111 Kerosene
0.2 E//1 All cations are present as nitrates and MO is present as Na molybdate.

This EZ-waste mimic 11 was converted into H3P04 (85% by weight)
A pale green clear solution was obtained upon mixing with 8Qm/. This corresponded to a NaNO3:H3PO4 mo/l/ratio of 3:l. The solution was concentrated by evaporation in a flask of No. 21 by heating to dryness with a heating cap, and along with its water removal,
671.1% by weight of HNO, which could be formed by the quantitative reaction of NaNO3 and H3PO4, was distilled off. This is the conversion rate, i.e. 2/2 of the conversion of NaNt03 to Na2HP transport.
Corresponds to 3 or more. At that time, the maximum temperature in the flask is approximately 3
It was 00'C. The residue consists of a pale pink solidified melt;
This could be easily crushed and powdered.

The water-insoluble component of this residue was 12%, and the pH of the solution was
was 7.5. The residue was further heated to 750° C., yielding 19% of insoluble components, and the pH of the aqueous extract was 9.5.

The same experiments were performed with pure NaNO3 and EZ-waste mimic without the addition of phosphoric acid and with pure NaNO3 with addition of H3PO4. The residues were heated to 300°C and 750°C, respectively.
heated to ℃. In all cases the water-insoluble components were less than 1%.

Example 2 110g of pale pink fine powder residue from Example 1 was added to CaO powder 4,67
g and baked at 750° C. for 1 hour. Na
Based on 2HPO4, the amounts of Ca,O used correspond to a total excess of 69%. (2) The water-insoluble component of the unbaked powder solid was 68.9%, and the aqueous extract was extremely alkaline and reacted (PH 12.5).

The same experiment using 24.51 g of Ca(OH) instead of CaO yielded 69.5% of the water-insoluble component in the calcined alkali metal/alkaline earth metal phosphate.

Example 3 Light pink fine powder residue 1 (l = 3.36 liters of MgO powder) from Example 1
．． ! 9 and baked at 750°C for 1 hour.

The water-insoluble component of this product was 925%, and the pH of the aqueous extract was 11.5.

The same experiment using Mg(OT() 23.55 f! instead of MgO) yielded 83.9% of the water-insoluble component in the calcined alkali metal/alkaline earth metal phosphate.

EXAMPLE An EZ-waste mimetic of the composition described in Cattle Example 1 was prepared from T(3PO4
(85% by weight) mixed with 203.5 g (NaNO3:
1-T3PO4 molar ratio of 2:1). Solution example 1
It was evaporated to dryness as follows, the residue reaching a temperature of approximately 3°C. By further heating the experimental sample of this residue, only a slight increase in water-insoluble constituents to 11.7% was obtained: Temperature Insoluble constituents of aqueous extract (°C)
(%) pH 4009,58 60011,211 7001,1゜7 115 The nitrate content of the residue heated to 400°C is
It was 30.6 mol% based on aNO3.

At 500°C, the residue is nitrate-free, which is due to Na2HP
Corresponds to the formation of O4.

Example 5 10 g of the finely powdered residue of Example 4 heated to 300°C,
CaO4,67,! i' mixed intimately. The amount of Ca0D corresponded to approximately 1.7 times the stoichiometric amount required for the formation of NaCaPO4.

Heat treatment of this powder mixture reduces the amount of insoluble components by 700%.
A significant increase of up to 799% was obtained. In the following table,
The results of parallel experiments without addition of CaO are described together.

Contains 400 68.0 12.5 Does not contain 400 9.5 8 Contains 600 73.1 12.5 Does not contain 6
00 10.8 Contains 11 700
79.9 Does not contain 13 7oO11,5
11 Nitrate and sulfite components of several g/IJ were found in the aqueous extract in all cases. Due to the excess amount of CaO, the solution was significantly alkaline and reactive (pH 12,
5-13).

Example 6 10 g of the finely powdered residue of Example 4 heated to 300°C was
gO3,36,! 9 and heated to 700°C.

As a result, the water-insoluble components increased to 94.8% as shown in the following table.

400 54-. 5 11600
90.512 700 94.8 11.5 Comparative values without the addition of Mg0 can be found in the table for the example cow.

In the samples heated to 600°C and 700°C, no nitrates and only traces of nitrites were detected in the aqueous extracts.

Furthermore, chemical analysis of aqueous extracts revealed Cs and S in alkali metal/alkaline earth metal phosphates calcined at 700°C.
It was found that out of the total amount of r, only 0.03% of CsQ, 14 and Sr were dissolved. X-ray analysis revealed that the water-insoluble compound r-Na4Mg(PO4)2 was formed as the main component, along with excess amounts of MgO and γ-Na3PO4. This compound crystallizes in an equiaxed lattice and contains 30.0% by weight Na.

This example shows sufficient water-soluble NaNO3 (solubility 92(1/
Starting material NaNO3 of water 11) in a highly economical manner
Even higher Na content (30%) than in (27%)
) to a water-insoluble sodium compound was obtained.

Example 7 10G of the fine powder residue from Example 4 heated to 300°C was
Stoichiometric amount of MgO1 to form aMgPO4
, 99,9 and heated to 700°C. The product is 9
3.4% insoluble in water, the aqueous extract had a pH of 11.5 and was nitrate-free.

Example 8 10 g of the finely powdered residue of Example 4, which had been previously calcined to 500°C, had a cao of 2.76,! i' and CaHPO4" 2
It was intimately mixed with 48g of H208 and baked at 700°C for 1 hour. The amount was calculated to quantitatively react Na2HPO4 to NaCaPO4. (2) The calcined product was 97% insoluble in water. X-ray analysis revealed that only an insoluble compound NaCaPO4 (Buchwalait) with an orthorhombic lattice structure was formed.

The alkali metal/alkaline earth metal phosphates prepared according to these examples could be incorporated in large amounts with sufficient compatibility into known binders, such as cements, bitueumens or organic plastics.

Claims (1)

[Scope of Claims] 1. A method for evaporating and concentrating nitrate-containing aqueous radioactive waste in the presence of phosphoric acid, converting the resulting phosphate into a dissolvable solid and solidifying the aqueous radioactive waste, in which the alkali metal phosphoric acid remaining after the evaporation and concentration is The salt residue is mixed with an alkaline earth metal oxide and/or alkaline earth metal hydroxide, followed by a temperature of 500 ml.
A method for solidifying nitrate-containing aqueous radioactive waste, which comprises heat-treating at ~950°C to convert it into a soluble alkali metal/alkaline earth metal phosphate. 2. The method of claim 1, wherein the molar ratio of nitrate to added phosphoric acid is adjusted to between 2 and 3. 3. The method according to claim 1 or 2, wherein the alkali metal phosphate residue is heated at 300 to 500°C before mixing with the alkaline earth metal oxide and/or alkaline earth metal hydroxide. . MgO and/or CaO or Mg (Oi
4. The method according to any one of claims 1 to 3, using ( ) and/or Ca(OH)2. 5. The method according to any one of claims 1 to 4, characterized in that the alkaline earth metal oxide mixture is (1) calcined dolomite, quicklime and/or magnesia. 6. The method according to any one of claims 1 to 5, characterized in that the alkaline earth metal is magnesium. 7. The alkali metal phosphate residue is additionally mixed with calcium hydrogen phosphate before calcination with the alkaline earth metal oxide and/or alkaline earth metal hydroxide. The method described in any of Section 6. 8. The alkali metal phosphate residue is mixed with calcium hydrogen phosphate and surface-active silicon dioxide powder before calcining with the alkaline earth metal oxide and/or alkaline earth metal hydroxide. The method according to any one of Ranges 1 to 7. 9. The method according to any one of claims 1 to 8, wherein the powder mixture comprising the alkali metal phosphate residue and the additive is heat-treated at 650 to 850°C. 10. The method according to any one of claims 1 to 9, wherein a powder mixture comprising an alkali metal phosphate residue and an additive is compressed into a molded body before heat treatment. 1.1. Ilt-soluble alkali metal/alkaline earth metal phosphates are leached stable matrix) embedded in IJ lux,
A method according to any one of claims 1 to 10.