The invention relates to the field of conditioning waste by cementation, i.e. by incorporation in a cement matrix.
More specifically, it relates to a method for conditioning an acid waste by cementation, this waste possibly being a liquid waste such as an aqueous effluent, a semi-liquid waste such as a sludge, or a solid waste such as rubble, or a mixture thereof.
The invention finds particular application in the conditioning of acid waste produced by the nuclear industry and hence contaminated or potentially contaminated by radioelements, such as:
However, it can evidently be advantageously used to condition any other type of acid waste irrespective or origin (acid waste from the chemical industry, agri-food industry, test laboratories, etc.).
The conditioning of hazardous waste, nuclear waste in particular, by cementation is a conditioning method which has numerous benefits including simple implementation and relatively low cost (when compared with the costs of other conditioning methods).
The cementing of a waste involves the mixing of this waste with a cement material.
Cement materials containing hydraulic cements such as Portland cements, or containing blast furnace slag, which are conventionally used to cement waste, are highly basic materials.
The direct mixing of an acid waste with this type of material is difficult, even impossible, to carry out on account of the heat generated by the acid-base reaction, this heat being stronger the higher the acidity of the waste.
This is the reason why the state of the art recommends prior neutralisation of waste acidity by mixing the waste with sodium hydroxide (NaOH) and/or calcium hydroxide (Ca(OH)2) to prevent or at least limit the heat generated by the acid-base reaction (cf. for example C. Utton and I. H. Godfrey, Report by the National Nuclear Laboratory NNL (09) 10212, 29 Jan. 2010, hereafter reference [1]).
However, this neutralisation has a certain number of drawbacks which are greater the higher the acidity of the waste, such as:
There has been proposed in International application PCT WO 2004/075207, hereafter [2], a method for conditioning nuclear waste and in particular acid waste contaminated by actinides and/or transuranics, by cementation in a matrix of magnesium phosphate cement developed by Argonne National Laboratory under the trade name Ceramicrete™.
In this method also, provision is made for prior treatment of the acid waste by mixing the latter with magnesium oxide (MgO), which is one of the two components of Ceramicrete™, to bring the pH to a value of at least 5, and then mixing the waste with the components of the cement material.
Here too, this prior treatment intended to neutralise the acidity of the waste is both time- and energy-consuming, since the acid waste must be mixed slowly with the magnesium oxide to prevent the temperature of the resulting mixture being too high. For example, it is reported in Example 3 of reference [2], that to neutralise just 195 g of concentrated hydrochloric acid with 110.4 g of magnesium oxide—which corresponds to quantities far removed from those likely to be involved in waste cementation on an industrial scale—a time of 40 minutes is required.
Having regard to the foregoing, the inventors set themselves the objective of providing a method allowing acid waste and, more specifically, waste of high to very high acidity to be conditioned in a cement matrix, the method being free of any prior step to neutralise the acidity of this waste, without the absence of such neutralisation having a notable negative and/or uncontrolled impact on the setting of the cement material, on the reaction heat of this material and on the mechanical performance of the cement/waste composite obtained.
They further set themselves the objective that this method should be applicable to the manufacture of conditioning packages for acid waste, allowing a high incorporation rate of waste in a cement matrix with a view to minimising the number of conditioning packages for a given volume of waste.
As part of their work, the inventors have found that contrary to the teaching of reference [2], it is possible to incorporate highly to very highly acidic waste, such as concentrated nitric or sulfuric acid or sludge with high hydrofluoric acid content, in a magnesium phosphate cement material and directly, i.e. without a prior neutralisation step of the acidity of this waste, and without the setting of the cement material, the reaction heat of this material and the mechanical performance of the cement/waste composite being negatively and/or uncontrollably impacted.
It is upon these experimental findings that the invention is based.
A first subject of the invention is therefore a method for conditioning an acid waste by cementation, the acid waste being selected from among liquids having a pH of no more than 4, semi-liquids having a pH of no more than 4, solids of which the partial or full dissolution in water leads to a solution or suspension having a pH of no more than 4, and mixtures thereof, which comprises the steps of:
a) preparing a cement paste having as components at least: a magnesium phosphate cement and the acid waste; and
b) hardening the cement paste thus obtained, and which is characterized in that at step a), the cement paste is prepared without the acid waste being subjected beforehand to any treatment consisting in reducing the acidity thereof.
In the invention, by “magnesium phosphate cement”, it is meant any cement composed of a source of oxidized magnesium, i.e. in oxidation state+II, this source typically being a magnesium oxide (MgO) calcined at high temperature (of “hard burnt” or “dead burnt” type), pure or containing impurities of type SiO2, CaO, Fe2O3, AlO3, etc, and a source of water-soluble phosphate, this source typically being a phosphoric acid salt.
It is recalled that this type of cement, that is typically prepared by mixing the source of oxidized magnesium (in powder form) with an aqueous solution comprising the water-soluble phosphate, leads to the formation of a cement material via reaction between the source of oxidized magnesium (which is basic) and the source of water-soluble phosphate (which is acid), said sources of oxidized magnesium and water-soluble phosphate reacting together at ambient temperature to form a cement paste which sets rapidly.
Among the magnesium phosphate cements, preference is given to the cements composed of:
Among these phosphoric acid salts, preference is given to potassium dihydrogen phosphate.
In addition to comprising the magnesium phosphate cement and the waste, the cement paste may comprise at least one admixture selected from among plasticizers (whether or not water-reducing), superplasticizers, setting retarders, and compounds which combine several effects such as superplasticizers/setting retarders, as a function of the workability, setting and/or hardening properties it is desired to impart to the cement paste.
In particular, the composition may comprise a superplasticizer and/or a setting retarder.
Superplasticizers that are particularly suitable are high water-reducing superplasticizers of polynaphthalene sulfonate type.
Setting retarders that are suitable are particularly hydrofluoric acid (HF) and the salts thereof (e.g. sodium fluoride), phosphoric acid (H3PO4) and the salts thereof (e.g. sodium phosphate), boric acid (H3BO3) and the salts thereof (e.g. sodium borate of borax type), citric acid and the salts thereof (e.g. sodium citrate), malic acid and the salts thereof (e.g. sodium malate), tartaric acid and the salts thereof (e.g. sodium tartrate), sodium carbonate (Na2CO3) and sodium gluconate.
Amongst these, preference is given to hydrofluoric acid, sodium fluoride, boric acid and sodium borate.
When the cement paste comprises a superplasticizer, this preferably does not represent more than 4.5% by mass of the total mass of this cement paste, whilst when the cement paste comprises a setting retarder, this preferably does not represent more than 10% by mass of the total mass of said cement paste.
According to the invention, the cement paste may additionally comprise:
The terms “sand” and “gravel” are to be construed in the usual acceptance thereof in the field of mortars and concretes (cf. in particular standard NF EN 12620 concerning aggregates for concrete), namely that:
According to the invention, the cement paste typically comprises a water/magnesium phosphate cement mass ratio ranging from 0.10 to 1, preferably from 0.20 to 0.60 and better still from 0.30 to 0.55.
The water contained in the cement paste can come in full or in part from the acid waste if the latter is liquid, a semi-liquid waste or a solid waste that has previously been wetted. Therefore, the amount of mixing water which can be added to the magnesium phosphate cement and acid waste when preparing the cement paste is preferably adjusted taking into consideration the water content of the acid waste.
The preparation of the cement paste, or step a), can be conducted in several manners, in particular as a function of the form of the acid waste: liquid, semi-liquid or solid, and for a solid waste whether it is dry or wetted.
Therefore, for example, in a first embodiment of the method, step a) comprises the sub-steps of:
i) loading the magnesium phosphate cement and water into a container and mixing the cement and water until a homogeneous mixture is obtained;
ii) adding the acid waste in dry, wetted, semi-liquid or liquid form to the container; and simultaneously or successively
iii) mixing the mixture obtained at sub-step i) with the acid waste until homogenisation, whereby the cement paste is obtained.
It is to be noted that the source of water-soluble phosphate, which is contained in the magnesium phosphate cement, and the water can be added to the container separately or in the form of a solution previously prepared by dissolving the phosphate source in this water.
If one or more admixtures and/or sand and/or gravel are to be used, these can be added to the container at the same time as the magnesium phosphate cement and the water, and can be mixed with the cement and the water at sub-step i).
In a second embodiment of the method, step a) comprises the sub-steps of:
i) loading the acid waste in dry form into a container and mixing the waste until homogenisation;
ii) adding water and the magnesium phosphate cement to the container; and simultaneously or successively
iii) mixing the acid waste with the water and the magnesium phosphate cement until homogenisation, whereby the cement paste is obtained.
Here, too, the water-soluble phosphate source, which is contained in the magnesium phosphate cement, and the water, can be added to the container separately or in the form of a solution previously prepared by dissolving the phosphate source in this water.
If one or more admixtures and/or sand and/or gravel are to be used, these can be added to the container at the same time as the water and the magnesium phosphate cement, and can be mixed with the acid waste, the water and the cement at sub-step iii).
In a third embodiment of the method, step a) comprises the sub-steps of:
i) loading the acid waste in wetted, semi-liquid or liquid form into a container and mixing until homogenisation;
ii) adding the magnesium phosphate cement to the container and mixing the cement with the acid waste until a homogenous mixture is obtained;
iii) optionally adding water to the container (if the total amount of water required for mixing the magnesium phosphate cement is not contained in the acid waste), and, simultaneously or successively, mixing the mixture obtained at sub-step ii) with the water until homogenisation, whereby the cement paste is obtained.
If one or more admixtures and/or sand and/or gravel are to be used, these can be added to the container at the same time as the magnesium phosphate cement and can be mixed with the cement and the acid waste at sub-step ii).
In a fourth embodiment of the method, step a) comprises the sub-steps of:
i) loading the acid waste in wetted, semi-liquid or liquid form into a container and mixing the waste until homogenisation;
ii) adding the sand and/or gravel to the container and mixing the waste with the sand and/or gravel until a homogeneous mixture is obtained;
iii) adding water and the magnesium phosphate cement to the container; and simultaneously or successively
iv) mixing the mixture obtained at sub-step ii), with the water and the magnesium phosphate cement until homogenisation, whereby the cement paste is obtained.
Here, too, the water-soluble phosphate source, which is contained in the magnesium phosphate cement, and the water can be added to the container separately or in the form of a solution previously prepared by dissolving the phosphate source in this water.
If one or more admixtures are to be used, these can be added to the container at the same time as the water and the magnesium phosphate cement, and can be mixed at sub-step iv) with the mixture obtained at sub-step ii), the water and the cement.
In a fifth embodiment of the method, step a) comprises the sub-steps of:
i) loading the magnesium phosphate cement into a first container and mixing the cement until homogenisation;
ii) loading water and the acid waste in dry, wetted, semi-liquid or liquid form into a second container, and mixing the water with the waste until a homogenous mixture is obtained;
iii) transferring the mixture obtained at sub-step ii) from the second container to the first container, and simultaneously or successively
iv) mixing the magnesium phosphate cement with the mixture obtained at sub-step ii) until homogenisation, whereby the cement paste is obtained.
If admixtures and/or sand and/or gravel are to be used, these can be added to the first container at the same time as the magnesium phosphate cement and can be mixed with this cement at sub-step i).
Irrespective of the manner in which step a) is performed, the mixing operations can be carried out using a mechanical mixer such as a mixer device with one or more rotating impellers.
In addition, irrespective of the manner in which step a) is performed, the container in which this step is carried out may or may not be a container also to be used as conditioning drum.
Therefore, the method may additionally comprise, between steps a) and b), a step of draining the container in which:
The hardening of the cement paste, or step b), can be obtained by storing the conditioning drum at ambient temperature and under controlled hygrometry conditions.
This conditioning drum is hermetically sealed either between step a) and step b) or after step b).
As previously indicated, the acid waste is selected from among liquids having a pH of no more than 4, semi-liquids having a pH of no more than 4, solids of which the partial (if these solids contain insoluble matter) or total dissolution in water leads to a solution or suspension having a pH of no more than 4, or a mixture thereof.
If it is liquid, the acid waste can notably comprise or be composed of:
If it is semi-liquid, the acid waste may notably comprise or be composed of a sludge such as:
If it is solid, the acid waste may notably comprise or be composed of:
In all cases, the acid waste preferably represents from 5% to 70% by mass of the mass of the cement paste.
If the acid waste is a solid waste, then the method may additionally comprise a preliminary treatment to reduce the dimensions of this waste, for example mechanical treatment of crushing, fragmenting type or the like.
The method of the invention has numerous advantages notably including simplification and shortening of the conditioning time for cementing acid waste, and thereby allowing savings in time, energy and reactants without affecting the quality of the conditioning packages obtained.
Other characteristics and advantages of the method of the invention will become apparent from the following additional description referring to examples of embodiment of this method for cementing aqueous acid solutions and acid sludge.
This additional description is evidently given solely to illustrate the subject of the invention and is not to be construed as limiting this subject-matter.
A first series of mortars was prepared having the composition and characteristics given in Table 1 below.
For doing that, the solid constituents of these mortars (i.e. MgO, KH2PO4, borax and sand) were first mixed together in a mixer for 2 minutes to obtain a homogenous mixture, and the mixture thus obtained was mixed with an aqueous mixing solution for 30 seconds at slow speed, then 30 seconds at rapid speed and finally for 1 minute at slow speed.
Six different aqueous mixing solutions were used, namely:
The mortars were subjected to:
After hardening, they were also subjected to:
The results of these measurements and DTA are shown in
1° has no notable negative impact on the setting time of mortars for solutions having a pH equal to or higher than 2 (i.e. a concentration of nitric acid equal to or lower than 0.01 mol/L); on the other hand, an increase in setting time is observed for solutions having a pH equal to lower than 1 (cf.
2° leads to a reduction in the heat of hydration of mortars when the acid concentration of the aqueous solution is increased (cf.
3° induces a reduction in the compressive strength of mortars but that, irrespective of the nitric acid concentration, the compressive strength obtained is greater than 8 MPa which represents the desired minimum value of compressive strength (cf.
After hardening, the mortars were also characterized by X-ray diffraction (XRD).
As shown in
No trace of KH2PO4 is observed in the XRD diagram of the reference mortar, suggesting that this compound is fully consumed at the time of k-struvite formation.
Therefore, the addition of nitric acid to a mortar at the time of preparation thereof induces the formation of potassium nitrate.
The present example shows that the cementation of highly to very highly acidic waste produced by industrial processes using nitric acid, such as aqueous effluents derived from the refining of natural uranium concentrates or from the treatment of spent nuclear fuels, can be carried out directly, i.e. without any prior treatment of this waste intended to reduce the acidity thereof, and without shortening the setting time and without increasing the reaction heat.
A light reduction in mechanical properties is observed with an increase in nitric acid concentration. This is due to the fact that, since potassium dihydrogen phosphate reacts with nitric acid, it is partially consumed by this reaction and is hence less available to react with the magnesium oxide and to form k-struvite with the latter. An increase in the amount of phosphoric acid salt, in this case KH2PO4, incorporated in the cement paste, mortar or concrete should be sufficient to overcome this phenomenon.
A second series of mortars was prepared having the composition and characteristics given in Table 1 above, following the same operating protocol as indicated in Example 1 but using as mixing solution:
The mortars were subjected to measurements of setting time performed in the same manner as in Example 1 and, after hardening, to compressive strength measurements and DTA also performed in the same manner as in Example 1.
The results of the setting time measurements are given in Table II below, whilst the results of the compressive strength measurements and DTA are given in
This Table and
After hardening, the mortars were also characterized by XRD.
As shown in
A sludge containing hydrofluoric acid was cemented proceeding as follows.
First, corrosion products in the form powdery flakes and comprising on average: 17.6 mass % of fluorine, 4.4 mass % of nickel, 9.8 mass % of iron, 15.6 mass % of uranyl fluoride (UO2F2) and 33.3 mass % of uranium tetrafluoride (UF4), were mixed with water in a mass ratio of 1 to prevent any dispersion of the flakes into the surrounding atmosphere.
An acid sludge was obtained of pH 2 since the UF4 contained in the flakes reacts with water to release hydrofluoric acid following the equation:
UF4+2H2O→UO2+4HF.
A first mortar was then prepared having the composition and characteristics given in Table III below.
For doing that, the solid constituents of the mortar (i.e. MgO, KH2PO4, borax and sand) and the additional water were first mixed together in a mixer until homogenisation, after which the acid sludge was added to the mixer and the whole was mixed until homogenisation.
As reference, a second mortar was prepared of same composition and characteristics as the first mortar with the exception that it was free of corrosion products, i.e. flakes.
No notable difference was observed in terms of setting time and reaction heat between the first and second mortars.
After hardening of the mortars, the latter were cut into samples of cubic shape with sides of 4 cm and these samples were subjected to compressive strength tests using a manual press.
The results of these tests are given in
The water/cement mass ratio of the first and second mortars was high since it is 0.52, the effect of which is to reduce the compressive strength, an effect which is notoriously known for all cement materials and, in particular, for materials based on magnesium phosphate cements.
On the other hand,
Number | Date | Country | Kind |
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1873254 | Dec 2018 | FR | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FR2019/053005 | 12/10/2019 | WO | 00 |