WO2006090069A1

WO2006090069A1 - Method for the aqueous-phase synthesis of layered double hydroxide-type compounds

Info

Publication number: WO2006090069A1
Application number: PCT/FR2006/000417
Authority: WO
Inventors: Alain Seron; Fabian Delorme; Pierre Galle-Cavalloni
Original assignee: B.R.G.M. - Bureau De Recherches Geologiques Et Minieres
Priority date: 2005-02-25
Filing date: 2006-02-23
Publication date: 2006-08-31
Also published as: FR2882549B1; CN101151211A; FR2882549A1; EP1866251A1

Abstract

The invention relates to a method for the aqueous-phase synthesis of layered double hydroxide-type compounds from at least partially solid precursor elements. The inventive method is characterised in that it consists in: using natural minerals or industrial by-products as precursor elements; at least partially solubilising the precursor elements, such as to obtain a solution of divalent and trivalent cations; and performing a coprecipitation with a base comprising said solution of cations.

Description

PROCESS FOR AQUEOUS PHASE SYNTHESIS OF DOUBLE LAMELLAR HYDROXIDE TYPE COMPOUNDS

The present invention relates to a process for the synthesis in the aqueous phase of compounds of the Hydroxydes Doubles type

Lamellar (hereinafter referred to as HDL) from low cost products such as natural minerals or industrial waste.

It is known that HDL, which may be of natural origin, such as hydrotalcite, or synthetic, are products which have a structure in sheets, exchangeable ions compensating for a deficit of charges.

The general formula characterizing the products of the HDL family is as follows:

[[M (II)] n [M (III)] _z (OH) ₂ ] [A _z -Z] y -H ₂ O

/ n with:

M (II) representing a divalent cation which may be ions

Mg ²⁺ , Mn ²⁺ , Fe ²⁺ , Co ²⁺ , Cu ²⁺ , Ni ²⁺ , Zn ²⁺ , etc. M (III) representing a trivalent cation that can be Al ³⁺ , Cr ³⁺ , Fe ³⁺ , Mn ³⁺ , Co ³⁺ , etc .; Substitutions with monovalent or tetravalent cations can be observed in these structures. A, representing a negatively charged complex which can be in particular a compensating anion such as CO ₃ ^{2 "} , OH ^" , Cl ^" SO ₄ ^2" , BO ₃ ^" , F ^" , Cl ^" , Br ^" , ClO ₄ ^" , [FeCN ₆ ] ^" " Or a DNA molecule.

The properties of HDL-type compounds are directly related to their structure. They are particularly interesting because, on the one hand, of their ability to integrate, in it, a a multitude of divalent and trivalent cations, but also certain monovalent cations (Li) and, on the other hand, their ability to integrate a large variety of anions into the interlamellar space. Because of these properties HDL are likely to find direct applications in various fields, including that of pollution control and especially the treatment of pollution by heavy metals such as for example lead, zinc, or tin or those caused by anions, such as sulphates, arsenates, chromates, or any elements that may be trapped during HDL formation or during anion exchange processes in the structure of these minerals, as described in French Application No. 03.13647.

It has been proposed in the prior art various methods for producing HDL by synthesis.

Among the various processes used when it is desired to obtain pure products, the best known is that involving the coprecipitation of the constitutive cations of the HDL concerned.

Three techniques have been mainly proposed, namely: a titration with sodium hydroxide and / or sodium carbonate _;

a precipitation at constant pH under conditions of low supersaturation;

a precipitation at constant pH under conditions of high supersaturation. According to the first technique, a base (NaOH or Na ₂ CO ₃ ) is reacted with a solution of the cations that it is desired to integrate into the structure of the HDL, obtained by dissolving laboratory salts. Such a technique performed by varying the pH, leads to a sequential precipitation of the cations. For example, in the case of the Mg / Al system: Al (OH) ₃ precipitates as soon as pH = 4-4.5. The mixed hydroxide is obtained for a pH of between 7.5 and 8.5, whereas the hydroxide Mg (OH) ₂ alone would have been obtained with a pH greater than 9.5.

It is known that in order to precipitate HDL, the pH must be equal to or greater than the precipitation pH of the most soluble simple hydroxide. Precipitation at constant pH, under supersaturation conditions, can be carried out either by slowly adding a solution of NaOH and / or Na ₂ CO ₃ and a solution of the useful cations which one wishes to integrate into the HDL, to a large volume of water (conditions of low supersaturation), either by rapidly and directly adding the cation solution to that of NaOH and / or Na ₂ CO ₃ (conditions of high supersaturation).

It has also been proposed to carry out the synthesis of HDL by the hydrothermal route. For this purpose, mechanical mixtures of MgO and Al ₂ O ₃ or mixtures resulting from the decomposition of the corresponding nitrates have been used. The treatment of these mixtures, in an autoclave at 600K at pressures which may exceed 10OMpa, led to the production of a mixture of phases containing HDL. Furthermore, Japanese Patent JP 26418A discloses a process for producing hydrotalcite, namely an HDL compound in which M (II) is an Mg ²⁺ ion and M (III) is an Al ³⁺ ion, compensating anion A may be a carbonate ion, a sulfate ion, etc. This method provides for reacting magnesium ions and aluminum ions in the presence of an alkali in an aqueous solution, which alkali is calcium hydroxide. The various processes used according to the prior art have the disadvantage of being particularly expensive to implement, particularly because the precursor elements which have been used are themselves expensive, although so far, the many potential applications offered by HDL compounds have not been exploited as they should have been.

The object of the present invention is to provide a process for carrying out the synthesis of compounds of the double-layered hydroxide type at particularly low costs such that they make it possible to apply said compounds in various technical fields, where up to now their cost price paralyzed their development despite the high potential of these products.

The subject of the present invention is therefore a process for synthesizing, in the aqueous phase, compounds of the double lamellar hydroxide type starting from at least partly solid precursor elements, characterized in that:

- precursors are used for natural minerals or industrial byproducts,

at least partial solubilization of these precursor elements is carried out, so as to obtain a solution of divalent and trivalent cations,

a base co-precipitation of this cation solution is carried out.

According to the invention the precursor element may in particular consist of calcite, magnesite, rhodocrosite, dolomite, aluminum scrap, or aluminum-rich hydroxide sludge, aluminum base ores, or aluminum sulphate. According to the invention also, the pH value of the cation solution will be adjusted by adding a base in order to bring its value to that necessary to obtain a precipitation of the desired phase. Moreover, the ratio of the quantity of trivalent cations to the sum of the amount of divalent cations and trivalent cations [Mm / (Mn + M _π i)] will be adjusted by adding trivalent cations (Mm) to a value necessary to obtain the precipitation of the desired phases, this value generally being between 0.2 and 0.4. The added cations may result from the solubilization of industrial by-products such as, in particular, aluminum scrap, or aluminum-rich hydroxide sludge, aluminum base ores, or aluminum sulphate.

Of course the process according to the invention may be implemented with all types of minerals and / or industrial waste insofar as the latter may contain divalent and / or trivalent cations and able to be put in solution by acidic or basic attack.

The embodiments of the present invention will be described below, by way of nonlimiting examples, with reference to the appended drawing in which: FIG. 1 represents an X-ray diffraction diagram of a first example of HDL obtained according to the invention using precursors consisting of magnesite, and aluminum sulphate,

FIG. 2 represents an X-ray diffraction diagram of a second example of HDL obtained according to the invention using precursors consisting of calcite and aluminum nitrate, FIG. 3 represents an X-ray diffraction pattern of a third example of HDL obtained according to the invention using precursors consisting of rhodocrosite and aluminum nitrate. FIG. 4 is a scanning electron micrograph of an HDL obtained according to the invention from a calcite. (one cm representing 3.3 micrometers).

EXAMPLE I In a first example of implementation of the invention, a synthesis of HDLs was made from a natural mineral element of low cost, namely a purified magnesite (MgCO ₃ 91%).

Trivalent cations, in the form of aluminum sulphate, were added so that the value of the

M ₁₁₁ ratio M ₁₁₁ + M _n is in the range 0.2-0.4 which, as established, constitutes a favorable area for the formation of HDL type compounds. It could also be used according to the invention to do this to an addition of aluminum nitrate or an aluminum solution obtained from a waste rich in aluminum.

The solution of magnesite was obtained by attacking it with nitric acid. The solution thus obtained was then filtered in order to eliminate any insoluble residues, then an addition of aluminum sulphate was made, so as to bring the

₁₁₁ M ₁₁₁ M ratio -VM ₁₁ ^ _a ^ QO _will χ _eur, 24.

Co-precipitation was then carried out by adding to the solution obtained a mixture of sodium hydroxide and sodium carbonate in the presence of air. The product obtained at the end of this synthesis was characterized by X-ray diffraction, and the representative diagram of this structure is shown in FIG. 1. The synthesis made it possible to obtain a pure phase of the hydrotalcite type. This product shows a diffraction pattern characteristic of a well crystallized phase.

Furthermore, a sample of the HDL compounds obtained according to the invention was studied by transmission electron microscopy (TEM) and the presence of lamellar particles of very small dimensions was found.

(less than 50 nm).

In addition, measurements made on the HDL compounds thus obtained revealed that these were of a high whiteness, evidenced by the indices Rx = 96.1, Ry = 96.1, Rz = 95.8.

EXAMPLE II

In another example of implementation of the invention, a synthesis of HDL compounds has been carried out from another low-cost natural mineral element, namely a calcite (calcium carbonate) ground, in particular resulting from the grinding of a limestone rock.

In this embodiment, since the precursor element contained calcium, it was avoided, when the solution was put into solution, to provide on the one hand sulphate ions which would have caused the formation of gypsum. preferably to the desired HDL compounds and on the other hand carbonate or bicarbonate ions which would have caused the precipitation of calcium carbonate. For this purpose, the water used for dissolving has been previously degassed by bubbling nitrogen in order to remove the dissolved carbon dioxide. The passage of calcite into solution was carried out by attacking it with nitric acid. As in the previous example, a contribution of trivalent cations in the form of nitrate has been carried out

of aluminum, so as to bring the ratio M ₁₁₁ + M ₁₁ ^ _a value equal to 0.25. Co-precipitation was then performed by adding soda to the solution.

The product obtained at the end of this synthesis was characterized by X-ray diffraction, and the representative diagram of this structure is shown in FIG.

A second characterization was carried out using a scanning electron microscope (SEM) which made it possible to demonstrate the lamellar character of the product obtained. Large particles (of the order of 10 to 20 μm) and of small thickness (of the order of 0.2 μm) have thus been observed. A photograph of this HDL is reproduced in Figure 4.

On the other hand, while the carbonate constituting the precursor had a whiteness characterized by the indices Rx = 93, 1; Ry = 91.5; Rz = 88.3 it is found that the HDL compound obtained has much higher whiteness characteristics, namely Rx = 96.8; Ry = 9β, 4; and Rz = 95, l.

However, it is known that in the field of papermaking, when it is desired to obtain a paper of both quality and having good whiteness characteristics, a coating operation is carried out consisting in depositing on the surface thereof a product having both a lamellar structure (usually kaolin or talc) which will give it good qualities as to its appearance (smoothness) and a second product which will bring a great whiteness. We did hitherto called upon for this purpose to various products whose calcite (CaCOs) constituting precisely the precursor of the HDL obtained according to the invention. However, the lamellar nature of the structure of these and their high whiteness quality establish their great ability to at least advantageously replace the products previously used in the prior art.

EXAMPLE III In a third exemplary embodiment of the present invention, a precursor element was used for rhodocrosite (manganese carbonate). As previously it was put in solution by attack with a mixture of nitric and hydrochloric acid. Nitrate was then added

M ₁₁₁ aluminum, so as to bring the ratio - ^ /// ⁺ - ^ // to a value of 0.33.

Co-precipitation was then carried out by adding to the solution obtained a solution of sodium hydroxide. The product obtained at the end of this synthesis was characterized by X-ray diffraction, and the representative diagram of this structure is shown in FIG.

EXAMPLE IV

In a fourth embodiment of the present invention, as precursor elements, a zinc carbonate and an iron sulfate have been used.

The zinc carbonate was dissolved by etching with a nitric acid solution. An addition of iron (water-soluble) sulfate was then carried out to bring the ratio M (III) / [M (II) + M (III)] to a value of 0.33 and then co-precipitation with sodium hydroxide solution obtained.

As previously X-ray diffraction analysis showed that the product obtained at the end of this synthesis is an HDL.

EXAMPLE V

In a fifth exemplary embodiment of the present invention, as precursor elements, a magnesium carbonate and a metal aluminum rich waste product have been used.

The magnesium carbonate was dissolved by etching with a nitric acid solution. The metal aluminum was then dissolved by treating the waste with a sodium hydroxide solution. Co-precipitation was obtained by adding this solution to the previously obtained magnesium solution so as to bring the M (III) / [M (II) + M (III)] ratio to a value of 0.33. An X-ray diffraction analysis showed that the product obtained at the end of this synthesis is an HDL.

It has also been established that the process according to the invention can be carried out with all types of minerals and / or industrial wastes to the extent that they were likely to contain divalent and / or trivalent cations and able to can be dissolved by acid or alkaline attack.

The present invention is particularly interesting in that it allows the use of industrial waste in a direct way, that is to say without the latter having to undergo any purification, it being effected d ' itself according to the method.

Claims

1. A process for the synthesis in the aqueous phase of double lamellar hydroxide type compounds from at least partially solid precursor elements, characterized in that:

- precursors are used for natural minerals or industrial byproducts,

at least partial solubilization of the precursor elements is carried out, so as to obtain a solution of divalent and trivalent cations,

a base co-precipitation of this cation solution is carried out.

2. A process according to claim 1, characterized in that one of the precursor elements is a carbonate.

3. A process according to claim 2, characterized in that one of the precursor elements is calcite, or magnesite, or rhodocrosite or dolomite.

4. A process according to one of the preceding claims, characterized in that the pH value of the cation solution is adjusted by adding a base in order to bring its value to that necessary to obtain a precipitation of the sought phase.

5.- Method according to one of the preceding claims, characterized in that one adjusts the ratio of the amount of trivalent cations on the sum of the amount of divalent cations and trivalent cations [M _π i / (Mii + Min )] at a value between 0.2 and 0.4 by trivalent cation additions (Mm).

6. Process according to claim 5, characterized in that the added trivalent cations result from the solubilization of industrial by-products such as in particular aluminum waste, or aluminum-rich hydroxide sludge, aluminum base ores, aluminum sulphate.