WO2007080286A2

WO2007080286A2 - Coated iron sulphate particles and their use in the preparation of cements

Info

Publication number: WO2007080286A2
Application number: PCT/FR2006/002759
Authority: WO
Inventors: Vincent Morin; Hélène DEBEGNAC; Marc Biosa
Original assignee: Lafarge
Priority date: 2005-12-20
Filing date: 2006-12-15
Publication date: 2007-07-19
Also published as: FR2894956A1; WO2007080286A3

Abstract

The invention relates to a cement comprising coated particles, the said coated particles comprising: (a) a core comprising iron sulphate and/or tin sulphate and/or manganese sulphate; and (b) a shell comprising alginic acid or a derivative thereof.

Description

FERTILIZED IRON SULFATE PARTICLES AND THEIR USE IN THE PREPARATION OF CEMENT

TECHNICAL FIELD The present invention relates to coated particles of iron sulfate and / or manganese sulfate and / or tin sulfate, in particular coated with sodium alginate, suitable for the preparation of cements, in particular reduced-content cements. in chromium (VI). STATE OF THE ART

It is well known in the cement industry that the soluble chromium (VI) content should be as small as possible. Thus, current regulations (European Directive 2003/53 / EC) require that the concentration of chromium (VI) soluble in cement be less than 2 ppm.

A common method of obtaining a reduced chromium (VI) cement is to add ferrous sulphate (FeSO ₄ ) to the cement, since Fe ²⁺ ions can reduce Cr ⁶⁺ ions. Iron sulphate is usually added at the mill inlet in the case of open-circuit mills, or as a separator inlet in the case of closed-circuit mills, or directly to the finished product.

However, a problem with this method is the instability of ferrous sulphate in the storage conditions of the cement. Over time, ferrous Fe ²⁺ ions are converted into Fe ³⁺ ferric ions which are ineffective for the reduction of chromium (VI). Therefore, the amount of iron sulfate that is generally to be incorporated into the cement is typically greater by a factor of 30 than the amount of iron sulfate theoretically required to reduce soluble chromium (VI).

EP-A-1100758 discloses a process for the preparation of inorganic binders to which complexes are added organometallic compounds having reducing properties, making it possible to obtain inorganic binders with low or no content of chromium (VI). More specifically, the organometallic complexes of the document are formed with a polyhydroxycarboxylic or carboxylic acid having a backbone of 2 to 8 carbon atoms surrounding a central ion (Mn ²⁺ or Fe ²⁺ ). In particular, iron gluconate is provided. It is stated that these organometallic complexes make it possible to use an almost stoichiometric amount of iron relative to chromium, the stability of these complexes being indicated as excellent.

However, in this document, the ferrous ions being sequestered individually, it is necessary to use a large amount of organic molecules to hope to protect them from oxidation to ferric ions. In addition, the document does not provide quantitative data to evaluate the performance of this method in terms of stability of complexed ferrous ions and their ability to reduce chromium (VI). The question of the accessibility of these ions when they are supposed to reduce the chromium (VI) remains thus posed. Moreover, in fact, iron gluconate is not more stable than iron sulphate.

There is therefore still a need for a method for stabilizing iron sulfate and decreasing the amount of iron sulfate to be incorporated into a cement (for example to reduce the soluble chromium (VI) contained in this cement).

SUMMARY OF THE INVENTION

The invention thus provides coated particles, comprising (a) a core comprising iron sulfate and / or tin sulfate and / or manganese sulfate; and (b) an envelope comprising a natural or synthetic polysaccharide.

For the sake of simplification, all the rest of the description is established in relation to iron sulfate, which is the preferred chemical compound of the core of the coated particles according to the invention. However, it is understood that, in what follows, the iron sulphate may be replaced in an equivalent manner by manganese sulphate or sulphate of tin or a mixture of iron sulphate, tin sulphate and manganese sulphate, since these three chemical compounds are likely to reduce chromium (VI) and have a relative natural instability. Preferably, said polysaccharide is alginic acid or one of its derivatives, in particular sodium alginate, potassium alginate, calcium alginate, magnesium alginate, ammonium alginate or propylene glycol alginate. Sodium alginate is particularly preferred. Preferably, the iron sulfate is monohydrate, heptahydrate or as a mixture of iron sulfate monohydrate and heptahydrate.

Advantageously, the coated particles as defined above have a size d _{max of} less than about 200 μm, preferably less than about 100 μm.

Advantageously, the amount of polysaccharide of the envelope represents, relative to the weight of the core, an amount of between 0.05 and 5%, preferably between 0.2 and 2%.

Preferably, the coated particles according to the invention are stable to storage in a cement for a period of between 3 and 9 months.

Advantageously, the coated particles are in powder form, thus in dry form.

The invention also provides the use of polysaccharides for stabilizing iron sulfate.

The invention further provides a cement comprising coated particles as defined above.

According to an advantageous embodiment, the cement according to the invention is obtained from a cement free of coated particles, and contains a quantity of coated particles such that the amount by weight of iron sulphate is less than 0.02% , in particular less than 0.01%, and preferably less than 0.005% per ppm of soluble chromium (VI) present in said cement free of coated particles. According to an advantageous embodiment, the cement according to the invention contains a quantity by weight of iron sulphate of less than 0.3%, in particular less than 0.2%, and of preferably less than 0.1%, and having a chromium (VI) content of less than 2 ppm.

Said chromium (VI) content is expressed in relation to the dry weight of the cement, and is measured on cement juice as described in the draft standard PR NF EN 196-10 May 2005 "Test methods for cements - Part 10: Determination of the water - soluble chromium (VI) content of the cement.

The invention also provides the use of coated particles according to the invention or a cement according to the invention in the preparation of a concrete or a mortar.

The invention furthermore provides a process for preparing coated particles according to the invention comprising the following steps: - mixing of an iron sulphate powder with a polysaccharide solution, in a mixture of iron sulphate and polysaccharide; drying the mixture obtained in the preceding step in said coated particles. Advantageously, acid is added to the iron sulfate powder and the polysaccharide solution before the end of the mixing step.

According to a particular embodiment, said method comprises an additional step of final grinding and sieving, after the step of drying the mixture.

Preferably, in the process according to the invention, the mass concentration of polysaccharide in the polysaccharide solution is between about 0.01 and about 10%, in particular between about 0.1 and about 5%, and wherein The mass ratio between the polysaccharide solution and the iron sulfate at the kneading step is from about 5% to about 50%, particularly from about 10% to about 35%.

Preferably, in the process according to the invention, the temperature during the kneading step is the ambient temperature.

Preferably, the drying step is carried out at a temperature of between room temperature and about 70 ^° C, in particular at a temperature between room temperature and about 40 ⁰ C, for a period of between about 2 hours and about 5 days, at atmospheric pressure or under reduced pressure. The present invention makes it possible to overcome the drawbacks of the state of the art, and more particularly to reduce significantly, compared with prior art techniques, the amount of iron sulphate which must be incorporated into a cement in order to to reduce the soluble chromium (VI) contained in this cement, by incorporating only a moderate amount of an additive in the cement, while obtaining satisfactory results. Reducing the amount of iron used allows undeniable economic gains. The object of the invention is achieved by producing coated iron sulfate particles for incorporation into the cement, having the following characteristics: the iron sulfate is protected from the external environment by the coating and therefore remains essentially stable in the conditions of storage of the cement; the Fe ²⁺ ions are capable of being rapidly released (in particular in less than 10 minutes and preferably in less than 5 minutes) during the mixing of the concrete or the mortar, which then allows the reduction of the Cr ⁶⁺ ions; the cost of the coated particles bound to the raw materials and to the manufacture is low, which makes the solution proposed by the invention economically advantageous vis-à-vis the prior art. The invention thus provides a solution for stabilizing iron sulfate, without complex formation of iron sulfate, unlike the state of the art. The invention would likewise apply to tin, manganese or other metal sulphate and generally to sulphates and similar salts.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Coated particles.

The invention therefore relates to coated particles, comprising a core and an envelope, in which the core comprises iron sulfate and the shell comprises a natural or synthetic polysaccharide. Preferably, the core consists of iron sulfate. Preferably, the envelope consists of a polysaccharide. Other layers and / or components could possibly be present.

By "core" of the coated particles is also meant the center or the inner layer of said particles. By

"envelope" of the coated particles is also understood to mean the coating, the outer layer, the protective film or the shell of said coated particles.

By "natural or synthetic polysaccharide" is meant a polymeric organic molecule consisting of a chain of monosaccharide units. The viscosity of these polymers in solution at 1% by weight in water is variable, for example between 10 and 1000 cps.

By way of example, the viscosity of various alginates marketed by Degussa at 1% in water at 20 ^° C. is indicated below:

Satialgine S60: 34 cps; Algogel 3001: 59 cps; Satialgine S550: 300 cps; Algogel 3541: 440 cps;

Satialgine S1100: 610 cps.

Preferably, said natural or synthetic polysaccharide is capable of establishing, at multiple locations along the polymer chain, particularly ionic-type bonds with divalent cations, such as, for example, Fe ²⁺ ions,

Ca ²⁺ , Mn ²⁺ , Sn ²⁺ .

Examples of polysaccharides include pectins and alginates.

In what follows, a description is given in relation to the alginate, but any other polysaccharide is appropriate.

According to a preferred embodiment of the coated particles according to the invention, the polysaccharide is alginic acid, in particular in the form of sodium alginate. Sodium alginate is the sodium salt of alginic acid, which is extracted from brown algae (Phaeophyceae). It is also the most common form of marketing of alginic acid. Alginic acid is a natural polymer of formula

(C ₆ H ₇ NaO ₆ ) _n consists of two mono-saccharide units: D-mannuronic acid (M) and L-guluronic acid (G). The typical number of base units is about 200. The proportion of mannuronic acid and guluronic acid varies from one species of seaweed to another and influences, for example, the chemical properties, by varying the gelling power. or thickener of alginate. Preferably, in the invention, the ratio of the number of units M to the number of units G is between 0.5 and 1.5, and more particularly between 1 and 1.5.

The sodium alginate in solution forms, in the presence of divalent ions, a complex in gel form, also known as an "egg-box" assembly, in which said ions are sequestered. In the coated particles according to the invention, the alginic acid would establish bonds with the surface of the iron sulphate core, without establishing bonds with the Fe ²⁺ ions located inside the iron sulphate core. Thus, the alginate shell effectively protects the iron sulphate core from the environment under storage conditions, and is quickly and easily broken to liberate and render accessible the iron sulphate core under conditions of use. Indeed, the alginate is degraded in a basic medium. However, the cement produces a basic medium during its mixing, which causes the rupture of the envelope and / or the release of iron sulfate.

The use of sodium alginate also has the additional advantage that this product is common and inexpensive. The iron sulphate forming the core may be in its conventional forms, especially in monohydrate form, that is to say of formula (FeSO ₄ , 1H ₂ O), or in heptahydrate form, that is to say of formula (FeSO ₄ , 7H ₂ O). The heptahydrate form is the more commonly used in the field of cement, especially in the form of powder.

The coated particles generally have a particle size d _ma le (sieve pass) less than about 200 microns, preferably less than about 100 microns.

The amount of alginate in the coated particles is such that a protective film is created on the surface of the iron sulfate particles. For example, the amount of polysaccharide of the shell is, based on the weight of the iron sulfate of the core, an amount between 0.05 and 5%, preferably between 0.2 and 2%.

Polysaccharides are therefore useful for stabilizing iron sulfate. The coated iron sulphate is therefore substantially stable for a storage period of between 3 and 9 months, under the usual conditions for storing the cement, in particular in a silo or paper bag.

By "substantially stable" it is meant that sufficient Fe ²⁺ ions remain to reduce the amount of chromium (VI) to less than 2 ppm. Preferably, a proportion greater than 50%, in particular greater than 70%, in particular greater than 90%, of iron sulphate is still available at the time of maturity, that is to say has not undergone any oxidation of the ferrous ions.

Cement containing the coated particles.

The particles of the invention are useful as additives for cements.

By way of cement, it is possible to use any type of cement, in particular Portland, or composite cements, or any cement defined in the EN 197 standard.

The particles according to the invention can be added at any time during the manufacture of the cement. Particles can be added at the mill inlet in the case of open-circuit mills, or at the separator inlet in the case of closed circuit mills, or directly to the finished product. This latter solution is preferred in that it limits attrition actions on the surface of the particles. Particles can also be added in silos or in transport tanks, or in concrete or the final mortar.

The amount of particles is relatively small, compared to the amount of iron sulfate that was previously required to compensate for the loss of efficiency of iron sulfate over time.

The particles according to the invention have no noticeable effect on the conditions of use of the cement, which is therefore used quite conventionally.

Process for manufacturing the coated particles

The invention also provides a method of manufacturing the particles according to the invention, which comprises the following steps: - mixing of an iron sulfate powder with a polysaccharide solution, in a mixture of iron sulfate and polysaccharide; drying the mixture obtained in the preceding step in the coated particles. The method according to the invention may comprise an additional step of pre-grinding and sieving prior to the kneading step, so as to obtain iron sulphate powder from a stock of crude iron sulphate.

The starting iron sulfate powder may optionally be calibrated to obtain the desired final diameter. Since the amount of alginate is relatively small, the starting diameter is only slightly less than that of arrival.

The term "kneading" is understood to mean any technique leading to the formation of a protective film around the iron sulphate, especially the simple mixing, coating or encapsulation, whatever the material used, in particular a mixer or a bed of iron. fluidized air (with spraying of the polysaccharide solution on a suspension of iron sulfate particles).

According to one embodiment of the process according to the present invention, acid (in particular hydrochloric acid) is added. to the iron sulfate powder and the polysaccharide solution before the end of the mixing step.

The acid (in particular hydrochloric acid) makes it possible to deflate the chains of the polysaccharide by themselves, which has the effect of forming a denser protective film on the surface of the iron sulphate. The acid is added until an acidic pH is obtained, preferably less than 4.0 and in particular less than 2.0.

The preparation process according to the invention may optionally comprise an additional step of final grinding and sieving, after the step of drying the mixture, making it possible to obtain, from the coated particles, grinded and sieved coated particles.

According to a particular embodiment of the preparation method of the present invention, the mass concentration of polysaccharide in the polysaccharide solution is between about 0.01 and about 10%, in particular between about 0.1 and about 5%, and the mass ratio between the polysaccharide solution and the iron sulfate at the kneading step is between about

5% and about 50%, especially between about 10% and about 35%.

When alginate is fixed on the iron sulphate core, water molecules previously fixed by these alginate molecules are re-dropped. In general, the amounts of water and alginate are adapted to the specific surface area of the ferrous sulphate powder and to the quality of this powder.

The conditions of the process are in general the following: the kneading step is carried out for a sufficient duration, which can be between 2 and 30 minutes. Short periods are preferred. The temperature is generally room temperature, but may be higher if needed, or may be increased at the end of mixing.

The drying step is carried out at a temperature of between room temperature and approximately 70 ^° C., in particular between room temperature and 40 ^° C. for a time sufficient to remove the free water from the alginate. The drying can be carried out under reduced pressure (which significantly reduces the duration of drying). The end of the drying can be determined by visual inspection (the particles go from green to creamy white) or by loss of mass tracking (until the mass of the particles does not vary much more).

EXAMPLES

The following examples illustrate the invention without limiting it. Example 1. Synthesis of coated particles.

Coated particles according to the invention are prepared according to the following protocol: a) A solution of sodium alginate (supplier: VWR International / Prolabo, catalog number 27 660.183, viscosity equivalent to that of Satialgine S550 marketed by Degussa, cf. above) is prepared by slow dissolution hot (50 ⁰ C) of sodium alginate powder (1.2 g) in demineralized water (60 g). b) 250 g of iron sulphate heptahydrate (supplier: VWR International / Prolabo, catalog number 24 237.363) is crushed and sieved so that its particle size is less than 100 μm (the largest particle having a size of 100 μm ). c) This sieved iron sulfate powder is introduced into a Perrier or Lodige mixer. The sodium alginate solution is then added to this powder in a proportion of approximately 24% relative to the mass of iron sulphate. The dry amount of sodium alginate is about 0.5% of the iron sulfate mass. The whole is kneaded about 30 minutes to allow a homogeneous coating. d) The coating is further facilitated by the addition of hydrochloric acid just before the end of mixing (1 mL to 3 M of

HCl for 50 g of the iron sulfate / sodium alginate mixture. e) At the end of mixing, the final product is placed in an oven at 130 ^° C. for about 30 seconds. f) The product is then dried in an oven at 45 ° C for 2 days in order to evacuate the water contained in the protective film of alginate. g) The dried product is finally milled to obtain a particle size of less than 100 microns. This grinding step is a light grinding that does not break the protective alginate film.

Example 2. Reduction of chromium (VI) in a cement.

In this example, the reduction of chromium (VI) under different aging conditions of the cement is measured.

A CEM 1 52.5N cement containing 15 ppm of

Chromium (VI) soluble.

According to the tests, powdered iron sulphate or coated particles according to the invention are added thereto. In all cases, the iron sulphate, coated or not, which is used in addition with the cement, has a particle size smaller than

100 μm (maximum size of the particle).

After incorporating the powdered iron sulfate or the coated particles according to the invention into the cement, the latter is subjected to one or the other of two accelerated aging protocols described below. These two accelerated aging protocols constitute severe (extreme) storage conditions for cement. Also, the observation of a "good" behavior of a cement (in terms of low chromium content (VI)) in the conditions of accelerated aging means a fortiori that said cement will necessarily also have a good behavior under conditions of usual storage / storage.

When iron sulphate (coated or uncoated) must be added to the cement, the iron sulphate is mixed and homogenized with cement for 45 minutes with Turbula ^® before the aging of the product.

Aging protocol no. 1: high humidity

This test consists in putting in a tank a bed of cement powder containing iron sulphate: the mass of cement is approximately 600 grams and the height of the bed of material 1 cm. This tray is then placed in a chamber maintained at 20 ⁰ C and

100% relative humidity. The ferry stays 24 hours in this chamber, and then the measurement of soluble chromium (VI) is carried out.

We measure to 1 day.

Aging protocol n ° 2: silo simulation

This test is intended to simulate the storage conditions of the silo cement. The cement arrives relatively warm in the silo at the factory and remains confined. For this test, 1.5 kg of cement is introduced into a can of dimensions 108 mm × 136 mm (volume 1 L) which is then closed and placed in an oven regulated at 80 ^° C. The chromium content is measured ( VI) soluble at 7 and 28 days.

Chromium (VI) measurement procedure

The determination of chromium (VI) is carried out according to draft standard PR NF EN 196-10 May 2005 "Methods of testing cement - Part 10: Determination of the water-soluble content of chromium (VI) cement" . Experimental results

The two tables below list the experimental results of measurements of chromium (VI) soluble in cements subjected to the aging protocol No. 1 (Table 1) or No. 2 (Table 2).

In each case, the term "control" refers to a reference state of the cement (with or without iron sulfate or coated particles) which has not been aged. Measurements of chrome "light" (VI) soluble are therefore obtained on a cement unaged control or non-aged cement mixed with any iron sulfate immediately after mixing and homogenisation in the Turbula ^®.

Table 1

The results in Table 1 clearly demonstrate that, even under very unfavorable humidity conditions, the use of the coated particles according to the invention allows a much more effective reduction of the chromium (VI) content than the use of iron sulphate with equal or less iron sulphate content.

Table 2

The results in Table 2 demonstrate that the use of coated particles according to the invention makes it possible essentially to reduce all soluble chromium (VI) even after having subjected the cement to 28 days of accelerated aging by heat.

Example 3. Comparison between the use of iron sulphate and ferrous gluconate.

In this example, the relative efficiency of iron sulfate with respect to ferrous gluconate is compared for the reduction of soluble chromium VI.

Ferrous sulphate or ferrous gluconate is incorporated in a cement initially containing 13 ppm soluble chromium VI, for the same amount of Fe ²⁺ introduced (0.77 mM) in both cases. The final amount of chromium VI in the cement is measured according to the standard method described above. The results are shown in Table 3 below. It is found that, with equivalent amounts of ferrous iron introduced, ferrous gluconate is less effective than ferrous sulfate in reducing chromium VI. Table 3

Example 4. Minimum amount of coated particles according to the invention for a total reduction of chromium VI.

In this example, the remaining concentration of Chrome VI in a cement initially containing 15 ppm of Chrome VI, after addition of the coated particles according to the invention, at different concentrations, is measured according to the standard procedure described in Example 2. The results are reported in Table 4 below, in which the concentration of coated particles is represented by the concentration of FeSO ₄ introduced. The first line of the table represents the control (cement without coated particles).

Table 4

Example 5. Impact on the properties of use of cement. Tests have been carried out to evaluate the impact of the addition of alginate (sodium alginate Prolabo) to a cement on the properties of use of said cement. 0.1% alginate is incorporated in a cement as defined in Example 1, that is, that is, about 100 times more than the amount of alginate recommended for incorporation of coated particles according to the invention.

Curing time parameters (determination of consistency and setting according to EN 19G-3) (Table 5) and mechanical strength parameters, namely flexural strength (Rf) and compression (Rc) (Table 6). The tests are carried out on standard mortar.

Table 5

Table 6

It is found that alginate, even when present at such a high concentration, has no impact on the properties of the cement, which implies, a fortiori, that a much smaller quantity will not have impact. The use of the coated particles according to the invention will therefore have no impact on the properties of use of the cement.

Claims

A cement comprising coated particles, said coated particles comprising: (a) a core comprising iron sulfate and / or tin sulfate and / or manganese sulfate; and (b) an envelope comprising alginic acid or a derivative thereof.

The cement of claim 1, wherein the alginic acid derivative is sodium alginate, potassium alginate, calcium alginate, magnesium alginate, ammonium alginate or propylene. glycol alginate.

3. Cement according to claim 1 or 2, wherein the core of the coated particles comprises iron sulfate monohydrate or heptahydrate or a mixture of iron sulfate monohydrate and heptahydrate.

4. Cement according to one of claims 1 to 3, wherein the coated particles have a size d _max less than about 200 microns, preferably less than about 100 microns.

5. Cement according to one of claims 1 to 4, wherein the alginic acid or derived from the envelope is, based on the weight of the core, an amount of between 0.05 and 5%, preferably between 0, 2 and 2%.

6. Cement according to one of claims 1 to 5, wherein the coated particles are stable storage for a period of between 3 and 9 months.

7. Cement according to one of claims 1 to 6, obtained from a cement free of coated particles, and containing a quantity of coated particles such that the amount by weight of iron sulfate and / or tin sulfate and / or manganese sulfate is less than 0.02%, in particular less than 0.01%, and preferably less than 0.005% by ppm of soluble chromium (VI) present in said cement free of coated particles.

8. Cement according to one of claims 1 to 7, containing an amount by weight of iron sulfate and / or tin sulfate and / or manganese sulfate less than 0.3%, in particular less than 0, 2%, and preferably less than 0.1%, and having a chromium (VI) content of less than 2 ppm.

9. Cement according to one of claims 1 to 8, wherein the coated particles are present in powder form.

10. Use of cement according to one of claims 1 to 9 in the preparation of a concrete or a mortar.

11. Cement manufacturing method according to one of claims 1 to 9, comprising: the preparation of the coated particles; and - the mixture of the particles coated with cement.

12. The method of claim 11, wherein the preparation of the coated particles comprises the following steps: - mixing of an iron sulfate powder and / or tin sulfate and / or manganese sulfate with a solution of alginic acid or derivative, a mixture of iron sulfate and / or tin sulfate and / or manganese sulfate and alginic acid or derivative; drying the mixture obtained in the preceding step in the coated particles.

The process according to claim 12, wherein acid is added to the iron sulfate and / or tin sulfate and / or manganese sulphate powder and to the alginic acid or derivative solution prior to end of the mixing step.

14. The method of claim 12 or 13, comprising an additional step of final grinding and sieving, after the step of drying the mixture.

15. The process as claimed in one of claims 12 to 14, in which the mass concentration of alginic acid or derivative in the solution of alginic acid or derivative is between approximately 0.01 and approximately 10%, in particular between about 0.1 and about 5%, and wherein the weight ratio between the alginic acid or derivative solution and the iron sulfate and / or the tin sulfate and / or the manganese sulfate at the mixing step is from about 5% to about 50%, especially from about 10% to about 35%.

16. The method according to one of claims 12 to 15, wherein the temperature during the kneading step is the ambient temperature.

17. A method according to one of claims 12 to 16, wherein the drying step is performed at a temperature between room temperature and about 7O ⁰ C, especially at a temperature between room temperature and about 40 ⁰ C, for a period of between about 2 hours and about 5 days, at atmospheric pressure or under reduced pressure.

18. Cement according to one of claims 1 to 9, obtainable by the method of one of claims 11 to 17.