METHOD FOR PRODUCING NANOMAGNETITE

Abstract

The application relates to a method for preparing magnetite, comprising steps of:

a) reaction at a temperature of 100 to 500 C. of a material containing wstite with water, in order to obtain a solid comprising magnetite, and then
b) recovery of the magnetite in the form of particles wherein more than 25% by weight are of nanometric size.

Claims

1. A method for preparing magnetite comprising steps of: a) reaction at a temperature of 100 to 500 C. of a material containing wstite with water, in order to obtain a solid comprising magnetite, and then b) recovery of the magnetite in the form of particles wherein more than 25% by weight are of nanometric size.

2. The method according to claim 1, wherein the temperature of the reaction of step a) is 150 to 350 C.

3. The method according to claim 1, wherein the water is the water of an aqueous solution the pH of which is less than 7 when said aqueous solution is at 25 C. and at 1 bar.

4. The method according to claim 3, wherein the pH of the aqueous solution is from 2 to 5.

5. The method according to claim 4, wherein the pH of the aqueous solution is from 2.5 to 3.

6. The method according to claim 3, wherein the aqueous solution contains an organic acid comprising a group able to complex with the iron ions, such as the group COOH.

7. The method according to claim 1, wherein the pressure during the reaction of step a) is 5 to 700 bar.

8. The method according to claim 1, wherein the material containing wstite is steelworks slag.

9. The method according to claim 8, wherein the steelworks slag is a decalcified slag.

10. The method according to claim 9, comprising, before the step a), a step of putting a steelworks slag in contact with a decalcification aqueous solution with a pH of 1 to 6 in order to obtain a decalcified slag.

11. The method according to claim 9, wherein, during the step a), the pH of the aqueous solution is from 3 to 4, the pressure is from 100 to 200 bar and the temperature is from 200 to 300 C.

12. The method according to claim 1, wherein the step b) is carried out by magnetic separation.

13. The method according to claim 12, wherein the step b) comprises the substeps of: b1) optionally grinding of the solid comprising magnetite obtained at the step a), then b2) addition of water or an acid solution to the solid comprising magnetite in order to obtain a mixture, b3) magnetic separation of the mixture of the step b2) in order to obtain a solid, b4) addition of water or of an acid solution to the solid obtained at the step b3) in order to obtain a mixture, b5) magnetic separation of the mixture of the step b4) in order to obtain the magnetite in the form of particles wherein more than 25% by weight are of nanometric size.

14. The method according to claim 1, comprising, after the step b), a step c) of separation of the particles of nanometric size from the magnetite obtained at the step b), for example by high-gradient magnetic separation.

15. The method according to any of claims 1 to 7 and 12 to 14, wherein the material containing wstite is wstite.

16. The method according to claim 1, wherein the temperature of the reaction of step a) is 150 to 250 C.

17. The method according to claim 3, wherein the pH of the aqueous solution is from 2 to 4.

18. The method according to claim 3, wherein the aqueous solution contains an organic acid wherein the organic acid is acetic acid.

19. The method according to claim 1, wherein the pressure during the reaction of step a) is 10 to 400 bar.

20. The method according to claim 1, wherein the pressure during the reaction of step a) is 40 to 200 bar.

Description

[0091] The following examples and figures illustrate the invention.

[0092] FIG. 1 is an image obtained by FE-SEM (Zeiss Ultra-55 apparatus using secondary electron detection) of the solid obtained at the end of experiment 20 of example 1 with a magnification of 20,000 (reference of the Polaroid type).

[0093] FIG. 2 is an image obtained by FE-SEM (Zeiss Ultra-55 apparatus using secondary electron detection) of the solid obtained at the end of experiment 31 of example 3 with magnification of 50000.

[0094] FIG. 3 is an image obtained by TEM (transmission electron microscopy) (JEOL FEG 2100F apparatus used at 200 kV) of the solid obtained at the end of experiment 20 of example 1.

[0095] FIG. 4 is an image obtained by TEM (transmission electronic microscopy) (JEOL FEG 2100F apparatus used at 200 kV) of the solid obtained at the end of experiment 31 of example 3.

[0096] FIG. 5 shows the quantity of hygiene produced standardised by the theoretical concentration of ferrous oxide (FeO) in the samples (mol H.sub.2/kg FeO) as a function of the time in hours (h) for the steelworks slag used without prior treatment (squares) in water (experiment 33) or the steelworks slag previously decalcified by treatment with a solution of acetic acid and reacted in a solution of acetic acid at 1 mol/litre (rounds) (experiment 34) (example 4).

EXAMPLE 1: PREPARATION OF MAGNETITE FROM WSTITE

[0097] wstite (FeO 99.9%, Aldrich) was ground and sieved to a particle size of 50 to 100 m and a specific surface area measured by N2-BET(Belsorp-Max supplied by BEL Japan, Inc. III) of 0.70 m.sup.2/g. The state of oxidation of the iron in the starting material, quantified by Mssbauer spectroscopy (make Ortec) (was 91.6% Fe.sup.2+, 5.6% Fe.sup.3+ and 2.8% FeO. The mean state of oxidation of the iron corresponds to pure Fe(II), in accordance with the grade of FeO used.

[0098] For experiments 1 to 3, the wstite was used without prior grinding. For the other experiments, it was ground (50-100 m).

[0099] The aqueous solutions were aqueous solutions of acetic, oxalic or hydrochloric acid in the concentrations and at the pHs indicated in table 1, or water.

[0100] Experiments 1 to 19 were carried out in gold capsules 2 cm long, 4.0 mm outside diameter and 3.6 mm inside diameter. 80 mg of wstite and the aqueous solution were introduced therein with a ratio by mass of 1/1. The closed capsule was introduced into a reactor under pressure itself introduced into a furnace. Temperatures of 100 to 200 C. at a pressure of argon of 300 bar were used. At the end of the experiment, a compressed air flow was used to lower the pressure. The gaseous phase produced by the sample was recovered for analysis.

[0101] Experiments 20 to 22 were carried out in a 500 ml autoclave made from Hastelloy provided with two external ceramic heating bands for heating. The reaction medium was stirred at 800 revolutions per minute. This autoclave enables the production of hydrogen to be monitored in real time and therefore enables the reaction to be converted. The gaseous phase sample at high pressure and temperature taken off was condensed in a cold-water condenser and then analysed by gas chromatography. Samples of solution were also taken by means of a capillary and filtered on a 0.2 m pore titanium filter for subsequent analysis by inductively coupled plasma-optical emission spectrometry (ICP-OES) (Varian 720ES). The experiments were carried out with an FeO/water mass ratio of 1/200.

[0102] The components of the gaseous phase (H.sub.2, CO.sub.2, N.sub.2, O.sub.2, CO, CH.sub.4) were analysed with a gas chromatograph of the Clarus 500 type (Perkin Elmer) equipped with a polymeric column (Restek ShinCarbon) and a thermal conductivity detector (TCD). The temperature of the detector, of the injection system and of the furnace was respectively 250, 100 and 80 C. Argon was the eluent gas. Each sample of gas was analysed at least three times. With regard to experiments 1 to 21 conducted in the capsules, it was considered that the whole of the H.sub.2 is in the gaseous phase obtained after having pierced the capsule. With regard to experiments 20 to 22 conducted in the autoclave, the composition of H.sub.2 in the gas samples was determined while taking account of the proportion of H.sub.2 in the gaseous phase and in the liquid medium.

[0103] The proportion of iron in the aqueous solution was determined just after taking the aqueous solution with samples of 2 ml by UV spectroscopy after complexing with orthophenantroline. Stored in a refrigerator, all the samples were analysed once again by ICP-OES.

TABLE-US-00001 TABLE 1 Conditions of the experiments using wstite and proportion of H.sub.2 produced. c. T P Duration g H.sub.2/kg Exp. Acid (mol/l) ( C.) (bar) (h) pH FeO (g/kg)* conversion 1 MeCOOH 0.05 150 300 240 3 2.62 28% 2 Oxalic acid 0.001 150 300 240 3 0.24 3% 3 HCl 0.001 150 300 240 3 0.20 2% 4 MeCOOH 0.005 150 300 72 3.5 0.074 1% 5 MeCOOH 0.05 150 300 72 3 2.58 28% 6 MeCOOH 0.5 150 300 72 2.5 1.91 21% 7 MeCOOH 0.05 100 300 72 3 0.058 <1% 8 MeCOOH 0.05 200 300 72 3 5.34 58% 9 MeCOOH 0.05 150 300 24 3 1.34 14% 10 MeCOOH 0.05 150 300 3 3 0.068 <1% 11 MeCOOH 0.05 150 300 168 3 4.08 44% 12 MeCOOH 0.05 150 300 72 3 2.46 27% 13 MeCOOH 0.05 150 300 8 3 0.30 3% 14 MeCOOH 0.05 100 300 172 3 0.23 2% 15 MeCOOH 0.05 200 300 24 3 3.74 40% 16 MeCOOH 0.05 200 300 3 3 2.86 31% 17 MeCOOH 0.05 200 300 48 3 3.26 35% 18 HCl 0.001 150 300 72 3 0.084 <1% 19 HCl 0.001 200 300 72 3 0.19 2% 20 MeCOOH 0.05 150 160 48 3 8.06 87%*** 21 Water 150 150 64.5 6 0.26 3%*** 22 Water 300 180 144 6 2.18 23%*** c. = acid concentration T = temperature, P = pressure *mass of H.sub.2 produced measured by gas chromatography divided by the initial mass of material containing wstite ** conversion calculated from the mass of H.sub.2 produced ***The conversion differences observed in the capsules (experiments 1 to 19) or in the autoclave (experiments 20 to 22) for similar conditions of pressure, temperature and nature of the aqueous solution could be explained by the great difference in FeO/aqueous solution ratio (1/200 in autoclave and 1/1 in the capsules) and/or the almost absence of gaseous phase in the experiments carried out in capsules and/or especially in the absence of stirring of the reaction medium in the capsules.

[0104] The conversion of the reaction of step a) of the method (last column in table 1) was calculated by analysing the quantities of hydrogen (penultimate column in table 1), which are directly correlated with the quantities of magnetite.

[0105] Influence of the Nature of the Acid on the Conversion

[0106] The comparison of the results of experiments 1, 2 and 3 show that, at 150 C. and 300 bar, the proportion of hydrogen is ten times greater when acetic acid is used in place of hydrochloric or oxalic acid.

[0107] Influence of the pH and Temperature on the Conversion

[0108] The comparison of the results of experiments 4, 5 and 6 shows that, at a temperature of 150 C. and 300 bar and when acetic acid was used, the proportion of hydrogen is much greater at pH 2.5 or 3 than at pH 3.5. The optimum conditions appear to be achieved at these temperatures and pressure for a pH of 3. This is because the conversion into hydrogen is very different at a pH of 3.5 or 3 (respectively 1% or 28%), but is of the same order of magnitude at a pH of 3 or 2.5 (28% and 21%).

[0109] The results of experiments 7 to 17, in which an aqueous solution of acetic acid at pH 3 was used with a pressure of 300 bar, show that, for the same duration of reaction, the conversion of the reaction increases with temperature.

[0110] This observation is similar using water as the aqueous solution: the conversion into hydrogen is 3% at 150 C. (experiment 21), and 23% at 300 C. (experiment 22), that is to say nine times greater.

[0111] In water at pH 6 at two reaction temperatures (150 and 300 C.), the production of hydrogen was monitored over time (experiments 21 and 22).

[0112] At 150 C. (experiment 21), H.sub.2 was produced solely in the first ten hours of reaction in a proportion corresponding to a conversion of 3%. Small quantities of magnetite were identified by XRPD in the residual FeO medium. Between 10 hours and 65 hours of reaction time no H.sub.2 was produced.

[0113] At 300 C. (experiment 22), in the first ten hours of the reaction, the hydrogen production kinetics was four times greater than at 150 C. Unlike the experiment at 150 C., H.sub.2 was always produced after 10 hours of reaction, at a production level that gradually decreased with time. After 144 hours, a conversion of 23% was obtained.

[0114] It is therefore possible to carry out the reaction at pH 6, but it is necessary to use higher temperatures than those necessary at pH 3. A comparison of the results of experiments 20 and 21 shows that the conversion of the reaction is greatly influenced by the presence of acetic acid. This is because, at 150 C., when the aqueous solution is water, FeO practically did not react (experiment 21), whereas when the aqueous solution is an aqueous solution at 0.05 mol/litre of acetic acid pH of 3, the conversion was almost total in 10 hours.

[0115] A comparison of experiments 20 and 22 shows that, in order to increase the kinetics of the reaction, the use of an acid aqueous solution is much more advantageous than increasing the temperature.

[0116] The solid obtained at the end of the reactions was washed several times in water, ground and then analysed by X-ray powder diffraction (XRPD). The diffractograms were obtained with a D8 diffractometer (Bruker, CuK radiation) (2 scanning at 0.026, 8 seconds).

[0117] Some of the solid was kept unground for analysis by electron microscopy, scanning electron microscopy (SEB), field emission scanning electron microscopy (FE-SEM) (Zeiss Ultra-55 apparatus using both secondary electron detection and back-scattered electrons) and transmission electron microscopy (TEM) (Jeol FEG 2100F apparatus used at 200 kV). The two sets of apparatus were provided with an energy-dispersive X-ray spectroscopy (EDS) detector for chemical analysis. For field-emission SEM analyses, before the AuPd metallisation, the sample was either mounted on a double-face carbon based adhesive or incorporated in an epoxy resin and polished. For TEM analyses, a drop of the sample in powder form was dispersed in ethanol and deposited on a grid covered with carbon of the Lacey type.

[0118] Observation of the solids obtained by FE-SEM made it possible to distinguish the wstite and magnetite. FeO, with a higher average atomic number, the back-scattered electron flow is greater than that issuing from magnetite.

[0119] On a micron scale, the FE-SEM figures of the solid sampled in the autoclave during experiments 21 and 22 show that the oxidation of FeO into magnetite is mainly located in channels distributed homogeneously in the grains. The magnetite appears to nucleate at structural faults or cracks. The magnetite could be formed by a process of auto-oxidation of the FeO. Whatever the oxidation process that takes place, the formation of magnetite in the grains suggests that the kinetics of the reaction is not directly correlated with the specific surface area of the FeO used as the starting product. In other words, the grinding of the FeO grains to reduce the sizes thereof should not increase the kinetics of the reaction significantly.

[0120] The combined analysis of the FE-SEM images (FIG. 1, experiment 20) and TEM images (FIG. 3, experiment 20) afforded an estimation of the populations and distributions of the magnetite particles produced. The solid obtained contains magnetite in the form of particles with three populations of size: [0121] 10 to 20 nm, [0122] 100 to 200 nm, [0123] of micrometric size.

[0124] The analytical techniques and apparatus described in example 1 will be used in all the following examples.

EXAMPLE 2: PREPARATION OF MAGNETITE FROM A WSTITE/CaO MIXTURE

[0125] In order to simulate the behaviour of the wstite in steelworks slag, of which CaO is a majority component, experiments were carried out in gold capsules on wstite/CaO mixtures.

TABLE-US-00002 TABLE 2 Conditions of the experiments using a wstite/CaO mixture and proportion of hydrogen produced. c. T Dura- g H.sub.2/kg (mol/ ( P tion FeO - Exp. Material Acid L) pH C.) (bar) (h) (g/kg) 23 FeO/. MeCOOH 0.05 9.1 150 300 72 0.00051 Ca(OH).sub.2.: 1/1* 24 FeO/. MeCOOH 0.05 5.8 150 300 72 0.051 CaCO.sub.3.: 1/1* 25 FeO Water 0.05 3.0 150 300 72 2.47 reference) c. = acid concentration, T = temperature, P = pressure *mass ratios

[0126] The results in table 2 show that the addition of CaO, whether it be in the form of Ca(OH).sub.2 or CaCO.sub.3, inhibits the reaction, which would appear to be explained in particular by the increase in pH caused by these compounds.

EXAMPLE 3: PREPARATION OF MAGNETITE FROM STEELWORKS SLAG

[0127] Experiments were carried out on steelworks slag of the LAC type sampled at the Fos sur Mer site. This slag underwent ageing of two weeks in air on the site.

TABLE-US-00003 TABLE 3 Conditions of the experiments using steelworks slag. c. T P Duration Exp. Material Acid (mol/l) ( C.) (bar) (h) 26 Slag MeCOOH 2 150 300 72 27 Slag MeCOOH 4 150 300 72 28 Slag MeCOOH 2 300 300 72 29 Slag MeCOOH 4 300 300 72 30 Slag Water 250 180 72 31 Slag Water 300 180 72 32 Slag Water 350 180 72 c. = acid concentration, T = temperature, P = pressure

[0128] Experiments 26, 27, 28 and 29 were carried out in capsules in an acetic acid solution. Because of the presence of CaO and Ca(OH).sub.2 in the steelworks slag, high concentrations of acetic acid are necessary to achieve the pH range sought.

[0129] The solids obtained in the various experiments were analysed by XRPD. At 300 C., the initial FeO present in the slag was almost completely consumed and a very high proportion of magnetite was observed. At 150 C., the conversion of FeO is very small. The increase in concentration of acetic acid accelerates the formation of magnetite.

[0130] Experiments 30, 31 and 32 were conducted in an autoclave with samplings in the presence of deionised water. Because of the presence of CaO and Ca(OH).sub.2 in the steelworks slag, the reaction was conducted at a pH of between 11 and 12. The conversion levels calculated according to the measurement of H.sub.2 produced are, for experiments 30, 31 and 32, respectively 9%, 20% and 43% after 24 hours of processing. The temperature rise affords a significant kinetic acceleration.

[0131] A magnetic separation was carried out according to the protocol described previously. A first separation of the solid in suspension in water was carried out using a permanent magnet at ambient temperature and under ultrasound. A hydrochloric acid solution at 1 mol/litre was then added to the separated solid in order to improve the dissolution of the residual Ca phases and therefore the purity of the solid obtained in the end. A second separation step was then carried out in this suspension at ambient temperature and under ultrasound. The solids of the various separation steps were analysed by XRPD in order to quantify the proportions of magnetite and wstite thereof.

[0132] These analyses made it possible to estimate the solid obtained after reaction at a temperature of 300 C. and a pressure of 180 bar in the presence of water and following the processing described above that contains approximately 20% by weight wstite and 80% by weight magnetite (all size populations merged).

[0133] The combined analysis of the FE-SEM images (FIG. 2, experiment 31) and TEM images (FIG. 4, experiment 31) afforded an estimation of the populations and distributions of the nanomagnetites produced. The solid obtained contains magnetite in the form of particles with three populations of size: [0134] 10 to 20 nm, [0135] 100 to 200 nm, [0136] of micrometric size.

[0137] In order to estimate the proportions of each population, it is possible to use a method using intense field gradients by virtue of metal fibres immersed in the solution that flows in a strong magnetic field (0.5-1.2 T).

EXAMPLE 4: PREPARATION OF MAGNETITE FROM STEELWORKS SLAG PREVIOUSLY TREATED WITH ACETIC ACID

[0138] Experiments were carried out using steelwork slag, at a temperature of 250 C. and a pressure of 150 bar on previously ground samples and for a slag/solution mass ratio of 1/100.

[0139] In experiment 33, the slag was used without prior treatment. The reaction was carried out in the presence of water and at a natural pH of the slag in suspension lying between 11 and 12 for 72 hours.

[0140] In experiment 34, the slag underwent a prior treatment with an aqueous solution at 4 mol/litre of acetic acid at ambient temperature (25 C.). This treatment makes it possible to decalcify the slag and to double the concentration of iron oxide. The reaction at high temperature (250 C.) was carried out in the presence of an aqueous solution of acetic acid at 1 mol/litre. The prior decalcification of the slag makes it possible to work at a pH lying in the range from 2 to 4. The pH during the reaction at 250 C. was measured at between 3.5 and 4.

[0141] The proportion of hydrogen produced was monitored in accordance with the method described for example 1 and is supplied in FIG. 5. The results show that the conversion is appreciably improved by using a previously decalcified slag.

[0142] The solids obtained at the end of the reactions were treated as described for example 3. The analyses by powder X-ray diffraction made it possible to quantify the proportions of magnetite and wstite. The magnetite is in the majority and represents a proportion greater than 70% of the total mass of the solid.

REFERENCES

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