WO2012123184A1 - Coated iron oxide particles - Google Patents

Abstract

Process for producing particles having a coating composed of amorphous silicon dioxide and a core containing haematite and at least one further iron oxide selected from among magnetite and maghemite, in which magnetite particles and/or maghemite particles, an oxygen-containing gas or a hydrogen-containing gas, steam and a silicon compound are reacted at high a temperature, subsequently optionally cooled and the solid is separated off from gaseous substances.

Description

 Enveloped iron oxide particles

The invention relates to processes for the preparation of silica-coated iron oxide particles.

The inductive heating of compounds containing magnetic particles is a promising way for a rapid and gentle hardening of

Adhesive composites or for the crosslinking of polymers. It has proven to be advantageous to use coated magnetic particles. The shell is attributed the function, on the one hand to improve the incorporation into the networks and on the other to prevent unwanted growth of the magnetic phases.

In US4280918 there is disclosed a process for producing magnetic particles surrounded by a shell of silica, wherein in a dispersion containing magnetic particles having a positive zeta potential, there is a dispersion containing colloidal silica particles having a negative zeta potential. However, the particles thus produced, for example under shear conditions, do not have sufficient bonding of the magnetic core and the silica shell.

WO03 / 58649 discloses a process for the preparation of silica

coated iron oxide having a content of silicon of less than 20 wt .-% disclosed in which a silicate separation from water glass or silica sol on the iron oxide particles.

DE-A-102006054173 discloses a process for the preparation of porous magnetic silica particles. In this case, a pH-value modifier and an organic pore-forming agent are added to a liquid reaction mixture which contains magnetic particles and at least one compound serving as the silica source, and the resulting reaction mixture is subsequently spray-dried.

In WO02 / 09125 there is disclosed an oil dispersion crosslinking process for preparing spherical pore size magnetic silica particles comprising silica hydrosol formed by acid-catalytic hydrolysis of an aqueous silane dispersion and comprising a magnetic colloid or ferrofluid or Magnetic particles is mixed, in a water-immiscible dispersed organic phase and crosslinked during the dispersion process by adding a base.

EP-A-961653 discloses a process for the preparation of low Curie temperature core-shell particles wherein the shell is silica and the core is superparamagnetic iron oxides wherein a silica precursor is added to a dispersion of the core material, and subsequently by adding acid, the pH is lowered to 6 to 9.

WO2010 / 063557 discloses iron-silicon oxide particles having a core-shell structure having a BET surface area of 10 to 80 m ² / g, a shell thickness of 2 to 30 nm and an iron oxide content of 60 to 90 Wt .-%, wherein the core is crystalline and the iron oxides comprises hematite, magnetite and maghemite, the shell consists of amorphous silica and between shell and core at least partially present one or more compounds consisting of the elements silicon, iron and oxygen , It is obtained by a flame oxidation / flame hydrolysis process in which a mixture of an aerosol of a

 Iron oxide precursor and a vaporous silicon compound burns under defined conditions in a flame.

The majority of the processes mentioned in the prior art are those which take place in liquid, generally aqueous, phase. The process disclosed in WO2010 / 063557 is a gas phase process. The material thus produced is ideally suited for inductive heating.

The technical object of the present invention is to provide further methods resulting in coated magnetic particles. Here, the core, as in the case of the disclosed in WO2010 / 063557 gas phase method, in addition to magnetic iron oxides, also contain the non-magnetic hematite, since obviously the combination of magnetic iron oxides and hematite represents a particularly advantageous composition for inductive heating. A first object of the invention is a process for the production of particles with an envelope of amorphous silica and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite, wherein at a temperature of 800 to 1200 ° C, preferably from 900 to 1 100 ° C, comprising a stream

a) magnetite particles and / or maghemite particles

b) an oxygen-containing gas or a hydrogen-containing gas c) steam and

d) a silicon compound

is reacted, then optionally cooled and the solid separated from gaseous or vaporous substances.

The temperatures of 800 to 1200 ° C required in this process according to the invention can be provided for example in the form of an external heating. Preferably, they are formed by a flame resulting from the ignition of a mixture containing at least one hydrogen-containing fuel gas and an oxygen-containing gas. As hydrogen-containing fuel gases are hydrogen, methane, ethane and / or propane, wherein

Hydrogen is particularly preferably used. The molar ratio of O ₂ in the oxygen-containing gas / hydrogen-containing fuel gas is preferably 0.8 to 1.5, and more preferably 1.05 to 1.3.

This can be an internal or external flame. In an internal flame, the reaction of the starting materials, iron oxide or iron oxide and Siliciumdioxidprecursor, in the presence of the flame or their

Reaction products, such as water in the case of a hydrogen / oxygen flame. In an external flame, this only provides the temperature for the implementation of

The starting materials are ready while the reaction products are conducted separately from the reaction or are only subsequently combined with the products of the reaction.

Water vapor is formed by the reaction of the hydrogen-containing fuel gas and the oxygen-containing gas. This may be part or all of the amount needed in the process of the invention

Represent water vapor. The average residence time in the region of the reactor in which the temperature of 800 to 1200 ° C is preferably 10 ms to 1 s.

A second aspect of the invention is a process for producing particles comprising an amorphous silica shell and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite, in which

in a first region of a reactor at a temperature of 650 to 1000 ° C, preferably 700 to 850 ° C, comprising a stream

a) magnetite particles and / or maghemite particles

b) an oxygen-containing gas or a hydrogen-containing gas and c) water vapor

reacting, and in a second, the first subsequent, region of the reactor at a temperature of 400 to 800 ° C, preferably 600 to 650 ° C, wherein the temperature in the second region is lower than in the first region comprising a stream

d) the reaction mixture leaving the first region and a

 Silicon compound and

e) water vapor

brings to reaction and optionally the reaction mixture leaving the second region, preferably by feeding water, and subsequently separating the solid from gaseous or vaporous substances.

A third object of the invention is a process for the production of particles with an envelope of amorphous silica and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite, in which

in a first region of a reactor at a temperature of 650 to 1000 ° C, preferably 700 to 850 ° C, comprising a stream

a) Magnetitpartikel and / or Maghemitpartikel and a part of the total amount of a silicon compound and

b) an oxygen-containing gas or a hydrogen-containing gas and c) water vapor

brings to reaction and optionally in a second, the first subsequent, the region of the reactor at a temperature of 400 to 800 ° C, preferably 600 to 650 ° C, wherein the temperature in the second region is lower than in the first region, comprising a current

d) the reaction mixture leaving the first region and the remaining part of the total amount of the silicon compound and

e) water vapor

brings to reaction and optionally the reaction mixture leaving the second region, preferably by feeding water, and subsequently separating the solid from gaseous or vaporous substances.

The temperatures of 650 to 1000 ° C required in the process according to the invention can be provided for example in the form of an external heating. Preferably, they are formed by a flame resulting from the ignition of a mixture containing at least one hydrogen-containing fuel gas and an oxygen-containing gas. As hydrogen-containing fuel gases are hydrogen, methane, ethane and / or propane, wherein

Hydrogen is particularly preferably used. The molar ratio of O ₂ in the oxygen-containing gas / hydrogen-containing fuel gas is preferably 0.8 to 1.5, and more preferably 1.05 to 1.3.

This can be an internal or external flame. In an internal flame, the reaction of the starting materials, iron oxide or iron oxide and Siliciumdioxidprecursor, in the presence of the flame or their

 Reaction products, such as water in the case of a hydrogen / oxygen lamb. In the case of an external flame, this merely provides the temperature for reacting the starting materials, while the reaction products are conducted separately from the reaction or are only subsequently combined with the products of the reaction. FIG. 1 shows schematically a method with an internal flame, in FIG. 2 a method with an external flame. Where: A =

Maghemite and / or magnetite powder, B = air, C = hydrogen, D = silicon compound and E = cooling.

Water vapor is formed by the reaction of the hydrogen-containing fuel gas and the oxygen-containing gas. This may be part or all of the amount needed in the process of the invention

Represent water vapor. The average residence time in the region of the reactor in which the temperature is from 650 to 1000 ° C. is preferably 10 ms to 2 s. The average residence time in the region of the reactor in which the temperature is from 400 to 800 ° C. is preferably from 0.1 to 10 s.

The embodiment of the invention in which the silicon compound is introduced both in the temperature range of 650 to 1000 ° C as well as in the temperature range of 400 to 800 ° C, is preferably such that more than 50 mol% of

Silicon compound, more preferably 60 to 90 mol%, are introduced in the temperature range of 400 to 800 ° C.

In the inventive method, the magnetite and / or hematite particles can be used either in powder form or as a dispersion. Preferably, they are used as an aerosol, either as a solid / gaseous aerosol

powdered particles or as a solid / liquid / gaseous aerosol when using dispersions introduced into the reactor. For this purpose, the known to those skilled units, such as multi-component nozzles can be used. The carrier gas used is generally air or nitrogen. If a dispersion of maghemite and / or magnetite particles is used, their content in the dispersion is generally from 0.5 to 25% by weight, preferably from 3 to 10% by weight. The dispersion may contain dispersing additives such as polyacrylic acid and salts thereof. The simpler method variant, however, is the

To use maghemite and / or magnetite particles as powder.

The magnetite used in the process according to the invention and / or

Maghemite particles have, independently of each other, a middle one

Diameter of preferably 10 nm to 1 μηι and particularly preferably 100 to 500 nm. The silicon compounds used in the process according to the invention are hydrolyzable and / or oxidizable compounds. These are preferably selected from the group consisting of SiCl ₄ , CH ₃ SiCl ₃ , (CH ₃ ) ₂ SiCl ₂ , (CH ₃ ) ₃ SiCl, HSiCl ₃ , (CH ₃ ) ₂ HSiCl and CH ₃ C ₂ H ₅ SiCl ₂ , H ₄ Si, Si (OC ₂ H ₅ ) _4, Si (OCH ₃ ) ₄ or mixtures of the abovementioned. Particularly preferably, SiCl ₄ , and Si (OC ₂ H ₅ ) ₄ can be used. With very particular preference, SiCl ₄ can be used in the process steps which are carried out at the higher temperature, namely from 800 to 1200 ° C. or 650 to 1000 ° C. Si (OC ₂ H ₅ ) ₄ can be completely particularly preferably can be used in the process steps which are carried out at the lower temperature, namely from 400 to 800 ° C.

In the inventive method, the weight ratio of

Maghemite particles and / or magnetite particles to the silicon compound, calculated as Si0 ₂ , preferably 10:90 to 95: 5 amount. In general, high

 Induction temperatures sought, so that a weight ratio of 60:40 to 95: 5 is to be regarded as particularly preferred.

The embodiment of the invention in which the silicon compound is introduced both in the temperature range of 650 to 1000 ° C as well as in the temperature range of 400 to 800 ° C, is preferably such that more than 50 mol% of

Silicon compound, more preferably 60 to 90 mol%, are introduced in the temperature range of 400 to 800 ° C.

The particles produced according to the methods of the invention consist of a dense envelope of amorphous silica and a core containing hematite and at least one further iron oxide selected from magnetite and maghemite. The processes according to the invention are preferably carried out such that the core contains magnetite, maghemite and hematite. Particularly preferred is a

Weight distribution of 30 to 50% maghemite, 40 to 60% hematite and 5 to 20% magnetite, the sum of the proportions adding up to 100%. The BET surface area of the particles is preferably 5 to 80 m ² / g. The thickness of the shell is preferably from 3 to 40 nm. The crystallite sizes determined by Debye-Scherrer

Core constituents are preferably less than 100 nm. Particularly preferred is the crystallite size of hematite thus determined 40 to 70 nm, of magnetite 30 to 90 nm and of maghemite 20 to 40 nm. In contrast to those obtainable in the prior art with silica coated

Iron oxide particles allow the inventive method, the production of particles with particularly high heating rates. During production by the processes according to the invention, complex conditions occur at high temperatures

Transformations of the iron oxides, which apparently lead to a particularly favorable arrangement of the core components, which are attached via chemical bonds to the amorphous silicon dioxide shell. Ensure magnetite and maghemite the electromagnetic coupling of the energy into the core, while hematite provides for a high thermal conductivity.

Examples

analytics

The content of iron oxide is determined by digestion with NaOH, dissolution in dilute H ₂ S0 ₄ and subsequent iodometric titration. The BET surface area is determined according to DIN 66131.

The quantitative determination of the core components is performed by X-ray diffractometry (reflection, Θ / Θ diffractometer, Co-K _a , U = 40kV, I = 35mA, scintillation counter, simulated graphite monochromator, angular range (2Θ) / increment / measurement time: 10 - 100 ° / 0 , 04 ° / 6s (4h)). The hematite is clearly identifiable because of the freestanding reflexes. The maghemite is significantly detectable by the reflexes (1 10) and (211) in the anterior area of the bowel. Using the Rietveld method, a quantitative phase analysis is performed (error about 10% relative). Quantitative phase analysis is performed using set 60 of the ICDD database PDF4 + (2010). The quantitative phase analysis and the determination of the crystallite sizes are carried out with the Rietveld program SiroQuant®, Version 3.0 (2005).

 The thickness of the outer shell is determined by transmission electron microscopy (TEM). In addition, it is checked whether the shell is tight. By this is meant that upon contact of the particles with hydrochloric acid less than 50 ppm of iron are detectable. At room temperature, 0.33 g of the particles are brought into contact with 20 ml of 1 N hydrochloric acid solution for 15 minutes. Part of the solution is subsequently analyzed for iron by means of suitable analysis techniques, for example ICP (inductively coupled plasma spectroscopy).

The heating time up to a temperature of 100 ° C is determined in a silicone composition. The silicone composition is obtained by mixing 33 g ELASTOSIL ^® E50, Fa. Momentive Performance Materials, 13 g of silicone oil type M 1000, Fa. Momentive

Performance Materials, 4 g of Aerosil ^® 150, Fa. Evonik Degussa and 2.5 g,

according to 4.76 wt .-% silica coated iron oxide by means of a

SpeedMixers 2x30 sec. And 2x45 sec. Mixed at 3000 rpm. Subsequently, the silicone composition is applied in a thickness of about 1 mm on a glass slide. Of the Energy input occurs by induction in the alternating magnetic field of a frequency of 670 MHz with an electrical power of 2.5 kW.

 feedstocks

Maghemite: Auer-Remy, average particle size 20 to 50 nm, specific surface area> 30 m ² / g, approximately spherical.

Magnetite: Auer-Remy, average particle size 20 to 30 nm, specific surface area> 60 m ² / g, spherical.

Examples 1 to 3 are carried out by means of indirect flame, examples 4 to 6 by means of direct flame. EXAMPLE 1 In a first region of a flow-through reactor, a mixture of 1250 g / h of pulverulent maghemite and 4.5 Nm ³ / h of air is subjected to a temperature-controlled treatment in a high-temperature zone. The high-temperature zone is formed by an external hydrogen / oxygen flame, which results from ignition of 6.5 Nm ³ / h of hydrogen and 30 Nm ³ / h of air. The mean residence time of the reaction mixture in this first region is about 0.1 s. The temperature 50 cm below the burner mouth is 825 ° C.

 In the second area of the flow reactor become the flow of hot

Reaction mixture from the first region 0.6 kg / h of vaporous Si (OC ₂ H ₅ ) _{4 was added} together with 3 Nm ³ / h of nitrogen. The mean residence time of the

Reaction mixture in the second range is 0.9 s. The temperature 20 cm above the point of addition is about 640 ° C.

 Subsequently, the reaction mixture is cooled and the resulting solid is deposited on a filter of the gaseous substances.

The solid has an iron oxide content, calculated as Fe ₂ O ₃ , of 89% by weight. Its BET surface area is 30 m ² / g. The proportions by weight of maghemite,

Magnetite and hematite are 54, 9 and 37%. The heating temperature after 60 s is 312 ° C. After treatment with hydrochloric acid, 15 ppm iron is detectable.

Example 2 is carried out analogously to Example 1, but the atomization of the magnetite takes place in a reducing atmosphere. Furthermore, the flame parameters are selected differently than in Example 1, so that the temperature in the first region is lower. Example 3 is carried out analogously to Example 1, but a 1: 1 mixture, each 0.325 kg / h, of magnetite and maghemite is used.

Example 4: In a first region of a flow reactor, a mixture of 800 g / h of pulverulent maghemite and 4.5 Nm ³ / h of air are reacted in a hydrogen / oxygen flame. The flame results from the ignition of a

Mixture of 8 Nm ³ / h of hydrogen and 21 Nm ³ / h of air results. The average residence time of the reaction mixture in this first region is about 1.2 s. The temperature 50 cm below the burner mouth is 1061 ° C.

 Subsequently, the reaction mixture is cooled and the resulting solid is deposited on a filter of the gaseous substances.

The solid has an iron oxide content, calculated as Fe ₂ O ₃ , of 82% by weight. Its BET surface area is 30 m ² / g. The proportions by weight of maghemite, magnetite and hematite are 20, 62 and 18%. The heating temperature after 60 s is 352 ° C. After treatment with hydrochloric acid, 25 ppm iron is detectable. Example 5 is carried out analogously to Example 4, but magnetite is used instead of maghemite.

Example 6 is carried out analogously to Example 4, but a 1: 1 mixture of magnetite and maghemite is used and the atomization takes place in a reducing atmosphere. The starting materials and reaction conditions as well as the physicochemical properties of the powders obtained are summarized in Table 1.

The examples show that it is possible by means of the processes according to the invention to produce silica-coated iron oxide particles. The short-term high-temperature treatment makes it possible to modify the iron oxide particles used so that the core of the particles produced by the process according to the invention contains three iron oxide modifications, namely, irrespective of the iron oxide, magnetite, maghemite and hematite used. The composition of the core can be varied as shown in the examples and leads to high temperatures during inductive heating. Table 1: Starting materials and reaction conditions; physicochemical data

a) 90: 10 H ₂ / N _2; b) 50 cm below burner mouth; c) second area 20 cm above the addition point); d) heating time up to 100 ° C; 670 MHz; in silicone matrix;

e) 1: 1 parts by weight; f) TEOS = Si (OC ₂ H ₅ ) ₄

Claims

 Process for producing particles with an amorphous shell Silica and a core containing hematite and at least one other iron oxide selected from magnetite and maghemite, characterized in that one at a temperature of 800 to 1200 ° C comprising a current a) magnetite particles and / or maghemite particles b) an oxygen-containing gas or a hydrogen-containing gas c) steam and d) a silicon compound is reacted, then optionally cooled and the solid separated from gaseous or vaporous substances. Method according to claim 1, characterized in that the temperature of 800 to 1200 ° C is formed by a flame resulting from the ignition of a mixture containing at least one hydrogen-containing fuel gas and an oxygen-containing gas. Method according to claim 2, characterized in that the molar ratio of O ₂ in the oxygen-containing gas / hydrogen-containing fuel gas is 0.8 to 1.5. Process according to claims 1 to 3, characterized in that in the region of the reactor in which the temperature of 800 to 1200 ° C, the average residence time is 10 ms to 1 s. Process for producing particles with an amorphous shell Silica and a core containing hematite and at least one other iron oxide selected from magnetite and maghemite, characterized in that in a first region of a reactor at a temperature of 650 to 1000 ° C comprising a stream a) magnetite particles and / or maghemite particles  b) an oxygen-containing gas or a hydrogen-containing gas and c) water vapor  reacting, and in a second, the first subsequent, region of the reactor at a temperature of 400 to 800 ° C, wherein the temperature in the second region is lower than in the first region, comprising a stream comprising d) the reaction mixture leaving the first region and a  Silicon compound and  e) water vapor  brings to reaction and optionally the reaction mixture leaving the second region and then cools the solid of gas or  separated off vaporous substances. Process for producing particles with an amorphous shell  Silica and a core containing hematite and at least one other iron oxide selected from magnetite and maghemite,  characterized in that one  in a first region of a reactor at a temperature of 650 to 1000 ° C comprising a stream  a) magnetite particles and / or maghemite particles and part of the  Total amount of a silicon compound and  b) an oxygen-containing gas or a hydrogen-containing gas and c) water vapor  reacting and in a second, the first subsequent, region of the reactor at a temperature of 400 to 800 ° C, wherein the temperature in the second region is lower than in the first region, comprising a stream comprising d) the reaction mixture leaving the first region and the  remaining part of the total amount of the silicon compound and e) water vapor  brings to reaction and optionally the reaction mixture leaving the second region and then cools the solid of gas or  separated off vaporous substances. 7. Process according to claims 5 or 6,  characterized in that the temperature of 650 to 1000 ° C is formed by a flame coming out the ignition of a mixture which is at least one hydrogen  containing fuel gas and an oxygen-containing gas results. 8. The method according to claim 7, characterized in that the molar ratio of O ₂ in the oxygen-containing gas /  hydrogen-containing fuel gas is 0.8 to 1.5. 9. Process according to claims 5 to 8,  characterized in that  in the region of the reactor in which the temperature of 650 to 1000 ° C, the average residence time is 10 ms to 2 s. 10. The method according to claim 5 to 9,  characterized in that  in the region of the reactor in which the temperature of 400 to 800 ° C, the average residence time is 0, 1 to 10 s. 1 1. Process according to claims 1 to 10,  characterized in that  the maghemite particles and / or magnetite particles are introduced in the form of an aerosol. 12. Process according to claims 1 to 1 1,  characterized in that  the maghemite particles and / or magnetite particles have a mean diameter of 10 nm to 1 μηι have. 13. Process according to claims 1 to 12,  characterized in that the silicon compound selected from the group consisting of SiCl ₄ , CH ₃ SiCl ₃ , (CH ₃ ) ₂ SiCl ₂ , (CH ₃ ) ₃ SiCl, HSiCl ₃ , (CH ₃ ) ₂ HSiCl and CH ₃ C ₂ H ₅ SiCl ₂ , H ₄ Si, Si (OC ₂ H ₅ ) ₄ and / or Si (OCH ₃ ) ₄ . 14. Process according to claims 1 to 13,  characterized in that the weight ratio of maghemite particles and / or magnetite particles to silicon compound, calculated as Si0 ₂ , 10:90 - 95: 5. Process according to claims 5 to 14, characterized in that in the region of the reactor in which the temperature is from 400 to 800 ° C, more than 50 mol% of the silicon compound are introduced.