WO1989012887A1

WO1989012887A1 - Process for treating metal particles against corrosion and the particles thereby obtained

Info

Publication number: WO1989012887A1
Application number: PCT/EP1989/000684
Authority: WO
Inventors: Gilles Le Du; Pierre-André Marie Vincent MARI
Original assignee: Eastman Kodak Company; Kodak-Pathe
Priority date: 1988-06-24
Filing date: 1989-06-17
Publication date: 1989-12-28
Also published as: FR2633311A1; EP0424411A1

Abstract

This invention concerns a process of treating particles which are unstable in air and the particles obtained which exhibit good corrosion resistance. The method consists in treating the metal particles by a current of gas containing silane and hydrogen in a fluidised bed. The particles may subsequently be subjected to a current of oxygen, preferably in the same fluidised bed, to obtain a coating of silica around the particles. The method is especially useful in obtaining particles of iron for magnetic recording materials which are suitable for dispersion and are corrosion-resistant.

Description

PROCESS FOR TREATING METALLIC PARTICLES

AGENT AGAINST CORROSION AND PARTICLES OBTAINED The present invention relates to a process for obtaining and treating metal particles which are unstable in air, as well as the particles obtained, which have good dispersibility, increased resistance to corrosion and good resistance to aging. The invention relates more particularly to ferromagnetic metal particles which can be used for magnetic recording. Many metals exhibit critical air instability due to their reducing nature, the properties of their oxides and their sensitivity to various catalytic agents. This instability in air is often increased depending on the physical state of the metal and the specific surface it can offer to the ambient atmosphere. Consequently, if the particles are finely divided, the problem of instability in air is particularly critical, going so far as to manifest itself under the aspect of almost spontaneous flammability, in the case of pyrophoric metal particles.

Among the various materials known and used for magnetic recording, ferromagnetic metal particles have, compared with iron oxides in particular, the advantage of a magnetization at saturation and a particularly high coercive field. However, these metal particles are difficult to disperse (in the binders used to prepare the magnetic layers usually) and, above all, suffer from being very sensitive to corrosion by oxidation, given the oxidizable nature of metals, in particular iron.

Given the attractive magnetic properties of metallic particles, various treatments have been proposed to improve their dispersibility and to reduce their sensitivity to corrosion. It has for example been proposed to protect iron particles by coating with iron oxide. The constitution of this oxide coating consumes a non-negligible part of the iron of the particle and thus contributes to weakening the magnetic characteristics. But, above all, the permeability of iron oxides makes ineffective any attempt at final protection by the formation, on the iron particle, of a coating of iron oxide. Indeed, through this coating, the phenomena of cationic or anionic diffusion can continue, that is to say that the oxidation can continue and cause an uninterrupted decrease in the iron content of the particle and, thereby, continuously and irreparably worsening the weakening of the magnetic characteristics, in any case those of these characteristics which are directly related to the quantity of ferromagnetic metal. This is true in all cases of metallic particles protected from oxidation by an oxide layer of the same metal, where the desired properties are linked to the quantity of material.

In metallurgy, various techniques are known for passivating metals, that is to say making them insensitive to various corrosive agents. Some of these techniques use silicon compounds. These techniques have been applied to ferromagnetic particles intended for magnetic recording. It is in particular known to coat the goethite particles with silicates or silica gels which are then reduced at high temperature to give iron particles coated with silica. Such methods are for example described in the patents of the United States of America 4,334,933, 4,347,291 and 4,400,337. The disadvantage of these methods lies in the slowing action exerted by the silicon compounds on the process of reduction of goethite to iron: if the amount of silica formed is too large, the reaction can be blocked before being completed.

It is also known to passivate particles of ferromagnetic metals by forming on their surface a coating of organic silane by treatment of these particles with a solution of silane in an organic solvent. Such methods, described in US Patents 4,475,946 and 4,437,882, therefore involve the industrial use of organic solvents.

US Patent 4,746,547 describes a process for treating metallic particles smaller than 100 μm in size with gaseous silane. The silane decomposition reaction does not however take place in a fluidized bed, and the presence of hydrogen is not compulsory. This patent does not disclose the means of the invention making it possible to control the deposition of silicon on particles of very small dimensions and to avoid their agglomeration.

None of the aforementioned methods is entirely satisfactory as much by the results which they make it possible to obtain in terms of corrosion resistance and ability to disperse, as by their ease and their reproducibility in terms of industrial implementation.

The present invention relates to a method of treating metal particles which are unstable in air, which provides a solution to the abovementioned drawbacks. The method of the invention for treating metal particles consists in subjecting these particles to the action of silane gas, and it is characterized in that the particles are subjected to the action of a gas stream containing silane gaseous and hydrogen in a partial pressure pH / pSiH ratio between 10 ⁶ and 10? in a fluidized bed.

According to advantageous embodiments, the fluidized bed is heated to a temperature between 200 ° C and 400 ° C, and the fluidization speed is between 0.1 and 0.5 m / s.

According to another embodiment, the metal particles are ferromagnetic particles, and more particularly iron particles. The subject of the invention is also a process for the passivation of metal particles in which the metal particles carrying a silicon coating are oxidized, as obtained by the treatment process described above, to form on their surface a layer of silica.

The subject of the invention is also a process for obtaining treated particles of a metal from an oxide of this metal, in which particles of oxide of this metal are reduced by a gaseous reducing agent in a fluidized bed, then they are treated according to the treatment method described above.

According to a preferred embodiment, all of the methods of the invention can be carried out in a fluidized bed, in the same reactor, by carrying out the appropriate purges between each step. The gaseous reducing agent used to reduce the metal oxide is preferably hydrogen. Such a process is very advantageous in an industrial application, since it makes it possible to use the same reactor to prepare the particles and to treat them, by changing only the reactive gas, and not does not require further treatment of the particles.

The method is mainly applicable to obtaining iron particles intended for magnetic recording. The invention makes it possible to obtain ferromagnetic metal particles stabilized against oxidation with a reduced reduction in magnetization, suitable for dispersion in a binder and resistant to aging.

The method of the invention is however not limited to such an application. It can also be used in general to coat fine particles with a silicon layer.

In the process according to the invention, an H 2 / SiH 4 gas mixture is used in a fluidized bed. The use of the fluidized bed technique makes it possible to better control the coating of the particle, but it is obviously essential to properly control the amount of silicon formed on the particles. Indeed, the objective is to make a supply of silicon just necessary to coat the particles, without introducing an excess of silicon which, being housed between the particles, would cause an undesirable aggregation of the particles. In particular in the case of use for magnetic recording, such aggregates greatly hinder the subsequent formation of magnetic dispersions, because they require more intense mixing operations which destroy the morphology of the particles. This is in particular the case for needle-like particles which are most often sought for making the magnetic recording layers. Control of the amount of silicon formed on the particles is also necessary insofar as there is a threshold beyond which the corrosion resistance no longer increases, while the magnetic characteristics begin to degrade. The amount of silicon deposited during a given time depends on the ratio of partial pressures pH 2 / pSiH4, the fluidization speed and the treatment temperature.

The partial pressure ratio pH 2 / pSiH4 makes it possible to control the rate of decomposition of the silane, and therefore the rate of deposition of the silicon, the presence of hydrogen tending to slow down the reaction. The ratio of partial pressures pH 2 / pSiH4 is between

10 and 10, which corresponds to a silane content by volume relative to the volume of total fluidization gas of between 0.05 and 1%. The fluidization speed, which is the ratio of the total gas flow rate through the section of the reactor, itself depends on the size of the particles and on the nature of the carrier gas, as will be seen below. The temperature is preferably between 200 ° C and 400 ° C.

Figure 1 is a graph showing the relationship between the amount of silicon added to the particle and the residence time in the bed according to different characteristics of the fluidized bed (fluidization rates and silane concentrations) at 350 ° C.

FIG. 2 is a graph showing the effectiveness of the protection (evaluated by the oxidation of the particle) as a function of the amount of silicon added to the particle. FIG. 3 represents diagrams showing the structure of the inter and intragranular porosities before and after the treatment.

Figure 4 shows a diagram of the installation. In the graphs, the percentage of silicon is given by mass relative to the total mass of the particle, and possibly includes the silicate which could have been deposited on the starting metal oxide particles.

The fluidized bed technique, recommended in the process of the invention, consists in bringing a layer of particles 7 solids in a state presenting certain analogies with a fluid, thanks to the action of an ascending gas stream. The advantage of the fluidized bed technique is that it allows very good contact between the gas and the particles. Thus the reaction times can be short, which is interesting, in particular when, as in the case of magnetic particles, there is a risk of deterioration of the substrate.

A minimum speed of the gas flow (minimum fluidization speed) is necessary to obtain the suspension of the particles and the formation of the bed. But if the speed of the gas flow exceeds a certain limit (maximum fluidization speed), particles are entrained by the gas stream. The speed of fluidization governs the deposition of silane, but it itself depends on the size of the particles and the viscosity of the medium, determined by the nature of the carrier gas. The particles treated according to the process of the invention are in the form of powders consisting of porous grains having dimensions of 0.1 to 1 mm and pore sizes between 1 and 0.01 μm. This structure appears in FIG. 3, giving the inter and intragranular porosities of the powder. This pore size allows the silane to infiltrate inside the grains, to deposit on the individual particles. But on the other hand the grains themselves are large enough that one can use fairly high fluidization speeds. When the carrier gas is argon, the fluidization speed is preferably between 0.1 and 0.5 m / s.

Powders having the granularity characteristics indicated above are for example goethite powders prepared according to the procedure of French patent 2,487,326. However, any powder having such 8 particle size characteristics whatever its preparation process.

The process according to the present invention is carried out in a fluidized bed in a device mainly comprising: - A reactor (1) inside which the thermochemical reactions are carried out; this reactor consists of a stainless steel tube and has at its top a flange and a cover provided with a safety glass intended for the observation of fluidization. At its base, another interlocking flange receives the sintered metal distribution plate and ensures the assembly of the assembly on a base (2).

- The gas heater (3) whose function is to heat the different gases (H, N, air, argon, O) before their introduction into the reactor, thus bringing additional calories to the fluidized bed so as to facilitate the setting in temperature and its regulation. - A heating chamber (4) which surrounds the reactor in the closed position and which provides most of the calories necessary for heating and regulating the bed and which also serves as insulation. This enclosure consists of two half-shells each equipped with an electrical resistance.

- A dedusting cyclone (5) placed in the gas evacuation circuit at the outlet of the reactor. It acts by centrifugation and separates from the gas stream the fine particles of product entrained during fluidization.

- A cooler (6) placed in the gas evacuation circuit downstream of the dedusting cyclone, and which is used to cool the hot gases before they pass through the purifier. It can be a tubular exchanger composed of a series of small metal tubes traversed by hot gases and cooled externally by a circulation of cold water contained in a cylindrical envelope.

- A product recuperator (7). It mainly comprises a cyclone fitted with a pneumatic valve and a product recovery container. The product is recovered by pneumatic transport of the particles.

- A purifier (8) whose main role is to purify the gases from the two circuits

(cooling and recovery) by bubbling in water before any release into the atmosphere. This device also forms a liquid seal (non-return) isolating the contents of the reactor from the outside, in the event of a temporary stop of operation of the installation. For reasons of simplification, the device has been described only in its main functions. These devices, well known in the prior art, include other elements such as temperature and sampling probes arranged in particular in the reactor (1). In our case, it should be noted that the silane is introduced into the reactor without passing through the heater, by means of another conduit (9) communicating with the reactor, this due to the fact that the silane is a gas which is decomposes on heat.

In practice, the powder formed from the porous particles of particles is first swept with the gas chosen for the fluidization. Then, the cold gas mixture is introduced at the base of the diffuser surmounted by the powder. The gas mixture comprises silane SiH, hydrogen and an inert dilution gas which, preferably, is a rare gas such as helium or argon. The concentration of silane in the dilution gas is between 1% and 5% by volume. After the action of the silane gas on the particles, 10 followed by a nitrogen purge, a stream of oxygen can be passed through the fluidized bed so as to obtain a layer of silica around the particles. In a preferred embodiment, a stream of oxygen diluted in nitrogen with increasing concentration is used, from 0 to 3% at the start, up to 21%, at 80 ° C., for 1 hour.

In a preferred embodiment, the metal particles are prepared from a metal oxide by reduction with a gaseous reducing agent, in the same fluidized bed, this reducing agent preferably being hydrogen.

The following examples illustrate the invention. EXAMPLES

In all the examples, goethite prepared according to the procedure of BF 2 487 326 is used.

(Pingaud), in the form of needles about 0.4 μm long and of particularity about 20, coated with silicate at 7.5% relative to the mass of goethite. Example 1 After reduction of the goethite with hydrogen in a fluidized bed, the reduced powder is treated with silane with a fluidization speed of 0.5 m / s (a product composed of argon containing 3% of silane)% of SiH. and the temperatures are indicated in the table below, for 4 samples and one untreated control. The 5 samples are then passive with a current of O 2.

The table below gives the quantity of total silicon mass titrated, including the silicate initially deposited on the goethite needles, and the magnetic properties: coercive field, magnetization, middle, for the particles obtained, and for a control sample. prepared from the same goethite reduced by hydrogen in a fluidized bed, then passive with oxygen, but without the silane treatment. BOARD

Hc Im Shoulder width% Si (Total Temperature% SiH4

0 emu / g dosed) by mass ° C (mole / mole H)

Witness 1620-1660 100 0.57-0.59 1 / 1.2 / 0

1 1660 100-105 0.61 2.2 350 0.07

2 1670 110-120 0.6 3.3 350 0.2 -

3 1,725 120 0.61 3.5 350 0.35

4 1680 110-115 0.6 3.3 310 0.35

The magnetic properties are measured with a hysterometer under a magnetizing field of 4500 Oe. It can be seen that the magnetization and the coercive field are less reduced for the particles of the invention treated with silane before passivation, than for the passive control without treatment with silane. Example 2

It is possible to measure the protective qualities of an oxide layer on the needles by studying the oxidation kinetics (variation in mass per unit area) in a non-humid environment, of these needles. This was done at 130 ° C. The parabolic oxidation constant K can be measured as Δm / S = K- / t, where m is the mass of the particles, S their surface and t the oxidation time. Figure 2 gives the value of K as a function of the% in total Si titrated. It can be seen that the quality of the protection increases with the silicon content and stabilizes around 5% of Si. Example 3 The sample 2 and the control of Example 1 are used to prepare two compositions having the following formula.

Metal particles 73.6%

Estane 5715 ¹ 6.98%

VAGH ² 4.66% A Acciiddee oolléiiqquuee 0.73%

^Al 2 ° 3 5.88%

Cr ₂ 0 ₃ 1.1%

Myristic acid 1.45%

Gafac 5.51% i Polyester urethane supplied by B.F. Goodrich Chemical Co.

2 Polyvinyl acetochloride supplied by Union Carbide Corp.

3 Surfactant based on complex organic phosphates, supplied by General Aniline and Film

Corporation. The compositions are ground in a solvent mixture consisting of toluene, cyclohexanone and tetrahydrofuran (1/3: 1/3: 1/3 by volume), and the compositions are deposited on a support of polyethylene terephthalate. The products obtained are oriented and dried to obtain layers 5 μm thick, the magnetic properties of which are indicated in the following table: Samples Hc (Oe) Middle part Im S / N

(Emu / cm) Witness 1475 201.8 50 db

Invention 1,550 0.74 212.2 53.4 db

It can be seen that the properties of the sample of the invention are better than those of the control. Example 4 The porosity of the powders is measured after reduction and after deposition of silicon.

The process of the invention is carried out at 350 ° C., with a fluidization speed of 0.31 m / s and a percentage of silane of 0.9%. The diagrams of FIGS. 3a and 3b represent the distributions of the pore volumes as a function of the diameter of the pores. The measurements are carried out with a mercury porosimeter. We can observe two kinds of pores: the largest have a diameter between 50 and 120 μm, and the smallest have a diameter between 13 and

120 nm; it can be seen that the small pores do not disappear after the silane treatment, which means that the particles are not agglomerated.

Claims

H

CLAIMS - Process for treating metal particles which are unstable in air, which consists in subjecting these particles to the action of silane gas, characterized in that the metal particles are subjected to the action of a gas stream comprising gaseous silane

SiH, hydrogen in a pressure ratio

⁴ ₆ partial pH / pSiH between 10 and

10 ⁴ , and a dilution gas, in a fluidized bed. 2 - Process according to claim 1, wherein the concentration of silane in the dilution gas is between 1% and 5% by volume.

3 - Process according to any one of claims 1 or 2, wherein the dilution gas is argon or helium.

4 - Method according to any one of claims 1 to

3 in which the fluidization speed is between 0.1 and 0.5 m / s.

5 - Method according to one of claims 1 to 4, wherein the fluidized bed is heated to a temperature between 200 ° C and 400 ° C.

6 - Method according to one of claims 1 to 5, wherein the metal particles are ferromagnetic metal particles. 7 - Process according to claim 6, wherein the ferromagnetic metal particles are iron particles. 8 - Method for passivation of metal particles, in which the metal particles carrying a silicon coating are oxidized, as treated according to the method of any one of claims 1 to 7, in the same fluidized bed, by a gaseous oxidizing current to form a layer of silica on their surface. 9 - Process for obtaining treated particles of a meta from an oxide of this metal, in which particles of oxide of this metal are reduced by a current of a gaseous reducing agent in a fluidized bed, then they are treated in the same fluidized bed according to the method of any of claims 1 to 8. - The method of claim 9, wherein the gaseous reducing agent is hydrogen. - Method according to one of claims 9 or 10, wherein iron particles are prepared from alpha hydrated ferric oxide. - Particles of ferromagnetic metal stabilized against oxidation without reduction in magnetization, suitable for dispersion in a binder, and resistant to aging, prepared according to a process according to one of claims 1 to 11.