DE2524520C2 - Process for accelerated stabilization of pyrophoric iron powder - Google Patents

Description

The invention relates to a method for accelerated stabilization of pyrophoric iron powder by reaction with oxygen-containing gases.

For the production of magnetic tapes, iron in powder form can also be used as a magnetic material. The metal powder obtained from goethite in the pseudomorphic reduction has a large surface area (around 20 to 30 m ² / g). It's pyrophoric.

In order to be able to handle this metal powder in air, it must be stabilized. The stabilization can basically be achieved in two different ways - either by reacting with Oxygen or oxidizing agents in organic solvents, or by reaction with oxygen-containing ones Gases, e.g. B. oxygen-containing nitrogen. Both procedures must be at or near room temperature be carried out because above 75 ° C oxygen diffuses into the iron lattice, as from experiments on the Thermobalance can be closed. It is also known that this Stabilization at normal pressure or can be carried out close to normal pressure, but this should not be according to DE-OS 19 31521 be crucially important.

In practice, the stabilization can be achieved by reacting the iron powder with small amounts of oxygen in the Fluidized bed can be achieved, with the oxygen being part of the fluidizing gas stream as long as the powder is present is pyrophoric, this reaction is almost quantitative and has a heat of reaction of about 546 kj / mol O2 (obtained from calorimetric measurements) connected - a value that is very similar to the heat of formation of iron oxides

Because of this high enthalpy of reaction, the temperature rises considerably. On the other hand, the Heat of reaction is primarily removed from the reaction vessel by the gas stream. As a result, the Oxygen content of the gas should only be in the order of magnitude of 0.1 to 0.2 vol.%, Otherwise the temperature will increase would rise high. This is true at least when the reaction takes place under such conditions that other heat transport processes, e.g. B. Heat conduction through the vessel walls or cooling devices within of the fluidized bed, can be disregarded. Since during the stabilization process about 6 % By weight of oxygen are taken up, this stabilization process is quite time consuming.

The invention is based on the object of carrying out the stabilization in the shortest possible time.

According to the invention, this object is achieved by the reaction of the pyrophoric iron powder with the Oxygen-containing gases is carried out under increased pressure until there is a steep drop in temperature occurs, and then continues to work at or near normal pressure. Preferably will at a pressure of 1.2 to 50 bar, in particular 15 to 20 bar, worked

In the investigations that have led to the invention, it has been found that the time required is the The pressure of the reaction gas is approximately reciprocal

In addition to shortening the time required to stabilize a certain amount of powder used the inventive method offers the further advantage that the device, based on a certain throughput capacity can be made much smaller, which makes it cheaper.

An embodiment of the invention is shown in the drawing and will be described in more detail below described. It shows

F i g. 1 an autoclave arrangement for carrying out the process,

F i g. 2 a characteristic temperature-time curve the procedure,

Fig. 3 is a graphic representation of the product of the stabilization time (minutes, Fig. 2) and pressure (bar) as a function of the pressure during the stabilization process,
F i ξ. 4 ^ ine graphical representation of the iron content and the magnetic properties of the powders as a function of the pressure during the stabilization process and

F i g. 5a and 5b show the signal-to-noise ratio of magnetic tapes made with stabilized iron powder at 3 μm (Fig.5a) and 6μπι (Fig.5b) wavelength of a written sine wave signal as a function of the gas pressure during passivation. The bias noise was measured only at 8 μm wavelength.

All experiments were carried out with pyrophoric iron powder in the German patent application P 23 61 539.6-24 proposed method was obtained by reduction at 379 ⁰ C with 9m ³ H ₂ / h after. The starting material for this was FeOOH, doped with tin and neutralized with sulfuric acid

In Fig. 1, the pressure vessel 1 contains a mixture of about 0.13% by volume of oxygen in nitrogen. These Mixing was produced by filling the pressure vessel with 1 bar of air before 150 bar of nitrogen were filled. With valve 2 the pressure is reduced to 30 bar. At this pressure a Device for measuring the oxygen flow 3 calibrated

About 60 g of pyrophoric iron powder are introduced into the vessel 4 under a _{gentle stream of the N 2} / O ₂ mixture. After the autoclave 5 has been closed, pressure is applied to the desired value by actuating the pressure reducing valve 6. A gas flow directed from top to bottom is then allowed to act on the powder by closing valve 7 and opening valves 8 and 9 until a desired gas flow, measured with the rotameter 10 at normal pressure, is reached. The gas velocity measured with the rotameter was varied in relation to the pressure between 0.5 m ³ / h at 1.2 bar and 7.2 m ³ / h at 18 bar

Autoclave always the same at all pressures (approx. 7 cm / s). The speed is the same Order of magnitude as used in fluidized bed reactors. This means that the powder im Autoclaves would form a fluidized bed if the gas was reversed from bottom to top would flow. Because of the small dimensions of the autoclave, this could not be permitted. A Fluidized bed is however preferable for a technical system because of the heat exchange between the flowing gas and the powder is thereby improved. It should be noted that fluidization is a Powder filling only depends on the linear speed of the gas flowing through, regardless of the Pressure of the system.

As the gas flows through the powder filling, its oxygen content reacts with the powder, whereby forms a stabilizing layer on the surface of each individual particle and the corresponding Heat of reaction is released. This increases the temperature up to a value close to 40 ° C (F i g. ^), at which the Heat generated by the reaction is precisely balanced by the amount of heat generated by the gas flow transported away and diverted through the walls. As a result of thermal insulation, the Heat conduction very low, so that the steady-state temperature only depended on the oxygen content of the gas and due to the constant oxygen content The nitrogen-oxygen mixture was almost the same in all experiments.

After some time the temperature drops steeply (FIG. 2), although the gas stream is still oxygen At this point in time, the stabilization layer is almost completely formed. It then becomes that Gas flow gradually added more oxygen from the vessel by carefully opening valve 112 is opened until the gas finally contains around 20% by volume of oxygen. This second part of the stabilization is to be carried out at or near normal pressure for safety reasons, especially since its duration is only depends little on the gas pressure.

Finally, the powder is removed from the autoclave and placed in a glass tube that is covered with a glass frit is provided as a filter plate, treated with a stream of air to give the powder the required amount of water (about 0.5 to 1 wt .-%) can adsorb from the air. Since the mentioned second part of the If passivation is carried out at or in the vicinity of normal pressure, the moistening Jes Powder

Tabel

Electroacoustic values of tapes. Powder # 1 was stabilized in a fluidized bed in the conventional manner, all others under pressure. The signal strength is given in dB compared to a commercially available CrO ₂ tape (BASF C 401 R).

combine with it by supplying the required oxygen in the form of moist air

In Fig. Thus, in FIG. 2, the steep drop in temperature indicates the completion of the formation of the oxide layer on the surface of the particles. The maximum temperature was ailen Try 32 to 40 ⁰ C.

For comparison purposes, part of the powder was stabilized in the usual way in a fluidized bed with an N ₂ / O 2 mixture. This powder came from the same batch as the powder treated according to the invention.

The iron content of all powders was determined quantitatively by titration with ethylenediaminetetraacetic acid determined The starting material (before stabilization) had an iron content of 96.2% by weight. All powder were also examined for their static magnetic properties stabilized powders and a binder system made from practically uncrosslinked linear polyurethanes and vinylidene chloride-acrylonitrile copolymers that have been ground with glass balls, magnetic tapes manufactured and examined

In Fi g. 3 is the time it takes to stabilize, & h. the time t between the start of gas flow and the steep temperature fall (F i g. 2), multiplied by the pressure ρ ρ plotted as a function of pressure. Despite a certain spread, the product t ■ ρ turns out to be constant. This means that the time t decreases in inverse proportion to the pressure ρ

Fig.4 shows the iron content and the static magnetic properties as a function of pressure. No influence of pressure can be seen. In the The table below shows the results of acoustic measurements on the magnetic tapes.

F i g. 5a and 5b show the signal-to-noise ratio at 3 and μπι 6 μπι wavelength of an inscribed sine signal as a function of the pressure during stabilization. The bias noise was at 8 μm measured, but it depends very little on the wavelength. The tapes were made from the invention stabilized powders and a binder system made from practically uncrosslinked polyurethanes and Vinylidene chloride-acrylonitrile copolymers, which were ground with glass balls, produced.

The signal-to-noise ratio is given in dB compared to a commercially available CrO ₂ tape (BASF C 401 R). Again, no significant influence of the pressure can be seen.

j powder pressure Signal strength at wavelength 12 am 24 am 48 'in Alternating current - Guideline factor +6.2 _ - before May rush ϊ +5.6 +4.8 +3.4 at wavelength j No. bar 3 μm 6 am +6.0 +5.2 +3.7 8 in j 1 1 +2.8 +4.6 +6.0 +6.6 +4.7 -0.6 2.00 1 ² 1.2 +4.0 +4.6 +5.2 +5.2 +3.1 -0.6 2.26 1 ³ 2.5 +5.0 +5.6 -0.6 2.14 \ ^4th 5.0 +4.0 +5.6 -0.0 2.16 7.5 +4.4 +5.6 + 1.4 2.22

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Claims (3)

Patent claims: 1. Process for accelerated stabilization of pyrophoric iron powder by reaction with oxygen-containing Gases, characterized in that the reaction takes so long under increased Pressure is applied until there is a steep drop in temperature, and then at or near Normal pressure is continued. 2. The method according to claim 1, characterized in that at a pressure of 1.2 to 50 bar is being worked on. 3. The method according to claim 2, characterized in that at a pressure of 15 to 20 bar is being worked on.