JPS5819721B2 - Method for producing ferromagnetic metal powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing ferromagnetic metal powder, and particularly to a method for producing ferromagnetic metal powder used in magnetic recording media.

Ferromagnetic powders conventionally used as magnetic recording media include maghematite (γ-Fe2Q), cobalt-doped maghematite, and magnetite (Fe20
4) Cobalt-doped magnetite, magnetite bertholed compounds, cobalt-doped bertholed compounds of maghematite and magnetite, chromium dioxide, and the like are known.

Requirements for magnetic recording media are becoming increasingly strict, and ferromagnetic powders with characteristics suitable for high sensitivity and high density recording have recently been actively developed.

One of the target materials is ferromagnetic metal powder.

Ferromagnetic metal powder has high residual magnetism and is therefore promising as a magnetic powder for high-density recording media, but since it is a fine powder and has a large surface area, it has the disadvantage of being easily oxidized.

The present invention prevents oxidation and improves the properties of ferromagnetic metal powder produced by a wet reduction method through continuous treatment, thereby providing a ferromagnetic powder having excellent magnetic properties as a magnetic recording medium.

Conventionally, ferromagnetic metal or alloy powders have been produced by the following method.

(1) A method of thermally decomposing an organic acid salt of a ferromagnetic metal and reducing it with a reducing gas.

(For example, Special Publication No. 36-11412, No. 36-2223
(No. 0, No. 48-29280, etc.) (2) A method for reducing acicular oxyhydroxides, or those containing other metals, or acicular iron oxides obtained from these oxyhydroxides. .

(For example, Special Publication No. 35-3862, No. 37-1152
(3) A method of vaporizing a ferromagnetic metal in a low-pressure inert gas.

(For example, Special Publication No. 46-25620, No. 47-4131
(4) A method of thermally decomposing a metal carbonyl compound.

(For example, Japanese Patent Publications No. 39-1004, No. 40-3415, No. 45-16868, etc.) (5) A method in which ferromagnetic metal powder is electrodeposited using a mercury cathode and then separated from the mercury.

(Example: Special Publication No. 35-12910, No. 36-3860
(No. 45-19661, etc.) (6) A method in which a reducing agent is added to a solution containing a salt of a ferromagnetic metal to reduce the solution.

(For example, Special Publication No. 38-20520, No. 38-265
55, JP-A No. 48-82396, etc.) The present invention particularly relates to a method for producing a magnetic metal powder-containing composition suitable as a magnetic recording medium from magnetic metal powder obtained by the wet reduction method described in (6) above. However, the method using wet reduction method had serious difficulties.

In other words, since a large amount of water is present in the wet reduction method, it is very important to easily and economically remove this water without impairing the magnetic properties of the resulting powder. None of them were satisfactory.

Regarding this water removal, the following methods have been proposed in the past.

(1) Using a solvent such as acetone, the water-containing ferromagnetic metal powder is washed and the water is replaced with the solvent.

This method has the drawback of requiring a large amount of solvent and not completely replacing the water with the solvent.

(2) A slurry made by adding acetone to the dehydrated ferromagnetic metal powder cake is placed in a container and vacuumed.

Place in an oven, heat to about 150° C. under reduced pressure, and hold for several tens of hours (Japanese Patent Application Laid-open No. 49-41899).

The problem with this method is that it takes a lot of time to remove water, and it is also necessary to use acetone.

(3) After washing the water-containing cake of ferromagnetic metal powder obtained by the wet reduction method with a water-miscible organic solvent such as acetone, the water is removed by gently drying it in the air (US patent! , No. 206,338, No. 3,535, 1
04 etc.).

The problem with this method is that the risk of ignition becomes extremely high as the amount of ferromagnetic metal powder handled increases.

This is because the metal powder, which has a large surface area and is itself highly reactive, is exposed to air.

(4) After dehydrating the slurry of ferromagnetic metal powder produced by the wet reduction method and pulverizing it into flakes, it is supplied to a dryer with a heating surface, and the temperature of the heating surface is adjusted to 80°C under an inert atmosphere. Dry while maintaining the temperature at -250°C while stirring for at least two or more hours during the drying process (
JP-A No. 52-41154).

The problem with this method is that productivity is low because it is basically a batch process.

Furthermore, it is necessary to perform dehydration treatment and pulverization treatment, which increases the number of steps and equipment costs.

The present invention continuously processes ferromagnetic metal powder obtained by a wet reduction method in a completely closed system.
Dehydrate and heat-treat the powder without exposing it to air,
Furthermore, by performing a stabilization treatment, a magnetic metal powder suitable as a magnetic powder for magnetic recording media is provided, and many of the conventional problems described above are solved.

Since the magnetic metal powder of the present invention is produced by a continuous process, the entire process can be stabilized, so that the quality variation is reduced and the magnetic properties are uniform.

Other features and advantages of the invention will become apparent from the following description with reference to the drawings.

The present invention detects the oxygen concentration in the non-oxidizing gas that is passed through both processes when ferromagnetic metal powder produced by a wet reduction method is continuously dried and heat-treated in a non-oxidizing atmosphere.
When the concentration exceeds a specified concentration close to the upper limit (for example, explosive limit) that can avoid excessive oxidation of the ferromagnetic metal powder, a new non-oxidizing gas is added or the existing gas is replaced with a new non-oxidizing gas. This is a method of replacing it with sexual gas.

According to the present invention, the ferromagnetic metal powder, which is easily oxidized, is prevented from being exposed to excessive oxygen, the characteristics of the metal powder can be stabilized with high quality, and the safety of the process is further improved.

Preferably, both the drying and heat treatment steps are each connected to separate non-oxidizing gas circuits, so that the detection and control of the oxygen concentration is carried out separately.

The ferromagnetic metal powder produced by the wet reduction method has very high activity and is easily oxidized, so it is necessary to avoid contact of the ferromagnetic metal powder with oxygen as much as possible during the drying and heat treatment process.

However, it is uneconomical to carry out such a process in a high purity non-oxidizing atmosphere.

The method of the present invention can avoid the problem of excessive oxidation while allowing the presence of oxygen in the atmosphere during these steps.

The drawings are flow sheet diagrams illustrating preferred embodiments of the invention.

The present invention is a method for treating ferromagnetic metal powder produced by a wet reduction method, and it should be understood that any known method can be adopted as the wet reduction method.

In this case, the ferromagnetic metal powder is either freshly produced and dispersed or suspended in the solution, or is precipitated in the solution.

The present invention is a continuous processing method for such a solution (hereinafter referred to as solution).

Now, the solution flows into the intermediate stock tank A from the pipe 1 continuously or intermittently.

Tank A has a sealed structure, and in order to prevent oxidation of the ferromagnetic metal powder, a non-oxidizing gas is filled to prevent air from being mixed into the solution (nitrogen gas is used in this example).

This non-oxidizing gas is supplied through line 3.

The pressure and amount of the non-oxidizing gas shall be such that air will not flow into tank A, and the supply source shall be determined so that the internal pressure is higher than the external pressure.

A stirring blade 5 is installed in the tank A to prevent the ferromagnetic metal powder from settling.

The purpose of this medium stock tank A is to remove air before sending the solution to the next process, stir the whole solution to make it homogeneous, and also remove air from the solution. The aim is to stabilize the solution flow for subsequent steps by stabilizing the volume.

This improves the quality of the product and makes its properties uniform.

Next, the solution thus prepared is continuously supplied to the sedimentation tank B via the pipe 7 by the pump P1.

A level meter LC1 is installed in the intermediate stock tank A to detect the level of the solution, and when the amount of solution in the tank A decreases below a certain level, the pump P1 is stopped and the liquid is transferred to the sedimentation tank B. The liquid supply should be stopped.

Sedimentation tank B functions to cause the ferromagnetic metal powder to settle in the solution by its own weight, forming a ferromagnetic metal powder slurry for the purpose of increasing efficiency in the next drying process.

A slurry take-out pipe 11 is connected to the bottom of the sedimentation tank B, and a pipe line 13 for a separated water take-out port is connected to the top, and the ferromagnetic metal powder is made into a slurry by drawing out a considerable portion of the water in the solution. This reduces the drying efficiency and energy load of the next process.

The residence time of the solution in sedimentation tank B can be easily determined experimentally in relation to the sedimentation rate of the ferromagnetic metal powder, but powder for magnetic recording media usually has a residence time of about 10 minutes at the longest and about 3 minutes at the shortest. That's fine.

From the viewpoint of preventing oxidation, it is desirable that this time be as short as possible.

The sedimented and concentrated slurry is transferred to pipe 1 by pump P2.
1 and supplied to dryer C at a constant flow rate.

On the other hand, excess water overflows from the upper part of the tank and is drawn out to the pipe line 13 to be reused.

The dryer C has a heating jacket attached to the outer wall, through which an external heating medium such as steam is passed.

The heating temperature can be up to 300°C, but ideally the internal temperature is 250°C or less.

However, this internal temperature is not supplied only from the jacket, but also from the non-oxidizing gas.

That is, the heated non-oxidizing gas is continuously supplied from the heater H and moved to the outlet end (from left to right in the figure) while being stirred by the rotating stirring/transfer blade 15 inside the dryer C. Heat is applied in a countercurrent direction to a ferromagnetic metal slurry.

In this way, the slurry is gradually dried while flowing from the inlet to the outlet of the dryer C, but the powder surface from which water has been removed is protected by the non-oxidizing gas.

The rotation speed of the rotary blade 15 can be set arbitrarily, and is approximately 6 rpm.
The value up to is used.

The rotating blades perform the above-mentioned movement, increase drying efficiency, and prevent sintering between particles.

In this way, the ferromagnetic metal powder is made into independent discrete particles while being protected by an inert atmosphere, and can exhibit desirable magnetic properties as magnetic particles.

The completely dehydrated ferromagnetic metal powder protected by non-oxidizing gas is taken out from the dryer C into a pipe line 17, and sent to a heat treatment machine via a rotary valve R1.

The heat treatment machine is also supplied with non-oxidizing gas.

A heating jacket is installed on the outer wall of the heat treatment machine, and the temperature can be adjusted up to a maximum of 300°C.

This heat treatment machine adjusts the magnetic properties of the ferromagnetic metal powder, and is intended to have the effect of increasing the coercive force Hc in particular to make the powder advantageous for use in high-density recording media.

The residence time of ferromagnetic metal powder in the heat treatment machine is 1 to 3
It is about 0 minutes, preferably about 5 minutes.

A stirring blade 21 is also used in the heat treatment machine to perform uniform heat treatment.

The heat-treated ferromagnetic metal powder is continuously drawn out to the pipe line 23 by the rotary valve R2 and transferred to the product take-out tank E.
sent to.

The metal powder sent to tank E is easily ignited when exposed to oxygen, but is protected by non-oxidizing gas.

Therefore, if this metal powder is exposed to air when mixed with a binder resin for a magnetic recording medium, oxidation will rapidly proceed or there is a risk of ignition.

The product take-out tank E is a device for avoiding such danger by impregnating the ferromagnetic metal powder with an oxidation-preventing solvent (toluene in this example).

For this purpose, a solvent such as toluene is stored in tank El and is conveyed via line 25 to the top of tank E.

Since the ferromagnetic metal powder is sent in a highly heated state, a cooling jacket is provided on the outer wall of the tank E to prevent the temperature of the tank E from rising, and the tank E is cooled with tap water or the like 27 at a temperature of 25° C. or lower.

The flow rate of this cooling water is controlled by a flow meter Q.

More preferably, the cooling should be carried out immediately at the exit of the heat treatment machine, but considering factors such as cooling time, the space required for this would inevitably be large. Cooling is carried out simultaneously with impregnation to achieve space savings.

After the impregnation process is completed, the ferromagnetic metal powder protected by the solvent is drawn out to the pipe line 29 by the pump P3 and transferred to the next step.

The next step is not related to the subject matter of the present invention, but is a magnetic paint manufacturing step commonly used to manufacture magnetic recording media, in which ferromagnetic metal powder protected by a solvent, a resin binder, and a solvent are mixed. It is kneaded.

Since the magnetic powder obtained by the method of the invention is protected by a solvent, the paint manufacturing process can be carried out in batch mode.

Pump P3 is from tank E1 to tank. It is also shown as being used for supplying solvent to tank E, and also for the next process to combine the solvent from tank E' (
(see line 31), but these are optional.

Next, a supply circulation system for non-oxidizing gas to be supplied to each process will be explained.

In the drawings, it has already been mentioned that non-oxidizing gas is supplied to the dryer C and the heat treatment machine, but the supply and circulation routes are completely separate.

Circulation minimizes consumption of non-oxidizing gases. Although this is done to reduce the amount of heat used, the moisture content of the non-oxidizing gas that has passed through the dryer C and the heat treatment machine is completely different.
If these are dehumidified using a common condenser, complete dehumidification cannot be achieved.

Therefore, in this embodiment, by employing two separated circulation paths, increase in equipment costs such as addition of another dehumidifier and decrease in operability can be avoided.

Non-oxidizing gas (nitrogen) is fed to heater H via line 33.

The gas in this route only needs to be replenished, and most of it is in pipe 3.
5, and these combined gases are heated by heater H.

The heater H heats the inflowing gas to a temperature substantially equal to the temperature of the dryer C, and then sends the gas to the outlet side of the dryer C by the fan f of the pipe line 37.

The heating method of the heater H may be performed by selecting any means such as electricity, steam, or a heat medium.

In this example, a heat medium was used.

The gas flowing into dryer C heats and dehydrates the ferromagnetic metal powder slurry as described above.

The non-oxidizing gas that has thus come to contain a large amount of water is drawn out from the pipe 45 to the cyclone C1 and is first subjected to the separation of the entrained metal powder, and the remaining highly humid gas is extracted from the pipe 47,
It is sent to the condenser G via the filter F and the conduit 49.

Cooling water flows through the condenser G from a conduit 51, and non-oxidizing gas is dehumidified.

The separated water is recovered through line 53, while the dehumidified gas is recycled as described above.

On the other hand, the non-oxidizing gas to the heat treatment machine is supplied from its supply source through the line 39, and the majority of it is circulated from the heat treatment machine through the fan f of the line and the heating "f5 H'.
It first flows into the jacket of the heat treatment machine.

The non-oxidizing gas flowing down the thermal jacket flows into the heat treatment machine from the exit side to provide heat to the powder being heat treated and protection against oxidation.

This gas flows out from the powder inlet side into line 43 and then enters filter F where it is removed from the entrained ferromagnetic metal powder before being recycled to heater H1 as described above.

In the system of the present invention, since the ferromagnetic metal powder from which moisture has been removed is present in the dryer C and the heat treatment machine, the atmosphere surrounding the powder contains oxygen within the limit (explosive limit). ) or more shall not be included.

Therefore, in order to control the oxygen concentration in the dryer C and the heat treatment machine, an oxygen concentration detector 0 is provided, and this is connected to the pipe 55.
, 57 to the inlets of heaters H and H', respectively.

In this way, the detector O detects the oxygen concentration at each stage, and these are the specified concentrations (in this example, they are set at 25, which is the explosive limit).
When the amount of oxygen increases above this level, the non-oxidizing gas is automatically discharged from the system and new non-oxidizing gas is introduced.

For example, the detector 0 can operate the joint valve (2) in the conduit 59, 71 to discharge gas, and activate the joint valve V in the conduits 33, 39 to introduce new gas.

In this way, safety regarding explosions, fires, etc. within the system can be ensured.

[Brief explanation of drawings]

The drawing shows a flow sheet of the method of the invention. The main members in the figure are as follows. A: Intermediate stock tank, B: Sedimentation tank, C: Dryer, C
1: Cyclone, D: Multi-disc heat treatment machine, E: Product removal tank, El: Tank, F: Filter, G: Condenser, H: Heater, H": Heater, 0: Oxygen concentration detector, Q: Flowmeter, V: Valve, LC1 double level meter, P1~
P3: Pump, P4: Vacuum pump, R1-R2: Rotary valve, f: Fan, - (solid line) indicates the flow of the ferromagnetic metal powder solution, - - (done-dot chain line) indicates the flow of the solvent, -・−(Two-dot chain line) Indicates the flow of water, −・・・−(
(three-dot chain line) indicates the flow of non-oxidizing gas; ----(dashed line) indicates the path for oxygen concentration detection.

Claims (1)

[Claims] 1. When ferromagnetic metal powder produced by the wet reduction method is continuously dried and heat treated in a non-oxidizing atmosphere, the oxygen concentration in the non-oxidizing gas distributed in both of the above steps is detected, and the concentration is determined to be A ferromagnetic system characterized by newly supplying a non-oxidizing gas to replace a part of the non-oxidizing atmosphere in the system when the concentration exceeds a specified concentration close to the upper limit that can avoid excessive oxidation of the magnetic metal powder. Method for producing metal powder.