JPH0232324B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing ferromagnetic iron powder, and more particularly to a method for producing ferromagnetic iron powder that has applications as high-density magnetic recording media such as magnetic tapes, magnetic drums, and magnetic disks. Conventionally, ferromagnetic iron powder has been produced by reducing acicular oxyhydroxides and oxides such as α-FeOOH and α-Fe ₂ O ₃ (hereinafter referred to as acicular goethite, etc.) under heating. There are known ways to do this. However, in the conventional method, breakage, destruction, and even sintering of the raw material, such as needle-shaped goethite, inevitably occurred during reduction under heating. As a method to improve this, ferromagnetic metal powder was produced by immersing an oxide or oxyhydroxide in an aqueous solution containing boric acid or a borate, and then reducing it by heating.
42832 methods are known. In this method, a large amount of the solvent in which boric acid and borate are dissolved must be removed by evaporation, which requires a large amount of energy, and the boric acid and borate dissolved in the solvent must be removed by evaporation. When the solvent is evaporated and removed, it moves with the solvent to the evaporation interface, and a concentration distribution of boric acid and borate inevitably occurs inside the powder, and this concentration distribution is not made uniform and becomes non-uniform. Disadvantages have been recognized, such as having to be heated and reduced at a temperature, and as a result, this also causes a decrease in magnetic properties. The present invention aims to solve these problems, and by using zinc borate as a pre-treatment material for acicular goethite, iron powder useful as high-density magnetic recording media can be produced at low cost. The present invention provides a method for manufacturing. That is, the present invention involves mixing and dispersing acicular goethite with zinc borate and reducing the ferromagnetic iron powder by heating in a reducing atmosphere to obtain ferromagnetic iron powder. and preferably,
The heating and reducing process is (a) 350°C under a non-reducing atmosphere.
This is a method for producing iron powder, which comprises a step of heating to a temperature above .degree. C., and then (b) a step of heating and reducing in a reducing atmosphere. Mixing and dispersing needle-shaped goethite and other zinc borate can be carried out using a mixer such as a miller, a roll mixer, a stirring mixer, or a ball mill. Furthermore, what is useful when mixing and dispersing zinc borate is that the solubility of zinc borate in water is extremely low at room temperature, less than 0.1% by weight.
Water can be used as a dispersion medium. Regarding media other than water, if the medium does not dissolve zinc borate or has extremely low solubility of less than 1% by weight even if zinc borate is dissolved,
It is permitted to use these as dispersion media. This is because the dispersion medium is separated from zinc borate by perfusion or transpiration, but if the solubility is large,
This is because zinc borate migrates to the liquid side together with the dispersion medium during evaporation, and as pointed out in the above-mentioned Japanese Patent Publication No. 54-42832, unevenness occurs in the concentration distribution of zinc borate during transpiration. The amount of zinc borate added is in the range of 0.05/100 to 5/100, preferably 0.2/100 to 2/100, in terms of the atomic weight ratio of boron to iron.
Below the lower limit of this range, the effect is not significant, and above the upper limit, iron is diluted with zinc borate, causing a decline in magnetic properties. The ferromagnetic iron powder of the present invention is mainly composed of iron and contains 90% by weight or more. Needle-shaped goethite etc. mixed and dispersed with zinc borate are heated at 350°C in a non-reducing atmosphere, for example in air.
Above, it is preferably heated to 400 to 500°C for 30 minutes to 4 hours. This heating produces oxyhydroxide (α
-FeOOH) is converted into oxide (α-Fe ₂ O ₃ ),
Zinc borate seems to be compatible with needle-shaped goethite. After this step, the next reduction step is carried out, and the reduction step is a step in which H ₂ gas or a mixed gas containing H ₂ gas is applied to the goethite at 200 to 450°C, preferably 350 to 400°C. be. The supply rate of H ₂ gas to the goethite etc. is 0.1 to 100Nl/gr-Fe/ if expressed in gas hourly space velocity (GHSV).
hr, preferably in the range of 2 to 50 Nl/gr-Fe/hr. If the amount is less than this range, the reaction progresses slowly and is not practical, and if the amount is more than this range, the pressure drop in the reactor increases, so it is not necessarily suitable for reaction operation. Furthermore, if the reaction temperature deviates from the above conditions, the reaction progresses at a low temperature and takes a long time to complete, making it unrealistic. At a high temperature, the reaction speed is too fast, resulting in unnecessary particle damage and destruction. Furthermore, it tends to easily lead to sintering. In the present invention, coexistence of other elements, e.g.
Even if Si, P, Al, Ni, Cr, Mn, Cu, etc. coexist, the effects of the present invention are not hindered in any way. One feature of the present invention is that the reduction temperature can be lowered. According to the conventional method, if the reduction temperature is lowered, sufficient magnetic properties are not developed, but in the present invention, sufficient magnetic properties are observed even at a reduction temperature in the range of 350°C to 400°C. In addition, the form of iron powder obtained by the present invention includes:
According to high-magnification electron microscopy, it almost completely retains the morphology of the raw material, such as goethite.
Phenomena such as particle breakage, destruction, and even sintering between particles are hardly observed. The magnetic properties of this iron powder, for example, in terms of coercive force (Hc), vary depending on the particle size and acicular ratio, but when reduced at 375°C, it shows an extremely high value of Hc = 1340 (Oe). It is understood that it is an excellent product. This material satisfies the performance required as a material for high-density magnetic recording media, and the magnetic properties after making it into a magnetic tape also support this, and it has particularly good dispersibility and is of extremely high practical value. Next, the present invention will be specifically explained with reference to Examples. Example 1 3 gr of zinc borate (as 2ZnO・3B ₂ O ₃・3.5H ₂ O) was added to 100 gr of needle-shaped α-FeOOH, and the mixture was stirred for 30 minutes using a stirring mixer to convert the zinc borate into α-
Dispersed in FeOOH. Then, it was placed in a heating furnace and heated in an air atmosphere at a temperature of 450±5°C for 2 hours, then cooled and taken out. When this was observed using a high-magnification transmission electron microscope, α-Fe ₂ O ₃ with few holes was confirmed (Figure 1). Separately, when α-FeOOH without zinc borate was heat-treated under the same conditions, it was found that α-FeOOH had transformed into α-Fe ₂ O ₃ consisting of large holes (Figure 2). In this way, it was confirmed that zinc borate is already compatible with α-Fe ₂ O ₃ during heat treatment. The above is the heating process,
Next, proceed to the reduction step. Heating process obtained α-Fe ₂ O ₃ 50gr inner diameter 38 with reaction gas preheater
A reactor made of a stainless steel tube of 2 mm was filled, and H ₂ gas was introduced at a feed rate of GHSV=20 Nl−H ₂ /gr−Fe/hr, and reduction was carried out at 375° C. for 7 hours. After reduction, the reactor was purged with N ₂ gas and cooled, and then the reduced iron powder was taken out from the reactor and immersed in toluene.
A portion of this was spread over a vat of boronic acid to evaporate toluene. The magnetic properties of iron powder from which toluene had been evaporated (hereinafter referred to as air-dried iron powder) were measured. Magnetic measurements were carried out using a DC type magnetic hysteresis curve measuring device in a magnetic field with a maximum magnetic field of 3.7 KOe. Coercive force (Hc) = 1340Oe, saturation magnetization (σ _s ) = 171emu/
gr, squareness ratio (σ _r /σ _s ) = 0.60, and it was found that it exhibited very excellent magnetic properties. On the other hand, α-Fe ₂ O ₃ to which no zinc borate was added was reduced to iron under the same reducing conditions. The magnetic properties of the obtained iron powder are Hc = 520Oe, σ _s = 176emu/gr,
It was found that σ _r /σ _s =0.34, and the magnetic properties were extremely deteriorated compared to the present invention. As a result of observation using a high-magnification electron microscope, it was found that the iron powder of the present invention still maintained its acicularity even after being reduced to iron powder, whereas the iron powder of the comparative example was in a sintered state. It was observed that the acicularity was no longer maintained. Example 2 This example is an example in which B/Fe=0.2/100 and the heating conditions were 400° C. for 2 hours. Other conditions are the same as in Example 1. That is, the needle-like α
-Add 1.2g of zinc borate to 100g of FeOOH and stir for 30 minutes with a stirring mixer to α-
Dispersed in FeOOH. This was placed in a heating furnace and heated at 400±5°C for 2 hours in an air atmosphere, and then taken out. Example 1 α-Fe ₂ O ₃ 50gr obtained in the heating process
Reduction was carried out in the same reactor. When the magnetic properties of the obtained iron powder were measured, Hc = 1100 Oe, σ _s =
152 emu/gr, σ _r /σ _s = 0.54, and had sufficient performance as a high-density magnetic recording material. Example 3 This example is an example in which B/Fe=2/100 and the heating condition was 500° C. for 2 hours. Needle-like α-
To 100gr of FeOOH, 11.2gr of zinc borate and 1000gr of low conductivity water were added and mixed for 1 hour using a sieve machine. Then, after suction filtration using No. 5C paper (Toyoro paper product name), put it in a dryer and dry it at 150℃ for 10 hours to evaporate moisture, then put it in a heating furnace and dry it at 500±5 in an air atmosphere. After heating at ℃ for 2 hours, it was taken out. 50gr of α-Fe ₂ O ₃ obtained in the heating process was placed in the same reactor as in Example 1, and H ₂ gas was heated to GHSV=
Introduced at a feed rate of 20Hl−H ₂ /gr−Fe/hr,
Reduction was performed at 400°C for 5 hours. Regarding the magnetic properties of iron powder obtained by air drying according to Example 1, Hc=
1290 Oe, σ _s = 162 emu/gr, σ _r /σ _s = 0.58. The iron powder obtained in this example showed very little deterioration in magnetic properties due to aging. Examples 4 to 8 The magnetic properties described in Table 1 were obtained for iron powders obtained using the raw materials and conditions shown in Table 1. Conditions other than those in Table 1 were the same as in Examples 1 and 3.

【table】

[Table] Example 9 25g of magnetic iron powder obtained in Example 1 was added to a 25% methyl ethyl ketone solution of thermoplastic polyurethane resin.
The mixture was placed in a stainless steel container with 10 gr of methyl ethyl ketone, 38 gr of methyl ethyl ketone, and 0.05 gr of silicone additive, and treated with a paint conditioner using alumina beads as a dispersion medium for 8 hours to obtain a mill base.
To this mill base, 10 gr of a solution of the polyurethane resin in methyl ethyl ketone was added, and methyl ethyl ketone was further added to adjust the viscosity to obtain a magnetic paint. This magnetic paint was applied to a 12 μm thick reinforced polyethylene terephthalate film using a blade coater so that the thickness of the magnetic layer after drying would be approximately 4 μm, the magnetic powder was oriented through a magnetic field, and then dried with hot air. The tape was smoothed by passing it through a calender roll to obtain a tape for evaluation. The magnetic properties of this tape are Hc=1305Oe, Br/
It showed extremely good values of Be=0.83 and degree of orientation 2.5. The high degree of orientation of Br-Bs indicates that the acicularity is maintained until it is reduced to iron, indicating that the magnetic iron powder has excellent dispersibility. When treated with zinc borate, a magnetic powder with excellent dispersibility can be obtained. The degree of orientation was actually improved by 25% compared to treatment with boron oxide, which is another invention of the present inventors. Comparative Example 1 1000g of 0.5wt% boric acid solution was added to 100gr of needle-shaped α-FeOOH, mixed for 1 hour using a sieve machine, and filtered by suction. The amount of the suction-filtered filtrate was 500 g, and when the boric acid in the filtrate was analyzed by ICP, it was found that the concentration was almost the same as that of the added boric acid solution, and it was found that a considerable amount of boric acid was washed away by the filtration. Then put it in the dryer at 150℃.
After drying for 10 hours to evaporate moisture, it was placed in a heating furnace and heated at 450°C for 2 hours in an air atmosphere, then taken out. When this was observed using a high-magnification transmission electron microscope in the same manner as in Example 1, a considerable portion was found to be in a state similar to that shown in FIG. 2 of the attached drawings, and many large holes were observed. In other words, it was confirmed that the deposition of boric acid was not uniform and sufficient over the entire particle. α- obtained in the heating process
50g of Fe ₂ O ₃ was reduced in the same manner as in Example 1 at 375°C for 7 hours. Regarding the magnetic properties of the iron powder obtained by air drying according to Example 1, Hc = 858 Oe,
σ _s =165 emu/g, σ _r /σ _s ] 0.45, making it highly suitable as a magnetic iron powder for magnetic recording. In particular, Hc is 858 Oe, and iron powder for compact cassette audio metal tape is used.
It is a serious problem that the Hc value is far from reaching the standard value of 1100 to 1300 Oe. When the magnetic iron powder was observed with a high magnification transmission electron microscope, it was confirmed that it was also partially sintered. It is presumed that this is because boric acid inevitably migrated onto the particle surface during the drying process, making it substantially non-uniform. A magnetic tape for evaluation was prepared using the obtained magnetic iron in the same manner as in Example 9. The magnetic properties of this tape were Hc = 815 Oe, Br/Bs = 0.68, degree of orientation = 1.22, and the orientation was poor. In other words, due to the inevitable migration of boric acid during drying, the iron powder of this comparative example was partially sintered, so it not only failed in Hc, but also failed when making the tape. It was also found that the results were significantly inferior in terms of orientation properties, which are a very important factor. Comparative Example 2 The concentration of boric acid was 1wt%, and 500 g of a boric acid solution with a higher concentration than Comparative Example 1 and needle-like α-
FeOOH was mixed in a sieve machine. The result was a highly viscous clay-like mixture. The resulting paste was put into a dryer without being filtered, dried at 150° C. for 10 hours to evaporate water, heated in the same manner as in Comparative Example 1, and then reduced. The properties of the magnetic iron powder obtained after air drying are: Hc = 928Oe, σ _s = 155emu/g,
σ _r /σ _s =0.45, making it extremely unsuitable as a magnetic iron powder for magnetic recording. That is,
The obtained iron powder, especially in Hc, is still significantly lower than the standard value for audio metals, and has no industrial applicability. Next, a magnetic tape for evaluation was prepared using the magnetic iron powder according to the method of Example 9. The magnetic properties of this tape are Hc=870Oe, Br/Bs=0.68,
The degree of orientation was 1.22, and the orientation, which is an important factor in making a tape, was also very poor. Although the amount of mixed solvent was reduced, the loss due to filtration was substantially suppressed, the sintering of the particles progressed violently due to the inevitable migration of boric acid during drying, so the magnetic properties were of course unacceptable. It was also found that the orientation properties, which are a very important factor when making a tape, were also significantly inferior. Comparative Example 3 To 100 gr of acicular α-FeOOH, 1000 gr of a 0.5 wt% aqueous solution of zinc nitrate was added, mixed for 1 hour using a sieve machine, and filtered by suction. The filtrate after suction filtration is 500g.
When zinc in the filtrate was analyzed by ICP, it was found to be approximately the same concentration as the added zinc nitrate, and a considerable amount of zinc nitrate was washed away by filtration. Thereafter, it was placed in a dryer and dried at 150° C. for 10 hours to evaporate water, followed by heating, reduction, and air drying in the same manner as in Comparative Example 1 to obtain magnetic iron powder. The magnetic properties of the magnetic iron are: Hc = 870Oe, σ _s = 53emu/g,
σ _r /σ _s =0.43, and it was completely unsuitable as a magnetic iron powder for magnetic recording. Hc is 870Oe
It cannot be used as iron powder for metals. A magnetic tape for evaluation was prepared using the magnetic iron powder and the same method as in Example 9. The magnetic properties of this tape are Hc=825Oe, Br/Bs=0.65,
The degree of orientation was 1.19, and it was found that the orientation was poor. In other words, migration of zinc nitrate during drying causes sintering of the particles, which deteriorates the magnetic properties.
Of course, it failed in terms of Hc, and the orientation properties, which are a very important factor in making tape, were also extremely poor, and it was found that there was no industrial applicability. As shown in Comparative Examples 1 to 3 above, if a readily water-soluble boric acid (compound) such as boric acid is used in the known technology, it will only dissolve in the filtration and escape in vain without being deposited. The more fundamental problem is that regardless of whether filtration is performed or not, boric acid and other substances inevitably migrate to the particle surface during drying, resulting in a significant decrease in the magnetic properties of the product iron powder, especially Hc. It can be seen that the standard value of iron powder for metal tapes of 1100 to 1300 Oe cannot be reached, and that there is no industrial applicability for metal tapes.

[Brief explanation of drawings]

Figure 1 is a transmission electron micrograph (100,000x magnification) of the particle structure of α-FeOOH particles treated with zinc borate after the heating process. Figure 2 is a transmission electron micrograph (100,000x magnification) of the particle structure of untreated α-FeOOH particles after the heating process.
It is.

Claims (1)

[Scope of Claims] 1. When obtaining ferromagnetic iron powder by reducing an iron-based oxyhydroxide or oxide under heating in a reducing atmosphere, the oxyhydroxide or oxide is dry-processed with zinc borate. 1. A method for producing ferromagnetic iron powder, which comprises mixing and dispersing zinc borate, or mixing and dispersing zinc borate using a dispersion medium that does not substantially dissolve zinc borate, and then heating and reducing the mixture. 2 The process of thermal reduction is performed under (a) a non-reducing atmosphere;
The method according to claim 1, comprising the steps of heating to 350° C. or higher, and (b) reducing by heating in a reducing atmosphere.