JPH0369848B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to lepidocrocite (γ-FeOOH)
More specifically, the present invention relates to a method for producing lepidocrocite, which is suitable as a starting material when producing magnetic powder for magnetic recording media such as audio tapes, video tapes, and magnetic cards. (Prior art and problems to be solved) In general, magnetic iron oxide powder for magnetic recording media such as audio tapes, video tapes, and magnetic cards is
α-FeOOH (goethite) or γ-FeOOH (lepidocrocite) is used as a starting material, and it is sequentially subjected to treatments such as calcination (dehydration, hardening), reduction, and oxidation to form acicular γ-Fe ₂ O. ₃ (maghemite), or by obtaining Co-γ-Fe ₂ O ₃ with cobalt deposited on the particle surface by cobalt modification treatment. Further, metal powder can be obtained by further proceeding with the reduction without stopping the reduction at the magnetite (Fe ₃ O ₄ ) stage in the above-mentioned process. The magnetic properties of these magnetic iron oxide powders and metal powders depend on the properties of the above-mentioned starting materials, so in order to obtain magnetic iron oxide powders and metal powders suitable for magnetic recording media, starting materials with excellent properties must be used. There is a need to. This point is common between magnetic iron oxide powder and metal powder, and an example of magnetic iron oxide powder will be described below. Conventionally, magnetic iron oxide powder obtained using lepidocrocite (γ-FeOOH) as a starting material is more effective than the magnetic iron oxide powder obtained using goethite (α-FeOOH) as a starting material for end products such as audio tapes and video tapes. Although the recording material has excellent magnetic orientation, dispersibility, squareness ratio, and transfer characteristics, there is a problem that it is difficult to obtain microparticles with a large particle size distribution and excellent acicularity. The drawback was that it had a negative effect on the magnetic properties (coercive force, reversal magnetic field strength distribution, etc.). Conventionally, the method for synthesizing this repid crosite has already been published in Japanese Patent Publication No. 33-6734, Japanese Patent Publication No. 43-2214,
Although disclosed in Japanese Patent Application Laid-Open No. 47-40097, etc., usually
Lepidocrocite is produced by the following synthetic reaction. First, an aqueous solution of ferrous chloride (FeCl ₂ ) obtained using pickling waste liquid or the like is neutralized by adding an aqueous alkali solution such as caustic alkali or ammonia. At this time, the molar ratio of hydroxyl groups to iron (OH/Fe) is approximately 0.8 to
Mix the alkaline aqueous solution so that the ratio is 1.4. This is then converted into an oxygen-containing gas (air or oxygen)
oxidizes to form seed crystals. The reaction temperature at this time is approximately 10 to 25°C, and as the reaction approaches completion, the PH value of the reaction solution decreases, and when the alkaline content is completely consumed and the reaction ends, the PH value decreases to approximately 3.2 to 3.8. Go down. This reaction is called a seed crystal reaction. After that, the temperature is raised to 30-50℃ in an oxygen-containing atmosphere, oxygen-containing gas is blown in so that the pH value becomes 3.5-4.5, and an alkaline aqueous solution is added to remove the seed crystals generated in the above seed crystal reaction. Make it grow. This reaction is called a growth reaction. As this growth reaction approaches completion, the reaction rate slows down and the pH value increases. The reaction is terminated when the pH value reaches approximately 5.5, and needle-like lepidocrocites are obtained. However, in the above method, as mentioned above, the average particle size is small (the BET specific surface area, which is used as a measure of the average particle size, is 60 m ² /g or more),
Currently, goethite is frequently used because lepidocrocite, which has a narrow particle size distribution width, cannot be obtained and is unsuitable as a starting material for magnetic iron oxide powder for high-density recording. Among these problems, miniaturization of particle size is an essential issue in order to improve the S/N ratio and C/N ratio of VTRs and audio tapes. Therefore, the present applicant has been conducting research on lepidocrocite, which has many advantages as described above, in order to eliminate the disadvantages of the above-mentioned prior art. As a result, regarding the improvement of particle size distribution,
60-201377, patent application 1986-202267, patent application 1987-
203136, Japanese Patent Application No. 60-203137, and Japanese Patent Application No. 60-203922, regarding fine particle size and improvement of particle size distribution,
This was proposed in patent applications No. 61-7309 and No. 61-7310. Based on the above proposal, the present invention aims to provide a method for producing lepidocrocite which has excellent acicularity and particle size distribution and has a small average particle size suitable for high-density recording. . (Means for Solving the Problems) In order to achieve the above object, the present inventor has conducted extensive experimental research on conditions under which fine particles with a small particle size distribution can be obtained regarding various parameters in the seed crystal reaction and growth reaction. As a result, this is possible by appropriately controlling the PH value before starting the heating after the seed crystal reaction is completed, controlling the atmosphere during heating, and slowly controlling the addition rate of the alkaline aqueous solution during the growth reaction. find out,
This invention has been made. That is, in short, the present invention, when producing lepidocrocite by the above-mentioned seed crystal reaction and growth reaction, continues to blow oxygen-containing gas until the PH value after the seed crystal reaction is 3.5 or less, and the PH value is increased. When the blown gas is changed from an oxygen-containing gas to an inert gas and the temperature is raised after the . is 0.7
The feature is that the speed is controlled to be slow below PH/hr. The present invention will be explained in detail below based on examples. The following reaction can be used to synthesize γ-FeOOH by oxidizing FeCl ₂ . FeCl ₂ +2NaOH→Fe(OH) ₂ +2NaCl … 4Fe(OH) ₂ +O ₂ ―→ k ₁ 4γ−FeOOH＋2H ₂ O … 4FeCl ₂ ＋6H ₂ O＋O ₂ ―→ k ₂ 4γ−FeOOH＋8HCl … Via Fe(OH) ₂ When comparing the reaction with the reaction of directly oxidizing FeCl ₂ , the reaction rate is overwhelmingly faster. In other words, the reaction rate constants k ₁ and k ₂ are k ₁ 》
_k2 . In order to synthesize γ-FeOOH with a good particle size distribution, it is important to choose a fast seed crystal reaction and a slow growth reaction. In the present invention, a fast seed crystal reaction is selected to synthesize seed crystals of the same size as quickly as possible, a slow growth reaction is selected, and the number of seed crystals produced by the seed crystal reaction is not increased. It grows slowly (without creating new particles).
During the growth reaction, NaOH is added to neutralize HCl generated by the reaction, but if the addition rate of NaOH is too fast, the reaction occurs preferentially over the reaction, resulting in the formation of γ-FeOOH. becomes faster, new particles are generated, and the particle size distribution deteriorates. Therefore, the initial addition rate of NaOH in the growth reaction has a significant effect on the particle size distribution. Conventionally, the end of a seed crystal reaction was determined to be complete when the alkaline content was completely consumed and the reaction ended when the PH value of the reaction solution decreased to approximately 3.2 to 3.8 at the end of the reaction. Therefore, the temperature may start to increase when the pH value is around 3.8, and in this case, the particles start to grow rapidly, which results in fine particles with a small particle size distribution. Experiments have revealed that this is one of the reasons why this is not the case. Therefore, in the present invention, the oxygen-containing gas is continuously blown in until the PH value of the reaction liquid becomes reliably 3.5 or less at the end of the seed crystal reaction, and when the PH value reaches a predetermined value of 3.5 or less, the oxygen-containing gas is Stop blowing, then switch to an inert gas such as nitrogen and lower the liquid temperature to 35~35°C.
They decided to raise the temperature to 50℃. Here, the inert gas atmosphere during the temperature rise from the seed crystal reaction to the formation reaction is also very important. When the temperature is increased in an oxygen-containing atmosphere, the oxidation reaction of _FeCl2 progresses,
Be careful as this will worsen the particle size distribution. After the temperature is raised, the inert gas is stopped, oxygen-containing gas is blown in again, and an alkaline aqueous solution such as caustic soda or ammonia is added to perform a growth reaction. However, when adding an alkaline aqueous solution, conventionally it was added at a roughly constant rate (e.g. 80 g/min of 1.6 mol/alkaline aqueous solution);
In particular, in the present invention, in view of the fact that the pH value rises rapidly at the beginning of addition, which causes coarse growth of particles,
At the initial stage of addition, a slow growth reaction is caused around the seed crystal, and for this purpose, it is necessary to control the rate of increase in the pH value of the reaction solution to a slow rate of 0.7 PH/hr or less. Note that such control of the rate of increase in PH value is preferably carried out for about 60 minutes after addition of the alkaline aqueous solution. In this way, in conjunction with proper control of the PH value before the temperature rise, fine particles with the desired small particle size distribution can be obtained. In the present invention, manufacturing conditions other than those described above are the same as those of the conventional method, and these conditions are not particularly restricted. However, the theoretical amount of alkali required to convert ferrous chloride to ferrous hydroxide is added separately in the seed crystal reaction process and the growth process, but the amount added in the seed crystal reaction process is based on the amount of goethite. In order to prevent generation and obtain lepidocrocite particles with excellent acicular properties, the amount needs to be 0.4 to 0.7 times the theoretical amount. Of course, the present invention can be applied to various conventional synthetic reaction methods. For example, as a method previously proposed and disclosed by the present applicant, aging is carried out by holding at a pH value of 6.0 to 9.0 and a temperature of 10 to 50°C for 20 to 120 minutes before blowing oxygen-containing gas in the seed crystal reaction (patent application). (Sho 60-201377), PH after seed crystal reaction
A method of aging in which the value is 3.0 to 5.0 and the temperature is 10 to 50°C for 10 to 120 minutes (Japanese Patent Application No. 60-202267), after the growth reaction, the PH value of the reaction solution is 3.0 to 5.0. and a method of aging at a temperature of 10 to 50°C with stirring for more than 1 hour (patent application 1983-203137)
No.), the rate at which the alkaline aqueous solution is added to the initial ferrous chloride aqueous solution per ferrous chloride aqueous solution
0.12 minutes or less (Patent Application No. 60-203136),
A method for appropriately controlling the supply amount of oxidizing gas in three stages according to the oxidation reaction process (Patent Application No. 1983-203922),
Control the concentration of Si relative to the total solution in the mixed solution of ferrous chloride aqueous solution and alkaline aqueous solution in the range of 5 to 30 ppm, and adjust the Si concentration in the range of 5 to 30 ppm as the alkaline aqueous solution added in the growth reaction. (Patent application 1986-7309)
No.), a method for appropriately adjusting the average absorption rate of oxygen in the three stages of the first and second half of the seed crystal reaction and the growth reaction when supplying an oxygen-containing gas (Patent Application No. 7310/1982);
It can be applied to etc. When implementing the present invention, there are some points to keep in mind: When stirring the reaction solution during the seed crystal reaction, temperature raising, and growth reaction, the rotation speed of the stirring blade varies depending on the size of the reaction tank. , about 200 to 400 rpm is desirable. (Example) Next, an example of the present invention will be shown. Example While stirring a ferrous chloride aqueous solution 25 with a concentration of 0.97 mol/in a reactor maintained in a nitrogen gas atmosphere,
Sodium hydroxide aqueous solution with a concentration of 0.71 mol/42
was added, and while maintaining the liquid temperature at 15°C, the nitrogen gas was stopped and air was blown at 20/min for oxidation to perform a seed crystal reaction. When this seed crystal reaction was completed and the pH value fell to 3.2, the air supply was stopped, nitrogen gas was switched to the reactor at a rate of 5/min, and the temperature of the reaction solution was raised to 40°C. Thereafter, the nitrogen gas was stopped and air was blown at a rate of 3/min, and a sodium hydroxide aqueous solution with a concentration of 1.6 mol/min was added to start the growth reaction.
When adding this aqueous sodium hydroxide solution, the rate of increase in the pH value of the reaction solution for 60 minutes after the start of addition was 0.7PH/hr.
The reaction was completed while being controlled so as not to exceed. Comparative Example 1 When the PH value at the end of the seed crystal reaction was 3.8, 20/
A synthesis reaction was carried out under the same conditions as in the above example, except that air blowing of 1 min was stopped and nitrogen gas was changed to 5 min of nitrogen gas, and the temperature was raised from 15°C to 40°C. Comparative Example 2 The addition rate of the sodium hydroxide aqueous solution was increased for 30 minutes after the start of the growth reaction, and the pH value was increased from 3.2 to 3.7 in 30 minutes.
Under the same conditions as the above example except that the temperature was increased to
A synthetic reaction was carried out. In this case, the rising speed is 1PH/
It is hr. Comparative Example 3 In the example, the seed crystal reaction was completed and the PH value was
When the temperature dropped to 3.2, the temperature was raised to 40°C while blowing air at a rate of 3/min without switching the blowing air to nitrogen gas. Thereafter, the reaction was carried out in exactly the same manner as in the example, and γ-FeOOH was synthesized. The BET method specific surface area of each of the lepidocrocites obtained in Examples and Comparative Examples 1, 2, and 3 above was examined, and these lepidocrocites were calcined, reduced, and oxidized in a conventional manner to obtain γ. −
Fe ₂ O ₃ was produced, its specific surface area and coercive force were measured, and the γ-
The particle size distribution of Fe ₂ O ₃ was measured. Furthermore, the above-mentioned acicular γ-Fe ₂ O ₃ (maghemite)
A magnetic tape was prepared using the usual method of pulverizing the material, making it into a paint, coating it on a plastic base film, and orienting it in a magnetic field, and measured its magnetic properties. The above results are shown in Tables 1 and 2 as well as in FIGS. 1 and 2. As is clear from the same table, the specific surface area of the lepidocrocite obtained in the example of the present invention is larger than that of each comparative example. In other words, the particle size is small, and γ-Fe ₂ O ₃ using this as a starting material has a small particle size and excellent magnetic properties, as well as high sound quality and high image quality with particularly low S/N ratio and C/N ratio. audio and VTR tapes. In Comparative Example 3, although the specific surface area of lepidocrocite is large (the particle size is small), as shown in Table 2 and FIGS. 1 and 2, γ-Fe ₂ O Due to the poor particle size distribution of ₃ , γ−Fe ₂ O ₃
The specific surface area of the magnetic tape is small and the coercive force is poor, resulting in poor magnetic tape characteristics.

【table】

[Table] (Effects of the Invention) As detailed above, according to the present invention, when producing lepidocrocite, the PH value is properly controlled before the temperature increase performed after the seed crystal reaction is completed, and the temperature is increased. By controlling the atmosphere during the growth reaction and slowly controlling the addition rate of the alkaline aqueous solution during the growth reaction, we can produce repid crocite with excellent acicularity and particle size distribution and a small average particle size suitable for high-density recording. can be manufactured. Therefore, maghemite (γ-Fe ₂ O ₃ ) obtained using this as a starting material also has small particle size and excellent magnetic properties.
Furthermore, it becomes possible to manufacture high quality magnetic recording media.

[Brief explanation of drawings]

1 and 2 are diagrams showing the particle size distribution of γ-Fe ₂ O ₃ , FIG. 1 shows the case of Example, and FIG. 2 shows the case of Comparative Example 3.

Claims (1)

[Claims] 1. Add an alkaline aqueous solution such as caustic alkali or ammonia to a ferrous chloride aqueous solution by 0.4 to 0.7 times the theoretical amount required to convert the ferrous chloride into ferrous hydroxide, and then blow in an oxygen-containing gas. A seed crystal reaction is performed by generating seed crystals of lepidocrocite, then the oxygen-containing gas injection is switched to an inert gas and the temperature is raised, and then an oxygen-containing gas is blown in place of the inert gas again. In the method of performing a growth reaction in which the seed crystal is grown by adding the alkaline aqueous solution, the oxygen-containing gas is continued to be blown until the PH value after the seed crystal reaction is 3.5 or less, and the PH value reaches a predetermined value. After that, the temperature is increased by switching the blown gas, and when the aqueous alkaline solution is added in the growth reaction, the rate of increase in the PH value of the reaction solution at the initial stage of addition is a slow rate of 0.7 PH/hr or less. 1. A method for producing lepidocrocite, which is characterized in that it is controlled as follows.