JPH0329011B2 - - 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 iron oxide 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. It is produced by obtaining Co-γ-F 2 O ₃ (maghemite) or Co-γ-F ₂ O ₃ with cobalt deposited on the particle surface by cobalt modification treatment. In this case, the magnetic properties of the obtained magnetic iron oxide powder depend on the properties of the above-mentioned starting material, so in order to obtain magnetic iron oxide powder suitable for magnetic recording media, it is necessary to use a starting material with excellent properties. be. In this respect, conventionally, lepidocrocite (γ-
The magnetic iron oxide powder obtained using Goethite (α-FeOOH) as a starting material has a higher magnetic orientation than that of goethite (α-FeOOH) in magnetic recording media such as audio tapes and video tapes. Although it has excellent dispersibility, squareness ratio, and transfer characteristics, it has the problem of large particle size distribution, which has the disadvantage of adversely affecting the properties of the final product (coercive force, switching field strength distribution, etc.). . By the way, among the above starting materials, lepidocrocite is usually produced by the following synthetic reaction method. 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 ammonium. 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)
It is oxidized and a seed crystal is created by a seed reaction. The reaction temperature at this time is approximately 10 to 25°C. As the seed reaction approaches completion, the pH of the reaction solution decreases, and when the alkaline content is completely consumed and the reaction ends, the pH value will be approximately 3.2 to 3.8. After that, the temperature is raised to 30-50℃, and the PH value is about 3.5-4.5
A growth reaction is performed by blowing oxygen-containing gas and adding an alkaline aqueous solution so that the growth reaction occurs. 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, as mentioned above, the above method only yields repitdocrosite with a large particle size distribution, which cannot be said to have good properties, and furthermore, it is difficult to control the average particle size, so it is not suitable for the purpose of use. However, there was a drawback in that it was not possible to provide lepidocrocite having an arbitrary average particle size. For this reason, goethite (α-FeOOH) is currently frequently used as a starting material for producing magnetic iron powder. The present invention solves the drawbacks of the prior art, and produces a repito-crocite with excellent properties that have a small particle size distribution and stably have various required particle sizes. The purpose of the present invention is to provide a method for producing docrosite. (Means for Solving the Problems) In order to achieve the above object, in the present invention, when a ferrous chloride aqueous solution and an alkaline aqueous solution are mixed, green rust, which is a precursor of lepidocrocite seed crystals, is produced. We found that controlling the uniformity of this crystal grain size would bring about good results in the generation and growth of lepidocrocite seed crystals in the subsequent process. The main idea is to provide a ripening process to maintain the quality. The present invention will be explained in detail below based on examples. The ripening step in the present invention is carried out between the mixing step of adding an alkaline aqueous solution to the ferrous chloride aqueous solution and the seeding reaction step of oxidizing with an oxygen-containing gas to generate seed crystals of lepidocrocite through a seed reaction. The mixed aqueous solution is maintained at a pH value of 6.0 to 9.0 and a temperature of 10 to 50°C for 20 to 120 minutes.
Of course, since this is not a seed reaction, it goes without saying that it is carried out in a non-oxidizing atmosphere such as an inert gas atmosphere, and is carried out with stirring. The mixing and stirring state may be such that, for example, the rotation speed of the stirring blade is 200 to 400 rpm. Note that if the holding time is less than 20 minutes, there will be no effect of uniformizing the crystal grain size of Green Rust; however,
If the temperature is maintained for more than 120 minutes, the above effects cannot be expected any longer and the processing time will only be prolonged, so the period should be within the range of 20 to 120 minutes, preferably.
Hold for more than 30 minutes and less than 100 minutes. Furthermore, if the PH value and temperature of the mixed aqueous solution are outside the rising range, goethite may be generated or the acicularity of the particles may deteriorate, so it is necessary to carry out the process at a relatively low temperature and with a weak acid or weak alkali within the above range. In addition, by appropriately setting the above PH value, temperature, and holding time, it is possible to control the crystal grain size of green rust, and therefore, the solution concentration of ferrous chloride, the starting material, etc. In combination with the selection of conditions, it is possible to control the average particle size of the lepidocrocite obtained. The steps other than the aging step in the present invention are the same as conventional ones, and the conditions are not particularly limited.
However, the theoretical amount of alkali required to convert ferrous chloride to ferrous hydroxide is added separately in the seed reaction step and the growth step, but the amount added in the seed reaction step is determined to prevent the generation of goethite. In order to prevent this and obtain lepidocrocite particles with excellent acicular properties, the amount needs to be 0.4 to 0.7 times the above theoretical amount. (Example) Example 1 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 the mixture was maintained at a pH value of 7.6 and a temperature of 13°C for 60 minutes with stirring to age. After this ripening, air at 5 atm (gauge pressure) was blown at a rate of 20/min to oxidize and perform a seed reaction. During this time, the liquid temperature was controlled at 13°C. After confirming the completion of the seed reaction by measuring the PH value (PH = 3.2), the air blowing was stopped and nitrogen gas of 1.5 atm (gauge pressure) was blown at a rate of 3/min to inert the inside of the reactor. Create an atmosphere and stir while stirring 48
The temperature was raised to ℃. Then, instead of nitrogen gas, air at 5 atm (gauge pressure) was blown in at a rate of 3/min, and a sodium hydroxide aqueous solution with a concentration of 1.6 mol/min was added to the
The growth reaction was carried out by adding at a constant rate of g/min. This growth reaction ended when the PH value rose to 5.5. Example 2 While stirring a ferrous chloride aqueous solution 25 with a concentration of 1.0 mol/25 in a reactor maintained in a nitrogen gas atmosphere,
An ammonia aqueous solution 41 having a concentration of 0.8 mol/ml was added, and the mixture was maintained at a pH value of 7.5 and a temperature of 13° C. for 60 minutes with stirring to ripen. After this ripening, air at 5 atm (gauge pressure) was blown at a rate of 20/min to oxidize and perform a seed reaction. During this time, the liquid temperature was 13℃
was controlled. After confirming the completion of the seed reaction by measuring the PH value (PH = 3.4), the air blowing was stopped and nitrogen gas of 1.5 atm (gauge pressure) was blown at a rate of 3/min to inert the inside of the reactor. 45 while stirring and adjusting the atmosphere.
The temperature was raised to ℃. Then, instead of nitrogen gas, air at 5 atm (gauge pressure) was blown in at a rate of 3/min, and a sodium hydroxide aqueous solution with a concentration of 1.6 mol/min was added to the
The growth reaction was carried out by adding at a constant rate of g/min. This growth reaction ended when the PH value rose to 5.5. Comparative Example 1 In Example 1, the mixed aqueous solution of ferrous chloride and sodium hydroxide was immediately subjected to a seed reaction step without being aged, and thereafter the same treatment was performed. Comparative Example 2 In Example 2, the mixed aqueous solution of ferrous chloride and ammonia was immediately subjected to a seed reaction step without being aged, and thereafter the same treatment was performed. For each of the lepidocrocites obtained in the above four examples, the particle size of the particles was measured using an electron microscope, and the particle size distribution was determined. The results are shown in Table 1. The particle size distribution was expressed by the standard deviation value of the particle size distribution in the long axis direction of the acicular lepidocrocite particles, and the average particle size was expressed by the average value of the particle size distribution.

[Table] As is clear from the table, the particle size distribution of the obtained lepidocrocite is much smaller and more uniform in the case of the present invention example, which has a ripening process, than in the comparative example. This indicates excellent properties. In addition, the effect of this aging can be expected to be the same regardless of the type of alkali (sodium hydroxide or ammonia), and even if the average particle size is small (Example 1) or large (Example 2), A distribution is obtained. (Effects of the Invention) As described in detail above, according to the present invention, in the synthetic reaction of lepidocrocite, an aging process under specific conditions is performed before the seed reaction, so that the grain size can be set to any desired average particle size. Acicular lepidocrocite with excellent properties and extremely small distribution can be stably produced, and therefore magnetic iron oxide powder with excellent properties can be produced.

Claims (1)

[Claims] 1. After adding an alkaline aqueous solution such as caustic alkali or ammonia to a ferrous chloride aqueous solution, 0.4 to 0.7 times the theoretical amount required to convert the ferrous chloride to ferrous hydroxide, the pH value is 6.0 to 9.0. Aging is maintained at a temperature of 10 to 50°C for 20 to 120 minutes, then an oxygen-containing gas is blown in to generate γ-FeOOH seed crystals, and the above alkaline aqueous solution is added and an oxygen-containing gas is blown in. A method for producing lepidocrocite, which comprises completing the γ-FeOOH production reaction.