Abstract:
Improved acicular fine particles of ferromagnetic metal having superior powder characteristics and tape characteristics are provided, which fine particles are prepared by adding to an aqueous suspension of acicular iron oxide or oxyhydroxide, a solution of a different kind and non-alkali metal salt of an organic acid in place of conventional inorganic acids, followed by making the mixture basic and heat reduction.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to acicular fine particles of ferromagnetic metal for magnetic recording and a process for producing the same. 
     2. Description of the Prior Art 
     In recent years, magnetic powder having a high coercive force and a high spontaneous magnetization has been required for high-performance cassette tapes for audio, compact video tapes, etc., and as a product meeting such requirements, fine particles of ferromagnetic metal have been noted which are obtained by subjecting powder composed mainly of iron oxide or oxyhydroxide (which powder will hereinafter be often referred to as &#34;starting raw material&#34;) to heat reduction e.g. in a H 2  stream. In order to control the magnetic characteristics and the stability of oxidation resistance of the iron fine particles, a process has been proposed wherein one or more elements among a group of different elements (mainly metal elements) such as Ni, Co, Al, Si, etc. are adhered onto the starting raw material, followed by heat reduction to prepare fine particles of ferromagnetic metal. The process is concretely a process wherein an aqueous solution of a salt of the above different elements is added to an aqueous suspension of the above starting raw material, followed by changing the pH of the mixture to deposit and adhere the different elements in the form of hydroxide or the like onto the surface of the starting raw material, dehydrating and heat-reducing. In that process, inorganic salts such as chlorides, sulfates, etc. have so far been used as the above salt of the different elements. However, if chlorine ion, sulfuric acid ion or the like present in these inorganic salts remains on the surface of the particles after adhesion, it has a bad effect at the time of heat reduction step and makes the stability of oxidation resistance inferior. Thus, in order to remove them, particles after adhesion have so far been washed with water, but complete removal has been impossible that is, a considerable amount thereof usually remained on the surface of the material. Thus, various characteristics of the resulting fine particles of ferromagnetic metal was limited. The above bed effect which results at the time of heat reduction step refers concretely to sintering and tearing to pieces of the above particles, which will hereinafter be collectively referred to as collapse of particles. Such collapse reduces the uniformity of the particulate form and also produces inferior coercive force (Hc), and squareness (Rs), for powder characteristics, and inferior Hc and Rs for tape characteristics. 
     SUMMARY OF THE INVENTION 
     In view of the drawbacks of the above prior art, the gist of the present invention consists in using a metal salt of an organic acid as the salt of a metal to be adhered. 
     Namely the present invention resides in a process for producing acicular fine particles of ferromagnetic metal by adding a solution of a salt of a metal which is different from iron and exclusive of alkali metals (which metal will hereinafter be referred to as different metal), to an aqueous suspension of acicular iron oxide or iron oxyhydroxide and further adding a basic substance to make the resulting mixture basic and thereby deposit and adhere the hydroxide of the different metal onto the iron oxide or iron oxyhydroxide, followed by heat reduction, which process comprises using a metal salt of an organic acid as the above metal salt, and acicular fine particles of ferromagnetic metal thus obtained. 
     DETAILED DESCRIPTION OF THE INVENTION 
     As the iron oxide or iron oxyhydroxide used as the starting raw material in the present invention, materials composed mainly of other iron oxides. (e.g. α-Fe 2  O 3 ) or oxyhydroxides (e.g. γ-FeOOH) may also be used besides iron α-oxyhydroxide so long as they have acicularity. 
     As the organic acid salt of a different metal usable in the present invention, metal salts of formic acid, acetic acid, lactic acid, stearic acid, oleic acid, naphthenic acid, benzoic acid or the like are illustrated. Preferably metal salts of organic carboxylic acids of 1 to 20 carbon atoms, more preferably those of 1 to 4 carbon atoms and most preferably metal acetate may be used. 
     The different kind metals of these metal salts have no particular limitation, and one or more kinds of metals in a broad range excluding iron and alkali metals may be used. Examples of usable metals are Mg, Al, Cr, Mn, Co, Ni, Cu, Zn, Pd, Ag, Cd, Pb, Ca, Sr, Ba, Ti, Mo, Sn, Bi, Nb, Sm etc. Further, it is possible to coprecipitate iron salts and the different kind metal salts in combination. The reason why alkali metals are excluded is that they dissolve in an aqueous solvent in large quantities and hardly deposit on iron α-oxyhydroxide. 
     As the solvent for the above metal salts of organic acids, alcohols, esters, ketones, ethers or carboxylic acids of 1 to 4 carbon atoms or mixture thereof or mixtures thereof with water may be used besides water. 
     As the base used in the present invention, KOH, NaOH, aqueous NH 3 , NH 3 , gas, etc. are usable. In order to eliminate the effect of their cations remaining on the metal surface, aqueous NH 3  or NH 3  gas among the above bases may be preferable to use. This is because of the fact that ammonium iron is decomposed and separated at the time of heat reduction. 
     When the above bases are added, the pH of the above aqueous suspension of iron α-oxyhydroxide or the like is desirable to be adjusted to 8.5 to 12.0, preferably 9.0 to 11.0. Further, if desired, the temperature of the system is raised to 60° C. or higher, preferably 80° C. or higher. By raising the temperature, it is possible to crystallize the metal hydroxide precipitated in the vicinity of room temperature in a gel-like state and thereby make the adhesion state firmer. 
     The proportion of the weight of the element to be adhered to that of the starting raw material is preferably in the range of 0.5 to 15% by weight, more preferably 1 to 10% by weight, for controlling various characteristics of the aimed particles, and making the saturation magnetization of the particles higher and the adhesion of the metal more uniform. 
     The above heat reduction is usually carried out with H 2  gas in the temperature range of 300° C. to 600° C. 
     According to the present invention, since metal salts of organic acids are used, the radicals of the organic acids are decomposed and separated; harmful anions do not remain on the surface of fine particles of ferromagnetic metal; thus collapse of the particles at the time of heat reduction is few; and hence it is possible to prepare fine particles of ferromagnetic metal having a good uniformity, an improved squareness at the time of making tapes therefrom and an improved stability of oxidation resistance. Further, if a metal salt of acetic acid is used at the time of the adhesion, the dispersibility of the slurry is improved due to acetic acid ions to effect a more uniform adhesion; hence it is possible to obtain fine particles of ferromagnetic metal having more uniform magnetic characteristics. 
    
    
     The present invention will be concretely described by way of Examples. 
     EXAMPLE 1 
     Iron α-oxyhydroxide (water content: 80%)(300 g) was placed in a vessel and water (1.5 l) was added, followed by stirring for 2 hours, dropwise adding acetic acid (2 ml) to the resulting slurry to make its pH 3.0, further stirring, dropwise adding an aqueous solution obtained by dissolving nickel acetate Ni(OCOCH 3 ) 2 . 4H 2  O)(5.36 g) as a metal salt in water (100 ml), further stirring, dropwise adding aqueous NH 3  to adjust the pH of the mixture to 9.5, stirring for 30 minutes, raising the temperature up to 90° C. or higher, keeping the state for one hour, cooling to the room temperature, dropwise adding an aqueous solution of silicic acid (Si:1.0%)(140 g) for imparting heat resistance and sintering resistance to the resulting particles, filtering off and drying the particles and reducing the thus prepared material in H 2  stream at 500° C., to obtain fine particles of ferromagnetic metal. The magnetic characteristics of the magnetic powder are shown in Table 1 and the magnetic characteristics and oxidation resistance at the time of making tapes from the powder are shown in Table 2. 
     EXAMPLE 2 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution used in Example 1 was replaced by a solution obtained by dissolving nickel acetate (13.39 g) in water (250 ml). Various characteristics of the powder are shown in Tables 1 and 2. 
     EXAMPLE 3 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution used in Example 1 was replaced by a solution obtained by dissolving nickel acetate (26.78 g) in water (500 ml). Various characteristics of the powder are shown in Tables 1 and 2. 
     EXAMPLE 4 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution used in Example 1 was replaced by a solution obtained by dissolving nickel acetate (40.17 g) in water (750 ml). Various characteristics of the powder are shown in Tables 1 and 2. 
     EXAMPLE 5 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution used in Example 1 was replaced by a solution obtained by dissolving cobalt acetate (Co(OCOCH 3 ) 2 .4H 2  O)(5.34 g) in water (100 ml). Various characteristics of the powder are shown in Tables 1 and 2. 
     EXAMPLE 6 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution used in Example 1 was replaced by a solution obtained by dissolving copper acetate (Cu(OCOCH 3 ) 2 .H 2  O)(3.97 g) in water (100 ml). Various characteristics of the powder are shown in Tables 1 and 2. 
     EXAMPLE 7 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution used in Example 1 was replaced by solution obtained by dissolving zinc acetate (Zn(OCOCH 3 ) 2 .2H 2  O)(4.25 g) in water (100 ml). Various characteristics of the powder are shown in Tables 1 and 2. 
     EXAMPLE 8 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution used in Example 1 was replaced by a solution obtained by dissolving nickel formate (Ni(OCHO) 2 .2H 2  O)(3.98 g) in water (100 ml). Various characteristics of the powder are shown in Tables 1 and 2. 
     COMPARATIVE EXAMPLE 1 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution used in Example 1 was replaced by a solution obtained by dissolving nickel sulfate (NiSO 4 .6H 2  O)(5.66 g) in water (100 ml). Various characteristics of the powder are shown in Tables 1 and 2. 
     COMPARATIVE EXAMPLE 2 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution used in Example 1 was replaced by a solution obtained by dissolving nickel chloride (NiCl 2 .6H 2  O)(5.12 g) in water (100 ml). Various characteristics are shown in Tables 1 and 2. 
     COMPARATIVE EXAMPLE 3 
     Magnetic powder was obtained in the same manner as in Example 1 except that the metal salt solution was replaced by a solution obtained by dissolving cobalt sulfate (CoSO 4 .7H 2  O)(6.03 g) in water (100 ml). Various characteristics of the powder are shown in Tables 1 and 2. 
     
                       TABLE 1______________________________________Adhered substance, B - H characteristicsits amount adhered*.sup.1              Hc        (wt %)              (Oe)    σs (emu/g)*.sup.2                                 Rs*.sup.3______________________________________Example1      Ni(OCOCH.sub.3).sub.2              2.0     1510  163      0.512      Ni(OCOCH.sub.3).sub.2              4.9     1470  160      0.513      Ni(OCOCH.sub.3).sub.2              9.7     1350  154      0.494      Ni(OCOCH.sub.3).sub.2              14.5    1260  146      0.475      Co(OCOCH.sub.3).sub.2              2.0     1590  167      0.516      Cu(OCOCH.sub.3).sub.2              1.9     1440  153      0.507      Zn(OCOCH.sub.3).sub.2              1.9     1480  156      0.508      Ni(OCHO).sub.2              2.0     1490  160      0.50Compar.ex.1      NiSO.sub.4  2.0     1460  157      0.492      NiCl.sub.2  2.0     1470  158      0.493      CoSO.sub.4  2.0     1530  162      0.49______________________________________ *.sup.1 The &#34;amount adhered&#34;refers to the percentage by weight of a different kind metal component in the metal compound adhered, relative to iron α-oxyhydroxide. *.sup.2 σs: Specific magnetization *.sup.3 Rs: Squareness 
    
     
                                           TABLE 2__________________________________________________________________________Adhered substance,                Oxidationits amount adhered            Tape characteristics                             resistance*.sup.6        (wt %)            Hc(Oe)                 Br(G)*.sup.4                      Rs SFD*.sup.5                             (%)__________________________________________________________________________Example1    Ni(OCOCH.sub.3).sub.2        2.0 1440 2760 0.860                         0.480                             2.32    Ni(OCOCH.sub.3).sub.2        4,9 1390 2630 0.845                         0.515                             1.83    Ni(OCOCH.sub.3).sub.2        9.7 1250 2380 0.805                         0.570                             2.04    Ni(OCOCH.sub.3).sub.2        14.5            1150 2190 0.770                         0.625                             2.05    Co(OCOCH.sub.3).sub.2        2.0 1510 2820 0.850                         0.485                             2.66    Cu(OCOCH.sub.3).sub.2        1.9 1360 2540 0.835                         0.510                             2.07    Zn(OCOCH.sub.3).sub.2        1.9 1400 2580 0.840                         0.490                             2.78    Ni(OCHO).sub.2        2.0 1420 2670 0.850                         0.500                             2.4Compar.ex.1    NiSiO.sub.4        2.0 1360 2630 0.795                         0.560                             4.02    NiCl.sub.2        2.0 1370 2670 0.800                         0.555                             3.83    CoSO.sub.4        2.0 1430 2730 0.790                         0.560                             4.3__________________________________________________________________________ *.sup.4 Br: Remanent induction *.sup.5 SFD: Switching field distribution *.sup.6 The &#34;oxidation resistance&#34;refers to the percentage Br reduction. The measurement conditions of oxidation resistance are as follows: 50° C. RH 90%, one week. 
    
     As apparent from comparison of the data of Example 1 with those of Comparative example 1-3 in Tables 1 and 2, the magnetic powder of the present invention has increased Hc and σs and improved Rs, SFD and oxidation resistance.