JPS6114202B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing magnetic iron powder for use in magnetic recording materials, which has excellent dispersibility, orientation, and magnetic properties. Magnetic iron powder is attracting attention as a magnetic material for high-density recording, but in order to obtain acicular magnetic iron powder with excellent dispersibility and orientation, it is necessary to add a heat-resistant component to acicular goethite as a starting material. must be uniformly deposited in the required amount. This requirement for uniformity is much stricter than in the case of producing magnetic iron oxide powder. In other words, with magnetic iron oxide powder, the reduction is limited to magnetite (Fe ₃ O ₄ ), whereas with magnetic iron powder, the reduction must be reduced to almost iron, and the sintering between the acicular particles that progresses during the reduction process is intense. The quality of the final product, the magnetic iron powder, is greatly influenced by the amount of heat-resistant components added to goethite to prevent this and the uniformity of the adhesion.
In addition, the goethite referred to in the present invention is acicular hydrated ferric oxide (α-FeOOH), which can be reacted with iron salt alone or with the addition of salts of other metals such as chromium, nickel, cobalt, zinc, etc. Refers to what is obtained by precipitation. Although the process of coating the heat-resistant component on the starting material goethite is important, it must be an inexpensive method from an industrial standpoint. Therefore, using inexpensive water glass as a heat-resistant component,
Studies have been widely conducted on depositing SiO ₂ on the surface of goethite. Incidentally, it is known to use a dispersant such as a metaphosphate to ensure uniform adhesion, and this does not hinder the practice of the present invention. Dilute aqueous solutions of water glass are also used as dispersants in the alkaline region. Therefore, if water glass is used as a raw material for SiO ₂ , goethite can be uniformly dispersed in water, and therefore it is also possible to measure the uniformity of the deposition of SiO ₂ on the goethite surface. Goethite can be obtained by adding water glass to an aqueous suspension of goethite, stirring it, neutralizing the suspension, filtering it, and washing it with water to obtain SiO ₂ -adhered goethite. Because it aggregates, it is easier to wash with water.The lower the pH, the less sodium remains in goethite after washing with water. However, even after adjusting the pH to 3 to 5, washing with water, and drying, goethite still contains 400 to 1000 ppm of sodium.
There remains a problem with the extent of this. The remaining sodium must be removed as much as possible because it serves as a heat-resistant component and reduces the heat resistance of the deposited SiO ₂ . However, even if water washing is repeated under the above conditions, more than 400 ppm of sodium remains. Goethite with a heat-resistant component coated on the surface is 250~
Dehydrated at 400℃ to form hematite (α-Fe ₂ O ₃ ),
Furthermore, the hematite particles are baked at a high temperature. The firing temperature in this case is usually in the range of 350 to 850°C, particularly in the range of 550 to 700°C. Hematite calcined at high temperature is reduced in H ₂ to form magnetic iron powder. The reduction temperature is usually in the range of 300 to 500°C. In this case, if the game site
1.2 parts of SiO ₂ was deposited on 100 parts, and at this time 40 ppm
Let us consider the influence of this sodium on the heat resistance of SiO ₂ assuming that some sodium remains. The phase diagram of SiO ₂ −Na ₂ O is based on known literature (Journal
of American Chemistry, 61, 28 69
(1939)), but in order to read this phase diagram, first calculate the molar ratio defined by the following formula. SiO ₂ /SiO ₂ +Na ₂ O This is, in short, the composition of the SiO ₂ layer deposited on the surface of goethite in which Na remains, expressed in terms of molar ratio. In the above settings, if 100 parts by weight of goethite is used as the standard for calculation, Goethite 100 parts by weight SiO ₂ ^1.2 parts by weight Na ions .2 + X) = 400 Solving this to find X, it becomes X = 4.05 × 10 ^-2 parts by weight. When Na is converted to Na ₂ O, Na ₂ O = 4.05 x 10 ^-2 x 62/(2 x 23) = 5.46 x 10 ^-2 parts by weight. Therefore, on a molar ratio basis, SiO ₂ /0SiO ₂ +Na ₂ O = 1.2/60/1.2/60+5.46×10 ⁻²
/62 = 0.957. Now, if we read the phase diagram described in the above literature using this value, at about 780°C, the molten phase (Na ₂ O・
It can be seen that the phase separates into a solid phase (SiO ₂ ) and a solid phase (SiO ₂ ). Moreover, at this value of composition ratio, the molten phase
_Na2O.3SiO2 accounts for 12.6 parts by weight of _the total amount of _SiO2 - _Na2O . Therefore, when the adhering powder with this composition is heated to 780°C, the adhering phase separates into a molten phase (Na ₂ O.3SiO ₂ ) and a solid phase (SiO ₂ ), and this liquid phase becomes SiO ₂ It is easily understood that the particles of the starting material to which they are coated are melted or fused together. In addition, it is generally known that sintering occurs at a much lower temperature than fusion, and the residual sodium mixed in SiO 2 causes fusion of the SiO ₂ phase, not only at high temperatures of 780°C or higher, but also at temperatures _below this. It is easy to infer that this will occur. Naturally, magnetic iron powder requires that each of the acicular particles that are its constituent components are not fused together, but as mentioned above, the reduction obtained by heating and reducing the fused particles It can be seen that many of the particles of magnetic iron powder are fused to each other and have extremely poor dispersibility. In other words, even with heat treatment at this temperature, if only 400 ppm of sodium remains, the negative effect on sintering, especially the deterioration of the dispersibility of magnetic iron powder due to fusion between particles, cannot be ignored. is clear. As a result of various studies on the heat-resistant treatment of goethite using water glass, the present inventor has found that with simple cleaning, the residual sodium concentration in goethite adhered to water glass can be significantly reduced, and SiO ₂ can be uniformly treated. The present invention, which is a technique for depositing with good quantitative properties, has been completed.
That is, the present invention relates to a method for producing magnetic iron powder in which an aqueous suspension of goethite coated with water glass is separately heated and reduced, which is characterized in that calcium ions are present in the aqueous suspension. This is a method of manufacturing powder. As for the water glass, all of No. 1 to No. 4 water glass are used. The amount of water glass used is determined appropriately taking into account the size of the raw goethite particles and the goethite composition, but it seems that the lowest amount of 0.2 wt% is used in terms of Si. After adding water glass to the goethite suspension, it is preferable for the practice of the present invention to adjust the pH to 6 or higher. The slurry containing water glass is in the alkaline range, so it can be used as is, but it can be adjusted to pH by adding either hydrochloric acid, nitric acid, or sulfuric acid.
may be adjusted to about 6 to 8. The acid used for pH adjustment is preferably one that does not react with calcium ions coexisting in the present invention to form a hardly soluble salt. When the pH is lower than 6, the amount of acid used increases, and the solution must be neutralized with an alkali when disposed of after the deposition process. In order to coexist with calcium ions, calcium nitrate, calcium chloride, calcium hydroxide, etc. may be selected. Addition of calcium salts is industrially advantageous because Na ions in the water glass adsorbed on goethite are replaced by Ca ions, reducing the amount of residual Na and coagulating the slurry to facilitate filtering and washing. For example, the replacement of Na ions and Ca ions in water glass is described in the book by HTS, Britton.
Hydrogen Ions 3rd ed.Vol.2, pl06 (1942)
Shown at Chapman.Hall Ltd, London. Additionally, the addition of Ca salt has the advantage of coagulating a strongly dispersed suspension to facilitate washing with water. In the conventional technology without Ca, goethite water glass suspension coagulates and is easy to filter in the pH range of approximately 5 to 6.5, but in other regions it is strongly dispersed and becomes difficult to filter. I get used to it.
When calcium ions are made to coexist in an aqueous suspension by adding Ca salts in this way, Na ions in water glass are replaced with Ca, which reduces the negative effects of residual sodium on goethite. It coagulates the liquid, making washing and filtering extremely easy. Furthermore, since aggregation occurs even at pH 6 or higher, it is advantageous in terms of waste liquid treatment. As understood from the comparison between Examples 1 and 3 and Example 5, the aqueous suspension of goethite to which calcium ions are added has _a pH of 6 to 8. It also has the effect of keeping the amount of wear high. Example 1 Goethite synthesized in the alkaline region was washed with water to prepare a suspension consisting of 100 g of goethite and 2 parts of water. The pH of the suspension was 9.2. Next, an aqueous solution of No. 3 water glass containing 1.62 g of SiO ₂ was added to the suspension and stirred for 30 minutes. The amount added corresponds to 0.76 parts of Si per 100 parts of goethite.
The pH and viscosity were 10.8 and 10 cp, respectively. Next, nitric acid was added to adjust the pH to 7, and the mixture was stirred for 30 minutes. The viscosity was 8 cp. Thereafter, an aqueous solution containing 0.31 g of calcium chloride was added and stirred for 30 minutes. Addition amount is game site
Equivalent to 0.11 parts of Ca per 100 parts. The pH and viscosity were 6.8 and 330 cp. The total volume of the suspension at this time was 2.5. Agglomeration of the slurry was observed by adding Ca salt. Filter the agglomerated slurry and divide the resulting cake into 2
The cake was redispersed in water, stirred for 30 minutes, and filtered. The cake was dried and analyzed for composition of Si, Ca, and Na.
The slurry was filtered using 5C paper and a Buchner funnel with a diameter of 20 cm under reduced pressure using an aspirator.The filtering was completed in a very short time of 10 minutes each time, and the goethite was removed from the paper in the early stage of the filtering. There was no leakage into the liquid. The analytical values of goethite after washing with water were as shown in Table 1.

[Table] In other words, it can be seen that the amount of sodium remaining in goethite after washing with water is extremely small at 64 ppm. The goethite used has a long axis of 0.4μ and a short axis.
They were acicular particles with a diameter of 0.03μ and a specific surface area of 33m ² /g. Example 2 Goethite containing 0.5 parts by weight of Cr for 100 parts by weight of Fe synthesized in the alkaline region was washed with water,
A suspension consisting of 100 g of goethite and 2 parts of water was prepared. The pH of the suspension was 8.9. Next, an aqueous solution of No. 4 water glass containing 0.857 g of SiO ₂ was added to the suspension and stirred for 30 minutes. The amount added corresponds to 0.4 parts of Si per 100 parts of goethite. The pH and viscosity were 10.7 and 20 cp, respectively. Next, hydrochloric acid was added to adjust the pH to 6, and the mixture was stirred for 30 minutes. The viscosity was 225 cp and the goethite was aggregated. Thereafter, an aqueous solution containing 0.5 g of calcium nitrate was added and stirred for 3 minutes. The total volume of the suspension at this time was 2.5. The amount added corresponds to 0.2 parts of Ca per 100 parts of goethite. The pH and viscosity were 5.6 and 420 cp. Filter the agglomerated slurry and divide the resulting cake into 2
The cake was redispersed in water, stirred for 30 minutes, and filtered. The cake was dried and analyzed for composition of Si, Ca, Na, and Cr. In addition, the slurry should be prepared using 5C paper and caliber 20.
The filtration was carried out under reduced pressure with an aspirator in a cm Buchna funnel, and the filtration was completed in a very short time of 10 minutes each time, and there was no leakage of goethite from the paper into the liquid at the beginning of the filtration. Ta.
The analytical values of goethite after washing with water were as shown in Table 2.

[Table] In other words, it can be seen that the amount of sodium remaining in goethite after washing with water is extremely small at 42 ppm. The goethite used has a major axis of 0.45μ and a minor axis.
They were acicular particles with a diameter of 0.03μ and a specific surface area of 32m ² /g. Example 3 Goethite containing 1.2 parts by weight of Zn based on 100 parts by weight of Fe synthesized on the alkali side was washed with water to prepare a suspension consisting of 100 g of goethite and 2 parts of water. The pH of the suspension was 8.9. Next, an aqueous solution of No. 1 water glass containing 3.21 g of SiO ₂ was added and stirred for 30 minutes. Addition amount is game site
Equivalent to 1.5 parts of Si per 100 parts. The pH and viscosity were 10.6 and 15 cp. Next, hydrochloric acid was added to adjust the pH to 7, and the mixture was stirred for 30 minutes. The viscosity was 12 cp. Thereafter, an aqueous solution containing 0.6 g of calcium chloride was added and stirred for 30 minutes. The total volume of the suspension at this time was 2.5. The amount added is equivalent to 0.22 parts per 100 parts of goethite. PH and viscosity is 6.6, 420cp
The slurry was flocculated by adding Ca salt. Filter the agglomerated slurry and divide the resulting cake into 2
After stirring for 30 minutes, the cake was filtered and dried, and the composition of Si, Ca, Na, and Zn was analyzed. In addition, the slurry should be prepared using 5C paper and 20mm diameter paper.
The filtration was carried out under reduced pressure using an aspirator in a Buchna funnel of 1.5 cm, and the filtration was completed in a very short time of 10 minutes both times, and there was no leakage of goethite from the paper into the liquid at the beginning of the filtration. The analytical values of goethite after washing with water were as shown in Table 3.

[Table] In other words, it can be seen that the amount of sodium remaining in goethite after washing with water is extremely small at 73 ppm. The goethite used has a long axis of 0.5μ and a short axis.
They were acicular particles with a diameter of 0.033μ and a specific surface area of 32m ² /g. Example 4 Goethite synthesized in the acid side region was washed with water to prepare a suspension consisting of 100 g of goethite and 2 parts of water. The pH of the suspension was 5.4. After adjusting the pH to 8.0 by adding caustic soda, an aqueous solution of No. 3 water glass containing 1.62 g of SiO ₂ was added to the suspension and stirred for 30 minutes. The pH and viscosity were 10.0 and 12 cp, respectively. Next, nitric acid was added to adjust the pH to 7, and the mixture was stirred for 30 minutes. The viscosity was 12 cp. Thereafter, an aqueous solution in which 0.01 g of calcium hydroxide was dissolved was added and stirred for 30 minutes. The total volume of the suspension at this time was 2.5. The pH and viscosity were 9.3 and 320 cp. Si, Ca
The amount of Si added is 0.76 parts per 100 parts of goethite.
Equivalent to 0.054 parts of Ca. Washing and filtration are from Example 1.
The process was carried out in the same manner as in Example 3, and the permeability was very good as in Examples 1 to 3. The analysis values of goethite after washing with water were as shown in Table 4.

[Table] In other words, it is clear that the amount of sodium remaining in the goethite after washing with water is extremely small at 90 ppm. The goethite used has a long axis of 0.6μ and a short axis.
They were acicular particles with a diameter of 0.04μ and a specific surface area of 25m ² /g. Example 5 Same water washing game site as used in Example 1
A suspension consisting of 100 g and 2 parts water was prepared. The pH of the suspension was 9.2. Next, an aqueous solution of No. 3 water glass containing 1.62 g of SiO ₂ was added to the suspension and stirred for 3 minutes. The amount added corresponds to 0.76 parts of Si per 100 parts of goethite. The pH and viscosity were 10.8 and 10 cp, respectively. Next, nitric acid was added to adjust the pH to 9, and the mixture was stirred for 30 minutes. The viscosity was 9 cp. Thereafter, an aqueous solution containing 0.31 g of calcium chloride was added and stirred for 30 minutes. Addition amount is game site
Equivalent to 0.11 parts of Ca per 100 parts. The pH and viscosity were 8.8 and 120 cp. It was observed that the slurry flocculated by adding Ca salt. Filter the agglomerated slurry and divide the resulting cake into 2
The cake was redispersed in water, stirred for 30 minutes, and filtered. The cake was dried and analyzed for composition of Si, Ca, and Na.
Note that, as in Example 1, the slurry could be filtered in a very short time. The analysis values of goethite after washing with water were as shown in Table 5.

[Table] In other words, it can be seen that the amount of sodium remaining in goethite after washing with water is extremely small at 72 ppm. However, it has been shown that the amount of Si deposited in goethite is somewhat low. Example 6 The same water washing game site used in Example 1
A suspension consisting of 100 g and 2 parts water was prepared. The pH of the suspension was 9.2. An aqueous solution in which 1.0 g of sodium metaphosphate was dissolved was added to the suspension and stirred for 30 minutes. The amount added is equivalent to 0.3 parts of P per 100 parts of goethite. The pH and viscosity were 10.7 and 10 cp. Next, an aqueous solution of No. 3 water glass containing 1.62 g of SiO ₂ was added to the suspension and stirred for 30 minutes. The amount added corresponds to 0.76 parts of Si per 100 parts of goethite. The pH and viscosity were 10.9 and 7 cp, respectively. Next, nitric acid was added to adjust the pH to 7, and the mixture was stirred for 30 minutes. The viscosity was 8 cp. Thereafter, an aqueous solution containing 0.31 g of calcium chloride was added and stirred for 3 minutes. The total volume of the suspension at this time was 2.5. The amount added corresponds to 0.11 parts of Ca per 100 parts of goethite. PH and viscosity are 6.8,
It was 350 cp. It was observed that the slurry flocculated by adding Ca salt. Filter the agglomerated slurry and divide the resulting cake into 2
The cake was redispersed in water, stirred for 30 minutes, and then filtered. The solution was subjected to colorimetric analysis for PO ₄ ≡ along with the previous solution, and the cake was dried and analyzed for composition of Si, P, Ca, and Na. In addition, the slurry is made of 5C paper and caliber.
The filtration was carried out under reduced pressure using an aspirator in a 20cm Buchna funnel, and the filtration was completed in a very short time of 10 minutes each time, and there was no leakage of goethite from the paper into the liquid at the beginning of the filtration. Ta. The analysis values of goethite in the liquid and after washing with water are shown in the table.
6. It was as shown in Table-7.

【table】

[Table] In other words, it can be seen that the amount of sodium remaining in goethite after washing with water is extremely small at 70 ppm. Comparative Example 1 Using the water-washed goethite used in Example 1, a suspension consisting of 100 g of goethite and 2 parts of water was prepared. The pH of the suspension was 9.2. Next, an aqueous solution of No. 3 water glass containing 1.62 g of SiO ₂ was added to the suspension and stirred for 30 minutes. The pH and viscosity were 10.7 and 10 cp, respectively. Next, nitric acid was added to adjust the pH to 3.5, and the mixture was stirred for 30 minutes. The viscosity was 18 cp and the slurry was in a strongly dispersed state. The slurry was filtered, the resulting cake was redispersed in water from step 2, stirred for 30 minutes, and filtered.
A portion of the resulting cake was sampled for analysis.
The remaining amount was further redispersed in water from Step 2, stirred for 30 minutes, and then filtered. A compositional analysis of goethite at each washing stage was conducted. The filtration was carried out using 5C paper and a Buchner funnel with a diameter of 20 cm under reduced pressure using an aspirator. At the beginning of the process, goethite leaked from the paper into the paper, and the liquid was returned several times to perform the process, but it took a total of 16 to 20 hours. Further, the cake that had been washed twice was used as goethite, and an amount equivalent to 20 g was redispersed in water from step 2, heated to 90°C, stirred for 30 minutes, and then filtered. The total time required was 16 hours. Table 8 shows the analytical values for the washed cake.

[Table] In other words, it is clear that it is difficult to reduce the amount of sodium remaining in goethite to 400 ppm or less even if washing with water is strengthened. Comparative Example 2 Using the water-washed goethite used in Example 1, a suspension consisting of 100 g of goethite and 2 parts of water was prepared. The pH of the suspension was 9.2. Next, 1.62g of SiO ₂
An aqueous solution of No. 3 water glass was added to the suspension and stirred for 30 minutes. The pH and viscosity were 10.3 and 11 cp, respectively. Next, nitric acid was added to adjust the pH to 6.0, and the mixture was stirred for 30 minutes. The viscosity increased to 320 cp, and the slurry was visually observed to be agglomerated. The slurry was filtered, the resulting cake was redispersed in water from step 2, stirred for 30 minutes, filtered, a portion of the resulting cake was sampled for analysis, and the remaining amount was further redispersed in water from step 2. After stirring for 30 minutes, the mixture was filtered. The filtration was carried out using 5C paper and a Buchner funnel with a diameter of 20 cm under reduced pressure using an aspirator. The filtration was good and the filtration of each liquid was completed in 10 minutes. Further, the cake that had been washed twice was used as goethite, and an amount equivalent to 20 g was redispersed in water from step 2, heated to 90°C, stirred for 30 minutes, and then filtered. The extra time required is 15
It took minutes. Table 9 shows the analytical values for the washed cake.

[Table] In other words, the amount of sodium remaining in goethite remained as high as 630 ppm even when water washing was strengthened. Example 7 The goethite deposited in Example 1 was fired in air at 350°C for 2 hours using an electric furnace to form hematite, and further fired at 650°C for 4 hours to obtain a specific surface area of 30 m ² /
g of hematite was obtained. The hematite was heated at 450℃
Reduced to reduced iron powder in a H ₂ stream, replaced the atmosphere with N ₂ , left to cool to room temperature, and then 0.2% O ₂ in N ₂
A corresponding amount of air was mixed and aerated over the reduced iron powder for 2 hours, and then the amount of O ₂ was doubled until finally the entire amount was replaced with air to form an oxide film on the surface of the reduced iron powder. The magnetic properties of the magnetic iron powder are determined by the maximum magnetic field.
Coercive force (Hc) measured at 10KGauss
The magnetization was 1230 Oe, the magnetization (σs) was 166 emu/g, and the squareness ratio (R) was 0.51. Note that the specific surface area was 29 m ² /g. Add 50g of the magnetic iron powder to 25g of thermoplastic polyurethane resin.
% methyl ethyl ketone solution 20g, methyl ethyl ketone 75g, silicone additive as smoothing aid
0.1 g of each was placed in a stainless steel container, and dispersed for 8 hours with a paint conditioner using an alumina ball to obtain a dispersion, and then 20 g of the above 25% methyl ethyl ketone solution of the thermoplastic polyurethane resin was added. The mixture was further diluted with methyl ethyl ketone to adjust the viscosity to obtain a magnetic paint. This magnetic paint was applied to a reinforced polyethylene terephthalate film with a thickness of 12μ using a bar coater so that the dry film thickness was about 4μ, the magnetic particles were oriented using a magnetic field, and then dried with hot air and smoothed using a calendar roll. After the conversion, a magnetic tape for evaluation was obtained. When this was measured at a maximum magnetic field of 10 KGauss, good magnetic properties were obtained with coercive force (Hc) of 1170 Oe, residual magnetization (Br) of 2990 G, and Br/Bm = 0.82. Here, Bm is the saturation magnetization amount. Such high values of Br and Br/Bm indicate that the magnetic iron powder particles maintain good acicularity and there is no aggregation between the particles. Examples 8 to 12 Table 10 shows the results of the same treatment as in Example 7 using the goethites of Examples 2 to 6. All of the results show that the magnetic iron powder particles maintain good acicularity and there is no aggregation between particles that would adversely affect the dispersibility and orientation.

[Table] Comparative Examples 3 and 4 Table 11 shows the results of performing the same treatment as in Example 7 using the goethites of Comparative Examples 1 and 2 (all of which were third-washed goethites). In all cases, the dispersibility and orientation of the magnetic iron powder particles were inferior to those of Examples 7 to 12, which is considered to be an adverse effect of residual sodium.

【table】

Claims (1)

[Scope of Claims] 1. A method for producing magnetic iron powder in which an aqueous suspension of goethite coated with water glass is filtered, heated, and reduced, characterized by the presence of calciuion in the aqueous suspension. Method. 2. The method according to claim 1, wherein the aqueous suspension has a pH of 6 to 8.