GB2051025A - Process for producing hydrated iron oxide - Google Patents

Abstract

A hydrated iron oxide is produced by mixing an aqueous solution of a base with an aqueous solution of ferrous ions if desired with a small amount of zinc ion and oxidizing it while controlling the pH in a range of 5.5 to 7.5. A silicate can be added to all or part of the aqueous base solution. The aqueous solution of ferrous ions may contain zinc ions.

Description

SPECIFICATION Process for producing hydrated iron oxide The present invention relates to a hydrated iron oxide comprising goethite and lepidocrosite or a mixture thereof as a main component which is used as a raw material for producing a magnetic iron oxide for use in making a magnetic recording medium.

The conventional technology will be illustrated in respect of goethite. The conventional process is classified into two processes for producing goethite.

One is a reaction in the acidic condition and the other is a reaction in the alkaline condition.

The reaction in the acidic condition will be illustrated in detail. A 10 to 50% aqueous solution of a base, sufficient for neutralization of an aqueous solution of a ferrous compound, is added to said solution thereby converting ferrous sulfate into ferrous hydroxide. Air is fed into the aqueous slurry at 40 to 600C to bring about oxidation. During the oxidation, the blueish white colour of the precipitate is changed to green and then through a yellowish blue to yellow.

The pH decreases from 6-7 to 3-4. In order to convert the ferrous ion remaining in the slurry into goethite, an aqueous solution of a base is gradually added while maintaining pH at about 4. The reaction is completed in 15 to 20 hours.

Iron powder has been used instead of the addition of an aqueous solution of a base, but it requires a long reaction time and this process has seldom been employed in practice.

The reaction in the alkaline condition will be illustrated. An aqueous solution of a base, more than enough for the neutralization of the aqueous solution of ferrous sulfate, is added to said solution and an oxidation is carried out at a pH higherthan 11 at a temperature in the solution of 60 to 700C for about 15 hours to obtain goethite, as disclosed in Japanese Patent Publication No. 561011964.

The goethites obtained by either the abovementioned process have advantages and disadvantages. The goethite obtained in the acidic reaction has inferior purity (generally containing 0.5 to 1.0% of sulfur component as an impurity). Therefore, sintering tends to occur in the reduction and the oxidation for converting the product into y4erric oxide thereby imparting inferior magnetic characteristics.

On the other hand the cost of production is relatively low and accordingly this process is used on a large scale. The goethite obtained by the alkaline reaction has relatively high purity and little if any sintering occurs in the reduction and oxidation for converting the product into y-ferric oxide, so that superior magnetic characteristics are obtained. Therefore, the goethite can be used for a magnetic recording medium having high quality.

Since the pH in the production of goethite by the latter process is as high as 12 to 13.5, it is not easy to wash the product with water after the reaction, in comparison with the acidic process. Therefore, the cost of the product is higher. This means that the quality is high but the cost is also high.

Ways of achieving high performance and compact size for magnetic recording systems have been studied, and in particularto improve the SN ratio of the magnetic recording medium.

The particle size of the magnetic powder is closely related to the SN ratio of the magnetic recording medium. The SN ratio is increased by a decrease in the particle size. Therefore, it is desirable to obtain a magnetic powder having a small particle size to improve the magnetic recording medium. The particle size of the goethite used as a raw material for the magnetic powder can be controlled by selecting the reaction conditions. On the other hand when the particle size is smaller, the water washing treatment after the reaction is more difficult so that the cost is increased.

The present invention provides a process for producing a hydrated iron oxide by adding a base to an aqueous solution of ferrous ions and oxidizing it, wherein the pH of the reaction mixture is maintained in a range of 5.5 to 7.5 during the oxidation.

It is thus possible to produce a hydrated iron oxide such as goethite, useful as a raw material for a magnetic powder, having a smaller particle size which imparts high SN ratio for a magnetic recording medium, in an economical cost.

As described above, since the cost is high for the reaction in alkaline conditions, and a product having high quality, especially high purity, could not be obtained by the reaction in acidic conditions, it is necessary in accordance with the invention to carry out the reaction at near neutral pH. The impurities found in the acidic reaction are water-insoluble compounds having sulfur component which cannot be removed by washing the goethite with water after the reaction.

In general, it has been thought in an oxidation in the neutral conditions formed by adding a base to a ferrous salt, a grain magnetite is formed rather than goethite.

The inventors have found the conditions for forming goethite at a pH of 5.5 to 7.5 in the reaction system by modifying the method of adding the aqueous solution of a base to an aqueous solution of a ferrous ion as a main component and the oxidation of the product.

The inventors have further found that the coercive force can be increased and the time required for the formation of goethite can be shortened by incorporating zinc ions into the aqueous solution of ferrous ions used in the reaction system.

In the process of the present invention, the content of impurities contained in goethite can be small and sintering in the reduction and the oxidation reactions can be largely eliminated whereby excellent magnetic characteristics can be attained. The water washing of goethite after the reaction can be easily carried out whereby the cost of the product can be kept relatively low.

The reaction temperature can be lower than 500C preferably an ambient temperature of 25 to 35 C. The cost of energy can be reduced in comparison with the conventional process which are carried out at 40 to 70 C. The quality of the product is high. A magnetic powder of low cost and high quality can be produced.

In the process of the present invention, a hydrated iron oxide especially suitable as a raw material for a magnetic recording medium can be obtained by adding a silicate to all or part of the aqueous base solution so asto improve further the results obtained.

It has been known that the magnetic characteristics of a magnetic recording medium are improved by using a silicon component. For example, a compound having a silicon component as a main component is formed on the surface of the hydrated iron oxide and then a dehydration, a reduction and an oxidation are carried out as disclosed in Japanese Unexamined Patent Publication No. 12100/1978. It has been also known that a silicate is added in a production of goethite in an alkaline condition at pH of higher than 11.0 as disclosed in Japanese Unexamined Patent Publication No. 56196/1978.

In the process of the present invention, the object of the addition of a silicate is the same as that of the known process. However, the method of the incorporation of the silicate is different. That is, in the conventional technology, the effect for improving the characteristic has not been found at pH of the reaction mixture or the goethite slurry of lower than 8.0. In accordance with the process of the present invention, the effect for improving the characteristic is found by adding a silicate at pH of the reaction mixture of 5.5 to 7.5. This fact seems to be only the pH problem, however, this fact is remarkably important from an industrial viewpoint. A silicate is usually water soluble in an alkaline condition but is precipitated as a water insoluble compound.In the conventional technology, the compound having a silicon component is formed on a surface of goethite by utilizing this property or the silicon component is incorporated in a crystal during the reaction for forming goethite in a strong alkaline condition. In accordance with the process of the present invention, a silicate is added in a neutral or a weak acidic condition for forming goethite. Such possibility has not been considered by the conventional knowledge.

The process of the present invention will be further illustrated.

The aqueous solution of ferrous ion can be produced by dissolving a ferrous compound such as ferrous chloride, ferrous sulfate, ferrous nitrate etc.

in water. A concentration of the ferrous compound is from a saturated concentration to 0.5 wt.% preferably 5 to 40 wt.% especially 10 to 30 wt.%.

The base is preferably sodium hydroxide, carbonate or bicarbonate or potassium hydroxide, carbonate or bicarbonate or ammonium hydroxide.

The concentration of the base is usually 1 to 40 wt.% preferably 5 to 30 wt.% The oxidizing agent can be alkali chlorates, air, oxygen, ozone and alkali nitrates. The oxidizing agent is added at a ratio of more than a stoichiometric amount for converting a ferrous compound into a ferric compound. The oxidizing agent can be added before, during orafterthe mixing of the aqueous solution of ferrous ion with the base, since the oxidation is performed after forming ferrous hydroxide. That is, the oxidizing agent can be mixed with the base or a slurry of ferrous hydroxide.

The temperature for the oxidation is usually in a range of O to 800C preferably 5 to 600C especially 20 to 50 C. The conventional air bubbling oxidation method can be also employed.

The preparation of a hydrated iron oxide can be modified as desired.

The hydrated iron oxide is converted into magnetite by a reduction and magnetite is converted into y-ferric oxide by an oxidation. These products are called as the magnetic iron oxide powder.

The hydrated iron oxide is dehydrated by heating it.

The reduction of the hydrated iron oxide or the dehydrated iron oxide is usually carried out at 300 to 600 C preferably 350 to 400 C in hydrogen at 400 to 700 C in an inert gas with an organic compound such as alcohols, ketones, ether, esters, hydrocarbons as a reducing agent. The oxidation of the product is usually carried out by heating at 200 to 350cm preferably 250 to 3000C in air.

The particles of the magnetite obtained by a reduction of the hydrated iron oxide usually has a length of 0.1 to 2,u preferably 0.2 to 1, and an acicular ratio of 2 to 40 preferably 5 to 20.

The present invention will be illustrated by certain examples and references which are provided for purposes of illustration only and are not intended to be limiting the present invention.

EXAMPLE 1: Into 2.0 liter of a deionized water, 278 g. of ferrous sulfate was dissolved and the solution was stirred by a stirrer at a temperature of 250C + 1.0 C. A half of a solution obtained by dissolving 80 g. of sodium hydroxide and 30 g. of potassium chlorate in 1.0 liter of a deionized water, was gradually added during about 1 minute to the former solution. During the addition, pH of the solution was suddenly increased from about 3.0 to about 7.5 and then pH was gradually decreased to about 6.1 after 15 minutes. After 30 minutes, 0.5 liter of said remained solution of hydroxide and potassium chlorate was added to the reaction mixture at a rate of 2 ml per minute by a pump. It took about 250 minutes for completing the addition.During the addition, the reaction mixture was stirred and the pH change of the reaction mixture was automatically recorded. After about 200 minutes, the pH was gradually increased to 6.4-6.5.

The color of the precipitate was changed from blueish black to dark green and then further changed through yellowish blue to yellow. The end point of the rection should be conformed with the addition of the stoichiometric quantity of the base required for neutralizing ferrous sulfate. In the practice, the pH change was monitored and controlled to give about 7.0 with a small amount of the aqueous solution of a base.

In this example, about 20 ml of 2N-NaOH aqueous solution was added 20 minutes before the end of the reaction so as to complete the reaction for 300 minutes. At the end of the reaction, the pH was 7.14.

The resulting slurry of goethite was washed five times with water in a 100 liter tank by a decantation method, and then it was filtered and dried at 700C for 24 hours and pulverized to obtain about 86 g. of goethite powder. It was confirmed that most of the powder was mad of goethite (a-FeOOH) by X-ray diffraction method. It was also confirmed that the powdex was made of acicular crystals having an average length of about 0.3ill and an acicular ratio of about 10 by an electron microscopic observation. It was also confirmed that the powder had a specific surface area of 55.6 m2/g. by the BET method. It was also confirmed that the powder contains 0.07 wt.% of sulfur component by the fluorescent X-ray elemental analysis.

In a quartz boat, 10 g. of the resulting goethite powder was charged and the port was set in a reducing furnace. Firstly, it was heated at about 6000C in air for 1 hourto perform a dehydration and a heat treatment. (The heat treatment at 600 C results in an improvement of crystalline state and an improvement of magnetic characteristics of y-Fe2O3 obtained by a reduction and an oxidation of the product). Nitrogen gas was fed into ethanol in bubbling and was continuously fed into the reducing furnace at a flow rate of 0.3 liter/min. and the power was reduced at 400 C for 1 hour to obtain magnetite. The magnetite was cooled and then heated at 300 C for 1 hour in air to obtain y-ferric oxide.

Magnetic characteristics of the resulting y-ferric oxide were measured. The results are as follows.

Coercive force (Hc) 423 Oe Intensity of Saturated magnetization (sigma S) 75.6 emu/g.

Intensity of Residual magnetization (sigma R) 38.1 emu/g.

The specific surface area measured by the BET method was 32.7 m2/g.

According to an electron microscopic observation, it was confirmed that a sintering of the particles was not substantially found.

REFERENCE 1: The reaction was initiated with the same components as those of Example 1. After 80 minutes from the addition of a half of the aqueous solution of sodium hydroxide and potassium chlorate, pH was 4.0. The residual solution of sodium hydroxide and potassium chlorate was gradually added so as to maintain pH in a range of 3.5 to 4.0 thereby completing the formation of goethite. The reaction time was about 200 minutes.

The fundamental difference of this process from the process of Example 1 was the difference of pH in the addition of the aqueous solution of sodium hydroxide and potassium chlorate; pH was in a range of 6 to 7 in Example 1 whereas pH whereas pH was in a range of 3.5 to 4 in Reference 1.

In accordance with the process of Example 1, the resulting goethite was washed with water, filtered, dried and pulverized to obtain goethite powder.

It was confirmed that the powder contained 0.63 wt.% of a sulfur component by the fluoroescent X-ray elemental analysis.

In accordance with the process of Example 1, 10 g, of the resulting goethite powder was dehydrated, reduced and oxidized to obtain y-ferric oxide.

Magnetic characteristics of the resulting y4erric oxide were measured. As a result, a coercive force was only 256 Oe. It is considered that this is caused by a sintering phenomenon during the heattreatment at 6000C under the adverse effect of a relatively large amount of the sulfur component in the goethite. It was confirmed that a part of the particles is sintered to form different configuration by an electron microscopic observation.

The dehydration and the heat-treatment at 6000C were eliminated, but the resulting goethite was reduced at 400 C and oxidized at 3000C to produce y4erric oxide and magnetic characteristics were measured. The results are as follows.

Coercive force (Hc) 352 Oe Intensity of Saturated magnetization (sigma S) 74.1 emu/g.

Intensity of Residual magnetization (sigma R) 36.7 emu/g.

The specific surface area measured by the BET method was 25.3 m2/g.

As it is clear from the fact, the magnetic characteristics of y-fe rric oxide obtained from the goethite obtained in Example 1 are superiorto those of y-ferric oxide obtained from the goethite obtained in Reference 1.

EXAMPLES: In accordance with the process of Example 1, except adding 10 ml. of 1.0 mole aqueous solution of zinc chloride to the aqueous solution of ferrous sulfate, the reaction and the treatment were carried out under the same condition with the same compo nexts to obtain the goethite containing zinc component.

When zinc ion was added, the reaction velocity was slightly increased to complete the reaction for about 50 minutes shorter than that of Example 1.

According to the electron microscopic observation, the size of the particles was slightly smaller than that of Example 1. and the acicular ratio of the particles was slightly higher than that of Example 1. The specific surface area measured by the BET method was 62.8 m2/g.

In accordance with the process of Example 1, the product was dehydrated, reduced and oxidized to obtain y-ferric oxide.

Magnetic characteristics of the y-ferric oxide are as follows.

Coercive force (Hc) 451 Oe Intensity of Saturated magnetization (sigma S) 74.3 emu/g.

Intensity of Residual magnetization (sigma R) 38.0 emu/g.

The specific surface area measured by the BET method was 36.3 m/g.

In the examples, the reaction temperature was 25 C. When the reaction is carried out at 35 C, the reaction velocity is increased to complete the reaction for 150 to 200 minutes. It is preferable to control the addition of the aqueous solution of the base so as to maintain the minimum of pH to about 5.5. The particle size is usually reduced and accordingly, the condition is decided as desired. as the method of the oxidation, the potassium chlorate method desclosed in Japanese Unexamined Patent Publication No. 8029911975 was employed. It is also possible to employ the conventional air bubbling oxidation method.

In the reduction after the dehydration of goethite, the step of using nitrogen gas containing ethanol was employed as disclosed in Japanese Patent Pub lication No.24637/1978. it isalso possible to employ the reduction using hydrogen gas. It is possible to add about 80% of the total of the aqueous solution of the base at the initiation of the formation of goethite.

Magnetite may be included if it is to much. The reaction temperature is preferably lowerthan 500C since magnetite may be formed at highertemperature. In the last step of the formation of goethite, if pH is higher than 7.5 during the addition of the residual aqueous solution of the base, the reaction velocity is lowered to require long time for completing the reaction or the quality of the product is deteriorated.

Therefore, itis preferable to maintain the maximum of pH to about 7.5.

EXAMPLE 3: Into 2.0 liter of a deionized water, 278 g. of ferrous sulfate was dissolved and the solution was stirred by a stirrer at a temperature of 25 C + 1.0 C. A half of a solution obtained by dissolving 80 g. of sodium hydroxide and 30 g. of potassium chlorate in 1.0 liter of a deionized water, was gradually added during about 1 minute to the former solution. During the addition, pH of the solution was suddenly increased from about 3.0 to about 7.5 and then pH was gradually decreased to about 6.1 after 15 minutes. After 30 minutes, a solution obtained by dissolving 3.0 g. of sodium silicate solution (about 28% of SiO2 component) in said remained solution of sodium hydroxide and potassium chlorate was added to the reaction mixture at a rate of 2 ml per minute by a pump.It took about 250 minutes for completing the addition.

During the addition, the reaction mixture was stirred and the pH change of the reaction mixture was automatically recorded. After about 200 minutes, the pH was gradually increased to 6.4-6.5. The color of the precipitate was changed from blueish black to dark green and then further changed through yellow ish blue to yellow. The end point of the reaction should be conformed with the addition of the stoichiometric quantity of the base required for neutralizing ferrous sulfate. In the practice, the pH change was monitored and controlled to give about 7.0 with a small amount of the aqueous solution of a base.

In this example, about 20 ml. of 2N-NaOH aqueous solution was added 20 minutes before the end of the reaction so as to complete the reaction for 300 minutes. At the end of the reaction, the ph was 7.14.

The resulting slurry of goethite was washed five times with water in a 100 liter tank by a decantation method, and then, it was filtered and dried at 700C for 24 hours and pulverized to obtain about 86 g. of goethite powder. It was confirmed that most of the powder was made of goethite (a-FeOOH) by X-ray diffraction method. It was also confirmed that the powderwas made of acicular crystals having an average length of about 0.3# and an acicular ratio of about 10 by an electron microscopic observation. It was also confirmed that the powder had a specific surface area of 56.2 m2/g. by the BET method. It was also confirmed that the powder contains 0.83 wt.% of SiO2 component by the fluorescent X-ray elemental analysis.

In a quartz, port, 10 g. of the resulting goethite powder was charged and the port was set in a reduc ing furnace. Firstly, it was heated at about 600 C in air for 1 hourto perform a dehydration and a heat treatment. (The heattreatmentat600 C results in an improvement of crystalline state and an improvement of magnetic characteristics of y-Fe2O3 obtained by a reduction and an oxidation of the product). Nit- rogen gas was fed into ethanol in bubbling and was continuously fed into the reducing furnace at a flow rate of 0.3 liter/min. and the powder was reduced at 4000C for 1 hour to obtain magnetite.The magnetite was cooled and then heated at 300at for 7 hour in air to obtain y4erric oxide.

Magnetic characteristics of the resulting y-ferric oxide were measured. The results are as follows.

Coercive force (Hc) 447 Oe Intensity of Saturated magnetization (sigma S) 74.3 emulg.

Intensity of Residual magnetization (sigma R) 37.4 emu/g.

The specific surface area measured by the BET method was 36.2 m2/g.

According to an electron microscopic observation, it was confirmed that a sintering of the particles was not substantially found.

As it is found from the results, when sodium silicate was incorporated in the reaction for producing goethite, the coercive force is higher and the specific surface area is also larger. This indicates that the degree of the sintering phenomenon is lower.

In the example, the aqueous solution of sodium silicate was added to the half residual aqueous solution of the base. It is also possible to add the aqueous solution sodium silicate at the beginning orto a residual quarter aqueous solution of the base. The particle size is slightly varied.

Claims (6)

1. A process for producing a hydrated iron oxide by adding a base to an aqueous solution of ferrous ions and oxidizing it, wherein the pH of the reaction mixture is maintained in a range of 5.5 to 7.5 during the oxidation.

2. A process according to claim 1, wherein the oxidation is carried out at a temperature of 25 to 35 C.

3. A process according to claim 1 or claim 2, wherein sufficient base is added initially to said aqueous solution of ferrous ions so as to adjust the pH to within the range of 5.5 to 7.5 and further base is gradually added in said oxidation so as to maintain the pH in said range.

4. A process according to any preceding claim, wherein said aqueous solution of ferrous ions contains zinc ions.

5. A process according to any preceding claim, wherein a silicate is added to all or part of said aqueous base solution.

6. A process according to claim 1, substantially as herein described with reference to the accompanying drawings.