DE2349112A1 - METHOD FOR MANUFACTURING GOETHITE POWDER - Google Patents

Description

It 2622

SONY CORPORATION Tokyo / Japan

Process for the production of goethite powder

The invention relates to a method for producing goethite powder and in particular to a method for producing it of goethite powder, which is suitable for making maghemite powder with high coercive force.

In a magnetic tape suitable for use in an audio tape, a video tape recorder and the like, a needle-shaped maghemite powder (or gamma-Fe ₂ O ₃ powder) is generally used. However, since the coercive force (Hc) of the usual acicular maghemite powder is less than 400 oersted, it is not suitable for high recording density.

Needle-shaped maghemite powder, which is produced by the well-known process of reducing and oxidizing goethite powder (or Alpha-FeOOH powder) has a coercive force that depends on the particle size

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Depends on the particle shape and the particle size uniformity of the goethite powder / used as the starting material will.

There are known two generally used processes for producing a conventional goethite powder. With one of these In the process, air in the form of bubbles is passed through an iron sulfate and sodium hydroxide solution to produce goethite powder and to form sulfuric acid, and then the resulting sulfuric acid is converted into iron sulfate by iron converted. A disadvantage of this method is the excessively long time (typically a few days to a few months), which is required for the formation of Goethite. Another disadvantage of this method is that that the coercive force (Hc) of the maghemite powder produced by the resulting goethite is derived by the known method given above, under 37O Oersted lies.

In the other of these methods, air is blown in the form of bubbles through an aqueous iron salt and alkali hydroxide solution directed to form goethite powder. The maghemite powder, that of a resulting goethite derived by the above known method generally has a higher coercive force (Hc) than the maghemite powder, which is derived from the goethite obtained after the previous two-step process will. A disadvantage of this second method, however, is that it takes about 10 to 20 hours for formation of Goethite are required. Another disadvantage of this second method is that the hydroxide may gel and be non-uniform, especially when using a high density iron hydroxide suspension becomes very difficult to produce a goethite powder having a uniform particle size.

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The invention is based on the object of a method for To provide production of a goethite powder which is suitable for use as a starting material for producing a needle-shaped maghemite powder in a short time. That needle-shaped maghemite powder is said to have high coercive force of preferably over 400 oersteds and have a uniform particle size.

The invention provides a method for producing a goethite powder using an aqueous Iron hydroxide suspension created. The suspension is for a relatively short period of time in an inert Stirring the gas atmosphere. After that the resulting Suspension oxidized with oxygen to form goethite powder.

The invention is explained below with reference to FIGS. 1 to 5. for example explained. It shows:

Figure 1 is a graph showing the relationship between the agitation time in a nitrogen atmosphere and the Time required for a reaction according to the embodiment of Example 1 below is, emerges,

Figure 2 is a graph showing the relationship between the agitation time in a nitrogen atmosphere and the Coercive force (Hc) of a resulting maghemite powder according to the embodiment of the example it appears

Figure 3 is a graph showing the relationship between the stirring time in a nitrogen atmosphere and the time required for a reaction according to the embodiment of the following Example 2 of the invention, 409814/1024

Figure 4 is a graph showing the relationship between the agitation time in a nitrogen atmosphere and the Coercive force (Hc) of a resulting maghemite powder according to the embodiment of the example the invention is apparent, and

FIG. 5 is a diagram showing the relationship between the molar ratio of the OH ions to the Fe ions (OH ~ / Fe ⁺⁺ ) and the coercive force (Hc) of a maghemite powder derived from a goethite powder obtained according to the invention will be produced.

example 1

An alkaline solution containing potassium hydroxide in an amount of 392 g (7.0 moles) mixed with 800 ml of water, was placed in a reaction vessel.

279 g (1.4 mol) of ferric chloride (FeCl ₂ ^ H ₂ O) dissolved in 400 ml of water was added to the alkali solution with stirring to form a colloidal suspension of iron hydroxide. Previously, an inert gas, e.g. a nitrogen gas such as through. Bubbling is passed through the alkali solution at a rate of 3.0 liters per minute to remove the oxygen therein.

When the ferric chloride was added, nitrogen gas was passed through Bubbling (blowing) into and through the resulting solution in a reaction vessel while the Using this procedure, the temperature was maintained at 80 ° C with continuous stirring and a layer of nitrogen gas would develop above the surface of the resulting liquid reaction system maintained. This bubbling was continued until a milk-white, even collo-

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retroactively

changed

T349112

ideal iron oxide was obtained. The one required for the reaction Time (measured from the beginning of the n with the ferric chloride to a point when the formation of this colloidal iron oxide was considered complete) was about 3 hours.

Next, passed from nitrogen gas to air and the air blowing was started by the resulting liquid reaction system at the temperature of 8O ⁰ C with continuous stirring at a rate of 5.0 liters / minute and it was formed in 4.5 hours, a yellow goethite .

After rinsing with water, the goethite material was filtered, dried and pulverized, and it became 120 g of yellow Get goethite powder. The particles of this yellow goethite powder were needle-shaped, about a length of 0.5 microns and a needle ratio of about 20, where the needle ratio value is divided by dividing the average length of the particle by the average diameter of the particle is obtained. In addition, the particle size uniformity of the goethite powder was good.

2.0 g of this goethite powder was reduced at a temperature of 350 ° C. while bubbling hydrogen gas thereover at a rate of 1.0 liter / minute for one hour, after which the resulting product was oxidized by letting air at a temperature of 230 ° ° C was passed over it for one hour. As a result, a gray maghemite powder (Gamme-Fe _? 0, powder) was obtained. The coercive force (Hc), the square ratio (Rs) and the saturation magnetization (Gs) of the resulting gamma-Fe-O., Were 466 oersteds, 55% and 69.5 EMU / g, respectively.

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Another 2.0 g of the same goethite powder was reduced at a temperature of 380.degree. C. while bubbling the hydrogen gas over it at a rate of 1.0 liter / minute for one hour, after which the resulting product was bubbled over with air at a temperature of 250.degree ° C was oxidized for one hour to produce a maghemite powder. The coercive force (Hc), the square ratio (Rs) and the saturation magnetization (Gs) of this gamma-Fe ₂ O_ were 480 oersteds, 55% and 65.1 EMU / g, respectively.

FIG. 1 shows a diagram which shows the relationship between the stirring time in the nitrogen atmosphere, and the ordinate axis shows the required response time. Curve a shows the changes in time it took to form of goethite is required after the start of the oxidation reaction and curve b shows the changes in agitation time in the nitrogen atmosphere plus the time which is necessary for the formation of the goethite after the start of the oxidation reaction.

In this example, the time of the end of the oxidation reaction between air and colloidal iron oxide (that is the time when the iron ions (Pe) are essentially completely consumed) is determined in the following way: A 5 ml sample of the reacting liquid reaction system was released from the reaction vessel every 10 minutes Start of the oxidation reaction (i.e. after iron chloride has been added to potassium hydroxide with nitrogen sparging) and the sample was added to a dilute hydrochloric acid solution which removes oxygen was to prevent the nitrogen gas being passed from oxidizing, and the unreacted iron hydroxide in this sample was dissolved therein. The amount of hydrochloric acid in this dilute solution became so adjusted that the pH of the resulting solution

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was in the range of 1: 7. Then the iron ions became Fe found in the resulting solution by using a detection paper sensitive to Fe ions became. The paper used is commercially available under the name "Merckoquant Fe (Ε-Merck Company). The time at which no iron ion Fe could be detected in this sample was taken as the end time of the oxidation reaction viewed.

Fig. 2 shows the relationship between the stirring time in the nitrogen atmosphere and the coercive force (Hc) of the gamma-Fe-O_ derived from the resulting goethite powder as the raw material. As shown in Fig. 2, it is preferable to stir the solution for a time interval of about 2 to 5 hours in an inert gas (e.g., oxygen-free) to provide the desired goethite powder product for use as the starting material for the gamma-Fe ₃ O ₃ high coercive force of, for example, over 440 Oersteds.

Example 2

In order to compare the following conventional process for the production of goethite powder and "the property of the gamma * Fe-O ₃ obtained by the same process, the following example is described:

The aqueous solution of ferric chloride was mixed with the aqueous solution of potassium hydroxide and kept at the temperature of 80 C, air was passed through at a flow rate of 5.0 liters / minute. In this case, the stirring treatment was not carried out in the inert atmosphere as in the method of the invention. The reaction was complete in 9.5 hours and yellow goethite had been formed. After rinsing with water it was the material filtered, dried and powdered and you

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received 120 g of goethite powder. The particles of the resulting goethite powder were acicular with a length of about 0.8 mm and a needle ratio of about 10.

As in Example 1 of the invention, 3.5 g of goethite powder was reduced by hydrogen gas at a temperature of 350 ° C. and oxidized by air at a temperature of 230 ° C. to obtain gamma-Fe ₂ O ₃ . The coercive force (Hc), the square ratio (Rs) and the saturation magnetization (Gs) of the resulting gamma-Fe ₂ O ₃ were 387 oersteds, 54% and 67.1 EMU / g, respectively. As these results show, the coercive force (Hc) of the gamma-Fe ₂ O ₃ according to the method according to the invention is larger than that of the gamma-Fe ₃ O ₃ according to the conventional method.

Example 3

An alkaline solution containing 256 g (6.4 mol) of sodium hydroxide mixed with 600 ml of water, was placed in a reaction vessel.

445 g (1.6 mol) of iron sulfate (FeSO ₄ ^ H ₃ O) dissolved in 600 ml of water was added to the alkali solution with stirring to form an iron hydroxide suspension. In advance, a nitrogen gas was passed through the alkali solution at a flow rate of 2.0 liters / minute to remove the oxygen therein. At the same time and in the manner of Example 1, nitrogen gas (N_) was blown into the resulting suspension, which was kept at a temperature of 60 ° C. with continuous stirring until milky white, uniform colloidal iron oxide was obtained. The time required for this reaction was about 3 hours.

Then the nitrogen gas was switched to air, those with a flow rate of 1.0 liter / minute

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passed through the resulting suspension, which was kept at a temperature of 6O ⁰ C with continuous stirring to form yellow goethite in 6.5 hours by oxidation.

After rinsing with water, the material was filtered, dried and pulverized to give 140 g of yellow goethite powder to obtain. The particles of the yellow goethite powder were acicular with a length of about 0.5 microns and a Needle ratio of about 20 (measured as in Example 1). Also was the uniformity of the particle size of goethite powder good.

2.0 g of this goethite powder was reduced at a temperature of 350 ° C. while hydrogen gas was passed through at a flow rate of 1.0 liter / minute for one hour; thereafter the product was oxidized with air which was passed through at a temperature of 250 ° C. for one hour. As a result, the brown gamma Fe ₂ O ₃ powder was obtained. The coercive force (Hc), the square ratio (Rs) and the saturation magnetization (Gs) of this gamma-Fe ^ O ^ powder were 458 oersteds, 55% and 65.6 EMU / g, respectively.

Fig. 3 is a graph showing the relationship in this example between the stirring time in the nitrogen atmosphere and the required reaction time with (a) a constant density of the reaction suspension, (b) a constant response time and (c) a constant flow rate emerges from air. In this diagram, the abscissa shows the stirring time in the nitrogen atmosphere and the ordinate the time required. Curve a shows the changes in time required for the Formation of the goethite is required after the start of the air oxidation reaction, and curve b shows the changes the stirring time in the nitrogen atmosphere plus the

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Time taken for the goethite to form after the oxidation reaction has started is required.

Fig. 4 shows the relationship in this example between the stirring time in the nitrogen atmosphere and the coercive force (Hc) of the gamma-Fe ₂ O_ powder derived from the resultant goethite powder as the raw material. As shown in Fig. 4, it is preferable to stir the solution in nitrogen gas for 2 to 4 hours in order to obtain a desired goethite powder for use as a raw material for the _{high coercive force gamma-Fe 2} O ₃ of, for example, over 440 oersteds.

In Examples 1 and 3, the amounts of potassium hydroxide (KOH) and ferric chloride (FeCl ₂ * 4H ₂ O) and sodium hydroxide (NaOH) and iron sulfate (FeSO ₄ -TH ₂ O) were changed, ie, that
changed.

Example 4

that is, the OH ion to Fe ⁺⁺ ion ratio (OH ~ / Fe ⁺⁺ ) was

In an alkali solution containing potassium hydroxide in an amount of 336 g (6.0 mol) mixed with 900 ml of water, was placed in a reaction vessel.

334 g (1.2 mol) of iron sulfate (FeSO ₄ * 7H ₂ O) dissolved in 500 ml of water was gradually added to the alkali solution with stirring to form an iron hydroxide suspension. Previously, in order to remove the oxygen in nitrogen gas was passed through the alkali solution at a flow rate of 3.0 liters / minute. At the same time, in a manner similar to Example 1, nitrogen was blown into the resulting suspension at a temperature of 80 ° C. with continuous stirring until the milky white, uniform colloidal iron oxide was obtained. The time required for this reaction was about 3 hours.

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234S112

The mixture was then passed from nitrogen gas to air was passed at a flow rate of 5.0 liters / minute through the resulting suspension at a temperature of 8O ⁰ C with continuous stirring to form by oxidation of yellow goethite in four hours. '"

After rinsing with water, the material was filtered, dried and pulverized to give 109 g of yellow Goethite powder. The particles of the yellow goethite powder were needle-shaped with a length of about 0.4 microns and a needle ratio of about 15 (measured in Example 1). In addition, the particle size uniformity of the goethite powder was good.

2.0 g of this goethite powder was reduced at a temperature of 370.degree. C. with hydrogen gas being passed through at a flow rate of 1.0 liter / minute for one hour, and then the product was oxidized by letting air with a temperature of 250.degree C was bubbled through for one hour. A brown gamma Fe ₂ O ₃ powder was obtained. The coercive force (Hc), the square ratio (Rs) and the saturation magnetization (Gs) of this gamma-Fe ₂ Q_ powder were 447 oersteds, 55% and 72.0 EMu / g, respectively.

Example 5

An alkaline solution containing sodium hydroxide in an amount of 250 g (7.0 mol) mixed with 800 ml of water, was placed in a reaction vessel.

279 g (1.4 mol) of ferric chloride (FeCl ₂ · 4H ₂ O) dissolved in 500 ml of water was added to the alkali solution with stirring to form an iron hydroxide suspension. In advance, a nitrogen gas was passed through the alkali solution at a flow rate of 3.0 liters / minute to make the

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To remove oxygen in it. At the same time, a nitrogen gas was passed through the resulting suspension at a temperature of 60 ^° C. with continuous vigorous stirring through an ultrasonic device for 2.5 hours in order to obtain milky-white, uniform, colloidal iron oxide.

Thereafter , the nitrogen gas was changed to air, which was passed through the resulting suspension at a flow rate of 2.0 liters / minute at a temperature of 60 ° C. with continuous ultrasonic agitation, and yellow goethite was obtained in 4.0 hours.

After rinsing with water, this material was filtered, dried and pulverized to obtain 120 g of yellow goethite powder. The particles of the yellow goethite powder were acicular with a length of about 0.5 microns and a needle ratio of about 20 (measured as in Example 1). In addition, the particle size uniformity of this goethite powder was good.

2.0 g of this goethite powder was reduced at a temperature of 360 ° C. while bubbling hydrogen gas at a flow rate of 1.0 liter / minute for one hour, and then the product was oxidized by letting air having a temperature of 250 ° C was bubbled through for one hour. As a result, a brown gamma Fe _? 0 - powder obtained. The coercive force (Hc), the square ratio (Rs) and the saturation magnetization (Gs) of this gamma-Fe-O were 455 oersteds, 55% and 71.2 EMU / g, respectively.

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Example 6

The relationship between changes in molar ratio and coercive force (Hc) was examined in a gamma-Fe ₂ O_ powder derived from the goethite powder produced by the method of the invention and used as the starting material. The curves shown in Fig. 5 show the results. Curve a shows the case of goethite derived from potassium hydroxide and ferric chloride, and curve b shows the case of goethite derived from sodium hydroxide and iron sulfate. Fig. 5 shows that the gamma-Fe ₂ O ^ powder, which has a higher coercive force (Hc) than a common gamma-Fe ~ O_ powder, is thereby obtained

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can if one has a molar ratio (OH ~ / Fe) of over 2.5, and that a gamma-Fe-Oj powder having a coercive force (Hc) of over 440 oersteds can be obtained if one has a molar ratio (OH ~ / Fe) ranging from 3.2 to 4.5 in the case of goethite powder _{derived from potassium hydroxide (KOH) and ferric chloride (FeCl 2} * 4H ₂ O). This case is referred to herein as the "KOH-FeCl ₂ process". 5 further shows that gamma-Fe ₉ 0 powder, which has a higher coercive force (Hc) than the usual gamma-Fe ₂ O ₃ powder, can be obtained by using a molar ratio (OH ~ / Fe ⁺ ) of over 3, and a gamma Fe ₃ O ₃ powder which can have a coercive force (Hc) of over 440 Oe by having a molar ratio of 3.8 to 5.8 in the case of goethite powder derived from sodium hydroxide (NaOH) and iron sulfate (FeSO. 7H ₂ O). This case is referred to herein as the "NaOH-FeSO. Method".

While goethite powder, which was produced by the method of the invention, was used as the starting material for the production of an acicular gamma-Fe ₂ O, powder in the above examples, it can of course also be used as the starting material for the production of ring-shaped ones

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naohirägüoh changed

Magnetite powder can be used as indicated above. Description of the Preferred Embodiments

The invention relates to a method for producing goethite powder. The method includes mixing one aqueous solution of a water-soluble iron salt with an aqueous solution of an alkali hydroxide to form iron hydroxide to build. During this mixing, the liquid reaction system is in an inert (non-oxidizing) Atmosphere stirred. The resulting colloidal iron hydroxide suspension is then mixed with an oxygen, contains the gas for converting the iron hydroxide into goethite powder, is oxidized.

The alkali hydroxide is preferably sodium hydroxide or potassium hydroxide. The iron salt is preferably iron sulfate or ferric chloride. The stirring time for mixing the alkali hydroxide and the iron salt is preferably sufficient from about 2 to 5 hours, with a time of about 2 to 4 hours being allowed when iron sulfate is used as the iron salt is used. The molar ratio of the OH ions to the Fe ions (QH / Fe) is preferably more than 2.5 and if sodium hydroxide and iron salt are used, this molar ratio of OH ~ "ions to the Fe ions (OH / Fe) preferably from 3.8 to 5.8, and if potassium hydroxide and ferric chloride are used, this molar ratio OH ~ / Fe ranges preferably from 3.2 to 4.5. Mixing alkali hydroxide and iron salt and stirring the liquid reaction system and the

Oxidation to maghemite are preferably carried out at a Tempert IiM λ · t temperature in the range in which the formation and promotes the formation of magnetite is substantially prevented. For example, the temperature range for a sodium hydroxide and iron salt system is preferably about 40 to 60 ° C and for a potassium hydroxide and iron chloride system is preferably about 50 to 80 ° C.

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According to the invention, the solution containing iron hydroxide is stirred for 7 hours while being prevented from being oxidized, so that a uniform and finely divided colloidal iron hydroxide aqueous suspension can be obtained. The method of the invention has a number of advantages such as the following:

(1) The response time is shortened. The oxidation reaction time for the formation of goethite powder is determined by stirring in an inert (non-oxidizing) gaseous (preferably nitrogen) atmosphere according to the invention unexpected and even considerably shortened. Even the combined stirring time in such a gaseous one Atmosphere plus the oxidation reaction time, as FIGS. 1 and 3 show, will typically be a maximum of about 2 hours for the KOH-FeClj embodiment and by a maximum of about 4.5 hours for the NaOH-FeSO.-embodiment compared to the reaction time if no stirring treatment is carried out in an inert atmosphere, shortened.

(2) The coercive force (Hc) of the resulting maghemite powder will be raised. The maghemite powder (Gamma-Fe ^ O powder), derived from a goethite powder made by the method of the invention, unexpectedly has a high coercive force (Hc) typically in the range of about 440 to 480 oersteds suitable choice of stirring time in an inert atmosphere (preferably nitrogen). As FIGS. 2 and 4 show, such a high coercive force (Hc) can be achieved by stirring for about 2 to 4 hours in the inert Atmosphere in the NaOH-FeSO.-embodiment. In addition, one can choose the molar ratio appropriately OH / Fe in a goethite powder produced according to the invention, which is used as the starting material used to make a gamma-Fe-O-j powder

4 0 9 8 1 UI 1 0 2 U

will generate a higher maghemite powder coercive force than maghemite powder borrowed from a jib. As FIG. Shows, a molar ratio OH / Fe of over about 2.5 in the KOH-FeCl ₂ embodiment and a molar ratio OH ~ / Fe ⁺⁺ of over about 3 in the NaOH-FeSO ₄ embodiment can be selected. Thus, with a molar ratio OH ~ / Fe ⁺⁺ v ° ^{n 3} » ^{2 to 4} » ⁵ in the KOH-FeClj embodiment and with a molar ratio OH "" / Fe ⁺⁺ of 3.8 to 5.8 in the NaOH-FeSO ₄ -AuSführungsforxn produces a goethite powder which, when used as a starting material for the production of a gamma-Fe ^ O powder, _{produces a gamma-Fe 2} O ₃ powder which has a coercive force (Hc) of about 440 Oersted has.

(3) Uniform particle size in the resulting Goethig. The uniformity of the particle size of the The resulting goethite powder is made by stirring in the solution containing the iron hydroxide in an inert Contains (non-oxidizing) gaseous (preferably nitrogen) atmosphere, unexpectedly improved. This Process leads to uniform, fine-grained iron hydroxide particles. The improved evenness is confirmed by the electron microscope. The size of the iron hydroxide particles in aqueous Suspension made according to the invention cannot be measured because the iron hydroxide is in air oxidizes quickly, and therefore, when the suspension takes on a milky white color, stirring the suspension can terminates grounding, which also indicates that the iron-Fe ions are essentially completely consumed became. In the case of a magnetic tape made using a gamma Fe ^ O ^ powder produced by derived from a goethite powder which was manufactured according to the method of the invention, it is observed that that the orientation of the gamma Fe ^ Oo powder improves

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and the pressure level is low as desired.

(4) Easy oxidation reaction temperature control, In general, * the higher the temperature of the goethite oxidation reaction with oxygen-containing gas (preferably air), the higher the coercive force (Hc) and the better the magnetic properties obtained. Characteristically However, it is not yellow goethite, but black magnetite at reaction temperatures above 70 C as the reaction product obtained when sodium hydroxide is used as the alkali, as well as at the temperature of above 90 ° C if potassium is used as the alkali. Therefore, the temperature may cause an oxidation reaction when passing e.g. air through the iron hydroxide suspension which is produced according to the invention, usually about 40 to 60 ° C when sodium hydroxide is used, and usually about 50 up to 80 ° C if potassium hydroxide is used. On the other hand, an iron ion is necessary in an iron salt solution unstable and is converted to a ferric ion in hot water, and a hot alkali solution can be dangerous. Therefore, an iron salt solution and an alkali hydroxide solution are preferably used in the indicated relatively low temperatures mixed.

Preferably, an iron salt solution and an alkali hydroxide solution are used in the practice of the invention mixed at a relatively low temperature, and stirred in a nitrogen atmosphere to prevent oxidation, and then heated to a higher temperature before starting the oxidation reaction. Such a procedure favors the desired better results obtained using the invention as compared to the usual Maghemi production process, in which the Heating and air bubble formation are started at the same time when the iron salt solution and the alkali solution be mixed. 4098 14/1024

Claims (12)

2343 112 claims 1. A process for the production of goethite powder, characterized in that an aqueous iron salt solution with an aqueous alkali hydroxide solution are mixed to form an iron hydroxide suspension that the Iron hydroxide suspension is stirred in an inert atmosphere to form a fine hydroxide suspension form, and then the fine suspension of hydroxide is oxidized to form goethite powder. 2. The method according to claim 1, characterized in that the alkali hydroxide is sodium hydroxide. 3. The method according to claim 2, characterized in that the iron salt is iron sulfate. 4. The method according to claim 3, characterized in that the stirring time is two to four hours. 5. The method according to claim 3, characterized in that the molar ratio of the OH ~ ions to the Fe ions (OH "" / Fe ⁺⁺ ) is over 3. 6. The method according to claim 3, characterized in that - ++ the molar ratio of the OH ions to the Fe ions (OH ~ / Fe ⁺⁺ ) is 3.8 to 5.8. 7. The method according to claim 3, characterized in that the fine hydroxide suspension at a temperature of 40 to 60 ° C is oxidized. 8. The method according to claim 1, characterized in that the alkali hydroxide is potassium hydroxide. 4 0 9 8 1 U / 1 0 2 A 9. The method according to claim 8, characterized in that the iron salt is elsenic chloride. 10. The method according to claim 9, characterized in that the stirring time is 2 to 4 hours. 11. The method according to claim 9, characterized in that the molar ratio of the OH ions to the Fe ions (OH "/ pe ⁺⁺ ) is 3.2 to 4.5. 12. The method according to claim 9, characterized in that the fine hydroxide suspension at a temperature of 50 to 80 ° C is oxidized. 409814/1024 Blank page