NL8204902A - PROCESS FOR PREPARING GOETHITE. - Google Patents

Description

Method of preparing goethite.

The invention relates to a process for the preparation of goethite, which is useful as a raw material for magnetic materials, such as 3-iron powder, magnetite, -Fe 2 Og and the like.

When processing a powdered raw material into a useful product, it is well known that properties derived from the size, shape, crystallinity and the like of the particles play an important role. This also applies to magnetic materials.

The size, shape, crystallinity, and the like of goethite particles, which is a raw material, significantly affect the properties of a magnetic material obtainable from the raw material. The control of the size, shape, crystallinity and the like of goethite particles has been the subject of a great deal of work. Among the physical properties of goethite particles, it is considered the most difficult to control the particle size distribution.

For example, Japanese Laid-Open Patent Application 21720/1977 discloses a production method of goethite in which ferrous hydroxide is vigorously stirred for at least several hours in a non-oxidizing atmosphere in a uniform system, followed by oxidation of the uniform system thus prepared. However, it is difficult to convert a non-uniform flake containing ferrous hydroxide into a sufficiently uniform system merely by stirring the flake.

Another method is disclosed in Japanese Patent Laid-open No. 56196/1978, in which a non-uniform flake containing ferrous hydroxide is subjected to a neutralization reaction in the presence of a soluble silicate to make the flake uniform; and needle-shaped goethite particles are then subjected to a uniform growth reaction. When using the above method in 8204902 #. * 2, however, it is necessary to use soluble silicate in an amount of up to 0.1-1.7 atomic% in terms of Si based on iron. Moreover, Goethite absorbs the silicate and is thus converted into such a form as if it had been diluted by the silicate. As a result, the magnetic properties of iron powder obtainable by reduction of the above silicate-added raw material in a manner known in the art will be poor.

A further method is disclosed in Japanese Patent Laid-Open No. 59095/1977 in which goethite is produced by controlling the oxidation rate of ferrous hydroxide, for example, such that 8 wt%, 25-55 wt% and the remainder of ferrous hydroxide are Oxidized, in turn, at a 0.5-4 hour first stage, a 1.5-6 hour second stage, and a third stage complete the oxidation of ferrous hydroxide in its entirety. In the above method, it is necessary to change the oxidation rate in various ways in the course of the production of goethite, taking a relatively long time for the oxidation to complete. In addition, it is also required to precisely control the oxidation rate.

A still further approach is disclosed in Japanese Patent Laid-open No. 226,2377 / 1981 using seed crystals previously prepared at room temperature. However, this method poses such a potential problem that more magnetite than goethite can occur unless the reaction conditions, especially the temperature conditions, are carefully controlled.

An object of the invention is to provide an improved production method of goethite, which is useful as a raw material for the production of magnetic materials, such as iron powder, magnetite, Fe and the like.

8204902 3

Another object of the invention is to provide uniform goethite particles with an axial ratio of at least 7 and a specific surface area of 20-80 m2 per gram.

A further object of the invention is to provide the goethite with a well-controlled particle size distribution by granulating the oxidation rate of ferrous hydroxide with the crystal growth rate of goethite.

The above objects of the invention can be achieved by interrupting the feeding of an oxidizing gas to a suspension of ferrous hydroxide obtained by reacting a ferrous salt with an alkali metal hydroxide; especially while controlling the oxidation rate of ferrous hydroxide below 7% for each feed-feed-stop cycle of the oxidation gas and also adjust the feed-time of the oxidation gas within 10 minutes and equal to or less than a quarter of the feed-stop time per feed. - stop cycle of the oxidation gas.

Figure 1 is a micrograph (magnification 20-309S9) of goethite particles obtained by interrupted addition of an oxidation gas in Example 4.

Figure 2 is a graph showing the relationship between the slurry viscosity and the specific surface area of goethite particles at a slurry concentration of 1%, and Figure 3 is a micrograph (magnification 30,000 x) of goethite particles obtained by continuous addition of an oxidation gas in the comparative example .

The main feature of the invention is to supply an oxidation gas interrupted to a suspension of ferrous hydroxide. An interrupted supply of the oxidation gas can hardly balance the oxidation reaction of ferrous hydroxide with the growth reactions of obtained goethite particles, thereby yielding acicular goethite with a well-controlled particle size distribution.

8204902 f * 4

The oxidation gas useful in carrying out the process of the invention is an oxygen-containing gas. For example, oxygen, air, or a gas mixture obtained by diluting oxygen or air with another gas can be used effectively.

The interrupted supply of the oxidation gas can be carried out by repeatedly alternating an supply of the oxidation gas with stopping it.

According to the invention, it is preferable to make the supply time of the oxidation gas shorter than its supply stop duration per supply-supply stop cycle of the oxidation gas. Most preferably, the feed time of the oxidation gas is taken to be equal to or less than a quarter of the feed stop time per feed-feed stop cycle of the oxidizing gas. It is preferable to control the rate of oxidation of the ferrous hydroxide below 10%, and most preferably below 7% per feed-feed stop cycle of the oxidation gas. When the oxidation rate exceeds 10%, the above-mentioned oxidation reaction and growth rate are not balanced and thus such a high oxidation rate is undesirable for accomplishing the purposes of the invention.

The oxidation gas is supplied for a period of time sufficient to meet the oxidation rate described above. More specifically, the feed time is generally 20 minutes per feed-stop cycle of the oxidizing gas. The preferred feed time is within 10 minutes per feed-feed stop cycle of the oxidizing gas. When the oxidizing gas is supplied for a period exceeding the 20 minutes per supply-feed stop cycle, a longer period is required to complete each supply-feed stop cycle of the oxidation gas, resulting in a time consuming production of goethite, which production is impractical.

As each feed-feed stop cycle 8204902 becomes 4% 5 shorter, a different problem arises in the reactor in carrying out the oxidation reaction. Namely, if each feed-feed cycle of the oxidizing gas is shortened by interrupting the feeding of the oxidation gas to a slurry of ferrous hydroxide, a fresh feed of the oxidation gas is fed to the slurry, while the previous feed of the oxidation gas is still in the suspension has remained. If the above situation were to occur, the effects of the invention may not be attained. The lower limit of the time per feed-feed stop cycle may vary depending on the type of reactor used to oxidize the slurry. However, the above feed-feed stop cycle should be at least 5 minutes and preferably at least 10 minutes from a practical point of view.

To achieve the objects of the invention even more effectively, it is effective to add a non-oxidizing gas to the slurry and / or the reaction atmosphere while stopping the supply of the oxidizing gas. This comb is easily accomplished by feeding the oxidizing gas and non-oxidizing gas together to the ferrous hydroxide slurry and / or the reaction atmosphere and only by stopping the oxidizing gas supply or by of the oxidizing gas in the supply of the non-oxidizing gas.

The oxidation rate of ferrous hydroxide per feed-feed stop cycle of the oxidizing gas can be varied arbitrarily according to the invention by controlling the feed time of the oxidizing gas, the concentration of an oxidizing component in the oxidizing gas, the feeding rate of the oxidizing gas, etc. The size of the goethite obtained can be changed by varying the oxidation rate per feed-feed stop cycle of the oxidizing gas or the reaction temperature.

According to the invention, one is able to vary the size of goethite within a specific surface range of 20 - 80 m2 per gram measured by the BET isothermal absorption method and still keep the particle size distribution constant. As one feature of the invention, it can be mentioned that the resulting goathite particles have a crystallinity consisting essentially of acicular crystals with a needle ratio of 7 or more and substantially free of branches or shoots. Consequently, the viscosity of each goethite slurry is determined only by the dimensions of the goethite particles. When carrying out the process according to the invention, keeping the ratio of the supplying time of the oxidizing gas to the supplying stop duration period constant, the slurry viscosity increases linearly with respect to the specific surface (a physical quantity, which is the dimensions of the goethite particles) over a specific surface area of 25 - 60 m2 per gram, as illustrated in figure 2. In other words, it is possible to control the dimensions of the goethite particles by controlling the viscosity of a slurry of the goethite particles. This feature has not hitherto been predicted in the prior art, but is a merit typical of the invention. Needless to say, the viscosity control is carried out by controlling the rate of dehydration of ferrous hydroxide per feedstock feed cycle of the oxidation gas.

The term "ferrous salt" as used herein means a water-soluble ferrous salt, which may include ferrous sulfate, chloride, nitrate and the like. They may be used singly or in combination of two or more. It is sufficient to use the grade as it is obtained on an industrial scale, no special purification is required.

The effects of the invention can also be brought about by oxidizing a hydroxide obtained by coprecipitation of iron together with another metal, such as, for example, nickel, zinc, manganese, copper or chromium, to make goethite, that the other contains metal.

Any alkali metal hydroxide according to the invention can be used as long as it undergoes a reaction with a ferrous salt to form ferrous hydroxide. For example, one can effectively use an aqueous solution of potassium hydroxide or sodium hydroxide. The formation and oxidation reactions of ferrous hydroxide can be carried out at temperatures within the range of 20 - 80 ° C, preferably 30 - 50 ° C.

The otherwise obtained goethite particles are needle-shaped and have a well-controlled particle size distribution. They are useful as a raw material for magnetic materials, such as C1 iron powder, magnetite, and the like, which are obtained by removing the goethite particles

and optionally oxidize the otherwise reduced goethite particles in a manner known per se in the art. Example I

In a stainless steel vessel with an internal capacity of 18 liters and equipped with temperature controls and stirrers, 8 liters of caustic soda are added (concentration 1.8 mol per liter). Gas supplies arranged in concentric circles on the bottom of the vessel begin to supply nitrogen gas to the aqueous sodium caustic solution at a flow rate of 5.0 liters per minute.

8 liters of an aqueous solution of ferrous sulfate (concentration 0.226 mol per liter) are added while the sodium hydroxide solution is stirred. When the aqueous solution of ferrous sulfate is added to a large excess of the aqueous caustic solution, a neutralization reaction occurs spontaneously, whereby ferrous hydroxide is obtained as a suspension. Goethite is obtained by interrupted oxidation of the suspension. The temperature of the suspension is kept at 50 ± 1 ° C during the neutralization reaction and oxidation reaction.

In addition, through a gas supply device through which nitrogen gas can flow 8204902 m * 8, air is flowed at a rate of 2.0-3.0 liters per minute. The air supply time is set at 2 minutes, while the supply time of nitrogen gas without feed air is set at 18 minutes. In other words, the oxidizing gas is supplied for 2 minutes, while its addition is stopped for 18 minutes.

Thus, each feed-feed stop cycle of the oxidizing gas lasts 20 minutes. The ferrous content in each suspension is determined in terms of the concentration of divalent iron ions 10 by the titrimetric method, with potassium permanganate.

The oxidation rate is calculated using the following equation:

Total concentration 2+ of iron ions - Fe concentrate-

Oxidation rate tie x 100 (%] 15

Total concentration of iron ions When the slurry develops a vibrant yellow color, the feed-feed stop cycle of the oxidizing gas is terminated. At this step, the oxidation rate is 99.7%. The vessel is exposed to the ambient atmosphere and allowed to stand for 1 hour while stirring the suspension. The viscosity of the slurry is measured with a B-type viscometer after the slurry is left standing, as mentioned above. It amounts to 106 centipoises. Since the feed-feed stop cycle of the oxidizing gas was repeated thirty times in the present example, the total feeding time of the oxidizing gas is calculated as 60 minutes. The slurry contains needle-shaped goethite particles. After washing the slurry with water and then filtering, a wet cake is obtained.

The specific surface area of the goethite particles was measured according to the BET isothermal adsorption method using nitrogen gas. As a specific surface area, a value of 26.4 m2 per gram was obtained. The size of each particle was 10-13 in terms of length to width ratio and the length of each particle was 0.3-0.4 µm.

35 ..........

Examples II-IV

8204902 9

The production of goethite is carried out under the same conditions as in Example 1, but now the reaction temperature in Examples II and III is changed, and both the reaction temperature and the mode of supplying the oxidizing gas in Example 4 are stopped. Nitrogen gas supply is almost simultaneously stopped as the oxygen-containing gas in Example IV. Accordingly, the atmosphere in the reactor is considered to contain oxygen gas even though the supply of the oxidizing gas is stopped. The viscosity of a slurry of goethite and the specific surface area of goethite obtained upon completion of the reactions in each of the examples are shown in taibel A.

Table A

15 Ex. II Ex. III Ex. IV

Reaction temperature (° C) 40_30_40

Slush viscosity (cps) 158 150 118

Specific surface area (ma / g) _59.3_55.5_ 34.9

The slurry viscosity and specific surface area of the goethite obtained in each of Examples II-IV and the preceding Example I are deposited along the horizontal and vertical axes respectively, thereby obtaining a graph as shown in Figure 2. The graph shows, that the specific surface area linearly diminishes with the slurry viscosity. Although there was oxygen gas in the atmosphere in it

Example IV, the obtained goethite sufficiently fulfills the purposes of the invention. A tram-electron micrograph (magnification 30,000 x) of goethite particles obtained in Example IV is shown as Figure 1. Example V

In a stainless steel vessel with an internal capacity of 16 liters and equipped with temperature controls and stirrers, 6 liters of an aqueous solution of sodium hydroxide (concentration of 1.8 moles per liter) are added. By supplying gas supplies arranged in concentric 8204902 circles at the bottom of the vessel, the supply of nitrogen gas to the aqueous solution of sodium hydroxide is started at a flow rate of 5.0 liters per minute.

6 liters of an aqueous solution of ferrous chloride (concentration 0.225 mol per liter) are added while the aqueous solution of sodium hydroxide is stirred. When the aqueous solution of ferrous chloride is added to a large excess of the aqueous solution of sodium hydroxide, a neutralization reaction takes place spontaneously, whereby ferrous hydroxide is obtained as a suspension. By oxidizing the suspension intermittently with an oxidizing gas, iron oxide is obtained. The temperature of the suspension is maintained at 40 ± 1 ° C during the neutralization reaction and oxidation reaction. Air is then flowed at a flow rate of 2.5-3.5 liters per minute through a gas feeder through which nitrogen gas is passed. At the same time, the nitrogen gas flow rate is allowed to decrease from 5 liters per minute to 2.5-3.0 liters per minute. The supplying time of air is set at 2 minutes, while the supplying time of nitrogen is set at 28 minutes only at the flow rate of 2.5-3.0 liters per minute. In other words, the oxidizing gas is supplied for 2 minutes, while its supply is stopped for 28 minutes. Consequently, each feed-feed stop cycle is 30 minutes long. This interrupted supply of the oxidizing gas is repeated and the interrupted oxidation is stopped as soon as the slurry develops a vibrant yellow color. When the oxidation rate is measured at the above step by the same method as used in Example 1, the oxidation rate is found to exceed 99.8%. The vessel is exposed to the surrounding atmosphere and left for 1 hour while stirring the suspension. The viscosity of the slurry is measured with a B-type viscometer and the slurry is allowed to stand as noted above. It amounts to 133-35 centipoises. The specific surface area of the obtained iron 8204902 11 ° C oxyhydroxide in the present example is 34.8 m2 per gram. The total oxidation time is 60 minutes in the present example. The specific surface area of the iron-O * -oxydroxide obtained in the present example is almost equal to that of goethite obtained in Example IV, but the slurry viscosity of the ionO * -oxydoxide is greater than that of the goethite. An observation by a transmission electron microscope shows that each particle of the iron oxide is shorter in the width direction and has a longer length / width ratio compared to the goethite obtained in Example IV. The length to width ratios are 10 to 13 in Examples I to IV, while the length to width ratio is about 15 in the present example.

Example VI

Iron oxide is prepared according to the procedure of Example V, but now the feed time or the feed stop time of the oxidizing gas is changed to 2 and 8 minutes, respectively. The total oxidation time is 60 minutes. The viscosity of the obtained iron oC oxide hydroxide slurry is 193 centipoises, while the specific surface area of the iron oxide is 57.7 m2 per gram. Since the iron oxyhydroxide particles obtained in the present example have lengths spread over a wide range from less than 0.1 micrometers to 0.5 micrometers, it has been found that any oxidation gas feed duration is shorter. than a quarter of a feed stop period of the oxidizing gas is not preferred.

Comparative Example 30 A suspension of ferrous hydroxide is oxidized continuously for 600 minutes, which is equivalent to a total duration for all cycles in each of Examples I-IV. In this comparative example, the oxidation rate is precisely controlled so that it is kept as constant as possible in order to facilitate comparison of this comparative example with the above examples. The oxidation rate is measured every 20 minutes. The variation of the oxidation rate is kept as minimal as possible by adjusting the flow rate of the oxidizing gas. As a result, the oxidation rate is kept almost at a constant level. The viscosity of the iron oxyhydroxide slurry obtained is 165 centipoise, while the specific surface area of the iron O-oxyhydroxide particles thus prepared is 51 m2 per gram. As shown in Example III, the ion o (oxyhydroxide particles have both large and small particles. In the present comparative example, the procedures of Examples I - IV were carefully followed until the suspension of ferrous hydroxide was obtained. The reaction temperature was set at 40 ± 1 ° C.

As is apparent from the above results, the objects of the invention can be effectively achieved by interrupted feeding of an oxidizing gas.

Global rating of particle size and shape.

The mean length, standard deviation and coefficient of variation (= -x 100) mean length of the particles obtained in each of Examples II, III and IV as well as the comparative example are shown in Table 25B.

Ex. II Ex. III Ex. IV Comparison (not shown) (not shown (Fig. 1) example) ........; _ (Fig. 3) mean 30 length_ 0.151 _0.155_0.235_0.166 standard deviation 0.069 0.072_0.112_0.092 variation coefficient 35 (%) _ 45.7 46.5_47 / 7_55.4 8204902 13

According to Table B, the particles obtained in the comparative example have greater variation in length than the particles prepared in any of Examples II, III and IV pertaining to the invention. Furthermore, the coefficient of variation will increase as the length increases. However, the coefficient of variation of the particles obtained in Example IV of the present invention is 14% smaller compared to the result obtained in the comparative example, although the average length of the first-mentioned particles is longer than that of the particles obtained in the comparative example .

8204902

Claims (9)

1. Process for preparing goethite by oxidation of a suspension of ferrous hydroxide obtained by reacting a ferrous salt with an alkali metal hydroxide with an oxidizing gas, characterized in that the oxidizing gas is fed intermittently to the suspension. 2. Process according to claim 1, characterized in that the oxidizing gas is introduced intermittently, while inert gas is continuously supplied. Process according to claim 1, characterized in that the oxidizing gas and an inert gas are introduced alternately. 4. Process according to claim 1, characterized in that a supply time of the oxidizing gas 15 is used which is shorter than the supply stop duration of the oxidizing gas per supply-supply stop cycle of the oxidizing gas. A method according to claim 1, characterized in that the supply time of the oxidizing gas 20 is a quarter or less of the supply stop duration of the oxidizing gas per supply-supply stop cycle of the oxidizing gas. 6. Process according to claim 1, characterized in that an oxidation rate of ferrohydroxide of 10% or less is applied per feed-feed stop cycle of the oxidizing gas. 7. A process according to claim 1, characterized in that a feed time of the oxidizing gas is used of 20 minutes or less per feed-feed stop cycle of the oxidizing gas. 8. Process according to claim 1, characterized in that a supply-feed-stop cycle of the oxidizing gas is used of 5 minutes or longer. 9. Process according to claim 1, characterized in that an oxidation rate of ferrous hydroxide 8204902 Si is used of 7% or less per supply-feed-stop cycle of the oxidizing gas, a supplying time of the oxidizing gas of 10 minutes or less per supply-feed-stop cycle of the oxidizing gas and each feed-supply-stop cycle of the oxidizing gas of 10 minutes or longer. 8204902