WO1996013461A1

WO1996013461A1 - Method of producing magnetic iron oxide

Info

Publication number: WO1996013461A1
Application number: PCT/US1995/011791
Authority: WO
Inventors: Jame W. Krause; John A. Granberg; Gerald G. Endres
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1994-10-31
Filing date: 1995-09-18
Publication date: 1996-05-09

Abstract

A process for producing a magnetic iron oxide having a dark enough color to provide opacity to thin coatings of the magnetic oxide in a binder comprising the steps of: a) reducing α-Fe2O3 in a non-oxidizing atmosphere using an organic reductant at 250 to 600 °C to form an iron oxide precursor having 25 to 45 mole % Fe+2 based on total Fe (i.e., Fe?+2 and Fe+3¿); b) oxidizing the iron oxide precursor to form η-Fe¿2?O3 until carbon content is from 0.5-3 % by weight of the iron oxide particles; and c) heating the η-Fe2O3 in an inert atmosphere at a temperature of about 400 to 550 °C to a magnetic oxide particle having 10 to 40 mole % Fe?+2¿ based on total Fe.

Description

METHOD OF PRODUCING MAGNETIC IRON OXIDE

FIELD OF THE INVENTION

This invention relates to a process for preparing magnetic iron oxide particles.

BACKGROUND OF THE INVENTION

Magnetic iron oxide particles are frequently used in the magnetizable layer of memory storage elements such as video tapes, computer diskettes, etc. The magnetizable layer typically contains the magnetic particles, a polymeric binder, and other additives such as head cleaning agents.

Typically, production of magnetic iron oxides for use in magnetic recording materials begins with formation of iron oxyhydroxide (FeOOH) particles. These particles are dehydrated to nonmagnetic α-Fe₂O₃. The nonmagnetic α-Fe O₃ particles are reduced to magnetite (i.e., Fe₃O , which has a mixture of iron ions having +2 and +3 valences, typically being 33% Fe⁺² based on total iron content) using hydrogen or an organic reducing agent. The magnetite is then partially oxidized or completely oxidized to the magnetic iron oxide γ-Fe₂O₃. The magnetic iron oxide frequently is subsequently surface treated with Co^*2. Use of hydrogen as a reducing agent has several drawbacks, including safety issues arising from the use of highly flammable hydrogen gas and increased likelihood of sintering of the particles. To avoid the problems created by using hydrogen as a reducing agent, organic reducing agents may be used. Unfortunately, when an organic reducing agent is used to reduce the non-magnetic α-Fe₂O₃ particles, carbon containing residuals of the organic reducing agent may remain on the surface of the particles after the desired reduction occurs. These residuals often create an undesirable oily film on the particles which may interfere with subsequent processing. Thus, it would be desirable to find a convenient method of eliminating these carbon containing residuals. Recently there has been a trend toward using thinner coating thicknesses in video recording tape. Thinner coatings lead to less opaque coatings. However, a certain degree of opacity is required in most video recording tapes due to video recording machine operating requirements. Specifically, automatic stopping of the machine is triggered by light transmission through the tape. Thus, the ends of video tape are typically made more translucent than the body of the tape. As thickness of the magnetizable layer is decreased the amount of light transmitted through the video tape will increase. This increased light transmittance through the video tape makes the high light transmittance areas at the ends of the tape difficult to detect. The increased light transmittance could also cause unacceptable stoppages in the middle of the tape. To counteract this increasing light transmission, a darker magnetic layer is desired. Addition of carbon black powder has been one method for reducing light transmittance. Unfortunately, use of carbon black may adversely affect packing and orientation of the magnetic particles.

Increasing the content of Fe⁺² in the magnetic particles is known to create darker particles. However, if the amount of Fe⁺² becomes too high, magnetic properties of the particles will decrease. The Fe⁺² content may be present in the core of the iron oxide particles or may be added by surface modification. However, it is difficult to attain sufficient Fe⁺² content merely from surface modification because several iterations of surface modifying steps are required.

SUMMARY OF THE INVENTION

Applicants have discovered a new method of preparing magnetic iron oxide powders which eliminates substantially all of the carbon containing residuals of organic reductants and produces magnetic iron oxide powders, which have excellent magnetic and electromagnetic properties and a dark color (due to high Fe⁺² content). This method eliminates some steps which are required if the Fe⁺² content is provided primarily from surface modification and, therefore, requires less time and less energy. The process of this invention comprises the steps of: a) reducing α-Fe O₃ in a non-oxidizing atmosphere using an organic reductant at 250 to 600 °C to form an iron oxide precursor having 25 to 45 mole% Fe⁺² based on total Fe (i.e., Fe⁺² and Fe⁺³); b) oxidizing the iron oxide precursor to form γ-Fe₂O₃ until carbon content is from 0.5-3% by weight of the iron oxide particles; and c) heating the γ-Fe₂O₃ in an inert atmosphere at a temperature of about 400 to 550°C to a magnetic oxide particle having 10 to 40 mole% Fe⁺² based on total Fe. Preferably, the resulting magnetic oxide particle is subsequently surface modified with Co⁴² and, optionally, Fe⁺².

DETAILED DESCRIPTION OF THE INVENTION

The α-Fe₂O₃ particles used as starting materials in this invention may be derived by any known method. Preferably, these particles are produced by dehydration of α-FeOOH particles.

The FeOOH used in preparation of the α-Fe₂O₃ of this invention may be derived by any known method. For example, a ferrous sulfate may be reacted with a base to form ferrous hydroxide which is subsequently oxidized to α- FeOOH. As yet another option, a ferrous chloride starting material may be reacted with a base to form ferrous hydroxide which is oxidized to form γ-FeOOH

The iron oxyhydroxide (FeOOH) particles may optionally be treated with an anti-sintering agent such as phosphorous or silica. Preferably, the precursor iron oxyhydroxide particles are washed, filtered, and dried before further processing.

The FeOOH particles are dehydrated to α-Fe₂O₃. The dehydration step may occur in a calciner at temperatures from about 250 to 650°C, preferably 400 to 650°C, and most preferably 600 to 650°C. By using the higher temperatures not only are the particles dehydrated, but the crystal structure of the particles is also modified. The dehydration step may occur in an inert atmosphere or in air. In the first reaction step a) the α-Fe₂O₃ is reduced in a non-oxidizing (i.e., inert or reducing) atmosphere by an organic reducing agent to form an iron oxide precursor having both Fe⁺² and Fe⁺³, wherein the amount of Fe⁺² based on total Fe (Fe⁺² and Fe⁺³) is between 25 and 45%, preferably 30 to 42%, more preferably 35 to 41%. If the amount of Fe⁺² is too low, the particles produced will have a low magnetic moment. Higher levels of Fe⁺² usually can not be produced without encountering a problem of sintering of the particles. The temperature and time required for this reducing step depends in part upon the organic reducing agent. However, the reduction may occur at a temperature of between 250 and 600°C, preferably 400 to 500°C.

Any organic reducing agent known in the art may be used to reduce the α- Fe O₃. Examples of organic reducing agents include long chain carboxylic acids, their esters, in particular fatty acid glycerine esters; alcohols, especially higher alcohols, although lower alcohols such as methanol and isopropanol may be used; hydrocarbons; and polymers, preferably with a molecular weight in excess of 3000, such as polyethylene, polypropylene, polyesters, polyethers, polyamides, and polycarbonates. Long-chain fatty acid glyceride or a mixture of long-chain fatty acid glycerides work well at temperatures of at least 400°C. The long chain may be straight or branched but preferably should have at least 12 carbon atoms. Glycerides which are esters of acids having 16-18 carbon atoms are especially useful. Commercially available examples of suitable reducing agents include Empol™ 1014, 1016, and 1041; Hystrene™ 3695, 3675-C, 5469, and 9718; and Industrene™ 9018. Reducing with an organic reducing agent leaves a carbon- containing residue which may provide an undesirable oily film when the particles are further processed. The amount of organic reducing agent required varies depending on what organic compound is selected. However, for long chain fatty acid glycerides, the amount of organic reducing agent may be up to about 10% by weight of the iron oxide, preferably 4-7% by weight.

Optionally, the dehydration step may be carried out directly with the first reaction step a). In other words, the organic reducing agent may be mixed directly with the iron oxyhydroxide particles and the mixture is heated. What occurs in this situation is that the iron oxyhydroxide is first dehydrated to form α-Fe₂O₃, followed by reduction of some of the Fe⁺³ in the α-Fe₂O₃ to form the iron oxide precursor.

In the second step b), the iron oxide precursor is then oxidized to form γ-Fe₂O₃. Preferably, substantially all the Fe⁺² is oxidized to Fe⁺³, so that the iron oxide is primarily γ-Fe₂O₃. The amount of Fe⁺² is preferably less than 7%, more preferably less than 4%, of the total iron. The oxidation may occur under a variety of conditions, and the degree of oxidation is controlled by a balance of time, temperature, and oxygen content of the process gas. For example, as temperature increases, the time required to attain a given degree of oxidation decreases. Applicants have found that the oxidation may reasonably be undertaken by heating the iron oxide precursor particles in an oxygen containing gas, such as air, to about 200 to 370°C, prefably 275 to 350°C, for about 15 to 90 minutes, preferably 30 to 60 minutes. Using 1 to 10, preferably 2 to 7, standard cubic feet of air per pound (62-623, preferably 125-436 1/kg) of magnetic oxide works well.

Subsequent to the oxidation step, there is still a measurable amount of carbon containing residual. The amount of this carbon containing residual is determined indirectly based upon the amount of carbon in the oxidized particles. Preferably, the amount of carbon is in the range of about 0.2 to 5%, more preferably 0.4 to 3.5%, and most preferably 0.5 to 2% of the total weight of particles. To remove this undesirable residual carbon and to form darker magnetic pigment particles, the γ-Fe₂O₃ is heated or annealed in an inert atmosphere at temperatures of 400 to 550°C, preferably 450 to 510°C. When annealing in an inert atmosphere, secondary reduction occurs and the amount of Fe⁺² increases (and the amount of Fe⁺³ decreases) thereby creating darker magnetic oxide particles. This increase in Fe⁺² is caused by the secondary reduction of the γ-Fe O₃ and the concurrent oxidation of the carbon residuals to gaseous CO, CO₂, and the like. The gaseous oxidation products of the carbon residuals outgas from the particles. The specific surface area of the particles also decreases during the annealing step. The inert atmosphere is preferably nitrogen. The ratio of Fe⁺² to total Fe is in the range of 10-40%, preferably about 15-35%, more preferably about 20-30%, after annealing in an inert atmosphere.

Annealing in air would also serve to decrease or eliminate the undesired carbon containing residuals. However, when annealing is done in air the amount of Fe⁺² does not increase and, therefore, the particles do not become significantly darker.

The amount of carbon removed during the annealing step, whether annealed in air or in an inert atmosphere, is preferably at least 50%, more preferably at least 70%, and most preferably at least 80%, based on the amount of carbon present after the oxidation step b). Preferably no more than 0.5%, more preferably no more than 0.3%, carbon based on total weight of the iron oxide particles remains after annealing. Since the residual carbon is substantially used up by the annealing step, whether in air or inert atmosphere, there should be no oily film which might inhibit subsequent surface modifications.

To conserve the maximum content of Fe⁺² in the resulting magnetic iron oxide, it may be helpful to do subsequent handling of the iron oxide in water or in an inert atmosphere. Handling in inert atmosphere also avoids flammability hazards of the magnetic iron oxide. This restriction is less necessary after surface modification.

The resulting magnetic iron oxide may be surface modified with cobalt or iron ions, as desired. Any known procedure for surface modification may be used. For example, if only cobalt surface modification is desired, the following procedure may be used. While mixing a slurry of the magnetic oxide, a soluble cobalt salt and a compound which will insolubilize the cobalt ions, such as NaOH, are added to deposit cobalt-containing material onto the surfaces of the particles. Typically the slurry has about 5-15% solids. The homogeneous mixture is filtered, and the cake is heated in an inert atmosphere at 80 to 200°C to fix the surface deposit. Heating is discontinued before substantial diffusing of the cobalt ions into the particle cores. By adjusting the amount of cobalt salt in the slurry, the cobalt ions in the coating may be controlled within the desired range, preferably about 1- 10% of the total weight of the particles.

Similarly, if one desires to modify the surface with Fe⁺² and cobalt, a slurry of the magnetic oxides in is prepared. A compound, such as NaOH, is added which will precipitate the Fe⁺² and cobalt. The Fe⁺² is added to the slurry.

Following the addition of the Fe⁺², cobalt ions are added to the slurry. The slurry is then heated in an inert atmosphere.

Following the heating of the surface modified magnetic oxide, the slurry is cooled and washed with deionized water to decrease the remainder of the compound which precipitates the Fe⁺² or Co⁺² and any remaining salts. The magnetic iron oxide is then, preferably, filtered and dried.

Examples

Example 1 - Preparation of γ-Fe₂O₃ A ferrous sulfate solution was precipitated with NaOH to form ferrous hydroxide. The ferrous hydroxide was oxidized at a seeding temperature of 25°C followed by further oxidation at 75 °C to yield alpha iron oxyhydroxide with a specific surface area of 54 m²/g and proper shape and uniformity for producing magnetic iron oxide. The alpha iron oxide was washed and silicon/phosphorous adsorbed on the surface as an anti-sintering agent for particle shape retention during subsequent processing steps. The alpha iron oxyhydroxide was then dried in air at a temperature low enough to prevent dehydration to α-Fe₂O₃.

The alpha iron oxyhydroxide particles were dehydrated in a rotary calciner at between 630°C and 650°C for 31 minutes under a nitrogen purge to form α- Fe₂O₃

The α-Fe₂O₃ was reduced in a rotary calciner to magnetite using 5-6% by weight Hystrene™ 9718 (stearic acid/palmitic acid) based on weight of the iron oxide as a reducing agent at 470 to 490°C for 20-30 minutes with a nitrogen purge. The resultant iron oxide had a ratio of Fe⁺² to total Fe of between 37 and 40% as determined by titration with K₂Cr₂O₇. The iron oxide then was oxidized in a rotary calciner in air at between 320 and 340°C. The resultant magnetic iron oxide was primarily γ-Fe₂O₃ having only 1.2% Fe⁺² based on total amount of iron as measured by titration. The magnetic oxide had residual carbon in the amount of 0.74% by weight of total magnetic oxide measured using LECO™ CS-244 sulfur/carbon analyzer.

Comparative Example 2 - Heat treatment in Air

A portion of the magnetic iron oxide of Example 1 was heat treated in a rotary calciner at between 485 and 505°C with an air/nitrogen 50/50 purge. This heat treatment reduced the amount of residual carbon to prevent re-reduction of the magnetic oxide. The resulting magnetic oxide had no measurable amount of Fe⁺². The amount of residual carbon was reduced to 0.06% by weight.

Comparative Example 3 - Production of Pigments by Partial Oxidation An alpha oxyhydroxide was reduced as in Example 1. The resulting iron oxide was then partially oxidized in 50/50 nitrogen air at 150°C for 40 minutes. The resulting magnetic pigment had 21.3% Fe⁺² based on total amount of iron as measured by titration. The pigment also had 1.67% by weight residual carbon.

Example 4 - Heat treatment in an inert atmosphere

The remainder of the magnetic oxide of Example 1 was heated in a rotary calciner at a temperature of between 485 and 505°C with a nitrogen purge. The resulting magnetic oxide (which was discharged in air) had 13.5% Fe⁺² based on total amount of iron and had 0.06% by weight carbon based on total weight of the magnetic oxide.

Example 5 - Heat treated in inert atmosphere

Iron oxide pigments were prepared as in Example 1 except that oxidation occurred at 315°C for 30 minutes. The oxidized pigments had 1.7% Fe⁺² based on total iron content. The amount of residual carbon was 0.52% by weight of the magnetic oxide. These pigments were then heat treated as in Example 4. The resulting pigments (which were discharged uner water) had 17.4% Fe⁺² based on total iron. The amount of residual carbon was reduced to 0.08% based on weight of the magnetic oxide.

Example 6 - Heat treated in inert atmosphere

Iron oxide pigments were prepared as in Example 1 except that oxidation occurred at 315°C for 25 minutes. The oxidized pigments had 3.4% Fe⁺² based on total iron content. The amount of residual carbon was 1.02% by weight based on weight of the magnetic oxide. These pigments were heat treated as in Example 4. The resulting magnetic pigments (which were discharged under water) had 23% Fe⁺² based on total amount of iron. The amount of residual carbon was reduced during the heat treatment to 0.15% by weight of the magnetic oxide.

All of the samples from Examples 1-6 were tested for %Fe⁺², residual carbon content, σ,, and specific surface area. The results are shown in Table 1.

TABLE 1 MAGNETICS BEFORE SURFACE MODIFICATION

SA (m2/g) %C σ,(emu/g) %Fe⁺²

Example 1 43.6 0.74 70.9 1.2 (comparative)

Example 2 40.6 0.06 70.5 0.0 (comparative)

Example 3 42.7 1.67 65.2 21.3 (comparative)

Example 4 39.2 0.06 75.2 13.5

Example 5 — 0.08 — 17.4

Example 6 — 0.15 — 23.0 Comparative Example 7 - Surface modification

The magnetic iron oxide of Example 2 was dispersed in deionized water using a hammer-type mill. The dispersed slurry was transferred to a 50 gallon (189 1) stirred reactor with sufficient deionized water to have 5.5% solids by weight. NaOH was then added in sufficient amounts to precipitate Co⁺⁺ and Fe⁺⁺ ions and raise the normality to 1.0. While stirring the reactor and purging with nitrogen a 0.074 g/ml solution of Fe⁺² ions was added at a rate of 70 ml/min. The amount of Fe⁺² ions added was 4.0% based on weight of the magnetic iron oxide particles from Example 2 which were charged to the reactor. The reactor was then heated to 9PC for 30 minutes followed by cooling to 24°C. Another 4.0% of Fe⁺² ions were added to the reactor at a concentration of 0.074 g/ml and a rate of 70 ml/min. Again the reactor temperature was raised to 91 °C and held there for 30 minutes before cooling to 24°C.

After these initial iron precipitations and thermal treatments, 3.5% of Fe⁺² ions, based on weight of the magnetic iron oxide particles from Comparative

Example 2 which were charged to the reactor plus the weight of the first two Fe⁺² additions, at a concentration of 0.074 g/ml were added to the reactor at a rate of 70ml/min. When the addition of the Fe⁺² ions was complete, 1.75% cobalt ions, based on weight of the magnetic iron oxide particles from Comparative Example 2 which were charged to the reactor plus the weight of the first two Fe⁺² additions, at a concentration of 0.098 g/ml were added at a rate of 40 ml/min. The reactor was then heated to 91°C for 10 hours. The reactor was cooled to 24°C and another 3.5% of Fe⁺² ions at 0.074 g/ml and 70 ml/min were added. Another 1.75% cobalt ions at 0.098 g/ml and 40 ml/min were added next. The reactor was again heated to 91°C for 5 hours with a purge of nitrogen. The reactor was cooled to room temperature and the magnetic oxide was washed with deionized water to remove excess NaOH and remaining salts. The sample was filtered and dried in an inert atmosphere. Example 8 - Surface modification

This example shows the benefit in reduction of surface modifying steps required when secondary reduction is used. Magnetic iron oxide (125 g) from Example 4 was dispersed in 625 g of deionized water with a hammer-type mill. The dispersion was charged to a 2 liter stirred reactor and 615 g of deionized water was added to give about 8% magnetic oxide by weight. 162.5 g of 50% NaOH solution was added and the reactor was closed and purged with nitrogen gas. With the reactor agitation set a 800 rpm, 143.8 grams of ferrous sulfate solution containing 8.78 g of Fe⁺² (7.0% based on weight of magnetic oxide) was added at a rate of 6.3 ml min. Following the addition of the ferrous sulfate, 54.9 g of cobalt sulfate containing 4.39 g of Co(II) (3.5% based on weight of magnetic oxide) was added at a rate of 3.1 ml/min. The nitrogen purge was discontinued, the reactor sealed and the temperature raised to 85°C for 160 minutes. The reactor was cooled to room temperature, the slurry was transferred to a wash tank and washed with deionized water by dilution and decanting to a filtrate conductivity of less than 500 micro-siemens. This sample was filtered and dried in an inert atmosphere.

Example 9 - Surface modification Magnetic iron oxide (81.6 g) from Example 4 was dispersed in 638.4 g of deionized water with a hammer-type mill. The dispersion was charged to a 2 liter stirred reactor and 745 g of deionized water was added to give about 5% magnetic oxide by weight. 149.5 g of 50% NaOH solution was added and the reactor was closed and purged with moisturized nitrogen gas. With the reactor agitation set a 800 rpm, 73.5 grams of ferrous sulfate solution containing 4.49 g of Fe⁺² (5.5% based on weight of magnetic oxide) was added at a rate of 20.4 ml/min. Following the addition of the ferrous sulfate, 28.0 g of cobalt sulfate containing 2.24 g of Co(II) (2.75% based on weight of magnetic oxide) was added at a rate of 9.1 ml/min. The nitrogen purge was discontinued, the reactor sealed, and the temperature raised to 85°C for 160 minutes. The reactor was cooled to room temperature, the slurry was transferred to a wash tank and washed with deionized water by dilution and decanting to a filtrate conductivity of less than 500 micro-siemens. This sample was filtered and dried in an inert atmosphere.

Example 10 - Surface modification

Magnetic iron oxide (80 g) from Example 4 was dispersed in 504 g of deionized water with a hammer-type mill. The dispersion was charged to a 2 liter stirred reactor and 839 g of deionized water was added to give about 5% magnetic oxide by weight. 156.7 g of 50% NaOH solution was added and the reactor was closed and purged with moisturized nitrogen gas. With the reactor agitation set a 800 rpm, 11 1.4 grams of ferrous sulfate solution containing 6.8 g of Fe⁺² (8.5% based on weight of magnetic oxide) was added at a rate of 3 ml/min. Following the addition of the ferrous sulfate, 42.5 g of cobalt sulfate containing 3.4 g of Co(II) (4.25% based on weight of magnetic oxide) was added at a rate of 1.3 ml/min. The nitrogen purge was discontinued, the reactor sealed and the temperature raised to 85°C for 160 minutes. The reactor was cooled to room temperature, the slurry was transferred to a wash tank and washed with deionized water by dilution and decanting to a filtrate conductivity of less than 500 micro- siemens. This sample was filtered and dried in an inert atmosphere.

Comparative Example 11

The oxide from Example 1 was dispersed in deionized water using a hammer-type mill. The dispersed slurry was transferred to a 2 liter stirred reactor with sufficient deionized water to have 5.5% solids by weight. NaOH was then added in sufficient amounts to precipitate Co⁺⁺ and Fe⁺⁺ ions and raise the normality to 1.0. While stirring the reactor and purging with nitrogen, a 0.074 g/ml solution of Fe⁺² ions was added at a rate of 2 ml/min. The amount of Fe⁺² ions added was 4.0% based on weight of the magnetic iron oxide particles from Example 1 which were charged to the reactor. The reactor was then heated to 90°C for 1 hour followed by cooling to 24°C. Another 4.0% of Fe⁺² ions were added to the reactor at a concentration of 0.074 g/ml and a rate of 2 ml/min. Again the reactor temperature was raised to 90°C and held there for 30 minutes before cooling to 24°C.

After these initial iron precipitations and thermal treatments, 3.5% of Fe⁺² ions, based on weight of the magnetic iron oxide particles from Example 1 which were charged to the reactor plus the weight of the first two Fe⁺² additions, at a concentration of 0.074 g/ml were added to the reactor at a rate of 2 ml/min. When the addition of the Fe⁺² ions was complete, 1.75% cobalt ions, based on weight of the magnetic iron oxide particles from Example 1 which were charged to the reactor plus the weight of the first two Fe⁴² additions, at a concentration of 0.098 g ml were added at a rate of 1 ml/min. The reactor was then heated to 85°C for 10 hours. The reactor was cooled to 24°C and another 3.5% of Fe⁺² ions at 0.074 g/ml and 2 ml/min were added. Another 1.75% cobalt ions at 0.098 g/ml and 1 ml/min were added next. The reactor was again heated to 85°C for 4.5 hours with a purge of nitrogen. The reactor was cooled to room temperature and the magnetic oxide was washed with deionized water to remove excess NaOH and remaining salts. The sample was filtered and dried in an inert atmosphere.

Comparative Example 12 The iron oxide from Comparative Example 3 was surface modified as in

Example 8.

Example 13

The iron oxide from Example 5 was dispersed in 625 g of deionized water with a hammer-type mill. The dispersion was charged to a 2 liter stirred reactor and 615 g of deionized water was added to give about 8% magnetic oxide by weight. 155.2 g of 50% NaOH solution was added and the reactor was closed and purged with nitrogen gas. With the reactor agitation set a 800 rpm, 123.4 grams of ferrous sulfate solution containing 7.5 g of Fe⁺² (6.0% based on weight of magnetic oxide) was added at a rate of 6.3 ml/min. Following the addition of the ferrous sulfate, 43.5 g of cobalt sulfate containing 3.48 g of Co(II) (2.8% based on weight of magnetic oxide) was added at a rate of 3.1 ml/min. The nitrogen purge was discontinued, the reactor sealed and the temperature raised to 95°C for 210 minutes. The reactor was cooled to room temperature, the slurry was transferred to a wash tank and washed with deionized water by dilution and decanting to a filtrate conductivity of less than 500 micro-siemens. This sample was filtered and dried in an inert atmosphere.

Example 14

The iron oxide from Example 6 was surface modified as in Example 13.

Example 15 - Preparation of tape samples

Dispersions of the magnetic oxide pigments from examples 7-14 were prepared by adding 16 g of the magnetic iron oxide pigmients to 1.5g of dispersing agent (a blend of EMCOL™ and phosphorous based polyethylene oxide coupling agent) and 7.2 g of toluene. This mixture was placed in a media mill with 200g of stainless steel media. The mill was shaken for 25 minutes. The polymeric binder solution (7.25 g) and methylethylketone (8.1 g) were then added to the shaken mixture. The polymeric binder solution was prepared by mixing 122 parts of Vinylite™ from Union Carbide Co. with 36 parts dioctyl phthalate and 263 parts methylethylketone. The mixture, including magnetic iron oxide pigments and binder, was shaken for an additional 15 minutes and then coated onto a backing. The magnetic particles were oriented using a 1600 Gauss longitudinal orientation field. The handspreads were slit to 1/4" (0.635 cm) for testing of the magnetic properties.

The coated samples were tested for coercivity (He), squareness (Sq), switching field distribution (SFD), and retentivity (Br). The particles were tested before coating for the Fe⁴² content and saturation magnetization (σ_s). The results are shown in Table 2.

SUBSTITUTE SHEET (RULE ; !6)

TABLE 2 MAGNETICS AFTER SURFACE MODIFICATION

Precursor He Sq SFD Br σ,(emu/g) %Fe⁴² particle (Oe) (Gs)

CO Example 7 Example 2 791 0.85 0.28 1648 80.6 16.7

CD CO (comparative)

Example 8 Example 4 751 0.88 0.26 1620 81.5 17.6

CO

D Example 9 Example 4 717 0.88 0.25 1623 81.3 15.9 4 I

Example 10 Example 759 0.88 0.26 1677 81.5 18.7 |

33 c Example 1 ι Example 11 720 0.85 0.29 1552 79.8 16.5 m —r r (comparative) en

Example 12 Example 3 823 0.84 0.32 1156 73.9 23.9 (comparative)

Example 13 Example 5 702 0.87 0.28 1709 84.2 21.5

Example 14 Example 6 702 0.87 0.28 1563 85.3 25.7

_ _____ ___— — ——_——_. ___ ,

Claims

What is claimed is

1 A process for making magnetic iron oxide which comprises the steps of: a) reducing α-Fe₂O₃ using an organic reductant at 250 to 600°C to form an iron oxide precursor having 25 to 45 mole% Fe⁴² based on total Fe; b) oxidizing the iron oxide precursor to γ-Fe₂O₃ until residual carbon content is from 0.3 to 5% by weight of γ-Fe₂O₃; and c) heating the γ-Fe₂O in an inert atmosphere at temperatures of 400 to 550°C to produce a magnetic iron oxide having 10 to 40 mole% Fe⁴² based on total Fe

2 The process of claim 1 wherein the magnetic iron oxide has 15 to 35 mole% Fe⁺² based on total Fe.

3. The process of claim 1 wherein the organic reductant comprises a long-chain fatty acid glyceride

4 The process of claim 1 further comprising the step of preparing the α-Fe₂O₃ by dehydrating iron oxyhydroxide at temperatures between 600 to

650°C.

5 The process of claim 4 further comprising adding an anti-sintering compound to the iron oxyhydroxide prior to dehydrating

6 The process of claim 1 further comprising, after the heating step c), the step of depositing cobalt ions on the surface of the magnetic oxide particles and heating in an inert atmosphere to fix the surface deposit

7. The process of claim 1 further comprising, after the heating step c), the step of depositing Fe⁴² ions on the surface of the magnetic oxide particles and heating in an inert atmosphere to fix the surface deposit.

8. The process of claim 1 wherein the residual carbon content is from

0.4 to 4% by weight of the iron oxide particles.

9. The process of claim 1 wherein the residual carbon content after the heating step c) is less than 0.5% by weight of the magnetic iron oxide particles.

10. The process of claim 1 wherein the residual carbon content after the heating step c) is less than 0.3% by weight of the magnetic iron oxide particles.

11. The process of claim 1 wherein the heating step c) reduces the amount of residual carbon by at least 50%.

12. The process of claim 1 wherein the heating step c) reduces the amount of residual carbon by at least 70%.

13. The process of claim 1 wherein the heating step c) reduces the amount of residual carbon by at least 80%.

14. The process of claim 1 wherein the amount of organic reductant is up to 10% by weight of the α-Fe₂O₃.

15. The process of claim 4 wherein the amount of organic reductant is 4 to 7% by weight of the α-Fe₂O₃.