GB2076431A - Highly orientable iron particles for magnetic recording - Google Patents

Abstract

Acicular iron oxide particles are treated with an aqueous solution of sodium tripolyphosphate, and dried to produce a matrix. The oxide is reduced by subjecting the matrix, to a reducing gas at an elevated temperature. The reaction time and consumption of hydrogen gas during the reduction is decreased if the reduction is carried out in the presence of a dehydrating agent (e.g. calcium metal, hydride or carbide).

Description

SPECIFICATION Highly orientable iron particles This invention relates to a method of producing highlyorientable iron particles, by reduction of ferric oxide. The invention also relates to the use of a dess icant during the reduction.

The magnetic particles used in making magnetic recording elements, such as magnetic tapes, gener- ally consist of acicular gamma ferric oxide. It has long been recognized that iron itself would be superior to gamma ferric oxide with respect to signal-to-noice ratio, magnetic moment and coercive force.

8 Iron particles have not been used to any great extent in magnetic recording, despite the obvious advantages of iron, for the reason that in preparing the iron particles by known processes, the particles do not have the desired acicular shape and therefore are difficult to orient.

Chemical reduction processes have been used to prepare pure iron particles but the product may not have the desired acicular shape, and furthermore, the process is extremely expensive to carry out. The iron particles do not fit well into established proce dures for processing pigments and preparing tapes or other magnetic storage media.

Others have proposed gaseous reduction methods of iron oxide particles but such processes ordinarily result in a metallic alloy pigment which does not lend itself to being magnetically oriented. Presum ably, such gaseous reduction processes result in sin tering and the destruction of the desired acicular shape of the iron particles.

In accordance with the present invention, a method is provided for rendering a magnetic iron oxide precursor which can be subjected to gaseous solid state reduction to give a metallic magnetic alloy pigment of high energy and which can be incorporated into a magnetic tape coating formula tion with a high degree of magnetic orientability.

The present invention provides a method whereby an iron oxide particle having the desired shape can be coated with an inorganic surfactant layer, i.e.

sodium tripolyphosphate, and dried. This has the fol lowing advantages: - (a) Particles are kept separated; (b) Inter particle sintering by mass diffusion is pre vented, and (c) A material matrix is set up.

After standard solid state gaseous reduction, this material can be in the form of a magnetic metallic alloy and exhibits and/or possesses: (a) a higher degreed of magnetic orientation when incorporated into a magnetic storage medium than material which is untreated or treated with other organic materials; (b) mechanical integrity in the form of a material matric when still in the pigment state just after reduction; (c) a phosphate residue which is compatible with formulations used for incorporating magnetic pig ments into various information storage formats, and (d) a high degree of stability in the pigment form or final storage format.

Such a surfactant layer is assumed to be caused by a certain bonding between phosphorous and iron ions through the oxygen ions, although this invention is not predicted on any theory of operation. The thus coated material can then be subjected to a solid state gaseous reduction in the presence of a reducing agent, such as hydrogen, to provide a magnetic metal or magnetic metallic alloy which possesses a high degree of magnetic orientation when formulated into tape, and mechanical integrity. This matrix can be easily ground up to release the particles, and, in fact, the normal grinding of the pigment in the manufacture of magnetic tapes is adequate for this purpose so that a separate grinding step is not necessary. The amount of the matrix material is so small that it does not degrade the tape characteristics.

It should also be noted that the direct reduction of iron oxide with hydrogen is difficult. Iron oxide forms a compact mass and it is difficu It to secure intimate contact between the solid oxide and hydrogen. The process is very wasteful of expensive hydrogen; as the oxide reacts with hydrogen, water is formed which inhibits the reaction.

The second aspect of the present invention is to provide a desiccating agent in the reduction apparatus. By absorbing water of reaction as it is formed, the utilization of hydrogen is greatly enhanced, making the process more economically feasible. Further, the reaction time and/ortemperature can be reduced and the reaction goes more nearly to completion.

In the reduction of iron particles, the starting materials can be either red or yellow acicular ferric oxide or acicular gamma ferric oxide, and these are reduced to metallic iron in a stream of a reducing gas, e.g. hydrogen gas. Preferably this is conducted at a relatively low temperature to prevent sintering and to preserve the acicular particle shape. Normally, the temperature must not be over 450"C and preferably is not over 340"C. There is no lower limit as to the temperature except that the temperature must be high enough to make the reaction go fast enough to be economically feasible. Normally the minimum practical temperature of the conversion is about 275"C.

In carrying out the process, the iron oxide starting material is stirred with an aqueous solution of sodium tripolyphosphate and then dried. The proportion of the sodium tripolyphosphate to the iron oxide vary from ito 10% by weight and preferably to about 4%. In many instances, it is preferred to repeat the treatment prior to reduction. Thus, one can treat with the solution, dry the product and then treat again with the solution, followed by a second filtration, a second drying and subsequent reduction.

As already indicated, by provision of a desiccating agent in the reduction apparatus, water of reaction is absorbed as it is formed. As desiccants, calcium hydride, calcium carbide, metal calcium or mixtures thereof can be used. The preferred desiccant is calcium metal which is effective and inexpensive.

Another effective desiccant is a mixture of calcium hydride and calcium metal in a ratio of 3 to 1 to 1 to 3.

Normally about two parts by weight of desiccant are used per part of the iron oxide starting material (2:1) although this ratio is not critical and wide departures can be made; e.g., ratios of 0.65:1 to 3:1.

In carrying out the process when a desiccant is used, a dry matrix, produced in the manner described above is placed in a heated reaction vessel in the presence of the desiccant and, e.g. hydrogen is passed through the vessel.

Maximum utilization is obtained of the desiccant when a continuous feed process reduction is carried out using counter-current flow. Thus, incoming iron oxide is in proximity with used desiccant at one end of the system, while almost completeiy reduced iron is in proximity with fresh desiccant Alternate embodiments employ counter-current batch processing where in a series of stages fresh iron oxide is treated with used desiccant at a first stage and last stage iron reduction is carried out in the presence of fresh dessicant However, the benefits of the invention can be obtained to a large extent even in a batch process.

The matrix makes it easy to separate the spent desiccantfromthe reacted mass since the matrix particles are much courser than the desiccant particles and the latter may be removed by sifting.

The following Examples illustrate various preferred embodiments of the present invention, and by comparison with controls, advantages of implementation of this invention can be demonstrated.

Example 1 The starting material was yFe2Owhich had an average particle size of 1.0 micron by 0.15 micron. To 300 grams of the y ferric oxide, there was added a solution consisting of 7 litres of pure water with 12 grams of sodium tripolyphosphate dissolved therein. This was placed in a mixer and mixed overnight. The resulting mixture was then filtered to produce a moist filter cake and this was dried by placing it is a drying oven at 100"C overnight. The dried material was then placed in a porcelain combustion boat in an electric furnace, and hydrogen gas introduced into the tube at a rate of 2 SCFH. After the tube had been purged with hydrogen gas for ten minutes, the tube was heated to a temperature of 340"C for two to 20 hours.At the end of this time, the produce is allowed to cool to room temperature and oxygen is slowly introduced in the presence of an inert gas; this stabilizes the material.

Acontrol was prepared by reducing a second sample of the same magnetic oxide under exactly the same conditions except thatthe treatment with sodium tripolyphosphate was omitted.

The making of magnetic recording media, such as tapes or discs is well known to those skilled in thd art. Atypical teaching is found in the Ampex Graubert Patent No.3,320,090 wherein a mixture of two resins, phenoxy and polyurethane, serve to bind the magnetic particles to a backing material. In a typical manufacturing operation the magnetic pigment is first ground, e.g. in a ball or sand mill, with solvents to produce a pigment dispersion. The matrix material of the present invention is easy to break up and disperse in solvents in this usual preliminary pigment dispersing operation so that the reduced particles can be utilized in the usual tape making operation as soon as removed from the reactor in stabilized form without any additional processing.

The amount of the matrix is so small that it does not appear to degrade the tape.

The two samples were then made up into a magnetic tape formulation using a standard resin formulation such as described above. The first step of the tape making operation consists of grinding the stabilized particles in a solvent two produce a dispersion. Both samples of freshly coated tape were oriented by placing them in a magnetic field.

The magnetic properties of the tape were then tested with the following results.

Tripolyphosphate Control Treated Squareness (Mr/Ms) .80 .68 Coercivity (c) 960 Oe 960 Oe Saturation Moment 140 emulgm 135 emu/gm Squareness Sum 1.16 1.13 The use of a desiccant in the reduction is illustrated by Examples 2 to 14. Example 2 is a control in which no desiccant is used.

In each Example the method of Example 1 was exactly followed up to the stage of drying the moist filter cake in a drying oven at 100"C, overnight. The dried material was then mixed with the desiccant if used, and placed in a porcelain combustion boat in an electric furnace, and hydrogen gas introduced into the tube. After the tube had been purged with hydrogen gas for ten minutes, the tube was heated to a temperature and for a time period assetforth in the Examples. At the end of this time, the product is allowed to cool to room temperature and oxygen is slowly introduced in the presence of an inert gas; this stabilizes the material.

Example2 (Comparative Example-Control for Examples 3-14) Sodium tripolyphosphate treated tFe203 reduced in H2.

Weight of sodium tripolyphosphate treated zFe203 = 4.2 gm Hydrogen flow rate = 16 cu ft/hr Reduction time = 9 hrs.

Reduction temperature - 340 C Tape magnetics: Br/Bs = .80 Hc = 960 Oe Example 3 Sodium tripolyphosphate treated ,'Fe3O3 reduced in the presence of CaH2 and hydrogen.

Weight of sodium tripolyphosphate treated zFe203 = 4.4 cam Wt. of CaH2 = 8.8 gm Hydrogen flow rate = 2 cu ft/hr Reduction time = 7 hrs Reduction temperature = 340 C Tape magnetics: Br/Bs = .79 -.80 Hc = 970 Oe.

By comparison with Example 2 Example 3 shows that the hydrogen flow rate can be reduced by a factor of 8 and the reaction time reduced by employing calcium hydride.

Example 4 Sodium tripolyphosphate treated yFe2O reduced in the presence of CaH2 and hydrogen.

Weight of sodium tripolyphosphate treated eFe203 = 250 gm Wt. of CaH2 = 500 gm Hydrogen flow rate = 2 cu ft/hr Reduction time = 12 hrs Reduction temperature = 340"C Tape magnetics: Br/Bs = .77 -.80 Hc = 900 Oe Example 5 As received Fe2O3 reduced in H2 only. Ratios of CaH2/Fe2O3 from .65 to as much as 3/1 have been utilized and reduce the hydrogen consumption by 85% to 90%. Further, the ideal ratio of CaH2 to yFe2O2, which is 2/1, allows for repeated use of CaH2 and has been successively reused for up to three reductions.

Example 6 Sodium tripolyphosphate treated 7Fe2O3 = 5.5 gm WeightofCaH2 = 11 gm Reduction time = 8 hrs Reduction temperature = 3400C H2 flow rate = 2 cu ft/hr Tape magnetics: Br/Bs = .78 Hc = 980 Example 7 Weight of sodium tripolyphosphate treated yFe2O3 = 5.3 gm WeightofCaH2 = 11 gm Reduction time = 8 hrs Reduction temperature = 340"C H2 flow rate = 2 cu ft/hr.

Example 8 Same as Example 6 using same CaH2 as in 7 (third use). The oxide was reduced to only 88% of completion use. Same operating condition as Example 6.

Complete reduction can obtained using somewhat longer reduction times.

The above Examples show that the calcium hydride can be reused several times with little loss of efficiency.

Example 9 Sodium tripolyphosphate treated tyFe203 reduced in the presence of CaH2 and H2 is a rotating kiln with ounter-currnnt flow of iron oxide and desiccant: Kiln charge 5 kg CaH2 2.5 Sodium Tripolyphosphate-7Fe2O3 H2 flow rate 8 CFH Temperature 330"C Reduction time 19 hours Tape magnetic Hc 950 Oe Br/Br 0.78 - 0.80 Example 10 2.5 kg ofyferric oxide was mixed with 3.15 mg of calcium hydride and 1.05 kg of calcium. The oxide was then reduced at 330"C with hydrogen, flow rate 10 CFH, for 72 hours. The resulting product was cooled, stabilized and evaluated both as to the parti cles per se and magentic tapes made from the parti cles.

Example 11 2.5 kg of y ferric oxide was mixed with 2.15 kg of calcium and 2.1 kg of calcium hydride. This material was reduced at 330"C with hydrogen, flow rate 10 CFH, for 72 hours. The resulting product was cooled, stabilized and evaluated as in Example 2.

Example 12 2.5 kg ofyferric oxide was mixed with 3.15 kg of calcium and 1.05 kg of calcium hydride. The oxide was reduced at 330 C using hydrogen gas, flow rate 10 CFH, for 51 hours. The results product was cooled, stabilized and evaluated as in Example 2.

Example 13 2.5 kg of ferric oxide was mixed with 4.2 kg of calcium. This material was reduced at 330"C with hydrogen, flow rate 10 CFH, for 73 hours. The resulting product was cooled, stabilized and evaluated as in Example 2.

Example 14 2.5 kg of ferric oxide was mixed with 3.1 kg of calcium and 1.05 kg of calcium carbide. The oxide was reduced at 330"C with hydrogen, flow rate 10 CFH, for 72 hours. The resulting product was cooled, stabilized and evaluated as in Example 2.

The magnetic properties of the metallic iron particles made in accordance with the foregoing Examples 10-14 were measured and also samples of the various iron particles were combined with a resin binder and used to make a magnetic tape. The properties of the tape were measured. The following results were obtained, both on the iron particles as made in accordance with the foregoing examples and also with practical magnetic tapes fabricated and utilizing these particles.

MA EMETIC RESUL TS Desiccants * Powder Properties * * Tape Reluctant Square- Properties Example (Weight oS ness Square No Ratio) Hc(oe) (Emu/g) lBrlBs) Hchoe) ness 10 CaHCa (3/1) 999 148 0.47 960 0.81 11 CaH2lCa(1/1) 966 138 0.45 917 0.80 12 CaH2/Ca (1/3) 965 136 0.45 932 0.79 13 Ca 1011 140 0.46 939 0.79 14 Ca/CaC2 (3/1) 999 148 0.47 960 0.81 * Happ: 8Koe D.C.

* * Happ: 5Koe D.C.

Claims (16)

1. A process for reducing an acicular iron oxide to produce acicular metal particles suitable for magnetic recording comprising treating the oxide particles with an aqueous solution of sodium tripolyphosphate, drying the thus coated oxide to produce a matrix, and reducing the oxide by subjecting the matrix to contact with a reducing gas at an elevated temperature.

2. A process as claimed in claim 1, wherein the proportion of the polyphosphate to the iron oxide is from into 10% by weight.

3. A process as claimed in claim 1 or claim 2, wherein the iron oxide is y ferric oxide.

4. Aprocess as claimed in any one of claims 1 to 3 and having the following additional steps: (a) Cooling the matrix to ambient temperature, (b) contacting the matrix to produce stabilized iron particles, and (c) grinding the thus stabilized iron particles in a solvent two produce a dispersion of iron particles in a solvent medium.

5. A process, as claimed in any one of claims 1 to 4, wherein the matrix contact with hydrogen at an elevated temperature is effected in the presence of a dehydrating agent.

6. A process as claimed in claim 5, wherein the proportion of the dehydrating agent to the iron oxide matrix is from 0.65:1 to 3:1 by weight.

7. A process as claimed in claim 6, wherein the proportion of the dehydrating agent to the iron oxide matrix is 2 to 1 by weight

8. A process as claimed in any one of claims 5 to 7, wherein the dehydrating agent is calcium hydride, calcium carbonate, calcium metal, or a mixture thereof.

9. A process as claimed in any one of claims 5 to 8, wherein the dehydrating agent is calcium metal.

10. A process as claimed in any one of claims 5 to 8, wherein the dehydrating agent is calcium hydride.

11. A process as claimed in any one of claims 5 to 8, wherein the dehydrating agent is a calcium hydride and calcium metal mixture.

12. A process as claimed in any one of claims 5 to 8, wherein the dehydrating agent is a calcium carbide and calcium metal mixture.

13. A process as claimed in any one of claims 5 to 12, operated as a continuous process with oxide particles and dehydrating agent moving in counter currentflowthrough a heated reaction zone.

14. A process as claimed in any one of claims 1 to 13, wherein the coating and drying stages of the process of the matrix formation are repeated.

15. A process for reducing acicular ixon oxide to produce acicular metal particles substantially as hereinbefore described in any one of the Examples.

16. A magnetic recording element including acicular metal particles which have been prepared buy a process as claimed in any one of claims 1 to 14.