METHOD FOR PRODUCING OXIDE COATED IRON POWDER OF CONTROLLED RESISTANCE FOR ELECTROSTATIC COPYING SYSTEMS
United States Patent 3767477
Oxide coated iron powder required as a carrier in electrostatic copying systems has a uniform oxide film, and has a resistance which can be selectively controlled between 102 and 1010 ohms, when produced by a five step program of: Step 1 -- fluidizing iron powder in air while heating to a temperature between 500° and 750°F, to initiate an exothermic reaction; and thereafter to a higher peak temperature between 750° and 1600°F, desirably to about 950°F, principally by the heat of exothermically oxidizing iron, which can be supplemented by external heating. Step 2 -- introducing an inert gas such as N2 into the air and maintaining the temperature of the iron powder essentially constant at its peak temperature. Step 3 -- discontinuing the flow of air but maintaining fluidization by introducing a stream of inert gas such as N2, Ar or He while reducing the temperature to between 125° and 700°F. Step 4 -- introducing air into the inert gas and cooling the fluidized iron powder to a temperature between 25° and 50°F below that at the end of step 3. The temperature at the beginning of this step determines the resistance of the product, the lower the temperature the lower the resistance, and vice versa. Step 5 -- discontinuing the inert gas but maintaining fluidization in air while cooling the iron powder to a lower temperature suitable for removal.
US Patent References:
Magnetic testing material
Jacobs - April 1940 - 2197931
High frequency core material and core
Neighbors - December 1944 - 2365720
Magnetic testing material
Jacobs - April 1940 - 2197931
High frequency core material and core
Neighbors - December 1944 - 2365720
Inventors:
Mccabe, John M. (Rochester, NY)
Perlowski, John S. (Rochester, NY)
Schur, Ronald J. (Rochester, NY)
Perlowski, John S. (Rochester, NY)
Schur, Ronald J. (Rochester, NY)
Application Number:
05/212172
Publication Date:
10/23/1973
Filing Date:
12/27/1971
View Patent Images:
Export Citation:
Assignee:
Eastman Kodak Company (Rochester, NY)
Primary Class:
Other Classes:
423/634, 430/136, 427/213, 427/216
International Classes:
G03G9/107; C23C11/08
Field of Search:
148/6.35,20.3 117/31,DIG.6,26 423/634
Primary Examiner:
Whitby, Edward G.
Claims:
We claim
1. A method for producing oxide-coated iron powder of controlled resistance suitable for use as a carrier in an electrostatic copying system, said method comprising continuously fluidizing with gas a mass of iron powder in a reaction chamber while oxidizing said powder particles in a five step program consisting essentially of:
2. A method in accordance with claim 1 wherein said inert gas in steps 2, 3 and 4 is nitrogen.
3. A method in accordance with claim 1 wherein the temperature at point D is about 700°F for maximum powder resistance of between 109 and 1010 ohms.
4. A method in accordance with claim 1 wherein the temperature at point D is about 125°F for minimum powder resistance of 102 ohms.
5. A method in accordance with claim 1 wherein, in step 4, said fluidized iron powder is cooled to a temperature between about 25° and about 50°F below temperature D.
6. A method in accordance with claim 1 wherein the temperature D is above the ignition temperature of said iron powder, and wherein in step 4 said air is introduced into said stream of inert gas.
7. A method in accordance with claim 1 wherein the temperature D is below the ignition temperature of said iron powder, and wherein in step 4 said air supplants said inert gas, and the cooling of fluidized iron powder in air proceeds through steps 4 and 5 to said lower temperature suitable for removing the final oxidized product from said reaction chamber.
1. A method for producing oxide-coated iron powder of controlled resistance suitable for use as a carrier in an electrostatic copying system, said method comprising continuously fluidizing with gas a mass of iron powder in a reaction chamber while oxidizing said powder particles in a five step program consisting essentially of:
2. A method in accordance with claim 1 wherein said inert gas in steps 2, 3 and 4 is nitrogen.
3. A method in accordance with claim 1 wherein the temperature at point D is about 700°F for maximum powder resistance of between 109 and 1010 ohms.
4. A method in accordance with claim 1 wherein the temperature at point D is about 125°F for minimum powder resistance of 102 ohms.
5. A method in accordance with claim 1 wherein, in step 4, said fluidized iron powder is cooled to a temperature between about 25° and about 50°F below temperature D.
6. A method in accordance with claim 1 wherein the temperature D is above the ignition temperature of said iron powder, and wherein in step 4 said air is introduced into said stream of inert gas.
7. A method in accordance with claim 1 wherein the temperature D is below the ignition temperature of said iron powder, and wherein in step 4 said air supplants said inert gas, and the cooling of fluidized iron powder in air proceeds through steps 4 and 5 to said lower temperature suitable for removing the final oxidized product from said reaction chamber.
Description:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a novel method for producing oxide-coated iron (including alloy steel) powder of controlled resistance, suitable for use as a carrier of toner particles in an electrostatic copying system.
2. The Prior Art
Iron powder has been used in the past as a carrier for toner particles to be used in electrostatic copying systems, such as the well known xerographic process. Sometimes such iron powder is coated with a polymeric material which improves the electrostatic copying action. In such systems it is important that the iron powder have a controlled resistance which may have different values for different specific operations. The carrier resistance is a primary factor in determining image development characteristics, and results ranging from full fringing to full solid-area development, and points between, can be obtained depending on the mass resistance of the iron particles. The higher the powder resistance, the more fringy is the image, and vice versa. Control to secure the resistance for any desired image characteristic has been difficult, if not impossible, in the past.
SUMMARY OF THE INVENTION
In accordance with the present invention the iron powder required as a carrier in electrostatic copying systems has been greatly improved so as to have an extremely uniform oxide film on each particle, which is also uniform from particle to particle. Furthermore, the resistance of the oxide coated iron particles can be controlled to have selected specific values in the range between 10 2 and 10 10 ohms, which is uniform throughout various batches of powder; and the oxide coated iron particles also have improved ability to be uniformly coated with a polymeric material when such a coating is desired. This improved iron powder is secured by oxidizing iron powder while it is continuously fluidized with gas in a reaction chamber. A five step program is employed, with continuous uninterrupted fluidization, as follows:
Step 1 -- fluidizing the mass in a stream of air while heating to a temperature between 500°-750°F., desirably to about 650°F, to initiate an exothermic reaction therein; and thereafter to a higher peak temperature between 750° and 1600°F, desirably to about 950°F, principally by the heat of exothermically oxidizing iron, which can be supplemented by external heating.
Step 2 -- introducing an inert gas into the stream of air and maintaining the temperature of the iron powder essentially constant at its peak temperature. The time at this peak temperature determines the weight percent of oxygen pick up, but not the resistance.
Step 3 -- discontinuing the flow of air into the reaction chamber, but maintaining fluidization therein by introducing a stream of inert gas such as N 2 , Ar or He therein and reducing the temperature of the iron powder to a lower temperature between 125° and 700°F, e.g. about 560°F.
Step 4 -- introducing air into said stream of inert gas and cooling the fluidized iron powder to a temperature between 25°and 50°F below that at the end of step 3, and thusly continuing incremental increases of air until the temperature of powder is below its reignition point. The inert gas acts to prevent reignition while the temperature is above the ignition temperature, for example above about 500°F; below that temperature air can supplant the inert gas without danger of exothermic reaction.
Step 5 -- discontinuing the inert gas but maintaining fluidization by again introducing a stream of air into the reaction chamber and reducing the temperature of the iron powder to a lower temperature suitable for removing the final oxidized iron powder product from said reaction chamber.
In the program or cycle outlined above, the most critical point for assuring that the desired resistance will be obtained is at the end of step 3 and the beginning of step 4. The resistance can be any selected value within the range 10 2 to 10 10 ohms depending on the temperature at that point.
The fluidized oxidation process described above can be carried out by the apparatus described herein, or in the reactor described in U.S. Pat. No. 3,352,638 granted Nov. 14, 1967 to John S. Perlowski (one of the present inventors) and William E. Sillick, entitled "Process For Manufacturing Magnetic Oxide", and assigned to Eastman Kodak Company like the present application.
THE DRAWINGS
FIG. 1 of the drawing is a graph showing the five step method program of the present invention, wherein temperature is plotted against time;
FIG. 2 is a vertical sectional view showing a reactor for performing the method; and
FIG. 3 is a cross sectional view, partly in elevation, of apparatus for measuring the resistance of iron powder.
THE PREFERRED EMBODIMENT
In performing our novel method 500 pounds of Hoeganaes EH sponge iron powder (-80 +140 mesh U.S. Standard) are loaded into a fluidized bed reaction chamber 11 to provide a bed depth-to-width ratio greater than 1. The powder then is fluidized with air supplied at about 68 scf/min. by a conduit 13 and passing up through a perforate plate 15. The air may be at room temperature, or may be electrically or gas heated at 17 to 300°-400°F to expedite the process. The reactor is externally heated by the surrounding electrical resistance heaters 19, or by radiant heaters. Temperatures are measured by a thermocouple 20. The time in the various steps is not critical; but the longer the time at peak temperature in step 2, the greater the amount of oxidation occurring.
A program for producing iron powder having a resistance of 10 7 -10 8 ohms will be described to exemplify the principles of the invention:
Step 1 -- The temperature in the reaction chamber rises gradually to 630°F, at point A on the graph of FIG. 1 at which point an exothermic reaction of iron with the oxygen in the air commences, and the rate of heating rapidly increases spontaneously. At 700°F heating of the air at 17 is stopped. Time to point B can be as low as 11/2 hours with heated air, or as long as 5 hours with unheated air.
Step 2 -- When the temperature reaches 900°F at point B external heating is discontinued and the exothermic reaction is controlled so as to hold the temperature at 950°F ± 15°F to accomplish the desired controlled oxidation. This is done by introducing an inert gas such as nitrogen into the air stream so that the air to nitrogen ratio is approximately 1:3, and at the same time externally cooling the reactor by circulating air through an external cooling annulus 21. Time between points B and C can be about 33 minutes.
Step 3 -- After a suitable length of time, such as about 33 minutes, the oxidizing reaction is stopped at point C by discontinuing the flow of air and fluidizing the bed of iron particles only with an inert gas such as unheated nitrogen supplied at 68 scf/min., but without any interruption in fluidization. The bed of iron particles is cooled down over about 20 minutes to about 560°F at point D by the inert gas, and by continued circulation of cooling fluid outside the reaction chamber.
Step 4 -- At point D unheated air flowing at 25 scf/min. is again introduced into mixture with the nitrogen flowing at 68 scf/min. The temperature is reduced to about 30° below the temperature at point D over a short time, in this instance to 530°F at point E.
Point D is extremely critical to the process because the temperature at which the oxidizing gas mixture is first applied determines the bulk resistance of the final oxidized iron particles. Temperatures in steps 1 and 2 are less critical, as long as oxidation occurs. The purpose of the added N 2 is to prevent reignition of the iron powder at the higher temperatures at point D. At lower temperatures such as 150°F, where reignition cannot occur, one can switch directly to air without added N 2 .
Step 5 -- The cycle is then completed by discontinuing nitrogen addition and cooling the mass, while fluidized in unheated air at 68 scf/min., down to room temperature or any other desired temperature, such as 150°F or lower, at which it is feasible to remove the oxidized iron powder product from the reaction chamber.
When using a 560°F temperature at point D and 530°F at point E, the final product has a bulk resistance of 10 7 to 10 8 ohms. At other temperatures for point D ranging from 125°F to 700°F, controlled bulk resistances in the range of 10 2 to 10 10 ohms can be obtained; the lower the temperature at D the lower the bulk resistance.
For example, the following relationships have been found between temperature at point D and resistance:
Temperature °F Resistance -- ohms 150 10 2 -10 3 500 10 6 550 10 7 600 10 8 700 10 9 -10 10
The process steps described above produce similar results as to powder resistance when applied to other types of iron powder, such as non porous atomized iron powder, for example the -70 mesh (U.S. Standard) spherical steel powder sold by Whittaker Corporation (Nuclear Metals Division), West Concord, Massachusetts.
Resistance of powder is measured by the following device and procedure:
1. A resistance cell 25 comprises a 10 inches long, narrow horizontal brass trough 27 having a lead connected to one side of an electrometer 29 set to read voltage on the 30 scale (General Radio Company, type 1230A Electrometer). A pair of Teflon members 31,32 extend vertically upwardly on opposite sides of the trough and are coextensive therewith, with their top surfaces acting as horizontal tracks parallel to the trough.
2. A magnetic brush 33 comprises a shouldered handle 35 carrying a 900 gauss cylindrical magnet 37 one inch in diameter encased within a cylindrical brass guard 39 so that the sides and end of the magnet are enclosed. The guard has a lead which is connected to a second side of the electrometer.
3. 15 grams of powder are placed in a round aluminum weighing dish 21/2 inches in diameter, and the end of the magnetic brush is dipped into the center of the dish containing the powder until the end of the brush is in contact with the powder, but without compressing it, to attract a mass of powder.
4. Then the brush carrying the attracted powder sample on its end is removed and placed vertically in the resistance cell at one end thereof, with the brush's end surface spaced 5/32 inch from the brass trough but with the iron powder 41 in contact with the trough, and with the handle in contact with and riding on the tops of the Teflon tracks. The brush then is moved uniformly along the Teflon tracks (into the drawing paper) toward the other end of the cell to give the adherent powder a cylindrical shape and size which will be uniform each time a measurement is made.
5. When the brush has nearly reached the end of the cell its movement is stopped and, after a one minute wait, a circuit is completed through the brass guard, the adherent powder, and the brass trough, and the resistance on the ohmmeter of the electrometer is read.
6. The procedure is repeated and the two readings are averaged.
The five step oxidizing program described in detail above is particularly advantageous for producing oxidized core carrier particles, for electrostatic copying systems, having a selected specific resistance within the range of 10 2 to 10 10 ohms which has not been possible by other methods known to applicants at this time. Moreover, extremely good reproducibility has been obtained from batch to batch, when all batches are treated on the same program.
Other important advantages of the fluidization technique are as follows:
1. Fluidization is most effective in bringing a reactive atmosphere in innermost contact with the particulate materials. More conventional approaches such as tray or continuous belt roasting would yield a broader non uniform spectrum of results because top layers are exposed to more of the atmosphere.
2. Heat transfer rates are extremely good in fluidized beds. In addition to shortening process cycles, the high heat transfer provides very uniform temperatures, which are difficult to obtain in static beds of iron powder.
3. Mixing rates are high under fluidization which contributes to product uniformity.
4. Sintering together of the particles to form a cake is a serious problem during the oxidization of iron powder by some procedures. This problem is minimized in a fluidized bed.
5. The reaction is strongly exothermic, and from a safety viewpoint an inert gas can be quickly introduced into fluidized beds to stop the reaction.
6. Subsequent coating of the iron powder particles with a polymer is improved.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
1. Field of the Invention
The present invention relates to a novel method for producing oxide-coated iron (including alloy steel) powder of controlled resistance, suitable for use as a carrier of toner particles in an electrostatic copying system.
2. The Prior Art
Iron powder has been used in the past as a carrier for toner particles to be used in electrostatic copying systems, such as the well known xerographic process. Sometimes such iron powder is coated with a polymeric material which improves the electrostatic copying action. In such systems it is important that the iron powder have a controlled resistance which may have different values for different specific operations. The carrier resistance is a primary factor in determining image development characteristics, and results ranging from full fringing to full solid-area development, and points between, can be obtained depending on the mass resistance of the iron particles. The higher the powder resistance, the more fringy is the image, and vice versa. Control to secure the resistance for any desired image characteristic has been difficult, if not impossible, in the past.
SUMMARY OF THE INVENTION
In accordance with the present invention the iron powder required as a carrier in electrostatic copying systems has been greatly improved so as to have an extremely uniform oxide film on each particle, which is also uniform from particle to particle. Furthermore, the resistance of the oxide coated iron particles can be controlled to have selected specific values in the range between 10 2 and 10 10 ohms, which is uniform throughout various batches of powder; and the oxide coated iron particles also have improved ability to be uniformly coated with a polymeric material when such a coating is desired. This improved iron powder is secured by oxidizing iron powder while it is continuously fluidized with gas in a reaction chamber. A five step program is employed, with continuous uninterrupted fluidization, as follows:
Step 1 -- fluidizing the mass in a stream of air while heating to a temperature between 500°-750°F., desirably to about 650°F, to initiate an exothermic reaction therein; and thereafter to a higher peak temperature between 750° and 1600°F, desirably to about 950°F, principally by the heat of exothermically oxidizing iron, which can be supplemented by external heating.
Step 2 -- introducing an inert gas into the stream of air and maintaining the temperature of the iron powder essentially constant at its peak temperature. The time at this peak temperature determines the weight percent of oxygen pick up, but not the resistance.
Step 3 -- discontinuing the flow of air into the reaction chamber, but maintaining fluidization therein by introducing a stream of inert gas such as N 2 , Ar or He therein and reducing the temperature of the iron powder to a lower temperature between 125° and 700°F, e.g. about 560°F.
Step 4 -- introducing air into said stream of inert gas and cooling the fluidized iron powder to a temperature between 25°and 50°F below that at the end of step 3, and thusly continuing incremental increases of air until the temperature of powder is below its reignition point. The inert gas acts to prevent reignition while the temperature is above the ignition temperature, for example above about 500°F; below that temperature air can supplant the inert gas without danger of exothermic reaction.
Step 5 -- discontinuing the inert gas but maintaining fluidization by again introducing a stream of air into the reaction chamber and reducing the temperature of the iron powder to a lower temperature suitable for removing the final oxidized iron powder product from said reaction chamber.
In the program or cycle outlined above, the most critical point for assuring that the desired resistance will be obtained is at the end of step 3 and the beginning of step 4. The resistance can be any selected value within the range 10 2 to 10 10 ohms depending on the temperature at that point.
The fluidized oxidation process described above can be carried out by the apparatus described herein, or in the reactor described in U.S. Pat. No. 3,352,638 granted Nov. 14, 1967 to John S. Perlowski (one of the present inventors) and William E. Sillick, entitled "Process For Manufacturing Magnetic Oxide", and assigned to Eastman Kodak Company like the present application.
THE DRAWINGS
FIG. 1 of the drawing is a graph showing the five step method program of the present invention, wherein temperature is plotted against time;
FIG. 2 is a vertical sectional view showing a reactor for performing the method; and
FIG. 3 is a cross sectional view, partly in elevation, of apparatus for measuring the resistance of iron powder.
THE PREFERRED EMBODIMENT
In performing our novel method 500 pounds of Hoeganaes EH sponge iron powder (-80 +140 mesh U.S. Standard) are loaded into a fluidized bed reaction chamber 11 to provide a bed depth-to-width ratio greater than 1. The powder then is fluidized with air supplied at about 68 scf/min. by a conduit 13 and passing up through a perforate plate 15. The air may be at room temperature, or may be electrically or gas heated at 17 to 300°-400°F to expedite the process. The reactor is externally heated by the surrounding electrical resistance heaters 19, or by radiant heaters. Temperatures are measured by a thermocouple 20. The time in the various steps is not critical; but the longer the time at peak temperature in step 2, the greater the amount of oxidation occurring.
A program for producing iron powder having a resistance of 10 7 -10 8 ohms will be described to exemplify the principles of the invention:
Step 1 -- The temperature in the reaction chamber rises gradually to 630°F, at point A on the graph of FIG. 1 at which point an exothermic reaction of iron with the oxygen in the air commences, and the rate of heating rapidly increases spontaneously. At 700°F heating of the air at 17 is stopped. Time to point B can be as low as 11/2 hours with heated air, or as long as 5 hours with unheated air.
Step 2 -- When the temperature reaches 900°F at point B external heating is discontinued and the exothermic reaction is controlled so as to hold the temperature at 950°F ± 15°F to accomplish the desired controlled oxidation. This is done by introducing an inert gas such as nitrogen into the air stream so that the air to nitrogen ratio is approximately 1:3, and at the same time externally cooling the reactor by circulating air through an external cooling annulus 21. Time between points B and C can be about 33 minutes.
Step 3 -- After a suitable length of time, such as about 33 minutes, the oxidizing reaction is stopped at point C by discontinuing the flow of air and fluidizing the bed of iron particles only with an inert gas such as unheated nitrogen supplied at 68 scf/min., but without any interruption in fluidization. The bed of iron particles is cooled down over about 20 minutes to about 560°F at point D by the inert gas, and by continued circulation of cooling fluid outside the reaction chamber.
Step 4 -- At point D unheated air flowing at 25 scf/min. is again introduced into mixture with the nitrogen flowing at 68 scf/min. The temperature is reduced to about 30° below the temperature at point D over a short time, in this instance to 530°F at point E.
Point D is extremely critical to the process because the temperature at which the oxidizing gas mixture is first applied determines the bulk resistance of the final oxidized iron particles. Temperatures in steps 1 and 2 are less critical, as long as oxidation occurs. The purpose of the added N 2 is to prevent reignition of the iron powder at the higher temperatures at point D. At lower temperatures such as 150°F, where reignition cannot occur, one can switch directly to air without added N 2 .
Step 5 -- The cycle is then completed by discontinuing nitrogen addition and cooling the mass, while fluidized in unheated air at 68 scf/min., down to room temperature or any other desired temperature, such as 150°F or lower, at which it is feasible to remove the oxidized iron powder product from the reaction chamber.
When using a 560°F temperature at point D and 530°F at point E, the final product has a bulk resistance of 10 7 to 10 8 ohms. At other temperatures for point D ranging from 125°F to 700°F, controlled bulk resistances in the range of 10 2 to 10 10 ohms can be obtained; the lower the temperature at D the lower the bulk resistance.
For example, the following relationships have been found between temperature at point D and resistance:
Temperature °F Resistance -- ohms 150 10 2 -10 3 500 10 6 550 10 7 600 10 8 700 10 9 -10 10
The process steps described above produce similar results as to powder resistance when applied to other types of iron powder, such as non porous atomized iron powder, for example the -70 mesh (U.S. Standard) spherical steel powder sold by Whittaker Corporation (Nuclear Metals Division), West Concord, Massachusetts.
Resistance of powder is measured by the following device and procedure:
1. A resistance cell 25 comprises a 10 inches long, narrow horizontal brass trough 27 having a lead connected to one side of an electrometer 29 set to read voltage on the 30 scale (General Radio Company, type 1230A Electrometer). A pair of Teflon members 31,32 extend vertically upwardly on opposite sides of the trough and are coextensive therewith, with their top surfaces acting as horizontal tracks parallel to the trough.
2. A magnetic brush 33 comprises a shouldered handle 35 carrying a 900 gauss cylindrical magnet 37 one inch in diameter encased within a cylindrical brass guard 39 so that the sides and end of the magnet are enclosed. The guard has a lead which is connected to a second side of the electrometer.
3. 15 grams of powder are placed in a round aluminum weighing dish 21/2 inches in diameter, and the end of the magnetic brush is dipped into the center of the dish containing the powder until the end of the brush is in contact with the powder, but without compressing it, to attract a mass of powder.
4. Then the brush carrying the attracted powder sample on its end is removed and placed vertically in the resistance cell at one end thereof, with the brush's end surface spaced 5/32 inch from the brass trough but with the iron powder 41 in contact with the trough, and with the handle in contact with and riding on the tops of the Teflon tracks. The brush then is moved uniformly along the Teflon tracks (into the drawing paper) toward the other end of the cell to give the adherent powder a cylindrical shape and size which will be uniform each time a measurement is made.
5. When the brush has nearly reached the end of the cell its movement is stopped and, after a one minute wait, a circuit is completed through the brass guard, the adherent powder, and the brass trough, and the resistance on the ohmmeter of the electrometer is read.
6. The procedure is repeated and the two readings are averaged.
The five step oxidizing program described in detail above is particularly advantageous for producing oxidized core carrier particles, for electrostatic copying systems, having a selected specific resistance within the range of 10 2 to 10 10 ohms which has not been possible by other methods known to applicants at this time. Moreover, extremely good reproducibility has been obtained from batch to batch, when all batches are treated on the same program.
Other important advantages of the fluidization technique are as follows:
1. Fluidization is most effective in bringing a reactive atmosphere in innermost contact with the particulate materials. More conventional approaches such as tray or continuous belt roasting would yield a broader non uniform spectrum of results because top layers are exposed to more of the atmosphere.
2. Heat transfer rates are extremely good in fluidized beds. In addition to shortening process cycles, the high heat transfer provides very uniform temperatures, which are difficult to obtain in static beds of iron powder.
3. Mixing rates are high under fluidization which contributes to product uniformity.
4. Sintering together of the particles to form a cake is a serious problem during the oxidization of iron powder by some procedures. This problem is minimized in a fluidized bed.
5. The reaction is strongly exothermic, and from a safety viewpoint an inert gas can be quickly introduced into fluidized beds to stop the reaction.
6. Subsequent coating of the iron powder particles with a polymer is improved.
The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.