US20100183816A1

US20100183816A1 - Low velocity oxygen-fueled flame spray method and apparatus for making ferrite material products and products produced thereby

Info

Publication number: US20100183816A1
Application number: US11/675,707
Authority: US
Inventors: Danny R. Smith; Henry G. Paris; Jeffrey A. Merker
Original assignee: Steward Advanced Materials Inc
Current assignee: Steward Advanced Materials Inc
Priority date: 2006-02-17
Filing date: 2007-02-16
Publication date: 2010-07-22
Also published as: WO2007111793A3; WO2007111793A2

Abstract

A method for making ferrite powder may include providing ferrite feed materials in a form of particles, such as having different sizes and irregular shapes. The method may further include exposing the ferrite feed materials to a low velocity oxygen-fueled (LVOF) flame spray. This may provide a more spherical shape to irregularly shaped particles to thereby make the ferrite powder. An apparatus for making ferrite powder may include a feeder for ferrite feed materials and a LVOF flame spray system for exposing the ferrite feed materials to the flame spray.

Description

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/774,309 filed on Feb. 17, 2006, the entire disclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of ferrite products and, more specifically, to the field of production of ferrite products.

BACKGROUND OF THE INVENTION

Mixtures of iron oxide, oxides of transition metals, other metals, and semi-metals are basic components in the manufacture of many products that are used for electromagnetic interference (EMI) suppression filters, inductors, and reprographic system components, among others. Ferrite materials, commonly called spinel, may be produced by forming small particles of chemical oxides. Other crystal structures, such as garnet, and hexagonal ferrite, may be also be produced. Such spinel ferrites may be based on an iron oxide denoted by the formula, MeFe₂O₄, and may contain as the Me element, for example, some combination of substituted transition metals such as manganese (Mn), nickel (Ni), cobalt (Co), vanadium (V), as well as other oxides of metals such as magnesium (Mg), copper (Cu), aluminum (Al), strontium (Sr), and zinc (Zn).
These ferrites may also contain semi-metals such as silicon (Si), and other additives such as titanium (Ti), tantalum (Ta), niobium (Nb), vanadium (V), and even alkaline earth metals such as calcium (Ca). In some cases, alkali metals may reside in the ferrite or in other phases.
Although referred to as spinel, garnet and hexagonal ferrite, these materials may be complex multiphase materials containing phases such as FeO and glass formers that are used to control bulk electrical resistivity, eddy currents, frequency response of material impedance, magnetic hysteretic characteristics, total magnetic moment, and sintering characteristics.
The production of ferrites may be controlled intentionally to produce spinels of oxides of iron and other elements, which may occur in more than one valence state having controlled ferrimagnetic properties. In many applications a particular surface morphology and a specific size of powder may be required to achieve desired product properties.
To obtain these desired product properties in a final product shape, a ferrite, or precursor mixture, must first be produced. This may be accomplished by methods well known to persons experienced in the technology and may include processes such as chemical precipitation, the use of naturally occurring oxide ores, or conversion of aqueous solution of metal salts such as chlorides, and even melting the starting ingredients.
The overall elemental composition of the incipient ferrite spinel may be created by mixing exact proportions of metal oxides, or chemical precursors. The overall elemental composition may require grinding elemental oxides in a proper proportion into an intimate mixture of small particle size, and adjusting the composition of the final mixture, which may then be spray dried.
Such mixtures may be subjected to intermediate thermal treatments in rotary or fixed kilns to partially react them, or to produce particle sizes useful for subsequent mechanical processing. After thermal-mechanical processing, the materials may be ground again into small particle size, and the composition may be adjusted to meet a target terminal composition. The materials may thereafter be spray dried in an aqueous process to create an aggregate that may be mixed and blended, pressed into a shape, and sintered.
For some applications, such as reprographic use, the powder may be used in the spray dried and sintered form. In other applications, the powder may be used with, or without, spray drying when it is intended as an additive to a mixture of an inorganic or organic binder.
A process for producing a pre-reacted oxide powder is disclosed in U.S. Pat. No. 5,976,488 to Workman et al., the entire disclosure of which is incorporated herein by reference. While this process produces pre-reacted powder, its particular phase mixture is not typically suitable for direct use in ferrite products without further processing and thermal treatments. Moreover, the atmospheric conditions used to produce such powders may not be effectively controlled to achieve a desired oxidation state and phase composition of all the iron and other elements.
The process described in the Workman et al. patent produces particle morphology that is somewhat useful for carrier bead, but the ratio of ferri-magnetic spinel to non-ferrimagnetic oxides of iron and other elements leads to a magnetic moment that may be too low for use. Further, the ratio may also lead to a volume electrical resistivity that is not suitable. Accordingly, this product must be further processed to produce a useful carrier bead.
Additionally, the process described in the Workman et al. patent may produce detrimental particle shapes, such as broken particles with sharp edges and elongate particles. It may be difficult and costly to remove these irregular particles by traditional separation methods since they exist in a range of sizes. Further, a significant percentage of useful spherical powder may be lost when conventional separation methods are used. A negative economic impact is observed due to a small percentage of total powder that is usable.
A significant advance in the area of making ferrite powders is disclosed in U.S. Pat. No. 7,118,728 to Paris et al., assigned to the assignee of the present invention and the entire contents of which are incorporated herein by reference. In particular, the patent discloses a method for making ferrite powder by exposing ferrite feed materials to a plasma so that the irregularly shaped particles are made more regular in shape. In addition, U.S. Pat. Nos. 5,462,686 and 6,793,842 also disclose various approaches to producing a ferrite powder using thermal processing.
Despite continual developments and improvements in the manufacture of ferrite powders and articles, there still exists a need for such ferrite materials with enhanced properties, and there still exists a need for more efficient related manufacturing processes.

SUMMARY OF THE INVENTION

In view of the foregoing background, it is therefore an object of the present invention to provide a method and associated apparatus for making ferrite powder with enhanced properties and in an efficient manner.
This and other objects, features and advantages in accordance with the invention are provided by a method for making ferrite powder comprising providing ferrite feed materials in a form of particles, and exposing ferrite feed materials in the form of particles to a low velocity oxygen-fueled (LVOF) flame spray to thereby make the ferrite powder. The exposing may comprise axially introducing the ferrite feed materials in the form of particles to the LVOF flame spray. In addition, the LVOF flame spray may operate at a temperature of less than about 5,000° C., and with a flame velocity of less than about 1,000 feet per second, for example.
The ferrite feed materials prior to exposing may comprise irregularly shaped particles. The LVOF exposure may produce more spherically shaped particles from the irregularly shaped particles. Exposing the ferrite materials to lower temperature environment of the LVOF flame spray may enhance the economical efficiency of production of ferrite particles, and advantageously increase a yield of ferrite powder production.
The method may further include controlling at least one of providing and exposing to make the ferrite powder to have at least one of a predetermined phase ratio, surface morphology, density, magnetic moment, and volume electrical resistivity. For example, the controlling may comprise controlling at least one of a feed rate, an exposure time, and a temperature of the ferrite feed materials during the exposing. The controlling may also comprise controlling a composition of the ferrite feed materials. Moreover, controlling may comprise controllably supplying at least one additional material to the ferrite feed materials during the exposing. For example, the at least one additional material may comprise at least one of oxygen, hydrogen, an inert gas, and calcined ferrite feed materials. Controllably supplying may comprise coating the ferrite feed materials with at least one of a silicate, alumina, and an organo-metallic.
The ferrite feed materials comprise at least one of nickel ferrite particles, manganese ferrite particles, magnesium ferrite particles, strontium ferrite particles, and zinc ferrite particles. The ferrite feed materials may also comprise metal oxides. The method may further include sorting the ferrite powder to have particle sizes within a predetermined range.
Another method aspect is directed to making a ferrite article. The method may include providing ferrite feed materials in a form of particles, exposing ferrite feed materials in the form of particles to a low velocity oxygen-fueled (LVOF) flame spray to thereby make a ferrite powder, and forming the ferrite powder into a ferrite article. The ferrite articles may be carrier beads, an inert anode, or a body of an inductor, for example.
Yet another aspect of the invention is directed to an apparatus for making ferrite powder comprising a feeder for ferrite feed materials in a form of particles, and an LVOF flame spray system for exposing the ferrite feed materials to an LVOF flame spray to thereby make the ferrite powder. The LVOF flame spray system may axially introduce the ferrite feed materials in the form of particles to the LVOF flame spray. The LVOF flame spray system may generate the LVOF flame spray at a temperature of less than about 5,000° C., and at a flame velocity of less than about 1,000 feet per second, for example. The apparatus may further comprise a controller for controlling at least one of the feeder and the LVOF flame spray system to make the ferrite powder to have at least one of a predetermined phase ratio, surface morphology, density, magnetic moment, and volume electrical resistivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating a method for making ferrite powder according to the present invention.

FIG. 2 is a flow chart illustrating another method for making ferrite powder according to the present invention.

FIG. 3 is a flow chart illustrating yet another method for making ferrite powder according to the present invention.

FIG. 4 is a flow chart illustrating still another method for making ferrite powder according to the present invention.

FIG. 5 is a schematic block diagram of an apparatus for making ferrite powder according to the present invention.

FIG. 6 is Table 1 showing a summary of comparative trials.

FIG. 7 is Table 2 showing a comparison of powder properties.

FIG. 8 is Table 3 showing a comparison of costs.

FIG. 9 is a scanning electron microscope photograph of a ferrite powder produced in accordance with a prior art nitrogen plasma spray.

FIG. 10 is a scanning electron microscope photograph of a ferrite powder produced in accordance with a prior art argon plasma spray.

FIG. 11 is a scanning electron microscope photograph of a ferrite powder produced in accordance with the LVOF spray process in accordance with the present invention.

FIG. 12 is a scanning electron microscope photograph of a ferrite powder produced in accordance with a prior art nitrogen plasma spray and illustrating an unmelted particle.

FIG. 13A is a flowchart for an embodiment for making ferrite power according to the invention.

FIG. 13B is a flowchart for an embodiment for making ferrite power according to the prior art plasma spray process.

FIG. 13C is a flowchart for another embodiment for making ferrite power according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
Referring initially to the flow chart 20 of FIG. 1, a method for making ferrite powder is now described. From the start (Block 22) ferrite feed materials are provided at Block 24. The composition of the ferrite feed material may be controlled when the ferrite feed material is provided at Block 24.
The ferrite feed materials may be in the form of particles having different sizes and irregular shapes. The ferrite feed materials may, for example, comprise nickel ferrite particles or zinc ferrite particles. The ferrite feed materials may also comprise manganese ferrite particles or magnesium ferrite particles so that the ferrite feed materials are advantageously environmentally friendly. Those skilled in the art will appreciate that the ferrite feed materials may comprise any combination of manganese ferrite particles and magnesium ferrite particles. Those skilled in the art will also appreciate that the ferrite feed materials may, of course, comprise any other ferrite particles.
At Block 26, the ferrite feed materials are subjected or exposed to an LVOF flame spray. The exposure may, for example, be in the form of passage through an LVOF flame spray gun as will be understood by those skilled in the art. Other devices may also provide the LVOF flame spray exposure. The exposure of the ferrite feed materials to the LVOF flame spray advantageously provides a more spherical shape to irregularly shaped particles to thereby make the ferrite powder. More specifically, the irregularly shaped particles are melted into more spherically shaped particles. Making the particles more spherically shaped advantageously enhances the efficiency and economics of ferrite particle production. Further, exposure of the ferrite feed materials to the LVOF flame spray advantageously enhances surface characteristics of the ferrite powder. The ferrite powder may be made to have a predetermined phase ratio, surface morphology, density, magnetic moment, and/or volume electrical resistivity while maximizing yield and minimizing oxidation residue, for example.
A number of different variables of the ferrite powder production may be controlled during the exposure of the ferrite to the LVOF flame spray. For example, the feed rate of the ferrite feed material may be controlled, the exposure time of the ferrite feed material to the LVOF flame spray may be controlled, and the temperature of the ferrite feed material during exposure to the LVOF combustion-heated gas may be controlled at Block 28.
Further, a rate of heat transfer from the LVOF flame spray may be controlled to thereby control the temperature of the ferrite feed material. The volume of combustion gas, as well as the type of combustion and oxidation gasses, used during LVOF flame spray exposure may also be controlled. For example, the flow rate of the carrier gas may be regulated. The flow of combustion and oxidation gasses may also be controlled, as understood by those skilled in the art.
Additional materials may be controllably supplied to the ferrite feed materials during exposure to the LVOF flame spray at Block 30. The additional materials may, for example, include oxygen, hydrogen, inert gas, or any other suitable material, as understood by those skilled in the art.
The ferrite powder generally comprises different sizes. Accordingly, at Block 32, the ferrite powder is sorted to have particles sized within a predetermined range. At Block 34, the ferrite powder may be formed into a ferrite article, before stopping at Block 36. More specifically, the ferrite powder may be formed into carrier beads, an inert anode, or a body of an inductor, for example. Those having skill in the art will recognize that the ferrite powder may also be formed into any number of other articles.
Referring now additionally to the flow chart 40 of FIG. 2, another method for making ferrite powder is now described. From the start (Block 42), the ferrite feed materials are provided at Block 44. At Block 46 the ferrite feed materials are calcined. The ferrite feed materials are preferably provided by either spray dried powder that has been calcined, or uncalcined spray dried powder. This enhances densification of the ferrite feed material and further enhances the economic efficiency of production of the ferrite powder.
The ferrite feed materials may also be coated. The ferrite feed materials are preferably coated with a non-ferrite ceramic precursor, such as a silicate, alumina, an organo-metallic, or calcium, for example. Those skilled in the art will appreciate that the ferrite materials may also be coated with other materials that decompose to native oxides in fairly thin layers.
Coating the ferrite feed materials may be particularly advantageous when using the ferrite powder to form carrier beads. More specifically, coating the ferrite feed materials advantageously enhances resistivity for carrier beads. Coating the ferrite feed materials may also advantageously enhance other surface properties in applications where special surfaces may be required, such as sensors and catalysts, for example.
Coating may be a pretreatment of “virgin” ferrite feed materials, i.e., ferrite feed materials that have not yet been exposed to the combustion gases. Coating may also be a post treatment of ferrite feed materials that have been exposed to combustion gases to thereby create new ferrite feed material of coated ferrite particles. Coating the ferrite feed materials may advantageously enhance resistivity, breakdown voltage, increase carrier life, or create novel catalysts.
At Block 48, the ferrite feed materials are exposed to the LVOF flame spray, as described above. The ferrite powder may be sorted at Block 50, and formed into a ferrite article at Block 52 before stopping at Block 54.
Turning now additionally to the flow chart 60 of FIG. 3, another method for making ferrite powder is now described. Metal oxides are spray dried at Block 64, and exposed to the LVOF heated gas at Block 66. The ferrite powder is sorted at Block 68, and formed into a ferrite article at Block 70 before stopping at Block 72.
Referring now additionally to the flow chart 80 of FIG. 4, yet another method for making ferrite powder is described. From the start (Block 82) waste material is heated in a reactor while supplying oxygen at Block 84 to provide metal oxide. The metal oxide is exposed to the LVOF flame spray at Block 86. The ferrite powder is sorted at Block 88, and formed into a ferrite article at Block 90 before stopping at Block 92.
Referring additionally to FIG. 5, another aspect of the present invention is directed to an apparatus 100 for making ferrite powder, and is now described. The apparatus 100 illustratively comprises a feeder 102 for ferrite feed materials in a form of particles, such as having different sizes and irregular shapes. As described above, the ferrite feed materials may, for example, comprise nickel ferrite, manganese ferrite, magnesium ferrite, zinc ferrite, or any combination thereof, as understood by those skilled in the art.
The apparatus 100 also illustratively comprises an LVOF flame spray system 106 for exposing the ferrite feed materials to the LVOF flame spray, such as to provide a more spherical shape to irregularly shaped particles to thereby make the ferrite powder. The combustion and oxidation gasses are illustratively supplied to the LVOF system 106 from combustion and oxidation gas supplies 105. A valve (not shown) may be positioned between the gas supplies 105 and the LVOF system 106 to regulate the flow of gasses as will be appreciated by those skilled in the art. The LVOF flame spray system 106 may include a flame spray gun and associated equipment as available from Sulzer Metco (US) Inc. of Westbury, N.Y.
The feeder 102 illustratively feeds the ferrite feed material into the LVOF flame spray system 106. The ferrite feed material is exposed to the combustion gasses in the LVOF system 106.
The apparatus 100 further illustratively comprises a controller 108 for controlling the feeder 102 and the LVOF combustion system 106 to make the ferrite powder to have a predetermined phase ratio, surface morphology, density, magnetic moment, and/or volume electrical resistivity while maximizing yield and minimizing oxidation residue. The controller 108 may control the feed rate of the ferrite feed material from the feeder 102, the exposure time of the ferrite feed material to combustion in the LVOF system 106, and/or the temperature of the ferrite feed materials when being exposed to the heated gas. As indicated above, the rate of heat transfer from the hot gas may be controlled to thereby control the temperature of the ferrite feed material. As also indicated above, the volume of combustion gas, as well as the types of combustion and oxidation gasses used during LVOF exposure may also be controlled.
The apparatus 100 also illustratively comprises an optional second feeder 110 to supply at least one additional material to the ferrite feed material. As noted above, the at least one additional material may, for example, comprise oxygen, hydrogen, helium, inert gas, air, or any other material as understood by those skilled in the art. The controller 108 is illustratively connected to the second feeder 110 to thereby control variables, such as feed rate, for example, of the additional material. The apparatus 100 also illustratively includes a carrier gas supply 101 for supplying carrier gas into the stream carrying the additional material from the second feeder 110. The carrier gas supply 101 is illustratively connected to the controller 108.
The apparatus 100 further illustratively comprises a collector 111 downstream from the LVOF system 106 for collecting ferrite powder, and a sorter 112 downstream from the collector for sorting the ferrite powder to have particle sizes within a predetermined range. Those skilled in the art will understand that the predetermined range is dependent upon the article that is formed using the ferrite powder. The apparatus 100 also illustratively comprises a former 114 for forming the ferrite powder into a ferrite article, such as carrier beads, an inert anode, or a body of an inductor, for example.
The additional material may also illustratively be supplied from the second feeder 110 to a feed stream 119 from the feeder 102 to the LVOF system 106 (indicated by dashed arrow 117). The carrier gas supply 101 illustratively supplies carrier gas to the stream 117 carrying the additional material to the feed stream 119.
The ferrite powder may be recycled into the LVOF system 106, as indicated by the stream of dashed arrow 121. In such a case, the ferrite powder material is taken from the sorter 112 and sent to a coater 125. The coater 125 is illustratively connected to the controller 108. The additional material from the second feeder 110 is also added to the coater 125. Accordingly, the recycled ferrite powder and additional material from the second feeder 110 is supplied to the LVOF system 106.
The additional material may also be added from the second feeder 110 to the ferrite powder after it has been passed through the sorter 112. The additional material is preferably provided to another coater 126 (indicated by the dashed arrow 122). The ferrite powder is also illustratively supplied to the coater 126 from the sorter 112 (as indicated by the dashed arrow 123). Other additional material may also be added to the coater 126 from a third feeder 130, and may include metal salt solution, organo-metallic, inorganic coating, or any other material as understood by those skilled in the art. The third feeder 130 and the coater 126 are illustratively connected to the controller 108, and carrier gas from the carrier gas supply 101 is illustratively supplied to the feed stream 122 from the second feeder 110.
The method and apparatus 100 described above may be used to produce smooth, spherical ferrite powder from previously sintered feedstock. More specifically, the previously sintered feedstock may have a larger than desirable average particle diameter. The particles may be produced by first grinding the feed to a desired average diameter, classifying the feed to remove unwanted particles, then passing the feed through the gas heated by the LVOF flame spray process. Classifying the feed to remove unwanted particles is preferable, as exposure to the hot gas may not alter particle diameter.
Table 1 (FIG. 6) presents a summary of the trials that were performed when evaluating N₂Plasma, Argon Plasma, and LVOF processes. The equipment used to produce the nitrogen plasma sprayed product was a Sulzer Metco externally injected 9 MB gun while the argon plasma sprayed product was produced using an internally injected Praxair SG100 plasma gun. The LVOF system was a Sulzer Metco 6P-II gun using acetylene-oxygen combustion gases.
Table 1 illustrates that LVOF produces the best characteristics of any of the other methods described herein. One key benefit is minimal oxidation residue (that forms small particles of the size of smoke) with maximum bulk density and true density. The other benefit is that it produces a very low amount of total defective powder including unmelted powder and miss-shaped or irregular particles. For an application such as a carrier bead, ideally the total defective particles will be below 1% with zero being optimal. It is also significant that the LVOF material, as sprayed without any further refining, has a lower total percent defective than the finished product with any plasma method to date. It has also been seen that the ratio of the size of powder produced in the 90^thpercentile of mass to the size of powder in the 10^thpercentile of mass (d₉₀/d₁₀ratio) is not a process characteristic of either Plasma or LVOF. The parameters that produce good material with the LVOF method have broad ranges, compared to plasma methods, that allow good product to be made. This can also be seen by looking at the differences in the control parameters of Table 1. It was seen with the plasma processes that minor changes often resulted in the production of material that was out of specification for one of the key characteristics that were mentioned before.
The presence of oxidation residue, or particles of ceramic on the size of smoke particles causes several undesirable changes in properties. It increases surface area and decreases good flow properties of the powder, making it difficult to handle. Oxidation residue or (smoke particles) are difficult to directly quantify, therefore these properties, surface area, bulk density and flow rate are used to indicate its presence. The presence of this dust is most pronounced after the plasma or LVOF processing before subsequent steps of the methods.
Table 2 (FIG. 7) shows the surface area, yield and other properties of the powders after plasma or LVOF processing. The argon plasma method makes a very high surface area (a consequence of the hot plasma) followed by the nitrogen plasma process. Material made with the LVOF process shows the desirable lowest surface area. This is also manifested by the lower flow rate of the argon plasma processed powder compared to the powder processed by the LVOF. The final indicator of less “dust” is the higher yield. While the yield of the nitrogen plasma is similar to LVOF, nitrogen plasma processing produces much more defective powder and powder with lower true density.
Table 3 (FIG. 8) compares the energy costs that show that producing powder via LVOF is at least $0.18 per pound less than a plasma process. This cost advantage is even more significant when the recurring cost of supplies, equipment and repair are considered. The plasma process used an electric arc and complicated electrical insulation and cooling of anodes and cathodes is required. Electrode life is relatively short for the plasma gun. The yield is higher with the LVOF process, 13% better with LVOF on average; therefore similar savings in raw materials costs are gained.
Referring now additionally to the photomicrographs of FIGS. 9-12 further features are now described as relating to the effect of thermal spray processes and “dust” formation. FIG. 9 shows a plasma sprayed particle using nitrogen. FIG. 10 shows a plasma sprayed particle using argon. In contrast, FIG. 11 shows a particle made using the LVOF process. Although the size of the particles is near the limit of resolution of the scanning electron microscope, FIG. 9 shows prominent occurrence of very small powder on the surface of the large particle (see arrow “a”). Another consequence of the nitrogen plasma spray is a large amount of other residue noted by the arrow “b.” Note the relatively smooth surface of the nitrogen-processed plasma sprayed particle. FIG. 10 shows a comparable particle made using argon gas in the plasma process. Note the particle is quite irregular and has a lot of the small “dust” particles on it. This “dust” contributes to poor flow properties. Also note that the argon-processed plasma particle is not very spherical and does not have a very smooth surface.
FIG. 11 shows a comparable powder made using the LVOF process. Note that the surface is relatively clean compared to the plasma processed powder particles. Also note the highly spherical and smooth particle.
FIG. 12 shows an example of the unmelted particles seen at higher frequency in the nitrogen plasma spray processed powders.
One of the benefits of using the LVOF spray process is that it produces powder with better technical properties and a lower energy process than plasma spraying, for example. The LVOF spray process introduces powder axially to the temperature source to allow a more uniform temperature distribution than processes that inject powder perpendicular to the flow of gasses, for example. The advantage here is that the lower temperature may prevent the ferrite from oxidizing and vaporizing to form ultra-small particles on the order of size of “smoke.” Such small powder may cause particularly poor flow properties.
In contrast, the plasma spray process may require a very limited balance between the gas flow rate, temperature, and injection pressure to achieve spheroidization, but avoid excessive heat transfer that vaporizes part of the feed material. The LVOF method is more forgiving; it has a larger window of process variables that produces ferrite powder with high bulk and true density. In addition, the lower gas velocity may result in less particle distortion giving less total defective (non-spherical) powder.
LVOF processing develops heat through combustion gases, while plasma process spray guns use an electric arc to ionize a primary and secondary gas to form a hot plasma. As will be appreciated by those skilled in the art, the most common fuels used in LVOF are oxygen, hydrogen, MAPP (methylacetylene and propadiene), and acetylene. MAPP, hydrogen or acetylene are not typically run simultaneously. When LVOF is combined with a co-axial feed method to the gas flow it may allow a more consistent introduction of the particles to the heat source, but does not vaporize them as frequently occurs in axial injection inside the plasma gun. This improved consistency may reduce the total irregular particles (hollow and miss-shaped) to achieve powder with the same or better density.
The typical flame temperature for LVOF is ˜3000° C. compared to 15,000° C. for a true plasma temperature. Accordingly, the preferred temperature may be desirably less than about 5000° C. and more preferably about 3000° C. Because the gas velocity and temperature are lower, the flame velocity of LVOF is 600-700 ft/sec compared to 1500-1800 ft/sec for plasma and the flame is almost twice as long with LVOF. In other words, the flame velocity may be desirably less than about 1000 ft/sec, and more preferably in the range of 600-700 ft/sec. This difference in length results in a residence time almost five times longer than plasma spraying. The exact values for the flame temperature and velocity are dependant on the fuels being used and their proportions as will also be appreciated by those skilled in the art.
LVOF may also be useful to conventional “press-and-sinter” ceramic processes that are used to make dense ferrite and other ceramic bodies. It may be considered an economical and efficient replacement of the rotary kiln used to pre-sinter ferrite, or other ceramic powders after spray drying. It may be particularly useful to make magnetite powder directly. Hematite material run at various feed rates may produce powder 10% denser than calcined product with an EMU that is consistent for material that has been transformed to Magnetite. Since pre-sintering in a rotary kiln typically requires significant startup costs, LVOF can be used advantageously for short or custom runs of material.
Referring now additionally to FIGS. 13A-13C, further aspects of the LVOF methods are now described. The LVOF process is outlined in the left column (FIG. 13A) as compared to the conventional approach in the center column (FIG. 13B). The LVOF green process is in the right column (FIG. 13C). The flow of material according to the steps indicated on the flow charts of FIGS. 13A-13C are as follows.
At Block 152 raw materials are mixed and pelletized with water to produce calciner feed material. The materials are reacted with high heat in a rotary furnace to begin spinel formation and reduce variations in final firing at the calcine Block 154. At the ball mill Block 156 metal oxides are combined and milled in an aqueous suspension to achieve the desired particle size. The slurry is checked to verify composition and additions are used to achieve the desired rheology prior to the slurry being spray dried at Block 158 to achieve desired particle size. At the green screen Block 160 the spray dried powder is sorted/screened to achieve the correct particle size distribution prior to preliminary densification.
A bisque operation is performed at the Block 161 (FIG. 13A); however, in the LVOF process only, sorted powder is partially reacted in an air environment to improve particle integrity and eliminate additives.
A sinter is performed at Block 162 (FIG. 13B) wherein materials that follow the standard flow are sintered to final density at high temperature. At Block 164 material from the kiln is deagglomerated to eliminate the large chunks of material that form in the kiln. The purpose here is to eliminate satellites and agglomerates so that fired screening can be done efficiently.
The powder is screened at Block 166 to achieve the target particle size for the LVOF process (Block 167, FIGS. 13A, 130) or heat treatment (Block 168) in the case of the standard process (FIG. 13B). The LVOF (Block 167) may be considered a spheroidization process—it has the same benefits as plasma spraying on homogeneity but the operating and equipment costs are significantly reduced. LVOF processing also produces material that has a more uniform density.
The heat treatment at Block 168 may be a combination of annealing treatments that are designed to produce a final product that meets the magnetic and resistivity specifications. At Block 170 a sintered screening step is performed to produce the target particle size range for the customer application. The key outputs from this are d₅₀, d₉₀/d₁₀, and a desired percentage less than 20 μm, for example.
Classification (Block 172) is utilized to remove additional undesired material. This typically removes higher surface area particles such as including unmelted particles and particles that are less than 10 μm. Prior to shipment (Block 174) all characteristics are tested and verified.
In the LVOF standard process (FIG. 13A), the material after the bisque Block 161 is still comprised of metallic oxides in their natural form, the sintering process is only partially completed to maximize screening yields and to maintain the tightest possible distribution. If the sintering is performed more completely, or in different atmospheres the effect is powder that is agglomerated and difficult to break up and screen efficiently. The LVOF feed materials may not be ferrite, the partial sintering is not a means to produce spinel, the atmosphere and the temperature are controlled in a manner that allows densification to occur to the point that strength is increased but there is not any reduction of the metal oxide components.
In the case of the LVOF green process (FIG. 13C) the feed materials are mixed, unreacted oxide powders. The integrity of these particles is maintained through the use of binders that are added in the spray drying process.
Although the present LVOF thermal spray process described herein is advantageous for producing carrier bead, it may be used for producing other products as well, and as will be appreciated by those skilled in the art. In addition, to producing environmentally friendly materials, such as MnFerrite, MnMgFerrite, and Magnetite, the process may also be used to produce CuZn and NiZn materials, for example. The LVOF process can eliminate the sintering step in the production of powders. The LVOF process offers an advantage over the plasma spray approach because it recovers the off-size product and it provides flexibility to process different materials and formulations, for example.
Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed, and that other modifications and embodiments are intended to be included within the scope of the appended claims.

Claims

1. A method for making ferrite powder comprising:

providing ferrite feed materials in a form of particles; and

exposing ferrite feed materials in the form of particles to a low velocity oxygen-fueled (LVOF) flame spray to thereby make the ferrite powder.

2. A method according to claim 1 wherein exposing comprises axially introducing the ferrite feed materials in the form of particles to the LVOF flame spray.

3. A method according to claim 1 wherein the LVOF flame spray operates at a temperature of less than about 5,000° C.

4. A method according to claim 1 wherein the LVOF flame spray operates at a flame velocity of less than about 1,000 feet per second.

5. A method according to claim 1 wherein the ferrite feed materials prior to exposing comprise irregularly shaped particles; and wherein exposing produces more spherically shaped particles from the irregularly shaped particles.

6. A method according to claim 1 further comprising controlling at least one of providing and exposing to make the ferrite powder to have at least one of a predetermined phase ratio, surface morphology, density, magnetic moment, and volume electrical resistivity.

7. A method according to claim 6 wherein controlling comprises controlling at least one of a feed rate, an exposure time, and a temperature of the ferrite feed materials during the exposing.

8. A method according to claim 6 wherein controlling comprises controlling a composition of the ferrite feed materials.

9. A method according to claim 8 wherein controlling comprises controllably supplying at least one additional material to the ferrite feed materials during the exposing.

10. A method according to claim 9 wherein the at least one additional material comprises at least one of oxygen, hydrogen, an inert gas, and calcined ferrite feed materials.

11. A method according to claim 9 wherein controllably supplying comprises coating the ferrite feed materials with at least one of a silicate, alumina, and an organo-metallic.

12. A method according to claim 1 wherein the ferrite feed materials comprise at least one of nickel ferrite particles, manganese ferrite particles, magnesium ferrite particles, strontium ferrite particles, and zinc ferrite particles.

13. A method according to claim 1 wherein the ferrite feed materials comprise metal oxides.

14. A method according to claim 1 further comprising sorting the ferrite powder to have particle sizes within a predetermined range.

15. A method for making a ferrite article comprising:

providing ferrite feed materials in a form of particles;

exposing ferrite feed materials in the form of particles to a low velocity oxygen-fueled (LVOF) flame spray to thereby make a ferrite powder; and

forming the ferrite powder into a ferrite article.

16. A method according to claim 15 wherein forming comprises forming the ferrite powder into carrier beads.

17. A method according to claim 15 wherein forming comprises forming the ferrite powder into an inert anode.

18. A method according to claim 15 wherein forming comprises forming the ferrite powder into a body of an inductor.

19. A method according to claim 15 wherein exposing comprises axially introducing the ferrite feed materials in the form of particles to the LVOF flame spray.

20. A method according to claim 15 wherein the LVOF flame spray operates at a temperature of less than about 5,000° C.

21. A method according to claim 15 wherein the LVOF flame spray operates at a flame velocity of less than about 1,000 feet per second.

22. A method according to claim 15 wherein the ferrite feed materials prior to exposing comprise irregularly shaped particles; and wherein exposing produces more spherically shaped particles from the irregularly shaped particles.

23. A method according to claim 15 further comprising controlling at least one of providing and exposing to make the ferrite powder to have at least one of a predetermined phase ratio, surface morphology, density, magnetic moment, and volume electrical resistivity.

24. A method according to claim 23 wherein controlling comprises controlling at least one of a feed rate, an exposure time, and a temperature of the ferrite feed materials during the exposing.

25. A method according to claim 23 wherein controlling comprises controlling a composition of the ferrite feed materials.

26. A method according to claim 25 wherein controlling comprises controllably supplying at least one additional material to the ferrite feed materials during the exposing.

27. A method according to claim 15 wherein the ferrite feed materials comprise at least one of nickel ferrite particles, manganese ferrite particles, magnesium ferrite particles, strontium ferrite particles, and zinc ferrite particles.

28. A method according to claim 15 wherein the ferrite feed materials comprise metal oxides.

29. An apparatus for making ferrite powder comprising:

a feeder for ferrite feed materials in a form of particles; and

a low velocity oxygen-fueled (LVOF) flame spray system for exposing the ferrite feed materials to an LVOF flame spray to thereby make the ferrite powder.

30. An apparatus according to claim 29 wherein said LVOF flame spray system axially introduces the ferrite feed materials in the form of particles to the LVOF flame spray.

31. An apparatus according to claim 29 wherein said LVOF flame spray system generates the LVOF flame spray at a temperature of less than about 5,000° C.

32. An apparatus according to claim 29 wherein said LVOF flame spray system generates the LVOF flame spray at a flame velocity of less than about 1,000 feet per second.

33. An apparatus according to claim 29 further comprising a controller for controlling at least one of said feeder and said LVOF flame spray system to make the ferrite powder to have at least one of a predetermined phase ratio, surface morphology, density, magnetic moment, and volume electrical resistivity.

34. An apparatus according to claim 33 wherein said controller controls at least one of a feed rate, an exposure time, and a temperature of the ferrite feed materials during the exposing.

35. An apparatus according to claim 33 wherein said controller controls a composition of the ferrite feed materials.

36. An apparatus according to claim 29 further comprising a second feeder to supply at least one additional material to the ferrite feed materials.

37. An apparatus according to claim 29 wherein the ferrite feed materials comprise at least one of nickel ferrite particles, manganese ferrite particles, magnesium ferrite particles, strontium ferrite particles, and zinc ferrite particles.