CA1054837A - Spherical, void-free particle formation in spray-dried ferrites - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Ferrite beads prepared via spray drying of a slurry of mixed oxides have characteristic voids or dimples present in the dried particles. This invention relates to the preparation of a slurry of ferrite forming metal oxides in a liquid wherein the slurry has a viscosity at about 23°C of between about 55,000 centipoise and about 140,000 centipoise, spray drying the slurry to form substantially spherical metal oxide beads, and sintering the beads to form ferrite particles which are continuous in sur-face area and free of interior voids. Electrostatographic develo per mixtures prepared with said ferrite materials and processes of developing electrostatic latent images therewith are also dis-closed.

Description

~a5~837 D/73069 BACKGROUND OF THE I~VENTION
This invention relates in general to electrostato-graphy and in particular to a process for the preparation of ferrite carrier materials, to the ferrite materials so prepared, and to the use of said ferrite materials in electrostatography.
Ferrite materials are gaining ever increasing import-ance in the electronics industry and in the electrostatographic arts. Their use as low conductivity magnetic core materials and as carrier materials in electrostatographic developer materials is well known. Briefly, ferrites may be described in general as compounds of magnetic oxides containing iron as a ma~or metallic component. Thus, compounds of ferric oxide, Fe203, formed with basic metallic oxides having the general formula MFe02 or MFe204 where M represents a mono or divalent metal and the iron is in the oxidation state of +3 are ferrites. Ferrites are also referred to as ferrospinels since they have the same crystal structure of the mineral spinel MgA1204. However, not all ferrites are magnetic such as, for example, ZnFe204 and CdFe204. This lack of magnetic property is due to the configuration of the ferrite lattice struc-ture. Further, some ferrites, such as magnetobarite, BaFe12019, which exhibit permanent magnetic properties are referred to as "hard" ferrites. A "hard" ferrite is difficult to magnetize and demagnetize and thus is the type of ferrite that is desirable in a permanent magnet. A "soft'~ ferrite has the opposite property;
it is easily magnetized and demagnetized. The "softer" the ferrite material is, the better it is suited to various electrical devices

-2- ~ ~

105~ 7 D/73069 in which magnetization must be reversed very often per unit of time. If one plots the characteristics of a "hard" ferrite and a "soft" ferrite on a graph in which the imposed magnetic field forms the horizontal axis and the total magnetization forms the vertical axis, one obtains a characteristic curve resembling a thick S known as hysteresis loop. A `'hard" ferrite has a wide hysteresis loop and a "soft" ferrite has a narrow one. Since each traversal of a loop represents energy lost, a narrow loop is desirable in devices in which magnetization must be reversed frequently.
The ferrite materials of main interest in the electrosta-tographic arts are the "soft" ferrites. The 'Isoft'' ferrites may further be characterized as being magnetic, polycrystalline, highly resistive ceramic materials exemplified by intimate mix-tures of nickel, manganese, magnesium, zinc, iron or other suitable metal oxides with iron oxide. Upon firing or sintering, the oxide mixtures assume a particular lattice structure which governs the magnetic and electrical properties of the resulting ferrite.
The formation and development of images on the surface of photoconductor materials by electrostatic means is well known.
The basic electrostatographic imaging process, as taught by C. F.
Carlson, in U. S. Patent 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, ex-posing the layer to a light-and-shado~l image to dissipate the charge on the areas of the layer exposed to the light and develop-ing the electrostatic latent image by depositing on the image a D/73069 finely-divided electroscopic material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light-and-shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing steps.
Several methods are known for applying the electro-scopic particles to the electrostatic latent image to be de-veloped. One development method, as disclosed by E. N. Wise in U. S. Patent 2,618,552, is known as "cascade" development.
In this method, a developer material comprising relatively large carrier particles having finely-divided toner particles electro-statically coated thereon is conveyed to and rolled or cascaded across the electrostatic latent image bearing surface. The composition of the carrier particles is so selected as to tribo-electrically charge the toner particles to the desired polarity.
As the mixture cascades or rolls across the image bearing surface, ~.05~837 D/73069 the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier and excess toner are then recycled. This technique is extremely good for the development of line copy images.
Another method of developing electrostatic latent images is the "magnetic brush" development process as disclosed for example, in U. S. Patent 2,874,063. In this method, a developer material containing toner and magnetic carrier particles are carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carrier into a brush-like configuration.
This "magnetic brush" is engaged with the electrostatic image-bearing surface and the toner particles are drawn from the brush to the latent image by electrostatic attraction. Thus, a devel-oper mixture may be provided comprising a toner material and a carrier material which consists of particles which are magneti-cally attractable. Consequently, iron and magnetic ferrite materials have been employed as the carrier material in the elec-trostatographic arts.
In the past, ferrite materials have generally been prepared by dry and wet methods. The dry method involves the intimate mixing of pure oxides or carbonates of the desired lOS4837 D/73069 metallic constituents and causing the mixture to react at elevated temperatures to form the desired structure. This method requires extensive ball-milling of the oxides or car-bonates, usually dispersed in a liquid, until an efficient degree of mixing is obtained. The mixture is usually then dried, granulated, pre-sintered to form the desired structure, reground to attain a suitable particle size distribution, pressed or compacted with a binder material, and finally sintered or refired at temperatures above the pre-sintering temperature.
This method is undesirable in that it results in ferrite material of large crystallite or grain size having a high temperature coefficient of permeability or decreased tempera-ture stability. The wet method generally involves the forma-tion of an intimate mixture of the desired components by co-precipitation from solution. Usually, the components are dissolved as nitrates and co-precipitated as hydroxides, car-bonates or oxalates. The product, after filtration and washing, is then prefired, reground, sized, compacted with a binder, and finally sintered or refired at temperatures above the pre-2 sintering temperature. This method also has the disadvantage of resulting in ferrite materials of large crystallite or grain size having a high temperature coefficient of permeability or decreased temperature stability. Both the dry and wet methods have the further disadvantage of requiring compaction of the product with a binder prior to final firing which is a time consuming, expensive step and which limits the firing temperature ~054837 3069 and further causes bead to bead agglomeration and sticking of beads to surfaces of sintering equipment.
Other techniques of producing magnetic powder are known such as preparing a powdered alloy and mechanically disin-tegrating the alloy to magnetic particles and blowing the magnetic particles through a reducing gas flame at a temperature sufficient to melt the particles to spherical form, and cooling and collect-ing the particles so obtained as disclosed in U. S. Patent 2,186,659.
Even though this technique can produce spherical particles, to avoid undesired reactions such as oxidation of the particles a protective gas stream such as hydrogen or nitrogen is generally re~uired. Further, the product coming from the ball mill must be balled in a compressed gas flame and the ball material caught in a liquid bath~ In addition, the balled material thus produced must generally be mixed in a kneading machine with a binding medium, such as an artificial resin that can be solidified. After drying, the material must be compressed in a suitable manner.
Another process of preparing ferrite beads is disclosed by A. Berg et al in Canadian Patent 1,000,477 is~ued November 30, 1976. In this patent, the process comprises making ferrite materials by preparing a slurry of meta~ oxides in a liquid, spray drying the slurry of metal oxides to form metal oxide beads, and sintering the metal oxide beads to form ferrite beads. However, it has been found that a substantial portion of ferrite beads made by this process have a characteristic void or dimple present in the dried particle whereas .

O7_ ~,~, ferrite beads which are spherical in shape and continuous in surface area are desired. Previous attempts to reduce or eliminate voided areas in ferrite beads made by this process have not been successful. Since previously known ferrite preparation processes are deficient in one or more respects, there is a continuing need for an improved ferrite production process and improved ferrite materials.
Thus by one aspect of the present invention there is provided a process for making substantially spherical, void-free ferrite particles comprising preparing a slurry of ferrite forming metal oxides in a liquid wherein said slurry has a ~iscosity at about 23C of between about 55,000 centipoise and about 140,000 centipoise, spray drying said slurry of metal oxides to form substatially spherical metal oxide beads to form ferrite particles which are substan~ially shperical in shape, continuous in surface area, and free of interior voids.
By another aspect of the present invention there is provided a substantially shperical, void-free electrostatoyraphic ferrite carrier particle, said ferrite carrier particle having been made by a process comprising preparing a slurry of ferrite forming metal oxides in alliquid wherein said slurry has a viscosity at about 23C of between about 55,000 centipoise and about 140,000 centipoise, spray-drying said slurry of metal oxides to form substantially shperical metal oxide beads, and sintering said substantially shperical rnetal oxide beads to form ferrite particles which are substantially spherical in shape, continuous in surface area, and free of interior voids.
By still another aspect of the present invention there is provided an electrostatographic imaging process comprising the steps of forming an electrostatic latent image on a surface and developing said electrostatic latent imaye by contacting said electrostatic latent image with a developer mixture ~ -8-comprising finely-divided toner particles electrostatically clinging to the surfaces of carrier particles having a particle size of from about 30 to about 1,000 microns, each of said carrier particles comprising carrier particles having been made by a process comprising preparing a slurry of ferrite forming metal oxides in a liquid wherein said slurry has a viscosity at about 23C of between about 55,000 centipoise and about 140,000 centipoise, spray-drying said slurry of metal oxides to form substantially shperical metal oxide beads, and sintering said substantially spherical metal ,oxide beads to form ferrite particles which are substantially spherical in shape, continuous in surface area, and free of interior voids, whereby at least a portion of said finely-divided toner particles are attracted to and held on said surface in conformance to said electrostatic latent imag~.

-8a-lOS4837 These and many other aspects will become more readily apparent when the following specification is read and considered in the light of the attendant drawings in which:
Fig. 1 is a perspective view of a ferrite-forming metal oxide mixture spray-dried from a slurry having a viscosity of about 9,000 centiposes at about 23C showing that the beads have discontinuous surfaces and contain voids in their interiors.
Fig. 2 and Fig. 3 are perspective views of a ferxite-forming metal oxide mixture spray-dried from a slurry having a viscosity of about 131,000 centipoises at about 23C showing respectively that the beads have void-free interiors and continuous surface areas.
The foregoing features and others are accomplished, generally speaking, by preparing a slurry of ferrite forming metal oxides in a liquid wherein said slurry has a viscosity at 23C of between about 55,000 centipoise and about 140,000 centi-poise, spray drying the slurry of metal oxides to form substantially spherical metal oxide beads, and sintering the substantially spherical metal oxide beads to form ferrite particles which are substanitally spherical in shape, continuous in surface area, and _g_ l(~S4837 D/73069 free of interior voids. Thus, ferrite materials having the aforementioned desired properties may be prepared by the process of this invention wherein the viscosity of the slurry of ferrite forming metal oxides at 23C is at least about 55,000 centipoise and up to about 140,000 centipoise or about the practical limita-tions of the equipment employed in preparing the ferrite materials of this invention.
The metal oxide materials may be selected first on the basis of desired ferrite properties. In a preferred embodiment using a high speed mixer, the metal oxide starting materials are slowly added to a make-up tank while a deflocculent is added so that the solids are continually wetted out. A smooth, homo-genous slurry is generally formed after approximately thirty minutes of agitation depending upon the equipment capacity and the size of the batch prepared. If the finished ferrite is to be composed of several components for use as an electrostatographic carrier particle, it is usually desirable to achieve an intimate mixture of the metal oxide starting materials by this slurry preparation process. The actual degree of mixing achieved may be controlled by the choice of equipment used and selection of specific equipment operating parameters and/or slurry conditions such as mixing speed, mixing time, viscosity and temperature.
The metal oxide starting materials may be mixed in slurry form in any one of the following types of equipment such a ball-mill, vibrating pebble mill, high speed stirrer with counter turning rotor and blades, impeller mixer, high speed dispersator, and 1~)54837 ~
D/73069 other conventional mixing equipment. As an alternative, one may dry mix the metal oxide starting materials and combine the dry mixture at a later time with a liquid medium. Following the slurrying operation, it is generally preferred to screen the slurries prior to spray drying in order to eliminate any large solid particles which may be present as would plug a pressure atomizer.
A spray dryer designed for either spray nozzle atom-ization or spray machine-disc atomization or equivalent may be employed to dry the slurry of metal oxide starting materials.
A particularly desirable type of spray machine is one that is essentially a closed pump impeller driven by a variable speed drive and is commonly termed a spinning atomizer, disc or wheel.
The total systsm generally consists of a power-coolant-lubrication console, power cables, fluid transport hoses, and a variable speed motor drive with closed impeller. The high speed impeller uses the energy of centrifugal force to atomize the slurry. The particle size distribution obtained with this spray machine is generally narrow. In addition, product characteristics may be varied by the spinning atomizer design, speed and position in the chamber relative to air entrance. Preferably, when employing the spinning atomizer, the spray drying should have a large diameter configuration to avoid sticking of the atomized metal oxide particles to the dryer chamber walls. Slurries of metal oxides may be atomized using two-fluid nozzles where the atomizing force is pressurized air, single-fluid pressure nozzles where the atomizing l()S4837, D/73069 force is the pressure of the slurry itself released through an orifice, and centrifugal atomization by a spinning wheel or other suitable atomization method. The atomizing pressures, or the speed of rotation in the case of wheel atomization, and the slurry feed rates may be varied as a partial control of particle size. It is also possible to control the particle size of the spray dried metal oxide beads by varying the percentage of solids in the feed slurry. The atomizing force and feed rate should be adjusted to the configuration, size and volumetric air flow of a given drying chamber in order that atomized particles do not contact drying chamber surfaces while still wet. In accordance with the process of this invention the percentage of solids in the feed slurry may be varied from about ~0 to about 80 percent by weight of oxides slurried in the liquid medium. If a defloc-culent material is added to the metal oxide slurry, the concentra-tion of deflocculent may be varied from about 0.01 to about 2.0 percent by weight of the oxide sollds. Although considerablelatitude exists in regard to the metal oxide particle sizes em-ployed for the slurry, metal oxide particles having an average particle size less than about-25 microns are preferred to avoid high settling rates in the slurry.

It has been found that no binder material need be added to the feed slurry in order to preserve the shape and inte-grity of the atomized metal oxide beads formed during the spray drying and collecting steps of the process of this invention.
The elimination of a binder material in the formation of spray ~C~S4837 D/73069 dried metal oxide beads has been found to provide a denser and stronger ferrite material following sintering of the spray dried beads. The elimination of binder material from spray dried metal oxide beads is preferred because it has been found that binder material promotes bead-to-bead agglomeration or adherence to equip-ment surfaces during the sintering step. The spray dried metal oxide beads may be collected in drying chambers of suitable size.
Spray dried metal oxide beads have been collected in a chamber 30 inches in diameter and 6 feet in height, with volumetric air flow of about 250 cfm. With a system of this type, a product collection rate of about 30 pounds per hour may be maintained. The same metal oxide slurry may be dried in a chamber 12 feet in diameter and 20 feet in height, with volumetric air flow of about 12,000 cfm. When employing this latter system, a product collection rate of about 1500 pounds per hour of spray dried metal oxide material may be maintained. It has been found that both types of dryer systems will produce a spray dried metal oxide product in the size range for a particular electrostatographic use, for example, on the order of 50 to S00 microns. In addition, both co-current and counter-current drying systems yield satisfactory products. The temperature of the drying air may be varied from about 400F to about 900F at the inlet and from about 200F to about 700F at the outlet with satisfactory results.

Any suitable type of sintering furnace may be employed in the sintering step of the process of this invention. Typical sintering furnaces include a static furnace, a rotary kiln, a 1~4~37 D/73069 tunnel kiln, or an agitated bed furnace. The static furnace type will generally provide for long residence times. The tunnel kiln type of sintering furnace generally provides uniform product re-action, consistent residence time and high capacity throughput.
During the sintering step, a special media such as a flow promoting ingredient, for example, aluminum oxide, zirconium oxide, or other materials may be added in combination with the metal oxide beads in a quantity sufficient to minimize or avoid bead-to-bead aggl~meration and bead'to furnace wall sticking. Generally, a quantity of flow promoting ingredient in an amount of from about 0.5 parts by weight to about 2.0 parts by weight, based on the weight of the metal oxide beads, provides satisfactory results.
Preferably, the flow promoting inyredient is added to the metal oxide beads prior to sintering in the amount of about l part by weight to about l part by weight of the metal oxide beads and is larger than the size of the spray dried metal oxide beads because bead-to-bead agglomeration and bead to furnace wall sticking is substantially eliminated. Thus, if the spray dried beads are about lOO microns, the flow promoting ingredient should be about 600 microns. Further, such a flow promoting ingredient may also influence the electrostatographic properties of the ferrite carrier material. In addition, to further avoid or minimize metal oxide bead sticking to rotary furnace walls a scraping device may be employed individually or in combination with the flow promoting ingredient. In any event, the sintering of metal oxide beads should be under controlled conditions as to preserve the shape -:L~-~05~837 D/73069 and particulate nature of the beads while providing a uniform furnace residence time to produce maximum bead uniformity and desired properties.
When the sintered ferrite material is to be employed in the electrostatographic art, it is desirable that the ferrite material when employed as a carrier possess certain basic properties.
The ferrite carrier should have uniform electrostatographic properties such as triboelectric response, magnetic permeability, and electrical conductivity as to meet machine performance require-ments. The ferrite carrier should be substantially uniform in size and sufficiently dense individual beads in order to minimizepossible bead sticking to the photoreceptor. The ferrite carrier should have uniform surface characteristics with a minimum of surface contamination. Finally, the ferrite carrier should be of a uniform shape with maximum roundness and shpericity, continu-ous in surface area, and free of interior voids.
Firing of the metal oxide spray dried beads at elevatedtemperatures to induce reaction of the ferrite components is generally carried out at between about 1150 and about 1600C.
Actually, lower and higher temperatures may be used, but this is dictated by the processing time, the furnace materials of construc-tion generally available, the ferrite formulation and the resulting strength of the fired bead. Generally, if a nickel-zinc ferrite carrier material is fired at 1100C for less than one hour, the carrier material may lack mechanical strength and sufficient solid state reaction than if one chooses to fire at a higher temperature, lOS~137 D/73069 for example, 1400C or 1500C. This is particu7arly importantwith respect to the resulting mechanical strength of the carrier material. To achieve the desired electrostatographic carrier pro-perties, based on firing, the firing time, the firing atmosphere, and the temperature relationship is important to establish the minimum firing conditions relative to the bead strength. Optimum electrostatographic ferrite carrier properties are obtained at sintering temperatures ranging from about 1200C to about 1300C
with a residence time of about 60 to about 180 minutes. The pre ferred range of sintering temperatures is from about 1150C to about 1500C with a residence time of about 10 to about 180 minutes because the ferrite materials are magnetic, have a polycrystalline spinel structure, are highly resistive, and provide the maximum electrostatographic response. Satisfactory electrostatographic ferrite carrier properties are also obtained at sintering tempera-tures ranging from about 900C to about 1600C with a residence time of about 5 minutes to about 5 hours. In any event, the sintering conditions should be sufficient to provide the desired polycrystalline spinel ferrite structure.
The firing atmosphere used is also important in that it influences oxygen content and thus the oxidation state of the metal ions present in the forming crystal structure. Here also, the conductivity of the ferrite carrier is influenced by an oxygen rich or deficient atmosphere. An example of the influence of the firing atmosphere is clearly demonstrated in the preparation of a ferrous-ferric ferrite from ferric oxide. When the material is 1~4~37 D/73069 fired in an oxidizing atmosphere. Such as an ambient or oxygen-rich atmosphere, inferior magnetic properties are obtained whereas firing in a suitable reducing atmosphere such as one containing from about one to about eight percent by volume of oxygen provides acceptable magnetic properties.
Any suitable size of sintering furnace may be em-ployed in the sintering step of the process of this invention.
Static or tunnel furnaces are preferred because they generally provide a consistent residence time and uniformity of product reaction. Thus, metal oxide beads spray dried in accordance with the process of this invention may be successfully pro-cessed through a laboratory sized static furnace. Tonnage lots may be processed in a tunnel kiln, gas or electric fired, at high throughput rates. Where pre-sintering is desirable, the preferred conditions consist of pre-sintering the spray dried metal oxide beads in a furnace at about 900C to about 1300C
with about a 10 to 25 minute residence time because these condi-tions provide bead strengthening and densification which assists in preservation of bead shapeand integrity during the final sintering step. This pre-sintering procedure provides suffi-cient reaction time to insure desired electrostatographic and magnetic properties of the ferrite carrier material following the final sintering step. Following sintering, cooling with about 12 to 15 hour residence time generally provides transition from the firing temperature to that of the final cooling. This method of cooling generally provides retention of desired electrostatographic properties of ferrite carrier materials.

105~837 D/73069 Magnetic permeability, electrical conductivity, and triboelectri-city can be controlled by controlling the cooling rate. For example, the electrical resistivity may be decreased by two to three orders of magnitude by rapid cooling.
Surprisingly, it has been found that no binder material or additive other than a deflocculent need be mixed with the feed slurry of the metal oxide starting materials. Spray dried spheri-cal metal oxide beads formed in accordance with the process of this invention unexpectedly retain their shape and integrity during the gpray drying, collecting, classifying, and sintering steps.
The absnece of binder material benefits the process in that the slurry is less apt to clog nozzle orifices under pressure and also, equally important, the drying temperature is not thereby limited. That is, when a binder material is present, the drying temperature is usually limited to prevent loss of the binder by oxidation. In addition, the use of higher drying temperatures allows an increase in the slurry feed rate to the spray dryer.
If, however, a binder is employed it may comprise any suitable fugitive film forming material. Typical fugitive film forming binders include polyvinyl alcohol, dextrine, lignosulfonates and methyl cellulose.
In accordance with the process of this invention it has been found beneficial to employ a deflocculent when prepar-ing the metal oxide slurry. Any suitable deflocculent may be employed. Typical deflocculents include the ammonium or sodium salt of polymethacrylic acid, pyrogallic acid, tannic acid, and ~054837 /73069 humic acid and the salts of a mixed aryl hydroxy carboxylic acid and a complex inorganic acid. The preferred deflocculent is the sodium salt of polymethacrylic acid because it generally promotes the preparation of a concentrated metal oxide slurry having a solids content of up to about 80% by weight in water based on the total weight of the slurry. Further, in spite of this remark-ably high solids content, the metal oxide feed slurry may be pumped to the spray dryer and atomized without clogging in a pressure nozzle or wheel atomizer. In addition, where about 50 to about 500 micron beads are desirable, the high solid content of the metal oxide slurry contributes to attainment of such particle sizes. Further, the high concentration of oxides reduces the equipment and energy requirements necessary to form the particles.

Any suitable pigmented or dyed electroscopic toner material may be employed with the ferrite carrier materials pro-duced in accordance with the process of this invention. Typicaltoner materials include: gum copal, gum sandarac, rosin, cumaron-eindene resin, asphaltum, gilsonite, phenolformaldehyde resins, rosin-modified phenolformaldehyde resins, methaxrylic resins, polystyrene resins, polypropylene resins, epoxy resins, polyethy-lene resins and mixtures thereof. The particular toner materialto be employed obviously depends upon the separation of the toner particles from the ferrite carrier materials in the triboelectric series. As is well known in the art, sufficient separation should exist to permit the toner to electrostatically cling to the sur-face of the carrier. Among the patents describing electroscopic 1~54~3~
D/73069 toner compositions are U. S. Patent 2,659,670 to Copley; U.S.
Patent 2,753,308 to Landrigan; U. S. Patent 3,079,342 to Insalaco; U. S. Patent Reissue 25,136 to Carlson and U. S. Patent 2,788,288 to Reinfrank et al. These toner material generally have an average particle diameter between about 1 and about 30 microns. Generally speaking, satisfactory results are obtained when about 1 part toner is used with about 10 to about 200 parts by weight of carrier.
Nickel-zinc ferrite and manganese-zinc ferrite carrier materials produced in accordance with the process of this inven-tion are preferred because they have triboelectric properties which vary from 8 to 40 micro-coulombs per gram of toner depend-ing on the specific toner used. Generally, the triboelectric value of the ferrite carriers decreases as to the amount of iron oxide present is increased. Increasing the iron content beyond the stoichiometric amount of two moles per mole of diva-lent metal and firing at temperatures above 1200C induces the formation of divalent iron. The presence of divalent and tri-valent iron causes an increase in the electrical conductivity of the ferrite materials. Thus, the extent of divalent iron formed and the conductivity of the ferrite and resulting developed electrostatic latent image background desired may be controlled within broad limits. Therefore, a ferrite carrier material having high electrical conductivity generally provides a developed elec-trostatic latent image with low background.
Generally, the ability to magnetically hold a ferrite carrier material of the nickel-zinc ferrite type in a magnetic i~S~1~37 D/73069 bxush configuration diminishes as the nickel to æinc ratio is decreased in the composition. At the various firing conditions, a significant loss in magnetic permeability is noted at nickel to zinc ratios of less than about 0.3. In electrostatographic machine evaluations it is found that the nickel-zinc ferrite carrier materials provide optimum electrostatographic response when the nickel to zinc molar ratio of about 0.3 or greater is present in the ferrite formulations. In addition, ferrites represented by MlM2Fex04+ prepared in accordance with the process of this invention have satisfactory electrostatographic properties when employed as carriers for electrostatographic developers when Ml and M2 comprise between about 0.1 to about 0.9 moles of metal oxide such as those described above and both Ml and M2 total 1.0, and x comprises about 1.4 to about 4.0 moles of iron. All the ferrite carriers exhibit magnetic permeability adequate for magnetic brush operation when ~intered for about 5 minutes to about 5 hours at temperatures between about 900C and about 1600C.
In accordance with the process of this invention, it is possible to form spherical substantially void-free metal oxide spray dried beads by atomizing and drying a slurry of metal oxide starting materials without the addition of a binder material.
Thus, this process avoids the conventional requirement of mixing a binder material such as an artificial resin with a metal oxide slurry in order to form metal oxide beads -that are void-free, continuous in surface area, and retain their particulate shape and integrity after spray drying and sintering to convert them lOS41~37 D/73069 to ferrites. In addition, this process avoids the step of press-ing or compacting the metal oxide mixtures prior to sintering.
Further, firing temperatures between about 900C and about 1600C
may be employed in sintering the spray dried metal oxide beads without substantial bead-to-bead agglomeration since no binder material is used. This process also permits storage of the spray dried metal oxide beads prior to their sintering without problems of caking, bead fracture, or significant loss of physical, chemi-cal, and mechanical properties. In addition, sticking of beads to the surfaces of sintering equipment is substantially avoided.
Further, ferrite materials produced according to this process have been found to possess improved uniformity of particle size and particle size distribution. The uniformity of particle size that may be obtained by the process of this invention has been found to provide ferrite carrier materials which have properties that are extremely desirable when employed in electrostatographic development processes. This process further provides economic efficiency and simplicity in the production of ferrite materials, avoids agglomeration and clogging problems in processing equipment common to conventional methods of preparing ferrite materials, removes restrictions imposed on conventional methods of preparing ferrite materials, is capable of producing extremely small particle size ferrite materials and ferrite materials of a desired size, and is particularly advantageous in preparing ferrite materials ranging from about 50 to 500 microns. Finally, this process may be employed to form ferrite materials of various compositions and characteristics.

~05~837 The following examples further define, describe and compare exemplary methods of preparing ferrite materials accord-ing to the process of the present invention. Parts and percent-ages are by weight unless otherwise indicated. The examples, other than the control examples, are intended to illustrate the various preferred embodiments of the present invention.
In the following examples, the unit employed for spray drying is a Bowen Tower Laboratory Spray Dryer manufactured by Bowen Engineering Incorporated, North Branch, ~ew Jersey. This unit has a bottom chamber collector and a single cyclone collector.
This chamber collector is 30 inches in diameter and the vertical chamber height is 6 feet. Nozzle atomization is upward with a maximum vertical particle path height of about 8 feet. The incoming air is heated by direct gas firing.
EXAMPLE I
A powdered metal oxide and water feed slurry comprising about 100 pounds of about 62.6 percent ferric iron oxide having a particle size of àbout 0.5 micron, about 26.6 percent zinc oxide having a particle size of about 0.1 micron, about 10.8 percent nickel oxide having a particle size of about 10 micronsj and about 43.2 pounds of water is prepared using a high speed dispersator.
About 0.825 percent by weight, based on the weight of the solids, of Darvan 7 ~the sodium salt of a polymethacrylic acid available 2~ from the R. T. Vanderbilt Oompany~ is added to the oxide slurry mixture~ The slurry mixture is about 70 percent by weight of . ..
* ~rademark -23_ T~ ~

i~5~837 D/73069 solids. The slurry is screened using 80 mesh seives to remove non-wetted agglomerates characteristic of open tank mixing of oxides. The slurry is found to have a viscosity of about 9,000 centipoises at about 23C. This slurry is then fed to the sprav dryer at a feed rate of between about 45 and about 55 pounds per hour, a drying air input temperature of about 490-500F~ and an outlet temperature of about 310-330F The type of atomizer is a single-fluid pressure nozzle and the atomizing force is about 160-165 psig provided by moving cavity pump. Spray dried metal oxide beads of about lO0 microns are obtained. The dryer surfaces are dry. The beads collected in the dryer chamber are a dry, free-flowing powder, but are found to contain voids in the interior of the beads and to have non-spherical surfaces as depicted in Figure 1.
EXAMPLE II
A powdered metal oxide and water feed slurry comprising about 5600 grams of about 62.6 percent ferric iron oxide having a particle size of about 0.5 micron, about 26.6 percent zinc oxide having a particle size of about 0.1 micron, about 10.8 percent nickel oxide having a particle size of about lO microns, and about 1950 grams of water is prepared using a high speed dispersator.
About 0.29 percent by weight, based on the weight of the solids, of Darvan 7 (the sodium salt of a polymethacrylic acid available from the R~ T. Vanderbilt Company) is added to the oxide slurry mixture. The slurry mixture is about 65 percent by weight of solids.
The slurry is screened using 80 mesh seives. The slurry is found lOS4~37 D/73069 to have a viscosity of about 131,000 centipoises at about 23C.
'rhis slurry is then fed to the spray dryer at a feed rate of between about 45 and about 55 pounds per hour, a drying air input temperature of about 490-500F, and an outlet temperature of about 310-330F. The type of atomizer is a single-fluid pressure nozzle and the atomizing force is about 160 psig pro-vided by moving cavity pump. Spherical spray dried metal oxide beads of about 100 microns are obtained. The dryer surfaces are dry. The beads collected in the dryer chamber are a dry, free-flowing powder and are found to have void-free interiors and continuous surface areas as depicted in Figure 2 and Figure 3, respectively.
EXAMPLE III
A powdered metal oxide and water feed slurry compris-ing about 9600 grams of about 62.6 percent ferric iron oxide having a particle size of about 0.5 micron, about 26.6 percent zinc oxide having a particle size of about 0.1 micron, about 10.8 percent nickel oxide having a particle size of about 10 microns, and about 2850 grams of water is prepared using a high speed dispersator. About 0.33 percent by weight, based on the weight of the solids, of Darvan 7 (the sodium salt of a poly-methacrylic acid available from the R. T. Vanderbilt Co.) is added to the oxide slurry mixture. The slurry mixture is a~out 65 percent by weight of solids. The slurry is screened using 80 mesh seives. The slurry is found to have a viscosity of about 17,000 centipoises at about 23C. This slurry is then fed to the iO541~37 D/73069 spray dryer at a feed rate of between about 45 and about 55 pounds per hour, a drying air input temperature of about 500F, and an outlet temperature of about 310-330F. The type of atomizer is a single-fluid pressure nozzle and the atomizing force is about 125 psig provided by moving cavity pump. Spray dried metal oxide beads of about 100 microns are obtained. The dryer surfaces are dry. The beads collected in the dryer chamber are a dry, free-flowing powder, but are found to contain voids in the interior of the beads and to have non-spherical surfaces.
EXAMPLE IV
A powdered metal oxide and water feed slurry compris-ing about 14,000 grams of about 62.6 percent ferric iron oxide having a particle size of about 0.5 micron, about 26.6 percent zinc oxide having a particle size of about 0.1 micron, about 10.8 percent nickel oxide having a particle size of about 10 microns, and about 4800 grams of water is prepared using a high speed dispersator. About 0.28 percent by weight, based on weight of the solids, of Darvan 7 (the sodium salt of polymethacrylic acid available from the R. T. Vanderbilt Co.) is added to the oxide slurry mixture. The slurry mixture is about 65 percent b~
weight of solids. The slurry is screened using 80 mesh seives.
The slurry is found to have a viscosity of about 106,000 centi-poises at about 23C. This slurry is then fed to the spray dryer at a feed rate of between about 45 and about 55 pounds per hour~- a drying air input temperature of about 500F, and an outlet temperature of about 310-355F. The type of atomizer is 1054~37 /73069 a single-fluid pressure nozzle and the atomizing force is about 175 psig provided by moving cavity pump. Spherical spray dried metal oxide beads of about 100 microns are obtained. The dryer su~faces are dry. The beads collected in the dryer chamber are a dry, free-flowing powder and are found to have void-free interiors and continuous surface areas.
EXAMPLE V
A powdered metal oxide and water feed slurry comprising about 5700 grams of about 62.6 percent ferric iron oxide having a particle size of about 0.5 micron, about 26.6 percent zinc oxide having a particle size of about 0.1 micron, about 10.8 percent nickel oxide having a particle size of about 10 microns, and about 1900 grams of water is prepared using a high speed dispersator.
About 0.75 percent by weight, based on the weight of the solids, of Clay Deflocculent #5, (the ammonium salt of a mixed aryl hydroxy carboxylic acid containing a small amount of sodium salt of a com-plex inorganic acid, available from the R. T. Vanderbilt Co.) isadded to the oxide slurry mixture. The slurry mixture is about 65 percent by weight of solids. The slurry is screened using 80 mesh seives. The slurry is found to have a viscosity of about 66,000 centipoises at about 23C. This slurry is then fed to the spray dryer at a feed rate of between about 45 and about 55 pounds per hour, a drying air input temperature of about 500F, and an outlet temperature of about 310F. The type of atomizer is a single-fluid pressure nozzle and the atomizing force is about 150 psig provided by moving cavity pump. Spherical spray dried metal oxide beads of ~54837 D/73069 about 100 microns are obtained. The dryer surfaces are dry. The beads collected in the dryer chamber are a dry, free-flowing powder and are found to have v,oid-free interiors and continuous surface areas.
EXAMPLE VI
A powdered metal oxide and water feed slurry comprising about 21,000 grams of about 62.6 percent ferric iron oxide having a particle size of about 0.5 micron, about 26.6 percent zinc oxide having a particle size of about 0.1 micron, about 10.8 percent nickel oxide having a particle size of about 10 microns, and about 7200 grams of water is prepared using a ball mill mixing device.
About 0.33 percent by weight, based on the weight of the solids, of Darvan 7 (the sodium salt of a polymethacrylic acid available from the R. T. Vanderbilt Co.) is added to the oxide slurry mix-ture. The slurry mixture is about 64 percent by weight of solids.
The slurry did not require screening. The slurry is found to have a viscosity of about 46,000 centipoises at about 23C. This slurry is then fed to the spray dryer at a feed rate of between about 45 and about 55 pounds per hour, a drying air input temperature of about 500F, and an outlet temperature of about 260F. The type of atomizer is a single-fluid pressure nozzle and the atomizing force is about 170 psig provided by moving cavity pump. Spherical spray dried metal oxide beads of about 100 microns are obtained.
The dryer surfaces are dry. The beads collected in the dryer cham-ber are a dry, free-flowing powder and are found to have smaller voids in their interiors than those made with lower viscosity slurries.

~OS4~37 A powdered metal oxide and water feed slurry compris-ing about 21,000 grams of about 62.6 percent ferric iron oxide having a particle size of about 0.5 micron, about 26.6 percent zinc oxide having a particle size of about 0.1 micron, about 10.8 percent nickel oxide having a particle size of about 10 microns, and about 7200 grams of water is prepared using a ball mill mixing device. About 0.32 percent by weight, based on the weight of the solids, of Darvan 7 (the sodium salt of a polymethacrylic acid available from the R. T. Vanderbilt Co.) is added to the oxide slurry mixture. The slurry mixture is about 65 percent by weight of solids. The slurry did not require screening. The slurry is found to have a viscosity of about 91,000 centipoises at about 23C. This slurry is then fed to the spray dryer at a feed rate of between about 45 and about 55 pounds per hour, a drying air input temperature of about 500F, and an outlet temp-erature of about 280-340F. The type of atomizer is a single-fluid pressure nozzle and the atomizing force is about 170 psig provided by moving cavity pump. Spherical spray dried metal oxide beads of about 100 microns are obtained. The dryer surfaces are dry. The beads collected in the dryer chamber are a dry, free-flowing powder and are found to have void-free interiors and continuous surface areas.
EXAMPLE VIII
A powdered metal oxide and water feed slurry comprising about 42,000 grams of about 62.6 percent ferric iron oxide having 105~1~37 D/73069 a particle size of about 0.5 micron, about 26.6 percent zinc oxide having a particle size of about 0.1 micron, about 10.8 percent nickel oxide having a particle size of about 10 microns, and about 14,500 grams of water is prepared using a ball mill mixing device. About 0.32 percent by weight, based on the weight of the solids, of Darvan 7 (the sodium salt of a polymethacrylic acid available from the R. T. Vanderbilt Co.) is added to the oxide slurry mixture. The slurry mixture is about 65 percent by weight of solids. The slurry did not require screening. The slurry is found to have a viscosity of about 80,000 centipoises at about 23C. This slurry is then fed to the spray dryer at a feed rate of between about 45 and about 55 pounds per hour, a drying air input temperature of about 500F, and an outlet tempera-ture of about 310-330F. The type of atomizer is a single-fluid pressure nozzle and the atomizing force is about 150 psig provided by moving cavity pump. Spherical spray dried metal oxide beads of about 100 microns are obtained. The dryer surfaces are dry. The beads collected in the dryer chamber are a dry, free-flowing powder and are found to have void-free interiors and continuous surface areas.

EXAMPLE IX
Spray dried metal oxide beads prepared in accordance with the process of Example VIII are placed in a direct electric-fired static kiln. About 1 pound, per 1 pound of the metal oxide beads, of aluminum oxide particles having a diameter of between about 600 microns are included with the metal oxide beads.

10~4837 D/73069 Sintering of the oxide beads is conducted at a temperature of about 1300C for about two hours. After cooling, the sintered metal oxide beads are examined and found to have void-free i~teriors and continuous surface areas.
Although specific materials and conditions are set forth in the above exemplary processes of making ferrite materials by the process of this invention, these are merely intended as illustrations of the present inventionO There are other ferrite materials, solvents, substituents and processes such as those listed above which may be substituted for those in the Examples with similar results.
Other modifications of the present invention will occur to those skilled in the art upon a reading of the present disclo-sure. These are intended to be included within the scope of this invention.

Claims (27)

WHAT IS CLAIMED IS:

1. A process for making substantially spherical, void-free ferrite particles comprising preparing a slurry of ferrite forming metal oxides in a liquid wherein said slurry has a viscosity at about 23°C of between about 55,000 centipoise and about 140,000 centipoise, spray drying said slurry of metal oxides to form substantially spherical metal oxide beads, and sintering said substantially spherical metal oxide beads to form ferrite particles which are substantially spherical in shape, continuous in surface area, and free of interior voids.

2. A process for making substantially spherical, void-free ferrite particles according to Claim 1 including adding a deflocculent to said slurry of said ferrite forming metal oxides prior to spray drying said slurry.

3. A process for making substantially spherical, void-free ferrite particles according to Claim 2 wherein said deflocculent is added to said slurry of ferrite forming metal oxides at a concentration of from about 0.01 to about 2.0 per-cent by weight based on the weight of said metal oxides.

4. A process for making substantially spherical, void-free ferrite particles according to Claim 3 wherein said defloc-culent comprises the sodium salt of polymethacrylic acid.

5. A process for making substantially spherical, void-free ferrite particles according to Claim 1 wherein a dry mixture of ferrite forming metal oxides is added to said liquid in pre-paring said slurry.

6. A process for making substantially spherical, void-free ferrite particles according to Claim 1 including screening said slurry of ferrite forming metal oxides prior to spray drying said slurry.

7. A process for making substantially spherical, void-free ferrite particles according to Claim 1 wherein said slurry of ferrite forming metal oxides has a solids content of from between about 40 percent to about 80 percent based on the total weight of said slurry.

8. A process for making substantially spherical, void-free ferrite particles according to Claim 1 wherein said ferrite particles have an average particle size between about 50 microns and about 500 microns.

9. A process for making substantially spherical, void-free ferrite particles according to Claim 1 wherein said slurry of ferrite forming metal oxides is suspended in a heated gas stream until said liquid is removed from said slurry.

10. A process for making substantially spherical, void-free ferrite particles according to Claim 1 including adding a flow-promoting ingredient to said metal oxide beads prior to sintering said metal oxide beads.

11. A process for making substantially spherical, void-free ferrite particles according to Claim 10 wherein said flow-promoting ingredient is larger than the size of said metal oxide beads.

12. A process for making substantially spherical, void-free ferrite particles according to Claim 10 wherein said flow-promoting ingredient is added to said metal oxide beads in a quantity sufficient to minimize or avoid bead-to-bead agglomera-tion and bead to furnace wall sticking during sintering of said metal oxide beads.

13. A process for making substantially spherical, void-free ferrite particles according to Claim 1 including presintering said ferrite forming metal oxide beads at a temperature of between about 900°C and about 1300°C for between about 10 minutes and about 25 minutes.

14. A process for making substantially spherical, void-free ferrite particles according to Claim 1 including sin-tering said ferrite forming metal oxide beads at a temperature of between about 900°C and about 1600°C for between about 5 minutes and about 5 hours.

15. A process of making substantially spherical, void-free ferrite particles according to Claim 1 including sintering said ferrite forming metal oxide beads at a tempera-ture of between about 1150°C and about 1500°C for between about 10 minutes and about 180 minutes.

16. A process for making substantially spherical, void-free ferrite particles according to Claim 1 including sintering said ferrite forming metal oxide beads at a tempera-ture of between about 1200°C and about 1300°C for between about 60 minutes and about 180 minutes.

17. A process for making substantially spherical, void-free ferrite particles according to claim 1 including sintering said ferrite forming metal oxide beads in an atmosphere contain-ing from about one to about eight percent by volume of oxygen.

18. A process for making substantially spherical, void-free ferrite particles according to Claim l including cooling said ferrite beads for about 12 to about 15 hours.

19. A process for making substantially spherical, void-free ferrite particles according to Claim l wherein said ferrite beads comprise nickel-zinc ferrite.

20. A process for making substantially spherical, void-free ferrite particles according to Claim 19 wherein said nickel-zinc ferrite comprises a nickel to zinc molar ratio of at least about 0.3.

21. A process for making substantially spherical, void-free ferrite particles according to Claim 19 wherein said nickel-zinc ferrite comprises between about 0.1 to about 0.9 moles of nickel, about 0.1 to about 0.9 moles of zinc, and about 1.4 to about 4.0 moles of iron.

22. A process for making substantially spherical, void-free ferrite particles according to Claim 1 wherein said ferrite beads comprise manganese-zinc ferrite.

23. A process for making substantially spherical, void-free ferrite particles according to Claim 22 wherein said manganese-zinc ferrite comprises between about 0.1 to about 0.9 moles of manganese, about 0.1 to about 0.9 moles of zinc, and about 1.4 to about 4.0 moles of iron.

24. A process for making substantially spherical void-free ferrite particles according to Claim 1 wherein said slurry of ferrite forming metal oxides is substantially free of binder material.

25. A substantially spherical, void-free electro-statographic ferrite carrier particle, said ferrite carrier particle having been made by a process comprising preparing a slurry of ferrite forming metal oxides in a liquid wherein said slurry has a viscosity at about 23°C of between about 55,000 centipoise and about 140,000 centipoise, spray-drying said slurry of metal oxides to form substantially spherical metal oxide beads, and sintering said substantially spherical metal oxide beads to form ferrite particles which are substantially spherical in shape, continuous in surface area, and free of interior voids.

26. A substantially spherical, void-free electrostato-graphic ferrite carrier particle according to Claim 25 wherein said ferrite particle has been sintered at a temperature of between about 900°C and about 1600°C for between about 5 minutes and about 5 hours.

27. An electrostatographic imaging process comprising the steps of forming an electrostatic latent image on a surface and developing said electrostatic latent image by contacting said electrostatic latent image with a developer mixture comprising finely-divided toner particles electrostatically clinging to the surfaces of carrier particles having a particle size of from about 30 to about 1,000 microns, each of said carrier particles comprising carrier particles having been made by a process comprising preparing a slurry of ferrite forming metal oxides in a liquid wherein said slurry has a viscosity at about 23°C of between about 55,000 centipoise and about 140,000 centipoise, spray-drying said slurry of metal oxides to form substantially spherical metal oxide beads, and sintering said substantially spherical metal oxide beads to form ferrite particles which are substantially spherical in shape, continuous in surface area, and free of interior voids, whereby at least a portion of said finely-divided toner particles are attracted to and held on said surface in conformance to said elec-trostatic latent image.