Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2001-12-28
2003-12-09
Koslow, C. Melissa (Department: 1755)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S633000, C423S634000
Reexamination Certificate
active
06660242
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the production of powders composed of barium or strontium ferrite intimately admixed with ferric iron oxide to be employed as raw materials in the production of polymer-bound magnetic materials and ceramic permanent magnets.
2. Description of the Related Art
Barium and strontium hexaferrite powders, represented by the formula MO.(Fe
2
O
3
)
x
where M is barium or strontium and x is about 5 to 6, are crystalline compounds produced for incorporation into a polymeric matrix to form flexible magnetic materials or into ceramic permanent magnets.
The most prevalent current production method for barium or strontium hexaferrite powder is the preparation of an admixture of discrete particles of barium carbonate or strontium carbonate and discrete particles of ferric oxide, then reaction of the admixture at high temperature for a period of several hours. The barium or strontium carbonate and iron oxide may be admixed as dry powders, or they may be formed into an aqueous suspension and mixed prior to introduction into a heating device. In some cases, an aqueous suspension of barium or strontium carbonate and iron oxide is subjected to ball milling or some other grinding process to reduce the size of the individual particles prior to introduction into a heating device. The reaction between the alkaline earth metal carbonate particles and iron oxide particles proceeds at temperatures above about 900° C. and is believed to proceed stepwise with the formation of intermediate ferrite products prior to the formation of the final crystalline barium or strontium hexaferrite displaying the desired magnetic properties. This production method suffers from problems associated with (1) difficulty in achieving a uniform admixture of the discrete particles of the reactants, (2) long reaction times resulting from the slow diffusion of the reactants from the discreet solid particles, and (3) difficulty in assuring that the alkaline earth metal carbonate particles and the iron oxide particles are of a consistent, small size so as to yield a consistent number of surface contact sites which can become ferrite crystal nucleation sites under high-temperature reaction conditions.
Several techniques have been invented to try to overcome the shortcomings of the admixed discrete powder method for producing barium and strontium hexaferrites. U.S. Pat. No. 4,116,752 teaches preparation of strontium hexaferrite or barium hexaferrite particles by reacting iron oxide, iron hydroxide, or iron oxyhydroxide with strontium or barium carbonate, carboxylate, oxide, or hydroxide in the presence of strontium chloride or barium chloride flux. After reacting this admixture at temperatures up to 1300° C., the water-soluble barium chloride or strontium chloride flux is extracted from the ferrite product by washing with water. This procedure is taught to produce well-defined single crystals that are not agglomerated.
U.S. Pat. No. 4,062,922 teaches that strontium nitrate solution and ferric nitrate solution can be admixed to prepare a solution containing 10.8 molecular weights of iron for each molecular weight of strontium, and this admixture can be dried in a spray dryer. The dried material from the spray dryer is then heated in air at 600° C. for about 16 hours to obtain intimately mixed grains of iron oxide and strontium oxide less than 0.02 microns in diameter; further heating at 1000° C. for 4 hours yields ferrite powder of composition SrFe
10.8
O
17.2
, or SrO.5.4Fe
2
O
3
.
U.S. Pat. No. 4,062,922 teaches preparation of alkaline earth metal ferrite magnets beginning with nitrate solutions. U.S. Pat. No. 5,306,592, teaches coprecipitation of barium or strontium hydroxide and iron hydroxide to form a raw material for ferrite production. These patents teach that particles composed of co-precipitated alkaline earth metal salt and iron salt can be subjected to high temperature reaction conditions to yield alkaline earth ferrite powders with superior magnetic properties if the salts can be easily decomposed to the oxides by heating. U.S. Pat. No. 4,025,449 teaches that mixed hydroxide precipitates containing alkaline earth metal ions and ferric iron ions in the proportions necessary to produce alkaline earth metal hexaferrite can be recovered from solutions of soluble iron salts and soluble alkaline earth metal salts; insoluble hydroxide precipitates are formed when a solution of an alkaline earth metal salt and a solution of a ferric iron salt are admixed and then added to an alkali metal hydroxide solution. This precipitate can then be reacted at temperatures up to 1500° C., for times up to several hours, to yield consistent fine crystals of alkaline earth metal hexaferrite. Thus, it is well established that intimate admixtures of barium or strontium compounds and ferric iron compounds can be advantageously employed to produce barium or strontium hexaferrite crystals with superior magnetic properties; however, the prior art methods for achieving intimate admixture have suffered from various serious drawbacks including relatively expensive raw materials, the handling of aqueous suspensions of gelatinous precipitates, disposal or treatment of dilute salt solutions, and possibly the control of air emisssions of pollutants such as oxides of nitrogen.
Almost all of the iron oxide presently being used as a raw material in the production of alkaline earth metal hexaferrite powders is a by-product of the recovery of hydrochloric acid from spent steel pickle liquor. The surface of steel sheet is cleaned by contacting it with a hydrochloric acid solution which dissolves the scale and corrosion present on the steel surface; this yields a ferrous chloride solution called spent pickle liquor. The most widely practiced means of dealing with this spent pickle liquor has become pyrohydrolysis in a spray roaster or similar equipment at temperatures up to about 1000° C. to yield hydrochloric acid (hydrogen chloride gas generated during the reaction absorbed in water) for reuse in the steel cleaning operation, and a ferric iron oxide powder by-product. This pyrohydrolysis reaction takes place in the presence of water vapor and oxygen as described by the following equation:
2FeCl
2
+2H
2
O+½O
2
→Fe
2
O
3
+4HCl
Methods have been developed to produce ferric iron oxide derived from the spent pickle liquor that is acceptable for use in the ferrite magnet industry, as a pigment, and in other applications.
Since the iron oxide utilized as a raw material in the production of alkaline earth metal ferrite magnets is recovered from iron chloride solution by a pyrohydrolysis reaction, direct production of particles composed of an intimate admixture of strontium or barium ferrite and ferric iron oxide by pyrohydrolysis eliminates duplication of processing steps and overcomes many of the shortcomings of the admixed discrete powders method of producing alkaline earth metal ferrite magnetic powders.
U.S. patent application Ser. No. 09/643,894, assigned to Chemical Products Corporation, now abandoned, teaches that particles composed of intimately admixed alkaline earth metal ferrite and ferric oxide can be produced from a solution containing alkaline earth metal chloride and iron chloride. This chloride solution reacts to form alkaline earth metal ferrite and ferric oxide at temperatures above about 800° C., and preferably above about 1000° C., in an atmosphere containing oxygen and water vapor. Spray roasting a chloride solution containing alkaline earth metal ions and iron ions in a ratio of about 1 alkaline earth metal atom to about 11.5 iron atoms yields an intimate admixture of alkaline earth metal ferrite and iron oxide that can be advantageously employed in the production of alkaline earth metal hexaferrite magnetic powders.
European Patent Application number 1 090 884, to Sumitomo Special Metals Co., Ltd., published Apr. 11, 2001, teaches that the temperature in the reaction zone of a spray roaster can be increased sufficiently to form a magnetic p
Cook Jerry Allen
Mauldin Lloyd Ballard
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