Ferrites, processing

McKinley, J.P. Jenne, E.A. (1991) Experimental investigation and review of the solids concentration" effect in adsorption studies. Environ. Sci. Technol. 25 2082-2087 McKinnon, W. Choung, J.W. Xu, Z. Einch, J.A. (2000) Magnetic seed in ambient temperature ferrite process applied to acid mine drainage treatment. Environ. Sci. Techn. 34 2575-2581... 

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In commercial and polycrystalline ferrites processing, variables and resultant microstructures have important consequences on measured properties. Only a few will be mentioned here. For example, the addition of a few percent of cobalt to Ni-ferrite can increase its resistivity by several orders of magnitude by ensuring that the iron is maintained in the Fe " state. Similarly, it is important to sinter MnZn ferrites under reducing atmospheres to ensure that the manganese is maintained in the Mn state but not too reducing so as to convert the Fe to Fe . 

During this cooperative work with Stillwater Mining Company a variety of tests were conducted at RHTs research facilities, including a complete characterization of the Ca-ferritic processing slag and two rotary kiln tests at 1500°-1550° and 1650°C. 

On a smaller scale, the Ferrite Process is reported to be widely used in Japan to remove metals from laboratory wastewaters in universities and institutes [27]. The process, outlined in Fig. 6.7, relies upon the formation of magnetically... 

Fig. 6.7 Flowchart for the Ferrite Process heavy metals removal system, [27].

M-type ferrites are mainly used as permanent magnet material. They have largely replaced the alnicos as preferred permanent magnet material, as a result of the lower material and processing costs. These ferrites were first introduced under the trade name Ferroxdure, the isotropic form in 1952 (22) and the anisotropic (crystal oriented) form in 1954 (23), and are widely available commercially under various trade names such as Oxid and Koerox. They cover about 55% of the world market of permanent magnet materials, corresponding to 1100 million U.S. doUars (1991), as weU as 55% of the U.S. market, at 300 million. 

The magnetic properties of ferrites are intricately related to composition, microstmcture, and processing much more so than in the case of metals primarily because of the complex chemistry of the oxides and because of the ceramic processing requited to produce the finished parts. 

Liquid Nitriding. As in gas nitriding, the process is carried out below the austenite region, and hardening is associated with the formation of hard nitrides in the ferrite. Liquid cyanide salts are used with others to provide the source of nitrogen. 

Ferritic Nitrocarburizing. This process is similar to carbonitriding, except that it is carried out in the temperature range of the stabiHty of ferrite and carbide (<723° C). Therefore hardening is not by martensite formation, but because of the formation of very hard carbonitrides. 

Reaction 3 also occurs on cooling since the concentration of SO is very low at roaster temperatures of 950°C and approaches zero at 1000°C. Another important reaction that occurs during roasting is the formation of zinc ferrite, Zn0-Fe2 03 above 650°C (see Ferrites). Zinc ores contain 5—12% iron. Zinc ferrite forms soHd solutions with other spinels, such as Fe0-Fe203, and therefore the zinc—iron compositions formed are of indefinite stoichiometry. Ferritic zinc is difficult to solubilize in hydrometaHurgical leaching but several recovery processes are discussed below. 

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