Mixing, calcining and milling

Mixing is usually carried out in a ceramic ball mill with steel balls. The inevitable attrition of the mill balls leads to an addition of 0.5-1 wt% to the iron content for which allowance must be made. Care must be taken to maintain the quantity and size distribution of the mill balls and to remove those that have become so small that they cannot be separated from the slip or powder on sieving. 

If a low oxygen pressure is required during sintering nitrogen is injected at a point in the tunnel kiln where the temperature is approximately 1000 °C, and the 

In the cases of NiFe204 and the rare earth garnets for microwave use, in which the losses due to conductivity must be minimized, an oxygen atmosphere may be required for sintering so that the concentration of Fe2+ ions is reduced to a very low level. 

For hard ferrites for use as permanent magnets relative permeability is unimportant and is usually near to unity in the magnetized state. The important parameters are a high coercive field, a high remanent magnetization and the maintenance of a suitable combination of these properties over the operational 

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