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Ferrite Processing from a Mixing Perspective

Ferrite materials have a dark grey to black color, possess strong magnetic properties and are categorized into Hard and Soft sub-classes. They are classified as ceramics due to their composition being a combination of metallic oxides.

Hard ferrites have strong coercivity, showing relatively permanent magnetic properties. Soft ferrites have low coercivity, hence their magnetization can switch direction without much energy requirement or heat generation. This characteristic can be very useful. For this reason soft ferrites find wide applications in electronic industries.

Ferrites are generally synthesized using an iron oxide based powder through a solid phase reaction incorporating other metals, such as magnesium, zinc, manganese, nickel, cobalt etc. to achieve various desired properties. One critical requirement in the synthesis is a high degree of mix homogeneity which impacts on microscopic composition, consequently product quality.

Typical processing sequence in the manufacture of ferrite products are: mix powders, freeze recipe (by granulation), dry, calcine, mill, spray-dry, form and sinter to achieve the desired magnetic properties. Calcination at sub-sintering temperatures (800° C – 1100° C) is considered beneficial in pre-consolidating metals for subsequent sintering which occurs generally between 1200° C – 1400° C. Calcination also eliminates carbonates.

In graphical form, the steps are as follows:

Ferrite Processing Sequence

Mixing and Granulation

It has been noted that for many operations including ferrite manufacture, mixing without granulation causes unacceptable property variations. This problem can be attributed to dry powders having low inter-particle attraction, hence easy to shift positions leading to de-mixing. There can be substantial de-mixing in subsequent handling prior to and during calcination. For example, the showering action of a rotary calciner tends to segregate particles.

Granulation modifies ingredient interaction, limiting or freezing particle mobility, thereby stabilizing microscopic composition. Therefore, granulation immediately following mixing is preferred, and ideally as a cycle extension in the same processing equipment. Furthermore, since granules tend to flow more readily than powders, they help to minimize the possibility of calciner mishap where process upset can turn powders into a fused mass, which becomes difficult to handle.

In a counter-current, rotating pan mixer, it takes just one to two minutes to homogenize powders and ten to fifteen minutes total to prepare a batch of spherical green granules. Dry mixing is immediately and seamlessly followed by water addition, wet mixing and granulation. Granulating moisture level is nominally 12% to 20%, depending on raw materials and process. It has been observed that a certain threshold temperature is necessary to initiate granulation, perhaps implying the presence of some type of chemical reaction.

Since iron oxide powders are abrasive, granule drying should be done downstream of the mixer-granulator, but not in it. This is because cycle time extension from heat transport as well as from accelerated tool wear considerations makes this approach uneconomical.

To promote fast and uniform heat up in the calciner, it is desirable to have relatively small spherical granules in the order of 1/8” to less than1/4” and narrow size distribution. Having smaller granules also makes it easier for the subsequent milling step. It is not necessary for the granules to have a high degree of sphericity, as long as their shape does not impede movement or cause larger aggregates to form when wet. This is unlike granulation for the purpose of dry pressing, where both high sphericity and wider size distribution aid in filling all voids in the form.

With a Lancaster K-series mixer-granulator, it is possible to adjust mixing tool speed, moisture level and time to control granule size, the degree of granule consolidation, producing optimum granule densities and hardness. Within limits, drier, smaller and less dense granules are preferred because they economize energy usage, and residence time in downstream processing.