molinos planetarios de molienda
Similar to the centrifugal mills, planetary mills have grinding chambers, which are moved with a fixed eccentricity, assembled on a rotating centre wheel. Additionally the single mill body rotates on its centre axis in opposite or same direction as the centre wheel.
The differential revolution speed creates centrifugal forces, which effect a media motion similar to gravity ball mills, however, with much higher acceleration and the possibility to use small grinding media below 5 mm. By setting the revolutions of centre wheel and mill body, it is possible to create basically three different media motion types: cascade and cataract motion like in gravity ball mills or media rotation similar to centrifugal mills. Accordingly, different stress mechanisms can occur and flexibly being influenced during operation by variation of the two rotations. The machine is well balanced, but the machine design is elaborate. Particularly material feed and cooling water supply are very difficult. Thus, only batch type lab mills with smaller volume are used. Only few approaches to a design of high capacity continuous planetary mills are known, none are in industrial operation.
Dry Mill Stabilisation and Additives
The major restricting factor for comminution in the submicron range is the interactive forces of small particles, which are the reason for (re-)agglomeration and sticking. The grinding forces are absorbed by agglomerates, coating of the media or layers on mill parts. This problem is minor in the liquid phase, because the shear forces of the slurries are higher and the transport of additives to the particle surfaces is easier. For several materials, Nano-scaled particles can be obtained by wet grinding with agitated media mills. With a dry milling process it is not yet possible to get a sufficient effectiveness even in the coarser submicron range.
Additives help to control the adhesion forces between particles. They can prevent re-agglomeration of just broken particles and reduce the tendency to stick at the media or mill parts. Not only for the grinding process, but also for the classification it is substantially important to avoid agglomeration, because particles can only be separated if they are well dispersed.
The easiest way to stabilise is the control of the electric double layer (Zeta-Potential) by H+ or OH- ions (pH-value). This electrostatic stabilisation is mainly applicable in water based slurries. The sterical or electrosterical stabilisation is using long chained molecules, with are applied to the particle surfaces. Those surfactants can also be used for dry particle stabilisation. In comparison to liquid slurries, the quick transport and the homogeneous distribution of the additives to the particle surface is much more difficult.