(49a) Dry Fine Grinding: Aspects of Particle Stabilization in Mill Classifier Circuit

In many industrial applications the demand on minerals with increasing product fineness rises. The production of these materials is often performed in closed mill-classifier circuits which enable the production of materials with high product fineness and constant product quality. Thereby, the use of dry operated stirred media mills is a promising and emerging option for the energy efficient production of fine particles due to their high stress intensity as well as high stress frequency compared to conventional ball mills. Generally, in dry grinding processes fine powders are not easy to produce and to handle since attractive particle-particle forces become more decisive with decreasing particle size. These forces are leading to a variety of challenges, in particular caused by a high state of agglomeration, a decrease of the powder flowability as well as material adherences on the machine and plant equipment. Obviously, these aspects may complicate the comminution within a mill-classifier circuit on various levels, simultaneously: As the attractive particle-particle forces determine the particle and bulk properties, they have a strong impact on the stress mechanism inside the mill, the particle classification and final product filtration as well as on the material transport and retention time inside the single devices. In order to control these forces, so-called liquid grinding aids which reduce the particle-particle forces by adsorbing on the surface of the product particles are often used as stabilizing additives. As the name suggests, these additives are especially known for improving the grinding performance inside the mill. However, there is only very little information available how far liquid grinding aids affect the process in mill-classifier circuits. Therefore, this study examines the application of grinding aids in mill-classifier-circuits with stirred media mills on different levels:

In a first step, the stressing behavior inside a dry operated stirred media mill was investigated in dependence of the grinding aid as well as the most important process parameters like stirred tip speed, bead size and bead density. Therefore, limestone was ground in a batch operated 1.6 liter laboratory stirred media mill. It became clear, that those mill parameters which determine the stress energy of the grinding beads have a strong impact on the grinding performance inside the mill. Beyond that, an enhancement of the grinding process as well as its control is effectively achieved by an appropriate application of grinding aids. Thereby, a crucial role of the powder flowability was identified: As the grinding aid determines the flow behavior of the product powder and thus the bulk flow as well as particle capturing between the grinding beads, it influences different important values like the number of stressed particles at each grinding bead collision as well as the resulting stress intensity. Hence, the grinding result strongly depends on the applied grinding aid. Additives that show different impacts on the bulk properties require different optimal process settings. Hence, those parameters need to be adjusted to the applied grinding aid very carefully in order to ensure effective fine grinding. As a rule of thumb, lower stress energies and higher collision frequencies are more favorable when adding powder flow assisting additives.

In the second step, similar experiments were carried out in a continuously operated 6.6 liter horizontal stirred media mill. Thereby it was examined, how far findings from the batch grinding experiments may be transferred to continuous grinding. It is shown that the batch results may give good indications for optimizing continuous stirred media milling. However, an overlapping impact caused by the different powder flowabilities was identified: Since the flow behavior of the product powder determines the axial material transport through the mill, grinding aids may imply a strong impact on the material hold-up as well as retention time within the mill. Thereby, a high powder flowability is not necessarily leading to a lower mill retention time. This circumstance is mainly caused by a deflector wheel in the end of the mill which does not only hold back the grinding beads but also big particles that have not been comminuted satisfactory. Thus, the particle stabilization achieved by grinding aids is affecting the grinding result on different levels simultaneously. All of these issues need to be taken into account when designing dry fine grinding processes in stirred media mills.

In addition to the grinding experiments, the impact of different grinding aids on the classifying performance in a dynamic air classifier was investigated independent of the mill-classifier circuit. Within a systematic investigation, the process parameters of the air classifier were examined in dependence of the grinding aid species, grinding aid concentration as well as fineness of the limestone fraction. As the grinding aids strongly affect the attractive particle-particle forces and thus the agglomeration behavior of the product powder, they also influence the classification performance in dynamic air classifier. Consequently, grinding aids also work as dispersion additives or classifying aids. It is shown how several classification characteristics like cut size, separation sharpness and incorrect discharge depend on the grinding aid application. Thereby, an ideal operating range with regard to the particle stabilization was identified.

Finally, grinding results from a mill-classifier circuit consisting of a dry operated horizontal stirred media mill and a deflector wheel air classifier are presented. It is shown how far the grinding aids affect the product quality, product throughput and specific energy consumption. Thereby, the results are correlated with the findings from the investigation of the single units shown above. For an even more comprehensive understanding of the pilot plant, a material circulation factor is considered which describes the mean number of material circulations inside the plant. It enables a more realistic description of the residence of the particles within the pilot plant. Again, effects of the different grinding aids are identified and are correlated with the grinding results.