In the first phase of this project (2019 – 2023), three types of grinding aid additives in their pure form (n-Heptanoic acid, Diethylene glycol, and 1-Hexanol), which promote material bulk properties to various levels, were selected. The research aimed to assess the influence of these grinding aids by diverse dosages on both, grinding in ball mills and size classification in dynamic air classifiers. The focus was placed on determining how the grinding aid additives influence the grinding and classification process environment and on modifying selected models from literature for both, ball mills and deflector wheel classifiers. The models should account for the presence of grinding aids during simulating a dry-operated grinding plant in a flowsheet simulation.
During the second phase of the project (2023 – 2026), the research is focusing on identifying appropriate powder macroscopic bulk properties that accurately reflect both, the characteristics of particles and the presence of a particular type and dosage of grinding aid additives, as illustrated in Figure 1. In addition, the study aims to establish correlations between important microscopic particles properties, such as particle size, specific surface area, and particles specific surface energy, and bulk behavior of the powder. These correlations can be used to account for the impact of grinding aids within modeling grinding processes. Moreover, the bulk properties of the powder influence specific process characteristics, such as the discharge rate that can be achieved, the quantity of material hold-up in the grinding drum, and the residence time within the mill. Further, the knowledge about the material hold-up and residence time is required for modeling continuous grinding processes. These factors subsequently affect the quality of the final product (see Figure 1).
In order to understand the relationship between the powder’s bulk properties, the respond of the process in terms of resulting process characteristics, and the outcome of the grinding process, dry grinding experiments in an open-circuit continuous tumbling ball mill were performed. The trials were conducted until a stable operational state was reached. The outcomes of this series of trials provided data on the hold-up mass, discharge rate, and samples from product as well as hold-up material in the grinding drum. The gained samples underwent measurement to evaluate the characteristics of the particles and the powder and to correlate it to the system response. Based on this series of tests, couple of tests were selected for measuring residence time distribution. The results demonstrated that Diethylene glycol (DEG) led to a higher material hold-up compared to the product that did not contain grinding aid by the same feed rate. However, DEG resulted in a shorter median residence time with the highest maximum residence time and much finer product than that without grinding aid. On the other hand, Hexanol produced a product size and powder-to-void-ratio of almost 1 similar to that of the product without grinding aids, but this was achieved at a higher feed rate than that of the product without grinding aids. The comparison of the experimental data regarding residence time measurements with the results obtained from the one-dimensional axial dispersion model provided a reliable approximation, especially up to 80 % of the cumulative residence time distribution, showing slight deviations for all product formulations. The influence of grinding aids on the macroscopic properties of powder, as well as their incorporation into flowsheet modeling for a grinding plant, has revealed a dependable correlation between the powder flowability index (ffc) and the specific surface area. This correlation, which illustrate the effects of various grinding aids, can be represented by a power function. This function can be used in the ball mill model to predict dynamically changes in powder flowability as the particle size evolves during the grinding process.