(110c) Up Scaling the Isamill with the Enhanced Stress Model

The IsaMill is an often used type of wet operated stirred media mills in mining and minerals industry. Thus usually mill sizes of several cubic meters are used in the industrial scales. The optimum set-up of such a process is often connected with empirical approaches or rough scale-up rules depending on length to diameter ratios or surface area values. Therefore, experiments in a small scale were usually carried out to find optimum grinding media diameters and stirrer tip speeds. These parameters could be integrated into the so called stress energy, which could be compared to the kinetic energy of colliding grinding media [1, 2]. If this stress energy is plotted over the specific energy for a desired particle size or product quality minimum curves occur, which means there is a certain stress energy which leads to the lowest possible energy consumption for the desired product. If the process parameters are set to values that the optimum stress energy occurs and the power consumption of the mil isl at a high level, maximum production rates are the result. These stress energy curves change with the mill type and size, thus the stress energy figured out for a lab scale mill does not lead to optimum parameters in the production scale mill.

For Nano grinding in stirred media mills an enhanced stress model was introduced, which enables the prediction of optimum stress conditions different mill designs, sizes and materials on basis of geometrical data and material values as input parameters [3-6]. Therefore the median stress energy is used for different mill sizes and geometries, which could roughly be calculated by geometrical data of the mill. Besides the prediction of the optimum grinding conditions, with less experimental work the prediction of specific energy input for a given set of parameters and a determined product quality could be realized. This model was used for lab scale mills (grinding chamber volumes below 2 L) and especially for nano applications where the stability and rheology of the suspensions plays a major role.

Within this work the model is applied for the scale-up of a fine grinding process in a more industrial scale. Therefore experimental work was carried out at a 4 L and a 20 L IsaMill with dolomite as a model mineral. For both mills the classical optimization way with the stress energy curves was realized by the variation of the process parameters grinding media diameter and stirrer tip speed.

The ISA-mill has a special separator at the rotor to keep the grinding media away from the sieve. This separator transfers back lifting forces to the grinding media, which lead depending on the process parameter to heterogeneous grinding media distributions [7-9]. This grinding media distribution has a strong influence on the mill operation in terms of grinding efficiency, production rate and mill wear for instance. [8-9].

Besides the experimental work the enhanced stress model was proofed for this fine grinding process with those bigger mills in comparison to the results of lime stone grinding in a small lab scale mill (0.5 L). Therefore different parameters like the median stress energy or the different energy transfer factors were calculated. The experimental work was used for the proof of the validity of the model as well as a few selected results for the determination of parameters for the model.

References:

[1] A. Kwade and J. Schwedes, "Breaking characteristics of different materials and their effect on stress intensity and stress number in stirred media mills," Powder Technology, vol. 122, pp. 109-121, 2002.

[2] A. Kwade, "A stressing model for the description and optimization of grinding processes," Chem. Eng. Technol., vol. 26, p. 199/205, 2003.

[3] S. Breitung-Faes and A. Kwade, "Design of wet stirred media milling processes via a mechanistic model," in European Symposium Comminution and Classification, Göteburg/Schweden, 2015.

[4] S. Breitung-Faes and A. Kwade, "Use of an Enhanced Stress Model for the Optimization of Wet Stirred Media Milling Processes," Chemical Engineering & Technology, vol. 37, pp. 819-826, May 2014 2014.

[5] S. Breitung-Faes and A. Kwade, "Prediction of energy effective grinding conditions," Minerals Engineering, vol. 43-44, pp. 36-43, 2013.

[6] S. Breitung-Faes, "Estimation of product relating energy of wet operated stirred media mills in terms of process transfer to other mill geometries and sizes," Minerals Engineering, vol. 103, pp. 33-42, 2017/04/01/ 2017.

[7] D. Schons, A. Kwade, "The axial grinding media distribution in the IsaMill at different operating conditions", Comminution16, Cape Town (South Africa) 2016

[8] D. Schons, A. Kwade, Operation guidelines for the IsaMill considering the axial distribution of grinding media, IMPC16, Québec (Canada) 2016

[9] D. Schons, A. Kwade, Betrieb von Rührwerkskugelmühlen – Steigerung des Durchsatzes unter Berücksichtigung der axialen Mahlkörperverteilung (English title: Operation of stirred media mills – Improvement of the production rate considering the axial grinding media distribution), ProcessNet Annual Meeting in „Comminution and Classification“, Dresden (Germany) 2017