This project aims in developing a system engineering approach for understanding, optimizing and scaling industrial dry grinding processes, with a special focus on the manipulation of the material properties and, thus, the grinding and classification efficiency by the use of grinding aids. In terms of process units, the project deals with a pilot scale dry grinding circuit consisting out of a ball mill and air classifier. During milling operations, grinding aids affect the milling process mainly in: tendency of fine particle agglomeration; amount of material coated on equipment surfaces; powder flowability; total mass of product inside the mill and residence time; product fineness after grinding.
The project work for the initial three years was divided in four main work packages:
a. Identify which aspects of the ball milling process are affected by different GA
b. Identify which aspects of the air classification process are affected by different GA
c. Modify process models from the literature to account for the presence of GA, implement a flowsheet simulation tool and validate the flowsheet with mill-classifier circuit data
In the first year of the project (2020) batch grinding tests and powder flowability measurements of the product were conducted in order to assess grinding aid contribution to the breakage aspect of milling, without powder transport. The second project year (2021) focused on the air classification step of the circuit. Trials in two air classifiers, in laboratory and industrial scales, were conducted. It was compared which aspects of this process are influenced by grinding aids and which are determined by machine design.
The third project year (2022) dealt with two aspects. First, continuous milling trials in passage mode to study the effect of grinding aids on powder transport, mill holdup and process dynamics and stabilization. Second, modification of process models from the literature to consider the effect of GA. The main conclusion of the open-circuit milling trials can be summarized as:
a. Powder flowability should be kept within an intermediary range (easy-flowing). Both excessive and too low flowability should be avoided in order to improve throughout and process stability.
b. High flowability can be detrimental to the amount of stressed material and energy transfer from ball to product particles.
c. Beyond flowability GAs should be selected in order to reduce caking on equipment surfaces and reducing ball coating. Once an amount of powder is stressed between two balls, it should be readily fall off to allow another sample of powder to be stressed.
The mill model proposed is formulated as mechanistic population balance for media mills capable of predicting grinding of particles sizes from the lower millimeter size range down to the sub-micrometer scale. The mechanistic approach to media mill models is a very flexible tool that allows full separation of material and process aspects. The proposed mill model assumes stead-state operation, requires input from Discrete Element Method (DEM) simulations and accounts for impact of powder flowability on powder stressing.