A Systems Engineering Approach to Dry-Milling with Grinding Aid Additives

Executive Summary
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 grinding aids. Grinding aids are defined here as liquid or dry substances that are added to the process in order to increase the product throughput, decrease the specific energy consumption and/or to reach a certain product fineness. During milling operations, grinding aids impact powder material mainly in:
 Product fineness after grinding;
 Tendency of fine particle agglomeration;
 Powder flowability;
 Total mass of product inside the mill and residence time
 Amount of material coated on equipment surfaces.
In this project, dry grinding of the materials alpha alumina and calcium carbonate is studied. Three substance classes were adopted as grinding aids: An Alcohol, a Carboxylic acid and a Glycol. For the experiments, a 47-liter batch-wise or continuously operated ball mill as well as a reflector-wheel air classifier are selected.
In the previous year of the project (first year) 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. It was also conducted initial open-circuit milling trials to study the effect of grinding aids on powder transport, mill holdup and process dynamics and stabilization. It was observed that additives promoted a more stable and efficient process. Although it was also observed that excessively high powder flowability can result in process ineffectiveness by reduction of mill residence time and no reduction of product particle size.
This project year 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 aid and which are determined by machine design. The main conclusions of the work are as follows:
a. The classifier corrected cut-size (when no bypass of fines to the course outlet occurs) is not affected by the use of grinding aids, being a result of wheel speed and air volume flow rate;
b. Grinding aids promoted a reduction in bypass of fines to the course outlet for both classifier scales, although with smaller intensity at very small cut-sizes;
c. In the industrial scale classifier, it was not observed reduction on powder caking on the chamber walls with grinding aids tested. This fact seems to be due to the chamber design and formation of air stream dead zones, but more studies are required.
d. The fish-hook effect for ultra-fine particles can be reduced by grinding aids, but is also very effected by the machine design.
e. Grinding aids promoted an increase in separation sharpness for coarser size classes, reducing coarser residue on the fine product.