Vertical mill optimization at Nornickel Harjavalta

Abstract

The goal of this thesis is to find out optimum grinding parameters using literature study and laboratory scale experiments for Nornickel Harjavalta’s vertical stirred mill. The particle size in the feed to the SMD is a mixture of coarse and finer particles and the goal is 80 % passing size of under 30 microns.

The literature study focuses on the main factors affecting stirred media detritors (SMD). The research methods aim to figure out the parameters affecting stirred media mill grinding performance. This is conducted using particle size analysis on grinding and how parameters affect power consumption which is highly dependent on the tip speed of the impeller and bead charge. The grinding rate depends on what parameters are chosen to be used.

The recommended values for the SMD based on the laboratory mill results are bead charge of at least 50 %. This is needed to reach the production rate requirements which can be used with different slurry densities. Larger bead charges are more energy efficient in grinding. Bead density should be higher than used previously in production (3200 kg/m^3). For example, a bead density of 6000 kg/m^3 could allow the material to be ground faster than lower bead densities and should be used. A bead mixture of larger and smaller beads should be used with higher bead densities than used in the past for the SMD as this will allow smaller power consumption with a similar grinding speed. Stirrer speed should be as high as possible which is dependent on the bead charge. For 50 % bead charge RPM should be at least 141 but in reality, the RPM is higher for the target capacity and particle size reduction. Slurry density of 1431 kg/m^3 would be good for the grinding speed with dry solids feed rate of 7 t/h.