MechChem Africa September-October 2023

Cyclone technologies for efficient size, mass and density-based separation

Ernst Bekker, Product Specialist – Cyclones, for the Multotec Group, talks to MechChem Africa about hydrocyclones and dense medium cyclones: how they work, their different applications and some of the things to look at, and to avoid, for best possible separation, production, and recovery efficiencies.

“C yclones are divided into two main categories: hydrocy clones and dense medium cyclones, which are funda mentally different in terms of the principles that apply, and the mineral separation processes they can be used for,” begins Multotec’s Ernst Bekker. Hydrocyclones use water as the trans port medium. The solids that require separa tion are mixed with water to form a slurry, before being pumped into the cyclone near the top and at a tangent to initiate spiralling flow. A strong vortex forms at the centre of the cyclone, with an air core through its centre passing from the spigot at the bottom to the vortex finder at the top. “Centrifugal forces from the spiralling flow tend to throw particles to the outside, while drag forces from the water being pulled towards the air core at the vortex pull particles in the opposite direction. So a balance of the forces emerges,” says Bekker. “The coarser particles tend to be thrown

to the stream spiralling downwards on the outside, while the finer particles tend to migrate into water surrounding the air core, which is being forced upwards by the vortex. The water closest to the air core takes the finer particles up and out of the overflow at the top, while the coarse particles con tinue to flow down the outside of the spiral, passing through the spigot at the bottom as underflow,” he explains. It is this balance of the forces that dic tates the cut size of a hydrocyclone. “It is important to remember that a hydrocyclone predominantly separates the mineral par ticles in the slurry based on size, with larger coarse particles reporting to the underflow while smaller or finer particles are taken up and through the overflow,” Bekker informs MechChem Africa . Ideally, he says, Multotec recommends a ratio of solids to water of about 20% solids by volume in 80% water. “Sometimes people try to push more solids into the cyclone to raise the throughput of the plant, or tonnes/

m 2 of processing footprint. But this will tend to inhibit the performance of the hydrocy clone, so we never like to exceed 30% solids by volume,” he advises, adding that higher solid volumes tend to result in lower separa tion efficiency and poorer recoveries and/ or increased product circulation. “When multi-density particles enter the mix, then a hydrocyclone, in essence, sepa rates based on mass. With homogeneous ores, mass and size are directly related, because bigger particles always have bigger mass, so we can still use the term cut-size to describe the separation process. But when the densities are different, this is not the case. In a multi-density clas sification application, a larger fraction of the high-density material will report to the underflow compared to a mineral that has a lower particle density. This is a challenge for hydrocyclones in a multi-density environ

Multotec hydrocyclones are used across the world for applications in mineral processing and other industries. The industry-proven product range has been optimised over hundreds of applications to improve classification efficiency, provide greater lifespan and reduce energy requirements.

18 ¦ MechChem Africa • September-October 2023