At a cost of $1.5 million, an attritor mill is no mean investment, so any means of reducing their ability to hammer themselves to uselessness in short order would save considerable amounts of money in direct costs and lost productivity.

Engineers at CSIRO Energy & Thermofluids Engineering have impact crushers to look at designing to improve wear rates.

In Australia, impact crushers where size reduction takes place through blows from rotating hammers, are designed for use on non-abrasive material such as brown coal. However, in Australia they are used at a number of plants that run on abrasive high ash-content black coal.

As a result, the mills require frequent refurbishment to address excessive wear. At large plants, it is not unusual for mill maintenance costs to run to several millions of dollars or more per annum.

This simple premise sums up why, over many years, solutions to the excessive wear in attritor mills have not been readily identified. This excessive wear is often highly localised or, in multistage machines, occurs dominantly in a particular stage. In these cases, the problems are often related to poor internal flow patterns of solids and gas.

The observation scenario has been compounded by the option of trial remediation, which is often problematic as observing the internal flows in these machines is difficult and costly to undertake on plant that is in service.

CSIRO has overcome these obstacles to progress through a combination of what some call the esoteric science of computational fluid dynamics (CFD) and complex multiphase small-scale modelling techniques.

Either small-scale physical modelling based in a laboratory alone or combined with computational studies, provide a cost-effective means of understanding the internal flows and testing alternative solutions.

For CFD modelling, meshes can be produced with the aid of CAD geometric information. Then solutions can be proposed, manufactured on our in-house machine centres, and tried in the laboratory or on computer. A variety of changes can be evaluated quickly and at small incremental cost.

Typical solutions call for revised internal flow deflectors, changing the size, shape and orientation of the hammers, or other comparatively simple changes. In those cases, the model, either computational of physical, can be modified then tested and the changes evaluated.

More fundamental changes, say to shift the basic crushing mechanism from hammer-particle to particle-particle impact, require careful redesign and may require on-site testing with real materials to confirm the effectiveness of proposed changes.

To date, CSIRO have involved NRG, Queensland Alumina Ltd and Alcoa in the project.

As a part of a project to redesign impact crushers at NRG’s Gladstone power station, a half-scale metal model of one stage of a multi-stage impact crusher is being built to install in parallel with a full-scale mill on-site, following laboratory-scale modelling at CSIRO. This will provide direct confirmation of proposed design modifications and their effect on wear, using run-of-mine coal.