A 25-year project on thickener modelling that has already delivered huge value for the mineral processing industry has now entered a new phase. Jodie Parry investigates what the world’s first three-dimensional thickener model will provide industry.

Visual output from computational dynamics modelling of thickener performance. A thickener uses gravity to separate sludge from liquids. A central drum called the feed well swirls around a central pivot in the thickener tank. Sludge settles to the bottom and separates from the liquid.

In this model fluid flow is coloured according to particle diameter and liquid velocity:

• Average aggregate diameter (μM) is shown on a scale from 0 (blue) to 500 (red) on the left of the image.
• Liquor velocity (m/s) is shown on a scale from 0.00 (blue) to 0.34 (red) on the right of the image.

Fluid flows in a circular current from the feed well to the base of the thickener tank and then towards the outer edge of the thickener. Then liquor flows up the side of the tank and across the top of the tank towards the feed well. The liquor velocity is greatest near the central stirrer, especially under the feed well.

In this model, within the feed well larger particles are towards the bottom and smaller ones towards the top. The base of the thickener tank is coated with medium sized particles.

Computational fluid dynamics modelling of thickener performance.

Process productivity article from resourceful: Issue 6, March 2014

Water is used in processing most minerals, from simply washing away waste solids, to leaching of target metals with acidic or basic solutions. Having done this, how are the solid and liquid phases then separated quickly and efficiently?

Gravity thickeners (large tanks) are used widely for this and flocculants and suspensions assist in producing a clear liquid stream at the top of the tank and a thick sludge of settled solids, which is discharged from the bottom.

However, despite their importance and widespread use, thickeners can be erratic and inefficient.

Researchers from CSIRO are advancing the science and modelling of thickeners using mathematical and computational fluid dynamics, with great results.

According to an independent assessment, their work has already delivered well over US\$500 million in value to end users.

‘Thickeners suffer from limited throughput, poor solid-liquid separation, operational difficulties and high consumption of expensive flocculants,’ Dr Phillip Fawell, CSIRO project leader, says.

‘In tailings applications, they frequently fail to deliver the desired solids concentrations, leading to the waste of water and the need for excessively large tailings disposal dams that can cause environmental issues.’

In response, Dr Fawell and his team are creating the world’s first three-dimensional full thickener model as part of the existing multi-faceted AMIRA International project.

‘We are applying the models to systematic studies of different designs and operating conditions (changing solids concentration, volumetric flows, dosages) to establish best performance guidelines,’ he says.

‘Our team expects to deliver control strategies for dealing with specific issues or performance targets.

‘We’re also developing a new test that will give the parameters needed to characterise the settling and consolidation of a flocculated feed for process or control modelling from a series of simple cylinder tests.’

Dr Fawell believes this could end up being a new standard method for such testing.

The project, or series of projects, which has been running for over 25 years, has been strongly supported by the Australian and international minerals industries with sponsorship funding of over \$20 million since inception.

Sponsors benefit from the sharing of key information and technology from each of the projects, including online resources such as a knowledge base, a thickener self-appraisal tool, and process models and training presentations.

They can also engage the research team to work on their site-specific problems. This has resulted in significant improvements to the performance of more than 150 thickeners.

‘What has impressed me is how CSIRO has been able to pass on to sponsors their broader understanding of how flocculation should be done in gravity thickeners,’ Gray Bailey, Program Manager from AMIRA International, says.

‘There’s no better feedback than hearing from sponsors who have gone back to their sites, looked at their thickeners and said, “Now that makes sense!”'