Although a rake may appear to be a fairly straightforward piece of technology, when used in mineral processing facilities, the difference between a well-designed or poorly-designed rake can mean millions of dollars in scaled, damaged or poorly operating equipment.
Research is suggesting that even small changes to the size, shape, location and angle of the rake blades, and speed of the rake movement, could make a big difference.
Working through the Parker Centre for Integrated Hydrometallurgy Solutions, CSIRO researchers are exploring how variations in the size, shape and spacing of rake blades and changes in operating conditions can affect thickener performance.
A key focus of the project, which is partly funded through AMIRA International, has been the development of a pilot-scale model of a gravity thickener and a computational fluid dynamics (CFD) model of a thickener rake.
Rakes are an essential part of gravity thickeners — the large tanks into which a slurry of fine mineral particles is fed with the aim of separating the solids from the liquor.
Many thickeners have a rotating rake mechanism at the bottom of the tank, which usually consists of several arms with a series of flat blades attached. They work by moving settled solids towards the tank’s underflow — the ‘plug hole’ — and scraping accumulated solids off the wall of the tank.
As the rake blades pass through the slurry, they also help break up its structure and open channels that make it easier for liquor to escape.
Dr Murray Rudman, research coordinator of the Thickener Technology team, has been working on thickeners for more than 12 years and his research suggests that even small changes can make a difference.
For example, in conventional rake design every blade is the same size, so rake blades on the outside of the arm move faster than the inner blades and rake more material toward the underflow than the inner blades. This mismatch in blade delivery cannot be accommodated by the underflow, and slurry recirculates unnecessarily in the outer part of the tank.
The faster moving outer blades also encounter greater resistance.
Reducing the size of the outer blades reduces the resistance, making it easier for the rake to rotate, but without adversely affecting the movement of sediment.
The pilot-scale thickener — a two-metre diameter thickener tank — is a key element in the team’s research because it helps corroborate computational predictions.
“An important part in CFD modelling is to validate your predications against something you can measure,” Dr Rudman says.
“Once you have validated it against data that you trust, you can then apply it to full-scale thickeners with a lot more confidence.”
Ron Kahane, mineral processing consulting engineer, says the rakes research could make an important contribution to an industry that has already significantly benefited from CSIRO research into improving other aspects of thickener performance.
“The remaining area that still causes a lot of problems to industry with production losses due to equipment failures or blockages is the rakes, especially in industries where the rakes scale,” Kahane says.
This build up of material on the rake blades can completely fill the spaces between the rake blades.
Dr Rudman’s work suggests bigger rake blades and better spacing between the blades could reduce this problem.
“The more you understand something the more you’re likely to find the answers,” Kahane says.
“The depth to which this work has gone makes the causes of rake problems a lot clearer and indicates what things you might look for or develop to get around these problems.”