Tough break

Researchers claim a world first with technology to determine how diamonds react to stress and damage. Siobhan Wagner reports

Although diamond is the hardest naturally occurring mineral, it is very brittle. All stages of the mining process, from transportation to comminution — or resizing of the material — can damage the valuable product.

An important characteristic is the breakage function which gives a measure of how easily a diamond is damaged and has, until now, not been fully possible.

With the use of a compression testing machine, high-speed video and 3D imaging technology, researchers at South African company DebTech, an R&D arm of the world’s largest diamond producer De Beers, have for the first time been able to experimentally measure this function.

Garry Morrison, senior research scientist at DebTech, said the ability to characterise the breakage function of the ore body of a material enables appropriate comminution circuits for machines such as crushers and mills to be designed so they operate as efficiently as possible.

‘In the case of diamond mining, however, we must efficiently break the host ore body, while leaving the diamonds undamaged,’ he said. ‘It is only by understanding the breakage function that we can figure out how hard we can operate our comminution circuits without breaking diamonds, and hence losing revenue.’

for their experiments the researchers used a Zwick compression machine equipped with a 30kN load cell and Zwick’s testXpert software.

First they captured a 3D model of a particle using a DebTech technology called Vision Size Frequency Distribution (VSFD). The system uses images taken from cameras positioned at various angles around a particle. The recorded images can be combined to create a 3D model.

The team broke the diamond particle in the Zwick machine and simultaneously filmed the event using a high-speed video (HSV) camera.

‘We are able to use the first frame of the HSV sequence to align our 3D model of the particle in exactly the same orientation as it is in the real compression test,’ he said. ‘We can replicate the test in simulation using this information.’  

Morrison said that at the moment the team does not actually simulate the fracture of the particle, but stops at the point where failure occurs in the HSV sequence.  

‘We are then able to determine how the particle is loaded at the point of failure,’ he said. Loads can vary from tens to thousands of Newtons and each diamond behaves differently depending on its shape, its flaws and inclusions. Some break into two or three larger fragments, while others fracture into vast numbers of tiny fragments.

‘We can typically test 30 diamonds a day using the Zwick,’ said Morrison. ‘The time-consuming part is the characterisation of the size and shape of all the fragments after each test.’ An analysis of the size and shape distributions of the fragments gives the breakage function.

‘It has not been possible to do this before on the scale we are able to,’ claimed Morrison. ‘And this is down to the accuracy we can achieve with the technology.’

‘This is a huge step forward in the comminution world,’ he added.

Morrison said the new system makes a vast improvement in the diamond mining process.

‘Diamond mining is a trade-off between doing things as quickly, and therefore as severely, as possible and not damaging the product,’ said Morrison. ‘The Zwick machine is helping us to better understand the effects our processes have on our product.’