MechChem Africa

With metallurgical operations constantly looking to improve recoveries, the RhoVol densimetric measurement system from De Beers Technologies SA (DebTech) now makes it possible to determine material density on-site quickly and easily – allowing process plants to be quickly optimised for best results.

According to Grant High, research and development manager at DebTech, density is a key element in a range of mineral separation processes, and knowing the density profile of material entering a metallurgical process is critical in determining the way that process is set up to give best results.

DebTech Rhovol density volume mass shape classifier sampler“Traditionally, this is done by taking a sample and putting it through a sink-float process where heavy liquids are prepared at different densities and the sample is moved between these liquids to establish a density profile of that sample,” says High.

This absorbs time, effort and cost, and turnaround times are anything from a number of days to even weeks. Most mines or plants will not have these facilities on-site so will have to send samples away for testing. He adds that there are also health, safety and environmental issues associated with these liquids, as they can be toxic and carcinogenic.

The manual nature of the testing process also makes it susceptible to human error, often introducing levels of variability that undermine the effectiveness of fine-tuning back at the process plant.

“By the time the results of the sample tests are returned to the customer, it is often too late to adjust the set-up process with sufficient effect,” he says, “especially as the density profile of the material passing through the plant may have changed since the last sample was taken.”

He notes that it has been something of a ‘holy grail’ for the metallurgical industry to be able to measure the density of incoming material in real time, so that the plant’s settings can be optimised in time to make a difference. In search of solutions, some years ago DebTech developed the vision size frequency distribution (VSFD) technique. This is a camera-based system for measuring the size of particles. In this system, specially situated cameras take a number of silhouette photographs of a particle, each photograph contributing to the reconstruction of a 3D model of each particle. On this basis, the volume of the particles is used to generate a size distribution.

In the RhoVol, the VSFD breakthrough is combined with a rapid weight measurement device, so that both the mass and the volume can be determined with great accuracy and speed, and can then provide the density of every particle. The weight of the particle can be measured within 700 milliseconds to a very high accuracy level – with an error margin of just 0.5 %.

An added advantage of this technology’s accuracy is that large samples are no longer required. Instead, very precise and accurate results can be achieved with much smaller samples than are commonly used in traditional sink-float methods – where 25-30 kg samples are often required. In the testing of kimberlites, for example, the sample size when properly riffled could be as small as 400 g depending on the size fraction.

“The RhoVol’s high process rate of 1 000 particles per hour makes it possible for users to rely on getting useful results in just one hour, as we have found that 1 000 particles is a good benchmark for a sample’s representivity,” he says. “When that number of particles is exceeded, the results tend not to vary much as a result.”

Once the particles’ volume data has been captured, a range of other measurements and information can be derived about the shape of the particle – such as elongation, flatness and compactness. “While size counts, so does shape,” says High.

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