Modern Mining August 2015

PROCESSING

intensity multiplied by the gra- dient is called the force factor and is responsible for separating minerals. The Longi-Multotec WHIMS machines typically have a force factor of 200 mil- lion Gauss 2 /mm and this is a vastly significant difference when compared to a traditional low intensity wet drum mag- netic separator, which has only 20 000 Gauss 2 /mm. “Trials indicate that the WHIMS can recover particles down to a fraction size as small as 5 micron, a charac- teristic that is attributed to the exceptionally high force fac- tor. Particle sizes as small as 80 % passing 30 microns have been recovered. To put this in perspective, this particle size is finer than the diameter of a

40 micron human hair,” Slabbert points out. In some areas PGM ore has a head grade of between 1,4 and 2,5 mg/t and the tailings that have been processed in the test work to date have had a head grade of 0,8 to 1,5 mg/t. This is significant given the fact that through the conventional flotation process it is not eco- nomically feasible to attempt to float the tails again if below 3 mg/t. With the Longi-Multotec WHIMS, the chrome is removed so that a saleable product is recovered and, furthermore, the PGM grades are upgraded to above 3 mg/t. It then becomes eco- nomically feasible to refloat through existing capital infrastructure. This presents advantages over and above the costs associated with min- ing and the comminution process, which may well contribute to almost 60 % of the opera- tional costs of a plant. A major benefit of the Longi-Multotec WHIMS machine is that once the test work data has been verified the machine can be upscaled to a capacity of up to 300 t/h of sol- ids (or 1 000 m 3 /h of slurry) through a single machine. Multotec is able to supply vertical ring WHIMS machine sizes in the range of 1 t/h up to 300 t/h. “This high capacity completely overshadows the much lower maximum capacity of 30 t/h that a horizontal ring WHIMS can achieve. Extensive tests have shown chrome grades of 43,8 % are readily achieved with the vertical ring WHIMS, which is significantly higher than the industry standard of 42 %, and indicates that the percentage grade achievable on full

scale plants in all likelihood will be higher,” says Slabbert. In addition to its proven increased recovery capacity, the Longi-Multotec WHIMS machine provides industry with a number of attrac- tive benefits that are said to include ease of maintenance and operator friendliness, lower operating costs, environmentally friendly equipment operation and easy and reliable pro- cess performance. The vertical ring WHIMS makes use of only one coil, resulting in savings on electrical infrastructure and electrical consumption for decreased capital cost of ownership and opera- tional costs. By feeding into the slurry feedbox instead of directly onto the matrix, there is a better tolerance of feed fluctuation without the separation process being affected. Control of the machine is also improved and it is more reli- able resulting in better separation consistency. “Pulsation allows a cleaner product to be produced, with better upgrade ratios achieved under variable and controlled conditions. The matrix boxes in the rotor can be changed out in situ and individually after wear or damage to a box, eliminating a total rotor change out. This, again, provides significant operational savings and ease of maintenance,” adds Slabbert. Slabbert explains that most of the test work has been done repeatedly using the Multotec vertical ring WHIMS on different samples and different seams of UG2, LG6 and MG tailings derived from Western Limb of the Bushveld Complex material, with some tests having been conducted on the Eastern Limb material. 

Two large Longi-Multotec vertical ring WHIMS machines, each capable of processing up to 150 tons per hour of solids.

“Trials indicate that the WHIMS can recover particles down to a fraction size as small as 5 micron ... .” Willem Slabbert, Multotec Process Equipment

August 2015  MODERN MINING  45