COMMINUTION AND CRUSHING
The keys to the new Outotec HIGmill design are the agitator( grinding rotors) and grinding chamber stator rings themselves, just like the rotor and stator in a mechanical flotation cell
efficient, attrition grinding environment. Attached to the shell are GrindForce counter stator rings, again unique to the HIGmill design. Feed slurry moves upwards and passes into the grinding zones between these stator rings and the wall lining. Due to the vertical arrangement of the HIGmill, classification is conducted simultaneously throughout the grinding process with larger particles remaining longer at the peripheral where there is a high concentration of media particles, while smaller particles move upwards. The stator rings create separate grinding zones around each rotor, and the HIGmill has between 15 and 20 of these zones, more than any other stirred mill on the market. In addition, these distinct stator rings force slurry and media to each grinding zone, eliminating any possibility of particle short-circuit or dead zones within the grinding volume of media which occupies between 60-70 % of the total mill volume.
“ Ideas in physics can explain fundamental mechanisms of stirred milling, in particular the HIGmill which was developed over many years with the design motivation of power savings. The HIGmill’ s unique tubular vessel design with grinding rotors and stator rings operating in plug-type flow is ideally suited for efficient, stirred mill grinding of mineral particles in slurry streams.”
From a particulate technology point of view, the product quality and grindability in a HIGmill is determined by three main factors, namely; 1. shear and compression stress mechanisms generated by the grinding media; 2. frequency a feed particle is stressed; 3. intensity of each stress event.
GrindForce is the trade name for Outotec ' s new profiled grinding rotor for use in stirred milling, taking its name from the very successful FloatForce ® technology, which like GrindForce, is a newgeneration mixing mechanism. The key to the new HIGmill design is the agitator( grinding rotors) and grinding chamber stator rings themselves, just like the rotor and stator in a mechanical flotation cell. The heart of the flotation cell is this rotor-stator mixing mechanism, which mixes the slurry, disperses air and generates kinetic turbulent energy. Turbulence is needed in order to accelerate the particles and give them sufficient energy, so that good particle-bubble contact occurs. Similarly in a HIGmill, the GrindForce mechanism more efficiently agitates the ceramic grinding media for improved particle-media interaction.
The agitator of the first HIGmill installed in a metalliferous operation included a number of changes to the materials of construction compared to the rotors used in white minerals processing, yet the fundamental design and profile were more or less unchanged. The basic design was a flat one-piece disc with angle spoke arrangement, keyed to match the shaft, much the same as other stirred disc mills on the market today. However, Outotec’ s design engineers were striving to develop a totally unique technology to the current market norm and set about this task, utilising the internal comminution team’ s world class, in-house DEM capability. The DEM focused on the shear / spillage between the flat surface of a grinding disc and the media bed, plus the amount of media agitation that can be achieved by a flat disc.
The result of this work was a move away from the normal flat surface grinding discs to profiled grinding rotors with counter stator rings, together making up the GrindForce mechanism. The DEM showed with this new rotor design, the velocity of the grinding rotors and media bed in contact with the rotors would be roughly the same, dramatically reducing the shear / spillage which causes wear. A further significant benefit the DEM indicated was improved media movement with the rotors compared to flat discs. The DEM work indicated a flat grinding disc surface is not the ideal design for mixing, and the media mixing / agitation is what creates the attrition grinding in stirred mills by increasing the stress intensity and the frequency a feed particle is stressed by the media bed.
From the DEM modelling, full scale grinding rotors were produced and installed in a large metalliferrous concentrator and results monitored over an eight month period.“ The results were outstanding with our benchmark 4,000 operating hours between grinding rotor replacements being surpassed given only a third of the new GrindForce rotors required replacement after this time. The added effect of the GrindForce vane rotors on the process performance was excellent with a significant improvement in grinding efficiency to the target grind size.” This is due to the improved power transfer between the GrindForce rotor surface and the media bed mass, compared to flat surface discs. In a full scale HIGmill operation before and after the introduction of the latest GrindForce grinding rotors, only 9 of the 17 original flat discs had been replaced with GrindForce rotors.
“ Full-scale operations have proven that HIGmill performance, which is already industry leading, can be further improved along with important wear performance enhancements with the application of GrindForce rotors and stators. Now all Outotec HIGmill grinding mills are equipped with GrindForce technology which furthers the energy and wear performance gap between Outotec HIGmills and other stirred mill solutions.”
New crushing options
Last year, thyssenkrupp unveiled the huge 63-130 gyratory crusher, first at Bauma and then at MINExpo. Since then, the first two machines of thyssenkrupp’ s new flagship in primary crushing have been ordered. For this particular project the crushers will be supplied as part of two semimobile crushing plants for a large open pit copper mine.
The new machine despite a roughly 14 % larger mantle diameter of 3.3 m weighs in much lighter than its predecessor at 490 t. Its throughput of up to 14,000 t / h is more than 30 % higher than that of the predecessor model.
64 International Mining | MARCH 2017