Engineering & Mining Journal

MAR 2017

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ENERGY EFFICIENCY MARCH 2017 • E&MJ 47 www.e-mj.com the attachment rate to see if we could im- prove recovery," said Weber. The objective was to define an opti- mum rotor/stator profile with maximum energy efficiency and improved metal- lurgical performance. In addition to the shape of the rotor/stator, CFD modeling was used to optimize the number of rotor blades for maximum recovery. The bene- fits of including slots — openings in the stator — were also demonstrated. From this study, the nextSTEP rotor evolved into a unique shape with broad vanes. The broad vane was incorpo- rated to produce a slurry flow jet that was spread over a much wider area of the blade relative to typical tradition- al-shaped rotors. It has been demonstrated that the collision rate of bubbles and particles is proportional to the square root of the energy dissipation rate divided by the fluid viscosity within the rotor stator re- gion. According to FLSmidth's flotation experts, the high-velocity regions of the nextSTEP rotor extend over a much wid- er field than traditional rotor shapes that produce a concentrated jet of high-ve- locity flow. This spread-out flow results in a wider region of high energy dissipa- tion as the slurry passes from the rotor to the inner stator region. From this it is anticipated that broadening this re- gion of high energy dissipation with the nextSTEP rotor shape will increase the collision frequency of bubble and par- ticle and therefore positively influence flotation kinetic rate. Inclusion of an optimum number of blades was also considered in the new rotor design. CFD modeling showed that increasing blades from the previous standard FLSmidth design of six blades to eight or more resulted in an increased level of turbulent energy dissipation. The current nextSTEP rotor design has nine blades. The slots or openings in the stator broaden the pumped slurry flow pattern as the slurry passes through the slots. The change in slurry flow characteristics was predicted by CFD modeling, which also predicted that these design features would result in improved flotation kinet- ics. The predictions proved to be true as numerous laboratory, pilot and industrial tests showed that this design achieved superior recovery versus the previous standard technology. Through the various testing phases in the development of nextSTEP, rotor/stator absorbed power was monitored relative to the previous standard and other designs tested. The nextSTEP technology consis- tently reported the lowest absorbed power of all designs. During testing, the reduc- tion in power was attributed to more effi- cient transfer of power to the slurry, and did not result in increased sanding. In the development and evaluation phase of the nextSTEP mechanism, sig- nificant power savings were achieved for laboratory-scale to 660-m 3 flota- tion cells. The majority of this data was generated by comparing standard Dorr-Oliver forced-air mechanisms to the nextSTEP mechanism. Recent data, comparing the nextSTEP mechanism to competitors' forced-air machines, have shown similar results. In addition, according to FLSmidth, the system's design also results in im- proved, evenly distributed wear patterns, which can save money over the life of the rotor/stator and reduce downtime for re- pairs or replacements. The rotor can also be run in a reverse direction to further in- crease the life cycle of the mechanism. Weber also noted that because the components of the nextStep system are among the common wear items that must periodically be replaced in ongoing flota- tion operations, the installation of next- Step equipment can be carried out during routine, scheduled maintenance down- time. Once installed, no additional oper- ator training is necessary. "This is simply an optimization of a familiar technology," Weber said. The company believes the lower en- ergy consumption and superior metal- lurgical performance are an attractive proposition for any mining operation us- ing flotation technology. The nextSTEP design has been engineered to fit all siz- es of machine, from the smallest 5-m 3 cell up to FLSmidth's 660-m 3 SuperCell machine. The mechanism is interchange- able with FLSmidth Dorr-Oliver forced-air flotation mechanisms and can be conve- niently retrofitted to other existing flota- tion equipment. Re-thinking the Process As the worldwide mining enterprise moves to processing orebodies with lower head grades, the volume of ore that must be ground and floated to produce the same amount of prod- uct increases by at least an equiv- The current nextSTEP design incorporates slots in the stator (left) and a nine-blade rotor (right).

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