The focus on water and energy consumption has placed high demands on minerals processing suppliers. Against this arena, many mine owners have recognised the importance of looking at the total life cycle cost, not just the initial outlay in technology and equipment. The mine owners have realised that it is critical to examine the lifetime operation and maintenance of a flotation machine. Total life cycle cost analysis considers not only the initial investment but also the lifetime operation and maintenance costs.
In flotation, for example, research shows that roughly 60 to 80% of the total life cycle costs are spent on energy, while the initial investment comprises less than 10%. The relevant cost factors for a flotation plant are investment, energy, reagent consumption and maintenance. Investment costs have been based on the purchase of flotation technology only since the variation in infrastructure, installation and assembly costs is significant.
The most significant life cycle cost item in flotation operations is the cost of electricity. Thus the operational expenditures are influenced by the energy price and the energy efficiency of the equipment used for production. Energy prices are based on prevailing market forces and outside of a site’s control, however the purchase of energy-efficient flotation technology is not.
The following aspects are critical in energy efficiency:
- Air dispersion
- Rotational speed of the mechanism
- Component wear
Optimal air dispersion is one of the basic requirements for good metallurgical performance. Plants operating with forced air cells have often noticed that the best results are achieved using individual and varying air feed rate in each cell. In traditional flotation mechanisms, the air feed is limited by the reduction of power draw and mixing, or by reduced dispersion of air, making the froth surface unstable and causing the froth to collapse. Outotec ’s new rotor and stator design, FloatForce, extends the maximum air feed limit. As a result, the cell surface is steady in all situations and the pumping rate of the mechanism is only slightly affected by air. Because of the flat power curve, less power is needed when the mechanism is operated with little or no air. This allows smaller motors and benefits both in investment and operating costs.
FloatForce has been designed with independent slurry and air slots, therefore pumping is independent of air addition, ensuring air dispersion at all pumping rates. Site test results have also shown further advantages such as increased bubble/particle interaction, increased bubble/surface area flux and optimal bubble size distribution. FloatForce technology can be retrofitted onto existing equipment, as well as on new cells.
The rotational speed of the rotor is an important factor in electrical energy consumption. Studies on variable-speed drive (VSD) mechanisms have been conducted indicating that a reduction in the rotor speed may be possible without reducing the metallurgical performance of the flotation cell.
The rotational speed has a significant effect on the power draw of the mixing mechanism. The pressure difference over the mixing mechanism is proportional to the rotation speed squared and the volume flow rate is directly proportional to it. Thus the power draw of the mixing mechanism is proportional to the third power of the rotation speed. Consequently, a minor reduction in rotation speed may have no effect on process performance but a significant effect on the energy consumption. For example, 10% reduction in rotation speed roughly equals to 27% reduction in power draw.
A drive mechanism that enables the adjustment of the rotation speed may produce significant savings in electricity consumption. Outotec’s research centre has conducted studies, which have shown that a variable speed drive may have payback time of only a few months, if the process allows optimization of rotation speed. The main benefit of the new arrangement is the possibility to adjust the cell during normal operation. Float cells typically operate with Jg values in the range of 0.5 to 1.5cm/s where we see the ability to reduce the speed, which in turn can provide significant power savings.
The key mechanical aspect for flotation process efficiency is the proper condition of critical wear components. Missing or inferior rotor or stator parts make the cell surface wavy and cause the froth to collapse. Air dispersion is reduced and decreased pumping causes sanding. Experience has shown that non-standard or copied spare parts often have a shorter wear life and in some cases decrease the metallurgical efficiency.
One of the benefits of the FloatForce is the modular design of the stator, which ensures the parts requiring maintenance or replacement can be easily changed, as opposed to replacing the complete stator. Using CFD and focusing on issues such as the critical flow area, the new stator was designed to resist traditional wear patterns, the wear being on a small, well defined area. This minimised friction losses and impact angles, thereby ensuring less wear and longer life.
As each blade can also be separately maintained, it is safe and easy to handle (less hauling in confined spaces), faster to install and has less equipment downtime. The replacement of the blades for one stator, for example, can take 30 to 40 minutes.
It is well acknowledged that larger flotation cells have significant advantages in capital costs. If one selects 15x100m3 cells instead of five 300m3 cells, the initial investment cost is approximately 50% higher. The economic advantages also continue in areas such as maintenance, instrumentation and building costs since there are fewer units and the required footprint is smaller.