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Mining and metals refining
Aug 15, 2019

Regular maintenance inspections help achieve optimal metallurgical performance in flotation machines

Declining ore grades is one of the key challenges in the mining and minerals processing. The demand for more efficient technologies and higher performing equipment is increasing while resources continue to deplete. During hard times, major investments to the latest technology is not always possible, and mining operations seek ways to improve and optimize the performance of their existing assets.

In the past, maintenance was considered as a corrective action performed only when something breaks down. Today proactive maintenance is seen as a value adding process. The equipment must be as reliable as possible for the plant to achieve its production targets. The purpose of proactive maintenance is to prevent equipment breakdowns and increase availability and performance of the equipment at an optimal cost.

Flotation equipment inspections and metallurgical performance

The relation between regular maintenance and metallurgical performance in flotation machines has been proven to be true. An excellent way of ensuring that equipment is not contributing to any production loss is to undertake regular inspections of the mechanical and electrical components. These inspections should be conducted as frequently as practical, typically during every plant shutdown, and form a critical part of any site maintenance program.

Maintenance for flotation machines is often compromised because lack of resources and time to drain and clean the cells. On the other hand, flotation cell maintenance is not conducted in a timely manner because often site personnel are not sure what to look for when inspecting the equipment. Unless the problem is obvious, they are not sure at what point performance is negatively impacted.

Many items that need inspecting are detailed in OEM operating manuals. However, if there is any doubt, the manufacturer should be consulted, as they can advise on the required state and even provide training to ensure critical items are understood. The following article provide tips to what to look for during maintenance inspections while the flotation equipment is operating and during planned shutdowns when it is possible to take a closer look of the equipment.

Inspection while flotation cells are operational

A list of inspection items that can be observed while flotation cells are operational can be seen in the Table 1. Some of these checks and more specialized condition motoring exercises fall under the umbrella of the maintenance department. However, this should not deter anyone in the plant from reporting deviations they observe. Timely reporting of problems has prevented severe incidents on many occasions.

Table 1. Items that can be inspected while flotation cells are operational.

While the plant is operational, regular checks should be conducted on the flotation cell drive unit and process control instruments to ensure they are operational and giving a reasonable output. The check on instruments and associated control inputs can be as simple as looking at the trend in the measured and controlled variable in the control system and confirming that these readings are being measured correctly in the field. Problems with control loops not responding can be due to a particularly large disturbance or lack of tuning. They can also be due to problems with control equipment (reading or actuating in the field) and can be used as an indication of such.

Shutdown inspections

There are several items, especially those inside the tank, that can only be seen during shutdowns. Table 2 lists the items to be inspected during shutdowns. Some of these checks can be done from outside the tank (e.g. pulley alignment), whilst other checks, such as the condition of the mechanism and the dart plugs, require entry into the tank. For most of these checks, the site equipment isolation procedure should first be conducted to ensure the safety of the people conducting the inspection. It is also critical that the tanks have been drained and cleaned prior to the internal inspection being undertaken.

Table 2. Items that can be inspected while flotation cells when shut down.

Flotation mechanism inspection

The mechanism is the heart of a mechanical flotation machine and consists primarily of the rotor and stator. Most rotors and stators have a metal skeleton covered by an elastomer (typically natural rubber or polyurethane) to enhance wear resistance. In normal operation, wear will mostly occur on the back of the rotor leading edges, where the air forms a spinning vortex which generates the bubbles. Regularly reversing the direction of the rotor will spread this wear on both sides of the blade, effectively doubling the service life of a rotor that is only run in one direction.

When checking the rotor, whether the elastomer coating has worn to the point where metal skeleton is exposed is of concern. Ideally, rotors should be changed out well before this, as performance at this point will be sub-par. There is also a potential for component failure due to slurry or chemicals in the slurry eating away the metal skeleton. Measuring rotor blade thickness at its thinnest point should be undertaken and the manufacturer consulted to advise on the recommended thickness for change-out. When inspecting the rotor, look for areas with localized damage, such as missing chunks of elastomer. These generally have been removed by large impacts, which indicate the presence of foreign material such as rocks inside the flotation cell. Finally, the rotor should be checked for rocks or other foreign bodies blocking the air and slurry pumping slots (example Figure 1). If foreign bodies are occupying this space, it reduces rotor efficiency. If possible, these foreign bodies should be removed before the cell is restarted.

An example of debris lodged in air slots
Figure 1. An example of debris lodged in air slots (this will negatively impact air dispersion)

Dart valve assembly inspection

Dart valve assemblies should be give close examination when inspecting inside flotation tanks. Key areas to look for wear are on the plugs, dart seat plate, dart shaft guides and joins on the darts. If the plug and plate are worn, it will change the flow characteristics through the valves. Wear on the dart shaft guides (see Figure 2) can result in dart shaft being at an angle, which can affect the flow through the dart and may result in uneven wear and poor control level. In the extreme, if the darts are not aligned and seated correctly, they may not have an effective seal when fully closed and could even break, causing damage to the shafts, the actuators and the positioners. Typically, clearances between the shafts and the shaft guides should be less than 5 mm in total around the circumference of the shaft.

An example of worn dart guides
Figure 2. An example of worn dart guides. If these are not replaced, shaft damage, failure and damage to the positioner could occur.

The value of maintaining equipment

One of the reasons that mineral processors have a hard time when they want to spend money on maintaining flotation equipment is that it is often difficult to demonstrate the value in this expenditure. Munro and Tilyard (2009, Mill Operators' Conference) gave two examples of where operation gave flotation cells maintenance attention, with the results having a significant impact on the bottom line. In the first example, a company was operating with a large buildup of tramp material in their flotation cells. When this was identified and removed, the recovery went from 83% to 90%. In the second example, the condition of the mechanisms and level control equipment had been allowed to deteriorate. When they were replaced, a recovery improvement of 4% and a 2.5% improvement in product grade were observed. In both instances, the value in maintaining the equipment was the difference in productivity.

One of the principle difficulties in quantifying the benefit of maintaining flotation equipment is that wear, and equipment degradation occurs over a long period of time. Compounding this, flotation is a complex process with many variables affecting the outcome, with changes in operator-controlled variables (e.g. air, level and reagent dosage) and changes in ore properties (e.g. feed grade, mineral dissemination and hardness) sometimes occurring on an hourly basis. These can mask the effect that equipment wear can have on production, especially when it is in most cases a gradual change.

Outotec Equipment Inspections with clear reporting and expert recommendations

If you would like Outotec to assist you with preventive maintenance planning, please let us know and we can discuss a tailored solution to meet your needs. As an example, the Outotec Equipment Inspection Service provides clear data and expert recommendations to improve decision-making and help keep site operations running reliably to meet production targets. The Inspection Service comprises data analysis and preparation, on-site equipment evaluation, and a full inspection report with follow up as agreed. Outotec technicians use a mobile app to make the inspection and reporting process faster. This means customers get the results quickly and in a clear, accessible format that’s easy to share with colleagues. The app even allows the technicians to capture thermal images to support more accurate decision-making.

Comments Charlie Romano, Maintenance Superintendent at KCGM, Kalgoorlie, Australia: “It’s a useful report with a good overview. Outotec’s service is very professional. Since restarting with the Outotec flotation inspections, we have seen an improvement in equipment reliability”.

utotec technicians use a cloud-based mobile app
Figure 3. Outotec technicians use a cloud-based mobile app to make the inspection and reporting process faster.

Acknowledgements

The maintenance inspection tips in this article are based on the conference paper "Unlocking value through flotation equipment maintenance" presented at 47th Annual Canadian Mineral Processors Operators Conference, Ottawa, Ontario, January 20-22, 2015. The authors of the paper were A. Jalili, B. Murphy and P. Tolvanen from Outotec.

References

Munro, P.D. & Tilyard, P.A. (2009). Back to the future - why change doesn't necessarily mean progress. Proceedings Tenth Mill Operators' Conference (pp 5-11). Australian Institute of Mining and Metallurgy: Melbourne.

Murphy, B. (2013). If you can’t measure it, you can’t manage it. MetSoc News2,  p. 3 Australasian Institute of Mining and Metallurgy: Melbourne.

Murphy, B., O’Connell, S., & Heath J. L., (2014). Maximising value through maintaining your flotation equipment. Proceedings of the 12th AusIMM Mill Operators Conference, Australian Institute of Mining and Metallurgy: Melbourne.

Outotec (2018), Case study: Regular, systematic equipment inspections deliver useful site tool and greater reliability, https://www.outotec.com/references/equipment-inspections-for-greater-reliability/

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