Prolonging the life of cyclones

Dense-medium separation using cyclones is a complex process. Proper preventative maintenance has an influence on the efficiency of the operation.  Unfortunately, there are mines that don’t realise the value of this. Mientjie Kleinhans finds out what can and should be done.

Gravity differences play a big role in dense-medium separation (DMS) – the process of separating valuable material from waste. Faan Bornman, technology manager, Multotec, describes in his technical paper, ‘Practical Operational Aspects of Dense Medium Cyclone Separation’*, that the specific gravity difference between the two fractions must be large enough to achieve high efficiencies. “Heavy particles sink through the dense liquid and light particles float,” writes Bornman.

To keep dense-medium cyclones in ideal working conditions, there are a number of important factors to keep in mind in the day-to-day operation of coal dense-medium processing plants. Bornman explains that one such factor is that mines should ensure that there is proper oversize protection from foreign objects – e.g. steel – in the case where ceramic lined cyclones are used.

“There should be protection against oversized particles that are too large to be processed by the cyclone unit in order to avoid blockages. And a working pressure transmitter or pressure gauge will assist in determining if the volumetric flowrate to the cyclone unit is consistent,” he explains.

Another important factor to keep in mind is proper maintenance and operation of drain and rinse screen panels, magnetic separators, and pumps. “The densitometer and pressure transmitter should be calibrated on frequent intervals to ensure accurate readings,” says Bornman.

He adds that proper inspection at frequent intervals throughout the operational life of the unit will ensure the internal integrity of the unit, and proper desliming upfront from the cyclone unit will prevent misplaced fines to the cyclone unit.

When the cyclone is pump-fed, it is important to check for impeller wear and a decrease in operational pressure. “Also, ensure that the pressure gauge is present and in good working condition. Check for wear in the inlet nozzle, inlet head, lower cone, and spigot. Both the cyclone and the support system, such as the plant around the cyclone, should also be in a good operational condition.”

According to Bornman, maintenance should preferably be done on every maintenance shutdown in order to allow for internal inspection, adding that the inspection should include: Maintenance

• monitoring the internal condition to determine if there are any abnormal wear patterns forming, or if any tiles, if this is a ceramic cyclone, have been damaged.
• measuring the spigot diameter since the cyclone efficiency can become affected as the spigot diameter increases.
• looking at the vortex finder leading edge, which means looking from the spigot side into the vortex finder, to determine if the condition is still good. In the event that it is not possible to inspect from the spigot side, the vortex finder length can be measured from the top of the cyclone after the overflow pipe has been removed.  
• measuring the vortex finder length and spigot diameter.
• checking for excessive grooves in the cyclone body or vortex finder.
• checking for inlet and cone wear, uneven surfaces, and steps.

It is possible to prolong the life of a cyclone in DMS by following a few steps. Bornman says that there is alternative wear material available on the market for clients to try. “Some of this material may, however, not behave as expected in a dense-medium environment and it is, therefore, always good to first try it on a smaller part of the cyclone.” 

Clients sometimes replace the whole cyclone after they have noticed that the spigot has worn to unacceptable limits. “In some cases, the remaining part of the cyclone is still in an acceptable condition and could still have been used. Some suppliers like Multotec, supply oversize spigots, which cater for these options where only the spigot is restored to its correct diameter, thus allowing for extended use of the complete cyclone.”

Bornman says that the life of a cyclone will be shortened when the cyclone runs at excessive pressure. “Wear rate is a function of the change in operational pressure to a power factor of 2.5. If the operating pressure doubles, the wear rate increases fivefold.”

Cyclone types

There are various types of cyclones for different applications, and it is beneficial to use the correct cyclone originally designed for the processing plant. “In DMS, it is of utmost importance to ensure the cyclone diameter matches the solids feed rate and correct medium to ore ratios. Similarly, the cyclone should be operating at the correct operating head (pressure). A mismatch cyclone does not achieve these objectives

“The impact of not having the correct sized cyclone will result in loss of product and, thereby, loss in revenue. In some cases, the client is not even aware of this loss since the sampling of these streams is not always done correctly and, therefore, sampling errors being introduced into the results mask the loss of efficiency,” says Bornman.

He adds that with an incorrect selection of the cyclone, others areas can become affected, like the cyclone feed pump not operating in the correct operating range, drain and rinse screens not been able to perform optimally resulting in medium losses, and increased wear rates inside the cyclone unit itself, which increases cost of ownership.

There is high-chrome, ceramic-lined, and silicon carbide cyclones cyclone parts available. “The benefit of using the correct or ideal cyclone will ensure the optimum lifetime and performance of the cyclones.”

Cyclone problems

With DMS being such a complex system, there will be certain issues causing problems in the cyclone, such as built-up material, pressure problems and inconsistent feed in and/or out of the cyclones.

Bornman says that build-up of material, also commonly referred to as hang-up of material, can be due to a too large differential over the cyclone which can cause medium instability inside the cyclone.

Since the cyclone dimensions are fixed, and at some stage the cyclone has been operating at the correct pressure, sudden changes in operating pressure would be the result of the cyclone feed pump not operating correctly. “This is not to say the pump itself is the problem, as the problem often manifests itself before the pump – like fluctuating medium additions or solids feed rates into the process.”

When there is inconsistent feed in and/or out of the cyclones, it is important to remember that the cyclone can only produce what it receives, and if the solids discharge rates are inconsistent out of the cyclone and one has confirmed that hang-up is not the problem, then the feed into the cyclone circuit is the problem. “Erratic feed to plant tonnages is usually the cause of the cyclone not discharging correctly. Another possibility can be the design of the mixing box or modifications made to the mixing box, which can cause slumping of solids into the suction of the pump,” says Bornman.

He adds that one must also ensure that there is enough medium in the circuit to ensure a proper level in the correct medium tank and mixing box. 

In the technical paper, Bornman writes that slimes, defined as the -45 micron clay particles, cause viscosity problems in the medium, which influences the separation of near density particles. “It is recommended to add 2 m³ to 3 m³ of water per tonne of coal to the preparation screen for de-sliming. The bed depth should be less than 20 mm, depending on the top size of the feed material.”

Other challenges

Two other challenges that mine’s face is inadequate spares inventory and improper training. “Inadequate spares inventory is one of the problems observed in mines. In order to keep costs low, low spares inventory is one of the areas that is recommended. We see that if the spare part is not available, no or inadequate maintenance is then conducted on the cyclone. This has a twofold effect:  the cyclone can be sacrificed due to worn parts, which affects the performance of the unit and, since maintenance is not done, other problem areas are not identified in time.

“Unplanned breakdowns can also occur. The combined monetary effect of inefficient operation and the cost of the breakdown far exceed the value of adequate spares inventory. Proper training, (like the Multotec open day client training sessions) on the impact that an inefficient cyclone can have on the bottom line of a process can help to reignite the effort of doing proper maintenance on an ongoing basis,” concludes Bornman.