Introduction
Before going too far into this, it is important to note that smaller Hydrocyclones (also referred to as cyclones) do make a finer cut.
A finer cut means more material will report to the underflow. This comes from the increased effect of the centrifugal forces on the particles with the smaller diameter cyclone.
The idea of larger cyclones offering better performance than smaller cyclones at first comes across instinctively as not true.
With that being said, there are multiple metrics for determining an Ultra Fines Recovery System’s performance.
Amount of underflow material
Since the amount of underflow material is the most obvious performance metric, let's examine how significant the difference between a larger cyclone and a smaller cyclone actually is.
A 4” (100 mm) cyclone can operate with cut point (a corrected d50c) in the range of 23 microns (500 mesh), while a 15” (380 mm) cyclone would operate (at the same pressure) in the range of 38 microns (400 mesh).
This may seem significant, but in truth, it could represent less than 1% difference in underflow recovery.
In the end, it may be worth chasing 1 ton per hour (tph) of misplaced material per 100 tph, but that assumes everything runs perfectly all the time. With only a minimal change in underflow recovery, this may put more importance on the other factors.
Availability
As with any piece of equipment, availability is a metric for performance. If a cyclone is not available to run, it may shut down the whole operation. If a cyclone is not performing as designed, it cannot provide the required advantages and benefits.
A cyclone has no moving parts, so what would affect availability or performance?
The size gradation of the material handled by the cyclone drives performance more so than the mechanics. The top size of the cyclone feed needs to be able to pass through the apex, which is more restrictive in a smaller cyclone configuration.
If the cyclone underflow discharge plugs, material is forced into the overflow or else the cyclone becomes completely plugged.
In the case of the material short-circuiting to the overflow because of a plugged underflow, that portion of the stream reports directly to waste. For example, if you have a 20-cyclone cluster and only one cyclone blocks, then 5% of the flow remains untreated.
However, it does not take long for another and then another cyclone to block, and before you know it, you may have up to 50% of the flow going directly to the pond before it is noticed by an operator.
One blocked cyclone, in a bank of 20, will send approximately 5 tph to waste per 100 tph. It does not take long to eat up the recovery benefit of a smaller diameter cyclone.
Roping (shown on the left) and ideal Hydrocyclone underflow (right).
When a cyclone is fully blocked (no material can get in), the flow is distributed to the remaining cyclones in the cluster. Each cyclone will have additional material reporting to the underflow, and the overall operating pressure will increase.
Once the cyclone apex is overloaded, the unit will begin to rope (high solids percentage, which looks like a stand of rope coming out of the bottom of the cyclone). A roped cyclone’s separation is negatively impacted, and coarser material will report to the overflow. In this case, all the operating cyclones will be sending coarse material to the overflow waste stream.
A roped cyclone can go unnoticed or ignored because it is too much of a pain to fix. In addition, the remaining cyclones are more likely to either partially or fully plug.
A cascade of failures can happen in a short time during what would be considered a minor upset condition in the rest of the operation. Because of the limitation of the smaller cyclones to handle the influx of larger top-sized material, these units can become a constant source of trouble.
Partially or fully plugged cyclones do not clear themselves. They require downtime to disassemble the units enough to remove the plugged material. Added to the performance failure, now there is a loss of operation time and increased maintenance costs. At this point, the loss of material is minimal compared to the loss of production.
Just looking at these two metrics for performance — amount of underflow material and availability, it starts to paint a larger picture of what improved cyclone performance can look like.
The goal of an Ultra Fines Recovery System is to remove as much solids from a waste stream as possible. The goal of any system is high operational availability. In order to achieve those goals, the cyclones need to be operating properly.
Larger cyclones are more reliable than smaller cyclones when it comes to handling influxes of larger top-sized material without plugging. The loss of material with a larger cyclone may not be as significant when facing lost production hours due to plugging of smaller cyclones. Because of these factors, larger cyclones are the better option when it comes to long-term plant performance and reliability.
