Dewatering with Hydrocyclones
by Alan Bennetts
By understanding the various methods of dewatering, an operator can select the best option for an operation.
Hydrocyclones have traditionally been used for classifying, but they can also be used to produce a thick, dewatered underflow. This can be accomplished in two different approaches. The first approach uses a traditional hydrocyclone with key parameters altered to achieve the desired percent of solids in the underflow. This method can be used for dam building, with the coarse, underflow being used to help form the wall.
Sometimes called a separator, the second approach modifies the traditional cyclone. These modifications still allow the cyclone to perform a predictable classification while producing a stackable material.
Both approaches will be covered with a review of the advantages and disadvantages.
Roped discharged dewatering
This method consists of purposefully “roping” the hydrocyclone discharge. Roping occurs when the air core is unable to form due to an excessive amount of material in relationship to the spigot/apex opening.
Under normal cyclone operation, the air core is essential to the classification process (Figure 1) and a spray discharge (Figure 2A) is formed. The percent of solids in the underflow is normally in the range of 42 to 50 percent by volume. Through changing the spigot/apex opening, the actual underflow percent solids can be adjusted to a target range (which can be lower than 42 percent by volume). When a cyclone is operated with a spray discharge, the underflow is not sufficiently dewatered to form a stack of material.
Stepping away from the traditional method of operating the cyclone, further dewatering can be achieved by reducing the spigot/apex opening or roping (Figure 2C). This creates a choke point at the discharge that preferentially allows the solids to continue and forces the liquids toward the vortex finder. The percent of solids by volume range is increased to 52 to 56 percent, and this allows the underflow to be stacked into a pile.
The main advantage of this approach is that it does not require any additional components to the hydrocyclone, but it does have a number of disadvantages. First and foremost, it reduces the classification ability of a cyclone (Figure 3C). A roping discharge of the cyclone will still preferentially send coarse material to the underflow and fine material to the overflow (through the vortex finder), but the sharpness or efficiency is significantly diminished. Because of this, the roped discharge method of dewatering is best suited for tailings and primitive dam building.
Another significant disadvantage with this method is feed fluctuations will directly affect the performance. In a situation where the tons per hour of the solids decreases, the spigot/apex may not be small enough to force the cyclone into a roping condition. The percent solids by volume of the underflow will decrease, and the underflow will become a spray discharge. The additional water in the underflow will reduce or eliminate the stackable nature of the material.
The opposite situation is less of a concern. Since the spigot/apex is already acting as a bottleneck for the solids, an increase in the tons per hour of the solids will force the excessive material into the overflow. High recovery of solids into the underflow should never be a major focus when employing this cyclone dewatering method.
A third disadvantage of this method is a higher wear rate on the cyclone liners, especially the spigot/apex liner and the vortex finder.
Water siphon dewatering
Through the addition of an extended overflow pipe, siphon break control, and a discharge regulator (aka fishtail, lipseal, or flapper), a hydrocyclone can be used to dewater solids and still perform a sharp classification.
The extended overflow pipe is used to create a siphon through the vortex finder. The siphon draws water and fine material, and the length of the overflow pipe has an effect on the amount of siphon force created. When the overflow pipe is too short, a smaller amount of siphon force is created and decreases the ability to dewater the material. When the pipe length is too long, the siphon force will be too great. This will cause additional material to report to the overflow. Most overflow pipes are designed to extend 6 feet below the underflow discharge of the cyclone.
To provide a level of control, the siphon break control panel can adjust the siphon force. With the siphon break completely closed off, the full siphon force is applied, resulting in the driest underflow and the highest amount of fines sent to the overflow. If the siphon break is completely open, the least amount of the siphon force is applied. The additional water in the underflow will carry some of the fines and may cause issues when stacking the material.
The discharge regulator is designed to seal off the spigot of the cyclone to prevent an air core from being formed while allowing the material to discharge. Material will build up in the spigot until the weight of the material is enough to open the discharge regulator. Water alone should not be able to overcome the seal as long as the cyclone is operating between the range of 6.5-14.5 psi at the inlet.
When operating outside of the pressure range of 6.5-14.5 psi at the inlet, the performance of the cyclone can become unstable. At low pressure, the slurry can alternate between heavy discharge at the spigot to excessive material reporting to the overflow. High pressure generally causes heavy discharge through the spigot without dewatering the material.
Consistent dewatering of the material is the primary advantage of the water siphon method, even if the solids throughput is fluctuating. When properly equipped and maintained, this method will also produce the driest underflow. It will also run on water without having underflow discharge.
Another significant advantage is the cyclone can still perform a sharp size classification similar to a traditionally operated hydrocyclone. The water siphon provides the lift velocity that sends the fines and water to the overflow.
The vertical footprint caused by the overflow pipe and the requirement to break to atmosphere can cause installation issues. Since the overflow pipe is controlled length, it is required that it be allowed to break to atmosphere to prevent excessive siphon force from being formed. This is usually accomplished by having the overflow pipe discharge into a receiving box, which will drain into the required pipeline. The feed percent solids by weight limitation can also be seen as a disadvantage. Cyclones operating with the water siphon method work best between 0 to 25 percent solids (by weight). New feed into the cyclone above the 25-percent limit will decrease classifying sharpness. An additional disadvantage to this method is the increased initial price due to the added components.
This method is ideally suited for creating large stockpiles, feeding various classifiers, attrition cells, or any process equipment that will be adversely effected by excessive or fluctuating water content. The percent of solids by volume range is increased to 54 to 58 percent. Cyclones can be mounted to radial stackers or other structures to increase the capacity of the stockpile. Material can be loaded into a truck within 24 to 48 hours if the stockpile has adequate drainage for the remaining water. It is not recommended to have the underflow report directly to conveyor belt due to housekeeping issues caused during start-up and shut-down.
The roped discharge method is a quick-and-dirty approach to dewatering material without the need for sharp classification. As long as there is enough material to choke at the spigot, it will continue to produce a dewatered underflow. It has the advantage of a lower initial price, but may have a higher maintenance cost.
With the water siphon method, dewatering can occur with a number of advantages such as sharp classification and the ability to accommodate feed fluctuations within an acceptable operational range. It also provides a consistent underflow density. The tradeoff is the cost of the additional accessories and the larger installation footprint, when compared directly to a traditional cyclone. A lower maintenance cost will offset the initial price difference.
Alan Bennetts is the process application manager for Gallatin, Tenn.-based Weir Minerals Linatex.
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