The Importance of Recirculating Dewatering Screen Underflow

Alan Bennetts
By: Alan Bennetts January 25, 2021
Recirculating Dewatering Screen underflow offers high throughput rates while preventing material loss.

Dewatering Screens have become a mainstay in many industries because they are a cost-effective method of drying material after a wet process. In addition to discharging a drier product, there are a number of reasons why Dewatering Screens are a great addition to an operation, including:

  • Improved housekeeping
  • Water recovery
  • Reduced stockpiles

Housekeeping 

Housekeeping alone is a valid consideration for utilizing a Dewatering Screen. While other equipment removes enough moisture to allow for conveyor transport, the material still contains “free water", or water that is readily shaken off and not in the material itself. 

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Dewatering Screens discharge material with a moisture content as low as 7% in some applications. This reduction in moisture improves overall housekeeping, as the material coming off the screen is drier. 

Material spills at conveyor scraper and transfer points result in additional cleanup work and potential equipment damage. Through the reduction of moisture in the final product, Dewatering Screens reduce excessive spillage, as well as material sticking to the belt, and lower housekeeping time.

Water Recovery

Another reason Dewatering Screens have gained acceptance is because of the importance of water recovery. Obviously, in a wet plant water is an important component of the process. On many sites, the problem of too much or too little water is an unpleasant circumstance. Environmental regulations and limited availability of water force operations to manage water balances at a completely new level.  

A Dewatering Screen can keep more water in the system to be reused or to be directed more easily to water recovery systems. Retaining the water in the system decreases demands on limit supplies, while cleaner water meets release requirements.

Stockpiles

Stockpiles can be an important part of any operation, but at the same time, any product sitting around waiting to dry for transport is a delay in sales revenue. In addition, stockpiles can take up valuable real estate in tight spaced operations. 

Discharge from a Dewatering Screen becomes available sooner for hauling and reduces the stockpile footprint. While a stockpile may still have water drainage with a Dewatering Screen in place, the size of the stockpile is significantly reduced and more manageable. 

With the above reasons, it is easy to justify a Dewatering Screen's place in an operation. However, it is important to bring up the characteristics that make a Dewatering Screen what it is.  

Dewatering Screen Design

Traditional screens are designed to make a separation based on the ability of a material to pass through an opening. Dewatering Screens operate in opposition to that idea. 

The goal of a Dewatering Screen is to retain as much material as possible on the screen and only allow water to report to the underflow. 

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Unfortunately, there is always some material, smaller than the screen media opening, that travels with the water to the underflow. 

In cases where the material that follows the water is not usable material, dealing with the screen throughs is easy. The water and fine material can be treated as a waste stream and potentially subject to a water recovery system.

The more difficult situation is when the material lost to the underflow is considered product. It becomes a balancing act of compromise to achieve high throughput and dry discharge against loss of material. 

Larger screen media openings allow for a higher percentage of open area and a higher drainage rate on the screen deck. The higher drainage rate allows for a high throughout and dry discharge. 

Since the screen media opening is the limit on the size of material reporting with the water, more material is lost with larger screen media openings. Decreasing the opening size will decrease material loss but will also reduce the drainage rate. Depending on the moisture level in the feed, it can reduce throughput capacity.  

Recirculating the Underflow

In order to retain the high throughput and discharge a “drip-free” material, the compromise is larger screen media openings to achieve the higher drainage rate. Since this would create an unacceptable level of loss of good material, recirculating the underflow material prevents material loss while the screen maintains high throughput capacity.

The introduction of a sump and pump underneath a Dewatering Screen is one of the more common approaches because it offers multiple solutions to recapture the material. In the case of Ultra Sand Plants and Ultra Fines Recovery Plants, new feed can be introduced into the same sump. This approach makes for a natural recirculation of the desired material to minimize loss. The only loss with this recirculating load is now in the Hydrocylone or Separator™ overflow.  

The above video on how the McLanahan UltraDRY Modular Dewatering Screen works shows screen underflow being recirculated back to the sand screw that precedes it in the wet process. 

When the sump underneath the Dewatering Screen is not part of the feed system, there are a number of methods to return the desired material back into the system for recovery. 

Gravity flow of the Dewatering Screen underflow is the most desired method, as it does not require any additional equipment. This method does require the destination of the slurry to be lower than the Dewatering Screen’s sump discharge point. A small pump can direct the slurry material when that is the case. 

There are other methods available, but a pump provides a number of options for how the material can be re-introduced anywhere back in the process. A pump also allows the addition of a cyclone, which can return the solids while removing a majority of the water. The cyclone can discharge in a number of points back into the system, including onto the Dewatering Screen. The cyclone overflow can be used to maintain the level in the recirculating sump to further enhance solids retention.

A Dewatering Screen alone can improve a wet process in a number of ways, but the addition of an underflow recovery system takes it to the next level. The system provides increased recovery, improved water balance and the driest dewatering screen discharge.   

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Tags: Mining, Aggregates, Minerals, Dewatering
Alan Bennetts

Alan is the Global Product Manager for Washing and Classifying at McLanahan Corporation. He provides leadership, direction and oversight to the evaluation, design, development, engineering, training and support needed for McLanahan’s extensive washing and classifying equipment line. Alan has nearly 25 years of experience in the mineral and aggregate industry, having served in a wide array of roles with equipment manufacturers and mining companies throughout the United States. Alan is a 1996 graduate of the University of Montana, where he received his Bachelor of Science degree in metallurgical engineering.