Factors That Affect Rotary Scrubber Retention Time

April 12, 2022
Read this blog to learn about the factors that affect Rotary Scrubber retention time as well as how to increase or decrease the amount of time the material stays in the scrubber.

Rotary Scrubbers remove light, loamy clays and soft, soluble contaminants from aggregate and various ores. These machines can handle large feed sizes (12” or 300 mm or larger) at high capacities of up to 8,000 tph.

Rotary Scrubber.

How Does a Rotary Scrubber Work?

Rotary Scrubbers consist of a large cylindrical drum supported on trunnion rollers at both ends, allowing the drum to rotate. The inside of the drum features adjustable lifters for agitating the material and advancing or retarding material flow through the drum.

Material to be cleaned enters the Scrubber via a feed chute, where it is directed into the drum. Fresh water is added through the feed chute and/or through a spray pipe that runs through the drum.

As the drum rotates, the lifters lift the material and carry it up the side of the drum until it is dropped, impacting the material below. As the material tumbles upon itself, the deleterious material is broken down and removed from the ore. The dirt, loam and softer materials are dissolved into the solution. The material is conveyed through the drum by the addition of new feed material.

Top view of a Rotary Scrubber.

Since all material that enters a Rotary Scrubber must also exit the Scrubber, the clay and other deleterious material removed in the washing process will discharge with the clean rock or ore. This material can be screened at the end of the Scrubber by an optional dewatering screen extension and/or be discharged on to a vibratory rinse screen.

An optional desander section can be added in applications where the feed material contains sand. It is recommended that any minus 3/8” material be removed prior to the Scrubber to decrease the cushioning effect that occurs when this fine material is present. 

A Rotary Scrubber cleaning and discharging aggregate.

Factors that Affect Rotary Scrubber Retention Time

Retention time is the amount of time the material stays in the Rotary Scrubber, and it varies for each application. The longer the material stays in the drum, the more it is subjected to the material-on-material scrubbing brought on by the lifting and dropping action of the internal lifters.

There are several factors that affect Rotary Scrubber retention time, including:

  • Bed depth
  • Percent solids
  • Drum diameter and length
  • Specific gravity of the material
  • Tons per hour

Bed Depth

Bed depth is an important factor when considering material retention time. A deeper bed of material takes longer to move through the drum to the discharge end.  

The bed depth is dictated by the size of the feed and discharge openings. A smaller discharge opening will allow a deeper bed to build before the material overflows out the end. This opening can be adjusted smaller or larger to increase or decrease retention time via a weir at the discharge end. 

Percent Solids

The percent solids of the feed also has an effect on retention time. The higher the percent solids and the thicker the slurry feed, the longer the retention time will be. A feed with a thicker consistency will take longer to move through the drum.

Conversely, the lower the percent solids and the more water in the slurry, the shorter the retention time. The water in the feed will help to move the material through the drum quickly.

For optimum operation, a 50%-60% feed solids percentage is preferred. Feeds with a lower or higher percentage of solids can change the length of Scrubber required for the desired tonnage and ultimately affect the retention time. 

Drum Diameter and Length

The length and diameter of the drum itself plays a role in the overall retention time of the material. The longer and larger the diameter of the drum, the higher the retention time. 

Specific Gravity

Another factor that affects Rotary Scrubber retention time is the specific gravity of the material. Materials with a lower specific gravity move through the drum much faster than materials with a higher specific gravity.

Sand, for example, has a specific gravity of about 2.7, where some ores can have a specific gravity of 5.0 or more. The sand would travel through the Rotary Scrubber much quicker than the iron ore because it is lighter and flows more readily. 

Tons Per Hour

The desired tons per hour will also affect retention time. A higher tonnage will move through the drum much more quickly than a lower tonnage. 

Increasing or Decreasing Retention Time

If you need to increase or decrease the retention time of the material in your Rotary Scrubber, you have a few options.

First, you can adjust the weir located at the discharge end of the Scrubber. Raising the weir will increase material retention time, as it will allow a deeper bed of material to build before discharge, while lowering the weir will decrease retention time by lowering the bed depth.

The second way to increase or decrease retention time is to adjust the internal lifters to advance or hinder material flow. Setting the lifters in the advance position will push the material toward the discharge end more quickly, decreasing retention time.

Setting the lifters in the retarding position will push the material back toward the feed end, keeping it inside the drum for a longer period of time, therefore increasing retention time.

The lifters can also be placed in a neutral position, where the natural velocity of the feed material advances the material toward the discharge end. 

Ideal Retention Time

Longer retention time isn’t always better in every application. The ideal retention time for a material will vary based on the factors listed above: bed depth, percent solids, specific gravity, drum size and tons per hour.

To achieve the ideal retention time, Rotary Scrubbers are tailored to the application into which they are operating. This is why it is important to know the material characteristics and desired capacity when sizing a Scrubber for your site. A correctly sized Rotary Scrubber will provide a clean, washed material at the lowest cost per ton. 

Prevent Blinding and Plugging
Tags: Scrubbing

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