Attrition Cells are commonly used in the frac sand industry to improve the turbidity of the sand, but they can also be used in other aggregate applications, such as in the production of construction materials, industrial glass and specialty products, as well as in some mineral processes.
Attrition Cells are designed to remove deleterious material via particle-on-particle scrubbing. They provide an aggressive scrubbing action to remove even the toughest contaminants. They can handle very high plasticity clays that are out of the scope of other scrubbing equipment (such as Blade Mills, high pressure washers and wash screens) but are limited to fine material approximately ½” (12 mm) minus.
How Attrition Cells work
Attrition Cells consist of a vertical paddle shaft that is powered by a gearbox and rotates within a round, square or octagonal rubber-lined tank. The paddles are configured in an altering arrangement to facilitate the particle-on-particle scrubbing necessary to remove deleterious material. Feed material enters the Attrition Cell via an inlet and is scrubbed as it works its way toward the outlet.
To achieve the highest chance of inter-particle interaction, Attrition Cells need to be fed a dense material, somewhere in the 70-75% solids by weight range. For this reason, they are typically fed by Separators™, Hydrosizers™, Fine Material Screw Washers or Dewatering Screens.
After the scrubbing stage, the material should be directed to rinsing or washing equipment to remove the liberated contaminants from the product stream so they don’t dry back onto the material. Fine Material Screw Washers, Hydrocyclones and Hydrosizers™ are often used in this rinsing stage.
Most often, Attrition Cells are used in series to cut down on the amount of material bypass. In this configuration, the outlet of the first Attrition Cell is connected directly to the inlet of the second unit. When the material discharges from the first Attrition Cell, it immediately enters the second tank for further attritioning. Depending on the material, the scrubbing process can involve one or multiple Attrition Cells.
Retention time (how long the material is subjected to attritioning inside the cell) is determined based on the severity of the contamination but typically ranges from two to five minutes.
It is important to note that Attrition Cells are not to be used for product sizing or shaping.
However, Attrition Cells are an effective means for removing deleterious material that can cause a slew of issues in the final product or the end use.
Why to use an Attrition Cell
The main reason to use an Attrition Cell is to break apart deleterious material for removal with the washing process downstream so that the competent material can be sold and won’t break down once it’s on the product pile.
Because of their ability to break down deleterious material, Attrition Cells are used to:
Remove surface contaminants
Contaminants such as clays, oxides and chemicals that cling to the surface of particles can have a detrimental effect on the final product. Attrition Cells remove these surface contaminants by subjecting the material to aggressive particle-on-particle scrubbing, which liberates the deleterious material from the sound particles.
Increase product durability
Durability measures a sand’s competency. It indicates a particle’s resistance to the production of detrimental clay-like fines when subjected to mechanical methods of degradation. The higher the durability, the more resistant that particle is to degradation.
Attrition Cells improve the durability of a material in a couple of ways. First, they can break up clusters that are strong enough to pass through the sizing process but will fracture under stress. Clusters lower the durability of a final product, so breaking them up ahead of time is key.
Another way Attrition Cells can increase the durability of a final product is by breaking apart fractured sand. Internal fractures within a particle will lower durability, and Attrition Cells have some ability to break these particles along their fracture lines into smaller, more durable particles.
Pro tip: When addressing durability issues, Attrition Cells should be placed in the beginning or the middle of the process before final sizing takes place.
Turbidity measures the amount of soluble clays in a fluid. If a particle containing clay contaminants is added to a fluid, the clay can dissolve into the solution and cause turbidity issues
Attrition Cells can be used to remove heavy clay coatings that can cause turbidity issues. It is extremely important that the material is rinsed after the Attrition Cell to prevent the liberated contaminants from drying back onto the sand particles, which would then lower the sand equivalency.
Pro tip: When addressing turbidity issues with a single product, Attrition Cells should be placed toward the end of the process. When multiple products are being made, the Attrition Cells should be placed before the sizing stage.
Improve sand equivalency
Sand equivalency indicates the relative proportion of fines, clay and dust to sand-size particles; in short, it measures a sand’s cleanliness. Determined by the sand equivalent test, the higher the sand equivalent value, the less clays present in the material and the cleaner the material is considered.
If you have sand particles with a high abundance of clay, an Attrition Cell can remove the contaminants to improve the cleanliness of the sand. Again, it is important that the sand is washed or rinsed after an the attritioning stage to remove the liberated clays from the product stream so they don’t dry back onto the clean sand.
Scrub sand-sized particles
When an aggressive scrubbing is needed to remove tough, heavy clays from sand-sized particles, an Attrition Cell is the machine for the job. They are specifically designed to remove tough, high plasticity clays from materials less than ½” (12mm).
Keep in mind, though, that an Attrition Cell is not a sizing or classifying device. Sand tanks, sand screws and Hydrocyclones are the best option for classifying sand-size particles to meet product sizing specifications.
Prepare flotation feeds and reagent washing
Flotation is a method used for separating particles based on their hydrophilic (water wetting) or hydrophobic (water repelling) properties. It is particularly used in mineral processing applications. When air is added to the solution, hydrophobic particles are carried to the surface with the air bubbles, while hydrophilic particles sink to the bottom.
With the flotation process, chemicals, known as reagents, are added to the system to improve the separation of particles based on surface chemistry, specifically hydrophobicity and hydrophilicity.
To better allow the particles to attach to the air bubbles or for the reagents to coat the particle, the particles should be free from contaminants. Depending on the material, contaminant and/or application, Attrition Cells can scrub away these contaminants to reduce reagent consumption and improve recovery in flotation applications.
Whether you want to remove surface contaminants, scrub sand-sized particles, increase durability, improve turbidity and/or sand equivalency, or prepare flotation feeds and reagent washing, Attrition Cells can provide the aggressive scrubbing action needed to remove clays and other contaminants that can harm a final product.