Introduction
Good, clean sand is a general way of referring to a sand product with a high durability and low turbidity.
Durability
A sand with a high durability will resist breaking down or grinding up into finer particles. Whether for a construction or fractioning application, high durability is a key characteristic of quality level.
Turbidity
Turbidity, on the other hand, is not really about the sand itself but the contaminants with the sand. The turbidity test shows silt or clay, which stays suspended in water. These small particles have adverse effects on the final applications for these produced sands.
Making good, clean sand
While the objective is clear, the path to a good, clean sand can be difficult. One of the go-to pieces of equipment to help improve both durability and turbidity is an Attrition Cell.
McLanahan Attrition Cells are designed to scrub the surfaces of particles to remove deleterious materials to improve sand durability and turbidity.
How Attrition Cells improve sand durability
The particle-on-particle interaction, which happens inside an Attrition Cell, has a number of effects on the material.
Breaks up clusters
In some deposits, the particles form a cluster due to a type of cementation. The cluster is strong enough to pass through the sizing process intact, but when used under stress, the cluster breaks apart.
Though the particles themselves may have a high durability, the presence of clusters will lower the overall durability of any product. Breaking the clusters apart during processing is a key goal before they become part of the final product.
Breaks fractured sand
Similar to the issue of the presence of clusters, internal fractures to the grain will lower overall durability by breaking apart under pressure. While Attrition Cells do not exploit existing fractures to the same level an Impact Crusher can, they can break apart a significant level of them. Once the fractured sand is broken apart, the remaining particles should have a higher durability.
When faced with these two issues, it is best to have the Attrition Cells before a final product is created. As the clusters break apart or the grain fully fractures, the size gradation of the material will shift to a finer distribution. If the final product has a tight specification for each size gradation, running it through an Attrition Cell could knock it out of spec. Performing the final sizing process after the attrition scrubbing will allow the product to meet the required specifications.
How Attrition Cells improve sand turbidity
While the durability has a significant importance, the turbidity of a sand product is what the Attrition Cell Scrubber is most known for improving. A turbidity test is the measure of suspended particles, such as clay or silt, in distilled water. It is the measure of the light that comes through a sand and water suspension.
The turbidity specification is essentially an indication of how dirty the sand is. It is possible that an additional washing could reduce turbidity, but when the coating is persistent or heavy, attrition scrubbing can remove it. The particle-on-particle interaction creates a scrubbing action on the outside of the sand particle.
The clay and silt will most likely stay in the water once liberated from the sand. If the coating remains on the particle, it may simply flake off when dried, especially when exposed to elevated heat, such as in a dryer.
This will increase dust levels when handling the material and create issues with the final product. The presence of a coating on the material can lower the quality of any construction product where the sand is used.
When dealing with just a turbidity issue, the attrition process can focus on just the final product in an effort to minimize the cost associated with the scrubbing equipment.
Where to include Attrition Cells to improve durability and turbidity
Where an Attrition Cell fits in any process also has to do with what the material looks like before and what is done with the material after. Material preparation and rinsing are an equally important part of the attrition scrubbing process. Preparing the material before the Attrition Cell is important to ensure the highest performance within the unit.
Attrition Cells as part of a frac sand plant.
Considering silica sand with a specific gravity of 2.65, the ideal feed to a scrub cell is 70-75% by weight. By targeting the ideal percent solids, the highest chance of particle-on-particle interaction occurs while still allowing the material to flow through the cells.
A higher percent solids will put more of a load on the motor, which is indicated by an increase in amps. The load on the motor and gearbox is decreased by adding water at the feed end, but adding water increases the distance between particles. With increased distance, there is a decrease in the chance of particle-on-particle collisions.
Many applications will automatically monitor the amp draw on the motor and add water accordingly, which will protect the unit. Without monitoring the performance, it can be easy to let the slurry get too dilute.
Using a Separator™, a Dewatering Screen or some other method of increasing the percent solids above the 70-75% range before the Attrition Cell and diluting it down is a common way of optimizing the scrubber's performance.
With the clusters broken apart, internal fractures exploited or coatings removed, a final rinsing of the material after the Attrition Cell stage makes sure all the benefits of attrition scrubbing are realized. Any material that needs to be scrubbed should take into account the preparation, the effects of the scrubbing and the rinsing stage.
Durability issues should include Attrition Cells toward the front or middle of the process and definitely before final sizing.
Turbidity issues should include Attrition Cells where all waste, (other than the coatings), have been removed. In cases of multiple products, that may mean having the attrition scrub stage up front before the sizing stage. With a single product, the attrition scrubbing can be toward the end.
In all cases, a sufficient rinsing stage is required.
