When there is a lack of natural sand deposits the only solution are the sand made by crushing bedrock. But when we work with this kind of sands we have to take into account that crushed sands do not have the same behavior and before put this product on the market it is necessary to correct the form and the size to achieve a stable quality. Actually it is more difficult to control the variability of natural sand, especially, if the sand is not washed.
The production of crushed sand is usually part of an entire process which produces other products. This production needs several size reduction stages, and sand is the product of one stage.
Depending of the type of crusher used in the aggregates plant, the crushed sand will content more or less fines under 63 microns. It is possible to extract this fraction (<63 microns) using a cycloning process. The grains lower than 63μ are residues coming from the crushing process. This fraction can be included in sands for concrete products, if proportions are respected.
If the quantity of fines lower than 63μ is high this could cause:
- Reduction of adherence
- Increase of volume of binder and consumption of water
- Increase of consumption of cement
How to process crusher sand? Working with or without water?
It is possible to process the crushed sand without water, but working with water will facilitate to respect the customer’s exigencies related with cleanliness and size distribution. The remain waters coming from crushed sand washing must also be treated and it could be recycled and be reused in the plant.
Main Techniques for treating sands
Here the main techniques used in quarries for treating the sand:
- Wheel washer
This machine has used a lot to recover sand contained in remain water from crushed sand washing. This technique has been replaced by cycloning techniques which are more accurate. The wheel washer has several disadvantages:
- The particle size controlled is very high, around 150 microns. A lot of particles mesuring between 63 to 150 microns are lost during the process.
- The dewatering is bad quality
- It is not possible to send the remain waters directly to sludge treatment because of the presence of fine sand in the water
- Cycloning (Liquid – solid separation)
This is the best method for getting an accurate size product. This technique can be used for different applications depending on size of product required:
- Classic applications: sand classification at 63 microns
- Fine sands: sand classification between 15 and 30 microns
- Recovering fine sands after a wheel washer or a washing screw (classification at 63 microns)
- For industrial sands: between 100and 150microns
Several companies have manufactured the cycloning principles designing modules including cycloning and dewatering. These modules are known as Cycloning group or sand treatment module.
Using horizontal dewatering machines it is possible to get a classification at 2mm or more. In some cases several dewatering machines are installed in parallel, as you can see on the picture below.
The polyurethane modules used for the dewatering will determine the classification.