Using hydroclones to maximize production

As aggregate producers are faced with the challenge of processing increasingly difficult materials, the challenge for manufacturers is to ensure the technology available continues to evolve to meet these changing requirements. Specifications for crushed stone, sand and gravel are becoming increasingly stringent, which requires the introduction of more advanced technology.

The process commonly referred to as the “washing” of aggregate is more accurately described as hydraulic classification. When considering the sand fraction, introducing this wet process is required for a number of reasons:

■ Removal of clays and silts to ensure your final sand products meet the required product specifications – the most common application being sands for use in the manufacture of concrete.
■ Dewatering of the sand slurry to remove excess water and allow for the final product output to be sold or used in downstream processes as quickly as possible.

The use of wet processing technology for sand production is an established process across the world as it allows you to increase the efficiency of concrete production by giving you a sand product with greatly reduced volumes of material in the minus 200 mesh range.

In the United States, the most common equipment applied in sand washing operations are classifying tanks and sand screws.

Classifying tanks

Classifying tanks accept the material from your aggregate screening phase and operate using gravity as the means of separation. The material enters a large classification tank, which is also fed with a non-stop supply of fresh water to ensure a continuous overflow.

The principle of the classifying tank is that the heavier sand particles fall to the bottom of the tank while the very fine particles overflow and are removed. The sand product is then discharged from the classification tank to a sand screw where the product is dewatered and sent to stockpile.

Classifying tanks are used when you have an excess of material in the minus 200-mesh range. Simply using a sand screw on its own will not be enough to remove the excess fines from your material. This is becoming increasingly common as we are faced with material deposits with higher fines content.

The use of classification tanks is also common in manufactured sand operations: This involves the production of a dewatered fine material from waste derived from crushing operations. This material tends to have an excess of material in the minus 200-mesh range and so requires that a classification tank is introduced in addition to the sand screw.

For those currently using classification tanks, there are a number of limitations to the process that will be familiar. These systems require large volumes of water in order to ensure the continuous overflow required to remove the excess minus 200-mesh material.

Classification tanks also require a significant amount of space on site and, as a result of their size, can take a long time to install and commission. The use of gravity as the means of separation is also not ideal as it is not an accurate process – meaning you will sacrifice some of the plus 200-mesh material in order ensure that excess fines are removed.

This adds significant cost to your sand washing operations as the waste material is often re-processed to try and recover the quality sand lost during the initial processing phase. This not only limits your productivity but adds significant cost to your operation as a result of the unnecessary double handling of material.

Sand screws

Sand screws will accept material either directly from your screening operation or will be fed from your classification tank in applications where there is excess fine material in the minus 200-mesh range that needs to be removed.

The sand screw is equipped with an overflow weir at the feed point, which provides the mechanism for fines removal. The limited capacity at the feed point means that control over the volumes of water required for accurate material classification is very limited. The result of this is that quality sand is lost over the overflow weir and is sent to settling ponds or water treatment phase.

There are a number of negative impacts on your operation as a result of this:

■ By sending good quality sand to your settling ponds or water treatment phase you are missing out on revenue by sacrificing good material in order to maximize removal of minus 200-mesh fines.

■ This process typically requires the reprocessing of your waste material – introducing significant operational costs through the unnecessary double handling of material.

■ The time required for clearing out settling ponds to recover lost material may result in plant downtime – harming the overall productivity, efficiency and profitability of processing operations.

■ By sending excess material to settling ponds you require considerably more space to accommodate them – and the classification efficiencies decrease as the proportion of fines in your feed material increases.

The second issue that will be familiar to those operating sand screws is the high moisture content of the final sand product. Sand screws employ the Archimedes screw principle to achieve dewatering of your sand product. The operation of the screw pushes the sand fraction up the elevated chamber while the water and the finer particles fall to the overflow weir.

The sand product is typically discharged with between 23 and 30 percent moisture content, and when processing finer material this number can be significantly higher. The result of this is that the sand product needs to be stockpiled for a considerable amount of time before it can be sold.

This requires unnecessary double handling of this material as it needs to be moved from the stockpile area below the sand screw to a separate stockpile area. Over the course of time, the excess water runs through the material stockpile and the sand can then be sold or used in downstream processes.

The result of having to stockpile material before it can be sold or used in downstream processes is that you are not able to turn final products into revenue as quickly as you would like.

Also, as a result of not using sufficient volumes of water, fines are not efficiently removed. This means the final sand product is not optimized for concrete production – excess fines in your sand means increased cement volumes during the concrete production process, which adds significant cost to the operation.

Quantifying the loss

Taking all of this information into consideration, is it possible to quantify the loss of revenue from sand washing operations employing classifying tanks and sand screws?

Let’s look at a 200-tons-per-hour sand washing plant, operating for 2,000 hours per year for 400,000 tons annual production. One can conservatively estimate the fines loss from sand screws to be at least 2.5 percent of production. That equates to 5 tons per hour or 10,000 tons per year.

Assuming a price per ton of $10, this means the typical sand screw operation is losing $100,000 of quality sand to settling ponds. Of course, operators tend to re-process this material to recover the material they know to be lost. But what also must be factored in here are the costs of this re-processing.

If you are losing an average of two days production per quarter as a result of pond cleaning and recovery operations, this equates to a loss of 12,800 tons of sand production every year, or $128,000.

Solving the problem
 of fines loss

Having looked at the issues with traditional classification tanks and sand screws, it’s now time to take a look at a solution to these problems, starting with the loss of quality fines to settling ponds. This problem arises as a result of the use of gravity as the means of separation of excess fines and quality sand.

One solution to this is to use a hydrocyclone-based system. Through the specification of the correct hydrocyclone configuration, producers are able to ensure that all the excess minus 200-mesh material is removed while retaining the quality sand in the final product.

So how does a hydrocyclone work?

Material enters the top of the cyclone and is subjected to centrifugal force. The cyclone setup should only be specified after extensive material testing to understand the proportion of minus 200-mesh material that you want to remove and the desired capacity.

The force within the cyclone separates the minus 200-mesh material from the quality sand product and gives you the capability to control your cut point. The finer material exits the overflow at the top of the cyclone with the wastewater, and this is sent to settling ponds. The sand fraction exits the cyclone at the bottom, ready for dewatering and stockpiling.

■ You have now eliminated the loss of quality fines to settling ponds – not only maximizing product yield but also minimizing operational costs as a result of not having to clean out and re-process material from ponds regularly.

■ The space required to accommodate settling ponds can also be reduced as a result of the fact that it is now only the waste fines being sent to the ponds rather than quality sand.

■ You are able to set up the cyclone configuration to guarantee production of sand specifications in line with the requirements of customers or downstream processes – this helps to maximize the commercial value of your sand product and the efficiency of your downstream processes.

■ Another important impact is the reduction of health and safety risks on site thanks to the removal of the requirement to regularly clean out ponds for reprocessing.

Reducing the moisture

The other main issue experienced as a result of the use of sand screws is the high moisture content of the sand product – typically between 23 and 30 percent when it hits the stockpile.

This is a result of the inability of the sand screw to effectively dewater the product and results in unnecessary double handling of the material and having to store the material for a number of days before it can be sold or used in downstream processes.

The solution to this problem is to employ a high-frequency dewatering screen to dewater the sand product.

Once sand slurry with the minus 200-mesh material removed is discharged from the hydrocyclones, it should be delivered to a dewatering screen that is sized according to your specific capacity requirements. This allows for the production of a sand product with moisture content typically in the 12 to 15 percent range.

The net result of this is that the product is ready for market straight from the belt, which has a number of benefits for your sand washing operations:
■ You are able to sell your product immediately, turning your sand into revenue in the shortest possible time.
■ You are able to use the sand product immediately in downstream processes, such as concrete production.
■ You can eliminate the area required to stockpile the sand product for further dewatering.
■ You can eliminate the unnecessary double handling of material. The reduced vehicle movements may reduce operational costs, as well as health and safety risks on site.

---

Lane Southard is Regional Technical Engineer for CDE Global Inc. and is based at the North America office in Cary, North Carolina.


---

Coming soon: Making the case for
traditional wet-processing methods.

Photo courtesy of CDE Global