Environmental News

Developments in sustainable sand manufacturing

Manufactured sand is the leading growth area in aggregate for  construction. Historically,  manufactured sand has been a by-product of the crushing and screening process. Today, due mainly to environmental constraints,  the focus is shifting to the deliberate  manufacture of sand to fill demand that natural sand deposits cannot continue  to supply.

In recent years, there has been research on the manufacturing of sand and we now have a better understanding of how sand characteristics affect concrete properties. Remarkable progress has also been achieved in the development of new equipment technologies and process control. These factors have contributed decisively to the creation of technology and processes that can generate high quality manufactured sand.

Manufactured sand can be used to control the cost and/or quality of aggregate production. It has been reported, both in research and commercially, that manufactured  sand offers performance advantages to  concrete, asphalt and mortar mixes. Specific characteristics of high quality product will improve the desired properties.   

Much of the hesitation in the use of manufactured sand for concrete mixes  has been process-related. Production of manufactured sand has been seen more  as a by-product rather than for an intended purpose. Value can be added to the whole operation by changing the focus of the crushing plant from producing fines to  producing high quality manufactured sand.  

PROCESS REQUIREMENTS  OF MANUFACTURED SAND
The correct process for the production of manufactured sand is key to producing an aggregate of sufficient quality for  concrete production. The process must match the properties of the source rock. If the fragmentation of the source rock is such that the sand produced meets gradation requirements then the process can be simplified. In the case of source rocks with higher intrinsic strength, the process needs to be refined to produce the required shape and gradation requirements.

To determine the process requirements, an understanding of the source properties is required. Usual source property testing and petrographic analysis will give insights to the possible fragmentation. From this information, the right crushing and  classification equipment can be selected.

Crusher selection for manufactured  sand production depends on source rock properties such as fragmentation pattern and hardness and the scenario to which the end product will be applied. Two crushers can produce high quality sand for concrete and asphalt, eg autogenous VSIs and new generation, high speed cone crushers. Metso offers leading technology in both crusher types through Barmac VSIs and HP cone crushers.

The selection of a crushing circuit should be such that the quality of the source rock is taken into account. It has been found that the energy input is proportional to the strength of the rock. Higher strength rock requires more energy input to create surface area. The consequence is the production of amounts of filler too high for normal concrete production, or which may be out of specification. For successful use, the filler must be classified. A typical process flow would engage a VSI, high frequency screen and an air classifier to produce high quality fine aggregate products.

Engineering customer success
Metso has developed systems and processes to assist customers to find a market for high quality manufactured sand. After bringing together Metso?s wide experience in sand production, the resulting solutions offered have been optimised from customer feedback and field success.

The Metso equipment offering for manufacturing sand has been rationalised based on what works. Efficient crushing and screening systems are utilised to reduce production costs and to offer consistent product quality. This allows customers to drive down total production costs while maintaining high product yields.

THE REAL COST OF CLASSIFICATION
Wet processing is the most widely used technology today for controlling the fines in manufactured sand. However, many quarry operators face a challenge in rising processed water costs because of shortages or environmental concerns. Handling of the processed water by settling in ponds or using mechanical dewatering is space consuming and expensive. Many product specifications also require subsequent product drying to meet market expectations and performance requirements.

For most dryers, the average energy requirement for a one per cent reduction in moisture is seven litres of diesel. Acknowledging the effect this process has on the environment, and the bottom line, dry  classification techniques come to the fore.

The use of dry classification of fine sand and filler products is at the forefront of manufactured sand innovation. The ability to produce a range of value-added sand products in a variety of specialist applications is important in the aggregate industry today. Metso?s manufactured sand solutions generally engage the use of Nordberg air classification (AC) products.

Why is air classification recommended?
The key benefits of AC are:
? High yield of quality manufactured sand and minimal waste.
? Dry end product.
? Low operational cost.
? Low environmental impact, low  power consumption.
? Market opportunities for dry filler.
Metso has developed standard solutions for all process equipment required to perform the specific tasks its customers demand. These solutions are offered with portable as well as fixed plants to meet customer requirements more precisely. These solutions have been designed out of different needs for cut point, capacity and portability. They are all optimised in terms of process equipment, feed arrangement, air flow and pressure.

The use of AC to remove filler from a manufactured sand has allowed control  to be given to the sand manufacture  process. Sand produced through a crushing and screening plant can have the filler  removed to be utilised as a filler in  concrete mixes.    
A holistic view of concrete production needs to be taken by the aggregate industry. Producing aggregate of a higher quality will reduce the overall production cost. The improvement in quality not only affects production costs but failure in the field.

Jamie Lunam represents Metso New Zealand.

Leave a Reply