The use of manufactured sand has two main drivers.
First, there is a lack of natural sand reserves. Urban expansion, local legislation and environmental constraints make natural sand and gravel extraction expensive. Application processes for greenfield sand and gravel extraction are long, expensive affairs with a high chance of rejection.
The increasing difficulty in extraction can negatively impact on a producer?s bottom line. Concrete plants require a quality sand to optimise production and minimise cement usage. In many regions of the world, sand and gravel extraction is heavily taxed or banned to preserve remaining deposits. The industry must find alternatives to meet the growing demand for fine aggregates.
Urban expansion and depleted resources have increased the distance between point of extraction and point of use. Road congestion increases travel time from pit to batching plant for many operators and, coupled with a declining number of pits, increases end customer prices.
The second driver is economic. Many quarry operators have found that a processed high quality manufactured sand can improve their bottom line and significantly reduce the percentage of waste and low value by-products. Customer requirements for production below 20mm, especially for 7-10mm, mean that significant amounts of 0-7mm are produced as a by-product.
With the right process equipment, low value crusher dust can be processed into a high value product. Concrete, mortar and asphalt sand production depend on material characteristics and regional regulations. The benefits of this process are two-fold: the producer secures new product offerings and reduces low value product and the customer has access to a secure supply of sand with a stable grading curve.
END PRODUCT PERFORMANCE
Production of high quality manufactured sand requires specialist knowledge in fine crushing and separation technology to achieve end product performance.
Process is key to a marketable product. The production of quality manufactured sand, along with its process and process control, is integral to customer success.
The two factors of most interest to the end users of manufactured sand are compliance with local regulations and consistency of the product.
Compliance to manufactured sand gradation is required at the bottom of the curve and throughout the product envelope.
Product consistency allows a more predictable mix to be produced than is achievable with natural sand due to normal natural variations in dug sand gradation. This means that end users with manufactured sand do not have to allow a safety factor to mitigate for natural gradation variations and can achieve binder savings.
SAND SOLUTION EQUIPMENT
Operators have exacting requirements. Specifications vary across regions and successful implementation of manufactured sand differs between operators. Solutions should be modelled on best practice to meet market requirements.
Metso can provide crushing and separation solutions for greenfield sites or plant refinements to achieve well graded, quality manufactured sand.
Cone crushing technology gives producers high energy efficiency and capacity, good product gradation, high reduction ratio and low sensitivity to rock hardness.
Cavity design for cone crushers should be optimised so that crushing occurs along the full length of the crushing zone between the liners. This causes inter-particle crushing, leading to the highest possible generation of the required fines and cubically shaped products. Advances in high speed cones with high clamping forces offer the opportunity to produce well graded and shaped <20mm product including well graded manufactured sand.
Vertical shaft impact (VSI) crushing technology can process fine, unscalped, heterogeneous and irregular feeds. Unlike cones, VSI shaping is not influenced by the closed side setting (CSS) and shaping occurs throughout the product curve.
The sand produced from rock-on-rock crushing has a proven performance in concrete and mortar products and is renowned for its cubical shape and consistent gradation. The gradation and quality of the product from the VSI does not tail off with increased wear on parts. This leads to the highest possible quality of performance in concrete and mortar mixes leading to reduced production costs.
AIR CLASSIFICATION TECHNOLOGY
Globally, the trend is towards utilising dry classification solutions to produce manufactured sand. The dry separation process separates fine and coarse particles, allowing the percentage of superfines in manufactured sand to be reduced to meet specifications and achieve quality products.
For aggregate producers, Metso offers two classifier options. The Gravitational Inertial Classifier is typically fed 0-4mm or below and makes separations between 300 microns and 63 microns. This unit is ideal for the manufacturing of concrete and asphalt sands that require reduction in the amount of 63 microns present. It has an internal recirculating function that allows the efficiency of the separation to be adjusted dependent on the desired grading curve.
For finer separations, the Centrifugal Classifier can make separations between 100 microns and 20 microns. This makes the unit suitable to the production of very fine products such as lime, fly ash and fertilisers.
Metso?s air classifier range provides versatile, economic technology that can be combined with different crushing and screening equipment to produce a tailored plant. It allows for the production of dry products so there are no dewatering or drying costs. Process water is not required, which significantly reduces the quarries? water demand, settling pond requirements and makes arid regions viable. No moving parts and ceramic linings mean low maintenance and wear part costs. Highly efficient separations, even at very fine particle sizes, produces quality product and limits the amount of product in waste.
A NORWEGIAN SUCCESS STORY
In 2008, Norwegian quarry Velde Pukk, located on the outskirts of oil city Stavanger, boldly invested in a unique, complete manufactured sand process, using air classification technology. In co-operation with Metso, the quarry runs a successful sand plant, serving the region?s concrete and asphalt industries with high quality manufactured sand.
Superior particle shape, consistent gradation and flexibility to balance the sand mix with air classified fines, accurately and according to the customer?s needs, are the benefits of the manufactured sand.
Cement producers in Stavanger have made cement savings of 10 to 15 per cent with manufactured sand. In pretensioned concrete
element production, manufactured sand provides faster hardening to natural sand.
Velde Pukk pioneered the production of manufactured sand through its fractionalisation of fine sands with Metso?s air classification technology.
?Aggregates production is the heart of our operation but we prefer to be called an industrial park. By investing in manufactured sand, we changed our focus from being an aggregates supplier to a problem solver,? says managing director Egil Velde.
?With manufactured sand, it?s all about consistent moisture, gradation and shape. After testing, we can confidently say that we can accurately achieve the gradation that our customers require for their asphalt and concrete production.?
When Velde Pukk?s product manager Sven-Henrik Norman describes the quarry operations, key words like dry process, constant, predictable gradation and tailor-made sand recipes are repeated often.
?The starting point of our operation is a low moisture process with covered conveyors and crusher and screen buildings. This guarantees quality all year round. Our air classification system also utilises Metso?s dry process know-how,? Sven-Henrik explains.
During the building of the manufactured sand process, Velde Pukk created a complete knowledge centre, including a series of different sand mixture recipes.
?Based on our studies, the secret behind the success of manufactured sand lies in the shape. Besides the high cubicity, this sand has a rougher surface than natural sand, resulting in a better grip in concrete and asphalt,? Sven-Henrik explains.
?For the end user of concrete and asphalt, this means a more trouble-free operation, thanks to the improved consistency. Savings in cement consumption and faster hardening are big bonuses, too,? he adds.
THE FUTURE OF CEMENT AGGREGATES
Spenncon, a Stavanger-based producer of pre-tensioned concrete elements, switched from natural sand to Velde Pukk?s manufactured sand in 2009 and has used about 5000 tonnes per month ever since. The plant has found the change to be positive.
?Based on the experience gained in concrete element manufacturing, we can see that the future of aggregates for the concrete industry is in manufactured sand. It contains a constant, stable quality and gives us better control of our element production,? says Spenncon?s plant manager, Tore Aasen.
According to Aasen, the plant now requires 10 per cent less cement for concrete production.
?In addition, the pre-tensioned elements reach high strength 30 minutes earlier than when using natural sand. This has a positive impact on our productivity,? he adds.
David Morrow is the global business development manager for air classifiers and Eero H?m?l?inen is the marketing communications manager for Metso.