Metso has years of know-how in different solutions for sand crushing and classification

Niraj Srivastava Vice President, Technical Support, South East Asia, Japan and Korean, Metso
A factory-made product used in urban areas, the importance of manufactured sand is beyond doubt. From users? perspective, it is necessary for them to know the properties of the material and where it differs from natural sand. Through a conversation with Niraj Srivastava, Vice President, Technical Support, South East Asia, Japan and Korean of a multinational company Metso, a prominent suppliers of vertical shaft impactors, which produces artificial sand, ICR brings readers this information.

Why go for M sand when you can get natural sand?
The noticeable effects on environment and the eco-system caused by sand mining, together with the growing demand of aggregates to fulfill the construction requirements of the urban world has made it imperative to look for alternate solutions to the concern. That?s the reason; operators now wish to produce high-quality manufactured sand.

Manufactured sand is defined as a purpose-made crushed fine aggregate produced from a suitable source material. Production generally involves crushing, screening and separation into discrete fractions, recombining and blending may be necessary. Manufactured sand has a more predictable product consistency than natural sand variation. There are two main drivers.

Firstly, there is a lack of natural sand reserves. Urban expansion, local legislation and environmental constraints have made the extraction of natural sand and gravel an expensive activity. Application processes for greenfield sand and gravel extraction are often long, expensive affairs with a high chance of site applications being rejected. The increasing difficulty in extraction has had a negative effect on the bottom line for many producers. In many regions of the world, the extraction of sand and gravel is heavily taxed or banned completely, to try to preserve remaining deposits. The industry must find alternatives to meet the growing demand for fine aggregates.

The second driver is purely economic. A growing number of quarry operators have found that 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 less than 20 mm, especially for 5/20 mm, mean that significant amounts of 0/5 mm are produced as a by-product.

By utilising the correct process equipment, low value crusher dust can be processed into a high value, premium product. Concrete, mortar and asphalt sand production possibly depend on the material characteristics and regional regulations.

Manufactured sand shape, smooth surface texture, and grading have a significant effect on the performance of fresh concrete. Aggregate when blends with well-shaped, rounded, and smooth sand particles require less paste for a given slump than blends with flat, elongated, angular and rough particles, reduces inter-particle friction in the fine aggregate component of the concrete mix grading. Optimised manufactured sand with uniform grading results in aggregate blends with high packing and with low water demand. As a result, they are less expensive and will have less durability problems caused by the paste such as heat generation, porosity, and drying shrinkage.

The benefits of this process are two-fold: the producer secures new product offerings and reduces low value products; and the customer has access to a secure supply of sand with a stable grading curve.

What is the present status? What are the various options available?
Rising concerns about the alarmingly negative environmental impact, has seen strict measures making their way in the country. After banning mining of river sand and other minor minerals without the mandatory environment clearance, the National Green Tribunal (NGT) has also banned beach sand mining or removal of sand from river beds across the country without an environmental clearance. Yet, news about illegal sand mining keeps making its way to the national and regional dailies every now and then. The figure reported to have accounted for loss of minerals through such practices have run into multi-million tonnes.

In fact, the 2011 sand policy encourages establishment of manufactured sand units by giving top priority while allotting quarries. So, manufactured sand is the option. As natural sand is not replenished at the rate it is being used, the government has begun to promote manufactured sand. Experts vouch that manufactured sand is not only a viable alternative to natural sand, but is superior in many ways. Tests have revealed that characteristics of mortars and concrete using manufactured sand as fine aggregate are superior when compared to the natural sand as fine aggregate. Demand for manufactured fine aggregates for making concrete is increasing day-by-day as river sand cannot meet the rising demand. Because of its limited supply, the cost of natural river sand has sky rocketed and its consistent supply cannot be guaranteed. Under these circumstances, use of manufactured sand becomes inevitable.

What products do you are offering for making sand, capacities, product specifications, etc.?
Metso has years of know-how in different solutions for sand crushing and classification, and a good portfolio of related equipment to produce a high quality manufactured sand.

Sand crushing solutions from Metso: Shape is a major issue. This is where the crushing equipment?s role is critical to ensure an acceptable shape and size. Cone crushers, like the Metso Nordberg? HP Series???and GP Series??? enable the production of finer products together with coarser aggregates. They also lower the initial capital investment and save energy, due to the fewer crushing stages. They gives the producers high energy efficiency and capacity, good product gradation, a high reduction ratio and low sensitivity to rock hardness. Cavity design for cone crushers should be optimised to ensure that crushing occurs along the full length of the crushing zone between the liners. This causes inter-particle crushing to occur, 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 <20 mm product including well graded manufactured sand.

Studies show that the best end product for various purposes can be achieved with high-velocity impact crushing. The Metso Barmac? VSI vertical shaft autogenous impactor improves the soundness and shape of the material and produces a quality that is very close to natural sand. 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 is proven to have sound performance in concrete and mortar products. Such sand is well known 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. Sand classification solutions from Metso: Screens and classifiers are then used to remove the needed amount of fine particles and dust. Metso?s Triple Slope Screen (TS Screen) is used for final screening for manufactured sand as well as for plaster sand to take 2.36 mm down product. Metso Air Classifiers, which are environmentally friendly and economical to use, produce consistent sand quality.

Dual bearing TS screen ensures capacity and efficiency due to multi-slope profile with elliptical vibrating motion, which has 20-40 per cent more capacity compared to conventional inclined screen. Elliptical motion toward at feed end and backward at discharge end – particle retention time on screening surface more for efficient screening. Superior for aggregate and fines screening. Screening area varies from 7.6 to 25 sq m in double and triple deck option.

Metso?s air classification technology offers dry classification solutions to produce manufactured sand. The dry separation process separates fine and coarse particles. This allows a reduced percentage of super fines in manufactured sand. Classified manufactured sand for concrete has particles of equi-dimensional shape, an ?even? type particle size distribution. Thereby meeting specifications and achieving quality products.

Metso offers two classifier options to aggregate producers. The gravitational inertial classifier is typically fed 0/4 mm or less and makes separations between 300 microns (50#) and 63 microns (230#). These units are ideal for use in the manufacturing of concrete and asphalt sands that typically require a reduction in the amount of 63 microns (230#) present. It is designed with an internal re-circulating function that allows the efficiency of the separation to be adjusted depending on the desired grading curve. When finer separations are required, the centrifugal classifier can make separations of between 100 microns (140#) and 20 microns (625#). Making it suitable for production of very fine products such as lime, fly ash and fertilisers.

How is the process carried out once you identify the rock?
Geologist conduct geological survey and estimate the aggregate reserves and their quality; mining engineers mine that aggregate and bring it to the surface; while process engineers beneficiate mined aggregate by utilising controlled and optimised size reduction process with the help of crushing, screening and classification equipment in some cases to produce manufactured sand.

Reliability, performance and capacity wise, Metso crushing, screening and classifying equipment are already proven, as per IS383-1970 (Reaffirmed 2002). The quality of manufactured sand is classified in four categories – Zone 1, Zone 2 Zone 3 and Zone 4 depending upon purpose of use. Manufactured sand must follow these Zones. (Table 1) Higher the Zone numbers finer the gradation. In crushing and screening plant at second last stage, impact crusher, mainly a shaping machine produces cubical sand particle with smooth surface texture. To meet the above Zone, thus manufactured sand need to be classified with help of classifier.

List some of the users in India.
Mostly all three stage plants are supplied by Metso with Jaw + Cone + Barmac, produce Barmac sand along with other aggregate fractions. There are almost 18 air classifier AC 30 units installed/operational India by below named customers for producing classified sand.

Dilip Buildcon
Larsen & Toubro
Thriveni Earth Movers Ltd (TEMPL)
SEW Construction
VLS Rock Sand P Ltd
Ramky Infra
SGP Shrii Ent.
Beach Minerals Co
United Infra
Thomsun Granites

Table 1: Comparison of River Sand and Manufactured Sand