The interest in solutions for manufactured sand is increasing worldwide due to the lack of availability of natural sand and to restrictions being placed on its extraction. This is especially true in Sweden, where in 2004 the Swedish Government introduced a tax on sand from natural resources to safeguard the remaining deposits for future natural purification of fresh water aquifers. Although the tax was introduced some years ago, it has not yet had a full impact because deposits already in use in 2004 are still being excavated. However, new extraction permits are difficult to obtain, especially in the big urban areas. This has given the aggregate producing companies some time to adapt their production towards manufactured sand and many research programmes within the industry to phase out the use of natural sand have begun.
Sandvik conducted a survey of its main customers in Sweden and found that the range of specifications of the sand desired was so wide that the process solution would need to be something extra. This range occurs because while almost all aggregate business takes place on a local or regional market, the variations and demands are very wide, which is reflected in the existing (EN) norms. The big challenge for Sandvik was to devise a process that was versatile enough to handle all the variations for concrete sand that exists and also to meet the criteria that the process plant should be mobile so that it could be taken on the road to customers? sites. Another important physical consideration was the make up of the raw material. Factors such as mineral composition, mineral grain size and structure and mica content gave different demands on gradation and the amount of allowed fines in the sand.
As this project was aimed at providing customers with a solution to meet a demand, the market situation of the quarry was a key consideration. Previous studies have shown that a relatively coarse feed without fines (generally a competent product) is very suitable for making manufactured sand. However, a competent product is one that has been invested in with regards to crushing and screening and could be sold. Therefore, any further processing, such as the manufacturing of sand, must make economic sense, in other words, increasing the margin by more than the extra costs involved. In today?s economic climate and costs of energy and labour, this further refinement of competent products is rarely profitable.
A more attractive feed material is that produced from the primary crushing stage as its value is lower. This is where Sandvik focused its efforts. Since this feed is less refined, a successful manufactured sand plant must be able to handle a variation of feed material when it comes to factors such as gradation and natural fines content (natural fines is the fine material – <20mm – generated in the blast).
A conceptual study was done and the resulting design was fitted on an existing wheel-mounted crushing and screening unit. The plant was commissioned by Sandvik in April 2009 and was launched at the Swedish fair, MaskinExpo, in May.
The material enters at a feed station with an adjustable chute which controls the flow of fines to the second chute or a stockpile. In the second chute, the material is either sent to the crusher or bypassed, with a range of control from a complete by-pass of the crusher to all fines entering the crusher. The crusher is from the Sandvik CV series of Vertical Shaft Impactors (VSI), which have strong reputation in sand applications. The by-passed or crushed product then enters a triple-decked screen which is equipped with a very sophisticated gate system for adjusting the output, ie:
? The product from the top deck is always re-circulated back to the crusher.
? The middle deck product can be split in parts of one-sixth (1/6) for recirculation or product stacking.
? The bottom deck product can also be split in parts of 1/6 for recirculation or product stacking.
? The material passing through the bottom deck is fed in to the air classifier.
In total, all these flow adjustment abilities give theoretically more than 1000 different set-ups.
The advantage of this versatile plant is that any fraction smaller than 50mm can be fed into the plant and between one and three fractions can be taken out. The distribution between the product fractions can also be altered with the adjustment gates in the screen chute. This means that the distribution of fractions is not so dependent on the crusher product distribution.
The fine fraction (normally 0-2mm – 0-8mm) is subject to fines removal in the air classifier. The classifier is of the well-known compact high-efficiency type. The classifier is equipped with dual fans with individual drives. The fan motors are also fitted with frequency converters. The advantage of this is that the classifier can be adjusted immediately from the operating panel without any mechanical adjustment of internal rotating parts. This was shown to be a major advantage during the demo tour where the input to the classifier was very diverse. The separation cut sizes can be adjusted from 50-100 microns. This is favourable when the product is not only subject to concrete aggregates but also asphalt.
Since starting out on a country-wide tour of Sweden in May, the plant has been operated for about 500 hours in total at six different customer sites (as of October 2009). Technically it has been a success; all the demands that have been given have been met. The manufactured sand has been used in construction, house building and also for shotcrete applications.
SAND TESTING PROCEDURE
Can every rock type or material be upgraded to concrete sand quality? No, there are some rocks that contain a high proportion of mica that have been shown to be rather difficult to upgrade. However, in the Sandvik test centre there is a test procedure to check the possibility of upgrading the rock to sand bound for concrete. These test procedures involve checking the flow properties of the sand and microscopy. Through these checks, it can be established if the rock material can be upgraded to the necessary quality for sand bound for concrete. There are several ?waste rock? materials in many quarries today that can, by the right crushing, screening and dedusting process, be upgraded to quality sand for use in concrete.
In Figure 1 you can see the flow tester and a screen shot of the microscope analysis for F-shape.
If the flow test and the microscopy show that the rock material can be upgraded to quality sand, a trial batch of 200 kg of sand is produced in the laboratory through Sandvik VSI crushing and screening, including dedusting (see Figure 2). This batch of sand is returned to the customer, so that they can cast concrete and test its compression strength. This means that the customer is fully convinced that everything will be satisfactory before Sandvik designs and quotes the full-scale sand processing plant. When a customer purchases a sand processing plant from Sandvik, they receive help and advice to streamline their existing plant as well as training for their personnel in the use of the sand processing plant.
HOUSE BUILDING
As a part of the road show, Sandvik upgraded 2000 tonnes of aggregates with one customer from quarry fines of size 0-20mm. These aggregates were bound for making concrete on an industrial scale for house building.
The concrete batches, with fully crushed aggregates, comprised 15 per cent ordinary Portland cement, 78 per cent aggregates and seven per cent water, all by weight.
The concrete batches were tested after 28 days and had a compressive strength of 42 and 41 MPa. These figures compare very well with concrete fabricated with natural aggregates 0-16mm and with almost equal cement and water portions which normally have a compressive strength of an average of 40 MPa. There did not seem to be any major difference in the properties of the concrete when using fully crushed aggregates. Moreover, the cement costs are almost equal.
Another beneficial factor of the fully crushed aggregates in comparison to the natural aggregates when making concrete was that there was less variation in the particle size distribution in the fully crushed aggregates. This is an advantage when making batches of concrete on an industrial scale because it will save time as there will be less need to vary the concrete recipe – usually only one recipe is needed.
In total 1000 m3 of concrete with fully crushed aggregates has been used in house building. Because concrete lasts for more than 10 to 20 years, we do not have any comparable results between concrete that uses natural aggregates or the fully crushed aggregates in the same geographical area and in the building of comparable houses.
ENVIRONMENTAL IMPACT OF MANUFACTURED SAND
The conclusion of the roadshow (which will continue until at least the end of 2009) thus far is that Sandvik?s sand concept can meet all its customers? expectations. The versatile design allows Sandvik?s customers to try out new ideas for effective and profitable production of manufactured sand.
The results of the trials on the roadshow have shown that by processing aggregates in the correct way, Sandvik can design crushing and screening circuits that, in a couple of minutes, produce fully crushed aggregates equal to, or better than, what nature has produced in thousands of years.
This has a major beneficial environmental impact. The demand for natural sand extraction can be greatly reduced, leaving natural sand to play its vital role of filtering drinking water in natural aquifers.
This is being reflected politically in Sweden, where many communities and local authorities are stating in purchasing tenders that because of environmental issues they prefer manufactured sand to natural sand – even if there is a slight increase in cost.
Per Hedvall is the general manager (process technology) and global market support manager (rock excavating and processing expertise) and Bengt Olof Tjell is the test centre manager for rock excavating and processing expertise group in Sandvik Mining and Construction (Europe).