Sand Processing

Quality rock at the right price

While the real price of aggregates is falling, the quality requirements for quarried materials are becoming even stricter. This is a trend that has been clear in the industry for many years and looks set to continue.

The causes of this situation arise from both the supply side and demand side of the industry. On the supply side, the price reductions have mainly been the result of improved communication processes (the ability to crush or grind rock), improved equipment availability and general cost-cutting. New concepts such as mobile crushing have also made a contribution to more efficient crushing. These factors have collectively driven down the price per tonne of aggregate – and yet allowed profit margins to remain largely static.

On the demand side, customers are not being unreasonable when asking that aggregate meet exact specifications. There are clear economic factors which confirm the need for improved aggregate quality, mainly relating to the shape of end products (flakiness and elongation).

Surprisingly, the improved shape of aggregate has the potential to significantly reduce the total cost of concrete and asphalt. Despite only constituting a small percentage of asphalt’s total make-up (less than 10 per cent), the high cost of bitumen determines the cost of the entire surface layer. Therefore, if the use of higher quality aggregates can extend the life of a road – or lead to less bitumen being needed – the added investment will more than pay for itself.

Quality aggregate
It is important to understand what is meant by a quality aggregate. A quality aggregate should have an angular shape, but not be flaky or elongated. It must have a constant gradation (a series of gradual and progressive degrees, steps or stages) – and a high voids ratio, so when packed together in bulk material there are not too many holes that need filling with expensive bitumen.

A good quality aggregate, in terms of shape and size, has the ability to increase the wear resistance of the top layer. Not only does this save on maintenance costs, the aggregates bond more securely to the bitumen and this internal friction also increases the load resistance of the layer. Even at the construction stage, high quality aggregates have better workability and are easier to pave.

These requirements are not just important for asphalt – concrete’s needs are very similar. A good quality aggregate will have the impact of reducing the amount of cement needed and the finished concrete will be stronger. Again, the cement will be easier to work with and lay.

A good quality aggregate for concrete should have a rounded or square shape with smooth surfaces – not flaky or elongated. It too should have an even gradation, have only a small amount of sand on its surface, and again have low voids when packed in bulk. And because concrete involves the chemical reaction of cement with water, aggregates should not interfere with this process. Therefore a controlled microfines content is necessary, as clay and other organic microfines can be harmful in concrete.

Unlike asphalt, there is a range of other requirements for aggregates where differing strengths or classes of concrete are required. In practice, however, this is not always consistently controlled and concrete suppliers are known to produce their own ‘recipes’ when it comes to aggregate requirements. These tighter requirements are based on these firms’ own development work.

Planning and efficiency
Understanding the need to produce better quality aggregate is one thing, but what does that mean in practical terms for quarry owners? As an example, in a typical three-stage plant where 0-16mm fractions are needed – where there are no specific quality requirements – production can range between 210 to 321 tonnes per hour, depending on the running parameters of crushers and screens in the process. Minimum capacity is achieved using large settings on the crushers, creating high circulation at the tertiary stage. Maximum capacity is achieved by tightening the settings in the crushers for lower circulation. Screening is optimised and the setting corresponds to the maximum yield for 0-16mm end product.

The financial impact between these two extremes is dramatic, for with an investment of just 9 per cent, the corresponding improvement in sales revenue is around 53 per cent, but is only possible through process optimisation.

The key to effective process optimisation is twofold. Firstly, operators should run equipment constantly with the optimum parameters. Modern automation can make a major contribution here. Secondly, plant availability should be maintained at a high level. This requires the removal of all kinds of process-based disturbances, such as oversize boulders entering the crushers. Proper process planning and management as well as preventive maintenance are the key elements here.

Looking at the two process configurations and reviewing the proportion of the final product (0-16mm) that is produced at different stages of the process shows the benefits. Most of the final product (4-16mm) is produced in stages other than the tertiary. In fact, 20 per cent of the final product originates either in feed or primary crushing, which explains the poor flakiness of the final product, since the flaky primary product spoils the higher quality aggregate emerging from the tertiary stage.

This configuration is not the only option. The process could be conducted differently to produce 100 tonnes per hour less of 0-16mm than in the previous example – but its quality is considerably better. The reason for this lies in the origin of the 4-16mm product being 94 per cent tertiary cone, while 4-16mm fraction from the feed and the primary crusher is not allowed to enter the final product and spoil it. On the contrary, this product is recrushed to achieve a better shape, which, since this uses up capacity, leads to the reduction in hourly tonnage.

These examples clearly indicate two main findings. Firstly, that the system configuration must be correct in order to produce the right aggregate amounts and fulfill the specifications. If this is not done correctly in the first instance, later modifications – although possible – will be expensive.

Secondly, running the process using optimal parameters in the equipment can make a real difference in terms of financial success. Achieving this requires the
continuous monitoring and adjustment of the equipment and process. Additionally, high availability and utilisation rates are crucial, as changes in utilisation have a strong impact on profit.

Jarmo Eloranta is Metso Minerals’ vice-president of research, construction.

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