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Crushing, Drill & Blast, Tips & Advice














Three reasons for oversized rocks in the primary crusher feed

Oversized aggregate in the primary feed crusher can be incredibly disruptive at the start of any crushing circuit. Adrian Imre discusses the three critical factors that can prevent blockage in the primary feed.

In today’s extractive industry, there is a clear trend of increasing production capacity to reduce costs through economies of scale.

This comes with an ever-changing work environment, where sudden issues can arise and dramatically impact downstream processes. More demanding equipment needs and knowledge gaps caused by workforce turnover add to the challenges.

The primary crusher is a vital part of the extractive process, taking the run of quarry aggregate and reducing it to a size that can be processed by the downstream operations.

However, extractive operations frequently have issues related to the primary crusher: low throughput, low utilisation and high wear rate of liners. Changes in fragmentation and breakage characteristics of the feed are sometimes difficult to observe and, consequently, difficult to control. The presence of oversized boulders in the feed is readily apparent since the throughput immediately decreases and typically causes blockages in the crusher cavity. These blockages can damage the crusher and take time to clear, resulting in a serious bottleneck for the entire process.

To prevent this type of bottleneck, one key element that needs to be analysed is the quarry’s drill and blast program. There are three critical aspects that need to be examined to ensure your feed does not contain oversized rocks.

Changes in aggregate type, mass

Problem: The mass of material you are blasting (blasting polygon) appears to be the typical aggregate type, therefore the blast design was unchanged. After blasting, an unusual quantity of oversized aggregates is found in the muckpile.

"The primary crusher is a vital part of the extractive process"

Solution: Even though aggregate types can appear to be similar, only the details from a true drill and blast program can eliminate guess work. A trained geologist with local experience is a necessity and can predict the correct joint system parameters needed to optimise the program. In the design phase, input from the site geologist is a valuable resource to achieve good fragmentation results, as only an experienced geologist can distinguish aggregate types and alterations with the naked eye.

For the same aggregate type, changes in aggregate mass structure will induce changes in fragmentation of the blasted material. An area with big pre-formed blocks in the mass sometimes cannot be detected during drilling. Information related to mass structure can be used to adapt the drill and blast design to local conditions.

Maintaining a drill and blast database to record conditions for each blast, design and results will contribute to continuous improvement in blast outcomes when blasting in adjacent or similar areas. The best way to deal with variation in rock mass structure is to define aggregate domains based on blastability and have an adequate drill and blast design for each area.

Beyond the blasting polygon

Problem: The blast design is well adapted to current aggregate conditions; all rules of thumb were respected and the blasting pattern generates good fragmentation results in a quarry with similar aggregate characteristics. Despite this, the number of oversized boulders remains high, contributing to an increase in costs due to secondary blasting and low loading productivity.

Solution: A well designed blast can still produce oversized blocks, generally at the limits of the blasting polygon such as the back row against the final pit wall. In a typical production blast with no method of wall protection, damage caused by back break is produced beyond the polygon limits.

In general, the rule of thumb is that with the same conditions and blast design, the quantity of oversized boulders produced is proportional to the blasting polygon perimeter. Minimising the perimeter of the blasting polygon for the same amount of quarried material can be a very effective way of reducing the number and size of the oversized blocks generated by blasting. The solution is to increase the size of the blasting polygon as much as possible and to design polygons with regular (as close as possible to rectangular) shapes.

For a given aggregate reserve in one bench of the pit, increasing the blast size reduces the number of blasts, therefore decreasing the proportion of large rocks generated around the perimeter and improving overall fragmentation. It also saves resources such as time and preparation, making operations more efficient.

Lack of experienced operators

A primary crusher feed – such as that on a Metso Nordberg GP330 – is vital to the smooth running of the entire crushing circuit.
A primary crusher feed – such as that on a Metso Nordberg GP330 – is vital to the smooth running of the entire crushing circuit.

Problem: The occasional oversized block in the feed may not be an issue for the primary crusher, nor cause interruption or affect normal activity for a long period of time. However, over days/shifts, the number of boulders arriving to the crusher can dramatically increase without any notable operational changes in the quarry. This can lead to power spikes in the crusher, increase the wear and maintenance requirements, and coarsen the feed to downstream operations, which could affect circuit throughput.

Solution: In extractive operations, employee turnover is an important factor, as training quality for novice employees has a very high impact on day to day activities.

The shovel/excavator operators are trained to operate the equipment in safe conditions and with high productivity. Many inexperienced operators may think that since the equipment has a large bucket and can manage big aggregate boulders, it is OK to load the large boulders into haul trucks. This is a poor strategy, with high potential to produce adverse effects downstream, including damage to trucks or the primary crusher and reduction in hauling and crushing productivity.

The operators must be clear from the initial stage of their training that the use of larger equipment is for higher productivity, not to manage massive muckpile blocks. The best strategy to manage large boulders when they are detected in the pit is secondary blasting. Secondary blasting is the most efficient, affordable way to resize the boulders.

There are multiple ways to reduce the impact of a blast producing oversize boulders. Using a trained geologist to construct a proper drill and blast program, increasing the size of the blasting polygon and ensuring shovel/excavator operators are properly trained can help reduce oversized feed and unwanted lost production.

Metso’s Process Optimisation (PRO) group, with extensive drill and blast expertise, can solve issues with oversized feed appearing in the primary crusher. The PRO group is a global team of extractive industry professionals who provide expert consulting, laboratory services, hardware and software products to the extractive industry worldwide.












ABOUT THE AUTHOR
Adrian Imre

Adrian Imre is the principal drill and blast consultant for Metso.









Thursday, 21 June, 2018 04:44am
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