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Crushing rocks right

Optimisation of crushing rocks and processes in quarries is a surefire way of boosting production with benefits that extend from extraction of materials to the stockpile and sale of graded products.

Depending on the type of material available at the quarry, the process of sizing and shaping stones and aggregates is usually undertaken with the use of a series of crushers to break rocks into manageable sizes until they conform to the required specification.

Bottlenecks at any of the steps along the process can have a dramatic effect on overall production and will in the long run have a negative effect on the profitability of the quarry.

Alan Fletcher
Alan Fletcher, Metso Minerals general manager for construction in southern Africa

In order to address crushing and screening efficiency, the Aggregate and Sand Producers Association of Southern Africa (Aspasa) invited crusher specialist, Alan Fletcher, Metso Minerals general manager for construction in southern Africa, to address a special workshop for its members on crusher optimisation.

Getting it right

According to Fletcher the single most important aspect of rock processing is safety and training of all staff in the correct operation and procedures when operating heavy processing equipment. Thereafter it is important to understand the types of crushing processes that will be required to manufacture the products needed and procure the right equipment to deliver the required tonnages.

“Once we have established the type of product and tonnages required we can build the processing plant around those principals and work out what type and how many crushers will be required. The typical plant will have primary crushers at the first stage (usually a jaw crusher) then materials will go to secondary crushing (typically cone crushers) and on to tertiary crushers if finer materials are required. Quartinery crushing is sometimes used to make materials finer still.

“As a rule it takes comparatively little energy to break big lumps and considerably more for smaller stones each time the particle is reduced further. That is precisely why plants need to be designed for efficiency to work out the correct pinch point to break the material quickly and efficiently. It is also important to keep running the plant efficiently thereafter,” says Fletcher.

Ensuring efficiency

He added that regular checks and clearing needed to be done especially on feeders, screens and scalping screens. Even a 10% buildup of materials on a screen can have a big effect on efficiency as it will block 10% of the material, but in addition it will recirculate missed material and cause a buildup that will further compound the problem.

Screen size and stratification also determines the amount of material that is delivered to the crusher and it is therefore important that the screen is correctly specified to deliver the correct tonnages.

Furthermore, users must know that crushers need to be choke fed for optimum performance. It is therefore vital that feeders and screens deliver the right amount of materials to the crushers at all times. Moisture should also be kept to lower than 5%, fines should also be minimized and the use of water minimised to avoid wear on liners from the bottom. “If you screen right your crushing is right.”

It is always advisable to run a plant at 100% for less time, rather than running it on trickle feed for a long period of time as the low material levels may cause damage to the crusher – any crusher works better when full. Equally important is the selection of the right type of liner with the right type of profile to feed materials and ensure the correct movement within the process. Depending on the application there are course to fine profiles that can be specified to meet crushing requirements.

Upkeep is key

“Maintenance staff should monitor the wear pattern regularly to ensure it is even from top to bottom and also ensure that the liners are replaced at 25-40% of their thickness to prevent damage to the machine.

“They should also look at the wear pattern to ensure crushing is taking part across the entire section which determines the overall utilization you are getting out of crusher. Reputable companies will be able to look at liner and tell you the effectiveness of crushing taking place and be able to develop liners to address inefficiencies,” concluded Fletcher.

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