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Tips for designing an efficient crushing and screening operation

There are several considerations in designing an efficient crushing and screening plant. The first is the raw materials to be crushed. The quarry shot material should be analysed for maximum feed size, gradation, chemical composition, amount of clay, hardness and variations within the deposit or ledges.  A list of product sizes needs to be determined as well as the percentage of each product in the total production.

The quarry?s yearly production and operating hours need to be determined. An experienced crushing and screening expert with a computer flow simulation program to optimise the output and efficiency should check the entire system.

FEEDER AND HOPPER DESIGN

{{image2-a:r-w:200}}One of the first steps in the process is to select the equipment to load the primary hopper. The truck size and number of trucks or loaders needs to be determined to evaluate the hopper capacity. Typically, the minimum hopper live load capacity is about 1.5 times the size of the unit dumping in the hopper. Rock box hoppers are typically used in dump truck operations. Sloped side hoppers might be considered for sticky materials.

The types of primary feeders include:

?    Apron feeders. These should be rugged and need to be designed to handle the impact and lump size of the raw feed from the quarry. Many manufacturers have cast manganese pans attached to a crawler tractor chain with sealed rollers. Impact rails are also necessary to ensure the lump size will not destroy the feeder pans. The modern apron feeder with tractor chain is known to provide very long life and is trouble-free. The apron feeder can be speed controlled to match the primary crusher capacity. Many times apron feeders are followed by a heavy duty scalping screen to separate products or to remove harmful materials. The scalping screen allows material that is already sized for the final products to bypass the primary crusher.  
?    Wobbler feeders. These are typically used for separation of clays in the feed. These machines are self-cleaning and handle sticky clay with a common separation at 89mm or less. These feeders have a high investment cost but are found to be extremely effective with sticky materials. It is recommended the feed size be controlled if the wobbler is used as the primary feeder. Dumping large rocks directly on a wobbler can result in damage to the feeder. As with any scalping device, the wobbler feeder is most efficient when the material bed depth over the feeder is fairly consistent. It is common for an apron feeder to be used as the primary feeder transferring material to the wobbler feeder for separation. The wobbler feeder is typically operated at a fixed speed but may be turned on and off as needed.
?    Vibrating grizzly feeders. These are typically a small investment cost. These feeders perform a dual function of feeding/conveying and separating smaller material to bypass the primary crusher. The oversize materials are discharged off the feeder into the primary crusher. This machine has speed controls but cannot control the feed rate to the crusher as precisely as the apron feeder. Grizzly feeders are exceptional when processing free-flowing materials, due to their multifunction and price advantage over the apron feeder. The most prevalent G-force common to vibrating grizzly feeders is between four and five, which is sufficient to stratify the material bed and provide a feathering effect as the material passes over the grizzly section.

PRIMARY CRUSHERS
The types of primary crushers include:

?    Jaw crushers. These are used for extremely hard and abrasive materials. Reduction ratios are typically low at 6:1. However, jaw crushers are very durable, low maintenance and effective in nearly any type of friable material. Jaw crushers have a medium investment cost. Typically, 0.5hp (0.37kW) is required per tonne per hour of material produced. Jaw crushers are typically long-life machines. New technology has incorporated quicker adjustment time, tramp iron relief and automated product size setting.
?    Impact crushers. These are typically used for hard and medium abrasive stone. The primary impact crusher, also known as a primary breaker, has a reduction ratio of 20:1. Impact crushers are higher maintenance than jaws, although are still considered low-maintenance when accounting for the tonnage rate and overall productivity. Impact crushers are typically the lowest investment cost compared with other primary crushers with comparable feed size and capacity.
?    ?Andreas-style? impact crushers use 0.75hp (0.56kW) per tonne per hour of material produced. While the Andreas-style primary impact crusher has become very popular worldwide due to many time-saving maintenance features, it remains less efficient than the traditional primary impactor or impact breaker. All Andreas-style crushers impact and grind the material past heavy aprons, with sizing controlled by speed and the apron proximity to the rotor.

In recent years, new impact crushers have been developed as ?true impact? crushers. This equipment combines the time-saving features of the Andreas-style impactor and the traditional primary impactor. These machines are designed to bounce material off heavy aprons, ricocheting material back into the rotating rotor, producing more crushed material with less horsepower. These ?true impact? machines use 0.5hp (0.37kW) per tonne per hour. They will produce a higher percentage of finished products than the Andreas-style machines. The impact crusher is one of the most versatile crushers and offers the advantages of high reduction ratio with lower horsepower per tonne.
?    Hammermill (primary). Typically, hammermills are used for soft, non-abrasive and dry materials. Some non-clog models accommodate wet or sticky materials. Hammermills are used where higher reduction ratios are required. Normally they are used when a product must be reduced to a certain size for the next operation in the circuit. Hammermills are common in cement plants, where the top size of the primary crusher output must be a certain size for the next operation. Reduction ratios for this machine are typically 20:1. Hammermills are higher maintenance than the impact crusher due to the higher reduction ratios and the design of the machine. Hammermills will require approximately 2hp (1.49kW) per tonne per hour of material produced in a primary application. Hammermills are a larger investment than the impact crusher, although they are less than the jaw or gyratory.
?    Sizers. These machines are typically used for medium to soft non-abrasive material. Reduction ratios for this machine are approximately 5:1. They are typically lower maintenance than the impact crusher. Sizers have medium investment cost.
?    Gyratory crushers. These are used for hard and abrasive materials. Reduction ratios are typically 4:1 and maintenance is minimal. Gyratories have a high investment cost but are typically long-life machines. These machines use about 0.25hp (0.18kW) per tonne per hour of material produced.

In the primary crusher, consideration should be given to initial cost, reduction ratio, horsepower per tonne per hour, hardness, abrasiveness of the material, feed size input and product required. Power consumption per tonne produced has more recently become a very important factor in crusher selection.

Tightening the quarry shot pattern should mean less oversize material being fed to the primary crusher hopper. Not having to break or remove oversize rock in the primary hopper can dramatically improve efficiency in the system. Rock breaker hammers are definitely a necessity at many primary stations but at times can encourage the operators to put oversize rock in the hopper.

SURGE CONSIDERATIONS

{{image3-a:r-w:200}}In a stationary quarry set-up, there is typically a surge pile. The primary crusher is usually oversized for capacity and is operated at a lower efficiency than the rest of the plant. It is common for the primary sized material to be stockpiled in such a manner that it can be reclaimed from the bottom of the pile to feed the rest of the plant. This overcomes the fluctuations in the feed from the primary crusher. The surge pile is designed with a live capacity of two to three or more hours.

Scalping, sizing considerations

A scalping screen is typically fed material after it has passed through the primary crusher. The machine separates oversize materials that need further crushing. Many times a specific product is pulled from the second or third deck of this screen. Scalping screens normally contain two or three decks. Scalping screens are often inclined-type screens and are not generally used for precise sizing. Oversize materials from this screen are usually conveyed to the secondary or tertiary crusher to be crushed further. Usually a sizing screen follows the secondary crusher.

The sizing screen separates the material, with the top deck returning on a conveyor to the secondary crusher for another pass. The rest of the decks on the sizing screen are blended onto conveyors for stockpiling. In cases where a tertiary crusher is used, the secondary crushed material may be conveyed to the tertiary crusher and possibly to an additional screen or screens. All reputable screen manufacturers offer formulas for capacity but a knowledgeable specialist should also be consulted due to variables regarding the material characteristics.

Secondary crusher considerations are as follows:

?    Cone crushers. These are suited for crushing hard abrasive materials. They are compression crushers. Cone crushers are a high initial investment and low maintenance. Reduction ratios are typically up to 6:1. It should be noted that the cone crusher is limited to a certain top size material being fed to it. Cone crushers use about 1hp (0.75kW) per tonne per hour making a 1? (25mm) product. Typically, the cone will have about 15 to 20 per cent oversized product recirculated back to be re-crushed to size.
?    Roll crushers. These can be used to crush hard and abrasive materials. They are also compression crushers. Roll crushers are medium to high initial investment and are average maintenance. Reduction ratios are up to 3:1. This machine requires 1.25hp (0.93kW) per tonne per hour of material crushed making a 1? product. The product from the roll crusher has almost no oversize. The triple roll crusher is the same as previously stated except it has a reduction ratio up to 5:1. Roll crushers typically have lower tonnages than their counterparts. Roll crushers are most noted for their low production of fines and controlled output gradation.
?    Horizontal shaft impactors. These are typically used with soft or medium abrasive materials. This type of impact will crush abrasive materials, although the wear cost may be prohibitive based on the metallurgy of the wear parts selected. Horizontal shaft impacts have a reduction ratio of up to 12:1 in a secondary application. They are low cost machines with a minimum investment value and are capable of high tonnages. Typically, the horizontal shaft impactor will have about 15 per cent of the product that will need to be re-screened, then oversize material recirculated back to be re-crushed. Impact crushers require about 1hp
per tonne of material crushed making a 1? product.
?    Vertical shaft impactors. These are typically used in medium abrasive materials. These machines are usually installed as a tertiary (third crusher in the system). They are very good at fine crushing down to 1/8? (or 3.17mm) and providing a cubical product.
?    Hammermills. These are primarily used in soft and non-abrasive applications. They are usually installed to make finer graded finished products. They require high horsepower per tonne per hour produced. Hammermills require about 2hp (1.49kW) per tonne of material crushed making a 1? product. They have a low investment cost and are small in size. Hammermills have sizing grates through which the material must pass, yielding little oversize. The finer the desired product, the more horsepower is required.


COMPREHENSIVE ANALYSIS

There are many considerations in designing an efficient crushing and screening system. Doing a complete analysis of the equipment should include considerations for improving efficiency, minimising the number of pieces of equipment, analysing power consumption based on the volume of material produced, maintenance schedules, labour hours required to maintain equipment, capital costs and personnel requirements.

Lou Winchip and Gerry Mangrich are respectively the engineering manager and regional sales manager for McLanahan Corporation.

This article was published in Quarry Management (UK) in October 2012 and is reprinted with kind permission. Visit www.agg-net.com

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