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oversized rock crusher

blog: three reasons why your primary crusher feed contains oversized rocks

blog: three reasons why your primary crusher feed contains oversized rocks

In todays mining 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 an important part of the mining process, taking the run of mine (ROM) ore and reducing it to a size which can be processed by the downstream operations. However, mining operations frequently have issues related to the primary crusher: low throughput, low utilization 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 ore 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.

In order to prevent this type of bottleneck, one key element that needs to be analysed is the mines existing drill and blast program. There are three critical things that need to be looked at to ensure that your feed does not contain oversized rocks which can cause unplanned downtime.

Solution:Even though ore 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 optimize 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 ore types and alterations with the naked eye. For the same ore type, changes in ore mass structure will induce changes in fragmentation of the blasted material. An area with big preformed blocks in the ore mass sometimes cannot be detected during drilling. Information related to ore 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 ore domains based on blastability and have an adequate drill and blast design for each area.

Problem:The blast design is well adapted to current ore conditions; all rules of thumb were respected and the blasting pattern generates good fragmentation results in a mine with similar ore 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. Minimizing the perimeter of the blasting polygon for the same quantity of mined ore 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 ore 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.

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 mine. This has the potential to 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 mining 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 is able to manage big ore boulders, then it is okay 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 the 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 and affordable way to resize the boulders.

There are multiple ways that you can 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 that shovel/excavator operators are properly trained can help reduce oversized feed and unwanted lost production.

rock boulder crushers

rock boulder crushers

In mining and quarryinq, the mineral commodity as present in the ground may be composed of rock blocks that must be reduced in size, to suit market requirements. The rock in place will be blasted, loosened, and then hauled away to the first processing step-size reduction. As delivered from the blasting area the rocks will range in size from dust to blocks several feet on a side. The crusher employed for the initial size reduction will have a maximum size rock capacity, and when any oversized rock is delivered, it lodges in the crusher and the materials flow in the mine or quarry is halted.

Whenever rock passes through openings, there exists a possibility for blockage by oversize pieces. This is particularly true when dealing with rock undergoing blasting primary breakage or collapse and drawing in a caving mine. Crusher and grizzly openings are the first size restriction for the materials handling process and therefore may plug by bridging or by oversized material. When this happens, several courses of action can be taken, depending on the hangup accessibility.

Manual techniques like sledge hammers or pry bars may be used to break or loosen boulders, but take excessive manpower and time. Hydraulic and pneumatic impact breakers are available but they are expensive. Blasting is often viewed as the most economical and quickest method for boulder breaking, and also requires very little effort from personnel. However, the person doing the blasting may not be experienced with explosive use. Since a blast in a crusher can be carried out at any random time, the blast area may be difficult to keep clear of personnel and equipment. Equipment near the crusher or grizzly could be damaged from blasting. Although blasting is the quickest method it is also potentially the most dangerous and destructive method. With improved practices and technology in this area, it is hoped that accidents resulting form boulder blasting can be reduced.

The Bureau of Mines recognizes the hazards involved and initiated this project to examine present practices, regulations, and safety records so as to recommend safer procedures and present improved technology for boulder blasting in crushers. In addition, all available literature dealing with boulder handling practices was examined.

Field visits were conducted to meet with operators firsthand about problems in handling boulders and the development of their chosen procedures. These visits were nationwide and covered many different types of rock, mining methods, and boulder handling equipment.

Crushers are generally very large machines that subject rocks to compressive forces that cause rock failure and thereby size reduction. The various components of a crusher system will be described below as well as the problems of boulder handling in each.

There probably are as many kinds and types of material delivery systems as there are applications for them. The delivery systems generally are pan type feeders or vibrating grizzlies that transport the rock from the hopper to the crusher mouth. Pan feeders are basically steel segmented conveyor belts, while grizzlies, whether vibrating or stationary, are a set of parallel bars set apart from each other a distance which permits passage of only the maximum size rock that can be easily handled in the next stage of rock processing. Grizzlies are usually built heavier than necessary in order to withstand secondary blasting either above or below the grizzly level. Vibrating grizzlies utilize the motion to feed a crusher with properly sized rocks while allowing undersized material to bypass the crusher, decreasing required crusher capacity. Figure 1 shows a hopper and vibrating grizzly arrangement. Grizzlies can also be installed stationary as equally spaced parallel bars sloping from the dump to the crusher mouth as shown in Figure 2. Storage hoppers are not needed since oversize rock falls down the incline immediately after dumping. Oversized rock can bridge or hang-up in these feed systems as in Figure 3. The advantage in such a system is in dealing with a disruption in rock flow and hang-up before the crusher mouth with easier access to the problem area.

Rock can be broken by compression, impact, and attrition in the various types of crushers developed for mining. Crushers are selected on the basis of the material being crusherd, such as physical structure, geological classification, hardness, and the chemical constituents.

empty. This is a beneficial condition for blasting in that the existing air space acts as a cushion and therefore reduces excessive shock transfer to the crusher. This can offer some solace to the operator, but several crusher manufacturers contacted recommend blasting only as a last resort.

Gyratory crushers, shown in Figures 4 and 5, have a conical head with an eccentric movement inside an outer concave bowl. They are designed for high capacities. A straight vertical discharge prevents the packing of sticky materials (Pit and Quarry, 1977). Boulder blockage will occur if rock bridges the crusher perimeter and the spider supports that span the intake diameter. Obviously, any blasting can affect these supports and throw the cone out of alignment. The majority of the operating mechanisms are underneath and to the bottom side of the cone. Hence this area is susceptible to shock forces from boulder blasting which could cause cracking of the outer shell or loosening of the drive shaft.

Jaw crushers, shown in Figure 6, are the oldest type of crusher in use. They are economical when dealing with a coarse or blocky feed although capacities are low. The swing jaw, powered by an eccentric, moves downward and toward the stationary jaw to crush, then moves up and back to allow crushed material to exit. Slabby rocks tend to slide down the crushing chamber untouched until their width equals the discharge dimension. Jaws can be concave to encourage rock falling lower in the crushing chamber, preventing packing (Pit and Quarry, 1977).

The eccentric wheel mechanism location prevents easy access to a boulder. It also is most vulnerable to blasting effects. The swing jaw and main frame, as well as toggle bolts and bearings, are also susceptible to blasting damage.

Single roll crushers, shown in Figure 7, use shearing action to break soft rock. The main part is a knobbed roll where the knobs extend 3 to 4 inches beyond the roll surface. Rock is caught between the roll and the stationary breaking

plate where it is continually hit and pushed by the roll knobs. The breaking plate, hinged at the top with the lower edge set on springs, can back off if an uncrushable material enters (Pit and Quarry, 1977). Large boulders can easily bridge themselves over the roll and avoid contact. The crusher knobs must be kept sharp so that a boulder does not bounce off the knobs and avoid crushing. Roll crusher drive wheels and roll shafts are extremely vulnerable to blasting damage.

A typical single roll hammermill or impactor is shown in Figure 8. In the hammermill the feed drops free and is hit by the crushing surface traveling at high speed. The rock shatters on impact and pieces are thrown toward a breaker plate for further size reduction. Blockage can occur whenever a boulder does not fall far enough into the feed opening to be exposed to the crusher hammers; consequently, no work can be performed on the rock. In many cases, the crusher design offers a means of access to alter the boulder position or enable other remedies to be tried (Pit and Quarry, 1977). Boulder blasting in this type of crusher can cause damage to the impactor shaft or bearings.

Boulders originate in a mining operation during the primary blasting of the commodity mined. This blasting must adhere to accepted design and execution to result in good rock fragmentation. Long stemming or poor hole spacing, for example, contribute to the creation of boulders. Geology also enters in that hard lenses may occur and be difficult to break or easily dislodged from the bench. Joints, bedding planes, and clay seams in the rock are also factors that determine blasting fragmentation, and some mineral deposits may tend toward slabbiness or blockiness. As previously discussed, primary crushers are selected chiefly on the basis of the incoming material characteristics.

Most boulders are set aside from the loading area and are blasted later. Sometimes a boulder may be truck loaded inadvertently if hidden in the muckpile. Loading operator experience and close supervision can prevent this from happening too often. It is important, for safety reasons, that boulder blasting be done in the pit. Loading equipment should have a proper bucket size matched with the crusher feed in order to help eliminate the transportation of oversize rock to the crusher.

Material handling equipment in a mining operation must be carefully matched for peak efficiency. For example, large capacity loading equipment would not be practical if the crusher mouth could not receive the larger loaded rocks. Oftentimes this mismatch occurs at smaller mining operations where equipment may be secondhand and purchased at a lower price rather than to meet specific requirements. The occurrence of boulders is further compounded in that small operators usually give inadequate attention to blast design, and cannot afford to hire a knowledgeable blasting foreman. The operator may not know how often boulders get hung up in the crusher, or how much time is lost to remedy that condition; it is just an accepted part of the operation.

Blockage of the crusher feed can be caused either by bridging of material or by a boulder too big for the feed opening. If material is wet and sticky, several smaller rocks can bridge over the feed opening and avoid crushing.

This usually happens when the delivery system is stopped and rock can settle and key together forming a bridge. Once the feed system activates, the material under the bridge is drawn out. The problem is more prevalent in cold weather where ice can freeze rock together.

Another form of crusher feed blockage results from a large boulder that cannot enter the feed, as shown in Figure 9. Occasionally the boulder can be repositioned in the feed system so that a smaller dimension of the boulder can be nipped by the crusher mechanism. If the feed system does not have this flexibility, then the boulder must be moved by auxiliary means which will be discussed at length in later sections.

three reasons for oversized rocks in the primary crusher feed - quarry

three reasons for oversized rocks in the primary crusher feed - quarry

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.

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 quarrys drill and blast program. There are three critical aspects that need to be examined to ensure your feed does not contain oversized rocks.

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.

{{quote-A:R-W:300-Q:"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.

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.

{{image2-a:r-w:300}}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.

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.

Metsos 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.

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