The important of crushing your ore and rock fine and properly is often forgotten. The finer you crush, the higher your ball mill tonnage and capacity will be.  The effect of ball mill feed size and how it affects circuit throughput can be hard to estimate. Here we described a method of designing a crushing plant using power drawn and power rate to define reduction ratios in each stage of crushing. The plant power and power rates were computed from a Bond calculation as applied to the crushing plant feed and output sizes. A comparison of the low and high energy configurations.

We would design this plant differently today using energy parameters from the pendulum impact tests for calculations. It would only be necessary to use the Bond feed and product size calculation if no pendulum results were available.

Crushing Finer To Reduce Milling Costs

This new high energy or power rate crushing brings a different perspective to comminution flow sheet selection. Generally, up until the early 1960’s the classical flow sheet for a beneficiation plant was primary crushing followed by two stages of cone crushing in closed or open circuit, making feed for rod mills, followed by ball mills. The rod

Rock splitting utilizes a tensional force to crack the rock. This is achieved by inserting a split steel shank in a drillhole and driving a steel wedge between the center of the split shank thereby producing the tensile forces.

The advantages of the rock splitting technique over blockholing are the absence of explosives and the reduction of downtime provided there is quick and easy access to the rock splitting equipment.

The rock splitting technique does have some disadvantages. Drilling the required hole is time consuming and hazardous. Before any drilling is done, the boulder should be examined for stray powder and for the best hole location. The boulder, probably containing cracks from the primary blast, will be hazardous to drill in that, it could split without warning. When actually splitting the rock with the splitter, a safe place on the boulder must be available to work from so as to avoid injury. Alternatively the procedure may be done by remote control whereby the operator is removed from the breaking area.

One type of rock splitting machine is illustrated in Figure 13. This hand held device utilizes hydraulic pressure to expand a plug placed in a drillhole creating tensile failure in the rock. The

A rock grappler is a claw shaped device used for picking up boulders. The grasping mechanism of a grappler is generally activated by a pneumatic or hydraulic cylinder, as shown in Figure 17. The cylinder is connected to each half of the claw and when activated forces the claw shut. Grapplers are used to remove or reposition boulders; they can only break rock by lifting and dropping.

There are two types of lifting mechanisms that can be used with the grappler claw. One requires that the claw be mounted on the end of a large hydraulic boom. This gives the grappler good mobility and good response to controls.

The problem with this mechanism is that an extremely strong and large boom is needed. The other lifting mechanism is an overhead crane system which uses cables and pulleys. This mechanism is strong, but is slow and difficult to maneuver into the desired position.

A grappler is most commonly used with a gyratory crusher because of the greater difficulty of access to a jaw, impact, or roll crusher (refer to figures 5 through 8). A grappler is a capital intensive piece of equipment but

A sling is a device for removing or repositioning a boulder or object in a crusher. A sling is made of a length of chain or cable with loop ends as shown in Figure 18c. To remove or reposition a boulder a sling is laced around the boulder and the looped ends are connected to a boom, winch, or overhead crane and hoisted out of the crusher or moved to where the crusher can nip it. The advantages to this method are the protection to the crusher parts because explosives are not used. The disadvantages are that the crusher has to be stopped, a person has to get in the crusher, and the length of time to move or remove the boulder can be excessive. Also, if an angled grizzly feeds the crusher, it must be cleaned of all loose debris before someone can enter the crusher mouth. Operators have spent as much as half a work shift doing this task.

A hook is a device which is primarily used for re-positioning the boulder. Some of the more efficient hooks are shown in Figures 18a and 18b.

The hooks

The rock hammering method of boulder breaking is a process of hammering the boulder by either a hydraulic or pneumatic hammer until the boulder breaks. The hammer is usually mounted on a boom. The motor, hydraulic pump, and oil reservoir are usually located near the boom. Figure 14 shows a typical hammer installation.

The advantages of this method are continuous operation (minimal downtime for boulders), no need for explosives, safer environment for the operator, and the hammer can be used to clean grizzlies.

The major disadvantage of the rock hammering method is the initial capital cost of the hammer.

The breakers have developed into a third generation model which requires less maintenance than earlier models. A variety of breaker points are available in order to achieve optimum performance for different rock characteristics.

Rock breaker designs are of two basic types. One type utilizes high impact blows at low frequency while the other type produces a much lower impact blow delivered at a much higher frequency. Frequencies can range from 25 to 450 blows per minute with blow energies developing between 1,000 and 20,000 ft-lbs per blow.

A variety of breaker points are

Boulders can be broken up using either explosives or mechanical methods which can be implemented in many ways. Handling boulders hung up in crushers is inherently inefficient for the overall operation, and the techniques in handling them each have their own advantages and disadvantages.

Table 4 shows a comparison of explosive boulder breaking techniques. When these methods are used it is important to make certain that the crusher feed is empty of rock that could transfer shock energy from the explosive detonation and damage the crusher. The table summarizes economic evaluations of some blasting methods on the basis of drilling need, and type and amount of explosives needed to break a cubic yard of rock. For each method, either dynamite or binary (two component explosives) can be used. The methods will be explained in detail, but it must be emphasized that cost figures shown in Table 4 cannot adequately include the cost for any increased accident potential that could result.

This section covers techniques found in use in mines and quarries or likely to be in use. A later section describes new technology that are not presently well enough developed for use for boulder breaking.