Table of Contents
Introduction
Compared to the gyratory crusher, the cone crusher is characterized by its higher speed and a flat crushing chamber design which is intended to give a high capacity and reduction ratio for materials suitable to this type of processing. The aim is to retain material longer in the crushing chamber to do more work on material as it is being processed. A concept of how a piece of stone might flow through a secondary crushing chamber is shown in Fig. 45. The number of times material will be nipped during crushing will depend on material size, friability, and the geometry of the crushing chamber as well as its speed and eccentric throw. Cone crushers are usually specified in terms of closed-side setting (CSS) and a given quantity of material passing a particular square-mesh screen size. A criterion of product passing closed-side setting is often available. This value will vary according to the particular crusher design and the details of its application.
Essentially, cone crushers can be classified into distinct types, depending on their duty. Crusher sizes for all duties are described by terms arising out of common industry usage, such as 3-ft, 7-ft, etc., which refer to the crushing-head (mantle) diameter. At present the range of sizes available varies from approximately 2 to 10 ft diam. Weights vary in the range of 5 to 200 tons, and connected horsepowers range from 10 through 700. There are so many variations that specifics should be obtained from machine manufacturers.
The crushing chambers that are fitted to various machines are also often referred to by terms arising out of common usage, such as standard for secondary crushers, and short head for tertiary crushers, which refer specifically to the Symons design. Also used are more specific but equally vague descriptions, such as coarse, intermediate, and fine chambers. Some manufacturers such as Allis-Chalmers with their Hydrocone crushers refer to the model number of the machine by the feed opening and the diameter of the mantle. For example, a Model 10-84 Hydrocone crusher has an 84-in. diam mantle, and is capable of accepting a 10-in. diam sphere at the feed point to the crushing chamber. Rexnord and Telsmith define crushing cavities in their capacity tables as feed openings at minimum recommended discharge settings (closed-side settings). Each practice offers guidelines which should be carefully defined for the specific crushing problem. The utilization of a cone crusher depends on how well it is controlled and the features of the circuit in which it operates. Normally, it will operate in the range of 75 to 85% of the peak capacity.
It is likely that industry pressures will call for a standarized de
scription for the cone crusher in the not too distant future and these could be in terms of power as well as physical machine size. The reasons for this should be evident in the following discussion. Cone crushers are usually arranged in stages to effect an efficient utilization of applied crushing power. Experience has shown that the crushing process is more controllable if certain reduction ratios are adhered to for each stage. Technically, this is defined as that size of feed (in. or mm) of which 80% will pass, divided by the size of the product (in. or mm) which 80% will pass.
Secondary Crushers
These are the cone crushers which accept suitable feed size material either prepared by a primary crusher or occurring naturally, and reduce it to a size suitable for marketing in one stage, or make feed for subsequent crushing or grinding stages. Secondary crushers have feed openings of 4 to 25 in. in the larger (7-ft) models down to a 2¼ to 4 in. in the smallest 24-in. models. Reduction ratios normally range between 3:1 and seldom more than 5:1. In the 7-ft models, the cone crusher typically makes a product all passing 2 in., but this is dependent on the machine design and properties of the material being treated.
Secondary crushers most commonly operate in open circuit. They also are operated in closed circuit with a vibrating screen, depending upon product requirements and conditions where the reduction ratio does not cause excessive power draw and build-up of circulating load. Screening ahead of secondary crushers is generally recommended, especially where the feed contains more than 25% material smaller than the desired closed-side setting.
Tertiary Crushers
These are cone crushers that normally take secondary crusher product and reduce it to a marketable product or make it suitable for subsequent comminution steps. The reduction achieved is a function of the crusher design and the properties of the material to be treated. The reduction ratio is normally in the range between 1.5 and 2 to 1, and seldom more than 3 to 1.
The tertiary crusher normally operates in closed circuit with a vibrating screen and makes a product smaller than all passing ½ or 5/8 in.
Fine Crushers
Sometimes these machines are called sand crushers and are called by various manufacturer brand names, such as Gyradisc, Hydrofine, V.F.C., and others. A cross section of a Symons Gyradisc crusher is shown in Fig. 46. Essentially, these crushers would be used in a fourth-stage crushing operation, but could be called on to reduce a screened feed size fraction from a secondary or primary crushing operation.
This type of crusher normally receives feed no coarser than 1½ in. that is scalped of all over-sized material and operates in closed circuit with a vibrating screen. For average materials, typical product is ¼ in. top size, although a —10 mesh product can be produced on those materials having suitable characteristics. The crusher is normally operated in a separate circuit from the main crushing plant because of the variations in output rates that are caused by varying physical properties in the feed. The reduction ratio is generally less than 2-1, and circulating loads are generally high. More interparticle crushing takes place in these machines than with conventional cone crushers and capacities can be more sensitive to changes in moisture content, feed size graduation, and other physical properties.
Design Features
For the various applications, crusher manufacturers normally have a range of crushers of different sizes and power ratings to select from.
Each has design features which may be the most acceptable for the specific user. When considering that the crushing process takes place in the crushing chamber, cone crushers can be generally classified into two basic types: (1) large eccentric throw, flat, short-chambered crushers normally with a cantilevered shaft supporting a mantle such as the Symons (Rexnord) standard and short-head designs shown in Figs. 47 and 48 (the Telsmith crusher design (Fig. 49) is a variation of this concept), and (2) large eccentric throw, flat, short-chambered crushers that are normally fitted with a spider bearing at the top of the crusher so the main shaft carrying the mantle is supported from both ends. An example of the later type is the Hydrocone crusher (Allis-Chalmers) illustrated in Fig. 50.
It is not the purpose of this Handbook to draw conclusions on the most suitable concept, but the various features should be studied by plant design people to see which best fits a particular application. There are mechanical features in various designs that should be a part of any crusher selection. The more noteworthy features listed by a range of producers are discussed in the following.
Crusher Setting Adjustment. Crusher setting can be changed mechanically by screwing the bowl or top shell, or changed by direct hydraulic actuation of the crusher main shaft, or through hydromechanical devices. On some crushers, setting adjustments can be made while the crusher is operating under load, and some must have the feed temporarily interrupted while a setting adjustment is made. With some arrangements, setting changes can be made remotely and the setting can be monitored by instrumentation. The variations in size distribution of the crushed product for different closed-side settings in open-circuit and closed-circuit operations are shown in Tables 22 and 23, respectively.
Bearing Design. Some crushers use antifriction bearings such as the Telsmith design (Fig. 49), although at the time of this writing, most larger installations are using bronze or babbitt-type bearings.
Tramp Relief. This feature allows passage of uncrushable material without stalling the crusher. Essentially two types of systems are employed by the various types of crushers available on the market: (1) mechanical systems employing springs, and (2) hydraulic systems employing gas-charged accumulators. Both systems allow the passage of uncrushable material to be accommodated by letting the crusher open. This feature also helps prevent overstressing the crusher.
The hydrocone crusher (Fig. 50) uses a system involving a check valve and gas-charged accumulator to pass uncrushable material. The Symons and Telsmith designs use mechanical springs although in the 10-ft Symons design, gas-charged accumulators are employed.
Crusher Stalling Considerations. Crushers can be stalled by power failure, uncrushable material too large to pass the tramp iron protection devices, or harder feed that might occasionally come along. The crusher then has to be restarted by digging out. Hydraulically set crushers of the shaft-adjusted type can often be restarted by lowering the shaft at the same time as energizing the main drive motor. By using an automated-setting control or feed-rate control system, this problem can be minimized.
Crusher Selection
The selection of cone crushers has been more of an art than a science. Little material has been published of a technical nature which allows the process engineer to accurately apply cone crushing equipment. Nearly all manufacturers utilize some form of capacity tables,
based on previous experience, to predict or estimate crushing performance. Although this procedure serves as a reasonable guide to crusher selection, it should be used with caution because of the many variables involved in the crushing process. Most capacity tables, therefore, have a qualifying statement and refer the user to factory consultation for more accurate performance estimates. This, coupled with the fact that there is a wide range of crushing-chamber configurations available for a specific size machine, justifies careful evaluation of the estimated performance for a particular crusher installation.
The performance of cone crushers depends on a balance of operating variables regarding the actual crusher and the raw material to be crushed. Each crusher manufacturer has evolved a design that balances the variables to produce an efficient crushing action. The relative merits of each design is a subject of interesting and sometimes spirited discussion.
The operating variables may be classified into two categories: (1) crusher variables and (2) external variables, which are interrelated. The major crusher, or design, variables include crushing-chamber configuration, speed and eccentric throw, horsepower, and methods to fully utilize the available power. The external variables, or process considerations, include the following: (1) physical properties of the feed material, (2) raw material flow pattern, (3) arrangement of equipment in the crushing circuit, (4) surge capacity provisions, and (5) efficiency of sizing equipment such as various types of screens. The following discussion will deal with these variables, not necessarily in the order stated. However, the interrelation of the operating variables should become clear.
Feed Size and Capacity
The first consideration when selecting cone crushers is to determine the actual feed size and the required capacity. The feed size determination is very important since it is used to select the proper crushing chamber. If the chamber selected has too small a receiving opening, the feed entry will be restricted, resulting in lower capacity and under- utilization of the crushing chamber. If the chamber selected yields too large a receiving opening, the feed will be crushed only at the bottom and will result in excessive horsepower, pressures, wear, and reduced capacity. The proper chamber will yield the most efficient crushing results, with the feed being reduced in a series of four to five blows as it passes through the crushing cavity. Another important consideration is that the feed should not be too fine. Therefore, it is generally recommended that 10 to 20% of the feed (depending on friability) should be larger than the closed-side receiving opening, but smaller than 90% of the open-side receiving opening.
The rated capacity should be defined as to open-circuit or closed-circuit crushing. Generally speaking, secondary crushers are used for open-circuit, and tertiary and fine crushers are for closed-circuit crushing. Secondary crushers are typically rated in terms of open-circuit capacity. Occasionally, however, secondary crushers are operated in closed circuit. Tertiary and fine crushers are rated in closed circuit-capacity, that is, net finished tons of product which will pass a square screen opening of the same size as the closed-side setting of the crusher, assuming normal screening efficiencies.
Closed-circuit capacities are usually higher than open-circuit capacities since the near size particles in the feed are better utilized to fill the voids within the crushing chamber.
Symons Cone Crushers. Capacity charts for standard and short- head Symons cone crushers are provided in Tables 24 and 25. The capacities shown in these tables are based on results obtained from many installations worldwide, crushing a broad range of ores and rocks. Capacity, type of circuit, feed opening, and discharge setting nomenclature associated with these capacity tables are defined by illustration in Fig. 51.
Gyradisc Crushers (Symons). Capacities for different-size Gyradisc crushers producing various product sizes are shown in Table 26. These capacity data are based on closed-circuit operations.
Gyrasphere Crushers (Telsmith). Capacity charts for various models and sizes of Gyrasphere crushers are given in Table 27 for bowl settings varying from ¼ to 2½ in.
Hydrocone Crushers (Allis-Chalmers). Hydrocone crusher capacities for open-circuit and closed-circuit operations are listed in Table 28. Data are included for various crusher and product sizes.
Crusher Variables
The crusher variables which affect the performance of the cone crusher are discussed in the following paragraphs.
Gyrations per Minute. This relates to the number of strokes or impacts per minute as the material progresses through the crushing chamber. As mentioned before, the most efficient crushing action requires a series of four to five blows as a particle passes through the crushers. Practically speaking, the speed or rpm is relatively fixed and is an integral part of each particular design.
Stroke. This is the total amount of transverse motion of the crushing head during one crushing cycle. This, too, is fixed and related to the speed of the crusher and the length of the crushing chamber.
Reduction Ratio. This is defined as that size of the feed of which 80% will pass, divided by that size of the product of which 80% will pass. Various reduction ratios are characteristic of each machine design, but are limited by practical considerations of how much chamber length can be incorporated in a specific design. Speed and stroke are balancing factors.
Rate of Change of Reduction Ratio. This can be more simply understood as the relation of the length of the chamber or amount of reduction per inch of head. The length of the chamber is important as it determines how long the material will be in a position to be impacted by the head.
Slope of the Crushing Chamber. This will determine the gravity effect on the material which ultimately determines the rate of travel of the material through the crushing chamber. The steeper the slope, the faster the eccentric must gyrate in order to insure the proper amount of impacts for proper reduction and sizing in the parallel crushing zone.
From all of the foregoing discussion, one should conclude that all crushers of similar mantle size do not necessarily have the same productivity, and the design of a cone crusher is a sensitive, complex engineering problem. The chamber design and eccentric throw are vital and are set by the machine’s design stress limits. Consequently, if an eccentric throw is reduced or a chamber length is increased
and speed remains unchanged, it would be necessary to reduce the average power draft in the drive motor for the same stress level in the crusher.
Power. The actual power drawn by a cone crusher is a function of the variables discussed previously. The productivity of a cone crusher will be a function of the power transmitted to the material as it passes through the crushing chamber. The output of a crushing plant will be related to the aggregate sum of the crushing power employed in the facility. This means if the power consumed in the crusher or plant can be increased, the extra power will be reflected in terms of increased tonnage of the same product or a similar tonnage of a finer product, assuming other variables remain constant.
Bond’s third theory of comminution can be used with reasonable accuracy to predict the performance of the various crushing stages in the overall crushing plant. This has already been discussed in Chapter 2, Gyratory Crushers. This procedure can be a very useful engineering tool in calculating crushing horsepower requirements, which may vary considerably from the catalog ratings that make no allowance for feed size, product size, or work index. It is not always possible to correlate actual power consumed by an individual machine in a crushing plant versus theoretical power, but there has been good correlation based on the average total power on overall plant reduction for many large crushing plants. The reason for this is probably often due to limited accuracies of sampling, difficulties in measuring sizing efficiencies, and the necessity in many instances of utilizing a correction factor in the calculation for scalped feed. Changes to feed size, occurring more or less constantly from changing physical ore characteristics, further complicate the sampling problem.
Results from testing programs do seem to indicate that crusher productivity for the same product material size graduation is independent of eccentric throw or chamber configuration, provided the power drawn is constant. The following example suggests that each manufacturer applies its particular design in such a way to utilize the power available. That is, a balance is made between available power and the volumetric capacity of the particular crushing chamber.
An example of this is shown in Fig. 52 where three different 3-ft sized crushers were run, fed from the same feed bin. Accurate power, capacity, and screen analyses were taken when each crusher was choke fed. Curve 1 is for a short-head crusher, fed at 54.3 tph and drawing 48 hp with a 0.31-in. close-side setting and 2.25 in. eccentric throw. Curve 2 is for a fine-chambered crusher, choke fed at 61 tph and drawing 71 hp with a 0.31 in. close-side setting and 0.75 in. eccentric throw. Curve 3 is for a fine-chambered crusher, fed at 103.4 tph and drawing 92 hp with a 0.437 in. close-side setting and 1.0 in. throw. Productivity comparisons for the three crushers are illustrated in Fig. 53. Dividing the amount of different size material produced for each crusher by its power consumption gives the following horsepower per ton of material crushed values:
These power consumption values are very similar for the respective product sizes.
It is interesting to note that the smaller eccentric throw made a finer product. The crusher’s ability to draw power is affected by the eccentric throw, speed and the crushing chamber shape. The slope, length, contour, and speed of operation of the crushing surfaces will have an effect on the fineness of the product coming from the machine. This length also affects power draw in the crusher for a given setting and eccentric throw. All these aforementioned criteria set the volumetric capacity of a machine, sometimes called the choke-fed capacity.
Because of eccentric designs for the machines, a different proportion of the chamber is utilized to do work. The application trick is then to select the proper chamber configuration. As there is a volumetric capability for a particular crushing chamber configuration, and there is a power consumption drawn by the machine for a given set of feed conditions and reduction ratio, there will be a setting that gives the particular reduction for a choke-fed condition.
Other Variables
If maximum performance is to be maintained, the crusher has to be installed and operated at the maximum average power draft over the planned operating period. To accomplish this, as much feed material as necessary has to be available to be crushed. Since material increases in bulk during crushing, it is necessary to screen the already product-size material to allow for bulking during crushing. As discussed previously, factors most affecting power draw are:
(1) quantity of material being fed to the crusher, (2) size distribution and quantity of material in the crusher, (3) physical properties of material being fed to crusher, such as work index, fracture size, size distribution, moisture content, stickiness, etc., and (4) proper chamber design and selection.
In crusher-plant flowsheet design the component selection, arrangement, and location are decisions the plant designer will make. The size-distribution variations that are encountered in the feed to an individual plant can be considerable. Examples of a three-stage crushing-plant flowsheet, using two stages of cone crushers, to provide feed for single-stage ball-mill circuits are shown in Figs. 54 and 55. Surge bins and variable-speed feeders will even out the feed rate to the crushers. If crushers are fed directly from a screen, a sudden change of size makes it difficult to operate near maximum power rating.
Primary Crusher Effect. The primary crusher has a significant influence on the subsequent crushing stages. A primary crusher can be considered to be a feed preparation step for fine crushing. Mine run ore will often vary from day to day in size distribution (see
Fig. 56), and usually the finer the primary discharge product, the better will be overall plant performance. In secondary crushers the narrower the feed opening, the more will be the utilization of crushing-chamber volume and power. If a narrower range of feed-size variations can be achieved, a crusher-chamber shape can be used which allows the crusher to operate at a higher average power draw.
Feeding the Crusher. Many tests have been run to show that, for best power draw and lowest machine operating stress, the crusher should receive evenly distributed, coarse and fine material, around the crushing chamber. The two curves shown in Fig. 57 demonstrate good and bad feeding conditions, and illustrate the crushing force in a machine being tested in a commercial plant. The upper curve, with uneven feed distribution, shows the pulses from each crushing stroke reaching a peak of 8.8 units, and the average under the curve is 3.7 units. With better feed distribution, the lower curve, the peak came down to 5.3 units and the average force of 4.0 units. As average units are a measure of productivity, more production was obtained for less machine stress with the more even feed.
Feed Rate Control. Most materials processed in crushing plants are not homogeneous. Therefore, considering a constant feed rate to the vibrating screen or grizzly, there will be a varying quantity fed into the crusher. This reflects in varying power consumption and productivity from the machine. With these operating difficulties being identified, particularly in larger plants, more operators are specifying that there must be provision of surge capacity ahead of all cone crushers. Ideally, the cone crusher feed material should be drawn from this surge by slow-speed apron, pan, or belt feeders, depending on material top size, with the discharge dropping vertically into the cone crusher. Some plants have been designed with vibrating feeders taking material from the surge to the crusher, but careful attention needs to be paid to size segregation occuring on the vibrating-feeder pan due to material trajectory from the end of the pan. With some feed materials, agglomeration buildup on a vibrating-feeder pan can affect the discharge rate for constant operating speed and amplitude conditions.
In many newer crushing plants, crushers and screens are in separate buildings with their own surge provisions. These are sometimes called ganged crushing and screening plants. They have the advantage that a screen or a crusher can be maintained without shutting down its counterpart in the overall crushing-plant flowsheet.
Crushing Circuit
Generally, crushers in mining plants are producing material to be fed to rod mills or ball mills. In this case, the aim will generally be to produce the finest product out of the crushing plant, because crushing costs are considerably less than grinding costs.
In the case of some plants producing lumps and fines from high grade iron ore, the aim is to maximize the production of lumps since they normally sell for a premium price. In this situation, the crushers and plant arrangement should be so as to apply no more size reduction than is needed to reach required top size. The plant should be run at minimum economic circulating load, since fine material is produced in the recycling of oversize material.
One of the methods currently used to design crushing and screening flowsheets is to utilize crusher capacity tables and empirically derived application equations. This procedure is normally based on assumed size distributions and volumetric relationships. Such application criteria have been found to be erroneous when materials are harder and feed size is coarser than the average conditions assumed for such data. To overcome these inaccuracies, crusher selection should be made on the basis of power required by the material being crushed. Such application, to be accurate, should involve testing materials in a conventional cone-type crusher, preferably in closed circuit, so that accurate product-size data can be generated in the range that is expected in the commercial operation. Readings are taken during the tests on tonnage and power consumption when the circuit is in balance. Such energy information can then be extrapolated to predict performance on a commercial plant. Unfortunately, such tests are often difficult to arrange because of sample availability and the geographic location of a new plant.
Another method of applying cone crushers would be to determine the raw material’s work index by running Bond impact-crushing tests and Bond rod-mill grindability tests, and using the highest of these two work indexes as a basis for approximate design. This can be a safer approach than relying solely on manufacturers capacity tables. The following example illustrates the reason for adopting such an approach. An existing operating plant was making rod-mill feed nominally —5/8 in. with a total plant feed rate from the primary crusher product averaging 1,000 tph of —6 in. top size. Samples of this feed over several weeks of operation showed an average of 40% —5/8 in., which means that 600 tph of material must be crushed in two stages of crushing. If the standard capacity table for a 7-ft cone crusher is used, two machines are indicated for the job. However, this material had a work index of 18 kw-hr per ton, which is well above the average. Using the work index, the calculated power requirement would be approximately 1,300 hp. This is close to the average power that would be drawn by five 7-ft cone crushers. Therefore, two crushers drawing a total of 600 hp would have had no chance of performing this duty.
With closed-circuit crushing plants, the screening capacity can limit plant output if the efficiency or capacity of the unit is too low. In crushing plant circuits, the efficiency and sizes of vibrating screens can be determined from empirical design equations developed by various equipment manufacturers. Many of the factors employed are based on an individual’s experience in the specific application and with certain assumptions on the properties of material being fed to the screen. Normally, one presumes that the sizing of material being presented to the screen will be of a regular size and consistency.
In designing a crushing plant, one should remember that the most important design criteria include the following:
- There has to be enough crushing power to reduce the material through a given screen opening.
- The screening device has to have sufficient area and efficiency to get this material out of the plant, particularly in the case of a closed-circuit operation.
- The horsepower per ton injected to the crusher feed will have an effect on the circulating load generated in closed-circuit plants.
- If the circulating load is too high for closed circuiting the final stage of crushing, an additional stage might be designed into the flowsheet.
There are many plants operating today where design criteria have set impossible goals for the equipment selected. In the case of vibrating screens, often the operators have been forced to open up the screen aperture because they could not hold the circulating load and, in some cases, the screen was undersized in the first place for the plant design capacity. Plant designers should be aware that there is a crushing-limit size in closed-circuit plants that occurs when the circulating load tends to infinity. This is a condition where the sizing unit removes less material than is being fed into the circuit. One such condition is when there is 50% oversize in the crusher product and the sizing efficiency is down to 50%. Operating data and product size distributions for various cone crusher installations are listed in Tables 29 and 30, respectively.
The plant design engineer makes decisions on the flowsheet concerning the number of stages of crushing, the position of screens, surge capacities, conveyors, and other process equipment, based on his knowledge of the feed material, product specifications, and his appreciation of the art. Examples of crusher-plant flowsheets are shown in Figs. 58, 59, and 60.
Feed and Product Data
Typical size distribution data for copper and iron ores before and after crushing are tabulated in Tables 31 and 32, respectively. The size-distribution curves for new feed, crusher product before screening, and crusher product after screening for copper and iron ores are shown in Figs. 61 and 62. In both ores, crushing was accomplished using Gyradisc crushers.
Operating Conditions
For changes in the crusher setting, and therefore the reduction ratio, there will be a corresponding change in the crusher throughput and usually a change in the crusher power draft. In many crushing plants after experimenting with various crushing chambers, it was concluded that plant productivity only increases if a new crushing chamber generates a higher average power draft. Most reported that circulating loads in closed-circuit operations change with different settings and chamber configurations, but for constant power input, there seems to be a fixed quantity of screen undersize that is produced.
The power that will be drawn by a crusher will be variable and most affected by feed variations. To accomodate this, a high breakdown torque should be specified. For accurate specifications the crusher manufacturer should be approached. Even if a higher horsepower motor is bought than is specified for the machine, the crusher should not be operated above the manufacturer’s limit. On most crushers smaller than 5½-ft size, the drive from the motor is via V-belts to a sheave on the pinion-shaft. For larger crushers, the drive is direct mounted using a lower speed motor. This saves space and sometimes permits drive maintenance.
Automatic Control
Sensors are normally used to monitor the power drawn by the main drive motor on the crusher. If the crusher is not working according to preset conditions, an adjustment is made to one or both of the following: (1) the feed rate is increased or decreased or (2) the crusher setting is changed. Crushers operated in the automatic mode usually show a significant increase in throughput at a given product
size. With feed-rate control, the tonnage increase might be 10 to 20% and with automatic-setting control 20 to 50% increase over manually controlled crushers. In the case of setting control and, to a lesser extent, feed-rate control, automatic control also provides protection against overloading the crusher.
Some plants are being designed so that automation is applied to the crushing-plant system in its entirety. A few of these systems used
the mathematical modeling approach and have achieved varying degrees of success in controlling plant output. The variables in a crushing and screening plant are numerous, and in many cases, it may be difficult to define adequately the complex interrelationships between the process variables. Crusher-plant automation is an area where studies should be concentrated for future minimization of capital and operating costs.
