From the discussion so far it is obvious that two energy factors determine a crusher’s productivity:
(1) Total power drawn affects the quantity produced of any given size or range of sizes.
(2) The energy applied, previously defined as the crusher’s Power Rate (kilowatt hours per ton of feed) will determine the size reduction. This has just been clearly demonstrated by the pendulum tests.
In a previous paper, we have shown that the theoretical forces achieved in a crushing chamber are a direct result of the design geometry and driving power applied to the crusher. Machines from three manufacturers can theoretically apply different crushing forces to processed material, according to Table (1).
The energy intensity within the crushing chamber will differ for various machines. The wide range of eccentric throws and connected powers for various units will give different crushing impacts, which might be considered anologous to the pendulum. The angle of the mantle for the crushing head varies between manufacturers as does the length of the crushing zone from less than 200mm (8 inches) to more than 600mm (24 inches) but as has been indicated from the pendulum size distribution correlations it is the energy input to the feed material which determines the reduction and size distribution. Eccentric throw and chamber shape appear to have little influence.
Note: Some manufacturers do not give all this specification information in sales literature, and many of these measurements here were made in the field on operating equipment. Even if correct in one instance, the specifications could be different for different machines. For accurate data, the person studying such information should refer to the crusher manufacturer.
This information is only published so the reader can get an order of magnitude comparison for setting up his own technical study.
To make the finest product, we should be running the crusher with the lowest practical feed rate and applying the maximum crushing force. This will give the highest power rate but there are physical limits to operating in this mode, especially if the crusher is run at fixed setting. To make a coarse product the power rate should be no greater than that needed to generate that size. The power drawn in any case should be maximized to yield the greatest quantity of the required product.
There has been adequate reporting by people such as Szaj and Lynch that operating crushers at fixed setting most often demands that the average operating power be limited to only 50 or 60% of that connected, to guard against crusher stalling and minimize mechanical damage. Control devices such as those by Lindgren et al have been developed to maximize power drawn and productivity through fixed setting crushers.
The power drawn by a crusher can be changed by varying the feed rate and/or the setting, as shown in Figs. (11) and (12). A small change in setting can bring about a large change in power rate, Fig. (13). Changing feed rate near optimum power draw gives small change to crusher power rate and the reduction ratio remains approximately the same.
The rapid fluctuation in crusher power draw that is normally associated with high power rate operation
makes it desirable to fit a crusher with an automatic setting control system as shown in Fig. 14 to guard against mechanical overloading. At the same time this maximizes the power rate and average power drawn for a particular configuration.
It has been determined that within a cone crushing chamber, machines using automatic setting regulation can normally operate at more than twice the power rate compared to units operating in a fixed setting mode.
The device in Fig. (14) works by constantly trying to reduce the machine’s setting according to a program, thereby maximizing power drawn within preset limits. If the crusher overloads, it opens up according to another program. Feed rate can be another variable controlled within the same system. The combined setting/feed rate system can be appropriately coordinated to give a crusher power rate control, thus effectively determining reduction ratio as will be seen in more detail following.
It has been demonstrated that both capacity and quantities of finer sizes produced, Fig. (15), can be improved substantially by using automatic setting controls in a cone crusher.
It can be seen that the yield of minus 13 mm material for the automated set crusher is approximately twice that for the fixed set machine. Also notice that the per cent minus 13 mm and 9.5 mm in the crushers’ discharge is about 10% higher even though the crushers are operating at close to the same close side setting. One of the main reasons for the differential in this test is the larger eccentric throw in the fixed set unit. The smaller eccentric throw and automatically controlled setting increased power draw to give a much higher crushing force. This is analogous to a large pendulum energy in the laboratory tests.
By adjusting both the feed rate and power together, we can change the power rate and, hence, reduction, as is shown from a field test result, Fig. (16). The automated setting and power drawn was almost the same for both operating conditions. The large difference in reduction was due to the different power rates. Superimposed pendulum results again demonstrate the family curve relationships for the laboratory tests.
The operating conditions for the crushers are diagramatically shown in Figs. (17) and (18).
It is clearly shown that the same energy applied to more material results in a much coarser product. In these tests the percent minus 12.7 mm (½ inch) changed from approximately 95% to 65% passing when the feed rate was doubled (power rate was effectively reduced by half). The automated setting control adjusted the crusher to the same power draw setpoint (320 KW). Because there is an exponential relationship between power drawn and close side setting, there was only a small 1 mm change between the tests. If an application engineer had been using a catalog, he might have computed the size distribution as 60% passing the setting. Clearly there would be a considerable error if this happened.
Existing Crusher Application Techniques
The methods of applying crushing machinery have changed little during the last century.
Crushers are being engineered into flow sheets normally by reference to catalog data supplied from various manufacturers. This information does not make any allowances for hardness or feed size distribution, which have important bearing on crusher capacities and sizes produced. Many catalogs for approximately similar size machines give capacities that are roughly equivalent. There has previously been no published scientific way of comparing this data. The pendulum impact test can be used as a standard for this purpose.
Manufacturers have changed connected power and eccentric throws on various machines with and without published changes to production rates and product size distribution analyses. These changes have occurred over such a time span that most engineers are unaware of their significance. Two cases of commonly used catalogs should be cited to show how general the nature of published information really is.
For many years Allis-Chalmers published catalog information, similar to Table (2), showing various eccentric throws and powers connected to individual machines. Because capacity and reduction ratios changed for the particular throw and chamber configuration, it was up to the application engineer to come up with operating predictions. This was often a complex problem. If automation were applied to maximize the power drawn by the crusher, capacities could be often increased by up to 100% more than the published capacity, as already evidenced by Fig. (15), which shows capacities approximately 40% above the present-day catalog. With automatic setting control applied to a crusher, reduction ratios could also be increased substantially on a given feed size distribution. By employing the present standard catalog format of capacities related to close side set¬ting, no rational tool can be developed to accurately size the equipment.
Probably the most commonly used capacity and size graduation tables are those in the SME-AIME publication, “Mineral Processing Plant Design,” published in 1978. These are given for Rexnord’s “Symons” crusher.
In early 1980, new capacities and installed power were given by the manufacturer for these machines. A 25-year survey of the literature, Table (3), shows changes to power for these units. Capacities for different sized machines also changed during this period.
For most manufacturers, nearly all the capacities quoted give the average size distribution for the discharged product as approximately 60% passing a size equivalent to the close side discharge setting. This is regardless of the eccentric throw or physical size of the machine. Because of such complications, designers and operators are referred to the factory for more accurate capacity and size distribution information. With some operators having reported product coarseness as low as 14% passing the close side setting, and with some only making a net average of 40% passing twice the close side setting on vast tonnages of easy-to-crush materials, the inexact nature of catalog information calls for a more accurate approach to crusher application.
Many engineers and some design organizations use standard published catalog capacities and size distributions in computer plant-design programs. Since most crusher manufacturers prefer people to seek factory advice for accurate application, information in such standard computer design programs can and has resulted in significant errors to plant operating performance.
One such plant, which cost several hundred million dollars to construct, compared actual performance data against design after more than one year’s operation. This information is summarized in Fig. (19). In order to shield the plant’s identity, capacity has been scaled to 100,000 tonnes per day. The numbers and sizes of crushers and screens are not given but have been scaled to the 100,000-tonnes-per-day figure. The plant was originally designed assuming it could make the required mil! feed through 12.7 mm (½ inch) square screens operating 75 percent of total time or 18 hours per day in a 365-day year.
- The plant was conservatively designed assuming a coarser feed material — 10% minus 12.7 mm versus the 47% actual. This was not, however, enough for the plant to make the design tonnage even when operating for a longer availability period. If the fines had been in the design proportion, plant output would have only been approximately:
24 x 2778/0.9 x 0.88 = 65,200
tonnes per day at 88 percent availability or 34,800 tonnes short of design.
- The crushers are operating at lower capacities and at smaller settings than design.
- Closed-circuit screen productivity is approximately half design (31 versus 56 TPH per square meter deck). This is because of a buildup in near size particles just above the screen aperture dimension.
- The availability of the operating plant increases progressively from 80 percent for the secondary crushers to 88 percent for the tertiary screens. These screens are obviously the plant bottleneck, despite the fact that they are operating at a coarser aperture (14.3 mm) than design (12.7 mm).
- The closed-circuit crushers are making a considerably coarser product (45 percent minus 12.7 mm compared to design, 66.5 percent) despite the fact that they are operating at 6.5 mm setting. The reader should remember that most catalog data would give a standard product for this setting as 60 percent passing 6.5 mm and 95 percent passing 12.7 mm. This coarse product is the reason for the bottleneck in this plant and could have been predicted if the facility had been designed using crusher power rates to calculate reduction ratios.
The crushers in this plant were automated using variable feed rate to control power drawn at a fixed close side setting. First-stage reduction crushers were running at 60 percent average connected power and the closed-circuit crushers were run at 95 percent average connected power. The equipment therefore was running near maximum capability. Obviously we need better design methods for crushing and screening plants if such errors are not to be repeated.
New Design Methods
By using automatic setting and feed rate controls on a crusher to maximize power drawn and optimize the power rate at each stage, significant improvements can be incorporated into any crushing circuit design.
In a recent paper 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 is shown in Fig. (20). Obviously the automated plant in Case B will cost much less to construct than Case A, so automation is an economic necessity.
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.