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gyratory crusher - an overview | sciencedirect topics

gyratory crusher - an overview | sciencedirect topics

Gyratory crushers were invented by Charles Brown in 1877 and developed by Gates around 1881 and were referred to as a Gates crusher [1]. The smaller form is described as a cone crusher. The larger crushers are normally known as primary crushers as they are designed to receive run-on-mine (ROM) rocks directly from the mines. The gyratory crushers crush to reduce the size by a maximum of about one-tenth its size. Usually, metallurgical operations require greater size reduction; hence, the products from the primary crushers are conveyed to secondary or cone crushers where further reduction in size takes place. Here, the maximum reduction ratio is about 8:1. In some cases, installation of a tertiary crusher is required where the maximum reduction is about 10:1. The secondary crushers are also designed on the principle of gyratory crushing, but the construction details vary.

Similar to jaw crushers, the mechanism of size reduction in gyratory crushers is primarily by the compressive action of two pieces of steel against the rock. As the distance between the two plates decreases continuous size reduction takes place. Gyratory crushers tolerate a variety of shapes of feed particles, including slabby rock, which are not readily accepted in jaw crushers because of the shape of the feed opening.

The gyratory crusher shown in Figure 2.6 employs a crushing head, in the form of a truncated cone, mounted on a shaft, the upper end of which is held in a flexible bearing, whilst the lower end is driven eccentrically so as to describe a circle. The crushing action takes place round the whole of the cone and, since the maximum movement is at the bottom, the characteristics of the machine are similar to those of the Stag crusher. As the crusher is continuous in action, the fluctuations in the stresses are smaller than in jaw crushers and the power consumption is lower. This unit has a large capacity per unit area of grinding surface, particularly if it is used to produce a small size reduction. It does not, however, take such a large size of feed as a jaw crusher, although it gives a rather finer and more uniform product. Because the capital cost is high, the crusher is suitable only where large quantities of material are to be handled.

However, the gyratory crusher is sensitive to jamming if it is fed with a sticky or moist product loaded with fines. This inconvenience is less sensitive with a single-effect jaw crusher because mutual sliding of grinding surfaces promotes the release of a product that adheres to surfaces.

The profile of active surfaces could be curved and studied as a function of the product in a way to allow for work performed at a constant volume and, as a result, a higher reduction ratio that could reach 20. Inversely, at a given reduction ratio, effective streamlining could increase the capacity by 30%.

Maintenance of the wear components in both gyratory and cone crushers is one of the major operating costs. Wear monitoring is possible using a Faro Arm (Figure 6.10), which is a portable coordinate measurement machine. Ultrasonic profiling is also used. A more advanced system using a laser scanner tool to profile the mantle and concave produces a 3D image of the crushing chamber (Erikson, 2014). Some of the benefits of the liner profiling systems include: improved prediction of mantle and concave liner replacement; identifying asymmetric and high wear areas; measurement of open and closed side settings; and quantifying wear life with competing liner alloys.

Crushers are widely used as a primary stage to produce the particulate product finer than about 50100mm. They are classified as jaw, gyratory, and cone crushers based on compression, cutter mill based on shear, and hammer crusher based on impact.

A jaw crusher consists essentially of two crushing plates, inclined to each other forming a horizontal opening by their lower borders. Material is crushed between a fixed and a movable plate by reciprocating pressure until the crushed product becomes small enough to pass through the gap between the crushing plates. Jaw crushers find a wide application for brittle materials. For example, they are used for comminution of porous copper cake. A Fritsch jaw crusher with maximal feed size 95mm, final fineness (depends on gap setting) 0.315mm, and maximal continuous throughput 250Kg/h is shown in Fig. 2.8.

A gyratory crusher includes a solid cone set on a revolving shaft and placed within a hollow body, which has conical or vertical sloping sides. Material is crushed when the crushing surfaces approach each other and the crushed products fall through the discharging opening.

Hammer crushers are used either as a one-step primary crusher or as a secondary crusher for products from a primary crusher. They are widely used for crushing hard metal scrap for different hard metal recycling processes. Pivoted hammers are pendulous, mounted on the horizontal axes symmetrically located along the perimeter of a rotor. Crushing takes place by the impact of material pieces with the high speed moving hammers and by contact with breaker plates. A cylindrical grating or screen is placed beneath the rotor. Materials are reduced to a size small enough to pass through the openings of the grating or screen. The size of the product can be regulated by changing the spacing of the grate bars or the opening of the screen.

The feature of the hammer crushers is the appearance of elevated pressure of air in the discharging unit of the crusher and underpressure in the zone around the shaft close to the inside surface of the body side walls. Thus, the hammer crushers also act as high-pressure, forced-draught fans. This may lead to environmental pollution and product losses in fine powder fractions. A design for a hammer crusher (Fig. 2.9) essentially allows a decrease of the elevated pressure of air in the crusher discharging unit [5]. The A-zone beneath the screen is communicated through the hollow ribs and openings in the body side walls with the B-zone around the shaft close to the inside surface of body side walls. As a result, the circulation of suspended matter in the gas between A and B zones is established and the high pressure of air in the discharging unit of crusher is reduced.

Crushers are widely used as a primary stage to produce the particulate product finer than about 50100 mm in size. They are classified as jaw, gyratory and cone crushers based on compression, cutter mill based on shear and hammer crusher based on impact.

A jaw crusher consists essentially of two crushing plates, inclined to each other forming a horizontal opening by their lower borders. Material is crushed between a fixed and a movable plate by reciprocating pressure until the crushed product becomes small enough to pass through the gap between the crushing plates. Jaw crushers find a wide application for brittle materials. For example, they are used for comminution of porous copper cake.

A gyratory crusher includes a solid cone set on a revolving shaft and placed within a hollow body, which has conical or vertical sloping sides. Material is crushed when the crushing surfaces approach each other and the crushed products fall through the discharging opening.

Hammer crushers are used either as a one-step primary crusher or as a secondary crusher for products from a primary crusher. They are widely used for crushing of hard metal scrap for different hard metal recycling processes.

Pivoted hammers are pendulous, mounted on the horizontal axes symmetrically located along the perimeter of a rotor and crushing takes place by the impact of material pieces with the high speed moving hammers and by contact with breaker plates. A cylindrical grating or screen is placed beneath the rotor. Materials are reduced to a size small enough pass through the openings of the grating or screen. The size of product can be regulated by changing the spacing of the grate bars or the opening of the screen.

The feature of the hammer crushers is the appearance of elevated pressure of air in the discharging unit of the crusher and underpressure in the zone around of the shaft close to the inside surface of the body side walls. Thus, the hammer crushers also act as high-pressure forced-draught fans. This may lead to environmental pollution and product losses in fine powder fractions.

A design for a hammer crusher (Figure 2.6) allows essentially a decrease of the elevated pressure of air in the crusher discharging unit [5]. The A-zone beneath the screen is communicated through the hollow ribs and openings in the body side walls with the B-zone around the shaft close to the inside surface of body side walls. As a result, circulation of suspended matter in the gas between A- and B-zones is established and high pressure of air in the discharging unit of crusher is reduced.

Jaw crushers are mainly used as primary crushers to produce material that can be transported by belt conveyors to the next crushing stages. The crushing process takes place between a fixed jaw and a moving jaw. The moving jaw dies are mounted on a pitman that has a reciprocating motion. The jaw dies must be replaced regularly due to wear. Figure 8.1 shows two basic types of jaw crushers: single toggle and double toggle. In the single toggle jaw crusher, an eccentric shaft is installed on the top of the crusher. Shaft rotation causes, along with the toggle plate, a compressive action of the moving jaw. A double toggle crusher has, basically, two shafts and two toggle plates. The first shaft is a pivoting shaft on the top of the crusher, while the other is an eccentric shaft that drives both toggle plates. The moving jaw has a pure reciprocating motion toward the fixed jaw. The crushing force is doubled compared to single toggle crushers and it can crush very hard ores. The jaw crusher is reliable and robust and therefore quite popular in primary crushing plants. The capacity of jaw crushers is limited, so they are typically used for small or medium projects up to approximately 1600t/h. Vibrating screens are often placed ahead of the jaw crushers to remove undersize material, or scalp the feed, and thereby increase the capacity of the primary crushing operation.

Both cone and gyratory crushers, as shown in Figure 8.2, have an oscillating shaft. The material is crushed in a crushing cavity, between an external fixed element (bowl liner) and an internal moving element (mantle) mounted on the oscillating shaft assembly. An eccentric shaft rotated by a gear and pinion produces the oscillating movement of the main shaft. The eccentricity causes the cone head to oscillate between the open side setting (o.s.s.) and closed side setting (c.s.s.). In addition to c.s.s., eccentricity is one of the major factors that determine the capacity of gyratory and cone crushers. The fragmentation of the material results from the continuous compression that takes place between the mantle and bowl liners. An additional crushing effect occurs between the compressed particles, resulting in less wear of the liners. This is also called interparticle crushing. The gyratory crushers are equipped with a hydraulic setting adjustment system, which adjusts c.s.s. and thus affects product size distribution. Depending on cone type, the c.s.s. setting can be adjusted in two ways. The first way is by rotating the bowl against the threads so that the vertical position of the outer wear part (concave) is changed. One advantage of this adjustment type is that the liners wear more evenly. Another principle of setting adjustment is by lifting/lowering the main shaft. An advantage of this is that adjustment can be done continuously under load. To optimize operating costs and improve the product shape, as a rule of thumb, it is recommended that cones always be choke-fed, meaning that the cavity should be as full of rock material as possible. This can be easily achieved by using a stockpile or a silo to regulate the inevitable fluctuation of feed material flow. Level monitoring devices that detect the maximum and minimum levels of the material are used to start and stop the feed of material to the crusher as needed.

Primary gyratory crushers are used in the primary crushing stage. Compared to the cone type crusher, a gyratory crusher has a crushing chamber designed to accept feed material of a relatively large size in relation to the mantle diameter. The primary gyratory crusher offers high capacity thanks to its generously dimensioned circular discharge opening (which provides a much larger area than that of the jaw crusher) and the continuous operation principle (while the reciprocating motion of the jaw crusher produces a batch crushing action). The gyratory crusher has capacities starting from 1200 to above 5000t/h. To have a feed opening corresponding to that of a jaw crusher, the primary gyratory crusher must be much taller and heavier. Therefore, primary gyratories require quite a massive foundation.

The cone crusher is a modified gyratory crusher. The essential difference is that the shorter spindle of the cone crusher is not suspended, as in the gyratory, but is supported in a curved, universal bearing below the gyratory head or cone (Figure 8.2). Power is transmitted from the source to the countershaft to a V-belt or direct drive. The countershaft has a bevel pinion pressed and keyed to it and drives the gear on the eccentric assembly. The eccentric assembly has a tapered, offset bore and provides the means whereby the head and main shaft follow an eccentric path during each cycle of rotation. Cone crushers are used for intermediate and fine crushing after primary crushing. The key factor for the performance of a cone type secondary crusher is the profile of the crushing chamber or cavity. Therefore, there is normally a range of standard cavities available for each crusher, to allow selection of the appropriate cavity for the feed material in question.

Depending on the size of the debris, it may either be ready to enter the recycling process or need to be broken down to obtain a product with workable particle sizes, in which case hydraulic breakers mounted on tracked or wheeled excavators are used. In either case, manual sorting of large pieces of steel, wood, plastics and paper may be required, to minimise the degree of contamination of the final product.

The three types of crushers most commonly used for crushing CDW materials are the jaw crusher, the impact crusher and the gyratory crusher (Figure 4.4). A jaw crusher consists of two plates, with one oscillating back and forth against the other at a fixed angle (Figure 4.4(a)) and it is the most widely used in primary crushing stages (Behera etal., 2014). The jaw crusher can withstand large and hard-to-break pieces of reinforced concrete, which would probably cause the other crushing machines to break down. Therefore, the material is initially reduced in jaw crushers before going through any other crushing operation. The particle size reduction depends on the maximum and minimum size of the gap at the plates (Hansen, 2004).

An impact crusher breaks the CDW materials by striking them with a high-speed rotating impact, which imparts a shearing force on the debris (Figure 4.4(b)). Upon reaching the rotor, the debris is caught by steel teeth or hard blades attached to the rotor. These hurl the materials against the breaker plate, smashing them into smaller particle sizes. Impact crushers provide better grain-size distribution of RA for road construction purposes, and they are less sensitive to material that cannot be crushed, such as steel reinforcement.

Generally, jaw and impact crushers exhibit a large reduction factor, defined as the ratio of the particle size of the input to that of the output material. A jaw crusher crushes only a small proportion of the original aggregate particles but an impact crusher crushes mortar and aggregate particles alike and thus generates a higher amount of fine material (OMahony, 1990).

Gyratory crushers work on the same principle as cone crushers (Figure 4.4(c)). These have a gyratory motion driven by an eccentric wheel. These machines will not accept materials with a large particle size and therefore only jaw or impact crushers should be considered as primary crushers. Gyratory and cone crushers are likely to become jammed by fragments that are too large or too heavy. It is recommended that wood and steel be removed as much as possible before dumping CDW into these crushers. Gyratory and cone crushers have advantages such as relatively low energy consumption, a reasonable amount of control over the particle size of the material and production of low amounts of fine particles (Hansen, 2004).

For better control of the aggregate particle size distribution, it is recommended that the CDW should be processed in at least two crushing stages. First, the demolition methodologies used on-site should be able to reduce individual pieces of debris to a size that the primary crusher in the recycling plant can take. This size depends on the opening feed of the primary crusher, which is normally bigger for large stationary plants than for mobile plants. Therefore, the recycling of CDW materials requires careful planning and communication between all parties involved.

A large proportion of the product from the primary crusher can result in small granules with a particle size distribution that may not satisfy the requirements laid down by the customer after having gone through the other crushing stages. Therefore, it should be possible to adjust the opening feed size of the primary crusher, implying that the secondary crusher should have a relatively large capacity. This will allow maximisation of coarse RA production (e.g., the feed size of the primary crusher should be set to reduce material to the largest size that will fit the secondary crusher).

The choice of using multiple crushing stages mainly depends on the desired quality of the final product and the ratio of the amounts of coarse and fine fractions (Yanagi etal., 1998; Nagataki and Iida, 2001; Nagataki etal., 2004; Dosho etal., 1998; Gokce etal., 2011). When recycling concrete, a greater number of crushing processes produces a more spherical material with lower adhered mortar content (Pedro etal., 2015), thus providing a superior quality of material to work with (Lotfi etal., 2017). However, the use of several crushing stages has some negative consequences as well; in addition to costing more, the final product may contain a greater proportion of finer fractions, which may not always be a suitable material.

The first step of physical beneficiation is crushing and grinding the iron ore to its liberation size, the maximum size where individual particles of gangue are separated from the iron minerals. A flow sheet of a typical iron ore crushing and grinding circuit is shown in Figure 1.2.2 (based on Ref. [4]). This type of flow sheet is usually followed when the crude ore contains below 30% iron. The number of steps involved in crushing and grinding depends on various factors such as the hardness of the ore and the level of impurities present [5].

Jaw and gyratory crushers are used for initial size reduction to convert big rocks into small stones. This is generally followed by a cone crusher. A combination of rod mill and ball mills are then used if the ore must be ground below 325 mesh (45m). Instead of grinding the ore dry, slurry is used as feed for rod or ball mills, to avoid dusting. Oversize and undersize materials are separated using a screen; oversize material goes back for further grinding.

Typically, silica is the main gangue mineral that needs to be separated. Iron ore with high-silica content (more than about 2%) is not considered an acceptable feed for most DR processes. This is due to limitations not in the DR process itself, but the usual customer, an EAF steelmaking shop. EAFs are not designed to handle the large amounts of slag that result from using low-grade iron ores, which makes the BF a better choice in this situation. Besides silica, phosphorus, sulfur, and manganese are other impurities that are not desirable in the product and are removed from the crude ore, if economically and technically feasible.

Beneficiation of copper ores is done almost exclusively by selective froth flotation. Flotation entails first attaching fine copper mineral particles to bubbles rising through an orewater pulp and, second, collecting the copper minerals at the top of the pulp as a briefly stable mineralwaterair froth. Noncopper minerals do not attach to the rising bubbles; they are discarded as tailings. The selectivity of the process is controlled by chemical reagents added to the pulp. The process is continuous and it is done on a large scale103 to 105 tonnes of ore feed per day.

Beneficiation is begun with crushing and wet-grinding the ore to typically 10100m. This ensures that the copper mineral grains are for the most part liberated from the worthless minerals. This comminution is carried out with gyratory crushers and rotary grinding mills. The grinding is usually done with hard ore pieces or hard steel balls, sometimes both. The product of crushing and grinding is a waterparticle pulp, comprising 35% solids.

Flotation is done immediately after grindingin fact, some flotation reagents are added to the grinding mills to ensure good mixing and a lengthy conditioning period. The flotation is done in large (10100m3) cells whose principal functions are to provide: clouds of air bubbles to which the copper minerals of the pulp attach; a means of overflowing the resulting bubblecopper mineral froth; and a means of underflowing the unfloated material into the next cell or to the waste tailings area.

Selective attachment of the copper minerals to the rising air bubbles is obtained by coating the particles with a monolayer of collector molecules. These molecules usually have a sulfur atom at one end and a hydrophobic hydrocarbon tail at the other (e.g., potassium amyl xanthate). Other important reagents are: (i) frothers (usually long-chain alcohols) which give a strong but temporary froth; and (ii) depressants (e.g., CaO, NaCN), which prevent noncopper minerals from floating.

crush the curve | coronavirus (covid-19) testing in idaho

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crushing products size and shape -what to expect

crushing products size and shape -what to expect

I have madea number of general remarks regarding the character of product delivered by crushers of various types, and under different conditions of operation. Generalities are of value only if we have some standard to which comparisons may be referred; therefore, we should like to present more specific information on the kind of product to be expected from crushing equipment under average operating conditions. Much of the data on which sizing/designcurves and tables are based comes from operations involving those two very important types: gyratory and jaw crushers; therefore these curves and tables are more nearly representative of the work of these types than of rolls or hammermills. They may be used for these latter types however if due allowance is made for peculiarities of each type, as pointed out in the descriptions of the different machines.

The preparation of a set of product gradation curves involves a considerable amount of work in the collection of the necessary test data, and a certain degree of discrimination in sorting such data and weeding out erroneous results. There are several reasons why no set of product gradation curves can be regarded as more than reasonably close approximations. First among these is the variation in physical structure of the many materials for which crushers are used; rocks exhibit a high degree of rugged individualism in their reaction to crushing. This variation is frequently quite pronounced between different ledges in the same quarry.

Gradation of the crusher feed also has its effect upon the product analysis. This is true even of screened feed, although deviations from the average are not likely to be so wide as they are for unscreened material, such as quarry-run or mine-run rock. We have commented on other variable factors, such as choke versus regulated feed, straight versus curved concaves, and so forth.

Fortunately, most materials do follow a certain definite gradation pattern and, by averaging a large number of test results, it is possible to plot a group of curves which can be classed as fairly close approximations. Even though approximate, these curves are of great value in crushing-plant design, or in the solution of problems concerning additions or alterations in the plant flowsheet. They simplify the problem of selecting secondary and tertiary crushers, as well as elevating and conveying equipment, and they are invaluable in the calculation of screen sizes. In short, they eliminate much of the old-time guess work in the preparation of the plant flowsheet.

Gyratory and jaw crushers are always rated at certain open-side or close-side discharge settings. In order that we may select the particular curve, of a group of curves, which will most nearly represent the product of a crusher having any given discharge setting, it is important to know approximately what percentage of the total output will pass a screen opening of equal dimension. It was universal practice in past years to designate such screen openings as ring-size for the very logical reason that the leading screen of that day, the revolving type, was, almost without exception, fitted with sections having round holes. Now that the vibrating screen, with its wire cloth or square-punched steel plate sections, has pre-empted the field there is no longer any excuse for adhering to the ring-size product designation.Above is alist of the approximate percentages of product passing a square opening test sieve whose holes are equal to the discharge setting of the crusher. Several different conditions are tabulated, and each condition is accompanied by estimates for four different classes of material.

In gravel pit operations it will usually be found that some one of these listed base rocks will predominate, and no great error will be introduced if this predominant rock is used as the basis for product calculations. Most base rocks will be close enough in physical structure to one of the listed varieties so that the percentages can be used for them without serious error. The same statement applies to the product gradation curves to be discussed. It must be remembered that the entire process of securing and compiling data of this nature is, at best, one which is susceptible of only approximate results.

It was formerly the custom to consider one set of product gradation, or screen analysis, curves as being suitable to represent the products of both primary (unscreened) and secondary (screened) feeds, making no allowance for the undersize material which is always present, to some extent, in quarry-run and mine-run materials. The average quarry does not produce as much of this undersize rock as the average mine, but the usual practice in mining operations is to scalp off most of the undersize ahead of the primary crusher, whereas this practice is the exception rather than the rule in quarry operations. As a matter of fact, where the secondary crushers are fitted with straight concaves, or jaw plates, as used to be standard practice, the dif-ference between product curves on screened and unscreened feed was not significant, and no great discrepancy was introduced by considering them under the one heading.

With the introduction of non-choking concaves in the standard gyratory crushers and reduction crushers, and the development of high speed fine-reduction crushers with high choke points, it soon became apparent that there was a substantial difference in the screen analyses of the two kinds of product, that is, crusher products on unscreened and screened feeds. The difference is especially significant in the lower part of the curve, where undersize in the feed would naturally show up, and where the cleaner breaking of the non-choke crushing chamber would likewise be reflected.

Here above isshown a family of curves for primary crushing of unscreened feed, such as the average quarry-run material in which the undersize (minus crusher setting) rock is present in proportions normally resulting from blasting operations. The same curves may be used for mining operations with stationary bar grizzlies ahead of the primary crusher.

In such operations the amount of undersize going into the crusher will usually be about the same as for the quarry operation without pre-scalping. It should be noted that the test data on which these curves are based were taken from gyratory and jaw crusher operations, but, as we have stated before, they may be used for other types of crushers if allowance is made for the characteristics peculiar to each type. As a matter of fact, so far as crushers of the Fairmount single-roll type are concerned, there is a natural compensation which brings the curves fairly well into line. The Fairmount crusher is inherently a somewhat cleaner breaking machine than either the standard gyratory or standard jaw types, but the class of rock for which the former crusher is largely used is usually subject to greater than average degradation during the blasting and loading operations in the quarry, which tends to level out the difference in crushing performance.Using Crusher and Screen Charts

The method of using the curves is so simple as to require little comment. The vertical axes represent material sizes, which may be taken as either square or round openings; provided of course that the same shape of opening is used throughout any particular analysis. The horizontal axes represent cmmdative percentages passing corresponding screen openings. If we wish to check the product to be expected from a crusher set at some predetermined discharge opening, we first refer to the table showing the approximate percentage of product which will pass an opening equivalent to the crusher setting. This gives us a point in the group of curves which may, or may not, be exactly on one of them. In the latter case we interpolate by following an imaginary curve between the two curves on either side of our point. We can thus tabulate cumulative percentages passing all of the product sizes in which we may be interested. Non-cumulative percentages; which are important because they are used to determine expected amounts of specific products are simply the difference between the upper and lower cumulative percentages for the particular product limits under consideration.

For those not familiar with the use of product gradation curves an example may be helpful. Suppose that a tentative selection of a 3.5 open- side discharge setting has been made for a standard gyratory primary crusher to be used for crushing quarry-run limestone. Referring to the table which lists percentages of product passing an equivalent square opening, we find that 85 to 90% of the crusher product should pass a 3.5 square opening. Choosing the lower percentage, to be on the conservative side,, we follow the horizontal line, denoting the 3.5 product size in the curve chart, over to the vertical line marking the 85% value. We find that the point we have established does not fall directly upon any of the group of curves, but lies so close to one of them that it may be used without appreciable error into our calculations.

Let us suppose that we wish to know how much of the product of our primary crusher will be retained on a 1.5 square opening screen, so that we may estimate the size and number of secondary crushers required to recrush the plus 1.5 contingent. Following the curve down to the 1.5 line, we find that 43% of the primary crusher output may be expected to pass this screen opening; 57% will be retained, which means that we must provide secondary crushing capacity to take care of 57 tons for each 100 tons fed to the primary crusher.

Occasionally it happens that we wish to scalp off a salable product from the output of the primary crusher; for example, a plus 1.5 minus 3.5 material for highway base- rock. The difference between the cumulative percentages at the 3.5 and 1.5 points on the curve gives us the amount, of such product to be expected from the output of the primary crusher This is 85 minus 43, or 42% of the primary crusher product.

If our problem had covered a crushing condition calling for 80 instead of 85%passing the opening equivalent to the crusher setting, we would have found that our point fell exactly on a curve, regardless of what crusher setting we had selected. This is because all of the family of curves are based on the 80% line. Obviously a group of curves might be based on any percentage line, but it is usual practice to choose the 80 or 85% values.

It will be noted that the curves bend upward in very marked fashion above the 75-85% region. This simply reflects the tendency of practically all materials to slab, or spall, to some extent in the crusher. As a matter of fact, product gradation in this upper range (above the open- side setting of the crusher) is of a distinctly uncertain and variable nature, and about all that a group of curves can do is to reflect the general tendency. Fortunately the exact screen analysis in this fraction of the primary crusher output is recrushed in succeeding stages, and all that is required is to know approximately how much of it there will be to recrush.

Although the group of curves we have been considering are intended for calulations involving primary crushing operations, they may also be used for secondary crusher products in those cases where no screening is performed between primary and secondary stages. Such an arrangement is seldom encountered in modern plant design, except where large jaw crushers, set very wide, are followed by a secondary, usually of the standard gyratory type, to reduce further the very coarse output of the jaw crusher to a size which can be handled by the recrushing, screening, and elevating equipment in the balance of the plant. In such cases it is simplest to consider the two-stage set-up as a single machine with discharge opening equal to that of the secondary crusher.

The group of curves on the rightischarted from screen analyses of the products of crushers receiving screened feed. They are useful in predicting the character of output from secondary and tertiary crushers, and are of great value in the preparation of plant flowsheets, and in calculating vibrating screen capacities. Their use in the latter connection will be discussed in the screening section of this series.

There is no need for extended comment on this group of curves; the method of taking off cumulative percentages, and non-cumulative fractions, is exactly the same as for the chart we previously discussed. The difference in the shape of these curves is attributable to the absence of fines in the crusher feed, and to the cleaner breaking action of the modem reduction crusher.

The product gradation curves for screened feed, described under the preceding sub-heading, can be used as a basis for calculating approximate screen analysis of products from closed-circuit crushing stages, but the values cannot be taken directly from the curves.

For example, consider a crusher set to turn out a product 70% of which will pass a 5/8 square opening, and in closed circuit with a screen which is equipped to remove the minus 3/4 product. Thecurve shows that approximately 85% of the crusher product will pass the 3/4 square openings.

Suppose that we wish to know how much minus 0.25 fines we may expect from the circuit.We do not go to the curve which touches the 100 percent ordinate at the 3/4 value; we calculate the percentage from the same curve which was used to predict the proportion of minus 0.75 in the crusher discharge. This curve shows approximately 29 percent of minus 3/4 in the material as it comes from the crusher, or 29 tons of fines in each 100 tons of crusher output. But, for the circulating load, we are only interested in that fraction of the crusher output which will pass the 3/4 screen, which is 85 tons.That part of the product gradation curve which lies below the 85 percent valuerepresents the gradation of the finished product, and 29 tons out of each 85 would be minus 0.25.

Let x equal percentage of minus 0.25 in the finished product, then x:100=29:85 or x = 34.1 percent of minus 0.25 rock from the closed circuit operation. Any other size of product may be estimated in a similar manner. Note that if we had used a curve touching the 100 percent ordinate at the 0.75 value, we would have arrived at a value approximately 50 percent for the minus 0.25 fraction; a value which is obviously erroneous for rock of average characteristics. We will comment on closed circuit crushing, and upon certain assumptions which have to be made in closed circuit calculations, in a later discussion of reduction-crushing.

Although the long established practice of designating crusher products by ring-size is not compatible with present-day screening practice, there are occasions when it is desirable to convert our calculations from one shape of opening to the other. So far as the curves themselves are concerned, once we have established the shape of screen openinground or squarewe can use them for either so long as we stick to one shape throughout the process of taking off percentages-passing. If, as occasionally happens, we have to deal with both shapes of screen opening in the same set of calculations, one or the other of them must be converted to equivalent sizes of the opposing shape. For example, if most of the screen openings are to be square, but one or two of them must be round, the round-hole sizes should be expressed in terms of equivalent square openings.

Inasmuch as the table of crusher settings versus equivalent product percentages is based on square openings, it is necessary to convert to equivalent round openings before this table can be used for such openings.

Below is the information needed to make conversions from round to square holes, or vice versa. The two columns at the left showing equivalent sizes for flat testing screens, are the columns to use in connection with crusher product calculations.Admittedly, listings of equivalent round and square holes, such as we show in this table, can be only approximately correct for the many different materials with which we must deal in crushing and screening computations. The infinite variety of shapes encountered renders absolute accuracy an impossible attainment. Practical experience, however, indicates that the comparisons shown in our table are in most cases close enough for all practical purposes.

Product SizeCorresponding Size Holes Through a flat testing screen Allis-Chalmers vibrating screenRevolving Screen Round holes Square holesRound holes Square holesRound holes 1/83/325/321/85/32 3/83/327/323/161/4 1/43/149/321/41/16 1/21/411/321/123/8 3/83/107/163/81/2 1/43/81/23/163/18 1/21/101/41/25/8 3/21/25/81/1811/10 3/82/1011/106/83/4 11/105/83/411/107/8 3/411/107/83/41 7/83/415/187/81 1/8 17/81 1/1612/101 1/4 1 3/811 2/181 1/181 3/8 1 1/41 1/161 3/81 1/71 2/14 1 3/81 1/81 1/161 1/41 3/4 1 1/21 1/41 3/181 3/81 7/8 1 5/81 3/81 3/41 3/102 1 3/41 1/21 7/81 3/162 1/4 1 7/81 5/821 3/42 3/8 21 3/42 1/81 7/82 1/2 2 1/81 7/82 1/422 5/8 2 1/41 15/182 3/82 1/162 3/4 2 3/822 1/22 1/82 11/16 2 1/22 1/82 6/82 1/43 1/8 2 5/82 1/42 3/42 3/83 5/12 2 3/42 3/82 7/82 1/23 1/2 2 7/82 1/232 5/83 5/8 32 5/83 1/42 3/43 3/4 3 1/42 3/43 1/234 3 1/233 3/43 1/44 3/8 3 3/43 1/443 1/24 3/4 43 1/24 1/43 3/45 4 1/23 7/84 3/44 1/85 1/2 54 1/45 1/44 1/26 1/4 5 1/24 3/45 3/456 7/8 65 1/46 1/25 1/27 1/2 6 1/25 1/275 3/48 767 1/26 1/28 3/4 7 1/26 1/2879 3/8 878 3/47 1/210 8 1/27 1/49 1/47 3/410 1/2 97 3/49 1/28 1/411 1/4 9 1/28108 1/211 3/4 108 1/210 1/2912 1/2

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