Has Anyone Crushed a Stone in Their Feed Mill

Roll Crushers

A. Gupta , D.S. Yan , in Mineral Processing Design and Operation, 2006

6.1.3 Roll Crusher Circuit Design

Roll crushers are generally not used as primary crushers for hard ores. Even for softer ores, like chalcocite and chalcopyrite they have been used as secondary crushers. Choke feeding is not advisable as it tends to produce particles of irregular size. Both open and closed circuit crushing are employed. For close circuit the product is screened with a mesh size much less than the set.

Fig. 6.4 is a typical set up where ore crushed in primary and secondary crushers are further reduced in size by a rough roll crusher in open circuit followed by finer size reduction in a closed circuit by roll crusher. Such circuits are chosen as the feed size to standard roll crushers normally do not exceed 50   mm.

Fig. 6.4. Roll Crusher Design Circuit.

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Gyratory and Cone Crusher

A. Gupta , D.S. Yan , in Mineral Processing Design and Operation, 2006

5.1.2 Secondary and Tertiary cone crushers

Cone crushers were originally designed and developed by Symons around 1920 and therefore are often described as Symons cone crushers. As the mechanism of crushing in these crushers are similar to gyratory crushers their designs are similar, but in this case the spindle is supported at the bottom of the gyrating cone instead of being suspended as in larger gyratory crushers. Fig. 5.3 is a schematic diagram of a cone crusher. The breaking head gyrates inside an inverted truncated cone. These crushers are designed so that the head to depth ratio is larger than the standard gyratory crusher and the cone angles are much flatter and the slope of the mantle and the concaves are parallel to each other. The flatter cone angles helps to retain the particles longer between the crushing surfaces and therefore produce much finer particles. To prevent damage to the crushing surfaces, the concave or shell of the crushers are held in place by strong springs or hydraulics which yield to permit uncrushable tramp material to pass through.

Fig. 5.3. Sketch of a secondary cone crusher.

The secondary crushers are designated as Standard cone crushers having stepped liners and tertiary Short Head cone crushers, which have smoother crushing faces and steeper cone angles of the breaking head. The approximate distance of the annular space at the discharge end designates the size of the cone crushers. A brief summary of the design characteristics is given in Table 5.4 for crusher operation in open circuit and closed circuit situations.

Table 5.4. Design characteristics of Standard Symons cone crushers [4].

Design Characteristics	Open Circuit		Closed Circuit
	Maximum	Minimum	Maximum	Minimum
Size, mm	3050	600	3050	600
Crasher chamber size range, mm *	76-432	25-76	76-178	25-51
Discharge setting (closed side)	22-38.1	6.4-15.8	6.4-19	3.2
Power kW	300-500	25-30	300-500	25-30

*

Chamber sizes vary between 3-6 numbers within a particular designated crusher size to produce fine, medium or coarse sized product.

The Standard cone crushers are for normal use. The Short Head cone crushers are designed for tertiary or quaternary crushing where finer product is required. These crushers are invariably operated in closed circuit. The final product sizes are fine, medium or coarse depending on the closed set spacing, the configuration of the crushing chamber and classifier performance, which is always installed in parallel.

For finer product sizes, i.e. less than 6   mm, special cone crushers known as Gyradisc crushers are available. The operation is similar to the standard cone crushers except that the size reduction is caused more by attrition than by impact, [5]. The reduction ratio is around 8:1 and as the product size is relatively small the feed size is limited to less than 50   mm with a nip angle between 25° and 30°. The Gyradisc crushers have head diameters from around 900-2100   mm. These crushers are always operated in choke feed conditions. The feed size is less than 50   mm and therefore the product size is usually less than 6-9   mm.

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Mineral Processing

Swapan Kumar Haldar , in Mineral Exploration (Second Edition), 2018

13.3.1 Crushing

Crushing is accomplished by compression of the ore against a rigid surface or by impact against a surface in a rigidly constrained motion path. Crushing is usually a dry process and carried out on ROM ore in succession of two or three stages, namely, by (1) primary, (2) secondary, and (3) tertiary crushers.

13.3.1.1 Primary Crusher

Primary crushers are heavy-duty rugged machines used to crush ROM ore of (−) 1.5  m size. These large-sized ores are reduced at the primary crushing stage for an output product dimension of 10–20   cm. The common primary crushers are of jaw and gyratory types.

The jaw crusher reduces the size of large rocks by dropping them into a "V"-shaped mouth at the top of the crusher chamber. This is created between one fixed rigid jaw and a pivoting swing jaw set at acute angles to each other. Compression is created by forcing the rock against the stationary plate in the crushing chamber as shown in Fig. 13.9. The opening at the bottom of the jaw plates is adjustable to the desired aperture for product size. The rocks remain in between the jaws until they are small enough to be set free through this opening for further size reduction by feeding to the secondary crusher.

Figure 13.9. Schematic diagram showing principle of jaw crusher showing the path of lumpy feed ore to fragmented product crushed under high pressure of fixed and moving jaws.

The type of jaw crusher depends on input feed and output product size, rock/ore strength, volume of operation, cost, and other related parameters. Heavy-duty primary jaw crushers are installed underground for uniform size reduction before transferring the ore to the main centralized hoisting system. Medium-duty jaw crushers are useful in underground mines with low production (Fig. 13.10) and in process plants. Small-sized jaw crushers (refer to Fig. 7.32) are installed in laboratories for the preparation of representative samples for chemical analysis.

Figure 13.10. Medium-sized jaw crusher in operation in underground mine for crushing run-of-mine (ROM) ore before transferring to the surface.

The gyratory crusher consists of a long, conical, hard steel crushing element suspended from the top. It rotates and sweeps out in a conical path within the round, hard, fixed crushing chamber (Fig. 13.11). The maximum crushing action is created by closing the gap between the hard crushing surface attached to the spindle and the concave fixed liners mounted on the main frame of the crusher. The gap opens and closes by an eccentric drive on the bottom of the spindle that causes the central vertical spindle to gyrate.

Figure 13.11. Working principle of gyratory crusher for breaking lumpy ore pressed between a fixed jaw and rotating conical head.

13.3.1.2 Secondary Crusher

The secondary crusher is mainly used to reclaim the primary crusher product. The crushed material, which is around 15  cm in diameter obtained from the ore storage, is disposed as the final crusher product. The size is usually between 0.5 and 2   cm in diameter so that it is suitable for grinding. Secondary crushers are comparatively lighter in weight and smaller in size. They generally operate with dry clean feed devoid of harmful elements like metal splinters, wood, clay, etc. separated during primary crushing. The common secondary crushers are cone, roll, and impact types.

The cone crusher (Fig. 13.12) is very similar to the gyratory type, except that it has a much shorter spindle with a larger-diameter crushing surface relative to its vertical dimension. The spindle is not suspended as in the gyratory crusher. The eccentric motion of the inner crushing cone is similar to that of the gyratory crusher.

Figure 13.12. Schematic diagram depicting the basic elements and function of a cone crusher.

A working cone crusher (Fig. 13.13) can perform as a tertiary crusher when installed in a close circuit between secondary crusher and ball mill to crush any overflow material of vibratory screening.

Figure 13.13. A working cone crusher in a mineral process plant operation, performing both secondary and tertiary crushing functions.

The roll crusher consists of a pair of horizontal cylindrical manganese steel spring rolls (Fig. 13.14), which rotate in opposite directions. The falling feed material is squeezed and crushed between the rollers. The final product passes through the discharge point. This type of crusher is used in secondary or tertiary crushing applications. Advanced roll crushers are designed with one rotating cylinder that rotates toward a fix plate or rollers with differing diameters and speeds. It improves the liberation of minerals in the crushed product. Roll crushers are very often used in limestone, coal, phosphate, chalk, and other friable soft ores.

Figure 13.14. Conceptual diagram depicting the basic elements of a roll crusher.

The impact crusher (Fig. 13.15) employs high-speed impact or sharp blows to the free-falling feed rather than compression or abrasion. It utilizes hinged or fixed heavy metal hammers (hammer mill) or bars attached to the edges of horizontal rotating discs. The hammers, bars, and discs are made of manganese steel or cast iron containing chromium carbide. The hammers repeatedly strike the material to be crushed against a rugged solid surface of the crushing chamber breaking the particles to uniform size. The final fine products drop down through the discharge grate, while the oversized particles are swept around for another crushing cycle until they are fine enough to fall through the discharge gate. Impact crushers are widely used in stone quarrying industry for making chips as road and building material. These crushers are normally employed for secondary or tertiary crushing.

Figure 13.15. Schematic diagram showing the basic elements and function of an impact crusher.

13.3.1.3 Tertiary Crusher

If size reduction is not completed after secondary crushing because of extra-hard ore or in special cases where it is important to minimize the production of fines, tertiary recrushing is recommended using secondary crushers in a close circuit. The screen overflow of the secondary crusher is collected in a bin ( Fig. 13.16) and transferred to the tertiary crusher through a conveyer belt in close circuit.

Figure 13.16. Close circuit transfer of oversize material from secondary crusher collected in an ore bin (top) and autotransfer to tertiary crusher by a conveyor belt (bottom).

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Jaw Crusher

A. Gupta , D.S. Yan , in Mineral Processing Design and Operation, 2006

4.1.2 Jaw crusher circuits

Primary jaw crushers typically operate in open circuit under dry conditions. Depending on the size reduction required, the primary jaw crushers are followed by secondary and tertiary crushing. The last crusher in the line of operation operates in closed circuit. That is, the crushed product is screened and the oversize returned to the crusher for further size reduction while the undersize is accepted as the product. Flow sheets showing two such set-ups are shown in Figs. 3.1 and 3.2.

Jaw crushers are installed underground in mines as well as on the surface. When used underground, jaw crushers are commonly used in open circuit. This is followed by further size reduction in crushers located on the surface.

When the run of mine product is conveyed directly from the mine to the crusher, the feed to the primary crusher passes under a magnet to remove tramp steel collected during the mining operation. A grizzly screen is placed between the magnet and the receiving hopper of the crusher to scalp (remove) boulders larger than the size of the gape. Some mines deliver product direct to storage bins or stockpiles, which then feed the crushers mechanically by apron feeders, Ross feeders or similar devices to regulate the feed rate to the crusher. Alternately haulage trucks, front-end loaders, bottom discharge railroad cars or tipping wagons are used. In such cases, the feed rate to the crusher is intermittent which is a situation generally avoided. In such cases of intermittent feed, storage areas are installed and the feed rate regulated by bulldozers, front loaders or bin or stockpile hoppers and feeders. It is necessary that the feed to jaw crushers be carefully designed to balance with the throughput rate of the crusher. When the feed rate is regulated to keep the receiving hopper of the crusher full at all times so that the volume rate of rock entering any point in the crusher is greater than the rate of rock leaving, it is referred to as choke feeding. During choke feeding the crushing action takes place between the jaw plates and particles as well as by inter-particle compression. Choke feeding necessarily produces more fines and requires careful feed control. For mineral liberation, choked feeding is desirable.

When installed above ground, the object of the crushing circuit is to crush the ore to achieve the required size for down stream use. In some industries, for example, iron ore or coal, where a specific product size is required (iron ore −30+6   mm), careful choice of jaw settings and screen sizes are required to produce the minimum amount of fines (i.e. − 6   mm) and maximum the amount of lump ore within the specified size range. For hard mineral bearing rocks like gold or nickel ores where liberation of minerals from the host rock is the main objective, further stages of size reduction are required.

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Production of high copper concentrates—comminution and flotation (Johnson et al., 2019)

Mark E. Schlesinger , ... Gerardo R.F. Alvear Flores , in Extractive Metallurgy of Copper (Sixth Edition), 2022

3.2.2.3 Autogenous and semiautogenous mills

The crusher product is ground in an SAG or AG mill ( Giblett, 2019). AG mills crush the ore without the need for iron or steel grinding media. They are used when the ore is hard enough for the tumbling ore to grind itself. In SAG milling, ∼0.15   m³ of 13   cm diameter iron or steel balls are added into the mill per 0.85   m³ of ore (i.e., 15 vol.% "steel") to assist grinding. SAG mills are much more common.

The mill product is usually passed over a large vibrating screen to separate oversize pebbles from ore particles of the correct size. The correct size material is sent forward to a eccentric cone mill for final grinding. The oversize pebbles are recycled through a small ball mill for final reduction. This procedure maximizes ore throughput and minimizes electrical energy consumption.

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Screening

A. Gupta , D.S. Yan , in Mineral Processing Design and Operation, 2006

11.3

A gold ore was crushed in a secondary crusher and screened dry on an 1180  micron square aperture screen. The screen was constructed with 0.12   mm diameter uniform stainless steel wire. The size analysis of the feed, oversize and undersize streams are given in the following table. The gold content in the feed, undersize and oversize streams were; 5   ppm, 1.5   ppm and 7   ppm respectively. Calculate:

1.

The mass ratios of the oversize and undersize to the feed,

2.

Overall efficiency of the screen,

3.

Distribution of gold in the oversize and undersize streams.

Size micron	Cum mass % Retained
	Undersize	Feed	Oversize
3350	0	0	0
2360	4.0	7.8	20.0
1700	10.0	42.0	68.2
1180	63.4	78.9	86.7
850	84.0	89.4	94.2
600	94.0	99.0	97.2
425	96.0	100	98.0
212	100		100

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Process Control

A. Gupta , D.S. Yan , in Mineral Processing Design and Operation, 2006

18.10.1 Self Tuning Control (STC)

The self tuning control algorithm has been developed and applied on crusher circuits and flotation circuits [22-24] where PID controllers seem to be less effective due to immeasurable change in parameters like the hardness of the ore and wear in crusher linings. STC is applicable to non-linear time varying systems. It however permits the inclusion of feed forward compensation when a disturbance can be measured at different times. The STC control system is therefore attractive. The basis of the system is:

1.

on-line identification of process model,

2.

use of the control model in the process design.

The mathematical basis of the process is the least square model. When the time delay, t, is greater than zero, the process model is described in the predictive form [7].

The disadvantage of the set up is that it is not very stable and therefore in the control model a balance has to be selected between stability and performance. A control law is adopted. It includes a cost function C_F, and penalty on control action. The control law has been defined as:

(18.66) ${\text{C}}_{\text{F}}={\left[{\text{O}}_{\text{PR}}(\text{t}+{\text{t}}_{\text{d}}+1)-{\text{O}}_{\text{SP}}\right]}^2+{\text{Π O}}_{\text{c}}{(\text{t})}^2$

where	OSP	= output set point,
	Π	= penalty on control action.

To obtain maximum stability and therefore minimum variance of the output, a suitable form of equation has been derived as:

(18.67) $\begin{array}{l}{\text{O}}_{\text{C}}(\text{t})=\frac{\beta }{({\beta }^2+\Pi )}[\{{\text{α}}_1{\text{O}}_{\text{PR}}(\text{t})+\mathrm{..}+{\text{α}}_{\text{n}}{\text{O}}_{\text{PR}}(\text{t-n}+1)+{\text{O}}_{\text{Sp}}\}-\hfill \\ \text{β}[{\text{β}}_1{\text{O}}_{\text{C}}(\text{t-}1)+\mathrm{..}+{\text{β}}_{\text{P}}{\text{O}}_{\text{C}}(\text{t-P})]\frac{1}{({\beta }_0^2+\pi )}\hfill \end{array}$

where	OC(t	= output Controller at time t,
	OSP	= output ser point
	OPR (t)	= output (Process) at time t,
	ε(t)	= error at time t,
	t	= time,
	n	= order of system,
	k	= constant,
	α, β	= parameters
	π	= n + t − 1

The parameters α and β are estimated by setting up a suitable algorithm and calculated for different times. When π = 0, then n + 1 = 1. Eq. (18.67) acts as a self tuning regulator [6].

In some cases, like the control of a crusher operation, a constant term has to be introduced in Eq. (18.67) as at zero control action and the crusher operates at steady state.

A block diagram showing the self tuning set-up is illustrated in Fig. 18.27. The disadvantage of STC controllers is that they are less stable and therefore in its application a balance has to be derived between stability and performance.

Fig. 18.27. Block diagram of Self Tuning Control (STC) [6].

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Rural and Developing Country Solutions

Salah M. El Haggar , in Environmental Solutions, 2005

Bone Recycling

Bone recycling is a simple process where useful products can be extracted. Minerals such as calcium powder for animal; feed are extracted from the bone itself. The base material for cosmetics and some detergent manufacturing needs are extracted from the bone marrow.

The bone recycling process passes through seven stages starting from crushing and ending with packing. Figure 13.14 gives a schematic diagram showing the bone recycling process which goes through the following steps:

FIGURE 13.14. Bone recycling process.

1.

Crushing: Bones are conveyed through an auger from the receiving area to the crusher where bones are broken into pieces of about 10 cm in length.

2.

Cooking: In the cooker, crushed bones are subject to saturated steam supplied from a fire tube boiler via the steam line to cook bones with fat and protein and kill any bacteria or pathogens.

3.

Centrifugal separator: In the centrifugal separator unit, bone marrow and fats are expelled out of a perforated tank leaving the crushed bones in the bottom.

4.

Cooling: Crushed bones are cooled by circulating water in a cooler hopper. The circulating water in cooled in a forced draft-cooling tower.

5.

Fine grinding: The cooled bones are transferred from the cooler to the hummer mill using an auger to obtain finer grains of calcium powder.

6.

Cyclone separator: A two-stage cyclone separator is used to separate the calcium powder before backing.

7.

Packing: At this stage the fine grains are weighed, packed and are ready for marketing.

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Paleoarchean Crustal Evolution of the Bundelkhand Craton, North Central India

Pritam Nasipuri , ... Yashvardhan Gaur , in Earth's Oldest Rocks (Second Edition), 2019

4.1 Analytical Methods

Following the standard procedures in the Beijing SHRIMP Center, zircons were separated using a jaw crusher, disc mill, panning, and a magnetic separator, followed by handpicking using a binocular microscope. The grains were mounted together with the standard zircon TEM (417  Ma, Black et al., 2003) and then polished to expose the internal structure of the zircons. Cathodoluminescence (CL) imaging was conducted using a Hitachi SEM S-3000N equipped with a Gatan Chroma CL detector in the Beijing SHRIMP Center. The zircon analysis was performed using the SHRIMP II also in the Beijing SHRIMP Centre. The analytical procedures and conditions were similar to those described by Williams (1998). Analytical spots with ∼25   μm diameter were bombarded by a 3   nA, 10   kV O²⁻ primary ion beam to sputter secondary ions. Five scans were performed on every analysis, and the mass resolution was ∼5000 (at 1%). M257 standard zircon (561.3   Ma, U   =   840   ppm) was used as the reference value for the U concentration, and TEM standard zircons were used for Pb/U ratio correction (Black et al., 2003). Common Pb was corrected using the measured ²⁰⁴Pb. Data processing was performed using the SQUID/Isoplot programs (Ludwig, 2001a,b). Errors for individual analyses are at 1σ, but the errors for weighted average ages are at 2σ.

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Feasibility Study Plant Design

A. Ryan , ... D. Rogers , in Gold Ore Processing (Second Edition), 2016

3.6 Requirements for Blending and Surge Capacity

Coarse or fine ore storage can be required for a number of reasons:

•

A surge pile between crusher and milling circuit isolates the milling circuit from the generally lower availability achieved in crushing circuits.

•

Feed to the milling circuit needs to be steady and crusher discharge can often fluctuate somewhat, particularly with smaller crushing plants.

•

A stockpile can be used to blend ore from different sources. This is useful for flotation circuits where fluctuations in grade can change the mass balance and circulating loads around the plant. Blending can also be done on the ROM pad.

Once the decision has been made, a number of options exist for surge capacity:

•

The lowest cost alternative is to have no surge at all, but rather to have a crushing plant on line. This is workable for small-scale plant with single-stage jaw crushers as the availability of these simple plant is very high provided control over ROM size is maintained.

•

The second alternative is to use a small live surge bin after the primary crusher with a secondary reclaim feeder. Crushed ore feeds this bin continuously and the bin overflows to a small conveyor feeding a dead stockpile. In the event of a primary crusher failure, the crusher loader is used to reclaim the stockpile via the surge bin, which doubles as an emergency hopper.

•

For coarse ore, the next alternative is a coarse ore stockpile. Stockpiles of this type are generally 15–25% live and require a tunnel (concrete or Armco) and a number of reclaim feeders to feed the milling circuit.

•

Multi-stage crushing circuits usually require surge capacity as the availability of each unit process is cumulative. A fine-ore bin is usually required. Smaller bins are usually fabricated from steel as this is cheaper. Live capacity of bins is higher than stockpiles but they also require a reclaim tunnel and feeders.

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