Shaking Table

Shaking Table:-

Shaking Table is a kind of mineral processing equipment of fine materials according to weight. It is widely used for distilling tungsten, tin, molybdenum, aluminum, zinc, other rare metal and noble metal ore, and it is also applicable for distilling iron, manganese and coal. The Shaking Table has the advantages of high enrichment grade, high separation efficiency, easy operation, convenient adjustment, and it can distill the final concentrate and gangue in one step, etc.

Vibration table is a kind of mineral processing equipment of fine materials according to weight. It is widely used for distilling tungsten, tin, molybdenum, aluminum, zinc, other rare metal and noble metal ore, and it is also applicable for distilling iron, manganese and coal. The rocking bed has the advantages of high enrichment grade, high separation efficiency, easy operation, convenient adjustment, and it can distill the final concentrate and gangue in one step, etc. 

Working Theory of vibration Table:-

The mineral processing of vibration Table is carried out on the rifle bed, the mineral particle cluster is fed from the upper corner of bed, and the water gullet provides horizontal current. The bed does asymmetry movement, so the inertial strength and friction are produced. Under the function of weight of mineral particle and horizontal current, the materials forms several layers according to weight and particle size, and do lengthways movement along the bed and horizontal movement along the oblique bed surface. Therefore, the mineral particle of different weight and fineness flow down in the shape of sector along the catercorner lines, and are discharged through the concentrate and gangue terminal zones. Finally, the concentrate, mineral and gangue are separated.

Performance and Feature:-

The Shaking Table has the advantages of good separation capacity, stable performance, high efficiency, smooth, lower noise, easy operation and maintenance, etc.

 
 

Magnetic separation

Magnetic separation:

All materials possess magnetic properties. Substances that have a greater permeability than air are classified as paramagnetic; those with a lower permeability are called diamagnetic. Paramagnetic materials are attracted to a magnet; diamagnetic substances are repelled. Very strongly paramagnetic materials can be separated from weakly or nonmagnetic materials by the use of low-intensity magnetic separators. Minerals such as hematite, limonite, and garnet are weakly magnetic and can be separated from nonmagnetics by the use of high-intensity separators.

Magnetic separators are widely used to remove tramp iron from ores being crushed, to remove contaminating magnetics from food and industrial products, to recover magnetite and ferrosilicon in the float-sink methods of ore concentration, and to upgrade or concentrate ores. Magnetic separators are extensively used to concentrate ores, particularly iron ores, when one of the principal constituents is magnetic. See also Mechanical separation techniques; Ore dressing.

Principle of operation:

 When minerals are placed in a magnetic field, there are three reactions which may occur. First, they are attracted to the magnetic field. Second, they are repulsed by the magnetic field. And third, no noticeable reaction to the magnetic field occurs.

Particles that are attracted to the magnetic field are called magnetic. But, there are two classifications of magnetic particles, strongly magnetic particles, such as iron and magnetite, and weakly magnetic particles, such as rutile, ilmenite, and chromite. Strong magnetic particles may be easily separated with a separator having a low intensity magnetic field of 400 - 600 gauss. Paramagnetic particles (weakly magnetic) require a higher intensity magnetic field to separate them, generally ranging from 6,000 to 20,000 gauss.

Particles that are repulsed by a magnetic field are called diamagnetic. A line of separators called Eddy Current Separators, takes advantage of the diamagnetic particles, separating them from other material. One of the largest uses currently is in the recycling industry, where wire and metals made from copper and aluminum are separated from plastics. When product, such as aluminum, passes over the eddy current , the spinning magnets inside the shell generate an eddy current in the aluminum thus creating a magnetic field around the piece of aluminum. The polarity of the magnetic field of the aluminum is the same as the rotating magnets, causing the aluminum to be repelled away from the separator. Product such as plastic, glass, or other process materials simply fall off the end of the separator. An eddy current is defined as the currents caused by voltages induced by changing flux, and tend to oppose the change of the flux.

Non magnetic particles, such as gold, quartz, and pyrite, are not amenable to magnetic separation, but some magnetic material may be removed from the feed. For instance, in a few situations, plants using gravity concentration for recovering gold, used magnetic separators to remove the high concentration of magnetite that was recovered with the gold, prior to further processing.

Magnetic separation is generally a low cost method of recovery, unless high intensity separators are required. The electro-magnetic high intensity separators that produce 20,000 gauss, tend to be expensive. However, the rare earth magnetic separators are relatively inexpensive and can produce magnetic fields around 6,000 gauss. So, when looking for a process to recover valuable minerals, magnetic separation should not be overlooked, if some of the material is magnetic or para-magnetic.

Froth flotation

Froth flotation:

Froth flotation is a process for separating minerals from gangue by taking advantage of differences in their hydrophobicity. Hydrophobicity differences between valuable minerals and waste gangue are increased through the use of surfactants and wetting agents. The selective separation of the minerals makes processing complex (that is, mixed) ores economically feasible. The flotation process is used for the separation of a large range of sulfides, carbonates and oxides prior to further refinement. Phosphates and coal are also upgraded (purified) by flotation technology.

Principle of operation:

Froth flotation commences by comminution (that is, crushing and grinding), which is used to increase the surface area of the ore for subsequent processing and break the rocks into the desired mineral and gangue in a process known as liberation, which then has to be separated from the desired mineral. The ore is ground into a fine powder and mixed with water to form a slurry. The desired mineral is rendered hydrophobic by the addition of a surfactant or collector chemical. The particular chemical depends on which mineral is being refined. As an example, SEX is added as a collector in the selective flotation of galena and sphalerite, after the addition of other flotation reagents. This slurry (more properly called the pulp) of hydrophobic particles and hydrophilic particles is then introduced to a water bath which is aerated, creating bubbles. The hydrophobic particles attach to the air bubbles, which rise to the surface, forming a froth. The froth is removed and the concentrate (con) is further refined.

Flotation equipment:

Flotation can be performed in rectangular or cylindrical mechanically agitated cells or tanks, flotation columns, Jameson cells or deinking flotation machines.

Mechanical cells use a large mixer and diffuser mechanism at the bottom of the mixing tank to introduce air and provide mixing action. Flotation columns use air spargers to introduce air at the bottom of a tall column while introducing slurry above. The countercurrent motion of the slurry flowing down and the air flowing up provides mixing action. Mechanical cells generally have a higher throughput rate, but produce material that is of lower quality, while flotation columns generally have a low throughput rate but produce higher quality material.

The Jameson cell uses neither impellers nor spargers, instead combining the slurry with air in a downcomer where high shear creates the turbulent conditions required for bubble particle contacting.

Mechanics of flotation

The following steps are followed, following grinding to liberate the mineral particles:

• Reagent conditioning to achieve hydrophobic surface charges on the desired particles

                      

• Collection and upward transport by bubbles in an intimate contact with air or nitrogen
• Formation of a stable froth on the surface of the flotation cell
• Separation of the mineral laden froth from the bath (flotation cell)

Simple flotation circuit for mineral concentration. Numbered triangles show direction of stream flow, Various flotation reagents are added to a mixture of ore and water (called pulp) in a conditioning tank. The flow rate and tank size are designed to give the minerals enough time to be activated. The conditioner pulp is fed to a bank of rougher cells which remove most of the desired minerals as a concentrate. The rougher pulp passes to a bank of scavenger cells where additional reagents may be added. The scavenger cell froth is usually returned to the rougher cells for additional treatment, but in some cases may be sent to special cleaner cells. The scavenger pulp is usually barren enough to be discarded as tails. More complex flotation circuits have several sets of cleaner and re-cleaner cells, and intermediate re-grinding of pulp or concentrate.

 

Electrostatic separation

Electrostatic separation:

Electrostatic separation is a process that uses electrostatic charges to separate crushed particles of material. An industrial process used to separate large amounts of material particles, electrostatic separating is most often used in the process of sorting mineral ore. This process can help remove valuable material from ore, or it can help remove foreign material to purify a substance. In mining, the process of crushing mining ore into particles for the purpose of separating minerals is called beneficiation.

Generally, electrostatic charges are used to attract or repel differently charged material. When electrostatic separation uses the force of attraction to sort particles, conducting particles stick to an oppositely-charged object, such as a metal drum, thereby separating them from the particle mixture. When this type of beneficiation uses repelling force, it is normally employed to change the trajectory of falling objects to sort them into different places. This way, when a mixture of particles falls past a repelling object, the particles with the correct charge fall away from the other particles when they are repelled by the similarly charged object.

Principle of operation:

A process called "electrostatic beneficiation", which means charging them with static electricity and separating them by passing them through an electric field, as pictured in the next figure.

An electrostatic beneficiator works because different minerals have different electrostatic affinities -- will absorb different amounts of charge depending upon their composition, and hence are deflected different amounts by an electric field. After grains are sieved by size, they are placed through a beneficiator. After a few passes through beneficiators, we have separated different minerals fairly well. (There's no change in physical or chemical identity; there's only separation of minerals.)

Beneficiators typically use free-fall of grains through electric fields. However, some beneficiators slide the grains down a ramp, and some put them across a rotating drum with a certain electrostatic charge so that grains of a certain affinity will stick to the drum and others will fall to the ground due to gravity or the centrifugal force. Thus, beneficiation separates minerals according to their electrostatic affinity, as well as their different densities (with gravity or the centrifugal force).

The grains are charged by any of the following methods: charging the screen that sieves them, or charging another surface which they slide over, or a diffuse electron beam as they fall. The charging method can depend upon which minerals we want to separate, since different minerals have different responses to different methods (and indeed to different temperatures, too).

The resultant material is collected in different bins whereby the enriched portion of the desired mineral is called the "concentrate" and the rest of the output is called the "gangue" or "tailings".

While on Earth we're usually interested in just one mineral and one bin, on the Moon we will often be interested in using more of the material. With an electrostatic beneficiator we could have multiple bins at the bottom, as the mineral stream will split up into multiple streams depending upon the degree of attraction or repulsion of each mineral.

The electrostatic separation of conductors is one method of beneficiation; another common beneficiation method is magnetic beneficiation. Electrostatic separation is a preferred sorting method when dealing with separating conductors from electrostatic separation non-conductors. In a similar way to that in which electrostatic separation sorts particles with different electrostatic charges magnetic beneficiation sorts particles that respond to a magnetic field. Electrostatic beneficiation is effective for removing particulate matter, such as ash from mined coal, while magnetic separation functions well for removing the magnetic iron ore from deposits of clay in the earth.

Engineering Properties of Rock Materials

Point Load Strength Index:-
Point load test is another simple index test for rock material. It gives the standard point load index, Is(50), calculated from the point load at failure and the size of the specimen,  with size correction to an equivalent core diameter of 50 mm.
Fracture Toughness:-
Fracture toughness of rock materials measures the effectiveness of rock fracturing. It is typically measured by a toughness test. There are three fracture mode: (Mode I), (Mode II), (Mode III). Correspondingly, there are three fracture
toughness, KIC, KIIC and KIIIC.
Brittleness:-
Tendency of a material to fracture or fail upon the application of a relatively small amount of force, impact, or shock. Opposite of toughness.
Indentation:-
Swelling:-
Some rocks swell when they are situated with water. As discussed in Chapter 2,
swelling is governed by the amount of swelling montmorillonite clay minerals in the rock material.
Rock swelling is measured in confined and unconfined conditions. Unconfined
swelling is measured by the percentage increase of length in three perpendicular
directions, when a rock specimen is placed in water.Confined swelling index measures swelling in one direction while deformations in other two directions are constrained.

Mechanical Properties of Rock Material

Compressive Strength:-
Compressive strength is the capacity of a material to withstand axially directed
compressive forces. The most common measure of compressive strength is the uniaxial compressive strength or unconfined compressive strength. Usually compressive strength of rock is defined by the ultimate stress. It is one of the most important mechanical properties of rock material, used in design, analysis and modelling.
Young's Modulus and Poisson’s Ratio:-
oung's Modulus is modulus of elasticity measuring of the stiffness of a rock material. It is defined as the ratio, for small strains, of the rate of change of stress with strain. This can be experimentally determined from the slope of a stress-strain curve obtained during compressional or tensile tests conducted on a rock sample.
Similar to strength, Young’s Modulus of rock materials varies widely with rock type. For extremely hard and strong rocks, Young’s Modulus can be as high as 100 GPa. There is some correlation between compressive strength and Young’s Modulus, and discussion is given in a later section.Poisson’s ratio measures the ratio of lateral strain to axial strain, at linearly-elastic region.For most rocks, the Poisson’s ratio is between 0.15 and 0.4.
Stress-Strain at and after Peak:-
With well controlled compression test, a complete stress-strain curve for a rock specimen can be obtained.
Strain at failure is the strain measured at ultimate stress. Rocks generally fail at a small strain, typically around 0.2 to 0.4% under uniaxial compression. Brittle rocks, typically crystalline rocks, have low strain at failure, while soft rock, such as shale and mudstone, could have relatively high strain at failure. Strain at failure sometimes is used as a measure of brittleness of the rock. Strain at failure increases with increasing confining pressure under triaxial compression conditions.
Rocks can have brittle or ductile behaviour after peak. Most rocks, including all
crystalline igneous, metamorphic and sedimentary rocks, behave brittle under uniaxial compression. A few soft rocks, mainly of sedimentary origin, behave ductile.
Tensile Strength:-

Tensile strength of rock material is normally defined by the ultimate strength in tension, i.e., maximum tensile stress the rock material can withstand.
Rock material generally has a low tensile strength. The low tensile strength is due to the existence of microcracks in the rock. The existence of microcracks may also be the cause of rock failing suddenly in tension with a small strain.
Tensile strength of rock materials can be obtained from several types of tensile tests: direct tensile test, Brazilian test and flexure test. Direct test is not commonly performed due to the difficulty in sample preparation. The most common tensile strength determination is by the Brazilian tests.
Shear Strength:-
Shear strength is used to describe the strength of rock materials, to resist deformation due to shear stress. Rock resists shear stress by two internal mechanisms, cohesion and internal friction. Cohesion is a measure of internal bonding of the rock material.
Internal friction is caused by contact between particles, and is defined by the internal friction angle, φ. Different rocks have different cohesions and different friction angles. Shear strength of rock material ca be determined by direct shear test and by triaxial compression tests. In practice, the later methods is widely used and accepted.

4.1 Physical Properties of Rock Material

1. Density, Porosity and Water Content:-
Density is a measure of mass per unit of volume. Density of rock material various, and often related to the porosity of the rock. It is sometimes defined by unit weight and specific gravity. Most rocks have density between 2,500nd 2,800 kg/m3.
Porosity describes how densely the material is packed. It is the ratio of the non-solid volume to the total volume of material. Porosity therefore is a fraction between 0 and 1. The value is typically ranging from less than 0.01 for solid granite to up to 0.5 for porous sandstone. It may also be represented in percent terms by multiplying the fraction by 100%. Water content is a measure indicating the amount of water the rock material contains. It is simply the ratio of the volume of water to the bulk volume of the rock material.
Density is common physical properties. It is influenced by the specific gravity of the composition minerals and the compaction of the minerals. However, most rocks are well compacted and then have specific gravity between 2.5 to 2.8. Density is used to estimate overburden stress. Density and porosity often related to the strength of rock material. A low density and high porosity rock usually has low strength. Porosity is one of the governing factors for the permeability. Porosity provides the void for water to flow through in a rock material. High porosity therefore naturally leads to high permeability.
2. Hardness:-
Hardness is the characteristic of a solid material expressing its resistance to permanent deformation. Hardness of a rock materials depends on several factors, including mineral composition and density. A typical measure is the Schmidt rebound hardness number.
3. Abrasivity:-
Abrasivity measures the abrasiveness of a rock materials against other materials, e.g.,steel. It is an important measure for estimate wear of rock drilling and boring
equipment.Abrasivity is highly influenced by the amount of quartz mineral in the rock material.The higher quartz content gives higher abrasivity.Abrasivity measures are given by several tests.

4. Permeability:-Permeability is a measure of the ability of a material to transmit fluids. Most rocks, including igneous, metamorphic and chemical sedimentary rocks, generally have very low permeability. As discussed earlier, permeability of rock material is governed by porosity. Porous rocks such as sandstones usually have high permeability while granites have low permeability. Permeability of rock materials, except for those porous one, has limited interests as in the rock mass, flow is concentrated in fractures in the rock mass. Permeability of rock fractures is discussed later.
5. Wave Velocity:-
Measurements of wave are often done by using P wave and sometimes, S waves. P- wave velocity measures the travel speed of longitudinal (primary) wave in the material, while S-wave velocity measures the travel speed of shear (secondary) wave in the material. The velocity measurements provide correlation to physical properties in terms of compaction degree of the material. A well compacted rock has generally high
velocity as the grains are all in good contact and wave are travelling through the solid.
For a poorly compact rock material, the grains are not in good contact, so the wave will partially travel through void (air or water) and the velocity will be reduced (P-wave velocities in air and in water are 340 and 1500 m/s respectively and are much lower than that in solid).

Mohr Coulomb Failure Criterion

• The Maximum shear stress theory proposed by coulomb postulates that failure will occur in a material when the maximum shear stress at a point in a material reaches a specific value, which is referred to as its shear strength.
• Coulomb suggested that the strength (shear) of rock is made up of two parts, a constant cohesion and a normal stress dependent frictional component.
• Cohesion of a material is its minimum shear strength, and this shear strength increases with increasing normal stress.
• Cohesion also describes the limiting or minimum amount of shear stress needed to get a surface slipping when the normal stress is very tiny or (zero), or in other words in the absence of normal stress, failure will occur only if shear stress component will overcome the cohesion of that material.
• The Mohr Coulomb failure criterion expresses the relation between the shear stress and the normal stress at failure.
• The Mohr Coulomb criterion is developed for compressive stress only.
• The Mohr Coulomb criterion sets up the criterion for predicting weather the stresses are sufficient to overcome the frictional resistance to slip along a surface.
• Mohr Coulomb strength criterion assumes that a shear failure plane is developed in the rock material and when failure occurs the stresses developed on the failure plane are on the strength envelope.
• The normal stress acting across the plane of failure increases the shear resistance of the material.
• The Mohr Coulomb failure criterion represents the linear strength envelope that is obtained from a plot of the shear strength of a material versus the applied normal stress.
• Mohr envelope defines the limiting size for Mohr’s circle.
• σ-τ coordinate below the strength envelope represents stable condition.
• σ-τ coordinate on the strength envelope represents equilibrium condition.
• σ-τ coordinate above the strength envelope represents failure condition.

Discussion about Applicability of Mohr’s Coulomb Failure Criterion

• We estimate that the criterion is most suitable at high confining pressure when the material does in fact fail through developments of shear planes. At lower confining pressure and in the uniaxial case, the failure occurs due to gradual increase in the density of micro-cracks and hence we would not expect such type of frictional criterion to apply directly. However at the higher confining pressures the criterion can be useful.
• Despite the difficulties associated with application of this it does remain in use as a rapidly calculable method for engineering practice.

Machine study of a Disc Mill

DISC MILL:-

Disc mill can be for secondary or fine crushing but its use is limited to special applications only because capacity is low and it is not suitable for all types of ores economically.

Disc mills are popular in West Africa and the Sudan and operate with a greater component of shear than compression. A disc mill consists of a circular chamber made of cast iron or steel within which two Discs with a narrow gap between them are mounted face to face. The discs are grooved in order to provide a shear mechanism.

When grains are introduced into the centre of the mill, the discs shear the grains between them. One of the discs rotates and the grains revolve, working their way to the outer edge of the disc before dropping by gravity into a holding sack below.

The grains lodge in the rotating disc and are sheared by the grooves in the opposing Disc. As the grains move to the edges of the Discs, the grooves become shallower and reduce the size of the grains. The design of grooves follows a very old style developed for stone mills several thousand years ago.

Discs are usually about 200–300 mm in diameter. Discs are normally aligned in a vertical direction, but horizontal alignment is more convenient when the mill is run by a diesel engine. Disc mills can run as fast as possible but normally at about 2 500–3 500 revolutions/minute, as overheating of the discs limits the speed of the mill.

Frictional heating imposes power limits. For example, a Disc mill with 300 mm Discs cannot be driven by an engine with more than 12 kW. However, the speed of mill is not a critical factor to the mechanism of

grinding. Disc mills operate more effectively with soft and moist grains that shear easily than with hard and brittle grains. It is common in West Africa to add water at the time of grinding. The milled product has to be used very quickly in order to prevent fermentation.

The fineness of the flour ground is adjusted by increasing the pressure on the grain by narrowing the gap between the discs. This is done with a simple hand wheel connected to the outer disc by a shaft. The mill should not be run empty because grains in the mill are needed in order to lubricate the action and, thus, prevent wear. Excessive wear is caused when the discs come into contact with each other. A fine flour or meal from a disc mill is obtained by re-circulating the product in the mill for a second or third grind.

CONSTRUCTION:-

A disc consists of two saucer-shaped discs with their surface having specially shaped grooves the depth of which reduced towards the circumference. The discs are face to face mounted vertically or horizontally and revolve at different speeds and in the opposite directions. In most designs one of the discs is driven while the other flutters or gyrates during revolving. Our laboratory model has heat treated mechanite metal discs mounted vertically one revolving in a planetary manner always having a proper curvature with relation to the other which is stationary.

Like other crushing machines, a disc mill has not of the discs spring-loaded through a screw mechanics that helps in adjusting the set and also provides safety against un crushable lumps.

The material is fed through a hopper at the top and falls into the axial conic between the discs during revolving. Due to the centrifugal force the feed is pushed through the taper grooves towards the periphery and gets crushed progressively. the product is finally discharged peripherally and collected in a peripheral receptacle.

Side View:-

                           

Machine Study Of Ball Mill And Perform A Grinding Test On Given Sample.

APPARATUS REQUIRED:-

1. Denver Ball-mill
2. A sieve-set with a sieve-shaker
3. Feed sample
4. Tachometer
5. Torsion-balance/Electrical balance

BALL MILL:-

A ball mill is a type of grinder used to grind materials into extremely fine powder for use in mineral dressing processes, paints, pyrotechnics, and ceramics.

A ball mill, a type of grinder, is a cylindrical device used in grinding (or mixing) materials like ores, chemicals, ceramic raw materials and paints. Ball mills rotate around a horizontal axis, partially filled with the material to be ground plus the grinding medium. Different materials are used as media, including ceramic balls, flint pebbles and stainless steel balls. An internal cascading effect reduces the material to a fine powder. Industrial ball mills can operate continuously fed at one end and discharged at the other end. Large to medium-sized ball mills are mechanically rotated on their axis, but small ones normally consist of a cylindrical capped container that sits on two drive shafts (pulleys and belts are used to transmit rotary motion). A rock tumbler functions on the same principle. Ball mills are also used in pyrotechnics and the manufacture of black powder, but cannot be used in the preparation of some pyrotechnic mixtures such as flash powder because of their sensitivity to impact. High-quality ball mills are potentially expensive and can grind mixture particles to as small as 5 nm, enormously increasing surface area and reaction rates. The grinding works on the principle of critical speed. The critical speed can be understood as that speed after which the steel balls (which are responsible for the grinding of particles) start rotating along the direction of the cylindrical device; thus causing no further grinding.

Ball mills are used extensively in the Mechanical alloying process^[1] in which they are not only used for grinding but for cold welding as well, with the purpose of producing alloys from powders. 

The ball mill is a key piece of equipment for grinding crushed materials, and it is widely used in production lines for powders such as cement, silicates, refractory material, fertilizer, glass ceramics, etc. as well as for ore dressing of both ferrous non-ferrous metals. The ball mill can grind various ores and other materials either wet or dry. There are two kinds of ball mill, grate type and overfall type due to different ways of discharging material. There are many types of grinding media suitable for use in a ball mill, each material having its own specific properties and advantages. Key properties of grinding media are size, density, hardness, and composition.

Size: The smaller the media particles, the smaller the particle size of the final product. At the same time, the grinding media particles should be substantially larger than the largest pieces of material to be ground.

Density: The media should be denser than the material being ground. It becomes a problem if the grinding media floats on top of the material to be ground.

Hardness: The grinding media needs to be durable enough to grind the material, but where possible should not be so tough that it also wears down the tumbler at a fast pace.

Composition: Various grinding applications have special requirements. Some of these requirements are based on the fact that some of the grinding media will be in the finished product. Others are based in how the media will react with the material being ground.

Where the color of the finished product is important, the color and material of the grinding media must be considered.

Where low contamination is important, the grinding media may be selected for ease of separation from the finished product (i.e.: steel dust produced from stainless steel media can be magnetically separated from non-ferrous products). An alternative to separation is to use media of the same material as the product being ground.

Flammable products have a tendency to become explosive in powder form. Steel media may spark, becoming an ignition source for these products. Either wet-grinding, or non-sparking media such as ceramic or lead must be selected.

Some media, such as iron, may react with corrosive materials. For this reason, stainless steel, ceramic, and flint grinding media may each be used when corrosive substances are present during grinding.

The grinding chamber can also be filled with an inert shield gas that does not react with the material being ground, to prevent oxidation or explosive reactions that could occur with ambient air inside the mill.

OPERATING PRINCIPLE:-

Manual hand-cranked ball mills with spiral feed chute are used for fine grinding. The ball mill is a rotating cylindrical crushing device which contains steel balls which comminute the material through percussive, shearing and compressive (squeezing) forces. Rotating the drum results in a continuous cascading of the balls and material contained inside. The duration of milling is determined by the final grain-size desired for the ground product. Water flowing through the mill removes the fine material.

AREAS OF APPLICATION:-

Fine grinding of middlings, raw ore or pre-concentrates.

SPECIAL AREAS OF APPLICATION:-

For special grinding steps where it is important that the products remain free of iron, such as in grinding of graphite, hard stones of flint, granite, etc. are used instead of the balls.

TECHNICAL DATA:

Dimensions:

approx. 1.5 × 1 × 1 m

Weight:

approx. 150 kg

Extent of Mechanization:

manual to fully mechanized, depending on drive system

Power Required:

from 100 W up to several kW, e.g. approx. 7.5 kWh/t energy input to crush Volcanic sulfide ores, up to 50 kWh/t energy consumption for milling of hard quartzite and similar ores

Form of Driving Energy:

electric

Alternative forms:

manual, pedal drive, hydromechanic with water wheel

Mode of Operation:

semi-continuous/continuous

Throughput/Capacity:

1 t/h: 11 - 12 kW

Operating Materials:

Type:

Water grinding bodies (Zylpebs or balls)

Quantity:

bulk-volume approx. 25 - 45 % of mill capacity

ECONOMIC DATA:

Investment Costs:

manual ball mill: approx. 1000 DM when locally produced; Millan mill 500 US$,

Volcan mill:

10.000 US$, Denver mill: 22.000 US$ for mills with approx. 1 t/h throughput

Operating Costs:

replacement of worn milling balls, energy costs

Related Costs:

possibly thickener, since ground product is a slurry

MANUFACTURER: Millan, KHD, Volcan, Denver, Alquexco, Eq. Ind. Astecnia, IAA, Talleres Mejia, Buena Fortuna, COMESA, Met. Mec. Soriano, FAMESA, FAHENA, FIMA, FUnd. Callao, H.M., MAGENSA, MAEPSA, Met. Callao E.P.S.

REMARKS:

In autogenous grinding, only the feed material itself, in the absence of balls or other grinding bodies, is subjected to the rotation of the mill drum. The grinding is achieved as a result of the larger material grains functioning as the balls, crushing the smaller or softer feed components. An example where autogenous grinding is applied is in the liberation of loosely-consolidated gold-containing conglomerates.

All types of ball mills produce high proportions of fine-grained product. In the case of particularly brittle minerals such as scheelite, wolframite, cassiterite, sphalerite, etc., this readily leads to overgrinding, resulting in poor recovery of the valuable mineral. Under these conditions, grinding needs to be performed with care, including prescreening and intermediate screening of the fines, and recycling of the screened overs back into the mill.

When the ground product is discharged from the mill as a slurry, the heavy material components remain in the mill longer due to their increased resistance to the flow forces. Consequently, grinding must be conducted correspondingly carefully, or an alternative method of removing the ground product from the mill must be employed, such as screening.

CONSTRUCTION INFORMATION:

Wheel bearings from cars are suitable as bearings for hand-cranked ball mills.

With belt or chain-driven systems, the entire mill housing is rotated.

The optimal rotational speed (rpm) is 75 % that of the critical rotational speed, or that where the centrifugal force causes the mill balls to remain on the drum perimeter:

n in min-1
D = mill diameter in m

For this rotational speed, at 30 % degree of filling, the power can be determined by the following formula

P (kW) ~ 10 GK (t) × V D (m), where GK is weight of balls in 1000 kg

For 20 % degree of filling the power is about 10 % higher, and for 40 % degree of filling about 15 % lower.

The rotational speeds for coarse grinding lie somewhat higher than for fine grinding, to a maximum of

D diameter of ball mill <= D/20

Old rail sections, cemented Into place, provide an inexpensive ball-mill lining.

The ends of the mill housing can be placed on roller or ball bearings, or on other forms of rollers or tires, the latter form can also be used to drive the mill, allowing good access to the front and back ends of the mill for easier handling at the feed and discharge points.

SUITABILITY FOR SMALL-SCALE MINING:

Hand-cranked ball mills have a rather limited application due to their low throughput. Useful primarily for regrinding of middlings. Small mechanized ball mills are appropriate in small-scale mining operations where finely-intergrown ore requires a fine liberation grinding, in which case a good supply of replacement parts must be available. 

Schematic Diagram of Ball Mill:-

 

 

Machine Study Of A Hammer Mill

Apparatus Required:-

• Laboratory Hammer-Mill
• Feed Sample
• Meter-rod

Hammer Mill:-

Hammer mills are characterised by their considerable grain size reduction ratio.

They are suitable for handling clay of any kind, ceramic mixtures with a high content of aggregates and raw materials with a low to medium hardness.

These mills are composed of a thick sheet metal casing, housing on the inside a set of circular and lateral armours coated with wearproof material; a rotor carrying the grinding hammers that are made of a special long-life alloy; a lump crushing hammer unit and special steel sizing grids.

In a hammer mill, swinging hammerheads are attached to a rotor that rotates at high speed inside a hardened casing. The principle is illustrated in Figure.

The material is crushed and pulverized between the hammers and the casing and remains in the mill until it is fine enough to pass through a screen which forms the bottom of the casing. Both brittle and fibrous materials can be handled in hammer mills, though with fibrous material, projecting sections on the casing may be used to give a cutting action.

Both hammering and rolling can achieve the desired result of achieving adequately ground ingredients, but other factors also need to be looked at before choosing the suitable method to grind. Excessive size reduction can lead to wasted electrical energy, and unnecessary wear on mechanical equipment.

General Design:-

The major components of these hammer mills, shown in the picture, include: - a delivery device is used to introduce the material to be ground into the path of the hammers. A rotor comprised of a series of machined disks mounted on the horizontal shaft performs this task. - free-swinging hammers that are suspended from rods running parallel to the shaft and through the rotor disks. The hammers carry out the function of smashing the ingredients in order to reduce their particle size. -a perforated screen and either gravity- or air-assisted removal of ground product. Acts to screen the particle size of the hammer mill to ensure particles meet a specified maximum mesh size.

Procedure:-

• Identify each part of the hammer. Draw a sketch and lebel each part on it.

• Switch on the machine and study the function of each part of the machine. Note the r.p.m of the machine.

• Examine the feed for its size-range and recover the average maximum size in it.

• Feed the material slowly.

• Switch off the machine and recover the product and weight it.

• Transfer the product to a set of sieve and sieve for 20 minutes.

  
  

  Observations:-

  Machine Details which is shown above:-

Machine Name

Hammer Mill

Motor Power

5 h.p

Motor rpm

1420 rpm

Mill rpm

2130 rpm

Motor Pulley

18 cm

Mill Pulley

12 cm

Grate Opening

1 com

Full swing dia. of shaft + hammer

35 cm

No. of hammers

32

Capacity

200 tons/hr

Feed

Dolomite

Size of Feed

(-18.85 to +13.33) mm

Weight of Feed

1 kg

Machine study of laboratory Rolls-crushers.

Theory:-

Rock crusher, also known as ore crusher, plays a vital part in crushing various types of rocks and ores. There are two types of rock crushers in actual production, that is, primary crusher and secondary crusher. Secondary crushers mainly handle rocks of smaller particle size that have already been impacted and crushed from their original size. In recent years, with the rapid development of infrastructure projects, rock crushers have found more applications in various fields and industries, such as mining, chemical industry, road construction, metallurgy, construction and so on. In this passage, several types of commonly used secondary crushers will be listed as follows.

1. Impact crusher:-

According to the nature of the materials to be crushed in actual production, impact crushers can be used for secondary crushing as well primary crushing. During operation, materials will be first fed into the crushing chamber for continuous and repeated impacting and crushing by large hammers. When the materials have been crushed to the proper particle size, they will finally be discharged from the lower part of the discharge opening. In general, impact crushers can be divided into two types, including horizontal shaft impact crusher (HSI for short), which uses hammers that swing on a rotating shaft, and vertical shaft impact crusher (VSI for short), which “throws” the material against a surface by means of the rock’s velocity to break it apart.

2. Cone Crushers :-

As a type of frequently used secondary crusher, cone crusher’s working principle is quite similar to that of the gyratory crusher. Nevertheless, the difference is that the crushing chamber in a cone crusher isn’t as steep as it is in a gyratory crusher and the positions of the different crushing zones are more parallel.

3. Roll Crusher:-

Name of roll crusher properly indicates its operating mode. The rocks and ores to be crushed will be processed between two large steel wheels, which press down on the materials and then crush the materials into smaller particle sizes to meet customers’ specific requirements.

 

Roller mills are suitable machines for fine crushing, which leads to extreme exposure of the feed in case of somewhat coarse intergrown ore avoiding further fine milling.The roll crusher equipment is a very potential force in market development of the crushing plant, roll crusher plays an important role in the process of crushing materials, the material is repeatedly crushed by the broken parts. Compared with hammer crusher and impact crusher, in the crushing of materials, the roll crusher equipment is more suitable for crushing brittle materials. Roll crusher equipment has large crushing ratio and can up to 50. This roll crusher can be used to crushing limestone in cement industry. Any company and business that use of the roll crusher equipment or are using the production of roll crusher has a deeper understanding of the improvement the utilization of crushing materials. Spare parts of roll crusher equipment is easy to replace, and has a corresponding reduction in maintenance expense. This machine is suitable for a very large hardness of materials or used for soft materials. Roll crusher equipment in daily use, the maintenance of the machine must be aware of. In particular, the operators need to pay attention to maintenance of the roller of roll crusher equipment, because the easiest to wear and consumption is the roller.

OPERATING PRINCIPLE:-

The feed material is crushed between two counter-clockwise rotating rollers to a degree of fineness allowing it to fall through a slit at the bottom. In the event that the pressure becomes too great, the rollers deflect outwardly, increasing the gap between them and consequently also the final grain size.

AREAS OF APPLICATION:-

Crushing of brittle ore in preparation of hydromechanic gravimetric sorting of medium-sized grain fractions.

REMARKS:

• Roll crushers are known for producing a ground product with a very low proportion of fines.
• 30 - 200 g of hard-steel wear per ton throughput depending on hardness and tenacity of the feed material.
• The roll crushing of hard minerals (igneous rocks, hard ores, gravel sediments) uses smooth Jaws, whereas the rollers for crushing medium hard or soft material (e.g. Iimestone, anhydrite, sedimentary Iron ores, etc. or salts, clays, soft brown coal, etc.) are fluted or serrated.
• The roller diameters should equal approx. 20 times that of that largest grain-size contained in the fee.

SUITABILITY FOR SMALL-SCALE MINING:-

Roller mills are suitable machines for fine crushing, which leads to extreme exposure of the feed in case of somewhat coarse intergrown ore avoiding further fine milling.

 

Manufacturer:- Millan, Volcan, Denver

TECHNICAL DATA:

Dimensions:

approx. 0.7 × 0.7 × 1.5 m, roller die 25 cm or more

Weight:

approx. 350 kg

Extent of Mechanization:

 fully mechanized

Power Required:

starting at approx: 5 kW

Form of           Driving Energy:

electric motor, internal combustion engine

Alternative Forms:

possibly hydromechanic

Mode of Operation:

continuous

Throughput/Capacity:   

approx. 700 kg/in

Technical Efficiency:

degree of comminution between 3:1 and 4:1

ECONOMIC DATA:

Investment Costs:

Denver mill, 2 t/h: 18.900 US $, Volcan mill, 500 kg/h: 5000 US $, Millan mill approx. 6500 DM including motor fob La Paz

Operating Costs:

labor costs, energy costs, minimal wear  

Side View of Machine:-

            

Crushers and their types

Crusher:-

A crusher is a machine designed to reduce large rocks into smaller rocks, gravel, or rock dust. Crushers may be used to reduce the size, or change the form, of waste materials so they can be more easily disposed of or recycled, or to reduce the size of a solid mix of raw materials (as in rock ore), so that pieces of different composition can be differentiated.

Types of crushers:-

Crushers are divided into two main categories on the basis of crushing property.

• Primary crusher
• Secondary crushers

Primary Crushers:-

The primary crusher mainly refers to the jaw crusher and impact crusher. They reduce 1.5 meter feed to approximately 10-20 cm particles.In the designing of a crushing plant of any nature and size, to select the right type and size of primary gyratory crusher is of great significance. Generally speaking, this machine is the largest and most expensive single item in the plant; a mistake in the choice may lead to a full replacement. Therefore, you have to pay close attention when choosing primary crushers. The following tips may be helpful for the selection of primary crushers.

• The name, hardness, humidity of material
• Pay attention to the hourly, daily or yearly capacity
• The discharging granularity or the final particle size of the finished products
• Crushing machine type and size
• Feeding method

Secondary Crushers:-

Secondary crushers mainly handle rocks of smaller particle size that have already been impacted and crushed from their original size. They reduce feed in .5-2 cm particles.In recent years, with the rapid development of infrastructure projects, rock crushers have found more applications in various fields and industries, such as mining, chemical industry, road construction, metallurgy, construction etc.

 

Table Of Different Types Of Crushers:-

 

 

Jaw Crusher:-

A jaw or toggle crusher consists of a set of vertical jaws, one jaw being fixed and the other being moved back and forth relative to it by a cam or pitman mechanism. The jaws are farther apart at the top -than at the bottom, forming a tapered chute so that the material is crushed progressively smaller and smaller as it travels downward until it is small enough to escape from the bottom opening. The movement of the jaw can be quite small, since complete crushing is not performed in one stroke.

Gyratory Crusher:-

A gyratory crusher is similar in basic concept to a jaw crusher, consisting of a concave surface and a conical head;both surfaces are typically lined with manganese steel surfaces. The inner cone has a slight circular movement, but does not rotate; the movement is generated by an eccentric arrangement. As with the jaw crusher, material travels downward between the two surfaces being progressively crushed until it is small enough to fall out through the gap between the two surfaces.

 

 

Cone Crusher:-

 

A cone crusher is similar in operation to a gyratory crusher, with less steepness in the crushing chamber and more of a parallel zone between crushing zones. A cone crusher breaks rock by squeezing the rock between an eccentrically gyrating spindle, which is covered by a wear resistant mantle, and the enclosing concave hopper, covered by a manganese concave or a bowl liner. As rock enters the top of the cone crusher, it becomes wedged and squeezed between the mantle and the bowl liner or concave. Large pieces of ore are broken once, and then fall to a lower position (because they are now smaller) where they are broken again. This process continues until the pieces are small enough to fall through the narrow opening at the bottom of the crusher.

Impact Crusher:-

 

mpact crushers involve the use of impact rather than pressure to crush material. The material is contained within a cage, with openings on the bottom, end, or side of the desired size to allow pulverized material to escape. This type of crusher is usually used with so ft and non-abrasive material such as coal, seeds, limestone, gypsum or soft metallic ores. There are two types of impact crushers: horizontal shaft impactor and vertical shaft impactor.

Horizontal shaft impactor (HSI) / Hammer mill:-

The HSI crushers break rock by impacting the rock with hammers that fixed upon the outer edge of a spinning rotor. The practical use of HSI crushers is limited to soft mat erials and non abrasive materials, such as limestone, phosphate, gypsum, weathered shales.

Vertical shaft impactor (VSI):-

VSI crushers use a different approach involving a high speed rotor with wear resistant tips and a crushing chamber designed to 'throw' the rock against. The VSI crushers utilize velocity rather than surface force as the predominant force to break rock. In its natural state, rock has a jagged and uneven surface. Applying surface force (pressure) results in unpredictable and typically non-cubicle resulting particles. Utilizing velocity rather than surface force allows the breaking force to be applied evenly both across the surface of the rock as well as through the mass of the rock. Rock, regardless of size, has natural fissures (faults) throughout its structure. As rock is 'thrown' by a VSI Rotor against a solid anvil, it fractures and breaks along these fissures. Final particle size can be controlled by 1) the velocity at which the rock is thrown against the anvil and 2) the distance between the end of the rotor and the impact point on the anvil.

Mineral Sizers:-

The basic concept of the mineral Sizer, is the use of two rotors with large teeth, on small diameter shafts, driven at a low speed by a direct high torque drive system. This design produces three major principles which all interact when breaking materials using Sizer Technology. The unique principles are; The Three-Stage Breaking Action, The Rotating Screen Effect, and The Deep Scroll Tooth Pattern.

  

The Three-Stage Breaking Action: Initially, the material is gripped by the leading faces of opposed rotor teeth. These subject the rock to multiple point loading, inducing stress into the material to exploit any natural weaknesses. At the second stage, material is broken in tension by being subjected to a three point loading, applied between the front tooth faces on one rotor, and rear tooth faces on the other rotor. Any lumps of material that still remain oversize, are broken as the rotors chop through the fixed teeth of the breaker bar, thereby achieving a three dimensional controlled product size.

 

Reductoin ratio:-

It is ratio of maximum size of feed to the maximun size of product. The average number of pieces the raw feed rock is reduced into.

Reduction ratio= Maximum size of feed/Maximum size of product

Different reduction ratios are given;