In the field of extractive metallurgy, mineral engineering,mineral
processing, also known as mineral dressing or ore dressing,
is the process of separating commercially valuable minerals from
their ores.
History
Before the advent of heavy machinery the raw ore was broken
up using hammers wielded by hand, a process called "spalling". Before
long, mechanical means were found to achieve this. For instance, stamp mills
were used in Samarkand
as early as 973. They were also in use in medieval Persia. By the 11th
century, stamp mills were in widespread use throughout the medieval Islamic world, from Islamic Spain
and North Africa in the west to Central
Asia in the east. A later example was the Cornish
stamps, consisting of a series of iron hammers mounted in a vertical frame,
raised by cams on the
shaft of a waterwheel
and falling onto the ore under gravity.
The simplest method of separating ore from gangue consists
of picking out the individual crystals of each. This is a very tedious process,
particularly when the individual particles are small. Another comparatively
simple method relies on the various minerals having different densities,
causing them to collect in different places: metallic minerals (being heavier)
will drop out of suspension more quickly than lighter ones, which will be
carried further by a stream of water. The process of panning and sifting for
gold uses both of these methods. Various devices known as 'buddles' were used
to take advantage of this property.Later, more advanced machines were used such
as the Frue vanner,
invented in 1874.
Other equipment used historically includes the hutch, a
trough used with some ore-dressing machines and the keeve or kieve, a large tub
used for differential settlement.
Unit operations
Mineral processing can involve four general types of unit
operation: comminution – particle size reduction; sizing –
separation of particle sizes by screening or classification; concentration
by taking advantage of physical and surface chemical properties; and dewatering
– solid/liquid separation. In all of these processes, the most important
considerations are the economics of the processes and this is dictated by the
grade and recovery of the final product. To do this, the mineralogy of the ore
needs to be considered as this dictates the amount of liberation required and
the processes that can occur. The smaller the particles processes, the greater
the theoretical grade and recovery of the final product, but this however is difficult
to do with fine particles as they prevent certain concentration processes from
occurring.
Comminution
Comminution is particle size reduction of materials.
Comminution may be carried out on either dry materials or slurries. Crushing and grinding
are the two primary comminution processes. Crushing is normally carried out on
"run-of-mine" ore, while grinding (normally carried out after
crushing) may be conducted on dry or slurried material. In comminution, the
size reduction of particles is done by three types of forces: compression,
impact and attrition. Compression and impact forces are extensively used in
crushing operations while attrition is the dominant force in grinding. The
primarily used equipment in crushing are-jaw crushers, gyratory crushers and
cone crushers whereas rod mills and ball mills, closed circuited with a
classifier unit, are generally employed for grinding purposes in a mineral
processing plant. Crushing is a dry process whereas grinding is generally
performed wet and hence is more energy intensive.
Sizing
The simplest sizing process is screening, or passing the particles
to be sized through a screen or number of screens. Screening equipment can
include grizzlies, bar screens,wedge wire screens, radial sieves, banana
screens, multi-deck screens, vibratory screen, fine screens, flip flop screens
and wire mesh screens. Screens can be static (typically the case for very
coarse material), or they can incorporate mechanisms to shake or vibrate the
screen. Some considerations in this process includes the screen material, the
aperture size, shape and orientation, the amount of near sized particles, the
addition of water, the amplitude and frequency of the vibrations, the angle of
inclination, the presence of harmful materials, like steel and wood, and the
size distribution of the particles.
Classification refers to sizing operations that exploit the
differences in settling velocities exhibited by particles of different size.
Classification equipment may include ore sorters,
gas
cyclones, hydrocyclones, rotating trommels, rake
classifiers or fluidized classifiers.
An important factor in both comminution and sizing
operations is the determination of the particle size distribution of the
materials being processed, commonly referred to as particle size analysis. Many techniques for
analyzing particle size are used, and the techniques include both off-line
analyses which require that a sample of the material be taken for analysis and
on-line techniques that allow for analysis of the material as it flows through
the process.
Concentration
There are a number of ways to increase the concentration of
the wanted minerals: in any particular case the method chosen will depend on
the relative physical and surface chemical properties of the mineral and the gangue.
Concentration is defined as the number of moles of a solute in a volume of the
solution. In case of mineral processing concentration means the increase of the
percentage of the valuable mineral in the concentrate.
Gravity concentration
Gravity separation is the separation of two or more minerals
of different specific gravity by their relative movement in response to the
force of gravity and one or more other forces (such as centrifugal forces,
magnetic forces, buoyant forces), one of which is resistance to motion (drag
force) by a viscous medium such as heavy media, water or, less commonly, air.
Gravity separation is one of the oldest technique in mineral
processing but has seen a decline in its use since the introduction of methods
like flotation, classification, magnetic separation and leaching. Gravity separation
dates back to at least 3000 BC when Egyptians used the technique for separation
of gold.
It is necessary to determine the suitability of a gravity
concentration process before it is employed for concentration of an ore. A
criteria called as concentration criterion is commonly used for this
purpose. It is defined as:
Concentration Criterion (CC)= (SG of heavy mineral-SG of
fluid)÷(SG of light mineral-SG of fluid), where SG=specific gravity
- for CC > 2.5, suitable for separation of particles above 75 micron in size
- for 1.75 <CC< 2.5, suitable for separation of particles above 150 micron in size
- for 1.50 < CC < 1.75, suitable for separation of particles above 1.7 mm in size
- for 1.25 < CC < 1.50, suitable for separation of particles above 6.35 mm in size
- for CC < 1.25, not suitable for any size
Although concentration criteria is a useful rule of thumb
when predicting amenability to gravity concentration, factors such as particle
shape and relative concentration of heavy and light particles can have a dramatic
impact on separation efficiency in practice.
There are several methods that make use of the weight or
density differences of particles:
- Heavy media or dense media separation (these include, baths, drums, larcodems, dyana whirlpool separators, and dense medium cyclones)
- Shaking tables, such as the Wilfley table
- Spiral separators
- Reflux Classifier
- Jig concentrators are continuous processing gravity concentration devices using a pulsating fluidized bed.(RMS-Ross Corp. Circular Jig Plants)
- Centrifugal bowl concentrators, such as the Knelson concentrator and Falcon Concentrator
- Multi gravity separators (Falcon Concentrator, Knelson, Mozley (Enhanced Gravity Separator), Salter Cyclones (Multi-Gravity Separator) and the Kelsey Jig)
- Inline pressure Jigs
- Reichert Cones
- Sluices
- Elutriators
These processes can be classified as either density
separation or gravity (weight) separation.
In dense media separation a media is created with a density
in between the density of the ore and gangue particles.
When subjected to this media particles either float or sink depending on their
density relative to the media. In this way the separation takes place purely on
density differences and does not, in principle, relay on any other factors such
as particle weight or shape. In practice, particle size and shape can have an
impact on separation efficiency. Dense medium separation can be performed using
a variety of mediums. These include, organic liquids, aqueous solutions or
suspensions of very fine particles in water or air. The organic liquids are
typically not used due to their toxicity, difficulties in handling and relative
cost. Industrially, the most common dense media is a suspension of fine
magnetite and/or ferrosilicon particles. An aqueous solution as a dense medium
is used in coal processing in the form of a belknap wash and suspensions in air
are used in water-deficient areas, like areas of China, where sand is used to
separate coal from the gangue minerals.
Gravity separation is also called relative gravity
separation as it separates particles due to their relative response to a
driving force. This is controlled by factors such as particle weight, size and
shape. These processes can also be classified into multi-G and single G
processes. The difference is the magnitude of the driving force for the
separation. Multi-G processes allow the separation of very fine particles to
occur (in the range of 5 to 50 micron) by increasing the driving force of
separation in order to increase the rate at which particles separate. In
general, single G process are only capable of processing particles that are
greater than approximately 80 micron in diameter.
Of the gravity separation processes, the spiral
concentrators and circular jigs are two of the most economical due to their
simplicity and use of space. They operate by flowing film separation and can
either use washwater or be washwater-less. The washwater spirals separate
particles more easily but can have issues with entrainment of gangue with the
concentrate produced..
Froth flotation
Froth flotation is an important concentration
process. This process can be used to separate any two different particles and
operated by the surface chemistry of the particles. In flotation, bubbles are
introduced into a pulp and the bubbles rise through the pulp. In the process,
hydrophobic particles become bound to the surface of the bubbles. The driving
force for this attachment is the change in the surface free energy when the
attachment occurs. These bubbles rise through the slurry and are collected from
the surface. To enable these particles to attach, careful consideration of the
chemistry of the pulp needs to be made. These considerations include the pH, Eh
and the presence of flotation reagents. The pH is important as it changes the
charge of the particles surface and the Eh affects the chemisorption of
collectors on the surface of the particles.
The addition of flotation reagents also affects the
operation of these processes. The most important chemical that is added is the
collector, This chemical binds to the surface of the particles as it is a
surfactant. The main considerations in this chemical is the nature of the head
group and the size of the hydrocarbon chain. The hydrocarbon tail needs to be
short to maximize the selectivity of the desired mineral and the headgroup
dictates which minerals it attaches to.
The frothers are another important chemical addition to the
pulp at it enables stable bubbles to be formed. This is important as if the
bubble coalesce, minerals fall off their surface. The bubbles however should
not be too stable as this prevents easy transportation and dewatering of the
concentrate formed. The mechanism of these frothers is not completely known and
further research into their mechanisms is being performed.
Depressants and activators are used to selectively separate
one mineral from another. Depressants inhibit the flotation of one mineral or
minerals while activators enable the flotation of others. Examples of these
include CN-, used to depress all sulfides but galena and this
depressant is believed to operate by changing the solubility of chemisorbed and
physisorbed collectors on sulfides. This theory originates from Russia. An
example of an activator is Cu2+ ions, used for the flotation of
sphalerite.
There are a number of cells able to be used for the
flotation of minerals. these include flotation columns and mechanical flotation
cells. The flotation columns are used for finer minerals and they typically
have a higher grade and lower recovery of minerals than mechanical flotation
cells. The cells in use at the moment can exceed 300 m3. This
is done as they are cheaper per unit volume than smaller cells, but they are
not able to be controlled as easily as smaller cells.
This process was invented in the 19th century in Australia.
It was used to recover a sphalerite concentrate from tailings, produced using
gravity concentration. Further improvements have come from Australia in the
form of the Jameson Cell, developed at the University of
Newcastle, Australia. This operated by the use of a plunging jet that generates
fine bubbles. These fine bubbles have a higher kinetic energy and as such they
can be used for the flotation of fine grained minerals, such as those produced
by the isamill.
Electrostatic separation
There are two main types of electrostatic separators. These
work in similar ways, but the forces applied to the particles are different and
these forces are gravity and electrostatic attraction. The two types are
electrodynamic separators (or high tension rollers) or electrostatic
separators. In high tension rollers, particles are charged by a corona
discharge. This charges the particles that subsequently travel on a drum. The
conducting particles lose their charge to the drum and are removed from the drum
with centripetal acceleration. Electrostatic plate separators work by passing a
stream of particles past a charged anode. The conductors lose electrons to the
plate and are pulled away from the other particles due to the induced
attraction to the anode. These separators are used for particles between 75 and
250 micron and for efficient separation to occur, the particles need to be dry,
have a close size distribution and uniform in shape. Of these considerations,
one of the most important is the water content of the particles. This is
important as a layer of moisture on the particles will render the
non-conductors as conductors as the layer of the water is conductive.
Electrostatic plate separators are usually used for streams
that have small conductors and coarse non-conductors. The high tension rollers
are usually used for streams that have coarse conductors and fine
non-conductors.
These separators are commonly used for separating mineral
sands, an example of one of these mineral processing plants is the CRL
processing plant at Pinkenba in Brisbane Queensland. In this plant, zircon, rutile and ilmenite are
separated from the silica gangue. In this plant, the separation is performed in a number
of stages with roughers, cleaners, scavengers and recleaners.
Magnetic separation
Magnetic separation is a process in which magnetically
susceptible material is extracted from a mixture using a magnetic force. This
separation technique can be useful in mining iron as it is attracted to a
magnet. In mines where wolframite was mixed with cassiterite, such as South
Crofty and East Pool mine in Cornwall or with bismuth such as at the Shepherd
and Murphy mine in Moina, Tasmania, magnetic separation was used to separate
the ores. At these mines a device called a Wetherill's Magnetic Separator
(invented by John Price Wetherill, 1844–1906)[1] was used. In this machine the
raw ore, after calcination was fed onto a moving belt which passed underneath
two pairs of electromagnets under which further belts ran at right angles to
the feed belt. The first pair of electromagnets was weakly magnetised and
served to draw off any iron ore present. The second pair were strongly
magnetised and attracted the wolframite, which is weakly magnetic. These
machines were capable of treating 10 tons of ore a day.This process of
separating magnetic substances from the non-magnetic substances in a mixture
with the help of a magnet is called magnetic separation..
This process operates by moving particles in a magnetic
field. The force experienced in the magnetic field is given by the equation
f=m/k.H.dh/dx. with k=magnetic susceptibility, H-magnetic field strength, and
dh/dx being the magnetic field gradient. As seen in this equation, the
separation can be driven in two ways, either through a gradient in a magnetic
field or the strength of a magnetic field. The different driving forces are
used in the different concentrators. These can be either with water or without.
Like the spirals, washwater aids in the separation of the particles while
increases the entrainment of the gangue in the concentrate.
Automated Ore Sorting
See also: Sensor-based sorting
Modern, automated sorting applies optical sensors (visible
spectrum, near infrared, X-ray, ultraviolet), that can be coupled with
electrical conductivity and magnetic susceptibility sensors, to control the
mechanical separation of ore into two or more categories on an individual rock
by rock basis. Also new sensors have been developed which exploit material
properties such as electrical conductivity, magnetization, molecular structure
and thermal conductivity. Sensor based sorting has found application in the
processing of nickel, gold, copper, coal and diamonds.
Dewatering
Dewatering is an important process in mineral processing.
The purpose of dewatering is to remove water absorbed by the particles which
increases the pulp density. This is done for a number of reasons, specifically,
to enable ore handling and concentrates to be transported easily, allow further
processing to occur and to dispose of the gangue. The water extracted from the
ore by dewatering is recirculated for plant operations after being sent to a
water treatment plant. The main processes that are used in dewatering include
dewatering screens, sedimentation, filtering, and thermal drying. These
processes increase in difficulty and cost as the particle size decreases.
Dewatering screens operate by passing particles over a
screen. The particles pass over the screen while the water passes through the
apertures in the screen. This process is only viable for coarse ores that have
a close size distribution as the apertures can allow small particles to pass
through.
Sedimentation operates by passing water into a large
thickener or clarifier. In these devices, the particles settle out of the
slurry under the effects of gravity or centripetal forces. These are limited by
the surface chemistry of the particles and the size of the particles. To aid in
the sedimentation process, flocculants and coagulants are added to reduce the
repulsive forces between the particles. This repulsive force is due to the
double layer formed on the surface of the particles. The flocculants work by
binding multiple particles together while the coagulants work by reducing the
thickness of the charged layer on the outside of the particle.
Thermal drying is usually used for fine particles and to
remove low water content in the particles. Some common processes include rotary
dryers, fluidised beds, spray driers, hearth dryers and rotary tray dryers.
This process is usually expensive to operate due to the fuel requirement of the
dryers.
Other processes
Many mechanical plants also incorporate hydrometallurgical
or pyrometallurgical
processes as part of an extractive metallurgical operation. Geometallurgy
is a branch of extractive metallurgy that combines mineral
processing with the geologic sciences.
A number of auxiliary materials handling operations are also
considered a branch of mineral processing such as storage (as in bin design),
conveying, sampling, weighing, slurry transport, and pneumatic transport.
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