Mineral Processing Instruments
Mineral Processing Instruments
Introduction
Mineral
processing can involve four general types of unit operation, Such as
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. In this process, physical
and chemical methods of minerals are used as above mentioned. And several
methods are used for different applications and different minerals
- Comminution
- Sizing
- Concentration
- Gravity concentration
- Froth Flotation
- Electrostatic separation
- Magnetic separation
- Automated Ore Sorting
- Dewatering
For above
methods, different machines and instruments are used. Crushers, Mills, Spirals,
Sinking Tables, Magnetic separators, Vibrators and so on. Following chapters
will briefly explained about the
machines and instruments
which we can
find at the
Mineral Processing Laboratory
at Department of Earth Resource Engineering in Moratuwa University.
Jaw crusher
Jaw crushers
are often used as primary crushers and
are perhaps the most popular crusher worldwide. These compressive crushers are
suitable for most any type of material. In this type of crusher, reduction
takes place between a stationary jaw plate and a moving jaw plate. The moving
jaw plate is mounted on the pitman, which is given a reciprocating motion.
Crushing takes place when the pitman moves toward the stationary jaw,
compressing the material. There are two main types of jaw crushers - single
toggle and double toggle.
At the
bottom of the assembly, the pitman is held in position by a toggle plate. The
combination of eccentric motion at the top and rocking motion at the bottom
provides a positive downward thrust throughout the crushing chamber.
Single
toggle jaw crushers have better feed acceptance capability than the
corresponding double toggle crushers. Jaw crushers are reliable, robust
machines, offering a 6:1 reduction ratio in most applications, and will
accommodate hard, abrasive materials.
Freedom series jaw crushers
Designed to
be a higher performance primary crushing solution than has ever been
available, the Freedom
Jaw Crusher features
an increased crushing
capacity of 15 percent, while maintaining the same
physical and power footprint as other jaw crushers. That means the Freedom 4450
Jaw can produce what another manufacturer's 50" x 60" jaw would
typically produce. This
crusher is ideal
for a greenfield
site, as well
as replacing an existing primary crusher since it's designed to deliver
the lowest cost per ton available in the market.
Universal jaw crushers
Introduced
in 1906, McLanahan's Universal Jaw Crusher was one of the first overhead
eccentric jaw crushers. Since that time, this robust crusher has continued to
be the basis of the Universal Jaw Crusher design. Today, it is available with
either traditional hydshim adjustment or adjust-on-the-fly capabilities with
analog control. It can be used in a
variety of operations,
including rock quarries,
sand and gravel
operation, mining operations, C
& D recycling, construction aggregates, and by road building contractors.
Materials processed by the Universal Jaw Crusher include limestone, granite,
basalt, gravel, gold ore, copper ore, iron ore, limestone, gypsum, concrete
rubble and more.
Advantages
·
Can
be used to crush large range of minerals,
·
Have
a range of particle size reduction ratios
Disadvantages
·
Enormous
Noise
·
Large
Energy loss
·
Huge
Maintain cost
Figure 1: Jaw
Crusher
Ball Mill
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 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 and
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 over fall
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 wetgrinding, 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.
Principle
A ball mill works on the principle of impact: size reduction
is done by impact as the balls drop from near the top of the shell.
Applications
The ball mill is used for grinding materials such as coal,
pigments, and feldspar for pottery. Grinding can be carried out either wet or
dry but the former is carried at low speed.
Advantages
Ball milling boasts several advantages over other systems:
the cost of installation, power and grinding medium is low; it is suitable for
both batch and continuous operation, similarly it is suitable for open as well
as closed circuit grinding and is applicable for materials of all degrees of
hardness.
Disadvantages
Huge energy loss. It uses only 0.1% - 1% of energy from
supplied amount. All other 99% energy is waste.
Humphreys Spiral (Spiral Separators)
Spiral separators of the wet type, also called spiral
concentrators, are devices to separate solid components in
a slurry, based
upon a combination
of the solid
particle density as
well as the particle's hydrodynamic properties
(e.g. drag). The device consists of a
tower, around which is wound a sluice, from which slots or channels are placed
in the base of the sluice to extract solid particles that have come out of
suspension.
As larger and heavier particles sink to the bottom of the
sluice faster and experience more drag from the bottom, they travel slower, and
so move towards the center of the spiral. Conversely, light particles stay
towards the outside of the spiral, with the water, and quickly reach the
bottom. At the bottom, a "cut" is made with a set of adjustable bars,
channels, or slots, separating the low and high density parts. Many things can
be done to improve the separation efficiency, including:
·
changing the rate of material feed
·
changing the grain size of the material
·
changing the slurry mass percentage
·
adjusting the cutter bar positions
·
running
the output of
one spiral separator
(often, a third,
intermediate, cut) through
a second.
·
adding wash water inlets along the length of the
spiral, to aid in separating light minerals
·
adding multiple outlets along the length, to
improve the ability of the spiral to remove
heavy contaminants
·
Adding ridges on the sluice at an angle to the
direction of flow.
