APPARATUS REQUIRED:-
- Denver Ball-mill
- A sieve-set with a sieve-shaker
- Feed sample
- Tachometer
- 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:-
No comments:
Post a Comment