Aluminium Processing
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Transcript of Aluminium Processing
1. INTRODUCTION
Origin of the report
The Mumbai universities have assigned the task of submitting report in
the subject presentation and communication techniques as a part of
curriculums.
Purpose
The purpose of the report is to know the students how the alumina is manufactured, which type of process is include in manufacture in alumina. And to find out the other alternative process to manufacturing alumina to reduce the cost of product.
Scope
Page | 1
The scope of the report is to know the students how the alumina is
manufactured, which type of process is include in manufacture in
alumina. And to find out the other alternative process to manufacturing
alumina to reduce the cost of product.
Limitations
Due to lack of time the visit was not possible therefore the all data is secondary (got it from the internet and books).
Sources of Methods of collecting data
World wide web (w.w.w.) Direct Reference.
2. TRACING THE HISTORY OF THE ALUMINIUM MINING
Page | 2
Aluminium came late into the world of major metals for the reason that
it was an electrolytic process product and required the electric furnace
at a high stage of development for its production. The Industry is only
half a century old and the principal producing countries have
developed the production of aluminium on the basis of one process and
one ore: .the process being the Hall-Héroult process, in which the oxide
of aluminium, alumina, produced by .chemical treatment of the bauxite
ore, is reduced electrolytic al ly; and the ore being high-grade bauxite,
which is a mixture of hydrated oxides of aluminium containing si l ica,
ferric oxide and other impurities.
2.1. Definition of Aluminium
Aluminium or aluminum is a si lvery white member of the boron
group of chemical elements. It has the symbol Al, and its atomic
number is 13. It is not soluble in water under normal circumstances.
Aluminium is the third most abundant element
(after oxygen and si l icon), and the most abundant metal , in
the Earth's crust. It makes up about 8% by weight of the Earth's solid
surface. Aluminium metal is too reactive chemically to occur natively.
Page | 3
Instead, it is found combined in over 270 different minerals. [ 4 ] The
chief ore of aluminium is bauxite.
2.2. History of Aluminium
Ancient Greeks and Romans used aluminium salts as dyeing mordants
and as astringents for dressing wounds; alum is sti l l used as a styptic.
In 1761,Guyton de Morveau suggested call ing the base
alum alumine. In 1808, Humphry Davy identif ied the existence of a
metal base of alum, which he at f irst termed alumium and
later aluminum (see etymology section, below).
The metal was first produced in 1825 (in an impure form)
by Danish physicist and chemist Hans Christian Ørsted. He
reacted anhydrous aluminium chloridewith potassium amalgam and
yielded a lump of metal looking similar to tin. [ 4 2 ] Friedrich Wöhler was
aware of these experiments and cited them, but after redoing the
experiments of Ørsted he concluded that this metal was pure
potassium. He conducted a similar experiment in 1827 by mixing
anhydrous aluminium chloride with potassium and yielded
aluminium. Wöhler is generally credited with isolating aluminium
(Latin alumen, alum), but also orsted can be l isted as its
discoverer. Further, Pierre Berthier discovered aluminium in bauxite
ore and successfully extracted it. Frenchman Henri Etienne Sainte-
Claire Devil le improved Wöhler's method in 1846, and described his
Page | 4
improvements in a book in 1859, chief among these being the
substitution of sodium for the considerably more expensive
potassium. Devil le l ikely also conceived the idea of the electrolysis of
aluminium oxide dissolved in cryolite; Charles Martin Hall and Paul
Héroult might have developed the more practical process after Devil le.
Before the Hall-Héroult process was developed in the late 1880s,
aluminium was exceedingly diff icult to extract from its various ores.
This made pure aluminium more valuable than gold. Bars of aluminium
were exhibited at the Exposition Universelle of 1855. [ 4 7 ] Napoleon II I ,
Emperor of France, is reputed to have given a banquet where the most
honoured guests were given aluminium utensils, while the others made
do with gold.
Aluminium was selected as the material to be used for the 100 ounce
(2.8 kg) capstone of the Washington Monument in 1884, a time when
one ounce(30 grams) cost the daily wage of a common worker on the
project. The capstone, which was set in place on December 6, 1884, in
an elaborate dedication ceremony, was the largest single piece of
aluminium cast at the time, when aluminium was as expensive as
si lver.
The Cowles companies supplied aluminium alloy in quantity in
the United States and England using smelters l ike the furnace of Carl
Wilhelm Siemens by 1886. Charles Martin Hall of Ohio in the U.S.
and Paul Héroult of France independently developed the Hall-Héroult
electrolytic process that made extracting aluminium from minerals
cheaper and is now the principal method used worldwide. Hall 's
process in 1888 with the financial backing of Alfred E. Hunt, started the
Pittsburgh Reduction Company today known as Alcoa. Héroult's process
was in production by 1889 in Switzerland at Aluminium Industrie,
now Alcan, and at Brit ish Aluminium, now Luxfer Group and Alcoa, by
1896 in Scotland.
Page | 5
By 1895, the metal was being used as a building material as far away
as Sydney, Australia in the dome of the Chief Secretary's Building.
Many navies have used an aluminium superstructure for their vessels;
the 1975 fire aboard USS Belknap that gutted her aluminium
superstructure, as well as observation of battle damage to Brit ish ships
during the Falklands War, led to many navies switching to
all steel superstructures. The Arleigh Burke class was the first such
U.S. ship, being constructed entirely of steel.
Aluminium wire was once widely used for domestic electrical wiring.
Owing to corrosion-induced failures, a number of f ires resulted. This
discontinuation thus i l lustrates one failed application of the otherwise
highly useful metal.
In 2008, the price of aluminium peaked at $1.45/lb in July but dropped
to $0.70/lb by December.
2.3. Old Method of Aluminium
Manufacturing
It has already been noted above that there are two essential stages in
the production of ingot aluminium from the ore which is normally
bauxite but may be alternatively clay, Lucite, alunite, nephelin or other
minerals giving rise to aluminium oxide. These two essential stages
Page | 6
are the production of pure aluminium oxide from the ore and the
reduction, by fused electrolysis, of the oxide to yield pure aluminium.
In addition to these main processes there are ancil lary operations such
as the manufacture of synthetic cryolite for the molten electrolysis
bath and the production of anodes for the electrolysis cells.
Bayer Process(Old Process)
The Bayer process is the principal industrial means of refining bauxite to
produce alumina (aluminium oxide).
Bauxite, the most important ore of aluminium, contains only 30–54% alumina, Al2O3,
the rest being a mixture of silica, various iron oxides, and titanium dioxide.[1] The
alumina must be purified before it can be refined to aluminium metal. In the Bayer
process, bauxite is digested by washing with a hot solution of sodium, NaOH, at 175
°C. This converts the alumina to aluminium hydroxide, Al(OH)3, which dissolves in the
hydroxide solution according to the chemical equation:
Al2O3 + 2 OH− + 3 H2O → 2 [Al(OH)4]−
The other components of bauxite do not dissolve. The solution is clarified by filtering
off the solid impurities. The mixture of solid impurities is called red mud, and
presents a disposal problem. Next, the hydroxide solution is cooled, and the
dissolved aluminium hydroxide precipitates as a white, fluffy solid. Then, when
heated to 980°C (calcined), the aluminium hydroxide decomposes to alumina, giving
off water vapor in the process:
2 Al(OH)3 → Al2O3 + 3 H2O
A large amount of the alumina so produced is then subsequently smelted in the Hall–
Héroult process in order to produce aluminium.
Page | 7
(Fig 2.2.1) Bayer Process
2.4. Modern Method of Aluminium
Manufacturing
There are two stages of Aluminium Manufacturing
Page | 8
STAGE 1- Converting Bauxite to Alumina
STAGE 2- Converting Alumina to Aluminium
STAGE 1- Converting Bauxite to Alumina
STEP 1- Crushing and Grinding: Alumina recovery begins by passing
the bauxite through screens to sort it by size. It is then crushed to
produce relatively uniformly sized material. The ore is then fed into
large grinding mil ls and mixed with a caustic soda solution ( sodium
hydroxide) at high temperature and pressure. The grinding mil l rotates
l ike a huge drum while steel rods - rol l ing around loose inside the mil l -
grind the ore to an even finer consistency. The process is a lot l ike a
kitchen blender only much slower and much larger. The material f inally
discharged from the mil l is called slurry. The resulting l iquor contains a
solution of sodium aluminate and undissolved bauxite residues
containing iron, si l icon, and titanium. These residues - commonly
referred to as "red mud" - gradually sink to the bottom of the tank and
are removed .
(Fig 2.3.1) Crushing of Aluminium
Page | 9
STEP 2-Digesting: The slurry is pumped to a digester where the
chemical reaction to dissolve the alumina takes place. In the digester
the slurry - under 50 pounds per square inch pressure - is heated to
300 °Fahrenheit (145 °Celsius). It remains in the digester under those
conditions from 30 minutes to several hours.
More caustic soda is added to dissolve aluminum containing compounds
in the slurry. Undesirable compounds either don't dissolve in the
caustic soda, or combine with other compounds to create a scale on
equipment which must be periodically cleaned. The digestion process
produces a sodium aluminate solution. Because all of this takes place
in a pressure cooker, the slurry is pumped into a series of " flash tanks"
to reduce the pressure and heat before it is transferred into " settl ing
tanks."
(Fig 2.3.2) Digestion of Aluminium
STEP 3-Settling: Settl ing is achieved primarily by using gravity,
although some chemicals are added to aid the process. Just as a glass
of sugar water with fine sand suspended in it wil l separate out over
time, the impurities in the slurry - things l ike sand and iron and other
trace elements that do not dissolve - wil l eventually settle to the
bottom.
The l iquor at the top of the tank (which looks l ike coffee) is now
directed through a series of f i lters. After washing to recover alumina
and caustic soda, the remaining red mud is pumped into large storage
ponds where it is dried by evaporation.
Page | 10
The alumina in the sti l l warm liquor consists of t iny, suspended
crystals. However there are sti l l some very fine, solid impurities that
must be removed. Just as coffee fi lters keep the grounds out of your
cup, the fi lters here work the same way.
The giant-sized fi lters consist of a series of "leaves" - big cloth fi lters
over steel frames - and remove much of the remaining solids in the
l iquor. The material caught by the fi lters is known as a " fi lter cake" and
is washed to remove alumina and caustic soda. The fi ltered l iquor - a
sodium aluminate solution - is then cooled and pumped to the
"precipitators."
(Fig 2.3.3) Settl ing of Aluminium
STEP 4-Precipitation: Imagine a tank as tall as a six-story building.
Now imagine row after row of those tanks called precipitators. The
clear sodium aluminate from the settl ing and fi ltering operation is
pumped into these precipitators. Fine particles of alumina - called
"seed crystals" (alumina hydrate) - are added to start the precipitation
of pure alumina particles as the l iquor cools. Alumina crystals begin to
grow around the seeds, then settle to the bottom of the tank where
they are removed and transferred to " thickening tanks." Finally, it is
f i ltered again then transferred by conveyor to the "calcination kilns."
Page | 11
(Fig 2.3.4) Precipitation of Aluminium
STEP 5-Calcination: Calcination is a heating process to remove the
chemically combined water from the alumina hydrate. That's why, once
the hydrated alumina is calcined, it is referred to as anhydrous
alumina. "Anhydrous" means "without water."
From precipitation, the hydrate is f i ltered and washed to rinse away
impurities and remove moisture. A continuous conveyor system delivers
the hydrate into the calcining kiln. The calcining kiln is brick-l ined
inside and gas-fired to a temperature of 2,000 °F or 1,100 °C. It slowly
rotates (to make sure the alumina dries evenly) and is mounted on a
ti lted foundation which allows the alumina to move through it to
cooling eqipment. (Newer plants use a method called fluid bed
calcining where alumina particles are suspended above a screen by hot
air and calcined.)
The result is a white powder l ike that shown below: pure alumina. The
caustic soda is returned to the beginning of the process and used
again.
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(Fig 2.3.5) Calcination of Aluminium
STAGE 2- Converting Alumina to Aluminium
Smelting: In 1886, two 22-year-old scientists on opposite sides of the
Atlantic, Charles Hall of the USA and Paul L.T. Heroult of France, made
the same discovery - molten cryolite (a sodium aluminum fluoride
mineral) could be used to dissolve alumina and the resulting chemical
reaction would produce metall ic aluminum. The Hall-Heroult
process remains in use today.
The Hall-Heroult process takes place in a large carbon or graphite l ined
steel container called a " reduction pot". In most plants, the pots are
l ined up in long rows, called potl ines.
The key to the chemical reaction necessary to convert the alumina to
metall ic aluminum is the running of an electrical current through the
cryolite/alumina mixture. The process requires the use of direct current
(DC) - not the alternating current (AC) used in homes. The immense
amounts of power required to produce aluminum is the reason why
Page | 13
aluminum plants are almost always located in areas where affordable
electrical power is readily available. Some experts maintain that one
percent of al l the energy used in the United States is used in the
making of aluminum.
The electrical voltage used in a typical reduction pot is only 5.25 volts,
but the amperage is VERY high - generally in the range of 100,000 to
150,000 amperes or more. The current f lows between a
carbon anode(positively charged), made of petroleum coke and pitch,
and a cathode (negatively charged), formed by the thick carbon or
graphite l ining of the pot.
When the electric current passes through the mixture, the carbon of
the anode combines with the oxygen in the alumina. The chemical
reaction produces metall ic aluminum and carbon dioxide. The molten
aluminum settles to the bottom of the pot where it is periodically
syphoned off into crucibles while the carbon dioxide - a gas - escapes.
Very l itt le cryolite is lost in the process, and the alumina is constantly
replenished from storage containers above the reduction pots.
The metal is now ready to be forged, turned into alloys, or extruded
into the shapes and forms necessary to make appliances, electronics,
automobiles, airplanes cans and hundreds of other familiar, useful
items.
Aluminum is formed at about 900 °C, but once formed has a melting
point of only 660 °C. In some smelters this spare heat is used to melt
recycled metal, which is then blended with the new metal. Recycled
metal requires only 5 per cent of the energy required to make new
metal. Blending recycled metal with new metal al lows considerable
energy savings, as well as the efficient use of the extra heat available.
When it comes to quality, there is no difference between primary metal
and recycled metal.
Page | 14
The smelting process required to produce aluminum from the alumina is
continuous the potl ine is usually kept in production 24 hours a day
year-round. A smelter cannot easily be stopped and restarted. If
production is interrupted by a power supply fai lure of more than four
hours, the metal in the pots wil l solidify, often requiring an expensive
rebuilding process. The cost of building a typical, modern smelter is
about $1.6 bil l ion.
Most smelters produce aluminum that is 99.7% pure - acceptable for
most applications. However, super pure aluminum (99.99%) is required
for some special applications, typically those where high ducti l ity or
conductivity is required. It should be noted that what may appear to be
marginal differences in the purities of smelter grade aluminum and
super purity aluminum can result in significant changes in the
properties of the metal.
Page | 15
(Fig 2.3.6) Smelting of Aluminium
3. RESOURCES USED IN
ALUMINIUM
MANUFACTURING
There have been essential ly no fundamental changes in process or
equipment since the beginning of the industry, although great
improvements have been made in the design of the furnaces and in the
electrodes.
Page | 16
3.1. Raw Materials
The aluminium industry has been oriented with reference to the raw
materials bauxite, electric power, coal and caustic soda used in the
preparation of alumina for electrolysis, and carbon for the manufacture
of electrodes.
One metric ton of aluminium requires three to four horse-power years
(roughly equivalent to 20,000-25,000 kWh) of electrical energy, four to
f ive tons of bauxite, four to f ive tons of coal, a substantial amount of
water, about one ton of caustic soda (of .which the greater part is,
however, recoverable) and 0.5-0.6 tons carbon electrodes--that is to
say, in all more than ten tons of material plus a large quantity of
power. These are optimum figures under the best production conditions
with the best materials.
Labour requirements depend upon the scale of operations, in the
largest plants being about 5 to 6 man-hours per ton of alumina
produced and about 30 man-hours per ton of aluminium produced from
alumina. In smaller plants the labour may work out as high as double
this f igure.
With the above starting point it is apparent that aluminium can be
produced at the cheapest rate where the combination of cheap power,
good bauxite and cheap coal exist: since there is, however, no region
in the world where these three raw materials are coincident--in other
words, since no one country enjoys sufficient preferential advantages
to grant it a monopoly--the process of aluminium production based on
the Hall-Héroult process and on bauxite has involved one main
dichotomy, it having been found more economical to produce the
alumina for reduction dose to coal rather than to bauxite, whilst the
electrolytic reduction plants must, of course, be located close to the
Page | 17
source of power. This dichotomy immediately introduces the factor of
transport, which becomes a further vital "raw material."
3.1.1. Availability of Power
In the early days of the aluminium industry power was almost
universally construed in terms of water power; hydro-electric energy
was then becoming an important factor in industrial economy and
strenuous efforts were made by the major industrial nations to secure
favorable locations and development of water power. At that time the
modern conception of electric power stations with alternative sources
of energy based on alternative fuels such as powdered coal, mineral
oi l , or even peat on a large scale (as used in certain regions of the
U.S.S.R.) was non-existent and hydraulic power represented a unique
advantage in the newly developing electrolytic process industries in
which aluminium was one of the major products. Capital investment in
hydraulic power stations varied greatly with the natural resources but
viewed as a long-term project it was always a good economic
proposition and in favorable instances . it provided remarkably cheap
power, as for instance, on the Saguenay river or at Niagara. In Europe,
conditions were, of course, at their best in Switzerland and Norway.
Since the situation of the source of power is nearly always unfavorable
for the location of a major industrial undertaking, power derived from
hydraulic stations was commonly relayed to selected centers which
could serve as the loci for assembly of the other raw materials and for
the erection of works and their concomitant towns and satell ite
industries.
These factors, with their geographic and economic background, are
complementary to the technical aspects of the use of water power in
the cheapest manner. An important consideration in all electrolytic
processing is the necessity for a continuous supply of electric energy
(in contrast with part-time uti l ization for engineering and municipal
requirements, e.g., for l ighting and heating) which implies a very high
Page | 18
load factor. From this angle the needs of an electrolytic industry such
as aluminiumare both favorable and unfavorable as regards power
consumption: on the one hand, the power undertaking can count on
disposing of an otherwise unused capacity and on the other hand, it is
essential that the production .cost of the energy shall be low and the
power supply steady. A hydro-electric system normally expects to
provide for commitments for a certain amount of f irm power which has
to be based upon the lowest level of water during the year, taking into
account normal storage provision. Due to irregular volume of water
f low, both annual and seasonal, the minimum may be exceeded during
certain years and for certain months in every year and this excess
power, known as secondary power, has been uti l ized successfully in the
electrolytic industry and to some extent in the aluminium industry. In
this industry, however, most of the big producers have their own power
undertakings or own at least a controll ing interest in them.
3.1.2. Coal & Fuel
Fuel is one of the most important materials consumed in the production
of aluminium, being essential to f ire the kilns in the production of
alumina. Unti l the outbreak of the second world war, the only fuel used
in the manufacture of alumina was coal; but during the war years and
particularly in America, some shift has taken place towards other fuel
sources, for example, natural gas, oi l or electricity may also be used in
the production processes according to the situation of the works
relative to a supply at a favorable cost.
In Europe the most favorably situated countries from the angle of coal
supply for alumina production were Germany and England, with Italy as
a runner-up. In the United States a good supply of high-quality coal is
available in the State of Washington and there is plenty of lower grade
bituminous coal for most of the plants producing alumina, although oil
was substituted for coal at four or f ive plants distributed through the
States in recent years.
Page | 19
(f ig 3.1) Coal
3.1.3. Caustic Soda
In general the aluminium industry has concerned itself only with the
mining of bauxite and the manufacture of alumina and metall ic
aluminium and has not entered into the production of the caustic soda
uti l ized in the alumina process. Both the processes used on a large
scale for the production of caustic soda, namely the soda-ammonia and
the electrolytic process,. Belong essential ly to the heavy chemical
industry and large quantities are available at a low price. The raw
material for caustic soda production, which is chiefly soda ash, is
available in abundant supply. This material also enters directly into the
process of alumina production by the Bayer method in some instances.
Page | 20
(f ig 3.2) Caustic Soda
3.1.4. Electrode Carbon
The anodes used in the electrolysis cells are prepared from a mixture
of petroleum coke and ordinary pitch, roughly in 70-30 ratio, but this
f igure varies according to the quality of the coke. Petroleum coke is a
by-product of petroleum refining, being actually the residue after the
volati le and fuel oi ls have been withdrawn. Before it is ready for
service for the production of anodes. The crude materials from the
petroleum sti l ls have to be calcined and this process requires special
plant and consumes one-fifth to one-quarter of the weight of crude
coke processed in fuel supply.
(f ig 3.3) Electrode Carbon
Page | 21
3.1.5. Bauxite
Chemically, bauxite consists of hydrated aluminium oxide together with
oxides of iron si l icon, t itanium and other elements, and varying small
percentages of clay and other si l icates. Physically, bauxite can be as
hard as rock or as soft as mud. Its colour may be buff, pink, yellow,
red, white or various combinations of colours.
Bauxite is the ore that is converted to alumina and then to aluminium.
Australia is the world's leading producer of bauxite, producing 33% of
the world’s bauxite production in 2007 (63 MT). Its deposits are among
the largest in the world (estimated at more than seven bil l ion tones of
commercial grade ore),
Second only to Guinea. Bauxite is mined by open cut methods in the
Northern Territory, Queensland and in Western Australia.
(f ig 3.4) Bauxite
3.1.6. Alumina
Page | 22
Alumina is made from bauxite at refineries in the Northern Territory,
Queensland and Western Australia. Around two and a half tonnes of
bauxite are needed to produce one tonne of alumina. Australia is the
world's leading producer of alumina, producing 30% of global output.
The majority of the world's alumina production is made by a process
known as the “Bayer Process”, which was developed in 1889 by German
scientist, Karl Bayer. This complex refining process produces a fine
white anhydrous aluminium oxide powder, known as alumina AL203.
The “Bayer Process” begins by grinding the bauxite and mixing it with
caustic soda to form slurry. The aluminium component is dissolved out
of the bauxite in a series of steam-heated digesters. The l iquid passes
into large settl ing tanks where the impurities, including sand and iron
oxide, settle out. The aluminate l iquor is f i ltered, cooled and “seeded”
with crystals of aluminium hydroxide to aid precipitations of alumina
hydrate. The hydrate is then fi ltered, washed and passed through
rotating calcinating kilns operating at high temperatures to produce
the white powder known as alumina.
More than 90% of the world's alumina production is used to make
aluminium. Small quantities are also used as an abrasive, absorptive
and refractory in the chemical ceramics and glass-making industries.
(f ig 3.5) Alumina
Page | 23
3.2. Main Chemical & Mineralogical
Constituents of Bauxite
TAA (Total Available Alumina – 42-43%)
This is very much important for cost effective production in Alumina
Refinery.
Gibbsite (Al203 3H20- 32-34%)
Gibbsite is crystall ine euhedral with variable grain sizes. It is more
easily soluble in Caustic Soda (< 135 oC) and the most preferred
bauxite. % Gibbsite of TAA should be 80-82%. When the bauxite is
predominantly Gibbsitic, the digestion can be carried out at relatively
low temperature i.e. Atmospheric Digestion (A.D.) at 105 oC and Low
Pressure Digestion (L.P.D.) at 140-145 oC, Energy consumption is less in
case of A.D. or L.P.D.
Boehmite (Al203 H20 – 9-11%)
It is soluble in Caustic Soda at higher temperature and pressure. %
Boehmite of TAA should be 18-20%. For boehmite bauxite high
temperature digestion at 240-245 oC is preferred. Energy consumption
in alumina refinery is higher in case of such bauxite. It is tough to
digest and result in low extraction efficiency.
Diaspore (0.8-1.2%)
It is soluble in caustic soda at very high temperature (greater than
300oC) and pressure. It is not preferred.
Page | 24
Silica (3-4%)
Caustic loss in alumina refinery is basically due to reaction of si l ica,
alumina and caustic leading to formation of desil icated product (DSP,
Sodalite or Can crinite) which is insoluble during digestion process.
Kaolinite (Al4 (Si4O10) OH)
It is reactive part of si l ica causes loss of both alumina and caustic soda
as well contaminate product and forms scales.
Quartz (SiO2 – 1.0%)
Non reactive part of si l ica but at high temperature a certain amount of
quartz can also dissolve in the l iquor.
Iron (15 – 18%)
Iron in bauxite is essential ly insoluble during digestion process and
increases mud load. Iron is distributed in 60:40 proportions as
Hematite and Goethite in most of Indian Bauxite.
Goethite (Fe203 H20- 5-7%)
Goethite in Iron should be 30-35%. Goethite causes settl ing problem
and alumina locked in Fe-minerals is diff icult to extract. Unreacted
Goethite can be colloidal in nature and result in settl ing trouble. It
also acts as an active seed for auto precipitation in the mud circuit.
Higher Goethite/Hematite ratio wil l result in higher potential for this
problem.
Hematite (Fe203 – 9-10%)
Higher proportion of hematite in iron is good for settl ing.
Vanadium (V205 – 0.25-0.30)
Page | 25
It is a valuable by product from bayers alumina refinery. More
vanadium dissolves at high temperature digestion and if not removed
forms harmful Vandate compound.
Gallium (Ga)
Found in traces (15 to 150 ppm) Bayer l iquor has been found world
richest source of gall ium.
Organic Matters (0.02-0.5%)
increases impurity level and affect alumina precipitation. Organic
matter as organics is highly soluble in Bayer l iquor and forms sodium
oxalate.
Page | 26
4. PROCESSES WHICH TAKES
PLACE IN THE
MANUFACTURING OF
ALUMINIUM
4.1. Hammer Mill
The purpose of Hammer Mil l area is to reclaim and grind 3” – 4”
Bauxite to ½” size (95%) by Hammer Mil l and transport it to Bauxite
day bin.
There are three Hammer Mil l circuits each containing two
Hammer Mil ls namely: -
Hammer Mil l # 1 & 2 - Sweetening Bauxite
Hammer Mil l # 3 & 4 - Normal Bauxite
Hammer Mil l # 5 & 6 - Normal Bauxite
Hammer Mil l # 7 - Normal Bauxite
Page | 27
Equipment provided in each circuit are Bauxite Feeder (Reclaim &
Emergency) 2 No’s, Hammer Mil l Feed Conveyor, Magnet & Metal
detector on Conveyor, Vibrating Screen, Hammer Mil l , Hammer Mil l
discharge Conveyor, Sump Pump in Conveyor Pits, Dust collection
system (Bag Filters), Weight-o-meter on Hammer Mil l discharge
Conveyor (Old circuit only), Air Blasters.
Hammer Mil l Description: It contains a high-speed rotor turning a
cylindrical body. The shaft is horizontal. Here the bauxite is dropped
into the top of the casing and gets broken by set of swing hammers
pinned to a rotor disk. Bauxite shatters into pieces, which fly against a
l iner inside the casing and breaks into small fragments. This in turn is
further crushed by the hammers and pushed through cage bars, which
covers the discharge opening.
The crit icality of the operation is to maintain the size of the hammer
mil l discharge, it should be +1/2” up to 5%. But in the raining season it
increases to 10%.
For health and safety purpose chemicals and water are spread in the
unloading area at the Wagoner end, for suppressing the fine dust of
bauxite.
Trouble shooting practices are for oversize product, abnormal sound
coming from Hammer Mil l , abnormal vibration in Hammer Mil l , bauxite
not coming through feeders to feed end conveyor of Hammer Mil l ,
misalignment of Conveyors and hammer Mil l tripping frequency.
Page | 28
(f ig 4.1) Hammer Mil l
4.1.1. Specification of Hammer Mill
Motor capacity-270 hp
No. of hammer -80
Grinding capacity-150 t/h
Load current-47.7 amp
Type – f lood coupling
4.1.2. Operation of Hammer mill During Rainy Season
Due to wet, bauxite operation becomes diff icult as plugging of feed and
discharge chute, etc. occurs.
To avoid diff iculty fol lowing steps are taken-
1\2’’cage bars are replaced by 1’’.
1\2’’ bypass screen is replaced by 1’’.
Feed rate is reduced
Page | 29
There are eight numbers of ball mil ls, which operate at different
capacities. The 1/2" size of bauxite from the hammer mil l is stored in
the day bin. Then with the help of belt conveyor it is feed into the ball
mil l . The load shell of belt conveyor takes care of feed rate of the ball
mil l . Out of eight ball mil ls, seven ball mil ls always remain in l ine. Here
wet grinding with the spent l iquor in the ball mil l is not done. For wet
grinding maximum speed of ball mil l should be 70-75% of its crit ical
speed.
4.1.3. Critical Speed
The crit ical speed is the theoretical speed at which the centrifugal
force acting on a ball mil l in contact the mil l shell at the height of its
path equals the force on it due to gravity.
The expression for crit ical speed is-
NC = 76.6/D1 / 2
Where NC = Crit ical Speed, D = Diameter of ball mil l in feet.
Ball changed inside a ball is 50% of its volume that is called bed
height which gives maximum capacity. The discharge of the ball mil l is
solid 50% with –200 mesh size (60-65%). From the discharge point of
view ball mil ls are of two types. In the first type the discharge of ball
mil l is passed through the simple sieve of –200 mesh size. The
underflow goes to the desil ication unit where as overflow again goes
into the ball mil l . In the other type the discharge of the ball mil l is
passed through the sump tank to the primary cyclone. The overflow of
the PC goes to secondary cyclone feed tank (SCFT) & underflow goes to
the ball mil l . From the SCFT slurry is feed to the secondary cyclone.
The overflow of the SC goes to the sump tank. The density of the
Page | 30
discharged slurry is maintained at1.82 kg/l. The description of
different ball mil ls is given below.
BALL
MILL NO
BAUXITE
RATE(T/HR)
LIQUOR
RATE
MOTOR
H.P.
TYPE
1 15 9-10 350 Overflow
2 30-32 16-18 900 Internal Grate
3 30-32 16-18 900 Internal Grate
4 40 17-19 1400 Overflow
5 50 23-25 1400 Overflow
6 50 23-25 1400 Overflow
7 70 28-30 2000 Overflow
8 50 25 1400 Overflow
(Chart 4.1)
The product quality of ball mil l plays an important role in the digestion
process. So the size of the ball & speed of the mil l is very crit ical. Lots
of sound pollution is there in ball mil l area. The level of sound isnearly
105 dB. Some of the area of troubleshooting is slurry density, which
becomes sometimes low. This can be removed by regular monitoring of
the ball mil l discharge.
Page | 31
4.2. Desilication
The product of the ball mil l (bauxite slurry) of al l ball mil ls is passed
through a desil ication heater where slurry is preheated up to 98°C by
means of steam of 2.4 kg/cm 2 pressure & temperature of 130 °C. This
slurry is passed through six no of desil ication tanks to provide holding
time of 10-12 hrs. Ball mil l no 4, 5, 6, 7&8 are used for normal bauxite
grinding & ball mil l 1, 2& 3 for the grinding of Gibbsitic bauxite. The
main reaction-taking place in the tank is
5[Al2O3 .2SiO2 .2H2O]+2Al(OH)3+12NaOH
2[3Na2O.3Al2O3 .5SiO2 .5H2O] + 9H2O
The temperature maintained for this reaction is nearly 98 °C. The slurry
first comes in the desil ication tank no 1, and then it is passed through
two heaters.
After passing through heater it goes to desil ication tank number 2, 3,
4, 5 & 6, where temp maintained is around 98°C and holding time is
12hrs to 14hrs. The maintained temperature and holding time is the
crit ical parameters for the desil ication tank. The main problem of the
desil ication area is the scaling in desil ication heaters. Life of heater is
30 days. After every 30 days heater is being cleaned with chemicals.
Efficiency of heater is around 60 % & level maintained in desil ication
tank is 90%. We can increase the service l ife of heater by using some
other metal instead of mild steel, l ike Al al loys, which are less
corrosive than mild steel. Also the speed of the slurry which passes
through heater is important parameter to be governed. The desil ication
product gets stick to the tank wall, which are removed after every 5 to
6 months. The main problem of desil ication area is the low pick up of
desil ication heater temperature.
Page | 32
4.3. Digestion
Two methods are in use for the digestion of bauxite-
Namely “One stream” and “Two stream” process.
One Stream: bauxite-l iquor slurry is heated indirectly to the digestion
temperature by passing through in series connected autoclaves .
Two Stream: the digesting l iquor is divided into two unequal stream.
The main stream (80-85%) is heated step-by-step in tubular heat-
exchangers with steam from flash tanks. The remaining l iquor is led to
the wet grinding of bauxite and fed.
LTD DIGESTION: In old technology 3 units of digestion are used to
digest alumina in caustic And each units are high temp type digestion
but in modern technology two type of digestion is used. First one is low
temp digestion and second is high temp digestion. This modification in
process is done on the basis that try hydrated alumina digests at lower
temp and pressure in caustic at 145 oC and12 bar while in high temp
digestion it digests at 245 oC and 36kg/cm2.
In this preheated slurry is fed to low temp digestion where it mixed
with 600psi and 290 gpl caustic and temp and pressure is maintained
Page | 33
at 145 oC and 12 bar and digested slurry is fed to pressure decanter
where we use flocculant to increase settl ing rate .and overflow of P D is
fed to flash tank while under f low is fed to 3 units of high temp
digestion using geho pumps.
(f ig 4.2) HTD Circuit Diagram
Page | 34
240-242OC
TO SAND CLASSIFIER
230 - 240 M3/Hr
DILUTION FROM
WASHER
70-80M3/Hr
190-205 M 3/Hr
80-82 o C
GEHO PUMPBOOSTER PUMP
LIQUOR HEATER
40% BAUXITE SLURRY FROM BALL MILLS
45-55 M3/Hr
TO BOILER 14-15 TPH
CONDENSATE FLASHING
30 psi STEAM TO WASHER 1.4-2.4 TPH100 psi 3.4-3.6 TPH
104-105OC 40% BAUXUITE
SLURRY DENSITY 189
TEMP. 95-96OC
DESILICATOR HEATER
78-80OC
CAUSTIC LIQUOR FROM EVAPORATION C255-
258gpl
155-165 M3/Hr
64-66 o C
CONDENSATE PUMP
BLOW OFF PUMP
106-108 OC
FLASH VAPORS FLASH TANKS
SLURRY HEATERS
CONDENSATE POTS
260-270 OC STEAM
42.5 Kg/Cm2
#2
DIGESTION FEED TANK
DIGESTER
ISH
ISH COND. POT
123
CONDENSATE TO BOILER
45 - 50 M 3/Hr
DISILICATOR TANK
8
9 268 7 5 4 3 1
6 5 4 3 2 17
BLOW OFF
TANK
#2 #2 #2 #2 #2 #2#1 #3 #4 #6 #7#5
240-242OC
Crea ted by : PANKAJ PANDEYH T D CIRCUIT DIAGRAM
HTD Digestion
Underflow of P D of LTD is pumped into the H T digestion having two
streams and direct heating system through slurry heaters where
Alumina content of bauxite is dissolved into caustic solution at 242 oC
temperature and 36 Kg per sq. cm pressure, in digestion I and II .
Digestion I I I is of single stream and indirect steam reaction occurring in
digestion is
Al2O3.3H2O + 2NaOH 2420C & 36Kg/cm2 2NaAlO2 + 4H2O
The above reaction is an endothermic reaction and requires heat for
reaction to take place. Two methods are in use for the digestion of
bauxite, namely “two stream” and “one stream” process.
Two streams is a process in which the digesting l iquor is divided into
two unequal streams. The main stream (80-85%) is heated step-by-step
in tubular heat exchangers with steam from flash tanks. The remaining
l iquor is led to the wet grinding of bauxite and fed finally to the
digesters. This method is used in digester I & I I . In one stream method
bauxite l iquor slurry is heated indirectly to the digestion temperature
by passing through in series connected autoclaves. This method is used
in digestion I I I .
To push the bauxite slurry into the mixing tee Geho pump is used. The
Geho pump has a diaphragm, which sucks and creates the adequate
pressure. Geho pump is used for high pressure and low flow. The
capacity of the Geho pump of digestion area I I and I is 17.75-35.5
M3 /Hr. and 133.8-153.9M 3 /Hr. respectively.
Page | 35
Caustic addition (290gpl)
67-70 oC
Bxt-Slurry
(f ig 4.3) Caustic Addition
The spent l iquor comes in the test tank where the concentration of the
caustic is maintained at 290 gpl by the addition of fresh caustic 900
gpl. Then with the help of pumps it is sent to the low-pressure heater
and medium pressure heater, before it comes to the mixing tee. From
the mixing tee it (Bxt-slurry l iquor) goes to the process slurry heater
where temperature is increased to 193 oC. Then it goes to ISH, where
Page | 36
Test tankPrimary
Booster
Charge Pump 20-22Kg/m3 Low Pressure
Heater
Booster
PumpMedium Pressure
Injection
Pump 69Kg/cm2Mixing Tee
Process Slurry
Indirect Steam Heater
193oC
Live Steam
Digesters 35 kg/cm2
230oCLive steam from Boiler at 42 bars
Tail Valve 35Kg/cm2
Flash Tank 1 to 7Blow OffBlow Off
PumpCLARIFICATIO
l ive steam is used to raise the temperature up to 230 oC. And finally the
mixture is passed through the digesters at 242 oC and 36Kg/cm 2
pressure. Holding time in digesters is 2hrs. The digested boehmite
bauxite slurry is f lashed in successive stages of f lash tanks and finally
the pressure is brought down near to atmosphere.
Sweetening is done in f lash tank 2, 3 or 4 at temp. 150 oC. in
sweetening process slurry is heated from 65 oC to 100oC by means of
2.4Kg/cm2 steam. And this slurry is fed to desired flash tank inlet l ine
in digestion unit I , I I , I I I through centrifugal pumps where it mixes with
the digester normal bauxite slurry. The Gibbsitic bauxite has higher
solubil ity at lower temperature (140 oC) resulting in increasing the
finished A/C ratio and enhanced rate of alumina dissolution. The main
advantages of this process are in increased rate of alumina dissolution
only through marginal addition steam requirement and higher l iquor
productivity.
The major operating parameters are steam temp., digestion temp.,
slurry flow, digestion pressure, steam pressure, back pressure
(pressure of control valve), heater temp. The main crit icality of the
operation is scaling in the heater pipe, vessel and pipelines, leakage
through joint, vent, pipeline, gaskets and welding joints. Scaling in the
pipe is removed by caustic cleaning. And crit ical equipments are
charging pump, booster pump, injection pump, Geho pump, heater and
agitator.
The heat recovery system of the digestion area is heater temp. Pick-up,
l ine and vessel insulation, steam line safety valve and use of steam
trap system. The safety equipment is pressure safety valve; if pressure
wil l increase it wil l open to release the pressure. Other arrangements
are facil it ies for tripping of booster pump, Geho pump, digestion slurry
control valve and injection pump.
Page | 37
Condensate generated from the process slurry flashing in series of
f lash tanks is pumped to boiler house for its heat recovery and
returned back to clear water header, used in refinery in pump gland
sealing and cooling tower make up.
The condensate management includes flash tank level to control. Some
of the preventions require during the operation are restricting moisture
to enter into the air, no choking of valve, smooth air supply, restricting
flow of wrong signals from control room, condensate contamination,
slurry leakage and slurry coming out from relief tank bottom. The whole
process of digestion area is DCS controlled. We can see and control al l
parameters through DCS.
4.4. Clarification
Purpose: To efficiently separate sand contents and red mud from
Pregnant Liquor, to remove maximum fine suspended mud solid
particles from Pregnant Liquor, to recover physical soda of red mud by
counter current washing and maximum possible economical recovery of
bound soda of mud by means of causticization process, f i ltration of red
mud slurry Techno-economically and environmentally viable disposal of
red mud.
Page | 38
4.4.1. HRD(High Rate Decanter)
Inside HRD sedimentation process takes place where separation of mud
and pregnant l iquor take place.
In HRD, the feed slurry is admitted tangential ly into the unit (feed well)
at a depth of 2 – 3 feet below the surface of l iquid. On entrance, the
slurry spread rapidly through the cross section of the settler. Liquor
than flows upward to be withdrawn at the overflow launder, and the
solid settles down to the bottom. The flocculent used for the settl ing of
solid particles are polymers (Flomina Al99F).The upper zone is free
from particles and increases sl ightly in solid below the entrance of the
feed. Particles settle in this zone by free settl ing. Below the dilute
zone is a compression zone in which the concentration of solids
increases rapidly with distance from the boundary between the zones.
The rakes, which operate in the bottom of the compression zone, tend
to break the flock’s structure and compact the underflow to a solid
contact. The main purpose of the rake is to keep mud pump able, to
bring mud towards pumping end and to remove trapped l iquor between
mud portable (underflow). In practice a clear overflow can be obtained
if the upward velocity of the l iquid in the dilute zone is less than the
minimum terminal velocity of the solid at al l point in the zone. Upward
velocity of the l iquid is directly proportional to the overflow rate. If
solid wil l be more then load on the rake wil l increase and in that case
dosing of f locculent is to be reduced.
Components of HRD are drive assembly, feed well, cable torque
mechanism and overflow launder.
Flocculants: It consolidates the underflow mud slurries. It neutralizes
the charges on the slurry, which in turn agglomerates to form flocs
each containing many particles.
Page | 39
4.4.2. Sand Classifier
The sand from the pregnant l iquor separates in cyclone by cyclonic
effect and comes in screw conveyor where the leaching of soda from
sand takes place and the sand collected in hopper is disposed off
through dumpers. The overflow of f irst screw conveyor is collected in
1 s t wash tank from where it is pumped in one of the settler in l ine. The
overflow of second screw goes either in settler or washer.
4.4.3. Washer
This system leaches out caustic from mud slurry and disposes l iquor
free mud. There are 7 Washers in one series. HRD underflow is
introduced into the 1 s t Washer feed tank. Polymer is added with the
feed. A counter current washing by the hot water takes place here.
The overflow of Washer # 1 is the final product of washing operation
and contains recovered caustic. Dilution is pumped to digestion area in
order to maintain L to P concentration. Wash water temperature is
maintained around 90 oC into last washer for effective leaching of soda.
Mud from last washer is pumped to drum fi lter directly. The other
streams received in washer circuit are drum fi ltrate l iquor, f ine seed
wash caustic zed l iquor, sand classif ier drained l iquor & fi lter press
washed cake.
The diameter of the washer is 125’ and they are 7 in number. Other
equipment associated with the washer is overflow pump, wash water
tank & underflow pump, dilution tank & dilution pumps and drum fi lter
heater (3 in nos).
Page | 40
4.4.4. Drum Filter
Filtration is the removal of solid particles from a fluid by passing the
fluid through a fi ltering medium or septum on which the solids are
deposited.
Drum fi lter is a continuous vacuum fi lter i .e. f i ltration is done under
vacuum. Drum fi lter is a drum rotating about a horizontal axis with a
portion of drum submerged in the slurry to be fi ltered in a vat.
Agitators in the vat keep the slurry thoroughly mixed. The drum
surface is divided into number of longitudinal sections comparatively
shallow in depth, each of which is connected by pipes to a common
stationery fi lter valve mounted on the end of one of the trunions on
which the drum rotates. The fi lter valve is connected to a vacuum
system. The drum suction is equipped with a perforated plate, which
forms the outer cylindrical surface of the drum and supports the fi lter
medium. The drum is then covered with a suitable fi lter cloth by
lateral caulking.
As the drum rotates through the slurry in the tank, the l iquid is sucked
through the cloth into the drum piping and out through the fi lter valve.
The solids are trapped on the surface of the fi lter cloth, forming what is
known as fi lter cake and deposit on the outer surface of the rotating
drum, where it is subjected to water spray (Temp. is 90 oC). Air is then
passed through the cake by the suction as the rotation cycle
progresses, to remove residual cake moisture as much as possible.
As the fi lter section rotates towards the cake discharge point the fi lter
valve shuts off the vacuum on that particular section ready to
discharge. Mud cake is f inally discharged to hopper after being led off
the drum over a small rol ler and scarped by a string scrapper. When
the mud hopper gets full , the mud is emptied out in dumper stationed
below the hopper. The mud is then disposed off in Mud Yard. The
product of the drum fi lter should be of below specifications:
Solid after f i ltration - 70%
Page | 41
Leachable Soda - 1.0%
4.5. Precipitation
Precipitation of alumina tri-hydrate from seeded caustic aluminate
l iquor is an important step of the Bayer’s process for production of
alumina from bauxite. The importance of precipitation step arise from
the fact that by and large it determines the ultimate characteristics of
the product (calcined alumina) and much of the success of the plant’s
performance depends on this aspect.
During precipitation precipitate of alumina tri hydrate gets deposited
on seed crystals as well as on new nuclei that get formed during the
precipitation cycle. The new nuclei also grow by fresh deposition of
precipitate. There are four main mechanisms taking place during a
precipitation process. They are nucleation, crystal growth,
agglomeration and crystal breakage.
Nucleation is a special condition characteristic by the spontaneous
generation of very fine crystals, of submicron sizes and as a
consequence there is a rapid increase in the production of sub sieve
particles. Controlled nucleation is very necessary to produce just the
required quantity of new seed crystals for maintaining the precipitation
Page | 42
process in a balanced state. Uncontrolled nucleation along with in
sufficient agglomeration and /or crystal growth would lead to seed
balance getting disturbed and this can’t be tolerated especially in a
plant that produces coarse sandy alumina.
Growth is the mechanisms by which particles are enlarge by the
addition or deposition of newly precipitated alumina tri hydrate into the
original crystals. The conditions required here are more or less similar
to what are required to favor agglomeration, which is also a process of
enlargement of particles. Large particles are produce predominately by
growth are dense and strong and distinct from those produced by
agglomeration, which are generally weak.
Agglomeration is also a mechanism whereby larger particles are
produced. But as distinct from the mechanism of straight growth, in
this case the enlargement is achieved by smaller particles coll iding and
then getting cemented together by precipitation of hydrate between
the coll iding particles that is by interstit ial deposition of hydrate.
In addition to the above mentioned mechanisms there is yet a forth
one, crystal breakage which could occur at any stage in a precipitation
process. It is a mechanism not directly caused by the precipitation
process but all the same influences the end results of the process. Due
to the shear forces created by agitation and turbulence, either the
newly deposited particles are sheared-off or the particles are
continuously created. Thus during a precipitation process two opposing
mechanisms must proceed in such a way as to permit the circuit to
remain in balanced state, that is, just the amount of coarse particles to
yield a final product of acceptable granulometry and just the amount of
f ine particles to provide adequate surface area for further
precipitation, while achieving maximum possible yield from the l iquor.
And also it is equally important for the operator to ensure that as the
particles are enlarged the final material attains adequate strength to
withstand breakdown during subsequent operations.
Page | 43
Our aim in alumina plant precipitation area is to achieve maximum
yield with coarse aluminum tri-hydrate particles and lesser f inishing
A/C ratio. Amount of alumina tri-hydrate extracted per l iter of pregnant
l iquor in nothing but yield. Yield=L to P caustic (L to P A/C- Spent A/C)
4.5.1. Hydrocyclones
There are three batteries of cyclones: Product cyclone, Coarse cyclone
and Swing cyclones. The Swing cyclone is the stand by for both Product
and Coarse. One battery consists of 24 cyclones with feeding
arrangement, under f low and over f low piping. The product cyclone
under flow is collected in Product cyclone under flow tank and then
pumped to product deliquoring fi lter. The coarse cyclone under flow is
collected in coarse cyclone under flow tank and PT under flow is also
pumped into the tank. The slurry from this tank is pumped to coarse
deliquoring fi lter Mts and a volume of 1440 Cu.mts. The feed to all PT’s
are fed from cyclone over f low tanks and is controlled and adjusted by
online flow meters and C/V. The overflow goes to associated ST’s.
PT Under flow is pumped by PT Under flow pumps to coarse seed
cyclone under flow tank. There are three sets, each set contain two
pumps.
(f ig 4.4) Cyclone Separator
Page | 44
4.5.2. Secondary Thickeners
There are four ST’s in plant. Normally three are inl ine and one is in
stand by mode. The tank diameter of ST’s is 12.8 Mts and volume of
1560 Cu.mts. The overflow of all ST’s goes to TT. ST Underflow is
pumped by ST Under flow pumps to Fine seed slurry tanks. There are
four sets, each set contain two pumps.
(f ig 4.5) Thickener
4.5.3. Terminal Thickeners
There are two TTs installed in plant, normally one in l ine and one is
stand by. The thickeners have diameter of 38 Mts and operating volume
of 6700 Cu. Mts. The TT feed is combined overflow from the three ST’s
in l ine and spent l iquor from fi ltration area. The over f low is collected
in a collection box, Where flocculent is added to achieve good settl ing
and low over f low solids. The TT’s are equipped with a rake mechanism.
The torque and motor current are monitored on a regular basis. The
over f low of TT goes to spent l iquor tank.
Page | 45
4.6. Calcinations
Calcination is another important step in the Bayer’s process, where
alumina tri-hydrate is calcined to the final form, alumina possessing
certain desired characteristics. This step is essential because for the
conduct of electrolysis al l material added to the cells must be
completely free from water whether chemically bound or absorbed on
the surface.
Thermal dehydration of alumina tri-hydrate starts at 180-290 0C and is
practically over at about 600 0C. However, alumina produced at these
temperatures has a highly activated surface and as such tends to
absorb considerable amounts of moisture when in contact with the
atmosphere. Hence the dehydration step has to be followed by
energetic heating to temperature of more than 1100 0C at which its
property of absorbing moisture from the atmosphere is reduced to a
level considered safe enough for the operation of electrolytic cells.
In HINDALCO there are two types of calciner. These are as fol lows:
1. Gas Suspension Calciner (GSC) 2. Flash Calciner
The main components in the G.S.C.System comprises of:
Page | 46
Hydrate Feed System
Ventury Type Flash Drier
2-Stage Cyclone Pre-heater(P01-P10&P02)
Gas Suspension CalcinersPO4.
Disengaging Cyclone or Separating Cyclone (PO3).
4-stage Cyclone Cooler (C01, C02, C03 &C04).
Secondary Fluid Bed Cooler (K01&K02).
Oil Heating System
Dedusting and Dust Recycling System.
Gas suspension Calciner
Preheated and partly calcined alumina at 300-400 0C enters the reactor
in a direction parallel to the conical bottom. Preheated air for
combustion is introduced at about 800 0C through a single pipe in the
bottom of the reactor. The velocity of the air at the inlet is sufficient to
ensure proper suspension of the particles over the entire cross-section
of the reactor at ful l and partial capacity.
After a few seconds’ exposure to 1100-1250 0C, the calcined alumina is
entrained from the reactor by the gaseous mixture of water vapor and
combustion products.
Page | 47
(f ig 4.6) Calciner
4.7. Evaporation Unit
Evaporation Technology: Evaporation is the process in which the
change of state from liquid to gas can occur at any temperature up to
boil ing point. At any one time, a variable population of molecules in a
l iquid wil l have sufficient energy to escape into the atmospheric. The
rate of evaporation rises with increased temp. Because as the mean
kinetic energy of the l iquid’s molecules rises and so wil l and so wil l be
the number of molecules possessing enough energy to escape. In
Page | 48
general,the lower the boil ing points, the faster the rate of evaporation,
though this also depends on the latent heat.
The Economy of Evaporation Unit depends upon the number of factors:
Mass Flow Specific Heat T 60
Evaporation Rate(TPH) =
Latent Heat 1000
Working of Evaporation Unit
In the plant during process water is directly added to Caustic or
indirectly as steam. So the Caustic concentration goes down. To
increase the concentration of Caustic, the water content inside the
Caustic should be removed, so three Evaporation Units are set. In all
Evaporation units, process is almost similar. Diluted Caustic comes to
unit, and goes to Barometric Condenser in which it creates vacuum,
then goes to preheater in which caustic is heated. This heated caustic
goes in f irst evaporator. In evaporators barometric condenser already
creates vacuum. From first to last evaporator caustic goes in the same
fashion and in last evaporator steam is used to heat the caustic, this
caustic then travels in reveres manner means from last to f irst
evaporator and vacuum is also goes on increasing from last to f irst
evaporator. Due to these effect caustic f lashes inside evaporator and
produces hot condensate and it is then cool down. The hot condensate,
which is formed inside evaporator, is used to heat incoming caustic
from first to last evaporator. So in this manner the consumption of
steam is less as compared to the evaporation rate achieved.
Page | 49
(f ig 4.7) Evaporator
Page | 50
5. GENERAL USE OF
ALUMINIUM
Auminium is the most widely used non-ferrous metal . Global production
of aluminium in 2005 was 31.9 mill ion tonnes. It exceeded that of any
other metal except iron (837.5 mil l ion tonnes). Forecast for 2012 is 42–
45 mil l ion tons, driven by rising Chinese output.
Aluminium is almost always alloyed, which markedly improves its
mechanical properties, especially when tempered. For example, the
common aluminium foils and beverage cans are alloys of 92% to 99%
aluminium. The main alloying agents are
copper, zinc, magnesium, manganese, and si l icon (e.g., duralumin) and
the levels of these other metals are in the range of a few percent by
weight.
Some of the many uses for aluminium metal are in:
Transportation (automobiles, aircraft, trucks, rai lway cars,
marine vessels, bicycles, etc.) as sheet, tube, castings, etc.
Packaging (cans, foi l , etc.)
Construction (windows, doors, siding, building wire, etc.)
A wide range of household items, from cooking
utensils to baseball bats, watches.
Street l ighting poles, sail ing ship masts, walking poles, etc.
Outer shells of consumer electronics, also cases for equipment
e.g. photographic equipment.
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Electrical transmission l ines for power distribution
MKM steel and Alnico magnets
Super purity aluminium (SPA, 99.980% to 99.999% Al), used in
electronics and CDs.
Heat sinks for electronic appliances such as transistors and CPUs.
Substrate material of metal-core copper clad laminates used in
high brightness LED l ighting.
Powdered aluminium is used in paint, and in pyrotechnics such
as solid rocket fuels and termite.
Aluminium can be reacted with hydrochloric acid or with sodium
hydroxide to produce hydrogen gas.
A variety of countries,
including France, Italy, Poland, Finland, Romania, Israel, and the
former Yugoslavia, have issued coins struck in aluminium or
aluminium-copper alloys.
Some guitar models sports aluminium diamond plates on the
surface of the instruments, usually either chrome or
black. Kramer Guitars and Travis Beanare both known for having
produced guitars with necks made of aluminium, which gives the
instrument a very distinct sound.
Sustainabil ity of Aluminium in Buildings
6. SUMMARY
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Although it can be seen that the environmental impact of Aluminium
production is far from negligible, the impact of bil l ions of people
constantly burning fossil fuels as a means of power and transport is far
more serious.
As long as every effort is made to replace / replant any environment
destroyed as a result of Bauxite mining, the biggest ecological cost of
Aluminium production is creating the power needed to refine it. Indeed
the Aluminium Oxide which the Hydrogen generator produces could be
turned back into Aluminium. Hopefully in the near future the energy to
do this wil l come from an environmentally friendly process (i .e. not
nuclear or fossil fuel power). Methods already exist to extract energy
from the sea and the wind. Methods may already exist (although not
publicly) to tap into the vast abundances of energy which exist
everywhere - after al l there is far more energy in one gram of matter
than all the power stations in the world produce in a year!
So the water / aluminum engine is not an ideal solution while we sti l l
produce electricity by non environmentally friendly means, but it is an
essential design which can give us some breathing (l iterally!) space
unti l our scientists come up with a method of tapping into the abundant
energy which surrounds us everywhere.
If you have the intell igence and knowledge to tackle these and other
crit ical problems then it is your duty to do so. After al l what is the
point in l iving out your l i fe ignoring the bigger picture, only for your
descendants and species to die out when the planet that supports them
is poisoned beyond repair.
7. GLOSSARY
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Agglomerate To from or collect in to rounded mass.
Air Blasters A strong gust or Air gust of wind.
Autoclave A strong Pressurized steam treated vessel.
Anthracite A dense shiny cold that has high carbon Content.
Barometric An instrument for measuring atmospheric Pressure.
Boster A device for increasing power or effectiveness.
Contaminates To make pure or under contact of mixture.
Colloidal A particulate matter so dispersed
Coarse A rough, especially to the touch
Cyclones A violet rotating windstorm
Condensate A substance formed by condensation
Clarification To make clear by removing impurities
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Calcinations To heat all substance of high temperature but
below melting point
Decanter A vessel used for decanting
Draft A current of air in enclosed area
Dilution Process of making weaker or less concentrated
Dehydration The process of removing water
Digestion The process of breaking down
Flomia Very aggressive solvent
Flashes To move or proceed rapidly
Gibbsite A mineral consisting of Hydrated alumina oxide.
Gheo Pump Piston Pump
Goethite A mineral consisting of hydrated alumina oxide
Hematite A brick- Red mineral that is ore of iron
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Hopper A funnel shape Container
Quartz A very hard mineral compound of si l ica
Sieve A utensil of wire mesh
Torque A turning or twisting force
Wagon A l ight Automotive transport vehicle.
8. References
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Professional or Personal web site
Aluminium Production Procedure- Oct 6, 2011-
http://anon99.tripod.com/water_engine/aluminium_production2.html
Features of Aluminium Oct 7, 2011-
http://www.rocksandminerals.com/aluminum/process.htm
Aluminium properties & Information Oct 7, 2011-
http://en.wikipedia.org/wiki/Aluminium
Aluminium Manufactures , Dealers, Exports Oct 8, 2011-
http://www.maharashtradirectory.com/catalogue/Alumina.htm
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