Ultrafine grained Fe-Cr-Ni austenitic stainless steels by ...
IT 283 Advanced Materials and Process I. Stainless Steels...
Transcript of IT 283 Advanced Materials and Process I. Stainless Steels...
IT 283 Advanced Materials and Process
I. Stainless Steels II. Alumina (Al2O3) III. Silicon (Si)
Prepared for
Professor Gary B. Paglierani
By
Muain Kiran
or the fulfillment of the requirements for the degree of Master of Science in Industrial
Technology in department of Industrial Technology,
California State University Fresno,
Spring 2005
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Table of Contents
STAINLESS STEEL…………………………………………………………………………….…………3
STAINLESS STEEL AT ELEVATED TEMPERATURES…21
ALUMINA (AL2O3)……………………………………………………………………….……….……………26
PROCESS………………………………………………………………………………………………………………30
SILICON (SI).……………………………………………………………………………………………..…………..39
BIBLIOGRAPHY……………………………………………………………………………………………….….52
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STAINLESS STEEL
HISTORY
Stainless steel is primarily when corrosion or oxidation are a problem. The
function that they perform cannot be duplicated by other materials for their cost.
Over 50 years ago, it was discovered that a minimum of 12% chromium would impart
corrosion and oxidation resistance to steel. Hence the definition “Stainless Steels”,
are those ferrous alloys that contain a minimum of 12% chromium for corrosion
resistance. This development was the start of a family of alloys which has enabled the
advancement and growth of chemical processing and power generating systems.
Subsequently several important sub-categories of stainless steels have been
developed. The sub-categories are austenitic, martensitic, ferritic, duplex,
precipitation hardening and super alloys. (sppusa.com)
The "discovery" of stainless steel occurred in the 1900 to 1915 time period.
However, as with many discoveries, it was the accumulated efforts of several
individuals that actually began in 1821. That year a Frenchman named Berthier
found that iron when alloyed with chromium was resistant to some acids. Others
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studied the effects of chromium in an iron matrix, but using a low percentage of
chromium. (ssina.com)
To be stainless steel, the chromium content needs to be at least 10.5%. In
1872, Messrs. Woods and Clark applied for a British patent for what they identified
as an acid and weather resistant alloy containing 30 to 35% chromium and 1.5 to 2%
tungsten. Then, in 1875, another Frenchman named Brustlein recognized the
importance of carbon levels in addition to chromium. Stainless steels need to have a
very low level of carbon at 0.15%. While many others investigated the chromium/iron
composition, the difficulty in obtaining the low carbon levels persisted for many years
until low carbon ferrochrome became commercially available. (ssina.com)
Discovery
In 1904, Leon Guillet published research on alloys with composition that
today would be known as 410, 420, 442, 446 and 440-C. In 1906, he also published
a detailed study of an iron-nickel-chromium alloy that is the basic metallurgical
structure for the 300 series of stainless steel. In 1909, Giesen published in England
a lengthy account on the chromium-nickel (austenitic 300 series) stainless steels.
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Also in England and France, Portevin published studies on an alloy that
today would be 430 stainless steel. In Germany, in 1908, Monnartz & Borchers
found evidence of the relationship between a minimum level of chromium (10.5%) on
corrosion resistance as well as the importance of low carbon content and the role of
molybdenum in increasing corrosion resistance to chlorides. (ssina.com)
Industrial Development
Harry Brearley, chief of the research lab run jointly by John Brown & Co.
and Thomas Firth & Sons, is generally accredited as the initiator of the industrial era
of stainless steel. Most of his work was on 430 (the chemical analysis was patented in
1919). The first product was table cutlery and it is still used today. (ssina.com)
General Information
The many unique values provided by stainless steel make it a powerful
candidate in materials selection. Engineers, specifies and designers often
underestimate or overlook these values because of what is viewed as the higher initial
cost of stainless steel. However, over the total life of a project, stainless is often the
best value option. (ssina.com)
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What is Stainless Steel?
Stainless steel is essentially a low carbon steel which contains chromium at
10% or more by weight. It is this addition of chromium that gives the steel its unique
stainless, corrosion resisting properties. (ssina.com)
Austenitic Grades
Austenitic grades are those alloys which are commonly in use for stainless
applications. The austenitic grades are not magnetic. The most common austenitic
alloys are iron chromium-nickel steels and are widely known as the 300 series. The
austenitic stainless steels, because of their high chromium and nickel content, are the
most corrosion resistant of the stainless group providing unusually fine mechanical
properties. They cannot be hardened by heat treatment, but can be hardened
significantly by cold-working. (sppusa.com)
Straight Grades
The straight grades of austenitic stainless steel contain a maximum of .08%
carbon. There is a misconception that straight grades contain a minimum of .03%
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carbon, but the spec does not require this. As long as the material meets the physical
requirements of straight grade, there is no minimum carbon requirement. (sppusa.com)
“L” Grades
The “L” grades are used to provide extra corrosion resistance after welding.
The letter “L” after a stainless steel type indicates low carbon (as in 304L). The
carbon is kept to .03% or under to avoid carbide precipitation. Carbon in steel when
heated to temperatures in what is called the critical range (800 degrees F to 1600
degrees F) precipitates out combines with the chromium and gathers on the grain
boundaries. This deprives the steel of the chromium in solution and promotes
corrosion adjacent to the grain boundaries. By controlling the amount of carbon, this
is minimized. For weld ability, the “L” grades are used. (sppusa.com)
All stainless steels are not produced as “L” grades. There are a couple of
reasons. First, the “L” grades are more expensive. In addition, carbon, at high
temperatures imparts great physical strength frequently the mills are buying their raw
material in “L” grades, but specifying the physical properties of the straight grade to
retain straight grade strength. An example of having cake and heating it too; this
results in the material being dual certified 304/304L; 316/316L, etc. (sppusa.com)
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“H” Grades
The “H” grades contain a minimum of .04% carbon and a maximum of .10% carbon and
are designated by the letter “H” after the alloy. People ask for “H” grades primarily
when the material will be used at extreme temperatures as the higher carbon helps the
material retain strength at extreme temperatures. (sppusa.com)
“Solution annealing” means only that the carbides which may have precipitated
(or moved) to the grain boundaries are put back into solution (dispersed) into the
matrix of the metal by the annealing process. “L” grades are used where annealing
after welding is impractical, such as in the field where pipe and fittings are being
welded. (sppusa.com)
Type 304
The most common of austenitic grades, containing approximately 18% chromium and
8% nickel. It is used for chemical processing equipment, for food, dairy, and beverage
industries, for heat exchangers, and for the milder chemicals.
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Type 316
Type 316 contains 16% to 18% chromium and 11% to 14% nickel. It also has
molybdenum added to the nickel and chrome of the 304. The molybdenum is used to
control pit type attack. Type 316 is used in chemical processing, the pulp and paper
industry, for food and beverage processing and dispensing and in the more corrosive
environments. The molybdenum must be a minimum of 2%.
Type 317
Contains a higher percentage of molybdenum than 316 for highly corrosive
environments. It must have a minimum of 3% “moly”. It is often used in stacks which
contain scrubbers.
Type 317L
Restricts maximum carbon content to 0.030% max. and silicon to 0.75% max. for extra
corrosion resistance.
Type 317LM
Requires molybdenum content of 4.00% min.
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Type 317LMN
Requires molybdenum content of 4.00% min. and nitrogen of .15% min.
Type 321, Type 347
These types have been developed for corrosive resistance for repeated intermittent
exposure to temperature above 800 degrees F. Type 321 is made by the addition of
titanium and Type 347 is made by the addition of tantalum/columbium. These grades
are primarily used in the aircraft industry
Martensitic Grades
Martensitic grades were developed in order to provide a group of stainless alloys that
would be corrosion resistant and hardenable by heat treating. The martensitic grades
are straight chromium steels containing no nickel. They are magnetic and can be
hardened by heat treating. The martensitic grades are mainly used where hardness,
strength, and wear resistance are required. (sppusa.com)
Type 410
Basic martensitic grade, containing the lowest alloy content of the three basic
stainless steels (304, 430, and 410). Low cost, general purpose, heat treatable
stainless steel. Used widely where corrosion is not severe (air, water, some chemicals,
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and food acids. Typical applications include highly stressed parts needing the
combination of strength and corrosion resistance such as fasteners.
Type 410S
Contains lower carbon than Type 410, offers improved weld ability but lower
harden ability. Type 410S is a general purpose corrosion and heat resisting
chromium steel recommended for corrosion resisting applications.
Type 414
Has nickel added (2%) for improved corrosion resistance. Typical applications
include springs and cuttlery.
Type 416
Contains added phosphorus and sulfer for improved machinability. Typical
applications include screw machine parts.
Type 420
Contains increased carbon to improve mechanical properties. Typical applications
include surgical instruments.
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Type 431
Contains increased chromium for greater corrosion resistance and good mechanical
properties. Typical applications include high strength parts such as valves and pumps.
Type 440
Further increases chromium and carbon to improve toughness and corrosion
resistance. Typical applications include instruments.
Ferritic Grades
Ferritic grades have been developed to provide a group of stainless steel to resist
corrosion and oxidation, while being highly resistant to stress corrosion cracking.
These steels are magnetic but cannot be hardened or strengthened by heat
treatment. They can be cold worked and softened by annealing. As a group, they are
more corrosive resistant than the martensitic grades, but generally inferior to the
austenitic grades. Like martensitic grades, these are straight chromium steels with no
nickel. They are used for decorative trim, sinks, and automotive applications,
particularly exhaust systems.
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Type 430
The basic ferritic grade, with a little less corrosion resistance than Type 304. This
type combines high resistance to such corrosives as nitric acid, sulfur gases, and many
organic and food acids.
Type 405
Has lower chromium and added aluminum to prevent hardening when cooled from high
temperatures. Typical applications include heat exchangers.
Type 409
Contains the lowest chromium content of all stainless steels and is also the least
expensive. Originally designed for muffler stock and also used for exterior parts in
non-critical corrosive environments.
Type 434
Has molybdenum added for improved corrosion resistance. Typical applications
include automotive trim and fasteners.
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Type 436
Type 436 has columbium added for corrosion and heat resistance. Typical
applications include deep-drawn parts.
Type 442
Has increased chromium to improve scaling resistance. Typical applications include
furnace and heater parts.
Type 446
Contains even more chromium added to further improve corrosion and scaling
resistance at high temperatures. Especially, good for oxidation resistance in sulfuric
atmospheres. (sppusa.com)
Duplex Grades
Duplex grades are the newest of the stainless steels. This material is a combination of
austenitic and ferritic material. This material has higher strength and superior
resistanceto stress corrosion cracking. An example of this material is type 2205. It is
available on order from the mills. (sppusa.com)
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Precipitation Hardening Grades
Precipitation hardening grades, as a class, offer the designer a unique combination of
fabricability, strength, ease of heat treatment, and corrosion resistance not found in
any other class of material. These grades include 17Cr-4Ni (17-4PH) and 15Cr-
5Ni (15-5PH).The austenitic precipitation-hardenable alloys have, to a large extent,
been replaced by the more sophisticated and higher strength superalloys.
(sppusa.com)
The martensitic precipitation hardenable stainless steels are really the work
horse of the family. While designed primarily as a material to be used for bar, rods,
wire, forgings, etc., martensitic precipitation hardenable alloys are beginning to find
more use in the flat rolled form. While the semi austenitic precipitation-hardenable
stainless steels were primarily designed as a sheet and strip product, they have found
many applications in other product forms. Developed primarily as aerospace
materials, many of these steels are gaining commercial acceptance as truly cost-
effective materials in many applications. (sppusa.com)
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Super alloy Grades
Super alloys are used when 316 or 317 are inadequate to withstand attack.
They contain very large amounts of nickel and/or chrome and molybdenum. They are
usually much more expensive than the usual 300 series alloys and can be more difficult
to find. These alloys include Alloy 20 and Hastelloy. (sppusa.com)
Stainless Steel
Group of corrosion resistant steels containing at least 10.5% chromium and
may contain other alloying elements. These steels resist corrosion and maintain its
strength at high temperatures. (ssina.com)
The chromium content of the steel allows the formation of a rough, adherent,
invisible, corrosion-resisting chromium oxide film on the steel surface. If damaged
mechanically or chemically, this film is self-healing, providing that oxygen, even in very
small amounts, is present. The corrosion resistance and other useful properties of the
steel are enhanced by increased chromium content and the addition of other elements
such as molybdenum, nickel and nitrogen. (ssina.com)
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There are more than 60 grades of stainless steel. However, the entire group can be
divided into five classes. Each is identified by the alloying elements which affect their
microstructure and for which each is named. (ssina.com)
Benefits of Stainless Steel
Corrosion resistance
Lower alloyed grades resist corrosion in atmospheric and pure water environments,
while high-alloyed grades can resist corrosion in most acids, alkaline solutions, and
chlorine bearing environments, properties which are utilized in process plants.
(ssina.com)
Fire and heat resistance
Special high chromium and nickel-alloyed grades resist scaling and retain strength at
high temperatures. (ssina.com)
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Hygiene
The easy cleaning ability of stainless makes it the first choice for strict hygiene
conditions, such as hospitals, kitchens, abattoirs and other food processing plants.
(ssina.com)
Aesthetic appearance
The bright, easily maintained surface of stainless steel provides a modern and
attractive appearance. (ssina.com)
Strength-to-weight advantage
The work-hardening property of austenitic grades, that results in a significant
strengthening of the material from cold-working alone, and the high strength duplex
grades, allow reduced material thickness over conventional grades, therefore cost
savings. (ssina.com)
Ease of fabrication
Modern steel-making techniques mean that stainless can be cut, welded, formed,
machined, and fabricated as readily as traditional steels. (ssina.com)
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Impact resistance
The austenitic microstructure of the 300 series provides high toughness, from
elevated temperatures to far below freezing, making these steels particularly suited to
cryogenic applications. (ssina.com)
Long term value
When the total life cycle costs are considered, stainless is often the least expensive
material option. (ssina.com)
Cycle of Stainless Steel
To ensure a high quality of life, the materials that we use as consumers and
manufacturers should meet not only technical performance standards, but have a
Long Service Life, be Usable in a Great Number of Applications, and be
Environmentally Friendly. Once their service is complete, they should be 100%
Recyclable, thereby completing the life cycle to be used once again. Stainless Steel
is such a material. (ssina.com)
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Stainless Steel Life Cycle
MELT
USE FABRICATE
SCRAP
The longevity of stainless is the result of the alloying composition and, therefore, it
has a natural corrosion resistance. Nothing is applied to the surface that could add
additional material to the environment. It does not need additional systems to protect
the base metal, the metal itself will last. (ssina.com)
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Stainless steel needs less maintenance and its hygienic qualities means that we
do not have to use harsh cleaners to get a clean surface. There is little or nothing to
dump into the drain that could have an environmental impact. (ssina.com)
Stainless steel products complete their service life. There is less concern about
disposal since this material is 100% recyclable. In fact, over 50% of new stainless steel
comes from old remelted stainless steel scrap, thereby completing the full life cycle.
(ssina.com)
STAINLESS STEEL AT ELEVATED TEMPERATURES
Stainless steels have good strength and good resistance to corrosion and
oxidation at elevated temperatures. Stainless steels are used at temperatures up to
1700° F for 304 and 316 and up to 2000 F for the high temperature stainless grade
309(S) and up to 2100° F for 310(S). Stainless steel is used extensively in heat
exchangers, super-heaters, boilers, feed water heaters, valves and main steam lines as
well as aircraft and aerospace applications. (ssina.com)
Figure 1 (below) gives a broad concept of the hot strength advantages of
stainless steel in comparison to low carbon unalloyed steel. Table 1 (below) shows the
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short term tensile and yield strength vs temperature. Table 2 (below) shows the
generally accepted temperatures for both intermittent and continuous service. With
time and temperature, changes in metallurgical structure can be expected with any
metal. In stainless steel, the changes can be softening, carbide precipitation, or
embitterment. Softening or loss of strength occurs in the 300 series (304, 316, etc.)
stainless steels at about 1000° F and at about 900° F for the hardenable 400 (410,
420, 440) series and 800° F for the non-hardenable 400 (409, 430) series (refer to
Table 1, below).
Carbide precipitation can occur in the 300 series in the temperature range
800 – 1600° F. It can be deterred by choosing a grade designed to prevent carbide
precipitation i.e., 347 (Cb added) or 321 (Ti added). If carbide precipitation does
occur, it can be removed by heating above 1900° and cooling quickly. (ssina.com)
Hardenable 400 series with greater than 12% chromium as well as the non-
hardenable 400 series and the duplex stainless steels are subject to embitterment
when exposed to temperature of 700 – 950° F over an extended period of time. This
is sometimes call 885F embitterment because this is the temperature at which the
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embitterment is the most rapid. 885F embitterment results in low ductility and
increased hardness and tensile strengths at room temperature, but retains its
desirable mechanical properties at operating temperatures. (ssina.com)
(Figure 1 taken from ssina.com)
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Table 1 (taken from ssina.com)
Short Term Tensile Strength vs Temperature
(in the annealed condition except for 410)
Temperature 304
& TS
ksi
316
YS
ksi
309
&
TS
ksi
309S
YS
ksi
310
&
TS
ksi
310S
YS
ksi
410*
TS
ksi
YS
ksi
430
TS
ksi
YS
ksi
Room Temp. 84 42 90 45 90 45 110 85 75 50
400°F 82 36 80 38 84 34 108 85 65 38
600°F 77 32 75 36 82 31 102 82 62 36
800°F 74 28 71 34 78 28 92 80 55 35
1000°F 70 26 64 30 70 26 74 70 38 28
1200°F 58 23 53 27 59 25 44 40 22 16
1400°F 34 20 35 20 41 24 --- --- 10 8
1600°F 24 18 25 20 26 22 --- --- 5 4
* heat treated by oil quenching from 1800° F and tempering at 1200° F
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Table 2( taken from ssina.com)
Generally Accepted Service Temperatures
Material Intermittent
Service Temperature
Continuous
Service Temperature
Austenitic
304 1600°F (870°C) 1700°F (925°C)
316 1600°F (870°C) 1700°F (925°C)
309 1800°F (980°C) 2000°F (1095°C)
310 1900°F (1035°C) 2100°F (1150°C)
Martensitic
410 1500°F (815°C) 1300°F (705°C)
420 1350°F (735°C) 1150°F (620°C)
Ferritic
430 1600°F (870°C) 1500°F (815°C)
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Alumina (Al2O3)
Bauxite and Alumina
Alumina (aluminum oxide Al2O3) is a fine white material similar in appearance to salt.
While alumina is also used in abrasive, ceramics and re factory industries, process was
designed to refine bauxite. Than, formed by weathering of sands and rocks millions of
years ago, increasing the alumina content as other more soluble elements were
removed. (qal.com)
Bauxite occurs close to the surface in seams varying from one meter to nine
meters, formed as small reddish pebbles (pisolites). Than, remove low-grade material,
and blending to provide a consistent grade. (qal.com)
The Process
The Process - an economical method of producing aluminium oxide - was discovered
by an Austrian chemist Karl Bayer and patented in 1887. (qal.com)
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The process dissolves the aluminium component of bauxite ore in sodium hydroxide
(caustic soda); removes impurities from the solution; and precipitates alumina
trihydrate which is then calcined to aluminium oxide. (qal.com)
A Process plant is principally a device for heating and cooling a large recirculating
stream of caustic soda solution. Bauxite is added at the high temperature point, red
mud is separated at an intermediate temperature, and alumina is precipitated at the
low temperature point in the cycle. (qal.com)
Bauxite usually consist of two forms of alumina - a monhydrate form Boehmite
(Al2O3.H2O) and a trihydrate form Gibbsite (Al2O3.3H2O). (qal.com)
Two grades of Weipa bauxite, the bulk of which is "monohydrate" grade bauxite.
Average analyses of Weipa bauxites
Constituent Monohydrate
Grade %
Trihydrate Grade
%
Al2O3
Available Al2O3
55
50*
50
44#
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Fe2O3
SiO2
TiO2
Other (mainly
H2O)
12
5
3
25
17
4
3
26
*40% from Gibbsite and 10% from Boehmite
# Gibbsite only is extractable in sweetening
Boehmite requires elevated temperatures (above 200°C) to dissolve readily in 10%
sodium hydroxide solution. (qal.com)
The trihydrate grade bauxite is mainly Gibbsite which dissolves readily in 10% sodium
hydroxide solution at temperatures below 150°C. (qal.com)
Consequently, monohydrate bauxite undergoes high temperature extraction under
pressure in digesters, while trihydrate grade material is added as a "sweetening
bauxite" to the flash tanks where temperatures are less than 200°C. (qal.com)
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Continue Process:
1- DIGESTION OF BAUXITE
Grinding:
Pisolitic, monohydrate-grade bauxite sized to a maximum of 20mm, is ground in 10 mills
(each with one compartment of rods and one of balls) to allow better solid liquid
contact during digestion. Recycled caustic soda solution is added to produce a
pumpable slurry, and lime is introduced for phosphate control and mud conditioning.
(qal.com)
Desilication:
The silica component of the bauxite is chemically attacked by caustic soda, causing
alumina and soda losses by combining to form solid desilication products. To
desilicate the slurry prior to digestion, it is heated and held at atmospheric pressure in
pre-treatment tanks, reducing the build-up of scale in tanks and pipes. Most
desilication products pass out with the mud waste as sodium aluminium silicate
compounds. (qal.com)
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Digestion:
Three digestion units; the monohydrate slurry is pumped by high pressure pumps
through two agitated, vertical digester vessels operating in series. Mixed with steam
and caustic solution, alumina in the bauxite forms a concentrated sodium aluminate
solution leaving undissolved impurities, (qal.com)
Principally, inert iron and titanium oxides and silica compounds. Reaction conditions
to extract the monohydrate alumina are about 250°C and a pressure about 3500
kPa, achieved by steam generated at 5000 kPa in coal-fired boilers.
Under these conditions, the chemical reactions are rapid:-
2NaOH + Al2O3.3H2O 2NaAlO2 + 4H2O
2NaOH + Al2O3.H2O 2NaAlO2 + 2H2O
By sizing the vessel to optimum holding time, about 97% of the total available alumina
is extracted and the silica content of liquor is reduced. (qal.com)
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Heat Recovery:
After digestion about 30% of the bauxite mass remains in suspension as thin red mud
slurry of silicates, and oxides of iron and titanium. The mud-laden liquor leaving the
digestion vessel is flash-cooled to atmospheric boiling point by flowing through a
series of flash vessels which operate at successively lower pressures. (qal.com)
The flash steam generated is used to preheat incoming caustic liquor in tubular heat
exchangers located parallel to the flash tank line. Condensate from the heat
exchangers is used for boiler feed water and washing waste mud. (qal.com)
Sweetening:
The trihydrate bauxite has separate grinding and pre-treatment facilities.
During the pass through the flash tanks, this additional bauxite slurry with high
trihydrate alumina content is injected to maximise the alumina content of the liquor
stream. This occurs in the appropriate flash vessels when the slurry from the digesters
has been cooled to less than 200°C. (qal.com)
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2 Clarification Of The Liquor Stream
Settlers:
Most red mud waste solids are settled from the liquor stream in single deck 40 metre
diameter settling tanks. Flocculants are added to the settler feed stream to improve
the rate of mud settling and achieve good clarity in the overflow liquor. (qal.com)
Washers:
The mud is washed with fresh water in counter-current washing trains to recover the
soda and alumina content in the mud before being pumped to large disposal dams.
(qal.com)
Slaked lime is added to dilute caustic liquor in the washing process to remove
carbonate (Na2CO3) which forms by reaction with compounds in bauxite and also
from the atmosphere and which reduces the effectiveness of liquor to dissolve alumina.
Lime regenerates caustic soda, allowing the insoluble calcium carbonate to be
removed with the waste mud. (qal.com)
Na2CO3 + Ca(OH)2 CaCO3 + 2NaOH
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Filters:
Settlers overflow liquor containing traces of fine mud is filtered in Kelly-type
constant pressure filters using polypropylene filter cloth. Slaked lime slurry is used to
produce a filter cake. Mud particles are held on the filter leaves for removal and
treatment in the mud washers when filters are sequentially taken off line. (qal.com)
Heat Interchange:
With all solids removed, the pregnant liquor leaving the filter area, contains alumina in
clear supersaturated solution. It is cooled by flash evaporation, the steam given off
being used to heat spent liquor returning to digestion. (qal.com)
Crystallisation:
Dissolved alumina is recovered from the liquor by precipitation of crystals. Alumina
precipitates as the trihydrate Al2O3 .3H2O in a reaction which is the reverse of the
digestion of trihydrate -
2NaAlO2 + 4H2O Al2O3.3H2O + 2 NaOH
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The cooled pregnant liquor flows to rows of precipitation tanks which are seeded with
crystalline trihydrate alumina, usually of an intermediate or fine particle size to
promote crystal growth. Each precipitation tank is agitated, with a holding time of
about three hours. During the 25-30 hours pass through precipitation, alumina of
various crystal sizes is produced. The entry temperature and the temperature
gradient across the row, seed rate and caustic concentration are control variables
used to achieve the required particle size distribution in the product. As correct
particle size is important to smelter operations, sizing is carefully controlled.. (qal.com)
Classification:
The finished mix of crystal sizes is settled from the liquor stream and separated into
three size ranges in three stages "gravity" classification tanks. The primary classifiers
collect the coarse fraction which becomes the product hydrate. The intermediate and
fine crystals from the secondary and tertiary classifiers are washed and returned to
the precipitation tanks as seed. (qal.com)
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Spent Liquor:
Spent caustic liquor essentially free from solid overflows from the tertiary classifiers
and is returned through an evaporation stage where it is re concentrated, heated and
recycled to dissolve more alumina in the digesters. Fresh caustic soda is added to the
stream to make up for process losses. (qal.com)
Calcinations Of Alumina
Washing:
A slurry of coarse hydrate (Al2O3.3H2O) from the primary thickeners is pumped to
hydrate storage tanks and is filtered and washed on horizontal-table vacuum filters to
remove process liquor. (qal.com)
Calcining:
The resulting filter cake is fed to a series of calcining units - an 1800 tones a day
circulating fluidised bed calciner or one of nine rotary kilns each 100m long and 4m in
diameter. The feed material is calcined to remove both free moisture and chemically-
combined water. Firing-zone temperatures above 1100°C are used, achieved by
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firing with natural gas. The circulating fluidised bed calciner is more energy efficient
than the older rotary kilns. Product sandy alumina particles are 90%+ 45 μm (microns)
in size. (qal.com)
Cooling:
Rotary or satellite coolers are used to cool the calcined alumina from the rotary kilns,
and to pre-heat secondary combustion air for the kilns. Fluidised-bed coolers further
reduce alumina temperature to less than 90°C before it is discharged on to conveyor
belts which carry it to storage buildings where it is stockpiled for shipment. (qal.com)
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Silicon (Si)
(chemicalelements.com)
Basic Information
Name: Silicon
Symbol: Si
Atomic Number: 14
Atomic Mass: 28.0855 amu
Melting Point: 1410.0 °C (1683.15 °K, 2570.0 °F)
Boiling Point: 2355.0 °C (2628.15 °K, 4271.0 °F)
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Number of Protons/Electrons: 14
Number of Neutrons: 14
Classification: Metalloid
Crystal Structure: Cubic
Density @ 293 K: 2.329 g/cm3
Color: grey
Atomic Structure
Number of Energy Levels: 3
First Energy Level: 2
Second Energy Level: 8
Third Energy Level: 4
(chemicalelements.com)
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Isotopes
Isotope Half Life
Si-28 Stable
Si-29 Stable
Si-30 Stable
Si-31 2.62 hours
Si-32 100.0 years
Facts
Date of Discovery: 1823
Discoverer: Jons Berzelius
Name Origin: From the Latin word silex (flint)
Uses: glass, semiconductors
Obtained From: Second most abundant element. Found in clay, granite, quartz, sand
(chemicalelements.com)
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History
(L. silex: silicis, flint) In 1800, Davy thought silica to be a compound and not an
element; but in 1811, Gay Lussac and Thenard probably prepared impure
amorphous silicon by heating potassium with silicon tetrafluoride.
(chemicalelements.com)
In 1824, a chemist by the name of Jons J. Berzelius isolated silicon in the form of
amorphous material by heating potassium with SiF4 and washing the reaction
products. A second allotropic form, crystalline silicon was first prepared by Jacque
Deville in 1854. Silicon in its content in the earth’s crust is exceeded only by oxygen.
Silicon consist of 3 stable isotopes, Si (92.23%), Si (4.67%), and Si (3.10%) and 12
artificial radioactive isotopes with mass numbers from 22 to 39 and a half-time from
0.10 sec to 1.6 x 102 years (Berger, 1997, p.56). (chemicalelements.com)
Silicon’s valence electron configuration is 3s23p2. Ionization energy for Si0 →
Si+ → Si2+ → Si3+ → Si4+ is 8.15, 16.34, 33.46, and 45.13 eV, respectively.
Electron affinity, Si0 + e- → Si, is 1.22 eV. Silicon atomic radius is 0.1175 nm;
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Si4+ radius is 0.039 nm (Berger, 1997, p.56). Because of this Silicon has become
one of the most studied materials among all known substances to humankind. The
reason is because silicon is the substance mostly use in modern electronic devices.
Silicon utilizes electrical, optical, photoelectrical, thermoelectric, thermal, mechanical,
and other properties. (chemicalelements.com)
The energy band structure of silicon was first studied by Herman and Jenkins.
They calculated that silicon energy gap is equal to 1.21 eV in the range of
temperatures from 4 to 1000 K. Crystal structure of diamond is exactly similar to
diamond. However, under pressure it’s between 11.2 and 12.5 GPa it acquires the
body-centered tetragonal structure. Lattice vibrations control thermal conductivity
of high purity silicon crystals. Study found that in the range of 100 to 1000K, it is
reverse proportional to temperature. Other study shows that thermal conductivity of
silicon is much lower than previously thought. (chemicalelements.com)
Semiconductor material such as silicon is mostly use in electrical application.
Intrinsic electrical conductivity of silicon at 300 K is close to 3.16 μS/cm. It has an
intrinsic charge carrier concentration of 1.02.1010 cm-3 3.265 which is in agreement
with the magnitude of 1.38.1010 cm-3. Silicon’s ohmic mobility is equal
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1450(300/T)2.6 for electrons and 500(300/T)2.3 for holes. The electrical
conductivity of silicon in liquid state near melting point is 1685 K 3.278, the
conductivity of solid silicon is close to 600 S/cm and it magnitude for melt is close to
12 kS/cm. (chemicalelements.com)
Liquid silicon’s conductivity is changing antibatically with temperature similar to
the behavior of metal. Solid and liquid density near melting point is close to 2.30 and
2.53 g/cm3. Dependence of the silicon refractive index on photon energy is in the
range of 0.47 to 1.13 eV, the refractive index, is changing from 3.443 to 3.553.
Silicon is virtually independent of temperature up to the its melting point therefore is
a diamagnetic material with the molar magnetic susceptibility of -4.9.10-5 SI units (-
3.9.10-6 cgs units).3.287 (chemicalelements.com)
Semiconductor material consists of silica, SiO2, which is mixed with coal or
wood chips and heated in a furnace at the temperature of 1800 to 2300 K to reach a
reduction of silica in reaction SiO2+2C→Si(melt)+2CO. The Mendeleev’s
periodic table is developing new ways in investigating the physical, chemical, and
technological properties of silicon. This is for the area where silicon has not been
fully studied. This area is what refers to as superconductivity. (chemicalelements.com)
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Sources
Silicon is present in the sun and stars and is a principal component of a class
of meteorites known as aerolites. It is also a component of tektites, a natural glass of
uncertain origin. (chemicalelements.com)
Silicon makes up 25.7% of the earth's crust, by weight, and is the second most
abundant element, being exceeded only by oxygen. Silicon is not found free in nature,
but occurs chiefly as the oxide and as silicates. Sand, quartz, rock crystal, amethyst,
agate, flint, jasper, and opal are some of the forms in which the oxide appears.
Granite, hornblende, asbestos, feldspar, clay, mica, etc. are but a few of the numerous
silicate minerals. (chemicalelements.com)
Silicon is prepared commercially by heating silica and carbon in an electric
furnace, using carbon electrodes. Several other methods can be used for preparing
the element. Amorphous silicon can be prepared as a brown powder, which can be
easily melted or vaporized. The Czochralski process is commonly used to produce
single crystals of silicon used for solid-state or semiconductor devices. Hyperpure
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silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a
hydrogen atmosphere, and by a vacuum float zone process. (chemicalelements.com)
Uses
Silicon is one of man's most useful elements. In the form of sand and clay it is
used to make concrete and brick; it is a useful refractory material for high-temperature
work, and in the form of silicates it is used in making enamels, pottery, etc. Silica, as
sand, is a principal ingredient of glass, one of the most inexpensive of materials with
excellent mechanical, optical, thermal, and electrical properties. Glass can be made in
a very great variety of shapes, and is used as containers, window glass, insulators, and
thousands of other uses. Silicon tetrachloride can be used as iridize glass.
(mineral.galleries.com)
Hyperpure silicon can be doped with boron, gallium, phosphorus, or arsenic to
produce silicon for use in transistors, solar cells, rectifiers, and other solid-state
devices which are used extensively in the electronics and space-age industries.
Hydrogenated amorphous silicon has shown promise in producing economical cells for
converting solar energy into electricity. (mineral.galleries.com)
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Silicon is important to plant and animal life. Diatoms in both fresh and salt
water extract Silica from the water to build their cell walls. Silica is present in the
ashes of plants and in the human skeleton. Silicon is an important ingredient in steel;
silicon carbide is one of the most important abrasives and has been used in lasers to
produce coherent light of 4560 A. (mineral.galleries.com)
Silcones are important products of silicon. They may be prepared by
hydrolyzing a silicon organic chloride, such as dimethyl silicon chloride. Hydrolysis and
condensation of various substituted chlorosilanes can be used to produce a very
great number of polymeric products, or silicones, ranging from liquids to hard, glasslike
solids with many useful properties. (mineral.galleries.com)
Properties
Crystalline silicon has a metallic luster and grayish color. Silicon is a relatively
inert element, but it is attacked by halogens and dilute alkali. Most acids, except
hydrofluoric, do not affect it. Elemental silicon transmits more than 95% of all
wavelengths of infrared, from 1.3 to 6.y micro-m. (mineral.galleries.com)
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Costs
Regular grade silicon (99%) costs about $0.50/g. Silicon 99.9% pure costs about
$50/lb; hyperpure silicon may cost as much as $100/oz. (mineral.galleries.com)
Handling
Miners, stonecutters, and others engaged in work where siliceous dust is breathed
into large quantities often develop a serious lung disease known as silicosis.
(mineral.galleries.com)
THE MINERAL SILICON
• Chemistry: Si, Elemental Silicon
• Class: Elements
• Subclass: Semi-metals
• Group: Carbon
• Uses: As an integrated circuit (IC) substrate and semiconductor.
• Specimens
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Silicon is rarely found in nature in its uncombined form. In fact it is amazing how
rare native silicon is with 25.7% of the Earth's crust being silicon. Silicon, binds
strongly with oxygen and is nearly always found as silicon dioxide, SiO2 (quartz), or
as a silicate (SiO4-4). Silicon has been found as a native mineral only in volcanic
exhalations and as tiny inclusions in gold. (mineral.galleries.com)
Of growing interest in rock shops, however, are laboratory-grown silicon boules.
Most such specimens are end fragments or flawed discards from the integrated circuit
industry. Silicon boules are grown (pulled) from a molten state from a seed crystal, in
such a way as to produce a single large crystal which must be completely without
crystal defects, or the entire boule must be discarded. Modern techniques can create
a single crystal several feet long and up to 10 inches in diameter. These large crystals
are sliced into very thin wafers, upon which complex integrated circuits can be etched.
The unused parts of the boule are often saved, and used as paperweights or
sometimes cut into bookends or other decorative items. (pearl1.lanl.gov)
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The word silicon (which is taken from the latin word for flint) can be confused with
other terms. One of these terms was already mentioned: Silicate (SiO4-4). Silicates
are minerals whose primary cation is the SiO4-4 ion group. Another confusing term is
silica. Silica is a term used by geologists for SiO2 or silicon dioxide in any form
whether it is in the form of quartz, or any of the Quartz Group members, or as a
segment of the chemistry of a silicate, or even as silicon dioxide dissolved in water. A
geologist might use the phrase, "The magma was rather poor in silica." Indicating an
SiO2 content that was lower than expected. Yet another term is silicone. Silicone is
a synthetic polymer that is made of silicon, carbon and oxygen and has many medical
and some industrial purposes. (pearl1.lanl.gov)
PHYSICAL CHARACTERISTICS:
• Color is iron-black, dark silver-gray to bluish brown.
• Luster is metallic.
• Transparency: Crystals are opaque.
• Crystal System is isometric; 4/m bar 3 2/m
• Crystal Habits are limited to microscopic crystals and inclusions.
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• Cleavage is absent.
• Fracture is conchoidal.
• Hardness is 7.
• Specific Gravity is approxiamtely 2.3.
• Streak is black
• Other Characteristics:
• Associated Minerals are limited to gold in which silicon has been found as
inclusions.
• Notable Natural Occurrences include Nuevo Potosi, Cuba; Tolbachik,
Kamchatka and Kola Peninsula, Russia.
• Best Field Indicator: Found with computer circuits etched on the surface!
(mineral.galleries.com)
Page 49 of 50
6. Bibliography
I. Pollack, Herman W. Materials Science and Metallurgy; 4th Edition. 1988
II. Paglierani, Gary. Industrial Materials and Processes. Department of
Industrial Technology California State University, Fresno, Spring 2005
III. Specialty Steel Industry of North America at
http://www.ssina.com/stainless/index.htm
IV. Stainless Plate Products, Inc at http://www.sppusa.com
V. Queensland Alumina Limited A.B.N. 98 009 725 044 at
http://www.qal.com.au/a_process/a_process.html
VI. Chemical Elements.com at
http://www.chemicalelements.com/elements/si.html
VII. Chemistery Divion at
http://pearl1.lanl.gov/periodic/elements/14.html
VIII. Amethyst Galleries' Mineral Gallery at
http://mineral.galleries.com/minerals/elements/silicon/silicon.htm
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