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


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http://www.chemicalelements.com/groups/metalloids.html

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


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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.


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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http://pearl1.lanl.gov/periodic/elements/8.html


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.


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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http://mineral.galleries.com/minerals/elements/class.htm#nonmetallics

http://mineral.galleries.com/scripts/item.exe?LIST+Minerals+Elements+Silicon

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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http://mineral.galleries.com/minerals/silicate/quartz/quartz.htm

http://mineral.galleries.com/minerals/silicate/class.htm

http://mineral.galleries.com/minerals/elements/gold/gold.htm

http://mineral.galleries.com/scripts/item.exe?LIST+Gift+Bookends+Silicon_Bookends

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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http://mineral.galleries.com/minerals/silicate/quartz/quartz.htm

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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!


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http://mineral.galleries.com/minerals/elements/gold/gold.htm

http://mineral.galleries.com/minerals/fablocal/kola.htm

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


VIII. Amethyst Galleries' Mineral Gallery at

http://mineral.galleries.com/minerals/elements/silicon/silicon.htm

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http://www.ssina.com/stainless/index.htm

http://www.qal.com.au/a_process/a_process.html

http://www.chemicalelements.com/elements/si.html


http://mineral.galleries.com/minerals/elements/silicon/silicon.htm

it 283 advanced materials and process i. stainless steels...

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