MANUFACTURE OF SULPHURIC ACID

OCCURRENCE OF SULPUR AND ITS PREPARATION

Sulphur occurs in nature in free-state (native sulphur) and its various compounds. Native sulphur

deposits occur in Karakum Desert of Turkmania in Uzbekistan and along the bank of Volga River.

The largest sulphur deposition other countries are in USA, Italy and Japan. Ninety percent of all the

elements of sulphur of world are obtained from sulphur bearing porous limestone in salt dome cap rocks of

Texas and Louisiana by Frasch process (the most economic method used to extract sulphur from underground

deposits).

SULPHUR FROM SMELTING

Sulphur recovery from hydrogen Sulphide (H2S) and Sulphurdioxide (SO2). Smelting is a process by

which metal is extracted by heating and melting and to enhance the melting we treat the metal object with

flux (separates metal from gangue i.e. undesired products).

SULPHUR FROM INDUSTRIAL GASES

Hydrogen sulphide is removed in purification of natural gas and in petroleum reforming, furnishing

either pure hydrogen sulphide or free sulphur depending upon the recovery system.

SULPHUR FROM PYRITE

The pyrite is roasted to obtain Sulphurdioxide. The pyrite is crushed to 35 to 200 mesh. It is fluidized in

reactor and is oxidized to sulphide particles to Sulphurdioxide.

SULPHURDIOXIDE FROM BY-PRODUCT

The steel industry consumes over (500,000 ton) large quantity of 60 0 be acid yearly for pickling (a

treatment on metal surface to remove impurities such as stains, inorganic contaminants, rust or scale from

ferrous metals, copper, aluminum etc.). The wastes are water solutions containing from 2 to 15 % Sulphuric

acid and 10 to 15 % ferrous sulphate.

PROPERTIES OF SULPHUR AT ORDINARY PRESSURES

Sulphur forms brittle yellow crystal having melting point 112.8 0C, density 2.01 g/cm3. It doesn’t

dissolve in water but dissolve in carbon disulphide and benzene. Sulphur is employed in rubber industry to

cure rubs. The crude rubber when cured requires its valuable properties only after being mixed with sulphur

and heated to a definite temperature. This process is called vulcanization of rubber. If the rubber is hardened

then it becomes an insulator called as Ebonite.

RAW RUBBER

Raw rubber also called Natural rubber, Indian rubber or caoutchouc, as initially produced, consists of

suitable polymers of organic compound isoprene _____________________, with minor impurities of other

organic compounds and water. Forms of polyisoprene that are useful as natural rubbers are classified as

elastomers. Currently, rubber is harvested mainly in the form of latex from certain trees. The latex is a sticky

milky colloid (a substance microscopically dispersed throughout another substance) drawn off by making

incisions into the bark and collecting the fluid in the vessels in a natural process called taping. The latex is then

refined into rubber ready for commercial purposes. Natural rubber is used extensively in many applications

and products, either alone or in combination with other materials. In most of its useful forms, it has large

stretch ratio, high resilience (ability of a material to absorb energy when it is deformed elastically and release

that energy upon unloading) and is extremely waterproof.

SULPHURDIOXIDE

Sulphurdioxide is an anhydride of sulphurous acid i.e. H2SO3 (H2O + H2SO4 = H2SO3). Sulphurous acid is

highly unstable.

Sulphurdioxide can attach oxygen transforming into sulphurtrioxide. Sulphurtrioxide is a colorless,

highly mobile liquid (capable of flowing, moving readily) with a density of 1.9 g/cm3. It boils at 44.7 0C and

freezes at 16.8 0C. Sulphuric acid dissolves sulphuric anhydride i.e. SO3 to form oleum or disulphuric acid or

pyrosulphuric acid or heptaoxodisulphate (VI) acid (H2S2O7). ________ is formed which corresponds in oleum

containing 45 % sulphurtrioxide.

INDUSTRIAL PRODUCTION OF SULPHURIC ACID

Two basic methods that are used for the industrial production of sulphuric acid are NITROUS METHOD

AND CONTACT METHOD. In both methods the first step is to obtain Sulphurdioxide which is converted to

sulphuric acid.

NITROUS METHOD

In this method nitrogendioxide is used to oxidize Sulphurdioxide to sulphurtrioxide which forms

sulphuric acid with water. Previously, this method was carried out in lead chamber and was called the

Chamber method. Later, nitrous units appeared in which the Sulphurdioxide is oxidized in packed towers by

nitrogenoxide dissolved in sulphuric acid. This is known as Tower method.

In chamber unit one cubic meter of chamber volume yields 5 to 8 kg sulphuric acid per day while in

tower unit up to 200 kg or more sulphuric acid is obtained per cubic meter. This produced sulphuric acid has

concentration of about 75 to 77 %.

Production of sulphuric acid by Nitrous Tower Method is complex. It can be represented by the

following simplified. The reaction that takes place is as follows:

Oxides of nitrogen N2O3 and NO2 can reaction with H2SO4.

N2O3 + H2SO4 =================== 2 (NO) HSO4 + H2O

2NO2 + H2SO4 =================== (NO) HSO4 + HNO3

In the first case the absorption of N2O3 by sulphuric acid, results in the formation of nitrosyl-sulphuric

acid and water while in second case nitrosyl-sulphuric acid and nitric acid is formed. The rate of reaction of

N2O3 (NO + NO2) with sulphuric acid is higher than that of NO2. If an excess of sulphuric acid is used in

absorbing the nitrogen oxide, the nitrosyl-sulphuric acid formed will dissolve in it. The solution is called nitrous

vitriol.

If SO2 is passed through tower containing a packing continuously wetted with nitrous vitriol added to

it, the following reaction takes place:

(NO)HSO4 + H2O ================= H2SO4 + HNO2

SO2 + H2O ================= H2SO3

H2SO3 + 2 HNO2 ================= H2SO4 + H2O

That is the nitrosyl-sulphuric acid is decomposed by water to sulphuric acid and nitrous acid. SO 2

combine with water and forms sulphurous acid which is then oxidized by nitrous acid to sulphuric acid, water

and NO-gas.

Thus, as the result of these reactions the sulphurdioxide oxidizes (liquid phase) and forms sulphuric

acid, while nitrogendioxide, which is contained in the nitrosyl-sulphuric acid is reduced to NO.

Thus reaction occurs in first tower through which the gas is passed called Glower’s tower (they

process sulphurdioxide into sulphuric acid).

THE CONTACT PROCESS

The contact process produces a number of grades of sulphuric oleum containing 20 % free

sulphurtrioxide, vitriol (92.5 % sulphuric acid and 75 % water) and battery acid which approximately has the

same concentration as vitriol oil but is of higher purity.

The contact process for manufacturing sulphuric acid consists of three stages:

Separation of impurities which are harmful to the catalyst from the gas.

Catalytic oxidation of sulphurdioxide to sulphurtrioxide.

Absorption of sulphurtrioxide to produce sulphuric acid.

The principle stage is that of catalytic oxidation of sulphurdioxide to sulphurtrioxide and the name of

this method is derived from this separation called contacting.

Catalytic oxidation is typical example of heterogeneous exothermic oxidation catalysis.

2 SO2 + O2 ===================== 2 SO3 + (2 x 96.7)KJ

S(c) + O2 (g) ===================== SO2 (g) , ∆H = -7900 Cal

SO2 + ½ O2 ===================== SO3 (g) , ∆H = -2300 Cal

The equilibrium constant for this reaction is calculated from partial pressures, according to Guldberg &

Waage’s Law of mass action may be represented/ expressed as;

Kp = PSO3/ PSO2 + PO 1/2

Values for this expression have been calculated and is based on our atmosphere here in table. Kp for any given

temperature:

TEMPERATURE KP

400 397

500 48.1

600 9.53

700 0.915

800 0.384

1000 0.1845

1100 0.0980

The contact process is made up of the following basic steps:

Purification of sulphurous gas (SO2) to remove catalytic poison.

Contact (catalytic) oxidation of SO2 to sulphuric anhydride (SO3).

Absorption of sulphuric anhydride (SO3) by sulphuric acid.

GAS PURIFICATION

It is carried out on Cottrell filter to remove catalytic poison and dust. Calatytic poison compound of

Arsenic and selenium (ArS2O3, SeO2).

CATALYTIC OXIDATION OF SO2

Oxidation of SO2 to So3 is reversible reaction. At 4000C, the equilibrium condition is favorable, being

almost 100%, the rate of attainment of this equilibrium is low. The rate at 5000C is 40 times as fast as that at

4000C.

Since the reverse reaction (SO3 ====== SO2 + ½ O2) doesn’t become appreciable until 5500C, it would

be advisable to run the reaction at this temperature in order to get as much conversion as possible to take

place quickly. There is here a conflict between favorable conversion equilibrium at low temperature and

favorable rate at high temperature.

By rewriting the expression for Kp in terms of mole fraction and total pressure for the equation (2 SO2

+ O2 ======= 2 SO3)

Kp= N∗nso32

nso22∗no2∗P

Where;

N = total moles

n = number of moles of each component

P = total pressure

From rearrangement,

nso32=nso22∗no2∗Kp∗P

N

From this expression, it would be clear that and increase either of SO2 or O2 will increase the

formation of SO3, thus illustrating the law of mass action.

STUDY THE FOLLOWING

FUNCTION OF TOWERS IN NITROUS PROCESS

REACTION WHICH ARE RESPONSIBLE FOR OXIDATION OF SO 2 TO SO3

THE STUDY ABOUT THE CATALYST OF CONTACT PROCESS

THE MAIN EQUIPMENT FOR THE PRODUCTION OF SULPHURIC ACID IN THE CONTACT PROCESS

THE MAIN REACTION AND HOW TO INCREASE THE PRODUCTION OF SO 3 FROM SO2

MANUFACTURE OF NITRIC ACID

GENERAL

Nitric acid is one of the most important mineral acid is HNO3, heavy colorless liquid whose density is

1.52 g/cm3 and produces fumes when exposed to air. It freezes at -470C and boils at 860C. Boiling of acid is

accompanied by its partial decomposition described by

4 HNO3 =========== 2 H2O + 4 NO2 + O2 – 259 KJ

The nitrogendioxide evolved/dissolved in acid, it gives it a yellow or reddish color depending on the amount of

nitrogendioxide present.

Nitric acid dissolves with water in all proportion. When dilute nitric acid is evaporated, it’s content in

the solution increases up to concentration 68.4% nitric acid, which corresponds to an azeotropic mixture with

a boiling point 121.90C.

There are two types of commercial nitric acid;

Dilute acid with 50-60% nitric acid.

Concentrated acid with 96-98% nitric acid.

Up to 1st World War nitric acid was made almost entirely from Chile saltpeter or Peru saltpeter (NaNO3

name has Chile as it is found in the country and to distinguish it from ordinary saltpeter potassium nitrate

(KNO3)).

PHYSICAL CHEMISTRY

MANUFACTURING PROCESS

Catalytic Ammonia Oxidation

This is an exothermic reaction between ammonia and oxygen. The reactions are shown below:

a. 4 NH3 + 5 O2 ================ 4 NO + 6 H2O + 907.3 KJ

b. 4 NH3 + 4 O2 ================ 2 N2O + 6 H2O + 1104.9 KJ

c. 4 NH3 + 3 O2 ================ 2 N2 + 6 H2O + 1269.1 KJ

d. 4 NH3 + 6 NO =============== 5 N2 + 6 H2O + 1810 KJ

Reactions (a) to (d) are practically irreversible and the course of overall process depends on the ratios

of their reaction rate.

In the absence of catalyst ammonia oxidation mainly result in the formation of N2 by reaction (c).

In manufacturing HNO3 ammonia oxidation by reaction (a) is desirable for obtaining maximum

oxidation of ammonia.

In the industry the degree of oxidation of ammonia to nitric acid (HNO3) with oxygen (O2) of air

reaches 98%. Platinum catalysts are used in the form of wire gauze made thin (0.06 to 0.09mm) wire with 40

to 42 holes per square centimeter. The gauze is made of platinum-rhodium alloys (5 to 10% Rh) which are less

subject to erosion during use and have longer period of service than the gauze of pure platinum. The rate of

oxidation of ammonia to NO over platinum catalyst is very high.

If time of contact between gas and catalyst is significantly increased, NO yield drops sharply as a result

of undesirable side reactions. The optimum contact time lies between 0.0001 and 0.0002 seconds.

The overall rate of oxidation process depending on the convertor design and operating conditions is

controlled by the rate of diffusion of ammonia from the core of the gas stream to the surface of platinum and

only when the agitation is very intense, the rate of reaction between oxygen absorbed on the platinum. The

intensity of catalyst is based on the rate of diffusion of ammonia in air can be found from equation;

G= DRTL

Where;

G is catalysis intensity

D is diffusivity of ammonia in air

R is universal constant

T is temperature

L is mean length of path of molecule in pores of catalyst

Catalysts are very sensitive to presence of impurities, which may be introduced into convertor with

ammonia air mixture. Phosphine which poisons the catalyst even when content in the mixture is as low as

0.00001% is especially toxic.

Under the conditions of ammonia oxidation process the platinum catalyst gradually becomes friable,

its mechanical strength drops and tiny particles of the catalyst are carried away by gas stream.

In units operating at atmospheric pressure and a total temperature of about 8000C losses of platinum-

rhodium catalyst amount to 0.04 to 0.06% per ton of nitric acid produced when the pressure and temperature

are higher catalyst losses are larger. Thus for instance, the unit operating at a pressure of 8 atm and

temperature of about 9000C, the catalyst attrition/losses may be as high as 0.3 to 0.49 per ton nitric acid

produced.

Oxidation of nitric oxide to peroxide is the second state manufacturing nitric acid.

2 NO + O2 ================= 2 NO2 + 112.3 KJ (indicates liberation of heat)

Below 1500C, the reaction is practically completely shifted towards formation of nitrogen peroxide. At higher

temperatures, the equilibrium shifts to left and at above temperature like 8000C, there is practically oxidation

of nitric oxide to nitrogen peroxide. For the great majority of reactions, raising the temperature leads to an

increase in the reaction rate, but oxidation of NO to NO2 doesn’t follow the general rule- the reaction rate

drops with an increase of temperature. Several hypotheses have been postulated to explain this phenomena;

one of them that most widely accepted is the oxidation of NO to NO2 proceeds through intermediate

formation of nitric acid dimers (a compound or molecule formed by combination or association of two

molecules).

2 NO ================== (NO)2 + Q

O2 + (NO)2 ============== 2 NO2 + Q

Formation of nitric acid dimers is a reversible reaction to the left. The equilibrium constant is lowered and the

equilibrium concentration of dimers in the gas mixture drops. The rate subsequent to oxidation of the dimer

to the nitrogen peroxide depends on the concentration of the dimers.

dGNO2

dt=k1∗p (N O2)∗p(O¿¿2)¿

It follows that the decrease in the rate of oxidation of HNO3 to peroxide when temperature is raised can be

explained as a sharp drop in the dimer’s concentration. An efficient means for speeding up gas phase reaction

including the reaction of oxidation of NO to NO2 is to increase the pressure. Thus a tenfold increase in

pressure will increase the rate of this reaction 1000 times.

Consequently by using a pressure of 10 atm. it is possible to reduce the oxidation time and size of the

equipment 1000 times (as compared with values of atmospheric pressure) in this case NO is practically

completely oxidized to NO2.

At low temperatures nitrogendioxide or peroxide forms (associates into) N2O4 by the reaction. N2O4 is

dimer.

2 NO2 =========== N2O4 + Q

The rate of this reaction is very high. With the increase of pressure, the equilibrium of reaction shifts towards

formation of N2O4. At 00C and atmospheric pressure, the degree of conversion of N2O4 is about 70%.

ABSORPTION OF NO2 (peroxide) BY WATER

Absorption of nitrogendioxide by water is final stage of the production of nitric acid. The following

reactions take place;

2 NO2 + H2O =============== HNO3 + HNO2 + Q (16 KJ)

N2O4 + H2O ================ HNO3 + HNO2 + Q (59 KJ)

The nitrous acid HNO2 is unstable and decomposes according to the equation;

3 NO2 + H2O =============== 2 HNO3 + NO + Q (75.8 KJ)

The overall equation for the process of absorption of NO2 by water is;

3 NO2 + H2O ================ 2 HNO3 + NO + Q (136KJ)

As absorption continues, the concentration of nitric acid increases and this slows the absorption of

nitrogendioxide. Absorption is speeded up by decreasing temperature and increasing the pressure. In unit

operating at atmospheric pressure with ammonia air mixture containing 10 to 12% ammonia, the absorption

of nitrogendioxide usually yields dilute nitric acid of 48 to 50% concentration. A pressure increase to 10 atm.

makes it possible to obtain nitric acid to 60 to 62% concentration.

Process for producing dilute nitric acid from ammonia can be divided into;

Atmospheric pressure

High pressure process

Combined process; in which oxidation is carried out at atmospheric pressure, while oxidation of NO

and absorption of nitrogendioxide by water are carried out at higher pressures.

In conversion of air-ammonia mixture to nitric acid at atmospheric pressure, the slowest stage is the

oxidation of nitric oxide to peroxide. Because of this, large size oxidation absorption towers must be used.

HYDROCHLORIC ACID

Hydrochloric acid is prepared by dissolving hydrogen chloride in water. At present the main method

of industrial manufacture of hydrogen chloride is its synthesis from hydrogen and chlorine.

H2 + Cl2 ============== 2 HCl + 103.6 KJ

This process is conducted in special plants in which a mixture of hydrogen and chlorine is continuously formed

and immediately burns in uniform flame. This results in a quite (without explosion) reaction.

The standing material for the production of hydrochloric acid and hydrogen is the electrolysis of

sodium chloride solution. Large amount of hydrochloric acid is also produced as a by-product in the

calcinations of organic compound.

RH + Cl2 ====== RCl + HCl (where R is organic radical)

Hydrochloric acid is a corrosive liquid with a pungent smell. Concentrated hydrochloric acid usually contains

about 37% of HCl. Its density is 1.19 g/cm3.

The former sulphate method for producing HCl which at present is employed only in laboratories is

based on the reaction of sodium chloride and sulphuric acid according to the reaction;

NaCl + H2SO4 ========= NaHSO4 + HCl

NaCl + NaHSO4 ======== Na2SO4 + HCl

The first reaction proceeds to a considerable rate with lsight heating, the second one only occurs at high

temperatures. Sodium sulphate is obtained as a by-product. If any solution is characterized by an identical

composition of the liquid and saturated vapor, and therefore, distilling without changing in their composition

is also called constant boiling or azeotropic mixture. The HCl solution boils at 110 0C with 20.2% HCl in the

liquid and vapor phase that is ___ assumes the azeotropic characteristic.

METAL CHLORIDES

Like the strong acid, HCl vigorously reacts with many metals and metal oxides. Its salts are called

chlorides. Most of them are soluble in water. The slightly soluble are AgCl, PbCl2, CuCl2 and HgCl2.

SODIUM CHLORIDE (NaCl)

NaCl or table salt is the starting material for the production of chlorine, hydrogen chloride, sodium

hydroxide and sodium carbonate (soda). It is used in dyes, soap and many other industries. It is also used in

food seasoning and preservation.

POTASSIUM CHLORIDE (KCl)

It is used in great amount in agriculture as fertilizer.

CALCIUM CHLORIDE (CaCl2.6H2O)

It is employed to prepare refrigerant. Anhydrous CaCl2 is widely used in laboratory practice for drying,

gases and dehydrating liquid in organic substance.

MERCURY CHLORIDE (HgCl2)

It is a corrosive sublimate and is very poisonous. Dilute sublimate (1:1000) are used in medicine as

strong disinfectants.

SILVER CHLORIDE

AgCl is less soluble salt of hydrochloric acid. Silver chloride is used to manufacture light sensitive

material.

The precipitation of AgCl when Cl- ion reacts with Ag+ ion is characteristic reaction for chlorination.

SYNTHESIS OF HYDROGEN CHLORIDE

Systhesis of hydrogen chloride is carried out using chlorine and hydrogen obtained by electrolysis of

aqueous salt solution. Hydrogen gas burns in chlorine exothermically by reaction

H2 + Cl2 ============ HCl + 184.2 KJ

The reaction constant K of this reaction of hydrogen chloride formation is given by

K p=PHCl2

pH 2+ pCl 2

---------------------------- (17.12)

K p=4 xe

( γ xe)2 -------------------------------- (17.13)

Where p denotes the partial pressure of component at equilibrium. xe is equilibrium conversion. Equilibrium

constant depends on temperature.

log K p=9586T

−0.44 logT +2.16−−−−−−(17.14)

Using equations (7.13) and (7.14), the dependence of xe on the temperature was calculated:

T0C xe17 2.54 x 10^(-17)

727 1.34 x 10^(-5)

1727 0.41 x 10^(-2)

2227 1.30 x 10^(-2)

The calculation shows that the equilibrium conversion increases above 15000C.

Industrially, the reaction is carried out tranquilly burning of hydrogen in chlorine at 300 to 24000C.

There is 5 to 10% excess of hydrogen in the reaction mixture to provide full utilization of chlorine.

The absorption of HCl made by any process liberates around 700 Btu/lb of HCl absorbed. The heat

must be taken away from the absorber or the efficiency will be low.

MANUFACTURE OF PHOSPHORIC ACID

Phosphoric acid is obtained from naturally occurring phosphates (phosphorite or apatite) by

extraction or by electro-thermal procedures. In the extraction process phosphoric acid is made from

phosphate mainly using sulphuric acid.

The phosphoric acid (solution) is separated from the precipitate (gypsum) by filtering. The precipitates are

washed with water fro complete removal of phosphoric acid from it. However, even after the washing

operation part of phosphoric compounds remain in gypsum called phosphoric gypsum for reaction.

Extraction process of phosphoric acid contains a maximum of 36% phosphoric acid. This is used for

making triple super phosphates and other fertilizers. Extraction process of phosphoric acid is sometimes

evaporated to increase the concentration from 50 to 75% in drum type concentrators, which are similar in

design and operation to the concentrated evaporation of sulphuric acid.

ELECTROTHERMAL PROCESS

Ca3(PO4)2 + 5 C + n SiO2 ======== P2 + 3 CaO.nSiO2 + 5 CO

In this process phosphoric acid is usually obtained in two stages. The first stage yield is elementary (yellow)

phosphorous. For this purpose phosphorite or apatite carbon (cone and anthracite) and SiO 2 (quartzite) are

charged into electric furnace. Upon heating the mixture to a temperature of 1500 0C the above reaction takes

place. Gas containing the phosphorous in vapor phase is withdrawn from the electric furnace at a temperature

of 6000C and cooled, first by steam then by water, the phosphorous condenses and accumulates under water

in the yellow phosphorous form. The residual gas which contains CO can be used as fuel oil or in the synthesis

of organic compounds.

In the second stage molten yellow phosphorous is pumped from heated storage tank through an

atomizer nozzle into combustion chamber when it burns to (oxidized) P2O5. The P2O5 readily combines with the

water contained in the flue gases to phosphoric acid.

Triple super phosphate is made by decomposing naturally occurring phosphate with the H3PO4

containing 70 to 80% phosphoric acid.

CaF(PO4)3 + 7 H3PO4 + 5 H2O ========= 5 (Ca(H2PO4)2 . nH2O) + HF + Q

The phosphorous is employed frequently as an intermediate product, being burned or oxidized to P2O5 which

is dissolved in water to form acids or other compounds.

Phosphoric acid (4 P + 5 O2 ===== 2 P2O5)

Meta phosphoric acid (P2O5 + H2O =========== 2 HPO3)

Para phosphoric acid (P2O5 + 2 H2O =========== H4P2O7)

Ortho phosphoric acid (P2O5 + 3 H2O =========== 2 H3PO4)

Apatite group comprises compounds of the A2(XO4)3 2 type in which cations are Ca+2, Pb+2 the addition of an

ion being F-1, Cl-1, O-2 and CO3-2.

Ca5(PO4)3F CO(PO4)3Cl

CaO 55% CaO 53.8%

P2O5 42.3% P2O5 41%

F 3.8% F 6.8%

SILICATE TECHNOLOGY

8O = 2s2, 2p4

14Si = 3s2, 3p2

SILICATE TECHNOLOGY

The properties of most silicate result from specific structure of their molecules, the basic structural

element being tetrahedric group S2O-4+4.

A characteristic feature of this structure is the high strength of the bond between the Si +4 and O-2 ions,

as a result of which most of the silicates are extremely hard and have high melting points.

RAW MATERIALS

The raw materials for silicate are:

Silicate materials (chiefly containing SiO2)

Aluminous materials (chiefly containing H2O3, clays and kaolin, bauxite)

Carbonate (salt of carbonic acid), limestone, marble and chalk, magnesite (MgCO3), dolomite (MgCO3

+ CaCO3)

Feldspar (K2O, NaO2) Al2O3 . 6SiO2 etc.

Other naturally occurring materials (granite, basalt, chrome iron ore, gypsum etc.)

The industry also utilizes slag and ashes and other chemicals like soda ash, sodium sulphate, potas, oxide of

lead, boron, organic pigments etc.

TYPICAL SILICATE TECHNOLOGY OPERATION

Sintering is highly important process reacting from heating a mixture of solid substances. In silicate

technology it is final inferring ceramics, refractory and cement, result in the formation of final high strength

ceramic substances and the minerals of cement clinkers. A distinction is made between solid phase and liquid

phase.

Solid phase sintering

Filling the space inside the grain (the pores) and interstitial spaces between grains. This occurs as a

result of increased mobility of atoms in the crystal lattice at high temperatures. Simultaneously, grains

re-crystallizes are same of the crystal grows in size at expand of others. The rate of solid-solid phase

sintering depends on grain size and on the presence of defects in the crystal lattice and it grows with

decrease in the size and increase in number of defects. The rate of solid phase sintering depends on

grain size and on the presence of defects in the crystal lattice and it grain with it.

Liquid phase sintering

It is a diffusion process and its course depends on the amount of molten mass which fills the space

between the grains and its properties. The sintering rate increases with decreased viscosity molten mass and

increase in its wetting ability.

Sintering is accompanied by formation of new chemical compounds resulting from the appearance of

liquid phase and reaction components of molten or sintered mass.

When sintered materials are cooled, a crystal lattice either of type which existed or a new one may be

formed. Under certain conditions, cooling a molten mass (substance) yields an amorphous substance- a glass,

and not a crystalline one. The presence of glass phase is a typical feature of many end-products of silicate

material/wave.

Ceramics

The name ceramics includes material and articles made from refractory substances such as clay and

carbides or oxides of certain materials, depending on their applications, they are distinguished by building,

refractory, chemically stable, domestic and technical ceramics.

Building ceramics includes brick, roofing and facing tiles, pipes and tubes. Refractory ceramics is used

for the internal lining of various furnace for instance blast furnace for steel making and glass furnace.

Chemically stable ceramics resist the action of chemically aggressive media not only at room temperature but

also at high temperature. Domestic ceramics includes fiancés and porcelain articles. Domestic ceramics are

used to make insulators, capacitors, motor for vehicles and aircrafts, spark plugs, high temperature crucibles

and thermocouple tubes.

The manufacturing of bricks involves the following steps

Preparing the ceramics mass

Moulding

Drying

Roasting (usually 9000C)

(minifying, calcinations and baking)

Clay Al2O3.2SiO2 or H4Al2Si2O9

White mica K2O.3Al2O3.6SiO2.2H2O or H4K2Al6Si6O24

Absestos CaO.3Mgo.4SiO4 or CaMg3Si4O12

Feldspar silicate containing alumina silicate. The most important common feldspar or the clase contains

potassium orthoclase -------------- K2O.Al2O3.6SiO2

K2O.Al2O3.6SiO2 + CO2 + nH2O ============== K2CO3 + 4SiO2 (n-2) H2O + Al2O3.2SiO2.2H2O

Pure kaolin occurs comparatively rarely. It is colored white and contains insignificant impurity of quartz sand.

Such kaolin is used for manufacturing of porcelain.

A number of silicate rocks like granites are used as building materials. Silicates are the starting

materials in the production of glass, cement and ceramics.

MARL

It is rock composed of calcite and clay particle (30 to 50%), outwardly it resembles limestone.

Talc: Mg3(OH)2Si4O10

FELDSPAR

Feldspar is generally used for three purposes:

In making the body composition of several types of porcelain, china earthenware and also in

preparation of glass and enamel.

An important ingredient in the glass sand batch.

A bonding agent in the manufacture of bonded abrasive like wheel and discs of garmet, corundum

emery etc.

The glass and ceramics industries are major consumers of feldspar and accounts for 95% of the total

consumption.

In ceramics bodies, the main vitrifying (fluxing) agent is feldspar. The majority of white ware bodies

contains a good proportion of feldspar. It acts as a flux. In ceramics industries, the flux is defined as the body

which develops glass phase. This is mostly provided by feldspar. Feldspar is used in varying proportions in

porcelain, china and earthenware. Earthenware contains an average of;

12% feldspar

25% ball clay

28% china clay

25% quartz

The proportion of feldspar varies in different products as given below;

Wall tiles 5%

Floor tiles 30%

Statutory porcelain 50%

Sanitary china and porcelain bodies 30%

In the glass industries also feldspar is used in varying proportions. It may contain 10 to 15% of the batch.

Feldspar in this industry is valued for two purposes, firstly it acts as flux and secondly for alumina contains

alumina products the resistance of glass to ________ binding and thermal shock.

Feldspar is used as bonding agent along with magnesium oxide, magnesium chloride and the other

synthetic glue in the manufacture of abrasive, discs and shapes of other shapes.

In the preparation of glazes the fine powder of feldspar is mixed with silica powder and thin slurry is

made. The fixed up goods are dipped into the slurry and dipped again. This imports glaze to surface, some

other gradients are also added to the slurry to import glaze and luster. Each pottery factory has its own

technique of preparing slurry which is regarded as trade secret.

FLUX

They are easily fusible materials like borax, iron oxide, lead oxide etc. the following are stages when

ceramics materials/products are fixed;

Dehydration at 1500C to 6500C.

Calcinations of CaCO3 at 6000C to 9000C.

Oxidation of ferrous organic material at 3500C to 9000C.

Silica formation at 9000C and higher.

Glaze temperature may be as low as ??0C and for some over glaze as high as 2000C for vitrification.

In the crushing of clays and processing of raw materials in the manufacture of cements suspension of various

concentration are made. To understand the characteristics of these suspensions, the knowledge of colloid

chemistry is necessary.

CEMENT ====== BONDING

The binding properties of cements are based on ability of silicates and aluminates of calcium to unite

with water and form high strength crystals.

The tri-calcium silicate hydrolyze according to equation;

3 CaSiO2 + 2 H2O ======= 2 CaOSiO2H2O + Ca(OH)2

The hydration of di-calcium silicate and tri-calcium aluminate takes place in the following way;

2 CaSiO2 + H2O =========== 2 CaSiO2.H2O

3 CaOAl2O3 + 6 H2O ============ 3 CaOAl2O3.H2O

1) Soluble contents dissolve in water and make it saturated solution.

2) During the 2nd stage, colloid, jelly-like, low solubility hydro-silicates, hydro-aluminates and hydro-ferrates of

calcium are formed (the reaction are given above to gather with CaO).

3) During the 3rd stage, the calcium hydroxide and hydro-aluminates of calcium crystallizes first. The

characteristic feature of the next stage of hardening are due to close packing, crystallization of mass of gel,

which consist of hydro-silicate of calcium drying in the mass.

The mechanical strength is due to inter growth and inter-wining of crystals.

Crystallization of mineral that solidifies at the highest rate that is of calcium silicate and the rate of

increase of strength of the cement rock depends on the content of this component.

G LASS

When the mixture of many silicates or silicate with silicon-dioxide is heated, transparent amorphous products of fusion called glasses are produced.

The glass do not have sharp melting and freezing points. Ordinary window glass or the glass used to make major part of the domestic glassware (bottles, jar, glasses etc.) consists chiefly of sodium and calcium silicate fused with silicon-dioxide.

The composition of such glass is approximately expressed by Na2O.CaO.6SiO2. The starting products of its production are white sand, soda, lime or chalk. The following reaction occurs when the mixture of these substances are fused.

CaCO3 + SiO2 ======== CaSiO3 + CO2

Na2CO3 + SiO2 ======= Na2SiO3 + CO2

Sodium sulphate and ________ is often used instead of soda. The coal reduces the sodium sulphate to sodium sulphite that enters into reaction with sand to form sodium silicate.

2 Na2SO4 + 2 SiO2 ======= 2 Na2SiO3 + 2 SO2 + CO2

QUARTZ GLASSIt can withstand higher temperatures than ordinary glass. It ____________ ultra violet rays which are

retained by ordinary glass. A very valuable property of quartz glass is its exceedingly low coefficient of thermal expansion _________ signifies that volume of quartz remain almost unchanged when it is heated or cooled. Consequently articles made from it can be greatly heated and than immersed in cold water, they do not crack. Short comings of quartz glass include the difficulty of working it and brittleness.

New building material known as glass fiber laminates are produced by combining glass fiber with ________________________. Their weight is one third to one forth of that of steel, but they are not inferior from them in strength. Glass fiber laminates are finding ever growing application in the motor vehicle, a aviation and ship building.

Glass fiber laminates are used for example to manufacture pipes with-standing a high hydraulic pressure and not subjected to corrosion. Glass exists because the viscosity grows very rapidly when molten glass is cooled so that crystallization has no time to occur. By introducing additional substance for accelerating ____________ into the starting substance and conducting the melting process indefinite condition it is impossible to obtain crystalline glass material known as glass ceramics ________________________.

As regard to the structure, ________ are fine crystals fused together by films of un-crystallized glass. They have strength, hardness, clinical and thermal stability. ____________ belongs to insulator with respect to electrical properties.

They can be used to make cheap and strong building material, electrical insulator, radio components apparatus (equipment) for chemical production.

REFRACTORY BRICKSRefractories enhances those materials which are used to withstand the effect of thermal, chemical

and physical effects that are _________ in ___________ procedure. Refractories are sold in the form of fire bricks, silica bricks, magnesite bricks, chlorite bricks. Silicon carbide refractories, _________ refractories, alumina silicate products and others.

In making refractories, the main material is ____________ adapt itself best to thermal, chemical and mechanical conditions to be meet with.

The _______ required to ____ together the particles of the refractories are kept minimum. Largely because of this there is ___ little vitrification.CHEMICAL PROPERTIES

The usual classification of refractories ________ then into acidic, basic and neutral groups although in many sharp __________ can not be made. Silica bricks are acidic and magnesite bricks are strongly basic, however fireclay bricks are generally placed in neutral group though they belong to either of these classes upon relative silica-alumina content.POROSITY

The porosity is directly related to many other physical properties of bricks including resistance to chemical attack. The higher the porosity of brick the more easily it is penetrated by molten fluxes and gases or a gases class of brick the one with lowest porosity has the greatest strength thermal conductivity and heat capacity.FUSION POINT

Before use, softening point of a refractoy must be determined. It is found by use of pyrometric cones of predetermined softening points. Most commercial refractories soften gradually over a wide range and has no sharp melting points because they are composed of several different minerals both amorphous and crystalline. Typical fusion points of refractories are variable in literature.SPALLING

A fracture or flakin off a refractory brick or block due to change in heat stresses or compression caused by heat is known as spalling.STRENGTH

Cold strength has only slight bearing on strength at high temperature. Although, most of application places refractires under compression _______, in rare cases they may be subjected to tension or stress alone. Resistance to abrasion or erosion is also very important for many furnace construction such as coke-oven walls and lining of discharge end of rotary cement kiln.RESISTANCE TO RAPID TEMPERATURE

The bricks with lowest thermal expansion and coarses texture are most resistant to rapid thermal change, less strain develops also.SODIUM CARBONATE

(SODA) Na2CO3

In the form of crystal hydrate, so do corresponds to formula Na2CO3.10H2O. But the crystal hydrate readily erodes, it loses its water of crystallization. Soda is one of chief products of the basic chemical industry. It is used in large amount by the glass, soap, pulp and paper, textile, petroleum and other branches of industry and also goes to prepare various salts of sodium.

It also finds domestic use mainly as detergent. At present, soda is produced commercially by solvay process based on formation of sodium hydrogen carbonate, when sodium chloride reacts with ammonium hydrogen carbonate in an aqueous solution. A concentrated sodium chloride is saturated with ammonia and then carbon dioxide obtained by calcinations of limestone is passed into it under pressure. Reaction of ammonia and carbon dioxide yields ammonium hydrogen carbonate.

NH3 + CO2 + H2O =========== NH4HCO3

The later enters into exchange reaction with sodium chloride NH4Cl and NaHCO3. Na4HCO3 + NaCl =========== NaHCO3 + NH4Cl

The sodium hydrogen carbonate has a comparatively low solubility in water, precipitates and is filtered off. When NaHCO3 is calcined, it decomposes into carbonate, water and carbon dioxide which is recycled.

2NaHCO3 ============= Na2CO3 + CO2 + H2OThe ammonia is recovered by heating the solution containing NH4Cl with slaked lime.

2NH4Cl + Ca(OH)2 ========== 2NH3 + CaCl2 + H2OIt is also recycled.

Thus the only residue of the process is CaCl2 that remains in solution after liberation of ammonia and has a limited application.

Sodium carbonate made by solvay process contains no water of crystallization is called soda ash.A part of sodium hydrogen carbonate is used without any further processing. It is employed in

medicine under the name of bicarbonate of soda and the baking of bread and in food industry in name of baking soda.