Chemical Industry

Chemical reactions and physical processes on a large scale to convert raw materials into useful products.

Conditions of the reactions are controlled to produce the best yield of product possible at an economic rate.

YIELD: Quantity of product formed Theoretical: Predicted by equation Actual: Quantity actually obtained

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Haber Process

Process to produce ammonia. Developed by German Chemist Fritz

Haber N2(g) + 3H2(g) 2NH3(g) H = –46kJmol-1

CONDITIONS FOR HIGH YIELD High pressure (less molecules on the

product side) Low temperature (forward reaction is

exothermic)

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Haber Process

ACTUAL CONDITIONS High Pressure (200-250

atmospheres pressure). If pressure is too high, expensive structural requirements are needed for the plant.

Moderately high temperature (~ 400oC). If the temperature is low then the yield is high, but it takes a long time for the reaction to produce the product (Rate low)

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Haber Process

Iron catalyst increases rate of forward and back reaction.

Yield of ammonia is approximately 45% of the theoretical yield

4

Haber Process

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Contact Process

Product of sulfur dioxide from sulfur or metal sulfides

S(s) + O2(g) SO2(g)

2ZnS(s) + O2(g) 2ZnO(s) +SO2(g)

Conversion of sulfur dioxide to sulfur trioxide

2SO2(g) + O2(g) 2SO3(g) H = –99kJmol-1

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Contact Process

Absorption of sulfur trioxide into concentrated sulfuric acid to form oleum

SO3(g) + H2SO4(l) H2S2O7(l)

REACTION THAT CONTROLS YIELD

2SO2(g) + O2(g) 2SO3(g) H = –99kJmol-1

CONDITIONS FOR HIGH YIELD High pressure Low temperature 7

Contact Process

ACTUAL CONDITIONS Atmospheric pressure. Yield is about

85-90% at this pressure. Costs to increase pressure are not offset by much greater yield.

Temperature: 450oC. Compromise between yield and rate.

Vanadium pentoxide catalyst (V2O5) increases rate of reaction.

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Contact Process

Sulfur trioxide is dissolved in concentrated sulfuric acid as it forms to maximise yield.

Acid is transported as oleum (less corrosive) and diluted as required by buyer which reduces transport costs.

H2S2O7(l) + H2O(l) 2H2SO4(aq)

600kJ of energy is released for every mole of acid formed. Some of this energy is used to produce electricity for the plant.

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Flow Diagrams

Used to represent the movement of materials through various components of the plant.

May include diagrams of equipment or show the process through a series of boxes and arrows. May show quantities of material and energy.

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Haber Process

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Contact Process

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Flow Diagrams

RAW MATERIALS Converted by chemical/ physical

means into useful products. Examples include coal, oil, natural gas, air, limestone, sand, metal ores, water

WASTE PRODUCTS No use or market for the product.

Disposal can be a problem if they are toxic or produced in large amounts.

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Flow Diagrams

BY PRODUCTS Not the main product, but do have

a use either within the plant or commercially. E.g. sulfur dioxide from metal smelters

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Metals

Most occur in the earth’s crust as minerals

The most common occurrences are K, Ca, Na, Mg as salts(Cl–, SO4

2–,

CO32–)

Al, Fe, Sn as oxides Zn, Ni, Pb, Cu as sulfides Au, Ag, Pt as the uncombined

metal 15

Metal Reactivity

When metals react they undergo oxidation (lose electrons) M Mx+ + xe

More easily a metal is oxidised, the less easily its ions are reduced to the metal

When determining the reactivity of a metal its reactions with water, acid and metal displacement reactions are considered

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Metal Reactivity

Example: Reactions of Calcium

Water: Ca(s) +H2O(l) Ca(OH)2(s) +

H2(g)

Acid: Ca(s) + 2H+(aq) Ca2+

(aq) + H2(g)

Displacement:

Ca(s) + Zn2+(aq) Ca2+

(aq) + Zn(s) 17

Metals from their Ores

Ore deposit is a region in the earth’s crust where the concentration of a metallic mineral is at a level where the extraction of the metal is commercially viable

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Metals from their Ores

Concentration of the mineral (removal of the gangue)

Conversion of the concentrate into a substance suitable for reduction. (Most common chemical process metal sulfide to metal oxide)

Reduction of the metal compound to metal via chemical means or electrolysis.

Refining the metal to remove trace impurities

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Production of Zinc

Zinc ore (zinc blende) is mined at Broken Hill (NSW) and Mt Isa (Qld). Contains approx 2-8% zinc

Crushed and ground into small particles at the mine ready for froth flotation

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Froth Flotation

Ore is added to tanks containing water, frothing agents and collector molecules (molecules with polar and non polar ends)

ZnS is attracted to the polar end of collector molecules and is carried to the surface of tanks on the froth when air is blown through the mixture. This is skimmed off.

Gangue remains on the bottom of the tank as a sludge 21

Froth Flotation

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Production of Zinc

The zinc sulfide is roasted in air to form zinc oxide

2ZnS(s) + 3O2(g) 2ZnO(s) + 2SO2(g)

Sulfur dioxide is used to make sulfuric acid for next step (Contact process)

Oxide is leached with sulfuric acid

ZnO(s) + H2SO4(aq) ZnSO4(aq) + H2O(l)

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Production of Zinc

Zinc powder is added to displace less reactive metals (Ag+, Cd2+, Cu2+). These are collected and processed.

Electrolysis of zinc sulfate Anode (Lead or silver/lead)

2H2O(l) O2(g) + 4H+(aq)

+ 4e Cathode (aluminium or zinc)

Zn2+(aq) +2e Zn(s)

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Production of Zinc

Overall 2Zn2+

(aq) +2H2O(l) 2Zn(s) +O2(g) + 4H+

(aq) The zinc produced is 99.95% pure

and requires no further purification

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Reduction using Electrolysis

Metals more active than zinc can’t be produced by electrolysis of aqueous solutions.

If a solution of a more active metal is electrolysed then

2H2O(l) + 2e H2(g) + 2OH–(aq)

occurs at the cathode in preference to the reduction of the metal.

A molten electrolyte is required with metals above zinc

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27

Reduction of Aluminium

Molten alumina Al2O3 is mixed with cryolite Na3AlF6, CaF2 and AlF3 .

This mixture has a melting point of ~1000oC compared to alumina which melts at 2030oC

This means the electrolysis is carried out at a lower temperature saving money

28

Reduction of Aluminium

Anode (Carbon) 2O2–

(l) O2(g) +4e

C(s) + O2(g) CO2(g) The anode is eaten away and

requires regular replacement.Cathode (Carbon lined steel

tank) Al3+

(l) + 3e Al(l) The aluminium forms below the

molten electrolyte and can be tapped off. 29

Reduction of Aluminium

Overall 4Al3+

(l) +6O2–(l) +3C(s) 4Al(l) + 3CO2(g)

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Chemical Reduction

Metals below aluminium can be produced by reduction with carbon

Iron: 3C(s) + Fe2O3(s) 2Fe(s) + 3CO(g)

Zinc: C(s) + ZnO(s) Zn(s) + CO(g)

These metal are more easily reduced than metals higher in the reactivity series

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Energy

The Reduction stage consumes most energy and so is the most costly stage of any metal production.

Electrolysis of a molten (non aqueous) electrolyte requires the more energy than other methods of reduction.

Consequently it is preferable (cost wise) if a metal can be either chemically reduced or produced by electrolysis of an aqueous solution.

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Energy

Chemical Reduction

Electrolysis of aqueous solution

Electrolysis of molten liquid

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Most Energy required: Most expensive

Least Energy required: Least expensive