2020

DPS BOKARO, SEC-IV

CHEMISTRY

DEPARTMENT

[EXTRACTION OF METALS -1] [Type the abstract of the document here. The abstract is typically a short summary of the contents of

the document. Type the abstract of the document here. The abstract is typically a short summary of the

contents of the document.]

Extraction of metals

The process of extracting metal ores buried deep underground is called Mining.

The metal ores are found in the earth’s crust in varying abundance. The

extraction of metals from ores is what allows us to use the minerals in the

ground! The ores are very different from the finished metals that we see in

buildings and bridges. Ores consist of the desired metal compound and the

impurities and earthly substances called Gangue. The extraction of metals and

its isolation occurs over a few major steps:

• Concentration of Ore

• Isolation of metal from concentrated Ore

• Purification of the metal

Aluminium:

• Bauxite AlOx(OH)3-2x [where 0 < x < 1]

• Kaolinite (a form of clay) [Al2 (OH)4 Si2O5]

Iron:

• Haematite Fe2O3

• Magnetite Fe3O4

• Siderite FeCO3

• Iron pyrites FeS2

Copper:

• Copper pyrites CuFeS2

• Malachite CuCO3.Cu(OH)2

• Cuprite Cu2O

• Copper glance Cu2S

Zinc:

• Zinc blend/Sphalerite ZnS

• Calamine ZnCO3

• Zincite ZnO

The ores obtained from the earth contain large quantities of foreign matter.

These unwanted impurities, e.g. earth particles, rocky matter, sand limestone

etc. present in an ore are called gangue or matrix

The process of removal of gangue from the ore is technically known as

concentration or ore dressing and the purified ore is known as concentrate.

Hand Picking

• The ore is separated from the main stock in a sufficient degree of purity by

simply picking it by hand and then breaking away the adhering rocky

materials with a hammer.

• This is done on the basis of differences in their colour, luster and lump

shape.

Gravity Separation or Hydraulic Washing (Levigation)

• This method of concentration of the ore is based on the difference in

specific gravities of the metallic ore and gangue particles.

• Generally metal ores are heavier than the gangue associated with them.

• By flowing the powdered ore in a current of water, the lighter rocky

impurities can be washed away and the ore particles are left behind.

• Generally oxide and carbonate ores are concentrated by this method. i.e.

haematite and cerussite.

Magnetic Separation

This method of separation is used when either the ore particles or the gangue

associated with it possess magnetic properties. For example, chromite

Fe(CrO2)2 being magnetic can be separated from the non-magnetic silicious

gangue by magnetic separation.

• This method is widely used for the separation of two minerals, when one

of them happens to be magnetic.

• The magnetic mineral can be separated from the non-magnetic one by

this method. For example, mixture of FeWO4(magnetic) and cassiterite

SnO2 (non-magnetic) are separated by this method.

Similarly, rutile TiO2 (magnetic) can be separated from chlorapatite, 3Ca3(PO4)2.

CaCI2 by magnetic separation.

FROTH FLOTATION METHOD

Froth flotation method is used to separate particles based on the

differences in the ability of air bubbles to selectively attach to

specific mineral surfaces in mineral-water slurry.

A finely crushed ore is suspended in water and a blast of air is

passed through the suspension. The wet particles to which air

bubbles are attached float as a froth which is skimmed, whereas

the particles to which air bubbles are not attached gets heavy

and sink to the bottom.

Principle:

The principle of froth floatation is that sulphide ores are

preferentially wetted by pine oil, whereas the gangue particles

are wetted by water.

Collectors are added to enhance the non-wettability of the

mineral particles. Examples of collectors are pine oil, fatty

acids and xanthates.

Froth stabilisers are added to stabilise the froth. Examples of

froth stabilisers are cresols, aniline.

If two sulphide ores are present, then it is possible to separate

the two sulphide ores by adjusting the proportion of oil to

water or by adding depressants. Example: For an ore

containing ZnS and PbS, the depressant used is NaCN. It

selectively prevents ZnS from coming to froth but allows PbS

to come with the froth.

METHOD

This method employs a mixture of water and pine oil which is

made to froth in a tank to separate sulphide ores. The

differences in the wetting properties of the ore and gangue

particles separate them.

A mixture of water, pine oil, detergent and powdered ore is

first taken in a tank. A blast of compressed air is blown

through the pipe of a rotating agitator to produce froth. The

sulphide ore particles are wetted and coated by pine oil and

rise up along with the froth (froth being lighter). The gangue

particles wetted by water sink to the bottom of the tank (water

being heavier). Sulphide being more electronegative attracts

the covalent oil molecules. The gangue being less

electronegative is attracted by the water. The froth containing

the sulphide ore is transferred to another container, washed,

and dried.

Role of depressants

In the froth floatation process, the role of the depressants is to

separate two sulphide ores by selectively preventing one ore from

forming froth. For example, to separate two sulphide ores (ZnS and

Pbs), NaCN is used as a depressant which selectively allows PbS to

come with froth, but prevents ZnS from coming to froth. This happens

because NaCN reacts with ZnS to form Na2[Zn(CN)4].

4 NaCN + ZnS → Na2[Zn(CN)4] + Na2S

Froth Stabilizers

As the name suggests, they stabilize the foam that rises to the top,

to make the separation easier. Pine oil, soaps, cresols etc are froth

stabilizers.

CONCENTRATION OF ORE BY LEACHING

It is often used if the ore is soluble in some suitable solvent. It is

chemical method used for purification of ore.

Bayer Process(leaching of aluminium)

The leaching of aluminium from bauxite on an industrial scale is known

as the Bayer process. This process will give us the end product of

Alumina (aluminium oxide) from bauxite. The other impurities of silica,

titanium oxide and varies other oxides will separate.

The first step is to heat the bauxite ore to temperatures of 200°c along

with a sodium hydroxide solution. This will convert the aluminium oxide

to a solution of sodium aluminate. The silica will dissolve itself in the

process. The chemical equation is

Al2O3 + 2 NaOH → 2 NaAlO2 + H2O

From the above solution, we remove solid impurities by filtration. Then

we bubble carbon dioxide through the aqueous solution of sodium

aluminate to neutralize it. And aluminium hydroxide precipitates as a

result.

2 NaAlO2 + CO2 → 2 Al(OH)3 + Na2CO3 + H2O

This hydrated alumina is then heated to 1470 K. This gives us a pure

form of aluminium hydroxide without the impurities.

2 Al(OH)3 → Al2O3 + 3 H2O

Leaching of gold

Gold is soluble in dilute cyanide solutions. Sodium or potassium cyanide

solutions have a preferential dissolving action on the small particles of

gold and silver over other materials usually found in gold and silver ores.

(Au + 2CN- [Au(CN)2]- + e-)x4

O2 + 2H2O + 4e- 4OH-

4Au + 8CN-+ O2 + 2H2O 4[Au(CN)2]- +4OH-

OR

4Au +8KCN +H2O + O2---� KAu(CN)2 +4KOH

Thus when fresh surfaces of gold are exposed to the action of cyanide in

an aqueous solution containing free oxygen, a gold cyanide compound

from which metal is obtained by replacement.

[Au(CN)2]- + Zn -------------� [Zn(CN)4]

2- + Au

• CONVERSION OF CONCENTRATED ORE INTO METAL OXIDES

1. CALCINATION

In this process the concentrated ore is heated in the

absence of air to a temperature below the melting point of

the metal.

Calcinations is done in a reverberatory furnace. In

calcinations-

a) Volatile organic matter is driven off.

b) Moisture present in the ore is expelled.

c) The ore becomes pores.

d) Water is removed from the hydrated oxides ores.

Al2O3.2H2O(bauxite) Al2O3 + 2H2O

Fe2O3.3H2O(limonite) Fe2O3 + 3H2O

e) Carbonate ores are decomposed to their oxides

ZnCO3(zincite) ZnO + CO2

MgCO3(magnesite) MgO + CO2

CaCO3(limestone) CaO + CO2

Calcined materials are mostly amorphous since

the calcination process activates materials by changing

their crystalline into amorphous structure, which provides

extra energy storage and activity increase.

2. ROASTING

In this process the ore is heated alone or mixed with other

materials under the regular supply of oxygen below the

melting point of the metal. Roasting is usually applied to

sulphide ore. During roasting

a) Moisture is driven away

b) Volatile impurities are removed

c) Organic matter is destroyed

d) Sulphur, arsenic, and other volatile matter present in

combined and free state are converted into volatile

oxides.

2ZnS + 3O2 2ZnO + 2SO2

4FeS2 + 11O2 2Fe2O3 + 8SO2

2HgS + 3O2 2HgO + 2SO2

Sometimes ore is mixed with suitable material and then

roasted in the absence of air

Ag2S + 2NaCl 2AgCl + Na2S

AgCl + 2Hg AgHg + HgCl

FLUXES

The ore even after concentration and calcinations still

contain certain impurities . there are certain

substances known as flux which when mixed with

calcinated ore combine with impurities to form easily

fusible materials known as slag

Impurity + Flux Slag

These slag are not soluble. They are lighter than the

molten metal and hence floats over its surface.

There are two types of flux-

1. ACIDIC FLUX

2. BASIC FLUX

Copper ores contain iron as the main impurity. Iron react with silica and

is removed as slag. Therefore, the slag formed in the blast furnace

is FeSiO3.

The reaction that takes place in the blast furnace is as follows:

FeO + SiO2 FeSiO3 (slag)

The calcium oxide reacts with silica (sand) impurities in the hematite,

to produce slag - which is calcium silicate.

Calcium oxide + silica → calcium silicate

CaO (s) + SiO2(s) → CaSiO3 (l)

This reaction is a neutralization reaction. Calcium oxide is basic (as it

is a metal oxide) and silica is acidic (as it is a non-metal oxide).

Q. What is the role of pine oil in froth floatation process?

Q. What is the role of silica in the extraction of copper?

Q. What is the role of depressants in froth floatation process?

Q. What is the role of dilute NaCN in the extraction of gold?

Q. Name the process of concentration,

(a) Based on densities

(b) Based on magnetic nature.

(iii) How an ore is concentrated by froth floatation process?

Q. Comment on the statement that- "All ores are minerals but all

minerals are not ores".

Q. Name a non-metallic element which

(a) forms acidic and neutral oxides

(b) Has a metallic lustre

(c) Is a liquid at ordinary temperatures

(d) Is a conductor of electricity

Q. Name a metal which occurs free in nature.

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