Bubbles of Oily Froth Can Lift Metal Ore Away From Worthless Rock

Pick up a handful of ordinary clay and you are holding a compound of aluminium. Aluminium is the most abundant metal in the earth's crust, and it is in the soil beneath almost every village and city. Yet no one extracts aluminium from clay. Metal companies instead dig up a particular reddish rock, bauxite, from a limited number of places.

The reason is cost. Clay holds its aluminium locked in a silicate that is very expensive to break apart. Bauxite holds aluminium as an oxide that can be processed profitably. Both are minerals of aluminium, but only bauxite is worth mining for the metal — only bauxite is an ore.

Even a good ore is never pure. It comes out of the ground mixed with sand, clay and rock. Before any metal can be extracted, that unwanted material has to be removed, and the method depends on the ore:

- Heavy ores can be washed free of lighter sand with running water
- Magnetic ores can be pulled out of the rock with a magnet
- Sulphide ores can be floated off in an oily froth while the sand sinks

This part builds the language and the first steps of metallurgy — the extraction of metals from their ores:

- Definitions: mineral, ore, gangue, flux and slag
- The ores of iron, aluminium and zinc, with formulae
- Concentration methods and the ores each suits
- Why crushing and concentrating come first

India mines all three metals covered here. Iron ore is mined in states such as Odisha, Jharkhand and Chhattisgarh, bauxite in Odisha and elsewhere, and zinc ore in Rajasthan. Each mine faces the same first problem: separating a valuable compound from a great deal of worthless rock.

One idea links everything that follows. Every step of metallurgy — concentrating, heating, reducing, refining — removes something unwanted as cheaply as possible. The chemistry chosen at each step is whatever does that best for the particular ore.

This page covers the first part of the ICSE Class 10 Chemistry chapter on metallurgy: minerals and ores, the ores of iron, aluminium and zinc, concentration of ores, and the purpose of pulverisation and concentration.

A mineral is any naturally occurring compound of a metal; an ore is a mineral from which the metal can be extracted profitably; gangue is the unwanted rock mixed with it; a flux is added to remove gangue; and slag is the fusible product they form.

The definitions:

- Mineral — a naturally occurring element or compound of a metal found in the earth's crust, with a definite composition
- Ore — a mineral from which a metal can be extracted conveniently and profitably
- Gangue or matrix — the earthy and rocky impurities, such as sand and clay, that are mixed with an ore
- Flux — a substance added during extraction that combines with the gangue to form a product that melts easily
- Slag — the fusible material formed when the flux reacts with the gangue

All ores are minerals, but not all minerals are ores.

- Bauxite is a mineral of aluminium and an ore — aluminium can be extracted from it profitably
- Clay is a mineral of aluminium but not an ore — extraction would cost far too much

How flux and slag work. In an iron blast furnace, the gangue is mainly silica, an acidic oxide. Limestone is added as a flux; it decomposes to lime, a basic oxide, which combines with the silica:

$${\text{CaCO}}_3\to \text{CaO}+{\text{CO}}_2$$
$$\text{CaO}+{\text{SiO}}_2\to {\text{CaSiO}}_3$$

Calcium silicate is the slag. It melts at the furnace temperature, floats on the heavier molten iron and is drained off separately.

Choosing a flux follows a simple rule.

- Acidic gangue, such as silica, needs a basic flux, such as lime
- Basic gangue, such as some metal oxides, needs an acidic flux, such as silica

Worked example — identify each term. In extracting iron from haematite in a blast furnace, sand is present in the ore and limestone is added.

- Ore: haematite
- Gangue: the sand, silica
- Flux: limestone, supplying lime
- Slag: calcium silicate

Worked check — balance the slag equation. Calcium $1=1$; silicon $1=1$; oxygen $1+2=3$ on the left and $3$ on the right. Balanced.

An everyday example. Blast-furnace slag, once a waste product, is now ground and used in making cement and in road construction. The flux turns a problem — solid rock that would clog the furnace — into a liquid that can be run off and even reused.

The boundary case in the definition of an ore. Whether a mineral counts as an ore depends on cost, not on chemistry alone. A mineral with too little metal to be worth extracting today could become an ore if metal prices rise or a cheaper method is found, so the word describes economics as much as composition.

Iron is extracted from haematite and magnetite, aluminium from bauxite, and zinc from zinc blende and calamine, each with a characteristic formula.

1. Ores of iron.

- Haematite — ${\text{Fe}}_2{\text{O}}_3$, a reddish-brown oxide and the main ore of iron
- Magnetite — ${\text{Fe}}_3{\text{O}}_4$, a black, magnetic oxide
- Limonite — $2{\text{Fe}}_2{\text{O}}_3\cdot 3{\text{H}}_2\text{O}$, a hydrated oxide
- Siderite — ${\text{FeCO}}_3$, a carbonate

2. Ores of aluminium.

- Bauxite — ${\text{Al}}_2{\text{O}}_3\cdot 2{\text{H}}_2\text{O}$, the main ore of aluminium
- Cryolite — ${\text{Na}}_3{\text{AlF}}_6$
- Corundum — ${\text{Al}}_2{\text{O}}_3$

3. Ores of zinc.

- Zinc blende — $\text{ZnS}$, a sulphide and the main ore of zinc
- Calamine — ${\text{ZnCO}}_3$, a carbonate
- Zincite — $\text{ZnO}$, an oxide

Grouping the ores by chemical type — which decides how each is treated later:

- Oxide ores: haematite, magnetite, bauxite, corundum, zincite
- Carbonate ores: siderite, calamine
- Sulphide ores: zinc blende

Worked example 1 — percentage of iron in haematite. Atomic masses Fe $56$, O $16$.

$$M=2\times 56+3\times 16=160\%\text{Fe}=\frac{112}{160}\times 100=70\%$$

Worked example 2 — percentage of iron in magnetite.

$$M=3\times 56+4\times 16=232\%\text{Fe}=\frac{168}{232}\times 100\approx 72.4\%$$

Magnetite is slightly richer in iron than haematite, although haematite is the more widely mined.

Worked example 3 — percentage of zinc in zinc blende. Zn $65$, S $32$.

$$M=65+32=97\%\text{Zn}=\frac{65}{97}\times 100\approx 67.0\%$$

Worked example 4 — percentage of aluminium in bauxite. Al $27$, H $1$.

$$M=2\times 27+3\times 16+2\times 18=138\%\text{Al}=\frac{54}{138}\times 100\approx 39.1\%$$

These are percentages in the pure compound. Real ore also contains gangue, so the metal content of rock as mined is always lower.

An everyday example. The red colour of much of the soil in southern and eastern India comes largely from iron oxide, the same compound as haematite. The soil is rich in an iron mineral, but far too dilute and mixed to count as an ore.

Two boundary cases.

- Magnetite is a mixed oxide, which can be written as $\text{FeO}\cdot {\text{Fe}}_2{\text{O}}_3$ — one iron(II) and two iron(III) per formula unit, which is how three iron atoms balance four oxygens
- Cryolite is listed as an ore of aluminium, but its main use in extraction is different — molten cryolite dissolves aluminium oxide in the electrolytic cell, as Part 3 explains

Heavy ores are washed free of lighter gangue in a stream of water, magnetic ores are separated from non-magnetic rock by a magnetic roller, and sulphide ores are carried off in an oily froth while the gangue sinks.

Concentration or dressing is the removal of gangue from the powdered ore.

1. Hydrolytic method — gravity separation.

- Principle: the ore particles are heavier than the gangue particles
- Process: the powdered ore is placed on a sloping, vibrating table or in a tank and washed with a stream of water. The lighter gangue is carried away; the heavier ore particles settle and are collected
- Suited to: heavy oxide ores such as haematite

2. Magnetic separation.

- Principle: either the ore or the gangue is magnetic, and the other is not
- Process: the powdered ore falls onto a conveyor belt moving over a magnetic roller. Magnetic particles cling to the belt longer and drop close to the roller; non-magnetic particles fall straight off further away, forming two separate heaps
- Suited to: magnetic ores such as magnetite, ${\text{Fe}}_3{\text{O}}_4$

3. Froth flotation.

- Principle: sulphide ore particles are wetted by oil, while gangue particles are wetted by water
- Process: the powdered ore is mixed with water and a little pine oil in a tank, and air is blown through to make a froth. Oil-coated ore particles cling to the air bubbles and rise with the froth, which is skimmed off. The water-wetted gangue sinks to the bottom
- Suited to: sulphide ores such as zinc blende, $\text{ZnS}$

Worked example — choose the method.

- Magnetite — magnetic: magnetic separation
- Zinc blende — sulphide: froth flotation
- Haematite — heavy oxide: hydrolytic method

Everyday versions of all three.

- Panning for gold in a river, where water washes away sand while heavy gold stays in the pan, is gravity separation
- Pulling iron filings out of sand with a magnet, a common school activity, is magnetic separation
- Washing an oily plate shows the idea behind flotation — oil clings to bubbles and lifts away, while grit stays behind

The boundary case — when physical methods are not enough. Bauxite is concentrated chemically, not by any of these three methods. It is dissolved in sodium hydroxide, which takes up the aluminium oxide and leaves the impurities behind — Baeyer's process, described in Part 3. Physical separation works only when ore and gangue differ in density, magnetism or wettability; when they do not, chemistry has to do the job.

The link to the ores section. Each method matches a type of ore from the previous section: gravity for heavy oxides, magnetism for magnetite, flotation for sulphides. Knowing an ore's formula tells you which concentration method to choose.

Pulverisation frees ore particles from gangue and gives them a large surface area, and concentration removes the gangue, so later heating and reduction use far less fuel and flux and give purer metal.

Pulverisation is the crushing and grinding of large lumps of ore into a fine powder, in crushers and grinding mills.

Purposes of pulverisation:

- Frees the ore from the gangue — in a large lump, ore and rock are locked together; in a fine powder, many particles are either mostly ore or mostly gangue, so they can be separated
- Makes concentration possible — flotation, magnetic separation and washing all need small particles
- Increases surface area, so that later reactions such as roasting, calcination, dissolving and reduction happen faster and more completely

Purposes of concentration:

- Removes most of the gangue before any heating is done
- Saves fuel, because the furnace does not have to heat useless rock
- Reduces the flux needed and the slag produced, since less gangue remains
- Improves the purity of the metal obtained
- Lowers transport costs, since the concentrate is much smaller than the raw ore

Worked example — what concentration saves. Suppose $1000\text{ kg}$ of crushed rock contains $150\text{ kg}$ of zinc blende, and froth flotation produces $200\text{ kg}$ of concentrate holding all of that zinc blende.

$$\text{zinc blende in the rock}=\frac{150}{1000}\times 100=15\%$$
$$\text{zinc blende in the concentrate}=\frac{150}{200}\times 100=75\%$$
$$\text{mass to be heated}=\frac{200}{1000}=\frac{1}{5}\text{ of the original}$$

The furnace now handles one-fifth of the mass for the same amount of zinc blende — a direct saving in fuel, flux and furnace time.

Worked example — the zinc in that concentrate. Using the $67.0\%$ zinc in pure zinc blende from the previous section:

$$150\times \frac{65}{97}\approx 100.5\text{ kg of zinc}$$

The metal content does not change during concentration — only the useless rock is removed.

Where these steps fit in the whole extraction:

- Crushing and pulverisation
- Concentration of the ore
- Conversion to oxide by roasting or calcination
- Reduction of the oxide to metal
- Refining of the impure metal

The next part of the chapter takes up the last three steps.

An everyday example. Whole spices ground to powder in a kitchen mixer release their flavour into cooking almost at once, while whole spices take much longer. A larger surface area speeds up any process that happens at a surface, and roasting ore in air is exactly such a process.

The boundary case. Concentration by physical methods does not change the chemical nature of the ore — zinc blende is still zinc sulphide afterwards. Only the proportion of ore to gangue changes, which is why a separate chemical step is still needed before reduction.