Roasting of iron ore

Heating in air: drier ore, more iron, fewer impurities

1. Iron ores and extraction of iron

2. More iron, with no iron added?

Pure

FeCO3 FeCO X_{3}

, the iron compound in siderite, is about 48% iron. After strong heating in air it ends up about 70% iron. Nobody added iron.

Pick the right answer

Predict: where does the higher percentage come from?

A. Heating makes new iron atoms
B. Heating reduces the ore to iron metal
C. Gases leave the solid, and the iron stays

الشرح

Heating in air cannot create iron atoms and does not reduce the ore. Gases (

CO2 CO X_{2}

, water vapour) leave the solid, so the same iron mass sits in a smaller total mass and its percentage rises.

3. What roasting is

Roasting

The ore is strongly heated in air.

Physical: crushing, sintering, concentrating → Chemical: roasting

Ore dressing: improving the physical, then the chemical properties of the ore

Roasting improves the chemical properties of the ore. It is part of ore dressing, before reduction.

4. Purpose 1: more iron in the ore

Purpose (a): drying the ore and expelling humidity, to increase the percentage of iron in the ore.

Equation 1

Siderite decomposes

aligned&FeCO3(s) →[Δ]\\&FeO(s) + CO2(g)aligned

Thermal decomposition:

CO2 CO X_{2}

gas leaves the solid.

5. Why the iron percentage rises

Follow 2 mol of pure

FeCO3 FeCO X_{3}

all the way to

Fe2O3 Fe X_{2} O X_{3}

(Fe = 56, C = 12, O = 16).

	Total mass	Mass of Fe	% Fe
Before: 2 FeCO₃	232 g	112 g	48.3%
After: Fe₂O₃	160 g	112 g	70.0%

Rounded atomic masses. The book's labels: 48.5% (FeCO₃) and 69.6% (Fe₂O₃).

The iron mass does not change. The total mass falls, so iron's share rises.

6. Purpose 2: oxidizing impurities

Purpose (b): oxidation of some impurities such as sulphur and phosphorus.

S(s) + O2(g) \to[Δ] SO2(g) S (s) + O X_{2} (g) Δ SO X_{2} (g)

True or false?

Roasting melts sulphur and phosphorus out of the ore.

الشرح

Sulphur and phosphorus are not melted out: they react with the oxygen of the air and are oxidized to

SO2 SO X_{2}

and

P2O5 P X_{2} O X_{5}

Balance the equation

Balance the phosphorus equation.

P(s) + O2(g) \to P2O5(s) □ P (s) + □ O X_{2} (g) □ P X_{2} O X_{5} (s)

Balanced equation

4P(s) + 5O2(g) \to 2P2O5(s) 4 P (s) + 5 O X_{2} (g) 2 P X_{2} O X_{5} (s)

الشرح

4 P and 10 O on each side.

7. Decomposition or oxidation?

Sort each roasting reaction by what happens to it.

Sort into groups

Sort the reactions.

$FeCO3 \to FeO + CO2 FeCO X_{3} FeO + CO X_{2}$ $2FeO + 1/2O2 \to Fe2O3 2 FeO + \frac{1}{2} O X_{2} Fe X_{2} O X_{3}$ Limonite loses $3H2O 3 H X_{2} O$ on heating $S + O2 \to SO2 S + O X_{2} SO X_{2}$ $4P + 5O2 \to 2P2O5 4 P + 5 O X_{2} 2 P X_{2} O X_{5}$

True or false?

Roasting reduces the ore.

الشرح

Roasting is heating in air: substances break up or gain oxygen. Removing oxygen from the iron oxides (reduction) comes in the next stage.

8. Check yourself

Pick the right answer

Limonite,

2Fe2O3*3H2O 2 Fe X_{2} O X_{3} \cdot 3 H X_{2} O

, is roasted. Why does the iron percentage of the solid rise?

A. Water vapour leaves; the iron stays in less total mass.
B. The heat reduces $Fe2O3 Fe X_{2} O X_{3}$ to iron.
C. Oxygen from the air turns into iron.
D. Iron from the furnace walls joins the ore.

الشرح

On roasting, limonite loses its water of crystallization as vapour:

2Fe2O3*3H2O \to 2Fe2O3 + 3H2O 2 Fe X_{2} O X_{3} \cdot 3 H X_{2} O 2 Fe X_{2} O X_{3} + 3 H X_{2} O

. The iron mass stays the same in a smaller total mass, so the iron percentage of the solid rises.

True or false?

In roasting, sulphur impurity is reduced to

SO2 SO X_{2}

الشرح

S + O2 \to SO2 S + O X_{2} SO X_{2}

sulphur gains oxygen from the air, so it is oxidized, not reduced.

9. Summary

$FeCO3 FeCO X_{3}$ (48% Fe) → $FeO FeO$ ( $Δ Δ$ , $CO2 CO X_{2}$ leaves)
$FeO FeO$ → $Fe2O3 Fe X_{2} O X_{3}$ (about 70% Fe) ( $+O2 + O X_{2}$ of the air)

Siderite in roasting: gases leave, oxygen is added, iron stays

Roasting: the ore is strongly heated in air.
(a) Drying the ore and expelling humidity, to increase the percentage of iron.
(b) Oxidation of some impurities such as sulphur and phosphorus.
Iron stays; gases ( $CO2 CO X_{2}$ , water vapour) leave. The ore is not reduced here.

Back to the hook: can you explain how siderite becomes richer in iron without any iron added?

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