Question 9
Give reasons for the following pertaining to Hall Herault's process.
(i) The fusion temperature of the electrolyte has to be lowered before conducting the electrolytic reduction.
(ii) The constituents of the electrolyte in addition to one part of fused alumina contains three parts of cryolite and one part of fluorspar.
(iii) A layer of powdered coke sprinkled over the electrolytic mixture, protects the carbon electrodes.
(iv) It is preferred to use a number of graphite electrodes as anode, instead of a single graphite electrode.
(i) Fused alumina has a melting point around 2050°C. Such high temperatures, create the below difficulties:
- A large amount of electrical energy is required to liberate Al2O3 and maintain the electrolyte in molten state at that temperature.
- The liberated Al metal [m.p. 660°C] may also tend to volatilize out and get wasted.
Hence the fusion temperature of the electrolytic mixture has to be lowered.
(ii) The reasons for addition of mainly Cryolite [in a higher ratio] and Fluorspar to the electrolytic mixture are:
- They lower the fusion point of the mixture i.e., the mixture fuses [melts] around 950°C instead of 2050°C.
- They enhance the mobility of the fused mixture by acting as a solvent for the electrolytic mixture. Thus cryolite in the molten state of subdivision dissolves aluminium oxide.
- Addition of cryolite enhances the conductivity of the mixture. Since pure alumina is almost a non-conductor of electricity.
(iii) The layer of powdered coke is sprinkled over the electrolytic mixture as:
- It prevents burning of carbon electrodes in air at the emergence point from the bath.
- It minimizes or prevents heat loss by radiation.
(iv) The oxygen evolved at the anode escapes as a gas or reacts with the carbon anode. The carbon anode is thus oxidized to carbon monoxide which either burns giving carbon dioxide or escapes out through an outlet. [2C + O2 ⟶ 2CO; 2CO + O2 ⟶ 2CO2] The carbon anode is hence consumed and renewed periodically after a certain period of usage. It is therefore preferable to use a number of graphite electrodes as anode instead of a single electrode.
Chapter Overview: Metallurgy
Metallurgy is the process of extracting metals from their ores and refining them for use. Metals occur in nature as free elements (gold, silver) or combined in ores (oxides, sulphides, carbonates). The extraction process depends on the metal's reactivity: highly reactive metals (Na, Al) are extracted by electrolysis, moderately reactive metals (Fe, Zn) by reduction with carbon, and least reactive metals (Cu, Hg) by simple heating. The general steps include mining, crushing and concentration of ore, extraction (reduction), and refining. Students must understand the extraction of aluminium (electrolysis of alumina in cryolite), iron (blast furnace), and zinc (reduction of ZnO with carbon). The reactivity series determines the method of extraction: metals above carbon require electrolysis; metals below carbon can be reduced by carbon. The chapter also covers alloys (mixtures of metals or metals with non-metals), their composition and uses, and the concept of corrosion and its prevention.
Key Concepts & Reactions
| Term / Process | Details |
|---|---|
| Ore | Mineral from which a metal can be profitably extracted |
| Gangue | Earthy impurities present in the ore |
| Calcination | Heating ore strongly in limited air to remove moisture and CO2 |
| Roasting | Heating ore in excess air to convert sulphides to oxides |
| Smelting | Reduction of metal oxide using carbon or CO in a furnace |
| Flux | Substance added to remove gangue by forming fusible slag |
| Slag | Fusible product of flux + gangue (e.g., CaSiO3) |
| Alloy | Homogeneous mixture of two or more metals (or metal + non-metal) |
Must-Know Concepts
- Blast furnace reactions for iron extraction: Fe2O3 + 3CO → 2Fe + 3CO2
- Coke burns: C + O2 → CO2; then CO2 + C → 2CO (reducing agent)
- Limestone acts as flux: CaCO3 → CaO + CO2; then CaO + SiO2 → CaSiO3 (slag)
- Aluminium extraction: Al2O3 dissolved in cryolite (Na3AlF6), electrolysed at ~950°C
- Common alloys: Brass (Cu + Zn), Bronze (Cu + Sn), Steel (Fe + C), Stainless Steel (Fe + Cr + Ni + C)
- Corrosion prevention: painting, galvanising, electroplating, alloying
Calcination vs Roasting
| Feature | Calcination | Roasting |
|---|---|---|
| Air Supply | Limited or no air | Excess air |
| Ore Type | Carbonates and hydrated ores | Sulphide ores |
| Gas Evolved | CO2 and water vapour | SO2 |
| Example | ZnCO3 → ZnO + CO2 | 2ZnS + 3O2 → 2ZnO + 2SO2 |
Important Diagrams to Practice
- Labelled diagram of the blast furnace with temperature zones and reactions
- Electrolytic cell for aluminium extraction (Hall-Heroult process)
- Flowchart of general metallurgical steps: ore → concentration → extraction → refining
Common Mistakes
- Confusing calcination (limited air, carbonates) with roasting (excess air, sulphides)
- Saying "coke reduces iron oxide" directly (CO is the actual reducing agent, not C)
- Writing wrong alloy compositions (brass is Cu+Zn, NOT Cu+Sn which is bronze)
- Forgetting the role of cryolite in aluminium extraction (lowers melting point)
Scoring Tips
- Draw and label the blast furnace diagram neatly - this is a very common question
- Write balanced equations for each zone of the blast furnace
- Memorise alloy compositions and uses as a table for quick revision
- Link extraction method to position in the reactivity series
Frequently Asked Questions
Why can't aluminium be extracted by carbon reduction?
Aluminium is above carbon in the reactivity series, so carbon cannot reduce Al2O3. Electrolysis of molten alumina (with cryolite) is required to extract aluminium.
What is the role of limestone in the blast furnace?
Limestone (CaCO3) decomposes to CaO, which acts as a flux. It combines with silica (SiO2, the gangue) to form calcium silicate slag (CaSiO3), which floats on molten iron and is removed.
Why are alloys preferred over pure metals?
Alloys are harder, stronger, more resistant to corrosion, and have better properties than pure metals. For example, stainless steel resists rusting unlike pure iron, and brass is harder than pure copper.