Concentration of Ore
Ores extracted from the earth's crust are rarely pure. They contain the desired mineral mixed with unwanted rocky material called gangue (pronounced 'gang'). Before any chemical extraction can take place, the ore must be concentrated — that is, the proportion of the valuable mineral must be increased by removing as much gangue as possible. This preliminary stage is called concentration of ore (also known as ore dressing or beneficiation) and is a purely physical process that does not change the chemical identity of the mineral. Three major methods are used depending on the physical and chemical properties of the ore and its gangue: gravity separation (exploiting density differences), froth flotation (exploiting differences in surface wettability, especially for sulphide ores), and leaching (selective chemical dissolution, used for aluminium, silver, and gold ores). Choosing the correct method is critical to the economics of metal production — using the wrong technique can result in loss of valuable metal or excessive energy costs. Understanding concentration is foundational to the broader study of metallurgy. The concentrated ore, called the concentrate, is then subjected to further steps such as roasting, smelting, or electrolytic refining. Students should note that concentration only enriches the ore; it does not produce the free metal. The purity of the concentrate directly affects the efficiency and cost of all downstream processes.
- Gravity Separation (Hydraulic Washing)
- Froth Flotation (for Sulphide Ores)
- Leaching (Chemical Concentration)
- Comparison and Selection Criteria
Gravity Separation (Hydraulic Washing)
Gravity separation relies on the difference in specific gravity (density) between the ore mineral and gangue. Crushed ore is placed on a vibrating, inclined table called a wilfley table, or carried in a stream of water over a sloping surface. The denser ore particles (e.g., cassiterite SnO₂, density ~7 g cm⁻³) settle and are retained, while the lighter silica gangue (density ~2.6 g cm⁻³) is washed away. This method is especially effective for native metals and dense oxides or carbonates. It is energy-efficient but not selective enough for finely disseminated sulphide ores.
Froth Flotation (for Sulphide Ores)
Froth flotation is used when the ore mineral and gangue have very similar densities but different surface chemistries. The crushed ore is mixed with water and several chemical reagents in a large tank: (1) Collectors (e.g., sodium ethyl xanthate, C₂H₅OCS₂Na) adsorb onto the sulphide mineral surface, making it hydrophobic (water-repelling). (2) Frothers (e.g., pine oil or MIBC — methyl isobutyl carbinol) stabilise the air bubbles without making the froth too stiff. (3) Activators and depressants selectively enhance or suppress flotation of specific minerals. Air is blown in through a rotating impeller. Hydrophobic sulphide particles (e.g., PbS — galena; ZnS — sphalerite; Cu₂S — chalcocite) attach to rising air bubbles and float to the surface as a stable froth, which is skimmed off. Hydrophilic gangue particles remain in the pulp at the bottom. The process can achieve concentrations of >25% Cu from ores containing as little as 0.5% Cu.
Leaching (Chemical Concentration)
Leaching uses a selective chemical solvent to dissolve the desired mineral while leaving gangue undissolved. Three important examples are: (a) Bauxite (Al₂O₃·2H₂O) — Bayer Process: crushed bauxite is digested in hot concentrated NaOH solution (170–200 °C, 15 atm). Al₂O₃ dissolves as sodium aluminate, NaAl(OH)₄, while iron oxide impurities (red mud) do not dissolve and are filtered off. The filtrate is cooled and seeded with Al(OH)₃ crystals to reprecipitate pure Al(OH)₃, which is then calcined to give Al₂O₃. (b) Silver ores: Treated with dilute NaCN solution in the presence of air; Ag dissolves as [Ag(CN)₂]⁻ (silver cyanide complex). Zinc dust is then added to displace metallic silver. (c) Gold ores (MacArthur–Forrest / cyanide process): Gold dissolves in dilute NaCN solution with oxygen, forming [Au(CN)₂]⁻. Gold is recovered by cementation on zinc or by electrowinning. These processes are highly selective but raise significant environmental concerns due to toxicity of cyanide leachates.
Comparison and Selection Criteria
The choice of concentration method depends on: (i) density difference between mineral and gangue — gravity separation requires Δρ > ~1.5 g cm⁻³; (ii) surface properties — froth flotation requires the mineral surface to be chemically responsive to collectors; (iii) ore grade and particle size — very low-grade, finely disseminated ores favour hydrometallurgical leaching; (iv) economic and environmental constraints. In practice, two or more methods may be combined. For example, a copper sulphide ore may first undergo gravity separation to remove coarse gangue, then froth flotation to concentrate the remaining sulphide fraction.
The Key Equation
- \ce{Al2O3} — Aluminium oxide (alumina) (mol): The principal mineral component of bauxite ore, containing Al³⁺ in an ionic lattice.
- \ce{NaOH} — Sodium hydroxide (caustic soda) (mol): The selective leaching agent; hot concentrated solution (170–200 °C) dissolves Al₂O₃ but not Fe₂O₃ impurities.
- \ce{Na[Al(OH)4]} — Sodium tetrahydroxoaluminate (mol): The soluble aluminate complex formed; on cooling and seeding it decomposes back to pure Al(OH)₃ precipitate.
The Key Equation
- \ce{Au} — Gold (mol): The target metal; dissolves oxidatively due to strong complexation by CN⁻ ligands (formation constant Kf ≈ 10²⁰).
- \ce{NaCN} — Sodium cyanide (mol): Leaching agent; CN⁻ is a strong σ-donor ligand that stabilises Au⁺ as a linear dicyanoaurate complex.
- \ce{Na[Au(CN)2]} — Sodium dicyanoaurate(I) (mol): The water-soluble gold complex from which gold is later recovered by zinc cementation: Zn + 2[Au(CN)₂]⁻ → 2Au + [Zn(CN)₄]²⁻.
- \ce{O2} — Oxygen (mol): Acts as the oxidising agent, enabling oxidation of Au(0) to Au(I) during leaching.
Worked Example
Problem. A sample of bauxite ore contains 52.0% Al₂O₃ by mass (molar mass = 102 g mol⁻¹). After the Bayer process leaching step, all the Al₂O₃ dissolves to form Na[Al(OH)₄]. Calculate the mass of NaOH (molar mass = 40 g mol⁻¹) required to completely leach the Al₂O₃ from 500 g of this bauxite ore. State any assumptions made.
Answer. 204 g of NaOH is required.
Origins · 1 minKarl Josef Bayer and how this was discovered
Karl Josef Bayer, an Austrian chemist working in Russia, developed the Bayer process in 1887–1892 while employed at a textile factory in Saint Petersburg. He discovered that aluminium hydroxide precipitated from alkaline solution was a much purer product than that obtained by earlier methods, and that the caustic soda could be recycled. His two-stage process — dissolution in hot NaOH followed by precipitation by seeding — became the foundation of the entire modern aluminium industry, which today produces over 130 million tonnes of alumina per year using his method. Bayer originally invented his process not to produce aluminium metal, but to supply pure aluminium hydroxide as a mordant for dyeing cotton fabrics — the connection to the aluminium industry came later as the Hall–Héroult electrolytic smelting process was developed independently at almost the same time.
Related Concepts
- Roasting and Calcination of Ores — After concentration, sulphide ores are roasted in air to convert them to oxides before smelting — the immediate next step following froth flotation.
- Reduction and Smelting of Metal Oxides — Concentrated and roasted ores are reduced by carbon, CO, or more reactive metals in the smelting stage to yield impure metal, which is subsequently refined.
- Electrolytic Refining of Metals — High-purity metals like copper and aluminium obtained from concentrated ores are further purified by electrolysis — understanding concentration efficiency directly impacts refining costs.
- Complex Ion Formation and Stability Constants — Cyanide leaching of gold and silver depends on the very large formation constants of [Au(CN)₂]⁻ and [Ag(CN)₂]⁻ — a direct application of ligand field and equilibrium chemistry.
- Properties and Reactions of Aluminium — The Bayer leaching process exploits aluminium's amphoteric character — understanding why Al₂O₃ dissolves in NaOH but Fe₂O₃ does not requires knowledge of periodic trends in metal oxide basicity.