(a) Explain the differences between ore, ore minerals and gangue, and discuss their significance in mineral processing. (20…

(a) Ore, ore minerals and gangue, and their significance in mineral processing (20 marks)

Differences.

• Ore is a natural aggregate of minerals from which one or more metals can be extracted at a profit. It is an economic term, so the same rock may be ore or not depending on price, grade and technology.
• Ore minerals are the metal-bearing minerals in the ore that are of value, such as chalcopyrite (Cu), galena (Pb), sphalerite (Zn), hematite and magnetite (Fe), and chromite (Cr). This is a mineralogical term.
• Gangue is the valueless mineral matter, such as quartz, calcite and feldspar, that is mixed with the ore minerals and is discarded. The terms are relative: baryte or fluorite may be gangue in one deposit and ore in another.

Significance in mineral processing.

• Nature of the ore mineral and gangue decides the method. Liberation, by comminution (crushing and grinding), frees the ore minerals from the gangue, and the grain size and intergrowth decide how fine the grinding must be.
• Separation exploits differences in properties: gravity (density, as in jigs and tables for cassiterite and gold), magnetic (magnetite), electrostatic, and froth flotation (sulphides, with reagents).
• The result is a concentrate, rich in ore minerals, and tailings, the gangue. The concentration ratio and recovery decide the economics, since fine grinding and separation cost money.
• Smelting and refining follow, and gangue forms slag, so a high gangue content raises the cost of transport and smelting.
• Tailings must be managed for the environment, and may hold valuable by-products.

(b) Tenor of an ore: economic viability and determination (15 marks)

Definition. Tenor is the average grade of the ore, that is, the percentage (or g/t for precious metals) of the metal in the ore.

Influence on economic viability.

• Revenue per tonne rises with tenor, and a higher grade lowers the cost per unit of metal, because less rock is mined, milled and smelted.
• The cut-off grade, the minimum tenor at which a block pays for its mining and processing, separates ore from waste. A rise in the metal price lowers the cut-off and adds to reserves.
• Tenor works with tonnage and recovery: a low-grade deposit can pay if it is very large and mined cheaply (open-pit porphyry copper, at 0.4–1% Cu), whereas a small deposit needs high grade.
• By-products such as gold and silver improve viability, while deleterious elements (arsenic, sulphur, phosphorus) attract penalties.

Methods of determination.

• Sampling: channel, chip, grab and drill-core samples, with proper spacing, followed by splitting and reduction.
• Assaying: chemical, fire assay, atomic absorption, ICP and XRF, on the prepared samples.
• Estimation: tenor is the weighted average of the assays, weighted by sample length and width, using polygons, sections or geostatistics (kriging).

(c) Reserve calculation (15 marks)

Volume V = 2 million m³ = 2 × 10⁶ m³, ore density = 2.5 t/m³, and grade = 3% Cu.

(i) Proven reserves of ore.

Tonnage = volume × density = 2 × 10⁶ × 2.5 = 5 × 10⁶ tonnes, that is, 5 million tonnes of ore.

(ii) Copper in the reserves.

Contained copper = tonnage × grade = 5 × 10⁶ × 3/100 = 1.5 × 10⁵ tonnes, that is, 150,000 tonnes of copper.

The proven reserve is 5 million tonnes of ore and it contains 150,000 tonnes of copper metal, before allowing for mining and processing losses; the recoverable copper will be lower than this by the mining and metallurgical recovery.