Lesson Notes By Weeks and Term v5 - Grade 11

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Performance objectives

• Define the lithosphere and its components.
• Explain how mineral deposits form in South Africa.
• Describe the chemical reactions for extracting gold and using coal.
• Discuss the environmental impacts of mining.
• Identify sustainable energy alternatives.

Lesson summary

The lithosphere, the Earth's solid outer layer, is a treasure trove of mineral and energy resources crucial for South Africa's economy and the daily lives of its citizens. From the gold that built Johannesburg to the coal that powers much of our electricity, understanding the chemical systems within the lithosphere is essential. Mining and energy extraction processes profoundly impact our environment, economy, and society. This week, we will focus on the composition of the lithosphere and how we extract and utilize these vital resources, paying particular attention to the chemical reactions involved and their environmental consequences.

Lesson notes

2.1 The Lithosphere: Composition and Structure The lithosphere is the rigid outer layer of the Earth, composed of the crust and the uppermost part of the mantle. It's broken into tectonic plates that move and interact, shaping the Earth's surface and influencing the formation of mineral deposits.

Rocks: Rocks are aggregates of one or more minerals.

The three main types are: Igneous Rocks: Formed from the cooling and solidification of molten rock (magma or lava). Examples include granite (intrusive) and basalt (extrusive).

Sedimentary Rocks: Formed from the accumulation and cementation of sediments (weathered rock fragments, mineral grains, or organic matter). Examples include sandstone, shale, and limestone.

Metamorphic Rocks: Formed when existing rocks are transformed by heat, pressure, or chemically active fluids. Examples include marble (from limestone) and quartzite (from sandstone).

Minerals: Minerals are naturally occurring, inorganic solids with a defined chemical composition and crystal structure.

Common minerals in South Africa include: Quartz (SiO 2 ): Found in many rock types. Feldspar (e.g., KAlSi 3 O 8 ): A major component of igneous and metamorphic rocks.

Calcite (CaCO 3 ): The main mineral in limestone and marble.

Gold (Au): A precious metal found in alluvial deposits and within certain rock formations like the Witwatersrand Basin.

Platinum Group Metals (PGMs): Found in the Bushveld Igneous Complex. 2.2 Formation of Mineral Deposits Mineral deposits are concentrations of minerals that are economically viable to extract. South Africa is particularly rich in certain types of deposits due to its geological history.

Magmatic Deposits: Formed from the cooling and crystallization of magma. The Bushveld Igneous Complex, one of the largest layered igneous intrusions in the world, is a prime example. As the magma cooled slowly, different minerals crystallized and settled out at different levels, creating layers rich in platinum, chromium, and vanadium.

Hydrothermal Deposits: Formed from hot, aqueous solutions that circulate through rocks, dissolving and transporting minerals. As the solutions cool or react with surrounding rocks, the dissolved minerals precipitate out, forming veins or disseminated deposits. The Witwatersrand Basin gold deposits are thought to have formed, at least in part, from hydrothermal activity.

Sedimentary Deposits: Formed from the accumulation of sediments. Coal deposits are formed from the accumulation and compression of plant matter over millions of years. Banded iron formations are another example, formed from the precipitation of iron oxides in ancient oceans.

Placer Deposits: Formed by the concentration of heavy minerals by flowing water. Gold nuggets are often found in placer deposits in rivers and streams. Diamonds are also found in alluvial and marine placer deposits along the West Coast. 2.3 Extraction and Processing of Mineral Resources Gold Extraction (Cyanidation): Gold is often found in low concentrations in ore. The cyanidation process is used to dissolve gold from the ore: 4Au(s) + 8CN - (aq) + O 2 (g) + 2H 2 O(l) → 4[Au(CN) 2 ] - (aq) + 4OH - (aq) This reaction shows that solid gold (Au) reacts with cyanide ions (CN - ) in the presence of oxygen and water to form the soluble gold cyanide complex, [Au(CN) 2 ] - . Zinc dust is then added to precipitate the gold: [Au(CN) 2 ] - (aq) + Zn(s) → [Zn(CN) 4 ] 2- (aq) + Au(s) Worked

Example: Suppose you have 1000 kg of gold ore that contains 0.001% gold by mass. How much gold, in grams, can theoretically be extracted using cyanidation?

Calculate the mass of gold in the ore: Mass of gold = (0.001/100) * 1000 kg = 0.01 kg Convert kg to grams: Mass of gold = 0.01 kg * 1000 g/kg = 10 g Therefore, theoretically, 10 grams of gold can be extracted from 1000 kg of this ore. This calculation doesn't account for process inefficiencies, so the actual yield will likely be lower.

Coal Combustion: Coal is burned to generate electricity in many South African power plants. The main reaction is the combustion of carbon: C(s) + O 2 (g) → CO 2 (g) + Heat This reaction releases energy in the form of heat, which is used to boil water and generate steam to turn turbines and produce electricity.

However, coal also contains sulfur, which reacts with oxygen to form sulfur dioxide: S(s) + O 2 (g) → SO 2 (g) SO 2 is a major air pollutant that contributes to acid rain. Worked

Example: A power plant burns 1000 kg of coal. The coal contains 1% sulfur by mass. How much SO 2 is produced, assuming complete combustion of the sulfur?

Calculate the mass of sulfur in the coal: Mass of sulfur = (1/100) * 1000 kg = 10 kg Convert kg to grams: Mass of sulfur = 10 kg * 1000 g/kg = 10000 g Calculate moles of sulfur: Moles of S = 10000 g / 32 g/mol = 312.5 mol From the reaction S(s) + O 2 (g) → SO 2 (g), 1 mole of S produces 1 mole of SO 2 .

Therefore, moles of SO 2 = 312.5 mol.