Chemistry · Chemistry of the elements · Grade 7

Mineral resources

From which ore, and how, are Muruntau gold and Almalyk copper extracted?

The situation

Every tonne of ore mined at Almalyk in the Tashkent region contains only 3–4 kilograms of copper, and the Muruntau gold pit in the Navoi region is more than 650 metres deep. Phosphorite is mined in the Kyzylkum, potassium salt and gas in Kashkadarya, and coal in Angren. What minerals do mineral resources consist of, how do we identify them, how is metal separated from ore, and what mark does this leave on nature?

Topic

Mineral resources and the deposits of Uzbekistan. Map of the mineral wealth of Uzbekistan: Muruntau gold, Almalyk copper, Kyzylkum phosphorite, Tyubegatan potassium salt, Shurtan gas, Angren coal

Key ideas

Minerals and ores
Deposits of Uzbekistan
Enrichment and the ecological footprint

Formulas

ω (Cu) = \frac{A_{r} (Cu)}{M_{r} (CuFeS_{2})} = \frac{63.5}{183.5} \approx 34.6 %

The mass fraction of an element in a mineral — copper in chalcopyrite.

CaCO_{3} + 2 HCl \overset{}{\to} CaCl_{2} + CO_{2} ↑ + H_{2} O

Calcite “boils” in acid — this is how carbonate minerals are identified.

m (ore) = \frac{m (metal)}{ω \cdot η}

$ω$ is the fraction of metal in the ore, $η$ is the recovery: from an ore with 0.4 % copper, ≈ 0.3 t of ore is needed per 1 kg of copper.

R = 1 - e^{- k t}

Flotation: the fraction recovered in time $t$ ; $k$ depends on the particle size, the collector and the air.

2 CuFeS_{2} + 4 O_{2} \overset{}{\to} Cu_{2} S + 2 FeO + 3 SO_{2}

Roasting the concentrate releases SO₂ — it is captured and made into sulfuric acid (otherwise it causes acid rain).

Theory

Mineral, rock and ore

A mineral is a chemical compound or element formed in nature, with a definite composition and crystal structure: calcite $CaCO_{3}$ , pyrite $FeS_{2}$ , native gold Au. A rock is a mixture of minerals (granite: quartz, feldspar, mica). An ore is a rock from which it is worthwhile to extract a metal or a valuable substance. The geological classes of compounds are: native elements (gold, silver, sulfur), sulfides (pyrite, chalcopyrite), oxides (magnetite $Fe_{3} O_{4}$ ), carbonates (calcite), halides (halite NaCl, sylvite KCl) and phosphates (apatite $Ca_{5} (PO_{4})_{3} F$ ). Mineral resources are metallic (metals), non-metallic (phosphorite, potassium salt, table salt, limestone, marble) and combustible (coal, oil, gas).

How to identify minerals

Colour is often deceptive: pyrite shines like gold (“fool's gold”). Reliable properties are: streak — the mark left on a porcelain plate (pyrite — greenish black, gold — yellow); hardness — the Mohs scale from 1 (talc) to 10 (diamond), pyrite 6–6.5, gold 2.5–3; density — gold 19.3 g/cm³, pyrite 5.0; magnetism — magnetite is attracted to a magnet; acid — carbonates “boil” in HCl, releasing CO₂; and flame colour: sodium — yellow, potassium — violet, calcium — brick red, copper — green. Halite and sylvite look very much alike — the flame tells them apart at once.

Deposits of Uzbekistan

Muruntau (Navoi region) — gold: discovered in 1958, mined since 1967, the open pit is ≈ 5.5 × 3.4 km and more than 650 m deep; Navoi Mining and Metallurgical Combine. Almalyk (Tashkent region) — copper, gold, silver, molybdenum; the ore of the Kalmakyr and Yashlik deposits contains ≈ 0.4 % copper. Angren — brown coal. Kyzylkum Phosphorite Complex (Konimex district, Jeroy-Sardara deposit) — phosphorite, ≈ 16–19 % P₂O₅. Dehkanabad (Kashkadarya) — sylvinite, potassium fertiliser. Shurtan, Mubarek, Gazli, Surgil (Ustyurt) — natural gas; the Shurtan and Ustyurt gas chemical complexes make polyethylene from it, and at Mubarek sulfur is obtained from the gas. Uchkuduk — uranium. Marble — at the Gazgan deposits in the Nurota district of the Navoi region. (The locations on the map are approximate.)

Ore dressing: crushing and flotation

More than 99 % of copper ore is useless rock (mainly quartz and silicates). So the ore is dressed: first it is ground to ≈ 0.1 mm in a crusher and a ball mill, and then flotation is carried out. A collector (xanthates) is added to the mixture of water and ore — it makes the surface of the sulfide minerals water-repellent (hydrophobic); air bubbles stick to them and carry them up in the froth. Quartz is hydrophilic — it stays at the bottom (tailings). A particle that is too large cannot stay on a bubble, one that is too small attaches slowly, and waste rock is carried into the froth together with the water. The concentrate is then roasted, smelted and purified by electrolysis.

The ecological footprint and how to reduce it (practical)

Hundreds of tonnes of rock are dug up for one tonne of metal: the open pit, waste-rock heaps, tailings (slurry) ponds holding residues of reagents and metals, dust, a large consumption of water, and SO₂ from roasting. Ways to reduce the footprint include recycling (remelting aluminium saves ≈ 95 % of the energy used to produce it from ore — International Aluminium Institute: ≈ 8.3 MJ/kg instead of 186 MJ/kg), extracting additional metals from waste, converting SO₂ into sulfuric acid, recirculating water, and reclamation of pits and heaps (spreading soil and planting greenery). In the practical, choose a product, calculate how much ore and waste rock is mined, increase the share of recycling and compare the footprint.

Safety and the model

Safety: wear goggles when working with hydrochloric acid and do not spill drops; do not taste minerals (halite included); put out the spirit burner with its cap. Model: the hardness and density of the minerals are reference values; the locations of the deposits are approximate; flotation follows first-order kinetics (the rate constants are schematic), the ore is taken to be 0.5 % Cu, the time is sped up 30 times, and the chalcopyrite particles in the vessel are drawn in larger numbers so that they can be seen; in the ecological footprint the ore composition, the recovery and the proportion of waste rock are typical (model) values, and transport and processing stages are not included.

Did you know?

The Muruntau gold deposit was discovered in 1958 and has been mined since 1967; the pit is ≈ 5.5 km long, ≈ 3.4 km wide and more than 650 metres deep.
According to the International Aluminium Institute, remelting aluminium saves about 95 % of the energy needed to produce it from bauxite; on average 4–5 t of bauxite give 2 t of alumina, and that gives 1 t of aluminium.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Sulfuric acid and the contact process — Grade 8
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