Heavy Metals: Properties, Extraction, Uses & Environmental Impact

Why Study Heavy Metals?

Heavy metals are elements with high atomic weights and densities. They are essential for industries (e.g., copper for electricity, iron for steel) but can be toxic to humans and the environment if not handled properly. This unit covers:

• Their natural sources (ores) and extraction methods.
• Chemical properties (reactions, oxidation states, complexes).
• Uses in daily life (alloys, medicines, electronics).
• Environmental and health risks (pollution, bioaccumulation).

1. Copper (Cu)

Key Facts

• Atomic number: 29
• Electron configuration: [Ar] 3d¹⁰ 4s¹
• Common oxidation states: +1, +2 (most stable)
• Color: Reddish-brown (metallic luster)
• Conductivity: Best conductor of heat and electricity among metals.

Occurrence & Extraction

Copper is found in nature as sulfides (e.g., chalcopyrite, CuFeS₂) and oxides (e.g., cuprite, Cu₂O). Extraction involves:

1. Crushing and concentration (froth flotation).
2. Roasting (converts sulfide to oxide): $$2CuFe{S}_2+O_2\to C{u}_{2}S+2FeS+S{O}_2$$
3. Smelting (in a blast furnace): $$C{u}_{2}S+O_2\to 2Cu+S{O}_2$$
4. Electrolytic refining (purification).

flowchart TD
    A["Crushed Ore"] --> B["Froth Flotation"]
    B --> C["Roasting (Cu₂S + O₂)"]
    C --> D["Smelting (Blast Furnace)"]
    D --> E["Electrolytic Refining"]
    E --> F["Pure Copper"]

Chemical Properties

• Reactivity: Less reactive than iron but reacts with acids in presence of oxygen: $$2Cu+O_2+2H_{2}S{O}_4\to 2CuS{O}_4+2H_{2}O$$
• Complex formation: Forms blue complexes with ammonia (e.g., [Cu(NH₃)₄]²⁺).
• Displacement reactions: Can displace less reactive metals (e.g., Ag⁺, Hg²⁺).

Uses

Environmental Impact

• Mining waste pollutes water (e.g., acid mine drainage from Cu²⁺).
• Bioaccumulation: Toxic to aquatic life (e.g., kills algae, fish).
• Solution: Recycling copper reduces environmental harm.

2. Zinc (Zn)

Key Facts

• Atomic number: 30
• Electron configuration: [Ar] 3d¹⁰ 4s²
• Common oxidation state: +2
• Color: Bluish-white (tarnishes to gray).
• Reactivity: More reactive than copper but less than iron.

Occurrence & Extraction

Zinc is found as sulfides (sphalerite, ZnS) and carbonates (smithsonite, ZnCO₃). Extraction:

1. Roasting: $$2ZnS+3O_2\to 2ZnO+2S{O}_2$$
2. Reduction with carbon (in a furnace): $$ZnO+C\to Zn+CO$$
3. Electrolytic refining (for high purity).

flowchart TD
    A["Zinc Ore (ZnS)"] --> B["Roasting (ZnO + SO₂)"]
    B --> C["Reduction (ZnO + C)"]
    C --> D["Zinc Vapor"]
    D --> E["Condensation"]
    E --> F["Zinc Metal"]

Chemical Properties

• Reactivity: Reacts with acids to give hydrogen: $$Zn+2HCl\to ZnC{l}_2+H_2$$
• Amphoteric oxide: Reacts with both acids and bases: $$ZnO+2HCl\to ZnC{l}_2+H_{2}O$$ $$ZnO+2NaOH\to N{a}_{2}Zn{O}_2+H_{2}O$$
• Complex formation: Forms [Zn(NH₃)₄]²⁺ (colorless).

Uses

Environmental Impact

• Mining: Releases toxic SO₂ during roasting.
• Recycling: Zinc is 100% recyclable (reduces mining waste).
• Health: Excess zinc causes nausea; deficiency leads to poor immunity.

3. Mercury (Hg)

Key Facts

• Atomic number: 80
• Electron configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s²
• Common oxidation state: +2 (Hg²⁺), +1 (Hg₂²⁺, dimeric).
• State: Liquid at room temperature (only metal).
• Toxicity: Highly poisonous (neurotoxin).

Occurrence & Extraction

Found as cinnabar (HgS). Extraction:

1. Roasting: $$HgS+O_2\to Hg+S{O}_2$$
2. Condensation: Mercury vapor condenses to liquid.

Chemical Properties

• Low reactivity: Does not react with oxygen or water.
• Amalgam formation: Alloys with other metals (e.g., dental amalgam: Hg + Ag + Sn).
• Complexes: Forms [HgCl₄]²⁻ (linear geometry).

Uses

Environmental & Health Risks

• Toxicity: Causes "mad hatter disease" (neurological damage).
• Bioaccumulation: Methylmercury (CH₃Hg⁺) accumulates in fish (e.g., tuna).
• Ban: Many countries restrict mercury use (e.g., thermometers replaced by digital).

4. Silver (Ag)

Key Facts

• Atomic number: 47
• Electron configuration: [Kr] 4d¹⁰ 5s¹
• Common oxidation state: +1 (Ag⁺).
• Color: Silvery-white, highly reflective.
• Conductivity: Second-best conductor after copper.

Occurrence & Extraction

Found as native silver (Ag) and sulfides (argentite, Ag₂S). Extraction:

1. Cyanide process (for ores): $$4Ag+8NaCN+O_2+2H_{2}O\to 4Na[Ag(CN{)}_2]+4NaOH$$
2. Displacement with zinc: $$2Na[Ag(CN{)}_2]+Zn\to N{a}_2[Zn(CN{)}_4]+2Ag$$

Chemical Properties

• Tarnishing: Forms Ag₂S (black layer) with H₂S.
• Photography: AgBr decomposes to Ag (black) on light exposure.
• Displacement: Can be displaced by copper: $$2AgN{O}_3+Cu\to Cu(N{O}_3{)}_2+2Ag$$

Uses

Environmental Impact

• Mining: Releases cyanide (toxic to aquatic life).
• Recycling: Silver is highly recyclable (e.g., from old cameras).

5. Iron (Fe)

Key Facts

• Atomic number: 26
• Electron configuration: [Ar] 3d⁶ 4s²
• Common oxidation states: +2 (Fe²⁺), +3 (Fe³⁺).
• Color: Grayish-white (rusts to reddish-brown).
• Abundance: Most used metal (steel production).

Occurrence & Extraction

Found as oxides (hematite, Fe₂O₃; magnetite, Fe₃O₄) and carbonates (siderite, FeCO₃). Extraction (blast furnace):

1. Coke (C) reduces Fe₂O₃: $$F{e}_2{O}_3+3CO\to 2Fe+3C{O}_2$$
2. Limestone (CaCO₃) removes impurities (forms slag, CaSiO₃).

flowchart TD
    A["Iron Ore (Fe₂O₃)"] --> B["Coke (C) + Limestone"]
    B --> C["Blast Furnace"]
    C --> D["Molten Iron (Pig Iron)"]
    D --> E["Steel Production"]

Chemical Properties

• Reactivity: Reacts with acids and oxygen: $$Fe+2HCl\to FeC{l}_2+H_2$$ $$4Fe+3O_2\to 2F{e}_2{O}_3(rust)$$
• Complex formation: Forms [Fe(CN)₆]⁴⁻ (Prussian blue).
• Oxidation states: Fe²⁺ (green), Fe³⁺ (brown).

Uses

Environmental Impact

• Mining: Deforestation, soil erosion.
• Rust: Corrodes infrastructure (costly repairs).
• Solution: Galvanization (Zn coating), paints.

Comparison Table: Heavy Metals

Exam Tip: How to Score Full Marks

1. Diagrams: Draw labeled flowcharts for extraction (e.g., copper smelting, blast furnace).
2. Equations: Balance all chemical equations (e.g., roasting, displacement reactions).
3. Applications: Link uses to properties (e.g., "Copper is used in wiring because it conducts electricity well").
4. Environment: Mention one environmental impact and one solution for each metal.
5. Comparison: Use tables to compare ores, extraction, or uses (e.g., Zn vs. Fe reactivity).
6. NEB-style questions:
   • Short answer: "Why is zinc used for galvanizing iron?" → Answer: Sacrificial protection (Zn reacts first).
   • Long answer: Describe the cyanide process for silver extraction in 5 steps with equations.
   • Numerical: Calculate % purity of copper from mass data (use stoichiometry).

Practice Questions (NEB Style)

1. Short Answer (3 marks) Describe the role of limestone in the blast furnace extraction of iron.

2. Long Answer (7 marks) Explain the cyanide process for extracting silver from argentite ore. Write balanced equations for each step.

3. Numerical (5 marks) A sample of zinc ore contains 65% ZnS by mass. If 100 kg of ore is processed, calculate: a) Mass of ZnS in the ore. b) Mass of ZnO produced after roasting (assume 100% conversion). c) Mass of Zn metal obtained if the reduction is 85% efficient.

4. Comparison (4 marks) Compare the extraction methods of copper and zinc. Why is electrolytic refining used for both?

Answers to Practice Questions

1. Limestone (CaCO₃) decomposes to CaO, which reacts with silica (impurity) to form slag (CaSiO₃), removing impurities from molten iron.

2. Cyanide Process Steps:

   • Ore crushed and treated with NaCN + O₂ + H₂O → Na[Ag(CN)₂].
   • Zn added to displace Ag: 2Na[Ag(CN)₂] + Zn → Na₂[Zn(CN)₄] + 2Ag.
   • Ag is filtered and melted.

3. Calculations: a) Mass of ZnS = 65 kg (65% of 100 kg). b) ZnS + O₂ → ZnO + SO₂ → 65 kg ZnS × (81.4/97.4) = 54.5 kg ZnO. c) ZnO + C → Zn → 54.5 kg × 0.85 = 46.3 kg Zn.

4. Comparison:

   • Copper: Roasted to Cu₂O, then smelted to Cu₂S → Cu (self-reduction).
   • Zinc: Roasted to ZnO, then reduced with carbon (not self-reducing).
   • Electrolytic refining is used for both to remove impurities (e.g., Cu²⁺ → Cu at cathode; Zn²⁺ → Zn).