UPSC Prelims 2021 Q17: Copper Smelting Pollutants and Slag — Explained
Why is there a concern about copper smelting plants?
1. They may release lethal quantities of carbon monoxide into environment.
2. The copper slag can cause the leaching of some heavy metals into environment.
3. They may release sulphur dioxide as a pollutant.
Select the correct answer using the code given below.
- 1 and 2 only
- 2 and 3 only
- 1 and 3 only
- 1, 2 and 3
Answer: (b) 2 and 3 only
Statement-wise explanation
- 1: incorrect — Copper is smelted from sulphide ores such as chalcopyrite (CuFeS₂). Air or oxygen is blown in to oxidise the sulphur and iron, so the off-gas is mostly sulphur dioxide, not carbon monoxide. Carbon monoxide comes from burning fuel without enough oxygen, as in blast furnaces or vehicle engines. It is not a recognised hazard of copper smelters, and the word 'lethal' overstates the case.
- 2: correct — Copper slag is the iron-silicate waste left after smelting. It still holds traces of copper, zinc, lead, arsenic, cadmium and chromium, and rain or groundwater can leach these out of dumped slag. In the Sterlite (Thoothukudi) case, about 3.5 lakh tonnes of slag were found dumped along the Uppar river. The Madras High Court noted arsenic above limits, and high cadmium, chromium, copper and lead, in some nearby drinking-water wells.
- 3: correct — When sulphide ore is smelted and converted, its sulphur leaves the furnace as sulphur dioxide (SO₂) gas. Modern plants capture most of it and turn it into sulphuric acid, but leaks and stack emissions still happen. SO₂ causes respiratory illness and acid rain. In 2013 the Tamil Nadu Pollution Control Board ordered Sterlite shut after residents complained of an SO₂ leak.
How to eliminate
Start with the chemistry. Copper ore is a sulphide, so smelting it must release sulphur as SO₂. Statement 3 is almost certainly true, and that rules out the option without it. Slag is a metal-bearing waste, and heavy metals leaching from dumps is a standard concern for every metallurgical waste, so Statement 2 is also safe. That leaves Statement 1. Its word 'lethal' is the giveaway. Carbon monoxide comes from incomplete burning of carbon fuels, not from oxidising a sulphide ore, and the claim of lethal quantities is extreme. Drop Statement 1 and only one option is left. Fact to remember: sulphide-ore smelting (copper, zinc, lead) means SO₂ in the air and heavy metals in the slag.
Know more
How copper is smelted and what it releases
Most copper comes from sulphide ores, mainly chalcopyrite (CuFeS₂). After the ore is concentrated, it goes through these steps:
- Matte smelting: the concentrate is melted with silica flux. Some iron and sulphur are oxidised, which leaves a copper–iron sulphide 'matte'.
- Converting: air or oxygen is blown through the molten matte. Iron sulphide oxidises into iron-silicate slag plus SO₂. Copper sulphide then oxidises into blister copper (about 99% Cu) plus more SO₂.
- Refining: blister copper is cast into anodes and refined by electrolysis into cathode copper.
| Output | Nature | Environmental concern |
|---|---|---|
| Sulphur dioxide | Gas from the oxidised sulphur | Respiratory illness, acid rain; usually captured and made into sulphuric acid |
| Copper slag | Iron-silicate solid waste | Leaching of Cu, Zn, Pb, As, Cd, Cr into soil and water if dumped |
| Phosphogypsum | Waste from the sulphuric-acid and phosphoric-acid units at integrated plants | Needs lined ponds; can contaminate groundwater |
| Fugitive dust | Particles from handling and furnaces | Heavy metals deposited on soil nearby |
Uses of slag: sand blasting, a substitute for fine aggregate in concrete, road construction and blended cement. Using it this way cuts the volume that gets dumped.
Indian context: Hindustan Copper (the public-sector firm, with mines and plants in Jharkhand, Rajasthan and Madhya Pradesh) and private smelters such as Hindalco (Dahej) and Vedanta's Sterlite (Thoothukudi) have dominated output. Sterlite was closed in May 2018 after protests over pollution. The Madras High Court upheld the closure in 2020.
Exam link: the same pattern holds for zinc and lead smelting. A sulphide ore means SO₂ in the air and heavy metals in the waste.
In the news
The Vedanta-Sterlite copper smelter at Thoothukudi stayed in the news through 2020–21. The Madras High Court upheld its closure in 2020, citing slag dumping and pollution, and in April 2021 the Supreme Court let it run its oxygen plant on its own during the COVID-19 second wave.