Chalcopyrite

The most important method for copper extraction from chalcopyrite is pyrometallurgy. Pyrometallurgy is commonly used for large scale, copper rich operations with high-grade ores.[24] This is because Cu-Fe-S ores, such as chalcopyrite, are difficult to dissolve in aqueous solutions.[25] The extraction process using this method undergoes four stages:

(1) Isolating desired elements from ore using froth flotation to create a concentration

(2) Creating a high-Cu sulfide matte by smelting the concentration

(3) Oxidizing/converting the sulfide matte, resulting in an impure molten copper.

(4) Refining by fire and electrowinning techniques to increase purity of resultant copper [25]

Chalcopyrite ore is not directly smelted. This is because the ore is primarily composed of non-economically valuable material, or waste rock, with low concentrations of copper. The abundance of waste material results in a lot of hydrocarbon fuel being required to heat and melt the ore. Alternatively, copper is isolated from the ore first using a technique called froth flotation. Essentially, reagents are used to make the copper water-repellent, thus the Cu is able to concentrate in a flotation cell by floating on air bubbles. In contrast to the 0.5-2% copper in chalcopyrite ore, froth flotation results in a concentrate containing about 30% copper. [25]

The concentrate then undergoes a process called matte smelting. Matte smelting oxidizes the sulfur and iron[26] by melting the flotation concentrate in a 1250°C furnace to create a new concentrate (matte) with about 45-75% copper. [25] This process is typically done in flash furnaces. To reduce the amount of copper in the slag material, the slag is kept molten with an addition of SiO₂ flux[26] to promote immiscibility between concentration and slag. In terms of byproducts, matte smelting copper can produce SO₂ gas which is harmful to the environment, thus it is captured in the form of sulfuric acid. Example reactions are as follows: [25]

(1) 2CuFeS_{2 (s)} +3.25O_2(g) -> Cu₂S-0.5FeS_(l) + 1.5FeO_(s) + 2.5SO_2(g)

(2) 2FeO_(s) + SiO_2(s) -> Fe₂SiO_4(l)[25]

Converting involves oxidizing the matte once more to further remove sulfur and iron, however the product is 99% molten copper.[25] Converting occurs in two stages: the slag forming stage and the copper forming stage. In the slag forming stage, iron and sulfur are reduced to concentrations of less than 1% and 0.02%, respectively. The concentrate from matte smelting is poured into a converter that is then rotated, supplying the slag with oxygen through tuyeres. The reaction is as follows:

2FeS_(l)+3O_2(g)+SiO_2(s) -> Fe₂SiO_4(l) + 2SO_2(g) + heat

In the copper forming stage, the matte produced from the slag stage undergoes charging (inputting the matte in the converter), blowing (blasting more oxygen), and skimming (retrieving impure molten copper known as blister copper).[25] The reaction is as follows:

Cu₂S_(l) + O_2(g) -> 2Cu_(l) + SO_2(g) + heat [25]

Finally, the blister copper undergoes refinement through fire and/or electrorefining. In this stage, copper is refined to a high-purity cathode.[25]

Chalcopyrite is an exception to most copper bearing minerals. In contrast to the majority of copper minerals which can be leached at atmospheric conditions, such as through heap leaching, chalcopyrite is a refractory mineral that requires elevated temperatures as well as oxidizing conditions to release its copper into solution.[27] This is because of the extracting challenges which arise from the 1:1 presence of iron to copper,[28] resulting in slow leaching kinetics.[27] Elevated temperatures and pressures create an abundance of oxygen in solution, which facilitates faster reaction speeds in terms of breaking down chalcopyrite's crystal lattice.[27] A hydrometallurgical process which elevates temperature with oxidizing conditions required for chalcopyrite is known as pressure oxidation leaching. A typical reaction series of chalcopyrite under oxidizing, high temperature conditions is as follows:

i) 2CuFeS₂ + 4Fe₂(SO₄)₃ -> 2Cu²⁺+ 2SO₄^2- + 10FeSO₄+4S

ii) 4FeSO₄ + O₂ + 2H₂SO₄ -> 2Fe₂(SO₄)₃ +2H₂O

iii) 2S + 3O₂ +2H₂O -> 2H₂SO₄

(overall) 4CuFeS₂+ 17O₂ + 4H₂O -> 4Cu²⁺+ 2Fe₂O₃ + 4H₂SO₄[27]

Pressure oxidation leaching is particularly useful for low grade chalcopyrite. This is because it can "process concentrate product from flotation"[27] rather than having to process whole ore. Additionally, it can be used as an alternative method to pyrometallurgy for variable ore.[27] Other advantages hydrometallurgical processes have in regards to copper extraction over pyrometallurgical processes (smelting) include:

• The highly variable cost of smelting
• Depending on the location, the amount of smelting availability is limited
• High cost of installing smelting infrastructure
• Ability to treat high-impurity concentrates
• Increase of recovery due to ability of treating lower-grade deposits on site
• Lower transport costs (shipping concentrate not necessary)
• Overall lower cost of copper production [27]

Although hydrometallurgy has its advantages, it continues to face challenges in the commercial setting.[28][27] In turn, smelting continues to remain the most commercially viable method of copper extraction.[28]