Copper Smelting Chemistry Tutorial

Key Concepts

• Rocks are mixtures of minerals.
• A mineral can be made up of uncombined elements or compounds.
• Although copper can be found in nature as the uncombined element, it is often found in nature as compounds.

  Common Copper Minerals
  Classification	Mineral name	formula	appearance	% copper
  element	native copper	Cu	reddish, metallic	100%
  oxide	cuprite	Cu2O	red, earthy	89%
  	tenorite	CuO	grey black	80%
  sulfide	chalcocite	Cu2S	dark grey, metallic	80%
  	covellite	CuS	indigo blue	66%
  	bornite	Cu5FeS4	golden brown, earthy	63%
  	chalcopyrite	CuFeS2	golden yellow, metallic	35%
  carbonate	malachite	Cu2CO3(OH)2	bright green, earthy	58%
  	azurite	Cu3(CO3)2(OH)2	blue, glassy	55%

• A copper ore is a mixture of copper minerals and other materials in the rock called gangue, from which it is economically viable to extract the metal.

  A typical copper ore contains between 0.5% and 2.0% copper.

• A high grade ore produces a high yield of metal.

  A low grade order produces a low yield of metal.

• Copper can be extracted from sulfide ores (usually higher grade ores) by direct smelting.

  The earliest evidence of copper smelting occurs in Serbian artefacts dating from around 5000 BC.
  Copper can be extracted from oxide ores using electrolysis (electrowinning) for low grade ores, or by the carbon reduction method of smelting for higher grade ores.

• The process of extracting copper from higher grade sulfide ores involves:

Example: Extraction of Copper From Chalcopyrite (CuFeS₂)

1. Mining : digging or blasting to obtain the copper ore from the surrounding rock
2. Crushing : mined copper ore is crushed into very small pieces
3. Grinding : crushed copper ore is ground into a powder
4. Concentrating by flotation (also known as froth flotation or as ore flotation).

   Mining produce ores containing 2% or less of copper.

   Concentrating the ore by froth flotation can result in ores with up to 35% copper.

   • A collector oil, such as an organic xanthate or thiophosphate, is added to the powdered ore and adheres strongly to the chalcopyrite particles making them water repellant.
     This collector oil does not adsorb so strongly on other minerals such as silicate minerals so these particles will not repel water.
   • This mixture is fed into a water bath containing a foaming agent such as crude cresol or pine oil (soap is not a suitable foaming agent in this case).
   • Jets of air are forced through the water bath.
   • Water repellant chalcopyrite particles stick to the foam bubbles and float to the surface making a froth.
     Gangue (waste) falls to the bottom and is removed.
   • The froth which contains the copper is skimmed off the surface and the now enriched or concentrated ore is taken away for roasting.
     The water bath mixture is recycled.
   • The concentrated (enriched) ore can contain up to 35% copper and a number of impurities such as antimony, arsenic, and lead.

5. Roasting : reduces impurities in the copper ore and produces calcine (a mixture of products)^*

   • Concentrated (enriched) ore is heated to between 500^oC and 700^oC in air.

     Some impurities in the ore, such as arsenic and antimony, are oxidized and form volatile gases which can be removed:

     4As + 3O_2(g) → 2As₂O_3(g)

     4Sb + 3O_2(g) → 2Sb₂O_3(g)

   • Roasting the ore containing chalcopyrite, CuFeS₂, also produces sulfur dioxide gas and a mixture of compounds called calcine.

     One of the reactions in the formation of calcine is shown below:

     2CuFeS2(s)	+	4O2(g)	→	2FeO(s) + Cu2S(s)	+	3SO2(g)
     chalcopyrite	+	oxygen	→	calcine	+	sulfur dioxide

   • Calcine, a mixture of solids including copper oxides, sulfides and sulfates, can then be smelted.

     eg, copper(I) oxide can be produced from copper(I) sulfide:
     2Cu₂S + 3O_2(g) → 2Cu₂O + 2SO_2(g)

6. Smelting with Fluxes^*: converts the calcine to matte (a mixture of copper sulfides and iron sulfides)
   Calcine is heated to over 1200^oC with fluxes such as silica (SiO₂) and limestone (CaCO₃).
   • Calcine melts and its compounds react with the fluxes.
   • Any copper(I) oxide present will be converted into copper(I) sulfide during the smelting process because copper has a higher affinity for sulfur than it does for oxygen.

     for example: Cu₂O + FeS → Cu₂S + FeO

   • Impurities form a slag which floats on the surface and is easily removed.

     One of the reactions to remove iron impurities is shown below:

     FeO	+	SiO2	→	FeO.SiO2
     iron impurity	+	silica	→	slag

     (FeO.SiO₂ can also represented as FeSiO₃)

   • After the slag is removed, the resulting product, called matte, is a mixture of copper sulfides (mostly Cu₂S) and impurities such as iron sulfides.
     This matte, containing about 40% copper, can then be fed to a converter.

7. Conversion of Matte into Blister Copper (impure copper)

   • Air is blown through the molten matte which converts iron(II) sulfide to iron(II) oxide and sulfur dioxide

     2FeS + 3O₂ → 2FeO + 2SO_2(g)

     Iron(II) oxide, FeO, slag is skimmed off.

   • Relatively pure copper(I) sulfide, Cu₂S, accumulates in the bottom of the converter.
   • Air is blasted through this copper(I) sulfide to reduce the copper and oxidize the sulfur to sulfur dioxide:

     reaction	Cu2S	+	O2	→	2Cu	+	SO2
     oxidation state (number) of copper	+1				0			decrease in oxidation state: copper is reduced
     oxidation state (number) of sulfur	-2						+4	increase in oxidation state: sulfur is oxidized

   • The impure copper produced by the converter is referred to as blister copper because bubbles of sulfur dioxide gas on the surface of the copper look like blisters.
   • Blister copper contains 97-98% copper.

     Impurities in blister copper can include gold, silver, antimony, arsenic, bismuth, iron, lead, nickel, selenium, sulfur, tellurium and zinc.

   • Blister copper can be caste into blocks to undergo electrorefining (electrolytic refining).

8. Refining: increases the purity of the copper by removing impurities.
   (see electrorefining)