Chemistry by Inam Jazbi: Metallurgy of Copper

Extraction of Copper from Sulphide Ore

Ore used and Type of Metallurgy

Copper is extracted mainly from sulphide ore, copper pyrite (Chalcopyrite), CuFeS₂ (CuS + FeS or Cu₂Fe₂S₄ i.e. Cu₂S + Fe₂S₃) which contain about 34% (6% in book) copper by Pyrometallurgy (Dry Process).

Main steps of Metallurgy

Extraction of Copper from sulphide ore involves the following steps:

1. Crushing of Ores into Powdered Form

The big blocks of ores are crushed into small pieces and then finally powdered.

2. Concentration of Crushed Ore

Ores are often contaminated with non-metallic and rocky impurities like sand (quartz), clay, mica, lime stone etc which are called gangue or matrix. The removal of unwanted impurities from the ore before metallurgical process is called Concentration or benefication or ore dressing. Copper pyrite is a low grade copper ore containing about 34% copper along with impurities such as sand, clay, lime stone etc.

The powdered ore is concentrated by froth floatation process (Selective Wetting) which is based upon preferential wetting of surfaces by liquids on account of the surface tension forces.

The powdered or crushed ore is concentrated by froth floatation process, in which the crushed ore is mixed with a mixture of Pine Oil or creosote oil and water and then thoroughly agitated with a blast of air. The ore is miscible (get wetted) with oil forming froth and rises to the top from where they are skimmed off and dried to get concentrated enriched ore whereas the gangue particles (impurities) are wetted by water and settle at bottom. Copper content is increased to 25-30% in this way.

3. Roasting

Roasting is a process in which the concentrated ore is strongly heated in a current of excess of air below the melting point of ore in the hearth of a large flat Reverberatory Furnace. Roasting not only dries the ore but also bring about following changes:

(i) Impurities like C, S, P, As,Sb in the ore are removed as their volatile oxides. It also removes organic matter

S	+	O₂	¾¾¾¾¾®	SO₂
4Sb	+	3O₂	¾¾¾¾¾®	2Sb₂O₃
4As	+	3O₂	¾¾¾¾¾®	2As₂O₃

(ii)Copper pyrite (ore) is converted into a mixture of Cuprous Sulphide and Ferrous Oxide. Some FeS present in the ore remains unreacted. At the end of roasting, the ore has been now contained Cu₂S, FeO and FeS.

4. Smelting (Formation of Molten Matte)

Smelting is the process of reduction of oxide of ore by reducing agent (like coke) under such conditions that metal is obtained in molten state.(In copper extraction, smelting serves to remove gangue).

A charge consisting of Roasted Ore, powdered Coke (reducing agent) and sand (flux) is strongly heated in a Water Jacketed Blast Furnace and a blast of hot air is blown at the lower part of furnace through tuyeres. During smelting coke burns releasing heat of combustion which serves to keep the charge in molten state.

The Ferrous oxide present in ore combines with sand (flux) to form fusible ferrous silicate (slag) which being lighter rises to the top and is withdrawn from the upper hole.

The molten mixture of Cu₂S and FeS is left behind after smelting which is called Molten Matte or Course Metal wchih contains about 45% copper. It being heavier, forms the lower layer and is withdrawn from the lower hole periodically.

5. Bessemerisation (Formation of Blister Copper)

The reduction of molten matte is carried out in a pear-shaped stainless steel furnace called Bessemer converter which is lined inside with MgO also provided with tuyers.

The molten matte is heated in Bessemer converter by blowing hot blast of air through pipes called Tuyers with more silica. The following changes occur in the converter:

(i) Air oxidizes most of FeS to FeO by direct oxidation which is removed as ferrous silicate (slag) by combining with silica. FeS is also converted into FeO by reacting with Cu₂O (formed by partial oxidation of Cu₂S).

(ii) After the iron has been removed, hot blast of air partially oxidizes Cu₂S into Cu₂O (leaving some unreacted Cu₂S). Now Cu₂O and remaining Cu₂S reduce each other (Auto or self reduction) without any external reducing agent to give metallic copper in molten state.

Blistered Copper

The molten metal produced is now cooled to solidify in sand moulds, the dissolved and hidden SO₂ gas escapes out, produces blisters (bubbles) on the surface of solid metal, so copper obtained is called BLISTERED COPPER which is 98-99% pure.

Since the reaction is exothermic, the heat liberated keeps the crude copper in the molten state. The completion of the reaction is indicated by the appearance of green flame produced by the vapourization of copper.

6. Electrolytic Refining of Blistered Copper

Blistered copper contains impurities mainly Ag, Au, Pt, Ni, Zn, Fe, As, Pb, As etc. Hence for electrical purposes, blister copper obtained after bessemerization is refined electrolytically to get copper of high purity (99.95-99.99).

The refining of Blistered Copper is carried out in a large Lead-Lined Tank using a series of anode of thick impure blister copper which places alternately with a series of cathodes of thin plates of 100% pure copper (The cathodes are coated with graphite so that the deposited pure copper may be removed easily). These electrodes are suspended in an electrolytic solution of CuSO₄ (15%) acidified with traces of dilute H₂SO₄ (5%).

On electrolysis, the atoms of anode (impure copper metal and other active metals such as Fe, Zn, Bi etc) are oxidized to their corresponding cations and pass into solution. The impurities of less active metals such as Ag, Au etc are not oxidized and settle down just below the anode as anode mud. The current of 1.3–1.5 volt is used for electrolysis which helps to deposit only copper on cathode leaving behind other metal ions in solution. The electrolytically refined copper is 100% (99.99%) pure. With the deposition of copper, cathode grows thick and anode becomes thin due to dissolution of the metal.

Electrolytic Reactions