Utilised throughout a variety of applications in modern society, copper and zinc is a thriving, gases-intensive industry.
Copper and Zinc rank as third and fourth in the top four most commonly used metals today and like fossil fuels, they are a finite resource.
The Earth’s reserves of copper have been projected to last only another 60 years and zinc less than 50, but based on reasonable extrapolation of 2% growth per year, both may run out before 2035.
Copper metal has been used by mankind for at least 10,000 years and alloyed with tin to make brass as early as 1500 BC. Incredibly, more than 95% of all copper ever mined and smelted was extracted since 1900. Impure forms of zinc were smelted and extracted in India around 1200 AD.
Copper is an excellent conductor of both heat and electricity, second only to silver, and these properties explain its widespread industrial use.
Copper is remarkably ductile and is easily worked, capable of being cold-rolled down to one fortieth of a millimetre in thickness. Its length can also be increased by cold drawing as much as 5,000 times, making copper an ideal metal for making wire.
The manufacture of electric motors, generators, transformers and cables, using copper windings and conductors, has driven rising demand for copper metal since electric power began to replace steam energy.
Air conditioning, refrigeration systems, automotive cooling radiators and home heating installations depend on the thermal conductivity of copper to optimise their efficiency. Copper is highly corrosion resistant and is easily joined making it indispensible for water reticulation, desalination plants and hydraulic systems.
Many countries use copper coins for the smaller denominations of their currency and as one of only two metals of colour, copper has long been used for decorative and architectural applications.
Telecommunication cables using copper conductors that criss-cross most developed countries are largely redundant and represent an overlooked source of high grade copper, because fibre-optic cables now outperform copper in this application.
About half of the zinc metal extracted worldwide is used as a corrosion-protective coating for iron or steel fabrications and constructions. Although pure zinc is more reactive than iron or steel, it forms a protective oxide and carbonate layer which protects the material underneath. The zinc is applied electrochemically by galvanizing, or as liquid zinc by hot-dip galvanizing or spraying.
Alloys of primarily zinc with small amounts of copper, aluminium, and magnesium are useful in die-casting and spin casting, especially in the automotive, electrical, and hardware industries. Such alloys have a low boiling point and comparatively low viscosity, making the production of small and intricate shapes possible.
Smelting is a form of extractive metallurgy used to extract pure metals from their ores. Most metal ores contain small percentages of metal atoms compounded with either oxygen as metal oxide, with sulphur as metal sulphide or with carbon and oxygen as metal carbonate.
Particles of these metal oxides, sulphides and carbonates are dispersed within rock formations and usually found in groups of metals.
For example; the main ores of zinc in nature are Calamine (containing zinc oxide) and Sphalerite (containing zinc sulphide) and mostly these occur in combination with the minerals of copper, lead, silver and iron.
Obviously the extraction of pure metals from such complex ores requires a knowledge of chemistry, without which it is impossible to predict if a given rock can be smelted or not, and what it will produce. Therefore, there is continuous debate to understand how the ancient people learned how to smelt.
Smelting of oxide ore applies heat and a chemical reducing agent, commonly a fuel that is a source of carbon such as coke, or in ancient times charcoal, to change the oxidation state of the metal ore. The carbon or carbon monoxide released by the burning fuel reacts preferentially with oxygen in ore to leave pure metal.
The carbon is thus oxidized, producing carbon dioxide and carbon monoxide that escape as gas. As most ores are impure, it is often necessary to use flux, such as limestone, to remove the accompanying rock gangue as slag.
Sulphide ores require additional processing to remove the sulphur, which is usually converted into sulphuric acid for use in subsequent refining processes.
The largest producers of copper ore are Chile, Peru, the US and China, but while annual fluctuations may shift the rankings below fourth, Chile leads by an enormous margin.
Many producing regions ship the ore elsewhere or smelt it without refining and considerable tonnage of ore and un-refined copper are processed in Japan, Peru, and Serbia.
Chalcopyrite (CuFeS2), currently the copper ore most commonly mined and accounting for about 50% of global production, is contaminated with iron and has a typical copper content of between 0.25% and 5%.
Such secondary sulphides are resistant to leaching with sulphuric acid, so are concentrated by grinding and froth flotation, then raising the copper content to the 20-40% range. When dried, this product is known as copper concentrate and is often traded as an intermediate product in its own right.
The extraction of pure metal from copper concentrate can be achieved either by traditional pyro-metallurgical or hydro-metallurgical methods, using a bacterial oxidation process to oxidise the sulphides to sulphuric acid. Only a limited amount of copper is produced using the hydro-metallurgical process.
Traditionally reverberatory smelters were used to form two liquids: one an oxide slag containing most of the impurity elements and the other a molten matte containing the valuable metal sulphide and some impurities.
Reverberatory furnaces were so named, because their design isolates the process material from contact with the fuel and transfers heat by reflection off the furnace roof.
The calcine mixed with silica and limestone flux is smelted at 1200°C and at this temperature, the exothermic reactions to proceed rapidly, reducing iron oxides and sulphides to slag which floats off and releasing sulphur dioxide gas. The matte produced now contains about 70% copper, but still as copper sulphide mixed with iron sulphide.
The mixture of copper and iron sulphide is further processed in a converter, of which the Pierce-Smith type has prevailed for many years, by blowing oxygen-enriched air through the molten matte.
In this first stage metal sulphide molecules are thus partially converted to metal oxide molecules, oxygen also reacting with the liberated sulphur atoms to form sulphur dioxide that forms about 10% of the converter off-gas. This is recovered to produce sulphuric acid for the later electro-refining process.
Again, silica flux is added to remove the iron oxide that forms as a result of the stronger affinity of oxygen for iron. The light slag formed is called Fayalite and is poured-off and additional matte added. This first stage of conversion is repeated many times until the converter is filled with copper sulphide.
The second stage of converting aims to oxidise the copper sulphide paste; again by blowing with oxygen-enriched air to produce blister copper, so named because the 95-98% pure copper ingots produced have a broken surface caused by escaping bubbles of sulphur dioxide gas.
The blistered copper is now transferred to an anode furnace to remove the residual oxygen. Natural gas or propane is blown through the molten copper oxide and when the metal is about 99% pure, the flame burns green indicating the copper oxidation spectrum.
Anodes cast from blister copper after reduction, are dissolved into a solution of 3-4% copper sulphate and 10-16% sulphuric acid. This electrolytic process uses cathodes made of thin rolled sheets of highly pure copper.
An electric potential of 0.2-0.4 volts causes the anode metal to dissolve and pure copper to be deposited onto the cathode sheets. More noble metal impurities like silver, gold, selenium and tellurium settle to the bottom of the cell as anode mud, while less noble constituents like arsenic and zinc remain in solution.
Copper cathode sheets are 99.99% pure and sold as a true commodity, deliverable to the metal exchanges in New York, London and Shanghai with dimensions of 96cm x 95cm x 1cm and mass around 100kg.
So called flash smelting techniques have been developed to optimise energy efficiency, by using the heat released during oxidation of the sulphur in the ore.
Total energy demand has thus been reduced from 30-40 GJ per ton of cathode copper, to around 20 GJ. In addition these newer processes, in use since the 1950’s, emit off-gas containing sulphur dioxide concentrations higher than 30% – thereby reducing the cost of sulphuric acid production.
Zinc was originally produced by thermal methods applying heat and carbon in a smelter, but the word smelting now includes the production of zinc through hydro-metallurgical means. Zinc has an interesting history, being responsible for the innovation of mock silver coins and mock gold.
It is difficult to smelt because it boils at a relatively low 907°C and so, could not be produced as a pure metal until the technique of reduction distillation had been mastered. Evidence shows that this was first achieved in India around 1200 AD.
Today zinc is produced by both pyro-metallurgical and hydro-metallurgical methods and both methods share a common first step: roasting.
The most important ore material for the production of zinc is zinc sulphide, commonly known as Sphalerite; it contains about 6-7% zinc and 33% sulphur. Sphalerite is usually concentrated using the froth flotation method to about 40-60% zinc before roasting.
The purpose of roasting is to convert the concentrate by oxidizing the sulphur into sulphur dioxide gas and the zinc sulphide into impure zinc oxide.
Fluidised Bed roasters operate at about 920-960°C, are capable of greater throughput, with greater sulphur removal and lower maintenance than earlier Multiple Hearth and Suspension roasters. Typically the concentration of zinc sulphide is reduced to 0.2–0.5%.
Currently about 10% of world zinc output is produced using this older technology because it is impossible to achieve greater than about 98% purity.
The older standard grade of zinc, know as Good Ordinary Brand (GOB) at 98.5% purity, is barely adequate for galvanisation but the metal required for die-casting alloys must be Special High Grade with an assay of 99.995% zinc.
The only pyro-metallurgical process still used in the US to smelt zinc, was developed by the St. Joseph Mineral Company in 1930. This system has the advantage of being able to smelt a wide variety of zinc-bearing materials, including electric arc furnace dust, but it cannot match the efficiency of the electrolysis method.
The process begins with a downdraft sintering operation. The sinter, which is a mixture of roaster calcine and Electric Arc furnace (EAF) calcine, is loaded onto a gate type conveyor and then combustion gases are pumped through the sinter.
The carbon in the combustion gases react with some the impurities, such as lead, cadmium, and halides and these impurities are driven off into filtration bags. The output called product sinter, usually comprises 48% zinc, 8% iron, 5% aluminium, 4% silicon, 2.5% calcium, and smaller quantities of magnesium, lead, and other metals.
It is then charged with coke into an electric retort furnace, where a pair of graphite electrodes produce current flow through the mixture. Electrical resistance from the coke raises the temperature to 1,400°C, producing carbon monoxide.
The carbon dioxide and zinc vapour flow to a vacuum condenser, where over 95% of the zinc vapour zinc is recovered by bubbling through a molten zinc bath.
Zinc is first leached from the purified zinc oxide (calcine) by a sulphuric acid solution and then separated in specialised electrolytic cells fitted with aluminium sheet cathodes.
Pure zinc metal is deposited onto the aluminium cathodes, oxygen is released at the anodes and the process forms sulphuric acid that is re-used in the leaching process.
Purified zinc metal is stripped mechanically every 24 hours from the zinc-coated cathodes.
Pressure leach process
This technique for zinc extraction was first applied successfully in Canada in 1981 and there are now three similar facilities operating worldwide.
Zinc sulphate solution is produced directly by oxidising bulk zinc concentrates under oxygen pressure of 1200 kPa absolute, at 150°C in sulphuric acid medium and the sulphur content precipitated as elemental sulphur.
The zinc sulphate solution thus produced is amenable to further processing for final zinc extraction, through conventional leach-electrowin units.