The elemental metal named zinc is listed on the Periodic Table as “Zn”, has atomic number 30 and melts at 420 degrees Celsius (788 degrees Fahrenheit). Zinc is the 24th most abundant element in the crust of the Earth. Its color is grey metallic and can be polished to a silver shine. In nature, zinc is not found as a pure deposit, but as a chemical compound. The most commonly exploited zinc ore is a sulfide named sphalerite, with a zinc concentration of around 61% percent. The largest deposits of zinc are found in North America, Australia and Asia.
Statuettes and other ornaments made of a zinc alloy and some ancient writings, such as Greek, Roman and Indian recordings, mention the use of zinc as early as the 5th century BC. Brass, an alloy made of copper and zinc, was already used in the 14th century BC in Palestine, containing 23% zinc in some specific cases.
The isolation of pure zinc was achieved, probably independently by several people, at the end of the 17th century to as late as the mid 18th century. The German chemist Andreas Marggraf normally gets credit for the discovery of pure zinc in 1746, even though others before him have claimed discovery. Marggraf is therefore credited more with carefully describing the process and its basic theory, than the actual discovery.
The Making of Zinc
Around 70% of the world’s zinc comes from mining, the remaining 30% originates from recycling zinc. Around 95% of the zinc is mined from sulfidic ore deposits and these mines are spread throughout the world. China is the largest producer of zinc, with around 29% of the global zinc output in 2010.
Zinc metal is mined by using conventional blasting, drilling and hauling techniques. Froth flotation is used to separate zinc from other minerals after grinding the ore. The final concentration of zinc is about 50% using this method, the remainder being sulfur and iron.
By a process known as froth flotation, zinc can be produced. Froth flotation is also used in the reduction of copper and lead ores. The process starts with grinding the zinc ore to fine material and subsequently mixing it with water, pine oil and flotation chemicals. The flotation chemicals attach themselves to any zinc particles present when the mixture is agitated with injected air. Bubbles rise up with zinc particles adhered to it, which are scooped up by scrapers, thus collecting zinc-laden froth.
To remove water and oils, the froth is filtered. The remainder is roasted at 1371 degrees Celsius (2500 degrees Fahrenheit) into solid blocks called sinter. All the material has been completely converted to zinc oxide.
In order to refine the zinc to a higher grade ore it is processed in a blast furnace fueled by electricity, coke or natural gas. As the furnace reaches temperatures of up to 1204 degrees Celsius (2200 degrees Fahrenheit) the zinc ore is melted. This process also generates carbon dioxide, which will reattach to the zinc as it cools to, once again, form zinc oxide. To prevent, or at least reduce, this reattachment, molten lead is sprayed on the zinc while it is still hot. The lead, because of its higher melting temperature of 550 degrees Celsius (1022 degrees Fahrenheit), dissolves the zinc. This mostly lead and zinc mixture is then carried to another chamber, where it will be cooled to 440 degrees Celsius (824 degrees Fahrenheit). Around this temperature, the zinc, because it is lighter, separates from the lead and is drained from the top. Subsequently it is cast into ingots. The lead is returned to the blast furnace to use again.
To reach an even higher grade zinc ore, the zinc is kept molten and undisturbed for hours. Iron and other contaminants will settle slowly to the bottom, thus allowing the almost pure zinc to be drained from the top.
With pyrometallurgical processes a maximum purity of 98% can be achieved. While this is high enough to use the zinc for galvanization, it cannot be used to die-cast alloys, which requires 99,995% purity.
The hydrometallurgical process, also known as electrolysis, is used a lot more than the pyrometallurgical process. It consists of four steps: leaching, purifying, electrolysis and casting.
In the first step, the zinc oxide is leached in a strong sulfuric. The result is a liquid called a leach product, which contains the zinc. It also produces a solid called a leach residue, which contains left-over metals (usually lead and silver) and is sold as a by-product.
The basic chemical process is described below:
ZnO + SO3 –> ZnSO4
The second step is called purification, because it removes certain elements from the zinc sulfate solution which can interfere with the electrolysis process. copper, cobalt, cadmium and nickel are removed and sold as by-products for further refining. The zinc sulfate solution must be very pure for electro-winning to be efficient.
Electrolysis is the third step, producing an almost 100% pure zinc deposit. The zinc is extracted from the zinc sulfate solution by electrowinning. An electric current is passed through the solution, which causes the zinc to deposit on aluminium sheets. Every day or two, the process is halted and the zinc-coated sheets are removed. Subsequently, the zinc is stripped from the sheets. About 3,900 KWh of electric power is expended producing one metric ton of zinc in this way.
The final step is to melt the high grade zinc and cast it into very high grade (99,995% purity) ingots, or directly alloy and cast it into ingots.
Zinc is usually transported in very large quantities on commercial shipping freighters, with the part between port and destination done by train or truck.
Galvanizing against Corrosion
Zinc is commonly used to coat iron or steel to protect these metals against corrosion. As it is more reactive than iron or steel, zinc will attract almost all oxidation until corrosion completely erodes the coated sheet. What is left is a surface protection layer of oxide and carbonate. This protection even functions after minor scratches and dents and can survive for many years.
Galvanization is used on metal roofing, bridges, guard rails, lightposts, heat exchangers and most visible to the consumer: car bodies.
Coating zinc on another metal is accomplished by electrolytic plating of the metal – much like chrome plating a metal – or dipping it into molten zinc.
Intricate Machine Parts
An alloy made of very high grade zinc and aluminium is used to create die-cast parts which require little machining before they are used in an assembly. By injecting the alloy under pressure into the cavity of a two-part steel die, it fills the entire void within the mold. After the metal cools and the die halves are taken apart, the resulting zinc-alloy part is very close to the desired shape.
Die-casting is used, among others, to create parts for aircraft, medical instruments and car parts like emblems and doorhandles.
An unique application of zinc uses its ability to transfer its corrosion resistance properties by electrical contact. In this manner, zinc is used as a sacrificial electrode. An example application for this kind of electrode is when it is attached to aluminium marine engines. Especially in salt water, the oxidation process of the metals on the ship forms a weak electrical current, which may lead to corrosion of the hull and engine parts. By having a zinc sacrificial electrode present, it sacrifices itself by corrosion, negating the electrical current and thus protecting the aluminium hull and/or engine.
Similarly, zinc is used as a component to produce batteries. In a dry cell battery, the zinc is housed in a metal can and creates a chemical reaction that results in a voltage potential between two contacts. An electrical device can be connected to the battery and powered by the electricity produced, until the available chemical reactants are spent.
One widely used alloy which contains a large amount of zinc is brass. Brass is an alloy of copper mixed with 3% to 45% zinc, depending on the type of brass. Brass is superior to copper in areas like ductability, strength and corrosion resistance. This makes it useful in water valves, musical instruments and communication equipment.
Other used alloys that contain substantial amounts of zinc include aluminium solder, commercial bronze and nickel silver. It is also the primary metal used in producing one cent coins in the United States. The zinc coin is coated with a layer of copper to give the false impression of a copper coin.
The trading of Zinc futures takes place mainly on the London Metal Exchange (LME). This exchange regulates and monitors (electronic) trading of various metal types.
Zinc futures prices are influenced by a number of factors. Global supply of zinc is a major price determining factor. In the case of over-production, prices will rapidly fall. Mining activities will consequently drop which will eventually cause a deficit in supply. This will again raise prices to a standard level and this cycle will repeat itself.
Substitutes can also greatly influence the demand for zinc. Metals such as aluminum and magnesium are alternatives as die-cast materials and as such can influence price movements of zinc. In the event of rising prices of aluminum and magnesium the demand for zinc will increase.
Production and refining methods for zinc are also influencing prices as these processes are becoming ever increasingly cost-effective. This will increase the supply of available zinc and thus lower the price of this metal.
Limited remaining deposits can cause difficulties on the supply of zinc in the future. In comparison to various precious metals and base metals, zinc has a lower return yield which is why very limited budget is spent on the exploration of new zinc deposits. This may cause a deficit in the long term, which will eventually raise prices.
LME Zinc Futures Contract
A Zinc Futures Contract on the London Metal Exchange (LME) has the following specifications:
|Contract Size||25 tonnes|
|Price Quotation||US dollars per tonne|
|Contract Months||Monthly: 7 out to 63 months|
|Tick Size||$0.25 per ton|