Extraction of Zinc

Metallurgy is the field of science that deals with the extraction of metals from ores that are naturally present in the environment. Most elements like metals are present in a combined state with other elements and they are called minerals.

An element can combine with a variety of other elements to form different minerals from them, only a few are good sources of that metal. Such sources are called Ores and the process of extracting metal ores buried deep underground is termed Mining. The metal ores are present in the earth’s crust in abundance. The extraction of metals from ores helps to use the minerals in the ground. The ores are very different from the finished metals that we use. Ores consist of the desired metal compound and the impurities present in them, some earthly substances present are called Gangue. The extraction of metals and their isolation follow these steps:

• The Concentration of Ore: The process of removal of the gangue from Ore is termed as Concentration of ore. This includes Froth Flotation Method, Leaching, Hydraulic Washing, Magnetic Separation.

• Isolation of Metal From Concentrated Ore: The second step involves the isolation of metals.

• Purification of the Metal: The metal extracted from its ore is usually impure in nature. This impure metal extracted is called crude metal. Refining is a process of removing impurities in order to obtain metals of high purity. The impurities can be removed from crude metal by various methods on the basis of the properties of the metal and the properties of its impurities.

Zinc Mining

It is the process of extraction of zinc from its ores, and the preparation of zinc metal for use in various products. The most commonly available ore in the extraction of Zinc is Zinc Blende also known as Zinc Sulphide (ZnS) and other ores include Calamine, Zincite, etc. Zinc Blende does not contain a very high percentage of zinc so it needs to be concentrated.  The best concentration method for zinc ore is froth flotation.

Procedure of Zinc Extraction

The Concentration of Ore: Froth Flotation

First of all, the ores are crushed into very fine powder-sized particles and are mixed with water. The mixture obtained is called Slurry. A Collector is used which will act as a surfactant and a chemical is added to the slurry. This process is done to enhance the hydrophobic nature of the mineral.

The slurry is now being converted into pulp. This pulp is now added to the container which is filled with water and then air jets are forced into it in order to create bubbles. The required mineral is now repelled by the water surface and gets attached to the air bubbles. As these air bubbles rise above the surface with mineral particles sticking to them, these are the froth. This Froth is now collected, separated, and is taken for the next process of refining and extraction.

The main principle involved in the process of Froth Flotation is the difference between the wetting ability of the ore and the remaining impurities. The particles are categorized into two types on the basis of their wetting ability: Hydrophobic and hydrophilic. If the minerals are of Hydrophobic nature then only can get attracted toward the froth and not with water. The deciding factor for the stability of the froth is the strength of the attachment of the bubble to the mineral.

Metal From Concentrated Ore

Roasting: After the concentration of ore, the next step involves the extraction of zinc by Roasting. The concentrated ore is finely divided into small pieces and is then suspended in a rising stream of air. The sulfur content can be reduced hugely with this process. This also converts the Zinc blende to ZnO which is more reactive. 

\[2ZnS + 3O_{2} \rightarrow 2ZnO + 2SO_{2}\]

Reduction:  Zinc oxide is mixed with powdered coke and heated to 1673K in a fire clay in which ZnO is reduced to zinc metal. The carbon from coke will act as a reducing agent in this reaction. The reaction involved in the final step of extraction of zinc is:

\[ZnO + C \rightarrow  Zn + CO\]

Purification: The last step involves the purification of Zinc, Zinc is purified by electrolytic refining. In this process, impure zinc is anode and cathode is of a pure thin sheet of zinc and the electrolyte used is \[ZnSO_{4}\] solution containing a small amount of \[H^{2}SO_{4}\]. On passing the electric current pure zinc gets deposited on the cathode side.

Zinc

• Zinc is a transition earth metal represented with the symbol ‘Zn’. It has an atomic number of 30. It is commonly used in the die casting process. Zinc is the world’s fourth most widely consumed metal followed by iron, aluminum, and copper.

• Zinc is Used as an Alloy: Zinc is mixed with copper (brass) and other metals to form material commonly used in vehicles and other household appliances. It is also used in the production of zinc oxide (the most important zinc chemical by volume of production), which is used as a protective skin ointment in rubber manufacture.

• Zinc is used to help in maintaining the balance of enzymes in the human body. Zinc deficiency can lead to hypogonadism in men, as well as mental lethargy, depression, and defects in the skin.

Did You Know?

Earlier days, extraction of zinc was very difficult compared to extraction of other metals because, at temperatures where metals such as iron begin to melt, zinc is in a gaseous state. Due to its low boiling point, at such temperatures, zinc will escape with exhaust gases.

Process of Zinc Extraction

Zinc concentrates, or zinc ores that have been enriched after removal from the mine, are delivered from all over the world in a granular bulk form. Enclosed conveyor belts transport the concentrates for temporary storage. Sulfuric acid is also shipped from the harbor, and it is the main byproduct of the production of zinc. In addition, zinc concentrates from various mines are temporarily stored. The crane picks up multiple types of concentrates, mixes them, and blends them to obtain the right mix, making for certain zinc refiners’ specific needs, ensuring the highest quality of refined zinc.

The mixed concentrates are transported on conveyor belts to the Roasters. In the roasting plant, the mix of zinc concentrates is introduced into fluidized bed furnaces with the addition of air. The sulfur contained in the zinc concentrates was released and oxidized at about 1,000 degrees Celsius. This gas is then purified in several phases and transformed into sulfuric acid to produce sulfur dioxide gas. Sulfuric acid is a crucial byproduct of the zinc production process, and it is an important base material for the chemical and pharmaceutical industries. 

The roasted zinc material is now mostly soluble in acid and is deposited towards the bottom of the roaster. Once cooled and ground, it is then pumped into the reservoir of the leaching plant. The roasted material from the mine is purified along with recycled material called volts oxide. Volts oxide is an important raw material for making Zinc. It is in the form of a powder produced from the recycling of zinc-coated steel. Both the roasted ore and the Volt's oxide are purified in the leaching plant by dissolving in sulfuric acid. Any accompanying impurities are precipitated. This leach solution, free of solid matter, is then heated and purified in three phases prior to proceeding to the production of metallic zinc. Metallic zinc production is based on electrolysis lead, and aluminum sheets are suspended in baths which are filled with the zinc-containing leech solution or electrolyte. 

The sheets are connected in an electric circuit where the lead sheet becomes the anode and the aluminum sheet becomes the cathode. The voltage drop across the anodes causes the zinc to deposit onto the aluminum sheets. The pure electrodeposited zinc is then peeled off the aluminum plates, washed, and transported to the melting furnace in the casting plant. The aluminum sheets will be put back in the electrolytic baths and reused to deposit more zinc from fresh zinc-containing electrolytes. The electrodeposited zinc from the electrolysis step is melted at over 420 degrees Celsius in induction furnaces and cast into slabs or Jumbo's weighing from 1 to 6 tons or in two unique shapes depending on its use and the needs of the customer. In some cases, additions of small amounts of aluminum or other metals are added to meet customer requirements. Strict quality controls are followed during the casting process including the regulation of returning scrap from the casting process to the melting furnace.