Zinc (Zn) is a bluish-white metal that holds a significant place in industrial manufacturing. This metal is widely employed for its ability to resist corrosion, primarily as a coating for steel and as a component in various metal mixtures. The process of transforming zinc from its raw state into a pure, usable commodity involves a series of complex physical and chemical engineering steps.
Sourcing the Raw Material
Zinc is not typically found in nature as a pure metal, but rather locked within mineral deposits. The vast majority of the world’s zinc supply originates from zinc sulfide ore, scientifically known as sphalerite. These ores are typically mined from underground deposits, often requiring extensive drilling and blasting operations. Zinc sulfide is frequently co-mined alongside other valuable metals, including lead, copper, and silver. The raw ore contains the zinc mineral chemically bonded with sulfur and mixed with significant quantities of unwanted rock material known as gangue.
Preparing the Zinc Concentrate
The first major processing step after mining involves physically separating the zinc-bearing minerals from the non-metallic waste rock. This process begins with mechanical crushing and grinding of the raw ore to reduce the material to a fine powder. The finely ground ore is then subjected to a technique called froth flotation.
In the flotation circuit, the powdered ore is mixed with water and specific chemical reagents that selectively adhere to the zinc sulfide particles. Air is then blown through the mixture to create a froth, and the mineral-coated zinc particles float to the surface where they are skimmed off. This physical concentration process yields a zinc concentrate, which typically contains 40% to 60% zinc.
The concentrated zinc sulfide (ZnS) cannot be directly processed into metal, so it must undergo a chemical transformation called roasting. Roasting involves heating the zinc concentrate in a fluidized-bed roaster to high temperatures, often around 950°C to 1000°C. This high-temperature reaction with air converts the zinc sulfide into zinc oxide (ZnO), a powdery substance called calcine, and releases sulfur dioxide gas. The sulfur dioxide is captured and converted into sulfuric acid, which is a valuable byproduct and helps mitigate atmospheric emissions.
Extracting Pure Zinc Metal
Once the calcine (zinc oxide) is produced, the next steps focus on extracting the pure zinc metal, primarily utilizing one of two major industrial methods. The dominant method worldwide is the hydrometallurgical or electrolytic process, often called the Roast-Leach-Electrowin (RLE) process, which accounts for over 90% of global zinc production.
Hydrometallurgical Process (RLE)
The RLE method begins with leaching, where the zinc oxide calcine is dissolved in a sulfuric acid solution. This chemical reaction forms an aqueous solution of zinc sulfate, separating the zinc from most of the solid impurities. The resulting solution still contains traces of other metals like copper, cadmium, and cobalt that could interfere with the final extraction. A purification stage is performed by adding zinc dust to the solution in a process called cementation, causing the unwanted metal impurities to precipitate out.
The purified zinc sulfate solution then moves to the electrowinning stage, a specialized form of electrolysis. Large electrolytic cells are used, where an electric current is passed through the solution between lead anodes and aluminum cathodes. The electrical energy causes the zinc ions to be chemically reduced, depositing pure metallic zinc onto the surface of the aluminum cathodes. This deposition process typically takes about 22 to 48 hours, after which the zinc sheets are mechanically stripped from the aluminum plates.
Pyrometallurgical Process
A less common alternative is the pyrometallurgical or thermal reduction process, which relies on high heat. In this method, the zinc oxide calcine is mixed with a carbon source, such as coke, and heated to extremely high temperatures, often exceeding 1400°C. The carbon acts as a reducing agent, chemically removing the oxygen from the zinc oxide to produce zinc vapor and carbon monoxide gas. Special furnaces, such as the Imperial Smelting Furnace, are designed to collect this zinc vapor. The vapor is then quickly condensed by cooling it with a spray of molten lead, converting the zinc vapor back into a liquid metal that is separated from the lead.
Final Purity and Common Uses
The zinc produced from the electrolytic cells or the thermal condensers is melted and cast into large ingots. This initial metal may undergo further refining, especially for sensitive applications that require the highest purity levels. Distillation refining, which involves re-vaporizing and condensing the liquid zinc, can be used to remove residual impurities like lead and cadmium. The result is Special High Grade (SHG) zinc, which typically achieves a purity level of 99.995%.
Approximately 50% of all zinc produced annually is used for galvanizing, a process where a protective zinc coating is applied to steel. The zinc coating sacrificially corrodes before the underlying steel, providing durable protection for infrastructure and automotive components. Another significant portion of zinc is utilized in die-casting, where it forms alloys with metals like aluminum and copper to create complex, high-strength parts. Zinc is also an active component in various types of batteries and is alloyed with copper to produce brass.