Zinc is one of the most widely used non-ferrous metals globally, ranking behind only aluminum and copper in production volume. This metal, recognized for its resistance to corrosion, is primarily sourced from the Earth’s crust as the sulfide mineral sphalerite (ZnS). Extracting usable zinc from this raw ore involves a complex, multi-stage process. This process transforms low-grade rock, often containing less than ten percent zinc, into a high-purity metal essential for modern industry.
Locating and Extracting Zinc Ore
Zinc deposits are typically found in polymetallic ore bodies, meaning the zinc often occurs alongside other base metals like lead, copper, and silver. Geologically, these deposits frequently fall into categories such as Volcanogenic Massive Sulfide (VMS) or Mississippi Valley Type (MVT) deposits, both formed by hydrothermal activity. The zinc content in the raw ore, known as the head grade, usually ranges between three and ten percent by weight.
The method of removing the ore depends on the deposit’s depth and geometry, with underground mining accounting for the majority of global production. For deep, high-grade deposits, miners commonly use cut-and-fill stoping. This involves removing the ore in horizontal slices and immediately replacing the excavated void with backfill, often a mixture of waste rock and cement. The backfill provides ground support and a stable working platform for the next slice of ore to be mined.
Shallower, lower-grade, and more widely disseminated ore bodies are typically mined using open-pit methods. This surface mining technique begins by removing the non-mineralized soil and rock, referred to as overburden, to expose the ore body. The exposed ore is then broken up using drilling and blasting. The resulting pit develops in a stepped pattern, known as benches, which maintain the stability of the pit walls and provide access for haul trucks to transport the broken ore.
Processing Ore into Zinc Concentrate
Once the bulk ore is extracted, it must be upgraded into a zinc concentrate before it can be economically shipped and refined. This beneficiation process begins with crushing and grinding, where powerful machines reduce the large ore rocks into a fine powder. This mechanical action is necessary to liberate the microscopic sphalerite particles from the surrounding waste rock, known as gangue.
The fine powder is then mixed with water and chemical reagents to create a slurry that is pumped into flotation cells. In this process, known as froth flotation, specific reagents adhere selectively to the sphalerite particles, making them water-repellent. Air is then bubbled through the mixture, and the treated zinc particles attach to the rising air bubbles, forming a mineralized froth layer on the surface. This froth is continuously skimmed off, resulting in a zinc concentrate that typically contains fifty to fifty-five percent zinc. The remaining waste material, or tailings, is separated and the concentrate is prepared for the final refining stages.
Refining Concentrate into Pure Zinc Metal
The zinc concentrate, primarily zinc sulfide, cannot be directly refined into metal and must first undergo a thermal treatment called roasting. This initial step involves heating the concentrate to temperatures exceeding 900°C in a fluid-bed reactor, which converts the zinc sulfide (ZnS) into zinc oxide (ZnO), also known as calcine. The sulfur is driven off as sulfur dioxide gas, which is captured and converted into commercial-grade sulfuric acid, a valuable byproduct. The calcine then enters one of the two major refining routes: pyrometallurgy or hydrometallurgy.
Pyrometallurgy
Pyrometallurgy involves smelting the calcine in a blast furnace, where the zinc oxide is reduced by carbon at extremely high temperatures. Due to zinc’s relatively low boiling point of 907°C, the metal vaporizes. It must then be collected and condensed in a separate chamber to produce liquid zinc.
Hydrometallurgy
The hydrometallurgical method, known as the Roast-Leach-Electrowin (RLE) process, is the predominant technique, accounting for over ninety percent of global primary zinc production. In this process, the zinc oxide calcine is dissolved, or leached, in a sulfuric acid solution to create an impure zinc sulfate solution.
This solution is rigorously purified by adding zinc dust, a process called cementation, which precipitates and removes trace metallic impurities like copper and cadmium that would otherwise interfere with the final step. The purified zinc sulfate solution is then routed into electrowinning cells, where a powerful electric current is passed between aluminum cathodes and lead alloy anodes. This current causes the high-purity zinc metal to deposit onto the aluminum cathodes, from which it is stripped, melted, and cast into marketable slabs or ingots, achieving a purity level often exceeding 99.99 percent.
Essential Applications of Zinc
The refined zinc metal is primarily valued for its exceptional ability to prevent corrosion, which is why its largest application is in the process of galvanizing steel. This involves coating iron or steel components with a thin layer of zinc, which acts as a sacrificial anode to protect the base metal from rust and degradation, significantly extending the lifespan of infrastructure and automotive parts.
Zinc is also a component in various metal alloys, most notably in the production of brass, which is a combination of zinc and copper valued for its strength and acoustic properties. Zinc is used extensively in die-casting alloys, such as Zamak, which are employed to create precision components for hardware and automotive industries. The metal also plays a continuing role in battery technology, particularly in alkaline and zinc-air batteries.