In the steel skeleton of a skyscraper, in the key components of a speeding car, and in the precision casing
of a household appliance, a silver-white metal silently plays the role of “guardian”. It is zinc. Zinc with its
excellent corrosion resistance, through the galvanization process, for the steel wearing invisible armor, so
that it is free from rust erosion, greatly extending the life of the material. The cornerstone of this “guardian”
power is modern zinc smelting technology - a sophisticated industrial alchemy that transforms complex ores
into highly pure metals.
The ore challenge: from symbiotic ore to pure zinc
Pure zinc ores rarely exist in nature. The most common type of zinc sphalerite (ZnS) is often closely symbiotic
with elements such as lead, copper, iron, silver, cadmium, arsenic and germanium. The core mission of zinc
smelting is to economically, efficiently, and environmentally friendly separate and purify zinc metal from these
complex ores, and ultimately produce Special High Grade Zinc (SHG) with a purity usually as high as 99.995%
or higher, to satisfy a wide range of high-end needs such as zinc plating, die-casting alloys, brass manufacturing,
and chemical raw materials.
Hydrometallurgy: the absolute mainstream of modern zinc smelting
Today, about 85% of the world's zinc is produced through the hydrometallurgical process, due to its high efficiency,
environmental friendliness (compared to the traditional thermal process) and adaptability to low-grade ores.
Roasting: Sulfur Conversion and Enrichment
The zinc sulfide concentrate first enters a boiling roaster. At precisely controlled temperatures (approx. 950°C)
and in oxygen-enriched air, the zinc concentrate is vigorously “boiled” and burned.
The core reaction: 2ZnS + 3O2 -> 2ZnO + 2SO2. The goal is to convert the zinc to acid-soluble zinc oxide (ZnO)
and to convert the sulfur to a high concentration of SO2 flue gas (which is used in the production of sulfuric
acid, an important by-product).
Key controls: temperature, oxygen concentration, residence time to ensure efficient zinc conversion and inhibit
the formation of harmful zinc ferrate.
Leaching: Dissolution trip in acid solution
Roasted sand (containing mainly ZnO) from roasting is mixed with spent electrolyte (dilute sulfuric acid) and reacts
in a large leaching tank: ZnO + H2SO4 -> ZnSO4 + H2O.
Complicated Challenge: In addition to ZnO, the roasting sand contains iron, copper, cadmium, cobalt, nickel, arsenic,
antimony and other impurity oxides. They also dissolve to varying degrees.
Neutralization and purification precursor: Through precise control of acidity (pH), temperature and addition of oxidizing
agents (e.g. MnO2), impurities such as iron, arsenic, antimony, etc. are induced to hydrolyze and precipitate, and are initially
separated.
Purification: solution in the “demining” action
The first paragraph in addition to copper and cadmium: CuSO4 + Zn -> ZnSO4 + Cu↓, CdSO4 + Zn -> ZnSO4 + Cd↓.
The second section in addition to cobalt and nickel: usually at higher temperatures and the presence of activators
(such as antimony salts, arsenic salts): CoSO4 + Zn + (activator) -> ... Co↓.
The zinc sulfate solution obtained by leaching (fresh solution) contains impurities (e.g. Cd, Co, Ni, Cu) that are lethal
for subsequent electrolysis. They must be completely removed by multi-stage deep purification.
Zinc powder replacement method: The core purification method. Zinc powder is added to utilize the difference in metal activity:
Strictly control the addition of zinc powder, temperature, stirring intensity, reaction time of each section to ensure the
deep removal of impurities, while minimizing the consumption of zinc powder and solution loss.
Electrolytic deposition: current casting of high purity zinc
The pure zinc sulfate solution enters the huge electrolytic tank. Inside the tank: an aluminum plate acts as the cathode
(deposits zinc) and a lead-silver alloy plate acts as the anode (corrosion resistant).
After energization (direct current): cathodic reaction: Zn2+ + 2e- -> Zn, high purity zinc is densely deposited on the
aluminum cathode.
Anodic reaction: 2H2O -> O2↑ + 4H+ + 4e-, generating oxygen and acid (H+) to maintain the acidity of the electrolyte.
Precise control: Current density, electrolyte temperature, zinc ion/acid concentration, and additives (e.g., gum, strontium salts)
are key to ensuring that the zinc cathode is flat, dense, and easy to strip, and that energy consumption is optimized. Periodically,
the zinc is stripped from the cathode (cycle time typically 24-48 hours) and melted and cast into ingots.
Pyrometallurgical smelting: an important addition for specific scenarios
While the wet process dominates, pyrometallurgical zinc smelting still has applications for specific feedstocks (e.g. high silica,
high iron, difficult to leach ores) or for integrated recovery (e.g. simultaneous lead and zinc smelting in ISP blast furnaces):
Closed Blast Furnace Zinc Refining (ISP): Processing of co-produced lead-zinc ores. Sintered lumps and coke are smelted in a
blast furnace, and zinc vapors are absorbed by lead rain sharply cooled at the top of the furnace to form a zinc-lead liquid,
which is then separated from the refined zinc in a distillation tower.
Electrothermal / Vertical Pot Distillation: High energy consumption and environmental pressure, gradually phased out or
only used in small-scale special scenarios.
Environmental protection and resource utilization: the core of sustainable
development
Modern zinc smelting faces serious environmental challenges and is a driver for technological innovation:
Flue gas treatment: SO2 from roasting must be efficiently recovered for acid production (>99.5% conversion rate), and NOx,
mercury, and particulate emissions must be strictly controlled.
Slag treatment and resource utilization: Leaching slag (containing zinc, lead, silver, iron, etc.) is a major challenge.
Mainstream process:
Rotary kiln volatilization (Waelz): Recovery of zinc, lead.
Hot Acid Leaching - Potash Alumina/Epilite/Hematite: Highly efficient recovery of zinc and precipitation of iron as stabilized
slag (building material utilization sought).
Oxygen Pressure Leaching: Direct treatment of zinc sulfide concentrate or difficult to treat slag, sulfur recovery in the form
of elemental sulfur, environmentally friendly.
Wastewater “zero discharge”: through neutralization and precipitation, membrane treatment and other technologies, to
achieve a high degree of recycling of wastewater.
Valuable metal recovery: Recovery of cadmium, cobalt, copper, indium, germanium and other rare and precious metals from
purification slag to enhance the value of resources.
Innovation Driven: Efficiency, Green and Intelligent
Zinc smelting technology continues to evolve:
High-efficiency equipment: large-scale boiling roasters, energy-efficient electrolytic cells, automated zinc stripping units.
Process Enhancement: Oxygen Pressure Leaching (OPL) technology promotion, Direct Electrolysis (DEZ) technology exploration,
and development of new high-efficiency purifying agents/additives.
Intelligent control: Process parameter optimization based on big data and AI, equipment predictive maintenance, energy
intelligent management.
Low-carbon smelting: green power application, process energy-saving technologies (e.g., deep recovery of waste heat), carbon
capture, utilization and storage (CCUS) exploration.
Conclusion
Zinc smelting, a modern industrial alchemy that combines chemical dissolution, ion exchange and current deposition,
transforms complex ores buried deep in the ground into the shiny metal that guards steel and drives industry. Wet smelting,
with its advantages of high efficiency and relative cleanliness, has become the absolute mainstream, and its core - deep purification
and precision electrolysis - reflects the pursuit of the ultimate. In the face of increasingly stringent environmental requirements
and resource pressures, technological innovation is driving zinc smelting to accelerate in the direction of greater efficiency,
cleaner and smarter. The birth of each piece of high-purity zinc ingot not only unites the wisdom of craftsmanship, but also
carries the important responsibility of building a solid defense for modern industry and promoting sustainable development.
Zinc, the silent “steel guardian”, the road of its forging is still continuously refined.