Deep in the earth's crust, in ancient veins, in discarded electrical appliances and galvanized steel,
there is a silver-blue metal - zinc. It is not as bright as gold and silver, but it builds the sinews of
modern industry; it is low-profile and hidden, but the process of extracting it is a grand project that
combines flames, solutions and wisdom. From the ore roaring mine to the precision operation of
the electrolysis workshop, the road of zinc extraction, human wisdom to wake up the earth's
treasures of the extraordinary journey.
Act I: The Gift of the Earth and the First Awakening of the Ore
Zinc rarely exists as a single substance. Its main carrier is sphalerite, a sulfide ore with a dark brown
to black luster, often symbiotic with galena (lead) and pyrite (iron). Other sources include rhodochrosite
and xenoliths. Zinc ore resources are widely distributed globally, with Australia, China, Peru, and the
United States being the main producing regions.
The mined ore first goes through the baptism of crushing and grinding. Giant crushers pulverize the
ore into small pieces, and ball mills further grind it into a fine powder, as if opening a cage for the
sleeping zinc. Then comes the delicate physico-chemical process of flotation. Specific chemicals are
added to the slurry, causing hydrophilic veinstone minerals to sink to the bottom, while hydrophobic
zinc-bearing minerals (mainly sphalerite) particles adhere to the rising bubbles and collect as a
zinc-rich zinc concentrate (typically 50-60% zinc). This is the critical enrichment step in the
extraction journey.
Act II: The Fire Road - Rebirth in Flames
For zinc sulphide concentrates, roasting is the “trial by fire”. In large roasters, such as fluidized roasters,
the concentrate reacts vigorously with air at high temperatures (around 950°C). The core objective is
desulfurization:
2ZnS + 3O₂ → 2ZnO + 2SO₂.
The product is a baked sand with a main component of zinc oxide (ZnO) and a flue gas rich in sulfur
dioxide (SO₂). This flue gas is by no means a waste product, but a valuable resource - fed into the contact
acid system for efficient conversion into sulfuric acid (H₂SO₄), the lifeblood of the industry, which realizes
the recycling of sulfur resources.
The next step in sand roasting is reduction smelting. The traditional method is to mix the roasted sand
with a carbonaceous reducing agent (coke or anthracite) in a high-temperature (above 1300°C) blast
furnace (similar to an ironmaking blast furnace):
ZnO + C → Zn(g) + CO
A key challenge is that zinc has a low boiling point (907°C) and is produced in vapor form at high
temperatures in the furnace. The zinc-containing vapors rise with the furnace gases and are “trapped”
and dissolved by the spray of liquid lead in a specialized condenser outside the furnace (e.g., a lead rain
condenser), forming a zinc-lead alloy. Subsequent cooling, zinc and lead due to differences in solubility
and separation, to obtain crude zinc (containing more than 98% zinc). This method has a long history,
but high energy consumption and certain limitations on the adaptability of raw materials.
Act III: The wet road - a delicate dance in solution
The mainstream of modern zinc extraction is the cleaner and more efficient hydrometallurgy (accounting
for more than 85% of global production), the core of which lies in the “acid leaching - purification
- electrowinning”.
Acid leaching: the mystery of dissolution
Roasted sand leaching: Roasted zinc oxide sand from roasting is the main raw material. It is put into
dilute sulfuric acid (waste electrolyte):
ZnO + H₂SO₄ → ZnSO₄ + H₂O
Zinc enters the solution in the form of zinc sulfate (ZnSO₄).
Direct Leaching: To reduce pollution and increase efficiency, various technologies have been developed for
the direct treatment of zinc sulfide concentrates (e.g., oxygen pressure leaching, atmospheric pressure
oxygen-enriched leaching). The core is the conversion of ZnS to ZnSO₄ solution in one step using oxygen
and sulfuric acid at high temperature and high pressure (or under specific conditions), with the by-production
of elemental sulfur (S⁰) or sulfuric acid, to avoid the generation of SO₂ fumes.
Purification: “cleaning” of the solution
The leaching solution (containing mainly ZnSO₄) usually contains a variety of impurity ions (iron, copper, cadmium,
cobalt, nickel, arsenic, etc.), which can seriously affect the quality and efficiency of the subsequent electrowinning
of zinc. Deep purification is the soul step of the wet process, usually using a combination of multiple stages:
Oxidation to remove iron and arsenic: Add oxidizing agent (e.g. MnO₂) and neutralize to remove iron and arsenic by precipitation.
Zinc powder replacement (antimony salt/arsenic salt purification method): This is the most central step. Zinc powder
and a small amount of activator (e.g. antimony salt or arsenic salt) are added to the solution under heating and stirring.
The zinc powder acts as a reducing agent, displacing and reducing impurity ions with a positive potential
(e.g. Cu²⁺, Cd²⁺, Co²⁺, Ni²⁺) from the solution to form a metallic or compound precipitate:
Cu²⁺ + Zn⁰ → Cu⁰ + Zn²⁺
Cd²⁺ + Zn⁰ → Cd⁰ + Zn²⁺
After several stages of fine purification, a new solution (ZnSO₄ solution) of pure composition, suitable for
electrowinning, is obtained.
Electrowinning: electric current “casts” pure zinc
The purified new solution is pumped into the huge electrolysis plant. This is where electrochemistry takes center stage:
Inside the electrolysis tanks, lead-silver alloy anodes and aluminum cathodes are arranged alternately.
When direct current is applied, the cathode (aluminum plate) undergoes a reduction reaction: Zn²⁺ + 2e- → Zn⁰,
and pure zinc metal is deposited on the surface of the aluminum cathode like a silver frost.
The anode undergoes an oxidation reaction: 2H₂O → O₂ + 4H⁺ + 4e-, precipitating oxygen and generating acid.
After 24-48 hours of deposition, the zinc-plated cathode is removed and the cathode zinc flakes are stripped off.
These zinc flakes are melted and cast into ingots, which are refined zinc with a purity of 99.995%. After consuming
the zinc ions, the solution in the tank is turned into acid-containing waste electrolyte, most of which is returned
to the leaching process for recycling.
Act IV: The Circle of Recycling - The Gift of Urban Mines
Zinc recycling is an important pillar of sustainable development. Zinc-containing wastes (e.g. galvanized steel
trimmings, die-cast zinc alloy waste parts, zinc-containing dust, used batteries) can be recast into recycled zinc
ingots or zinc alloys through the process of dismantling, sorting, and smelting (using induction electric furnaces
or blast furnaces). Approximately 30% of the world's zinc supply comes from recycled sources, significantly
reducing dependence on virgin minerals and the overall environmental footprint.
Epilogue: The pulse of a blue mineral
The extraction of zinc is a magnificent epic of mankind harnessing fire and water, fusing chemistry and electricity.
From the crushing and flotation of the ore, to the flames of the roaster and the dissolution of the acid leach tank,
to the subtle purification of the purification tank and the silent sculpting of the electric current in the electrolyzer,
every step of the process is characterized by the rigor of engineering and the wisdom of science. The fire road
shows the power under high temperature, while the wet process interprets the subtlety in solution. The regeneration
cycle, moreover, gives a green closed loop to this value chain.
These zinc ingots are the “blue armor guards” (galvanized steel) of the urban steel forests against rust, the zinc
alloy die-castings of the automobile engines that run with precision, the essential trace elements that safeguard
human health, and the battery material candidates for a clean energy future. Every pulse of the zinc extraction
industry is silently driving the sound operation and green transformation of modern society. It reminds us that
the blue treasures buried deep in the ground, skillfully awakened by human ingenuity, will eventually become
the cornerstones of our safe, efficient and sustainable future. The pulse of blue deposits will continue to beat
strongly.