Inside the furnace, the dark red lead concentrate tumbles and flows at a temperature of over a
thousand degrees, the impurities are captured tightly as green smoke, and the hot lead liquid
finally condenses in the mold to a cold silver-gray luster. In the control room, the process engineer
stares at the temperature curve and emission parameters on the screen and whispers to the
apprentice beside him: "Look at the real-time sulfur capture rate, 99.5%! Twenty years ago, this
green smoke was a piece of acid rain. Now the technology, not only to refine good lead, but also
to protect the blue sky." Lead smelting, this ancient technology, is undergoing a silent but thorough
technical revolution.
Core stage: the mainstream path of primary lead smelting
Modern primary lead smelting, mainly relying on the mature and reliable fire process, pursues a balance
between efficiency, recovery rate and environmental protection:
Sinter Roasting: Prologue to Desulfurization and Briquetting
Mission: To convert fine-grained lead concentrates (50-70% Pb) into hard, porous sintered briquettes with
significant removal of harmful sulphur.
Process: The concentrate is mixed with flux (limestone, quartz sand, etc.), reclaim, and water and spread on
the sinter. Upon ignition, air is drawn in from below and the sulfide (PbS) oxidizes to oxide (PbO) while releasing
large amounts of SO₂. The melt reacts with the impurities to form a low melting point slag phase, which binds
the material into lumps.
Key output: hard sintered lumps containing about 40-50% lead + high concentration of SO₂ fumes
(for acid production).
Blast Furnace Melting: Reduction and Separation Furnace
Core: High-temperature reduction reaction to change lead oxide into liquid metallic lead.
Charge: Sintered lumps, coke (fuel and reducing agent), flux (to adjust the slag pattern) are added from
the top of the furnace.
Storm clouds in the furnace:
High temperature zone (furnace belly): Burning of coke produces high temperatures (>1300°C) and the
reducing gas CO.
Reduction zone: CO reduces PbO to liquid lead: PbO + CO → Pb + CO₂. Partially unoxidized PbS also reacts
with PbO: 2PbO + PbS → 3Pb + SO₂.
Separation: the density of the largest liquid lead settled to the bottom of the cylinder; lighter slag floating
on it; between the two ice copper (trapping precious metals) or yellow slag (trapping arsenic and antimony)
to form an intermediate layer.
Product: Crude lead (containing about 95-98% Pb, with impurities such as Cu, As, Sb, Sn, Bi, precious metals, etc.)
is released at the bottom; slag (containing zinc that can be recovered) is released at the top; ice-copper/yellow
slag is periodically discharged from the intermediate layer.
Coarse lead refining: the ladder to purity
Coarse lead needs to be refined in several steps to remove impurities and reach a purity of more than 99.99%:
Molten precipitation of copper (plus sulfur to remove copper): Crude lead is slowly cooled down, copper and its
compounds due to the reduced solubility of the crystallization precipitation (molten precipitation), or the
addition of sulfur to generate Cu₂S slag skimming.
Alkaline refining (in addition to arsenic, antimony and tin): add saltpeter (NaNO₃) and caustic soda (NaOH) to the
molten lead. Nitrate oxidizes impurities, caustic soda and oxidation products react to form sodium salt slag (such
as Na₃AsO₄, Na₃SbO₄, Na₂SnO₃) is skimmed.
Addition of zinc to extract silver (Pikes method): Zinc is added to form less dense silver-zinc shells (Ag₂Zn₃) with the
gold and silver in the lead solution to the surface, which is repeatedly skimmed off. This is a key step in the recovery
of precious metals.
Vacuum dezincification (or oxidation dezincification): Removal of residual zinc. Vacuum distillation is highly efficient
and environmentally friendly, or air/steam is introduced to oxidize the zinc to form ZnO slag for skimming.
Electrolytic refining (ultimate purification): The fire-refined lead is cast into an anode, and a thin sheet of pure lead is
used as a cathode, and immersed in an electrolytic solution of hydrofluorosilicic acid (H₂SiF₆) and lead fluorosilicate
(PbSiF₆). When energized, the anode lead dissolves and purer lead precipitates at the cathode. Impurities such as gold,
silver, and bismuth remain in the anode sludge, which is an important precious metal resource. The cathode lead is
melted and cast into fine lead ingots (99.994%+).
Rising Power: Green Lead in Recycled Lead Smelting
Waste lead-acid batteries are the absolute main source of recycled lead (>85%), and its smelting is a model of
circular economy:
Pre-treatment: the art of dismantling and sorting
Crushing: Waste batteries are pulverized by a powerful crusher.
Sorting: Utilizing the difference of density, particle size and magnetism, efficiently separating:
Lead paste (heavy): Mainly containing PbSO₄, PbO₂, PbO (accounting for ~70% of lead content).
Lead alloys (heavy): grates, poles (containing Pb, Sb, Ca, Sn, etc.).
Plastics (light): Polypropylene (PP) cell shells, separators (recyclable).
Acid: Neutralized or recycled.
Lead paste smelting: the heart of the conversion
Mainstream - short kiln/rotary kiln smelting:
Lead paste is mixed with reductants (coke powder, coal), flux and smelted in a rotary kiln or short rotary kiln.
At high temperatures (~1200°C), the reduction reaction takes place: PbSO₄ + 2C → PbS + 2CO₂, PbS +
2PbO → 3Pb + SO₂; PbO₂ + C → PbO + CO, PbO + C → Pb + CO. Crude lead is produced.
The flue gas needs to be treated efficiently to recover SO₂ and to remove dust.
Explore - Wet Metallurgy:
Solid Phase Electrolytic Reduction: Lead paste is desulfurized (converted to PbCO₃ or PbO), and then directly
electrolytically reduced in alkaline or acidic electrolyte to obtain metallic lead.
Leaching-purification-electrolysis: Leaching of lead with specific solvents (e.g. citric acid, fluoboric acid),
purification of the solution and electrolytic deposition of lead. Cleaner, but scale and economics are still
being optimized.
Lead alloy component smelting:
Grids, etc. are usually melted in a reflector furnace, electric furnace, or converter, and the alloy lead is produced
directly after adjusting the composition.
Recycled lead refining:
The process is similar to primary crude lead refining (copper removal, impurity removal, etc.), but usually the impurity
spectrum is different (e.g., containing calcium and antimony), and the process needs to be targeted and adjusted.
The final output is refined lead or a specific grade of lead alloy.
Green smelting: the eternal battlefield of technological breakthroughs
The environmental challenges of lead smelting are centered on the treatment of waste gas (SO₂, lead dust), waste water
and solid waste (slag). Modern technology is building a tight defense:
Bronze wall of waste gas treatment:
SO₂ Recovery: High concentration SO₂ flue gas (>4%) generated from sintering/smelting is used to produce industrial
sulfuric acid by the mature double-rotation and double-absorption process, which realizes resource utilization. Low
concentration flue gas is desulfurized with high efficiency (limestone-gypsum method, sodium-alkali method, etc.)
to ensure compliance.
Particulate matter and lead dust control: High-efficiency electrostatic precipitator (ESP) removes coarse particles, and
baghouse dust collects fine particles and lead dust, and the emission concentration can be as low as <5mg/m³.
Control of non-organized emissions: airtight conveying, negative pressure operation, workshop ventilation and purification.
Wastewater closed-loop and deep purification:
Maximum recycling: Cooling water, slag flushing water, etc. are prioritized for recycling.
End treatment: Wastewater containing lead, cadmium and other heavy metals is treated by a combination of chemical
precipitation (e.g., lime, sulfide) + high-efficiency flocculation + multi-stage filtration (sand filtration, activated carbon
adsorption) + membrane technology (reverse osmosis) to ensure that it meets the standards for discharge or reuse.
Resource utilization of solid waste:
Smelting slag: after slow cooling and depletion treatment, it can be used as raw material for cement, road building
material or harmless landfill. Research and explore the further extraction of valuable metals (zinc, indium).
Anode sludge: a valuable by-product of electrolytic refining, an important raw material for extracting valuable metals
such as gold, silver, bismuth and antimony.
Recycled Plastics: PP plastics are cleaned and granulated for recycling.
Process innovation: moving towards cleaner and more efficient
Direct lead refining technologies: such as Kivcet, QSL and Osmert/Essa (top-blown submerged smelting) aim to combine
sintering and smelting, shorten the process, intensify SO₂ recovery, and reduce energy consumption and emissions.
Although not completely replacing blast furnaces, it represents the direction.
Oxygen-enriched combustion: Widely used to improve combustion efficiency, reduce flue gas volume, and lower
energy consumption.
Intelligent control: DCS system real-time optimization of process parameters (temperature, air flow, charging), to
improve efficiency, stability and environmental performance.
Conclusion: Furnace is not extinguished, evolution is not stopping
Fine lead ingots are glowing coldly on the conveyor belt, and the lab report shows that the purity far exceeds the
national standard. The engineer looks out of the window at the green trees and says to his young colleague, "This
furnace fire burns away pollution and refines responsibility. The standard of good lead has long been more than
just those few nines." From the ancient crucible to the intelligent smelting line, lead smelting in the flames
quenching itself. Increasingly stringent environmental regulations and the flourishing of the circular economy
are driving this change to a deeper level. Whether it is the purification of primary ore or the rebirth of used
batteries, the core proposition of modern lead smelting is not only to obtain the metal itself, but also how to
provide this indispensable basic material for the modern society with the smallest environmental footprint.
Every precise temperature control and every efficient sulfur capture is a solid step in the search for harmonious
coexistence between industry and nature. Lead smelting, a skill reborn from fire, is using science and technology
as a pen to write a green answer belonging to industrial civilization on the blueprint of sustainable development.