In the field of non-ferrous metal smelting, the fire refining process
for lead bullion has become an important technical means for
purifying precious metals due to its high efficiency and wide
adaptability. Especially when dealing with crude lead or lead
alloy containing copper, tin, silver and other impurities, fire
refining can realize directional separation and enrichment of
impurities through multi-stage oxidation, sulfidation and
melting precipitation reaction. In this paper, we will analyze
the core process and key technology of this process, and
provide reference for industry practitioners.
First, the underlying logic of the fire
refining process
Copper, tin, silver and other impurity metals in lead gold
nuggets can be selectively separated by temperature gradient
and redox reaction due to the differences in their
physicochemical properties. Example:
Copper: low solubility in liquid lead and easy to combine with
sulfur to generate copper sulfide float;
Tin: higher oxidation tendency than lead, preferentially oxidized
to SnO₂ into the slag phase;
Silver: needs to be extracted by higher oxidation potentials or
specific melts.
The core of the process design is to control the melting temperature,
oxygen potential and additives in stages, step by step “snip” the target
impurities, and ultimately obtain the purity of 99.9% or more of
the refined lead.
Second, the key process steps
1. Preliminary melting and copper removal
After the crude lead raw material is melted (the temperature is controlled
at 450-500℃), sulfur or pyrite (FeS₂) is added first. Elemental sulfur reacts
with copper to form copper sulfide (Cu₂S), whose density is much lower
than that of the lead liquid, forming a floating slag layer. More than 90%
of copper impurities can be removed by mechanical slagging or
decantation. This stage requires strict control of the amount of sulfur
added to avoid excessive sulfur residue affecting the subsequent process.
2 oxidative refining tin
Raise the temperature to 600-650 ℃ and drum into the air or
oxygen-enriched body, tin priority oxidation for SnO₂:
Sn + O₂ → SnO₂
The oxidized slag floats on the surface of the lead liquid in a porous
form and is removed by a continuous skimming device. This stage
requires precise control of the oxygen content to prevent excessive
oxidation of lead resulting in metal loss.
3. Extraction and deep purification of silver
The removal of silver requires the synergistic effect of **alkaline melting
agents (such as NaNO₃, NaOH) or lead oxide (PbO)**:
Alkaline oxidation: By adding sodium nitrate (NaNO₃) and sodium
hydroxide (NaOH), silver is oxidized to Ag₂O and reacts with NaNO₃
to form soluble silver nitrate, which is subsequently recovered by
electrolysis or displacement;
Parkes Process: Zinc metal is added to generate silver-rich zinc shells
(Ag-Zn alloy) by taking advantage of the high solubility of silver in
zinc, and silver ingots are obtained after separating the zinc by distillation.
4. Final refining and impurity monitoring
After multi-stage treatment, the lead liquid enters the final refining stage.
The generation of lead oxide is reduced by vacuum degassing or flux
covering, and a spectrum analyzer is used to monitor the impurity
content in real time to ensure that the purity of the finished lead
product meets the standard.
Advantages of the process and technological breakthroughs
Energy consumption and cost optimization
Staged temperature control technology reduces fuel consumption
by more than 20%;
Sulfurization-oxidation synergistic process increases copper and
tin removal rate to 98%;
Environmental protection upgrade
Copper, tin and silver enriched in the floating slag can be recovered
twice, realizing resource recycling;
Closed flue gas treatment system effectively captures lead dust and SO₂,
and the emission compliance rate is over 99%.
Automatic control
Online monitoring system dynamically adjusts melting parameters,
reducing the risk of manual intervention;
Intelligent slag removing robot reduces metal entrapment loss.
Fourth, industry applications and future trends
At present, the process has been widely used in renewable lead smelting,
lead storage battery recycling and multi-metal symbiotic ore treatment.
With the tightening of environmental regulations, the future iteration of
the technology will focus on:
Low-carbon smelting: Hydrogen fuel replaces traditional coal combustion,
reducing carbon footprint;
Deep separation of impurities: development of new melting fluxes to
achieve simultaneous removal of trace impurities such as antimony and bismuth;
Digital twinning: predicting slag phase composition through AI models
to optimize process parameters.
Conclusion
Through precise chemical reaction design and engineering control, the fire
refining process of lead nugget “separates” the impurity metals, and realizes
the efficient recovery of resources while improving the purity of lead. With
the popularization of clean production technology, this process will continue
to play a core value in the field of precious metal refining and promote the
industry to upgrade in the direction of greening and intelligence.