Crude tin refining is the core link in the tin metallurgy industry chain,
directly affecting the quality of the final tin products and comprehensive
economic benefits. With the electronic solder, photovoltaic welding tape,
tinplate and other high-end areas of tin purity (≥ 99.95%) requirements
continue to improve, as well as indium, bismuth and other strategic
metal resource value of the highlight, the modern crude tin refining
process is towards the direction of high-efficiency separation,
resource recycling, and intelligent control of the direction of the
continuous upgrading.
First, the technical value and necessity
of crude tin refining
Tin concentrate by reducing and smelting the output of crude tin
usually contains impurities 3% -8%, mainly including iron (0.5% -2%),
arsenic (0.1% -0.5%), copper (0.3% -1%), bismuth (0.05% -0.3%) and
trace indium, silver and other elements. These impurities will not
only reduce the conductivity, ductility and other physical properties
of tin, but will also trigger problems such as grain boundary corrosion
and solder brittleness in subsequent processing. Through the refining
process, not only can the purity of tin to 99.9% or more, but also to
achieve indium (electronic screen core materials), bismuth (low
melting point alloy), copper (industrial raw materials) and other
valuable metal directional recycling, so that the comprehensive
production cost reduction of 20% -35%.
Second, crude tin refining mainstream
process technology analysis
1. molten precipitation method in addition to iron and arsenic
The use of tin and iron, arsenic melting point differences (tin 232 ℃,
iron 1538 ℃, arsenic 817 ℃), in the 280-320 ℃ melting furnace, high
melting point impurities precipitated in the form of crystals. By
tilting the furnace body graded liquid release, can be separated
from the hard head slag containing 30% -50% of iron, arsenic
15% -25%, so that the iron content in the crude tin down to less
than 0.05%. The process energy consumption is as low as
50kWh/ton tin, with electromagnetic stirring technology can
improve the separation efficiency of 40%.
2. Centrifugal filtration to remove copper and antimony
Titanium alloy centrifuge is used to carry out multi-stage
separation at 240-260℃, utilizing the difference in density of
copper-antimony compounds (Cu6Sn5, SbSn) to realize gradient
removal. By controlling the rotational speed (1200-2000 rpm)
and temperature field distribution, the copper content can be
reduced from 0.8% to 0.003% and the antimony content from
1.2% to 0.01%. The centrifugal residue of copper enrichment
of 60%, can be directly used as raw materials for copper smelting.
3. Vacuum distillation of bismuth
Under the condition of 0.1-10Pa vacuum and 900-1000℃,
bismuth (melting point 271℃) is preferentially recovered by
gasification and condensation. The five-stage condensation
system collects bismuth hierarchically according to the
temperature gradient (600℃→200℃), and obtains crude
bismuth of 99.5% purity, with a recovery rate of over 98%.
The technology synchronizes the removal of lead, zinc and
other volatile impurities, so that the bismuth content in tin is ≤5ppm.
4. Electrolytic refining ultimate purification
Take crude tin as anode and high purity tin as cathode,
electrolyze in silicofluoric acid electrolyte (H2SiF6 80-120g/L,
Sn²+ 25-40g/L). Under the condition of current density
120-180A/m² and tank voltage 0.3-0.6V, the cathode deposits
tin with a purity of 99.99%, and silver, indium and other
precious metals are enriched in the anode mud. The use of
pulse current (frequency 50-100Hz) can inhibit the growth
of dendrites, current efficiency increased to more than 92%.
Valuable metal synergistic recovery
technology breakthrough
1. High-efficiency extraction of indium
Electrolytic anode mud is leached by sulfuric acid (pH=1.5,
80℃), and the indium leaching rate is over 95%. The
solution is selectively enriched by P204 extractant, and
indium sponge with 99.9% purity is obtained through
back-extraction and replacement. The innovative
introduction of ionic liquid extraction system reduces
indium recovery cost by 30%.
2. Bismuth refining upgrade
After the crude bismuth is oxidized and refined (600℃
oxygen blowing) to remove arsenic and tellurium,
99.995% refined bismuth is obtained by electrolysis (BiCl3-HCl
system). The new diaphragm electrolysis technology reduces
DC power consumption to 800kWh/ton, which is 45%
energy-saving compared with the traditional process.
3. Copper recycling
Centrifugal slag and melt precipitation slag are crushed and
flotation to obtain copper concentrate (Cu≥25%), which
enters the copper smelting system. Wet leaching-electrowinning
process can directly produce copper cathode with metal
recovery rate over 97%.
IV. Innovative application of intelligent
refining system
1. Digital twin process optimization
By constructing a three-dimensional simulation model of the
refining process, it simulates the impurity migration path and
metal phase change law in real time. After the application of
an enterprise, the melting temperature fluctuation is reduced
from ± 15 ℃ to ± 3 ℃, and the tin content of hard head slag
is reduced from 12% to 5%.
2. Machine vision impurity monitoring
Hyperspectral imaging system (wavelength 400-2500nm) online
analysis of melt surface composition, within 0.5 seconds to
identify the distribution of iron and arsenic impurities, to guide
the centrifuge speed dynamic adjustment.
3. Energy closed-loop management system
Waste heat recovery device converts the refining furnace flue
gas (400-600℃) into steam to drive the vacuum pump, which
reduces the comprehensive energy consumption of the system
by 25%. The proportion of photovoltaic DC power supply in
the electrolysis workshop is 30%, with an annual carbon
reduction of over 5,000 tons.
V. Green refining and sustainable development
1. Resourceful treatment of exhaust gas
After the smelting flue gas is removed by bag filter (99.9% efficiency)
and absorbed by lye (95% desulfurization rate), the carbon dioxide
capture system converts it into dry ice, which is used to cool down
ingot casting process.
2. Zero wastewater discharge system
Acid leaching wastewater recovers 80% sulfuric acid through diffusion
dialysis membrane, and concentrated water produces sodium sulfate
by-product through evaporation and crystallization. The reuse rate
of terminal water is over 98%, reaching GB/T 31962-2015 standard.
3. High-value utilization of solid waste
Refining slag (containing 2%-5% tin) is reduced and smelted to
produce recycled tin ingots, and the residue is made into permeable
bricks (compressive strength ≥ 30MPa), which are used in the
field of green building.