Gold electro-refining, as a key technology for modern precious
metal purification, is the core process for obtaining high purity
gold (Au≥99.99%). Against the backdrop of increasingly stringent
requirements for gold purity in the jewelry manufacturing,
electronics industry and financial reserves, electrochemical
refining technology has become the mainstream choice in the
global gold refining industry due to its excellent purification
efficiency and controlled process flow.
Electrolytic purification process principle
Electrorefining process is based on the physical characteristics of
electrochemical dissolution and deposition of metals, and realizes
the precise separation of impurity elements through the construction
of electrolysis system. Crude gold raw materials are melted and cast
into anode plates, which are jointly immersed with pure gold
cathodes into a special electrolytic solution. Under the action of
DC electric field, the gold at the anode enters the solution in an
ionic state, while the impurity metals are retained in the anode mud
due to the difference in oxidation potential. Gold with a purity of
99.99% or more is deposited directionally on the surface of the
cathode to form a refined gold ingot that meets LBMA standards.
The design of the electrolyte system directly affects the purification
efficiency, and a mixture of chloroauric acid (HAuCl₄) and hydrochloric
acid is usually used. The concentration of the solution needs to be
precisely controlled in the range of 80-120g/L for gold ions and
3-5mol/L for free hydrochloric acid, and phosphate buffers are added
to maintain the pH between 1.5 and 2.5. The temperature control
system keeps the electrolyte at a constant temperature of 45-65℃
to ensure the balance between the ion migration rate and the
deposition quality.
Details of refining process
Raw material pretreatment stage requires the initial purification of
crude gold by fire, through the high-frequency induction furnace
at a high temperature of 1200 ℃ melting, adding borax and other
slagging agents to remove most of the base metal impurities. The
molten metal is injected into the titanium alloy mold after vacuum
degassing, and cast into anode plates with a thickness of 15-25mm,
and its gold content needs to be raised to over 95% before entering
the electrolysis system.
The electrolyzer is made of polypropylene with anti-corrosion tank,
equipped with titanium-ruthenium-iridium composite anode frame
and pure silver cathode plate. The current density is set at 200-400
A/m² and the tank voltage is maintained in the range of 0.3-0.6V.
The anode dissolution cycle usually lasts 72-120 hours, and the
cathode is stripped when the thickness of the deposited layer reaches
3-5mm. During the process, the electrolyte gold ion concentration
needs to be monitored in real time, and the chemical balance is
maintained by replenishing chloroauric acid.
Anode mud treatment is an important part of the purification process,
the black mud rich in platinum, palladium and other precious metals
is leached by nitric acid, dissolved in aqua regia, and then separated
by ion exchange resin to realize multi-metal separation. The cathode
deposited gold is ultrasonically cleaned by deionized water,
remelted with high frequency under argon protection, and cast
into 1kg gold ingot in accordance with international standards,
and the total amount of impurities detected by GDMS is less
than 50ppm.
Analysis of key technical advantages
Compared with the traditional chemical refining method, the
electro-refining process has significant purity advantages. The
potential selectivity of the electrolysis process can effectively
separate silver (Ag), copper (Cu) and other associated metals,
and the removal rate of palladium (Pd), rhodium (Rh) and other
platinum group metals is over 99.8%. The refined gold ingots
are detected by glow discharge mass spectrometry, and the
content of non-precious metal impurities can be controlled
below 0.001%, which meets the requirements of aerospace-grade materials.
The closed design of the process greatly reduces the risk of
environmental pollution. The recycling rate of electrolyte reaches
more than 98%, and the waste gas treatment system effectively
controls chlorine gas emission through three-stage lye absorption.
Compared with the wet refining process, the consumption of nitric
acid per unit of gold production is reduced by 76%, and the
wastewater discharge is reduced by 90%, which is in line with the
European Union REACH environmental protection standards.
Intelligent equipment control improves production stability: DCS
system adjusts current density and electrolyte temperature in real
time to control cathode deposition rate fluctuation within ±2%.
The on-line XRF inspection device automatically analyzes the
purity of gold layer every 30 minutes, and with the mechanical
arm to realize the precise pick and place of cathode plate, the
whole system can realize uninterrupted operation for 120 days.
Process Innovation and Industry
Application
The application of new pulse electrolysis technology significantly
improves current efficiency. By setting a pulse frequency of
100-1000Hz, the grain size of the cathode surface is refined from
50μm in the traditional process to less than 10μm, and the density
of the deposited layer is increased by 30%. After a refinery adopted
this technology, the unit energy consumption was reduced by 22%
and the anode sludge generation was reduced by 18%.
In the field of electronics industry, electrorefined gold has broken
through the threshold of 6N level (99.9999%) purity. Through
ultra-pure electrolyte preparation and nano-filtration technology,
the sodium (Na) and potassium (K) content of semiconductor
bonding alloy wires is stably controlled below 0.1ppb. Aerospace-grade
gold target adopts vacuum electrolytic deposition process, and the
product density reaches the theoretical limit value of 19.31g/cm³.
The modularized design of process equipment promotes the
upgrading of the industry. The processing capacity of the standard
electrolysis unit is flexibly configured from 50kg/batch to
500kg/batch, and the precious metal recovery system can
synchronize the processing of four metals: gold, silver, platinum
and palladium. A new type of mobile refining unit integrates
electrolysis, testing and ingot casting functions, and has realized
one-stop purification of gold purity from 85% to 99.99% at the
mining site.
Quality control system construction
The raw material traceability system records the melting parameters
of each anode plate through laser coding technology, and the X-ray
fluorescence spectrometer (XRF) completes the screening of 32
elements at the casting stage. The electrolysis process establishes a
three-dimensional quality model, predicts the uniformity of cathode
deposition through big data analysis, and adjusts the spacing of the
pole plates and current distribution in a timely manner.
Finished product testing adopts inductively coupled plasma mass
spectrometry (ICP-MS) and fire assay gold method for dual verification,
and 41 impurity elements are trace analyzed. The internal quality of
gold ingots is detected by ultrasonic flaw detection to ensure that there
are no defects such as porosity and slag entrapment. Each batch of
products comes with an exclusive digital certificate, and the blockchain
traceability system can query the data of the whole refining process.
With breakthroughs in clean energy technology, innovative processes such
as photovoltaic electrolysis and bio-electrolysis are in the testing stage.
The new ionic liquid electrolysis system can operate at room temperature,
reducing energy consumption by 40%. In the field of precious metal
resource recycling, electro-refining technology is promoting the recovery
rate of gold from e-waste from 75% to over 95%, providing key technical
support for sustainable development.