As a key chemical raw material for electronic, medical and photographic industries, the precision
of silver nitrate preparation directly affects high-end processes such as semiconductor bonding
and film imaging. The global annual demand for silver nitrate is more than 3,800 tons, of which
75% is prepared through silver scrap recycling. This article will analyze the three core processes
of silver nitrate refining, technical difficulties and industrial upgrading trends, revealing the
manufacturing logic behind this white crystal.
Technical route of silver nitrate preparation
1. Chemical precipitation method - the upgrading of the traditional process breakout
Raw material pretreatment: silver waste (anode mud, waste catalyst, etc.) dissolved in nitric acid
(concentration 8-12 mol/L), control the reaction temperature of 60-80 ℃, the silver conversion
rate of > 99.5%;
Purification and removal of impurities: add sodium chloride to precipitate silver ions
(Ag⁺+Cl-→AgCl↓), and then selective reduction with ammonia-hydrazine hydrate system, lead,
copper and other impurities removal rate of > 98%;
Acidification and crystallization: concentrated nitric acid secondary acidification to pH ≤ 1,
vacuum rotary evaporator concentration to the solution density of 1.6g/cm³, cooling precipitation
needle crystals, purity up to 99.95%.
The process equipment investment is low (500,000 yuan / hundred tons of production line), but
the silver recovery rate of only 92-94%, gradually replaced by efficient methods.
2. Electrolysis method - the technological benchmark of the high-purity route
Electrolyte configuration: silver nitrate solution concentration control at 200-300g/L, free nitric acid
content of 5-8%, the temperature is maintained at 40-50 ℃;
Plate design: titanium-based platinum-plated anode (current density 800-1000A/m²) and pure silver
cathode spacing 30-50mm, tank voltage 2.5-3.2V;
Crystal growth: pulse current technology (on-off ratio 1:3) is adopted to reduce the risk of dendrite
generation and obtain cubic crystals (0.2-0.5mm) with uniform particle size and purity exceeding 99.99%.
Electrolysis silver direct yield as high as 98.5%, but higher power consumption (ton of product power
consumption of 3200kWh), suitable for pharmaceutical grade silver nitrate production.
3. Solvent extraction method - a breakthrough in green transformation
Extraction system construction: using bis (2-ethylhexyl) phosphate (D2EHPA) - kerosene mixed solvent,
the extraction rate of silver ions at pH = 2.5> 99%;
Back-extraction crystallization: back-extraction with 6 mol/L nitric acid, silver transfer efficiency of 97%,
mother liquor recycling 5 times after the impurity content is still less than 50ppm;
Tail liquid treatment: integration of ion exchange resin, to achieve copper, nickel and other metals
recovery, wastewater silver residue <0.1mg/L.
The process of silver comprehensive recovery rate increased to 99.2%, acid and alkali consumption
reduced by 40%, is becoming the mainstream technology.
Key control points of the preparation process
1. Accurate removal of impurity elements
Copper-nickel separation: add thiourea (0.5-1g/L) to form a complex, selective precipitation at pH=4;
Platinum group metal interception: chelating resin column adsorption of palladium and rhodium plasma
with saturation adsorption up to 120mg/g;
Trace iron control: ammonium oxalate co-precipitation to reduce Fe³+ concentration to below 0.3ppm.
2. Directional regulation of crystal morphology
Temperature gradient method: the bottom of the crystallization kettle was maintained at 45°C, and the top
was cooled to 5°C to form a vertical temperature field, and the crystal L/D ratio was optimized from 5:1 to 2:1;
Ultrasonic assistance: 40kHz ultrasonic waves crushed the crystal nuclei, which reduced the standard
deviation of crystal size from 0.15mm to 0.06mm;
Additive screening: polyvinylpyrrolidone (PVP) inhibits the growth rate of crystal surface, obtaining high
packing density (1.35g/cm³) products.
3. Full chain protection of production safety
Exhaust gas treatment: two-stage lye spray tower (NaOH concentration 10%) increases the nitric acid mist
capture rate to 99.8%;
Explosion-proof design: the reaction kettle is equipped with rupture discs (operating pressure 0.35MPa)
and nitrogen inerting system;
Intelligent monitoring: infrared thermal camera real-time detection of crystallization kettle temperature
field, temperature difference over 5 ℃ automatic alarm.
Industry pain points and technical attack direction
1. Adaptability of raw materials
Sulfur-containing silver sludge (S>5%) produces H2S when dissolving, and needs pre-oxidation treatment
(H2O2 addition of 3-5%);
Silver-resin composites need microwave breaking (800W treatment for 10 minutes) to release the
encapsulated silver particles;
Develop silver chloride direct ammonia leaching technology (NH3 concentration 6mol/L), skipping the
reduction and melting process.
2. Equipment corrosion and life bottleneck
Hastelloy C276 reactor corrosion rate in concentrated nitric acid ≤ 0.05mm / year, but the cost is 8 times
that of 316L stainless steel;
PTFE coated stirring paddle at 80 ℃ service life of only 6 months;
New silicon carbide ceramic evaporator corrosion resistance increased by 3 times, is a pilot application.
3. Environmental protection regulations force process upgrading
The unit consumption of nitric acid is reduced from 2.1 tons/ton of product in the traditional process to 1.4 tons;
The silver recovery rate was increased from 92% to 99.5%, reducing silver loss by 18 tons per year;
Waste acid regeneration system (MVR evaporation + catalytic oxidation) makes acid reuse rate over 85%.
Three major trends of industrial upgrading
1. Continuous production instead of intermittent operation
Tubular reactor realizes continuous operation of the whole process of silver dissolution-purification-crystallization,
and the production capacity is increased by 40%;
Online particle size analyzer (laser diffraction method) automatically adjusts the crystallization parameters, and
the product qualification rate is >99.9%;
After the renovation of a precious metal refinery, the labor cost of tons of products was reduced from RMB
1,200 yuan to RMB 400 yuan.
2. Deep penetration of intelligent control system
The digital twin platform simulates the crystallization kinetics of silver nitrate and optimizes the
combination of process parameters;
Machine vision recognizes crystal morphology and automatically sorts submicron defective products;
Blockchain technology traces the source of silver raw materials to meet the requirements of EU
conflict minerals regulations.
3. Closed-loop construction of resource recycling system
The silver recovery rate of electronic waste is increased from 75% to 95%, and 1.2kg of silver can be extracted
from each ton of waste circuit boards;
Breakthrough in silver recovery technology for medical X-ray film, and 60% reduction in treatment cost
of fixing solution;
The purity of regenerated silver nitrate from photovoltaic silver paste waste reaches 99.99%, which
has been commercialized.
Conclusion
The evolution of silver nitrate preparation technology is essentially the history of the improvement of precious
metal resource utilization efficiency. From the early workshop-style silver ingot nitration to today's intelligent
continuous production line, every 1 percentage point increase in the silver recovery rate is equivalent to 38
tons of new annual global silver supply (valued at 230 million yuan). Driven by the dual-carbon strategy, the
green preparation process, high-value regeneration of waste materials, and digital quality control are forming
a “technology triangle” to drive the industry toward the goal of “zero-waste”. When the demand for
photovoltaic silver paste is growing at 12% per year, and when flexible electronics put forward new
requirements for ultra-fine silver nitrate powder, this seemingly traditional chemical technology is
being revitalized in the field of high technology.