In recent years, the demand for high-purity silver remains robust due to the surge in electric and electronic equipment (EEEs). Although silver demand has outpaced mining production over the past four years, the global supply has managed to nearly meet this demand annually, thanks to effective silver recovery and refining practices.
Silver recycling plays a crucial role in satisfying future demand, particularly as the overall demand for metals escalates worldwide alongside technological advancements. The production of high-purity silver involves a range of methods and processes, including silver electro winning and electro refining. Exploring the intricacies of these processes and anticipating the future of silver production can provide valuable insights into this industry.
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Value of silver:
Silver, a precious metal valued at $25.21 USD/ozt as of December 18, 2023, serves a multitude of purposes, from adorning jewelry and silverware to enabling advancements in electronics and sustainable energy.
This versatile metal acts as a catalyst in the production of key industrial chemicals like ethylene oxide and formaldehyde, with the former consuming 10 million ounces of silver annually.
In the realm of renewable energy, silver plays a significant role in photovoltaic technology, with the solar panel industry consuming over 83 million ounces of silver in 2023. Leveraging its exceptional conductivity, silver finds widespread application in electric and electronic equipment, including printed circuit boards in electronic devices like phones, computers, and TVs, as well as membrane switches and RFID tags.
Despite the small quantities of silver required in these devices, it contributes significantly to their intrinsic value, prompting extensive recycling efforts and research into extraction methods. Due to the low silver content in silver ores, various extraction techniques have been developed to recover silver from electronic devices.
High- purity silver production:
- In 2023, 1016 million ounces of silver were mined out of the total supply of 1007 million ounces.
- Approximately 25% of silver is recovered from silver ores, while the remaining 75% is obtained as a by-product of metals like gold, copper, lead, and zinc.
- Copper Concentrates:
- Copper sulfide concentrates are smelted to produce blister copper, which contains 97-99% of the original silver content.
- During electrolytic refining, slimes containing impurities like silver are collected and smelted to obtain doré, a mixture of gold, platinum group metals, and silver.
- Doré is then electrolyzed to yield high-purity silver.
- Gold Concentrates:
- Cyanide leaching is commonly used to extract gold from ores.
- Gold recovery involves processes like the Merrill-Crowe zinc precipitation method or adsorption on activated carbon.
- The Merrill-Crowe process includes oxygen removal, addition of zinc powder, gold precipitate recovery, and smelting into gold doré bars.
- Silver Recovery:
- Silver can be leached using cyanide and recovered using methods similar to those for gold.
- Electrowinning, especially with emew technology, is an efficient and cost-effective alternative for silver recovery post-cyanide leaching.
- Lead Concentrates:
- Lead sulfide concentrates are roasted and smelted to produce lead bullion.
- Impurities like antimony, arsenic, silver, and tin are removed through various processes, including the Parkes process for silver removal.
- In the Parkes process, zinc is added to a molten lead/silver mixture, and the silver becomes concentrated in the zinc crust due to its higher solubility in zinc.
- The remaining lead-gold-silver residue undergoes cupellation at high temperatures to remove impurities, resulting in a high-purity gold-silver alloy.
- The gold is separated from the refined silver through a process called ‘parting’, involving digestion with nitric acid to dissolve the silver and precipitate it as silver chloride.
- Zinc Concentrates:
- Zinc sulfide concentrates are roasted and leached with sulfuric acid, dissolving most of the zinc.
- A residue containing 5-10% zinc, along with impurities like gold, lead, and silver, is formed.
- The residue is melted to create a slag, and through a process called slag fuming, zinc is vaporized, lead is converted and dissolves silver and gold.
- The metallic lead bullion obtained is refined to recover high-purity silver using the Parkes process.
Recycling Silver:
- Global Silver Usage in 2016:
- Approximately 55% of silver was utilized in industrial fabrication, including the photographic industry and Electronic and Electrical Equipment (EEEs).
- Just over 25% of silver was used in the manufacturing of jewelry and silverware.
- Recycling Methods:
- In the photographic industry, silver can be recycled from spent processing solutions through electrolytic methods.
- High-grade jewelry scrap can be re-alloyed, and silver can be recycled on-site by collecting and smelting the fine dust generated during polishing, known as ‘jewelry sweeps’.
- For low-grade silver scrap with minimal value, it is often returned to a smelter for processing.
- Methods like cyanidation, commonly used for gold recycling, are typically not cost-effective for silver scrap recycling.
Silver electrorefinning and silver purification:
The process of electrolytic refining of silver involves using anodes composed of crude silver and gold, with a specific composition. Anodes are created by melting silver and gold bullion in a furnace.
Thin sheets of 1000 fine silver serve as cathodes, where crystallized silver powder collects and is later removed for further processing. The silver powder is washed to eliminate impurities and acid, then melted down into bars.
The electrolyte used contains silver nitrate, free nitric acid, and liquid glue to enhance silver deposition on the cathode. To prevent silver from adhering to the cathodes, a “dope” solution is applied, consisting of silver nitrate, copper nitrate, and hydrochloric acid.
Anodes are replaced when silver depletion reaches around 10%, leaving behind crude gold for subsequent electrolysis.
Silver electrowinning:
Electrowinning is a highly effective method for silver purification and recovery, particularly from solutions containing silver, such as those from the photographic industry or in photovoltaic recycling. In the Merrill-Crowe process, gold and silver are precipitated from solution using zinc based on their nobility, as they prefer to remain in their metallic states. Using the principle of reduction potential, gold and silver can be electrowon from solution into high-purity products.
Emew technology enables the recovery of silver to a purity of up to 99.999%, even in the presence of base metals, effectively rejecting impurities like cadmium, copper, and lead. The use of emew polishing cells allows the electrolyte to be depleted to less than 10 ppm. This technology offers the advantage of recovering high-purity silver from various solutions, such as caustic cyanide PLS, nitric acid electrolyte, refinery by-products, effluent streams, scrap silver, and plating baths.
A notable precious metals refinery in India produces over one tonne per day of high-purity silver using emew technology, depleting silver in the electrolyte to less than 5 ppm and eliminating silver chloride precipitation. Direct electrowinning can be applied to a wide range of electrolytes containing silver, with emew technology capable of producing 99.99% silver from an electrolyte with as low as 10 g/L silver, providing a significant advantage over traditional silver refining methods.
The recovery of silver and silver recycling employs various methods such as the Parkes process, cupellation, parting, slag fuming, and electrolysis (electrorefining and electrowinning). These techniques are not only used for silver recovery from low-grade mineral ores but can also be applied to Electronic and Electrical Equipment (EEE) recycling, including solar panels and circuit boards. The recycling of silver from EEEs is becoming increasingly critical as metal demands rise due to technological advancements.
In the United States, the decreasing costs of new solar power plants compared to traditional energy sources like coal, natural gas, and nuclear power plants indicate a growing market for solar panel recycling. Research studies have shown that up to 95% of silver and copper can be recovered from spent solar cells, with a standard silicon solar cell module containing approximately 6 grams of silver. The global solar panel recycling market was valued at 63.8 million USD in 2016, with a projected compound annual growth rate (CAGR) of 22.1% until 2025, reflecting the potential for a prosperous recycling market in the near future.
According to the US Geological Survey, global known silver reserves total 530,000 tonnes, with 885.8 million ounces mined in 2016. At this extraction rate, the world’s silver reserves would be depleted in 20 years. To address this challenge, leveraging technologies like emew for silver recovery from various ores and Waste Electrical and Electronic Equipment (WEEE) will be crucial in meeting future silver demand. By utilizing emew technology after cyanide leaching or other concentrate leaching processes, it is possible to recover silver from a wide range of concentrations, ensuring a sustainable supply of high-grade silver for the years ahead.