Silver Recycling - Phoenix Refining

Silver has played a pivotal role in human civilization for over 6,000 years. Its historical significance is matched by its growing importance in today's world, where it finds applications in electronics, electrical equipment, photography, medicine, and wastewater treatment. Despite being more affordable than gold or platinum, silver is not as abundant in the earth's crust. It is typically found in ores mixed with gold, copper, lead, and zinc.

In 2014, global silver production reached 877.5 million ounces, but the availability of natural silver resources is on the decline. This scarcity has led to increased production costs, even though the market price for silver has dropped due to its expanded use across various industries. With silver's increasing demand, there is a pressing concern about its environmental impact, particularly its toxicity to marine life, microorganisms, and humans. This has highlighted the urgent need for effective recovery methods to handle silver from industrial waste and spent sources.

Recyling silver

The recycling of silver-containing materials involves processing a wide range of sources, including ores, electronic materials, waste jewelry, catalysts, and more. The complexity of this process arises from the different physical and chemical characteristics of each source. Initially, materials undergo pretreatment, such as washing, crushing, and physical separation, to concentrate the silver content. These methods yield low-purity metals, which are then processed using recovery techniques like leaching, adsorption, or chemical reduction to produce crude silver. Finally, refining processes increase the purity of silver to over 99.9%.

Silver Recovery Techniques:

1. Hydrometallurgical Process:

This process involves a series of leaches using acids or bases to extract silver. The metal is separated and concentrated through techniques like precipitation, solvent extraction, and ion exchange. The final recovery is achieved using methods like electro-winning, cementation, or crystallization.

2. Bio-Hydrometallurgical Process:

An emerging and environmentally friendly method, bio-hydrometallurgy uses microorganisms to extract metals from materials. Bacteria such as Acidithiobacillus ferrooxidans and Chromobacterium violaceum can mobilize silver from ores and electronic waste. Techniques like bio-oxidation and bio-sorption are employed for extracting silver.

3. Leaching:

Leaching dissolves silver from various materials using agents like cyanide, ferricyanide, thiourea, and thiosulphate. Each leaching agent has its advantages in terms of efficiency, cost, and environmental impact. For example:

Cyanide Leaching: The most common method, cyanide is highly efficient for dissolving silver but poses environmental risks.
Ferricyanide Leaching: This is an alternative that effectively dissolves silver sulfide.
Thiourea Leaching: Commonly used for mobile phone waste, thiourea leaches silver in the presence of iron ions.
Thiosulphate Leaching: This method is environmentally friendly and effective, especially in the presence of ammonia.

List of substituted leaching agents for silver leaching from primary and secondary sources are presented in Table 1

Table 1. List of silver leaching agents from primary and secondary sources.
Leaching agents	Efficiency (%)	References
Thiosulphate, EDTA, Cu(II), ammonia	100	Puente-Siller et al. (2013)
Thiosulphate	99	Jana et al. (2005)
Sodium thiosulphate, ammonia and sodium sulfite	97–99	Kononova et al. (2007)
Thiourea, sulfuric acid and ultrasound	98.6	Salim et al. (2005)
Nitric acid	96.1	Holloway et al. (2004)
Thiourea	95.6	Jana et al. (2008)
Thiosulphate and copper sulfate	95.1	Cui et al. (2011)
Ammonium chloride	95	Shaohua et al. (2011)
Hydrochloric acid and calcium chloride	92	Qing-hua et al. (2012)
Ammonium thiocyanate + oxygen pressure	88	Shenghai et al. (2011)
Sodium cyanide and hydrogen peroxide	87	Bahr (1990)
Sulfuric acid and hydrogen peroxide	85	Tao et al. (2003)
Sodium chloride, HCl and H2O2	82	Zhang et al. (2012)
Hydrochloric acid–hydrogen peroxide	80	Vazarlis (1987)
Chloride	60–80	Almeida and Amarante (1995)
Acidic thiourea-ferric iron	71.36	Ali et al. (2013)
Ferric-EDTA-thiosulphate	70	Jared and David (2013)
Thiourea and Fe3+	50	Li et al. (2012)
Bromination	50	Sylwia et al. (2013)
Ferricyanide and sodium cyanide	Unknown	Feng and David (2007)
Acidothiourea	Unknown	Manju et al. (2013)
Copper–ammonia-thiosulphate	Unknown	Jared and David (2013)
Aqueous ozone	Unknown	Viñals et al. (2005)
Ferric sulfate and sulfuric acid	Unknown	Morrison (1989)
Thiosulphate, ammonia and cupric ion	Unknown	Briones and Lapidus (1998)
FeCl3-HCl and Fe2(SO4)3-H2SO4	Unknown	Dutrizac (1994)
Thiourea, sulfuric acid and ferric ions (Acidothioureation)	Unknown	Chen et al. (1980)
Ammoniacal, bromine, iodine and hydrogen peroxide	Unknown	Starovoytov et al. (2007)
Hydrochloric acid and sodium chloride	Unknown	Nuñez et al. (1985)
Oxygenated aqueous ammonia	Unknown	Gryboś and Samotus (1983)
Thiocarbamide	Unknown	Lodeitshikov et al. (1995)
Acidic ferric chloride and copper chloride	Unknown	Barbara (1988)
Acidithiobacillus ferrooxidans (A. ferrooxidans)	98	Umesh and Hong (2013)
Mesophilic bacteria	Unknown	Pooley and Shrestha (1996)
Cyanogenic Chromobacterium violaceum, Pseudomonas fluorescens, and P. plecoglossicida	Unknown	Helmut et al. (2008)

Conclusion

Silver recycling is a complex but necessary endeavor, driven by both economic and environmental imperatives. Understanding the toxicity of silver and exploring innovative recovery methods are essential steps toward achieving effective and sustainable silver recycling. By advancing research and adopting safer technologies, we can better manage these precious resources and mitigate their environmental impact.

At Phoenix Refining, we are committed to leading the way in silver recycling with cutting-edge solutions designed to maximize recovery while minimizing environmental harm. Our state-of-the-art technologies and expert team ensure that we efficiently recover silver from both primary and secondary sources, adhering to the highest standards of safety and sustainability. By partnering with us, you can contribute to a greener future and make the most out of your silver resources. Visit Phoenix Refining to learn more about our innovative recycling solutions and how we can support your sustainability goals.

Syed, S. (2016). Silver recovery aqueous techniques from diverse sources: Hydrometallurgy in recycling. Waste management, 50, 234-256.