Silver refining

What is silver refining?

Silver refining is typically the final process in the production of high-purity silver suitable for sale in the market.  It is associated with purity and typically needs to meet a minimum of 99.9% purity and in most cases 99.99% purity and in some cases, 99.999% purity depending on the end-use.

The silver can be sold in many different forms in this purity such as bars, powder, granules and wire – in most cases the physical form of the silver process occurs after the silver refining process. However, there are different silver refining steps depending on the source of silver. Thus, it is important to understand the sources first before describing the silver refining processes.

Source: https://www.silverinstitute.org/mine-production

The two main sources of silver can be split into two categories:

1. Mined silver – called primary silver production
2. Recycled silver – called secondary silver
   

Source: Metals Focus, Bloomberg, Silver Institute

While different processing methods are used to concentrate the silver to a point where it can be refined, the ultimate refining step may be customized to process the silver feed from each type of source depending on the composition.

First, we need to understand what silver refining is. Different from other processes, such as calcinating or smelting, during the refining the chemical composition of the source metal doesn’t change and in most cases, will remain the same but the refining process will lead to a purer metal. Usually, the process of refining is carried out using hydrometallurgical processes and less commonly pyrometallurgical processes.

When we are talking about silver refining, we refer to the process of silver purification. Depending on whether the major metal is copper, gold, lead, or zinc, we decide on how the silver is going to be refined and what exact metallurgical process will be applied.

How is silver refined?

From Copper concentrates

Copper sulfide concentrates are smelted, resulting in blister copper, which contains approximately 97-99% of the silver that was in the original concentrate. The blister copper is then electrolytically refined and ‘slimes’ accumulate on the anode or at the bottom of the refining tank.

These slimes contain insoluble impurities, like silver. The slimes are collected and smelted, oxidizing all the metals except for gold, platinum group metals (PGMs), and silver. The metal that is recovered is called doré, which typically contains 0.5-5% gold, 0.1-1% PGMs; with the balance as silver. The doré metal is cast into anodes and electrolyzed in nitrate solution to obtain high-purity silver.

From Gold and Silver concentrates and ores

Cyanide leaching is often used to recover gold from its ores. Fine gold particles dissolve easily in cyanide, typically using NaCN concentrations of 0.02-0.05%; if the dissolved oxygen content of the solution is not high enough, aeration may be required.

4 Au + 8 NaCN + O2 + 2 H2O → 4 NaAu(CN)2 + 4 NaOH

Once dissolved in the cyanide, the gold must be recovered from the pregnant cyanide solution. Often, the Merrill-Crowe zinc precipitation process is used, or the adsorption of gold onto activated carbon.

The steps in the Merrill-Crowe process are oxygen removal, mixing in fine zinc powder, and gold precipitate recovery by filtration. The addition of zinc leads to the formation of a zinc cyanide complex and gold metal.

2 Au(CN) + Zn → 2 Au + Zn(CN)42-

Sulfuric acid is used to dissolve any zinc impurities that have precipitated with the gold. The final gold solids are smelted into gold doré bars.

Silver is also easily leached using cyanide and can be recovered using the same methods employed for gold recovery, as described above. However, electrowinning has proven to be an economical alternative, even more so when utilizing emew technology. Electrowinning can be used directly after the cyanide leach, resulting in fewer process steps, and therefore lower operating costs.

From Zinc concentrates

Zinc sulfide concentrates are roasted and leached with sulfuric acid. The sulfuric acid leach dissolves the majority of the zinc, leaving 5-10% in the residue, along with the impurities of gold, lead, and silver. The residue is either sold to lead smelter where silver recovery occurs in the same way as from lead concentrates or is fed to a fuming furnace. In the fuming furnace carbon as powdered coke or coal reduces the zinc to metal which vaporizes and separated from the gases leaving the furnaces as impure zinc oxide. More than 50% of the silver follows the zinc. The zinc oxide is returned to a zinc smelter and collected into a combined lead-silver residue that sold to a lead smelter.

From Lead concentrates

Lead sulfide concentrates are roasted and smelted to form lead bullion. The various impurities including antimony, arsenic, silver, and tin are removed by various different processes; silver is removed by the Parkes process. In this method of liquid-liquid (solvent) extraction, zinc is added to a molten lead/silver mixture and cooled slowly.

Because of zinc’s high melting point and lower specific gravity, it solidifies before the lead. The silver in the mixture becomes concentrated in the zinc crust since it is 3000 times more soluble in zinc than in lead.

Gold also reacts with the added zinc, and this gold-silver-zinc alloy is easily skimmed off of the liquid lead. The remaining lead-gold-silver residue is treated by cupellation, the process of heating to high temperatures (>800ºC) under strongly oxidizing conditions for impurity removal.

First, the antimony, arsenic, and zinc are oxidized and removed, followed by lead, with bismuth, copper, and tellurium being the last to be oxidized and removed as a slag known as “copper litharge”.

The remaining gold-silver alloy typically has a 99.9% purity. To separate the gold from the refined silver, a process known as ‘parting’ is used.

The most commonly employed method is digestion of the alloy with nitric acid wherein the silver is dissolved, the remaining gold is washed, and the silver precipitated from the washings as silver chloride, by salt addition.

From metal scrap

Approximately 55% of silver used globally in 2019 was used for industrial fabrication, this includes the photographic industry. Just over 25% of silver was used in jewelry & silverware manufacturing.

Source: Metals Focus, Bloomberg, Silver Institute

In the photographic industry, silver can be recycled from spent photographic processing solutions via electrolytic methods.

In the jewelry industry, high-grade jewelry scrap can be re-alloyed and the silver recycled on-site. The process includes collecting the fine dust that is generated when precious metals are polished and ground, known as the ‘jewelry sweeps’. The dust is smelted and refined to obtain pure silver.

Often, low-grade silver scrap has very low value and is returned to a smelter to be processed. Methods that are applied to gold recycling, such as cyanidation, are not usually economical for silver scrap.

Main processes used for silver refining

There are three main processes used for silver refining

1. Pyrometallurgical refining involves melting processes to separate pure silver from other metals.  Technically some of the pyrometallurgical techniques are pre refining steps to concentrate or separate silver and not technically refining.  In order to produce high purity silver in almost all cases, electrolytic refining is used.

a) Calcining concentrates the silver (and some of the other metals) by burning off any volatile materials including organic compounds.

b) Roasting can change the composition of the silver by converting say silver sulfide into native silver.  As it changes the chemical composition of the silver it may not technically be a refining technique but it is an important processing step in many silver-bearing ores and concentrates.

c) Miller Process involves removing the silver and base metals from gold by sparging chlorine gas through molten gold to form silver and base metal salts which are skimmed off the molten gold for processing. Silver chloride can be further processed by smelting techniques with zinc to form silver and zinc chloride or can be processed by the addition of sodium hydroxide and sugar/dextrose reduction. Again, while these processes are quite commonly used, they are complex involving several steps and not strictly speaking refining processes.

d) Fusion melting with lead to alloy the silver and pgms.  Zinc is used and removed in the slag leaving high-grade gold and silver which still need to be further refined.

e) Cupellation using lead which is oxidized and absorbed in a crucible. This method is really only used in assay techniques and not used on a large commercial scale for silver production.

2. Electrorefining involves electrolysis of a silver anode in silver nitrate, low-acid electrolyte to form pure silver crystals on the cathode.   The process can take several days and precious metals such as gold and platinum can be collected from the slime and sludge left behind when the silver and base metals dissolve in the electrolyte bath.  

The two main electrorefining processes are the Moebius and Balbach-Thum cells, though variations on these designs have been developed.  In most plants using electrorefining, the Moebius cells are used as they typically have a smaller footprint and are generally easier to operate and remove the silver from the cell.

In both of these types of cells, it is necessary to cast the impure silver into an anode form and then process it in the electrorefining cell.  The spent anode materials with the associated precious metals can be recast and refined again.  

The electrolyte is typically maintained at approximately 100g/L silver and when the soluble base metal impurities, such as copper, build up in concentration the electrolyte is removed.  Fresh electrolyte is prepared and added to the cells and the spent electrolyte containing silver, copper, and other impurities is further processed to recover the silver and return it to the anode melting stage for refining again.

The process can generally produce high-quality silver if the anode composition is managed carefully.  There are typically two main recycle streams for the silver:

1. Silver in the spent anodes

2. Silver in the spent electrolyte

which will typically require an additional 10-20% silver circulating load in the system depending on the purity of the anodes and process conditions used.  Particulate anodes have been developed to try and minimize the circulating inventory and automate the system somewhat.

The bagged anode compartment collects the precious metals as slime and keeps them separate from the silver crystal product. This works well to isolate valuable precious metals and requires periodic cleaning and materials handling.  If not maintained the anodes can passivate leading to a build-up in acid at the anode which can cause the product silver in the cells to partially redissolve.

The electrorefining cells achieve two important objectives in the same cell:

1. Separation of the silver and base metals from the gold and other precious metals, and;

2. Production of pure silver crystals by selective deposition from base metals in the background electrolyte

It is sometimes difficult to optimize both of these objectives in one piece of equipment.  For example, the gold and precious metals are trapped in the cell until they are released from the anode.  In order to do this as quickly as possible, the cells need to operate at a high current density on the anode.  The silver production at the cathode is then set by the dissolution rate at the anode.  For low purity anodes with base metal contamination, there is an imbalance of silver produced at the anode and anode silver removed at the cathode.

An illustration of the key steps of electrorefining is shown in the schematic below and described in more detail:

a) Silver feed material with associated precious and base metals are melted in an anode furnace with recycled spent anodes and silver recovered from the bleed electrolyte.

b) Periodically the electrorefining cells are stopped and the silver anodes are placed in the electrorefining cell.  The cells are restarted and current applied such that the silver dissolves slowly in the electrolyte with some of the palladium and the base metals.

c) The gold and platinum and other insolubles remain in mud which is captured in the anode bags.  The cells are again stopped and the slimes are periodically collected to send the gold and platinum for further processing.

d) The silver deposits on the cathode as a silver product and are removed from the cathode either manually or with automated scrapers and then taken out of the bottom of the cell.  Some of the silver is sold as a product and some is redissolved in nitric acid to generate silver nitrate electrolyte.

e) There is a natural imbalance in the system due to the fact that the silver is deposited at the cathode at a greater rate than it dissolves from the anode. In addition base metals impurities build up in the electrolyte.  The electrolyte needs to be balanced for silver and base metals impurities.  This is done by feeding the pure silver nitrate to the bath and removing base metals (and associated silver) from the electrorefining cell.  This feed/bleed system requires silver to be digested and used in a circulating loop.

f) The bleed electrolyte is processed to precipitate the silver using other more active metals such as copper or reagents such as sodium chloride/glucose and sodium hydroxide.  The silver is filtered in stages and dried and returned to the anode furnace again.

3. Electrowinning is focused on producing pure silver products and conditions are optimized for this.  The gold and precious metals are separated from the silver before the silver production starts in the digestion stage.  They are available for further processing immediately and do not enter the silver processing circuit which reduces the precious metals inventory significantly.  Over 99.9% of the silver is recovered in a single pass without the requirement to recycle impure silver to the melting stage.  This reduces the silver inventory in the circuit.

The silver is then electrolyzed to form silver crystals at the cathode and nitric acid at the anode.  In certain types of electrowinning cells, this acid can be available for reuse in the digestion circuit which closes the loop on the reagent consumption.

In emew electrowinning system, several factors are controlled on each individual anode/cathode pair rather than across a cell containing multiple anodes and cathodes in an electrolyte tank to ensure consistent and efficient operation including:

• • Flow Rate to each anode/cathode pair
  • Anode-cathode spacing on each anode/cathode pair
  • Voltages in each anode/cathode pair
  • Flush cycle to remove silver powder from each anode/cathode pair

In addition to the above controls in the emew system, the overall acid balance and silver depletion are carefully controlled to optimize conditions and produce a consistent silver product.  The conditions are fully optimized for silver production as the cells are not also performing the function of silver dissolution and pgms separation at the same time.

While electrorefining has historically been the main process used for refining silver, other methods which free up the valuable precious metals content quickly at the start of the process are becoming more popular.  In the emew silver refining process, this is particularly the case with high-value working inventory, as well as the ability to recycle and recover reagents, such as acid, in the electrowinning process.

Silver refining systems and refining equipment

As we can see, there are a variety of methods suitable for silver purification. The process design of a silver refining system will mostly depend on the type of concentrate, impurities, volumes, and purity requirements.

For example, if there are impurities in a solution that are less reactive than silver, the impurities might need to be removed first.  In many cases, impurities will affect the grade of the final product which denies the initial goals and operational requirements.

The concentration of the silver in the solution is also quite important. For example, in mining applications, conventional electrowinning can be used to recover silver and gold from cyanide-based leach solutions.  The precious metals are first dissolved or “leached” from the ore or concentrate into a solution using cyanide.  Then the electrowinning is used on Pregnant Leach Solution (PLS) to produce gold-silver alloy deposits.

There are other silver refining systems and technologies available that allow refining of silver up to .9999 grade, can work with very low metal concentrations, automated, and very safe to operate.

Silver refining through electrolysis – benefits and disadvantages

Silver refining with emew electrowinning

Silver refining demands technology that can achieve the highest purity coupled with advanced security features and low working capital. The proprietary emew silver process can produce up to .99999 silver.  This short animation explains the basic operating concepts of the fully automatic emew silver refining plant:

A completely enclosed system, emew eliminates acid mist and promotes a safer environment for operators and management. The emew silver refining system is low maintenance, worry-free, and fully automatic with minimal labour requirements.  There are no casting requirements for anode casting or associated fume handling or pollution abatement systems required. In addition to this, emew silver refining process doesn’t require caustic to control pH during the electrowinning.

The high-purity silver powder is automatically harvested using a flush through technology to a single collection point ensuring the utmost security, with no losses and simplified metallurgical accounting. A single batch can be processed in as little as 8 hours significantly reducing working capital or WIP which in itself can pay for the upgrade to emew.

Enhanced security, improved metallurgical accounting, and significantly reduced working capital make emew the ideal fit for your silver refinery or metal recovery operation. Do you have little space for an expansion or retrofit? These systems are compact with 1,000 kg/day production capacity fitting into as little as 25 m2 (269 ft2).

One of the major benefits for the small footprint plant is that it is fully automated which reduces the labour requirements while improving safety and security.  Monitoring every cell in the system provides 24/7 system performance accountability.

The future of silver

Recovery of silver and silver recycling is accomplished using various methods, including the Parkes process, cupellation, parting, slag fuming, and electrolysis (electrorefining and electrowinning). Silver is most often recovered from low-grade mineral ores, with less than 2% silver content. The techniques mentioned above, while employed for silver recovery from ores, can also be utilized for silver recovery from EEEs, including solar panels and circuit boards. Recycling silver from these EEEs will be a very important area for the future, as metal demands increase with new technological advancements. In the U.S. new solar power plants are now cheaper than new coal, natural gas, or nuclear power plants; and the price continues to drop. In Australia, China, India, Saudi Arabia, and the UAE, costs are dropping even faster. With so much growth in the industry, it means that there will be a need for more solar panel recycling in the coming years.

According to the Silver Institute, global demand for the silver forecast to rise 11% surpassing 1.025 billion ounces. The electrical and electronics sector will represent a significant portion of the total demand with the expected 300 million oz. for silver’s use in the sector. Solar is not far behind with the demand expected to grow to about 105 million oz. in 2021.

Based on the 2020 USGS report, silver production is trending upwards (USGS). In 2019 global silver production managed to increase to 26000 tons per year. At this pace silver will be depleted in about 20 years, if not sooner.

Harnessing the power of emew technology for silver recovery from a variety of silver-containing ores and WEEEs will help supply high-grade silver to meet the demand in the years to come. Following cyanide leaching, or other ore concentrate leaching processes, bleed solutions can be run through an emew system to recover silver from a wide range of silver concentrations. For more information on the power of emew for silver recovery and various case studies, please visit the Downloads section on our website.

Sources:

https://en.wikipedia.org/wiki/Refining_(metallurgy)
https://pubs.usgs.gov/periodicals/mcs2020/mcs2020-silver.pdf
https://www.silverinstitute.org/wp-content/uploads/2020/04/World-Silver-Survey-2020.pdf
http://algosonline.com/news/silver-recycling-market-emerging-trends-strong-application-scope-size-status-analysis-and-forecast-to-2025
https://www.northernminer.com/news/global-silver-demand-forecast-to-rise-11-in-2021-silver-institute/1003828143/
https://www.usgs.gov/

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