Case Study of emew Silver Process versus Conventional Electrorefining

Silver refining is a critical part of any precious metals refinery.  Often the higher value metals such as gold and the Platinum Group Metals (PGM’s) are associated with silver, which must be recovered separately as a by-product. 

As with other refined metals, the selling price of the final product is dependent on the purity, therefore precious metals refiners are inherently obsessed with purity as well as inventory (aka working capital). 

While LMBA Good Delivery Bars have minimum silver fineness of .9990, most silver refiners target a minimum silver purity of .9995, while .9999 and .99999 are not uncommon and can command a small premium depending on the application. 

Silver is commonly electrorefined using Moebius or similar electrolytic cells.  In these processes, impure silver anodes are placed in a bath of silver nitrate and when a voltage is applied across the electrodes, silver and other base metals are corroded into the electrolyte. 

Pure silver is deposited onto the cathode as a powder, while gold is left behind on the anode as a slime.  Pure silver powder is harvested from the cathode manually or using mechanical wipers that scrap the silver from the cathode, before being dewatered and washed. 

The concentration of copper and other base metals that are corroded into solution continuously build up to a steady state concentration which is controlled by way of an electrolyte bleed. 

The volume of bleed required depends on the concentration of impurities in the silver anode, and the predisposition of these metals to co-plate with silver thereby affecting the purity of the silver plated on the cathode. 

The bleed is treated  to recover silver separately from the base metals. 

The spent silver anodes are recycled to re-melt when the anodes have corroded to the point where they are almost falling apart (typically 48 hours). 

The gold slimes recovered from the spent silver anodes are melted into impure gold anodes for further refining into pure gold, while the spent silver anodes (typically 15-20% of the original anode weight) are re-melted into fresh anodes to start the electrorefining process anew. 

This method of conventional silver electrorefining has been used for many years, however there are several disadvantages that any silver refiner has to contend with:

• Silver purity is acutely affected by impurities in the feed material
• Silver is often manually harvested from the cathodes by operators
• Working capital is high due to the time (48h) required to recover gold from spent anodes, and continuous recycle of spent silver anodes
• Requirement for an anode casting circuit to feed silver to the electrorefining circuit
• Silver losses are common due to electrolyte bleed treatment, recycling of spent anodes and manual harvesting
• Make up solution is required to replace the bleed electrolyte
• Silver needs to be recovered from the bleed electrolyte – this typically requires messy and expensive chemical precipitation with chloride or cementation with copper.

A significant advancement to the silver refining process has been developed by emew.  The emew silver process uses nitric acid to digest the silver in the feed immediately freeing gold and PGM’s into the leach residue. 

The silver-rich electrolyte is fed to the fully automated emew electrowinning cells that are comprised of a pair of concentric, tubular electrodes through which the electrolyte is pumped at high velocity. 

The high flow of electrolyte across the cathode surface significantly improves mass transfer and enables high purity silver to be electrodeposited even in the presence of high concentrations of impurities such as copper. 

The pure silver deposited on the cathode is automatically harvested by a reverse flush cycle and collected in a secure, conical bottom tank before dewatering and washing. 

The emew silver process is well established with many industrial plants operating world-wide on both nitrate- (refining) and cyanide-based (mining) applications. 

The advantages of the emew silver process include:

• High purity silver (up to .99999)
• Lower inventory and working capital of silver and gold
• Simple, fully automated operation
• No requirement for anode casting and doré handling
• No spent anodes to recycle
• Smaller footprint than conventional electrorefining cells
• >99.99% of the silver is recovered without the need for cementation
• Wide operating range of silver and impurities
• Demonstrated ability to operate at 40% over design capacity
• No cementation required for the electrolyte bleed, and no cementation cake to reprocess
• Fully enclosed and automated system requiring less labour and resulting in fewer losses

The investment to upgrade an existing precious metals refinery from conventional electrorefining to emew electrowinning must be justified based on a case by case cost-benefit analysis.  Let’s take a look at a typical application. 

A precious metals refinery treats 500 kilograms per day of silver (5 million oz/a) using conventional Moebius electrorefining cells. 

The existing cells are nearing the end of their usable life, and the refinery manager is deciding between upgrading to a new electrorefining system, or the emew silver process. 

The current process using Moebius cells has a turnaround time of 48-72 hours from the time the silver (and gold) enters the electrorefinery as impure anode, until the gold is recovered from the spent anodes. 

Furthermore, the inventory required to produce 500 kg/d of pure silver is three times this amount, that is to say 1500 kg/d.  The turnaround time of several days coupled with the high silver inventory results in high working capital. 

Due to the presence of copper in the feed material, the current operation struggles to achieve .9995 silver purity so one of the requirements of the new system is to be able to consistently achieve .9999 silver. 

The existing anode casting circuit and silver harvesting is manually intensive, so the refinery manager, Melanie, is seeking to modernize and optimize operations in order to secure more business in the competitive refining world. 

Metallurgical accounting at the end of each month is difficult due to the high working capital, losses due to manual silver harvesting, not to mention the handling of brittle spent anodes and the anode bags of gold and PGM slimes. 

The refinery manager is sold on the benefits of emew in terms silver purity, no anode casting / handling and fully-automated harvesting, but she must be able to demonstrate lower working capital and reduced operating expenses in order to be considered for funding in the following year’s capital budget. 

The capital expenditure for an emew electrowinning circuit to produce 500kg/d silver (including polishing plant to treat electrolyte bleed and powder recovery system) is USD 453,500. 

Assuming a dore feed of 90% silver and 2% gold the savings in working capital alone are over USD 1.2 million per day (assuming silver price of USD 14.50/oz and gold price of USD 1200/oz).  Taking a more conservative approach, we shall consider only the working capital tied up in a day’s worth of silver, that is USD 466,000. 

Whilst it is true that the cost of this working capital or inventory can be financed it is also true that plant and equipment can be financed, and customers are always looking for faster returns on their gold and silver. 

For every additional 1% gold in the doré feed material, the additional working capital is equal in value to the emew silver plant.  Depending on how the inventory and capital equipment are financed, this has significant impact on the investment decision. 

Now let’s take a look at the breakdown of operating costs of emew compared to conventional electrorefining. 

Overall, the operating costs for emew are about 37% less than conventional electrorefining. 

The main operating cost of conventional electrorefining is labour, representing 36% of the total operating cost, followed by bleed reprocessing / waste treatment. 

Anode handling represents 6% of the total operating cost with conventional electrorefining – a cost that is not required with emew which uses an initial digestion stage as opposed to melting and anode casting. 

Melanie presented the following case to her management to include in the next year capital budget:

In addition to the financial benefit above the marketing department has indicated that production of 1 million ounces per year of .99999 silver could command a premium of $0.15/oz adding an additional $150,000/yr of revenue. 

While this is a bit speculative and needs some market testing, the markets requiring high purity silver are growing and include semiconductor, chemical vapor deposition (CVD), physical vapor deposition (PVD) display and optical applications.

In summary, the operating costs of emew are 37% less than conventional silver electrorefining, primarily due to lower labour and reprocessing / waste treatment requirements. 

Furthermore, emew does not require anode handling which has additional savings in terms of the associated capital and operating costs.  The savings in working capital between emew and conventional electrorefining offset the capital cost of the upgrade to emew, which also boasts additional benefits such as higher purity silver, enhanced security, less waste, fewer recycles, flexible operation, less susceptibility to impurities, and smaller footprint.