DOING SOME DIGGING Silver lining at the Mint

Yesterday in Ottawa, the ROYAL CANADIAN MINT officially opened its new silver refinery. For the first time since th...



Yesterday in Ottawa, the ROYAL CANADIAN MINT officially opened its new silver refinery. For the first time since the early 1980s, the Mint can now produce pure silver, at least four-nines fine (99.99%).

Squeezing a modern metallurgical plant into a heritage building in the centre of the nation's capital was no small feat. Executive director of bullion and refinery services David Madge said available space, environmental considerations and state-of-the art technology all played a part in choosing a plant supplied by PRIOR ENGINEERING ( of Switzerland.

The technology is conventional hydrometallurgy, but the plant is remarkable for its compact size and high degree of automation. All the process necessities fit into a space approximately 10 metres long, 3 metres wide and 3 metres tall. There are pressure reactors for making electrolyte, solution storage, two 1,000-L electrowinning cells, solution regeneration, and washing of the pure silver flakes. The refinery has a capacity of 2 million oz/year and the potential to expand to 10 million.

Next door is the silver furnace where scrap silver is pre-melted and cast into ingots. The ingots are sampled and assayed to determine their silver content; the refinery accepts feed of 80% Ag or better. The ingots are remelted and the liquid silver is granulated in a cold water bath to make feed for the refinery.

The granules are loaded into slender baskets suspended in the electrowinning cells. The baskets are partially submerged in silver nitrate solution containing 100-125 g/L Ag, and a current of 1,000 A is passed through the cells. The pure silver plates on titanium blanks and is automatically scraped off. The resulting flakes are collected in a screw conveyor and passed through a counter-current washing system. Water is removed from the clean silver in a centrifuge, and the metal is stored. The pure silver is cast into bars, wafers or grains.

Anode slimes are automatically vacuumed from the cells. Byproduct gold is collected and used at the Mint, and the copper byproduct will be tolled at a custom smelter.

Officials at the Mint are proud to once again offer silver refining. This makes their facility the refinery of choice for both gold and silver. Its reputation for purity, integrity and reliability only grows.

A visit to is well worth the time for coin collectors, serious investors and the curious among us.


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