Miller Process

The Miller process is named after its inventor, Francis Bowyer Miller, who trained to be an assayer in London before moving to Sydney at the age of 26. His elder brother was a professor of chemistry at King’s College and an assayer to the Royal Mint and the Bank of England. On arriving in Sydney in March 1854, Miller set up an independent assay practice. But almost before this could start operating, he was offered a full-time position at the newly established Sydney Branch of the Royal Mint, which had been established to refine gold from the recently discovered New South Wales and Victoria goldfields and to produce Sovereigns for use in the Australian colonies (though the bulk of gold from the 1850s gold rushes ended up in London).

Miller was clearly an innovative investigator, as witnessed by his paper on the identification of fake nuggets, which he presented to the New South Wales Philosophical Society in 1860. But it was seven years later, in June 1867, that he achieved gold industry immortality with the lodging of a patent in London for a new process for the refining of gold using chlorine gas. Within two years, the first full-scale plants employing the process had been successfully commissioned in the Sydney Mint and also in Auckland (gold having been discovered in New Zealand in 1861). Over the next several years, the Miller process was enthusiastically adopted by mints and refiners around the world. Many refiners installed Miller furnaces to replace their electrolytic refining tanks, which used the competing Wohlwill Process introduced in 1874.

It had long been known that by passing chlorine gas over heated gold concentrates, a water-soluble gold chloride could be formed and, by the mid-1860s, this approach had been used commercially in various mines in the United States, South Africa and Australia. Miller’s great insight was to realize that gold chloride is unstable above a temperature of ~400°C. So, passing chlorine gas through molten gold does not result in the formation of gold chloride. But many of the commonly occurring impurities in gold do form stable chlorides even above the melting point of gold. Some (e.g., silver and copper) form a slag layer, while others (e.g., lead and iron) are volatile and must be recovered from the furnace’s exhaust gases.

In 1995, Degussa AG estimated that two-thirds of all the gold that had ever been refined had been treated by the Miller process. The reasons for its success can be stated very simply: it is fast, low cost and produces a product (gold with a fineness of 995 or 996) that can be cast into large bars and sent to the market without further ado – the London Good Delivery bar! The only disadvantages of the Miller process are: firstly, it does not permit a separation of platinum (since the thermodynamic behaviour of platinum chloride is very similar to that of gold chloride) and, secondly, in contrast to the Wohlwill process, it cannot efficiently produce high-purity gold. For this reason, the electrolytic refining of gold became increasingly important again in the second half of the 20th century due to the growth in the market for 999.9 fineness gold, especially in East Asia.

Picture and historical text credit: Thanks to Megan Martin for her work on the biography of FB Miller which appeared in the Australian Dictionary of Biography, Supplementary Volume, 2005.