Froth Flotation and Broken Hill

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Taking a look at "Froth Flotation" the technology mentioned, but not explained to my satisfaction, around Broken Hill

Some years ago, I visited Broken Hill. It is an intriguing oasis, a place of interest after driving for so long through the outback. The streets are named after minerals and chemicals, and it has made a name for itself supporting artists. The Mullock heap has a memorial to industrial accidents and a restaurant - a creative use of the discards from mining. You'll find numerous musems around the way. In nearby Silverton there's the Mad Max Museum. Past that, there's there's quite a view and the Broken Hill / Umberumberka dam. I've not visited the picnic grounds that Doug Boleyn, retired Engineer, tells me are worth checking out. And locally you'll perhaps hear stories of how for a time only the sons of miners could work in mining at Broken Hill. But there was an exception - if the daughter of a miner married, her husband could work in the mines.

And you have the crusty old bush cockies, with their sneering arrogance about city folk - which you can take as part of the experience or be offended by them. But, when I heard them talking about where the ore came from to make the Harbour Bridge - I was able to challenge them on if they knew where the granite came for the pylons. They didn't know the answer - but I knew it - Moruya. It was one of the few times I was able to hold my ground with someone with a stronger personality in argument - and boy, did it feel GOOD.

Doug tells me it would probably have been made at Newcastle or Lithgow steelworks, in operation at the time. According to Wikipedia references, 79% of the steel for the Harbour Bridge was imported, and 21% from Australian sources. And, perhaps indeed, the ore for that steel would I imagine have come from Broken Hill. Still, when you look under the hood - it hardly seems that much to crow about.

But, visiting the museums, and hearing the details of the processes in mining, you hear stories about the missing ingredient behind sucessful mining - froth flotation. There's the claim that froth flotation was previously just a laboratory curiousity, and saw first commercial use at Broken Hill.

It seems the ore from Broken Hill had its own idiosyncracies, its own distinct challenges in extracting useful metal from the ore. But, given what I know about how detergents operate, how bubbles form, the story given seemed superficial. And, when I checked out the details in "Wonders Never Cease", the book of engineering acomplishments in Australia, its description was unsatisfying. Which is not to critice - they only had so much space to cover so much, after all. There's a bigger picture, and more to the story than Delprat and others as are mentioned there.

So, I did some digging. And here's my article, which while I've said a little about Broken Hill, I'm going to focus on the story - historical, chemical and mining - behind the development of froth flotation at Broken Hill.

While not "froth" flotation, there was another form of separation, still based on "flotation", noted in Wikipedia, a place Doug tells me he has visited:

The generally recognized first successful commercial flotation process for mineral sulphides was invented by Frank Elmore who worked on the development with his brother, Stanley. The Glasdir copper mine at Llanelltyd, near Dolgellau in North Wales was bought in 1896 by the Elmore brothers in conjunction with their father, William. In 1897, the Elmore brothers installed the world's first industrial size commercial flotation process for mineral beneficiation at the Glasdir mine. The process was not froth flotation but used oil to agglomerate (make balls of) pulverised sulphides and buoy them to the surface, and was patented in 1898 (revised 1901).

In trying to better illustrate what was going on, I'll first take a look at how detergents operate. Froth flotation is - in a sense - the reverse operation to detergents. Detergents dissolve oil in water, by surrounding little pockets of oil in shells of detergent. Detergents have water loving and hating ends - the water hating ends stick into the sphere of oil, so it is settled - and the water loving ends into the water - so it is happy to contain a sphere. This means that hot water and detergent will disolve away fat / oil on a frying pan, and will loosen dirt or stains attached to clothes, normally stuck there with something like oil.

Ore often contains fine particles of the mineral you want, and you can grind it up enough to expose surfaces of those particles. If the ore was in some sense a "solid solution", then you'd have to smelt or use chemical extraction dissolving things in their entirety. But, while the particles are not large enough to pick out by hand, at least they're not "dissolved" in each other at an atomic level.

But, at times the lumps of metal are large enough to metaphorically "pick out". The stuff you don't want, the contaminant so speak in mining jargon is called "gangue". Again, Wikipedia:

... galena, an ore of lead, is usually found in large pieces within its gangue, so it does not normally need extensive processing to remove it; but cassiterite, the chief ore of tin, is usually disseminated as very small crystals throughout its gangue, so when it is mined from hard rock, the ore-bearing rock first needs to be crushed very finely, and then has to be subjected to sophisticated processes to separate the ore.

It had been previously noted that metal and non-metallic fragments interacted with oil differently. Wikipedia:

it was well known that oil and oily substances had a selective affinity or attraction for, and would unite mechanically with, the minute particles of metal and metallic compounds found in crushed or powdered ores, but would not so unite with the quartz, or rocky non-metallic material, called gangue. It was also well known that this selective property of oils and oily substances was increased when applied to some ores by the addition of a small amount of acid to the ore and water used in process of concentration. Authors of early books on the flotation process related tales of froth flotation roots back to the writings of Herodotus, and listed a series of "flotation" patents back to the mid-nineteenth century.

In this sense, metals and metalic compounds are "hydrophobic" - they do not like water, while quartz and other materials are "hydrophillic", liking the company of water. We take it that anything that does not like water will like oil, and vice versa. Why the difference? It comes down to surface energy, the degree to which "surfaces" cost enery to form. When you form a crack in a material, you're forming a surface, and this is energetically less favourable. Metals are strongly bonded together ( this effect is stronger), and the whole lump of material is much more a coalesced mass as compared materials like quartz where the components have "come to an arrangement".

Wetting occurs when the solid has a higher surface energy than the liquid - water is a material strongly bound together, while oil is not so strongly bound. Hence - metal - with its higher surface energy - will preferentially wet with oil, because this has a lower surface energy.

If you had oil, water and detergent, you'd have globs of oil which the metal could find itself within, if the mineral particles were small enough. But bubbles of gas contain air - which is not water - something hydrophobic particles will be attracted to. Further, they can be bouyant, rising to the surface. So, froth flotation effectively exposes the metal particles to a large surface area that they are hydrophobically attracted to, which then naturally rises to the surface.

For the most part, metals and metalic compounds are hydrophobic to start with - but you can treat them chemically to improve this. Perhaps this is part of the story, where you treat the tailing with acid - though, equally, it could just be that the acid generates bubbles just where they're needed. Equally, earlier processes noted that you needed some oil to help things along - perhaps globs of oil operated as a carrier as mentioned earlier - though a froth which would rise to the surface was also crucial.

The other part of this story is one about Patents, who exactly did and discovered what - and that is its own quagmire. There wasn't just BHP and Delprat - but also Minerals Separation Ltd. Certainly, though, all the action - all the innovation - seems to have taken place at Broken Hill, and that part of the story stands. It was ( and still is ) a crucial technology in the history of mining in Australia.

There's some context - it was originally possible to mine the ore, and to start with, it was easy, but over time it became harder, with more metal lost to the "tailings".

The Australian Science and Technology Heritage Centre at the University of Melbourne captures some of the detail:

In the first five years of the twentieth century, Minerals Separation Ltd, a small London-based company, was involved in developing a revolutionary technique of ore extraction. Its work, along with that of others, emerged in response to 'the Sulphide Problem', seen by some historians as the most serious metallurgical crisis faced by any Australian mining field in the nineteenth century. Essentially the problem was that companies involved in mining silver, lead and zinc found that the deeper they mined the lode, the more difficult these metals were to extract and separate. The ore closer to the top of the lode was oxidised, and as such was easily smelted. In the deeper sections of the lode however, the minerals sought-after existed in sulphide form and there did not exist a method of extraction and separation that could be used economically. The deeper a lode was mined, the further diminished the percentage of useable ore. At many mine sites, piles of tailings grew higher and higher, loaded with seemingly irretrievable precious metals. The imperative to solve this problem - to somehow unlock these metals from the piles of worthless rock - was established.

Methods of separation using the specific gravity of each metal were tried but largely failed. Magnetic separation was tried but proved uneconomical. In 1900, the method of flotation as a means of separation emerged. This involved mixing water and chemicals with the ore so that certain mineral particles floated to the top and could be skimmed off easily. The original patents for the flotation process were secured by C. V. Potter in 1901 and by G. D. Delprat in 1902. Controversy over the originality of each soon followed and litigation ensued, lasting until 1907. The Potter - Delprat process, as it was referred to after 1907, proved unsuccessful on a large scale. In 1905, Minerals Separation Ltd improved the process in two ways. Firstly, by the addition of a small quantity of oil to the pulp and secondly by producing violent aeration via the mechanical agitation of the mixture. In this way they were able to separate the valuable material from the tailings. This process of Froth Flotation became the hallmark of Mineral Separation Ltd and constituted a major revolution in minerals technology. In 1905, this process was adopted at Broken Hill and effectively solved the problem of recovering the precious minerals from the tailing dumps.

All was not solved, however, for two problems persisted. Froth flotation was still unable to separate zinc from silver-lead conglomerates. Thus when the process was applied to newly-mined ore, unseparated concentrations of lead and zinc were produced. This lead to further developments in the Mineral Separation Process. F. J. Lyster (Zinc Corporation), T. M. Owen (Junction North) and Leslie Bradford (B.H.P) all made patentable improvements to the process at the mine sites themselves. These developments, in the direction of 'selective flotation', proved very successful.

Another player in the game was the Belgium-born chemist, Auguste De Bavay. In July 1906 he patented his 'skin' or 'film' flotation process and opened what was to become a successful plant at Broken Hill. In 1909, De Bavay started the public company, Amalgamated Zinc (De Bavay's Ltd), and enlarged the plant at Broken Hill. The sole business of companies like Amalgamated Zinc and the Zinc Corporation was to use the flotation process on the materials from the tailings dumps.

At some stage between 1910 and 1912, Minerals Separation Limited obtained the license to use De Bavay's Sulphide Process of ore extraction. It appears they actively traded patents, assayed ore samples at their South Melbourne flotation plant and installed their own plant and processes at clients' mine sites under license.

I don't have all the answers, but it's an intriguing story nevertheless. I've at least found a more satisfying perspective on froth flotation, satisfying that itch I first developed when I found out about it in Broken Hill.

Yes, there were improvements made at Broken Hill - but there were improvements made elsewhere, too. The true picture is somewhat more complex - but it remains fascinating. I hope you've been able to follow at least some of this article, and that it gives you more of a perspective when you're in Broken Hill, and see some of those videos!