Monday, 8 February 2016

Is the Autogenous Mill a Cornish invention?

The highest throughput grinding circuits in the mining industry use autogenous grinding (AG) or semiautogenous grinding (SAG) mills. An AG mill is a tumbling mill that uses the ore itself as grinding media. The ore must contain sufficient competent pieces to act as grinding media and preferably be high specific gravity which, for example, favors AG milling of iron ores and early deployment of AG mills can be attributed primarily to a need in the iron ore industry to economically process large quantities of ore in the late 1950s. Non-ferrous operations (mostly copper and gold) utilized AG milling to a lesser extent before recognizing that SAG milling was better able to handle a variety of ore types.
According to a well-known text book (now in 8th edition) the first paper describing ore as the grinding media was delivered to the American Institute of Mining and Metallurgical Engineers in 1908. However research by my old friend and CSM colleague Tony Clarke, now a respected Cornish mining historian, suggests that the autogenous mill might in fact have been used in Cornwall some 20 years before this.
Tony writes:
The treatment of 'roughs' (coarse sand middlings) had always been a constant though minor part of Cornish 19th century tin dressing operations, and was usually handled by small sets of stamps (often water-powered) working at a reduced height of drop for gentler pulverisation. After the mid-1870s, when the serious questioning of overall recoveries and tin losses came more to the fore, an alternative approach was tried, in the form of purpose-designed appliances such as grinding pans and barrel mills, the early development of the latter being outlined below.
In 1863 Williams and Price had proposed what was, in fact, a batch tube mill, with internal lifters and steel balls as a grinding medium. Apart from being used batchwise, this was essentially little different from far more recent ball mill design. Ten years later, Lundgreen's pulverising barrel (also a batch machine, carrying 2,000 lbs. of steel 'bullets') was in use in the U.S.A., handling 600 - 700 lbs. of charge in 70 minutes, and working at 26 r.p.m.
At this time, in Cornwall, a few different versions of grinding pans (with peripheral screen discharge) had made their appearance and were in use, but a new device was shortly to challenge them. The first really successful, continuous Cornish barrel mill was patented by Francis W. Michell and Thomas Henry Tregoning early in 1880, a prototype being erected at Wheal Peevor, near Redruth, a mine in which Michell held an interest. Its use was directed at the regrinding of 'burnt leavings' (i.e. the secondary residue after the re-dressing of cassiterite concentrate following calcination to remove arsenic), as well as coarse tin sand middlings. The machine consisted of a cylindrical barrel, 5 ft. 4 inches long by 3 ft. 4 inches diameter, with inlet and outlet trunnions, mounted horizontally on wooden stands and charged with about 15 cwts. (760 Kg.) of boiler plate punchings and odd scraps of iron and steel, revolving in bearings at 10 - 15 r.p.m. with a throughput of 5 - 8 tons per 24 hours.
Cornish barrel mill
Feed passed from the hopper with a little water, to be discharged continuously after grinding at the opposite end, and the inlet and outlet pipes were carried sufficiently far inside the cylinder ends to prevent the premature discharge of material by flushing. At the trial, a reasonable operating capacity was found to be 8 tons of 'burnt leavings' or 6 tons of tin 'roughs' per day. It was also found that any lighter, barren material was quickly ejected from the mill, so that power was not wasted grinding it. A marked feature was the thorough grinding of the feed material, with little or no unwanted oversize. The residence time in the mill could, of course, be controlled by feed rate and water flow, such that a degree of flexibility was built in. Consumption of iron was found to be about 1.8 lbs. (0.85 Kg.) per ton of feed, and discharged material fed directly into a round buddle (a large, circular pit with a gently-sloping floor - a typical Cornish tin dressing device in use at this time) was found to be satisfactorily separable.
By September 1882 some 30 of these pulverisers were said to be in use, and in 1884 the foundry firm of Bartles, in Redruth, purchased half of the patent rights, and became the solo manufacturers. One form of the mill was cast in ring sections (for ease of transport) instead of as a single cylinder.
There now came a very interesting modification. At the Mining Association and Mining Institute of Cornwall's July exhibition in 1888, the mill was demonstrated in use, but instead of scrap iron as a grinding medium, rough chunks of tin ore had been found to work just as well, with the double advantage of an increased cassiterite output, and complete freedom from abraded iron particles mixed in with the ground ore, providing their own subsequent, unwanted separation problems. In addition, the working speed had been increased to between 40 and 80 r.p.m.
The question now is, was Cornwall thus the birthplace of the autogenous mill? It is possible that a similar idea had been tried somewhere with quartz-gold ore, but this is outside my field of historical research, though I would be interested to hear of any examples via the medium of Barry's blog.
Tony Clarke, Camborne, Cornwall

Saturday, 6 February 2016

En route to the Indaba

I am on the train to London at present, and then this evening the BA flight to Johannesburg and on to Cape Town for this year's African Mining Indaba.
The annual Mining Indaba brings together the leading representatives from the mining investment community, the world's largest mining houses and Government ministries to share insights into how the sector can drive investments and capitalise on the opportunities available to Africa's mining industry.
Basically not my usual type of conference, but my 4th, and one that always springs a few surprises due to the sheer weight of numbers, usually around 7000 delegates in attendance at the Cape Town Convention Centre.
MEI is a media partner, and I will be reporting on my experiences when I return, and, speed of internet permitting, providing regular updates on the blog and on Twitter (#MiningIndaba via @barrywills).

Thursday, 4 February 2016

Last month's most viewed posts

The 10 most popular blog postings in January can be seen below, together with the date of the posting and the total number of comments on the post.

Let's start to take bad science seriously
18 January 2016 (15)
Return to Chingola
12 August 2012 (24)
Are these WASET conferences just a scam?
28 April 2013 (68)
A cautious welcome to 2016
4 January 2016 (3)
Last month's most viewed posts
7th January 2016 (0)
Who will be MEI's Young Person of 2015
11 January 2016
Latest design improvements increase spiral concentrator efficiency
1 April 2013 (19)
Eminent Biohydrometallurgist joins the Editorial Board of Minerals Engineering
11 July 2014 (1)
SRK Consulting says that waiting for the economic upturn will be too late
25 January 2016 (0)
MEI Conversations
5 April 2014 (0)

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Wednesday, 3 February 2016

Happy to live in Cornwall

Data collected over three years by the Office of National Statistics (ONS) has been used to measure the wellbeing of more than 300,000 people across the UK.
The resultant map shows that Cornwall is one of the the 'happiest' places to live in England.
The 'happiness map' of England (Cornwall far west)
I wonder why that might be:
Happy to be in Falmouth!
Looking forward to spreading a little happiness in Falmouth in June with the two MEI Conferences.

Monday, 1 February 2016

Biohydrometallurgy and Sustainability join forces in Falmouth

An exciting week awaits in Cornwall in June with the 2- day Sustainable Minerals '16 conference immediately following the 3-day Biohydrometallurgy '16 event.
The conference programmes have now been published, and there will be four very topical keynotes during the week, presented by high-profile figures in the industry.
The Biohydromet timetable has two keynotes, from Frank Roberto, of Newmont Mining Corporation, USA, and Dave Dew, of Dewality Consultants Ltd, UK. Frank will discuss the commercial heap biooxidation of refractory gold ores, while Dave will review the application of biohydrometallurgy to the treatment of base metal sulfide concentrates.
Timetabled in the Sustainable Minerals '16 conference are two keynotes, the first by Robin Batterham of the University of Melbourne, who will show how the mine of the future will be even more sustainable. On the second day Mike Battersby, of Maelgwyn Mineral Services, UK will discuss best practice in mineral processing. Mike was recently interviewed for Materials World magazine (January 2016), where he talks about his career and ideas on how to reduce energy use in the minerals industry.
The final day of Biohydromet '16 will provide a strong link into Sustainable Minerals '16, with presentations on the use of biotechnology in environmental issues, such as acid mine drainage. Sustainable Minerals '16 delegates will have the option of registering separately for this final day, and there are also discounts for those wishing to attend both conferences.
So all is set for a great week of presentations, and also networking opportunities, with the usual MEI coastal walks into old Falmouth, and a visit to the historic Camborne mining district on the Tuesday evening.
Biohydromet '14 delegates in Falmouth
Both conferences are appropriately sponsored by Outotec, which has recently been ranked for the second time as the world's third most sustainable company (MEI Online) and in 2015 strengthened its portfolio of gold processing technologies by acquiring Biomin's Biox bioleaching technology. Media partner for both conferences is International Mining, and for Sustainable Minerals Industrial Minerals is also a partner.
We look forward to a very productive week in June.

Thursday, 28 January 2016

Where are they now?

I get the impression, travelling around the world, that most mineral processors are at least aware of the MEI blog even though they may only visit it irregularly.
As the blog is very much about people, I thought it might be a good forum for catching up with people that you worked or associated with in the past and often wondered where they are now and what they are doing.
If you have somone you would like to catch up with, just leave a comment on this posting, and hopefully someone will see it and respond. I will start the ball rolling with the first request for info, and every few months I will highlight the posting to keep it active.