|Cornish barrel mill|
Monday, 8 February 2016
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.
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.
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