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Gold/Mining/Energy : Gold and Silver Juniors, Mid-tiers and Producers

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To: yard_man who wrote (38982)	4/26/2007 10:38:18 PM
From: E. Charters	of 78413

Yes, the pic was not of a ball crusher, which some people find interesting too, but in a different category. A company in the states did a lot of work on ball mills in the 1920's and 1930's. Hardinge. They developed the conical ball mill which necked down towards the discharge. It had a very small flat section at the feed end, after the apron part. For some reason they were supposed to be more efficient but the reason why is hard to fathom. A milling prof of mine said this was not so, but literature on the subject claims an increase. The supposed principle was that the first stage of reduction benefits from high drop and the finer sizes require less drop. But in fact I have heard the opposite principle, namely that more work is required to reduced finer sizes and the energy of reduction per root of two mesh reduction size is not proportional (linear) Ball mills are one of the most inefficient machines known to man. They get perhaps 1% efficiency. There is a vast difference in efficiency amongts mills. The reason for the inefficency is that rock is very strong in comression, but milling seeks to use impingement. Reduced area milling suraces get in the way of tension acting on the particle. In blasting where the particle movement is in the direction of more volume, the force works on tension and thus is maximally efficient. Despite the fact that dynamite is 5 times more expensive per kilowatt it is still 3 times more efficient to use powder to achieve size reduction. Thus blasting is 15 times better at utilizing power than impingment grinding. Many outfits are experimenting with very high pressure cornish type rolls and special metallurgy. High grinding rates can be achieved. One outfit in Argentina is talking about achieving 5000 tons per hour with roll type grinders. Conical Mills Rolls Modern HPGR

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