CHARACTER OF THE LINING
The material of which a lining is made affects the capacity of a tube mill by retarding or promoting the cascade of the pebbles; it likewise determines, to some extent, the constituents of the ore that receive the most grinding by performing the greatest grinding effect at the bottom of the mill where sulphides accumulate or nearer the surface where the lighter material is found. The liner which promotes the tumbling over or cascading of the pebbles causes grinding by impact while a liner that allows the slipping and sliding action of the pebbles grinds to a greater extent by attrition. In the first class we have silex blocks, all forms of liners with lifting bars such as the Komata and liners with a pebble surface such as the El Oro; in the second class we have the smooth liners whether made of cast iron, wrought iron, soft steel or manganese steel.
If we are grinding an ore in which a great proportion of the metallic contents is in the heavy sulphides we desire to use a liner that will assist the grinding of these heavier portions. The smooth liners which allow the bottom layers of pebbles to slip on the lining will grind these heavy sulphides which naturally seek the lowest level in the mill. This at least is the theory prevailing in the Tonopah district and has influenced their choice of this class of liner in several instances. This particular ore may act in this way but with a pulp containing 32 to 35 per cent, moisture with other ores the sulphides do not separate out from the mass of material even if the pulp is at rest and when the pulp is thrown about in the mill there is even less chance of this separation. There can be no doubt that when capacity is desired the liner should be such that it will assist the lifting of the pebbles.
The silex lining is made with silex blocks laid in cement. The blocks are usually 4 in. thick laid on edge and last from 7 to 11 months. The advantage of the silex lining is that it does not introduce objectionable material into the cyanide process while the main objection to its use is that the mill must be idle for at least 6 days while a new liner is being cemented in the mill. Steel or iron liners can be replaced in less than 12 hr., so that the lost time is a serious item, particularly in small installations with but one tube mill. Many mills have changed or are being changed from silex to some form of steel or iron lining.
Silex blocks weigh approximately 46 Ib. per square foot, the cost of lining a mill varying as follows :
When the Goldfield Con. used silex lining they lasted 7 months and were renewed with the following items of expense: LABOR
Removing and replacing manhole, removing
end liners $11.88
Removing pebbles 3.75
Removing old lining 11.25
Relining 63.76
Replacing pebbles 7.50
Total labor $98.14
SUPPLIES
Cast end liners $92.48
Silex, 17,710 Ib. at 2.634 cts. per pound 466.48
31 sacks cement at $1.10 per hundredweight 34.10
Total supplies $593.06
Total cost $691.20
The time involved is: Hours relining, 68; hours setting cement, 72; total hours lost, 140. While this is less time than is customary at other plants to allow the cement to set, there has never been any trouble on account of starting too soon.
The cost for lining seven 5 by 22-ft. tube mills with silex at the Montana-Tonppah mill, Nevada, is given by A. H. Jones 1 as follows : Labor at $4.50 a day $42.25
Cement, 28 sacks at $1.035 28.89
Silex, 6 tons at $42.80 256. 80
$327.94 for each mill
The average cost of renewing silex linings on the Rand is given at $525 and at the Nipissing mill at $625. The price of cast-steel or cast-iron linings for a 5 by 20-ft. mill will vary from $650 to $1,000, so that as far as the cost of lining alone is in question the silex is cheaper but as already mentioned the time lost during the relining must be taken into account.
