The tube mill or the ball mill is an ideal grinder for preparing ores for oil flotation because not only is it an excellent grinding machine but likewise it is a good agitator, thus combining the two features in one machine, and when the oil is added to the mill with the mill-feed no other method of mixing is required.

The degree of fineness required to save the highest economical percentage of the metallic contents of an ore will in great part determine the class of machine used for grinding. When a 60- or a 100-mesh product is required some type of ball mill or the Hardinge conical mill will no doubt be the favorite, the short cylindrical tube mill of big diameter and short length being a close competitor; when fine grinding is required the cylindrical tube mill from 12 to 18-ft. long will no doubt be preferred.

Most of the flotation plants are now concentrating base metals where as a rule fine grinding is not required but when this method of concentrating becomes common, for precious metal-bearing ores, finer grinding will be the rule. Tests made at a mill in Mexico indicated "that fine grinding was necessary for good results in flotation." When the mill heading was crushed to 60-mesh, the tailing from flotation assayed 0.08 oz. gold and 11 oz. silver; when the same ore was crushed to 100-mesh, the tailing assayed 0.04 oz. gold and 5 oz. silver; when the crushing was carried to 200-mesh, the tailing assayed 0.02 oz. gold and 3.75 oz. silver. 1 This experience will no doubt be duplicated in many localities so that the long cylindrical tube mill now used for preparing ores for the cyanide process can be used for grinding ores for flotation. Fig. 50 illustrates in detail the class of mill that will probably be used for grinding gold ores for flotation in closed circuit with a classifier.

A few examples of the degree of grinding and the machines used at various plants will serve to indicate present-day practice, at least for the flotation of base-metal concentrates.