In the early stages of the development of the flotation process for concentrating ores, the belief arose that at last a process of ore concentration was available for the successful treatment of the finest particles (1), which in water suspension are collectively known as "slime." It was thought that all mineral particles that are too fine to be recovered by gravity concentration and yet comprise a wide range of sizes float equally well. And, indeed, it is true that flotation is applicable to a wider size range than any of the three principal methods of gravity concentrationjigging, tabling, and vanning. Nevertheless the introduction of ball-mill grinding made it increasingly obvious that particles of all sizes do not float equally well or rapidly. The presence of an upper size limit beyond which flotation is impossible was recognized even before the advent of selective flotation (as contrasted with collective flotation). The existence of a lower size limit beyond which flotation is difficult was suspected, but the limitations of laboratory sizing technique made it difficult to more than surmise that particles of colloidal size are refractory to modern selective flotation operation (2).Definite reduction in floatability with reduction in particle size beyond a critical size of maximum floatability was recently demonstrated (3). The size limits within which recovery by flotation is good were found to be more or less peculiar to each mineral. In general, however, the optimum size of mineral particles for concentration by flotation is between 50 and 10 microns, and the recovery is markedly lower for particles finer than 5 microns.

In the beneficiation of ores by ore-dressing processes the treatment of the finest portion of an ore has always been a problem difficult, if not sometimes impossible, of solution. Flotation has proved no exception to this rule. In the "near-colloidal" size-range of mineral particles embracing the entire scale from 5 microns to the size of the truly colloidal particles recovery by flotation has been found to be very unsatisfactory (1, 5).A study directed to improve the flotation behavior of these particles has been carried on at the Montana School of Mines since 1931. It resulted in the discovery of a successful experimental solution of the problem of flotation of near-colloidal pulps (6). By adding the collector into the pebble mill, so that it is present while the minerals are being ground, satisfactory flotation results can be obtained. This holds true for a number of collectors that comprise a group of the most common ones and of some of their less common derivatives (6).The present paper deals with the investigation of the effect of some arbitrary variables on the flotation behavior of two mineral mixtures (galenaaplite and chalcocite-aplite) when the collector is present during the grinding.The variables investigated are: (a) the effect of increasing amount of collector; (b) the effect of some alkaline pH-modifying agents; and (c) the effect of substituting a "neutral" grinding atmosphere for g,ir atmosphere. The study of the effect of the three variables was confined to two collectors: potassium «-amyl xanthate and its oxidation derivative, «amyl dixanthogen («-amyl thioformate disulfide).