In the flotation concentration of ores, water-soluble substances such as xanthates, generally termed collecting agents, are used to prepare the surface of the minerals to be floated so that attachment will take place between air bubbles and the mineral particles. Very small quantities of xanthates (of the order of 0.05 lb. per ton of ore) are found entirely adequate to impart to the mineral particles a surface at which air will displace water to such an extent that efficient flotation is possible. At the present time the nature of the mechanism of this action is for the most part an unsettled matter.

It is well knotvn that particle size imposes limitations upon flotation, but despite the economic significance of this relationship, it has not been investigated by many wOrkers. Obviously, the maximum size of particles that can be floated is limited by the lifting power of the surface forces (5). It has been shown, also, that in the flotation of sulfide particles the ease of flotation decreases with decreasing particle size (6, 7, 8, 9). Thus, particles of a size near the maximum that can be floated should have the optimum floatability.

No abstract

On the contrary, if the thickness of the film is such that reflections of the zero and first orders differ by a full wave, bz will lag behind bl by PA, bo -(bl + bz + b3 + . . . . + bp). n, FIG. 2.Multiple reflections a t a filmed surface since its retardation involves not only that due to the thickness of the film, but also one moreretardation due to reflection into a rare medium at n dense medium boundary; likewise b, will lag behind bl by 2.3X or 3 X ; br by 3 .$A or PA, etc. X 2 The amplitude of the net reflection is then (bo + bl + b3 + . . . . ) -(bz + 6 4 + bg + . . . . )This, of course, represents the largest possible amplitude for a reflection, just as bo -(bl + bz + b3 + . . . . + bp)represents the smallest possible amplitude.

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.