Because of the extremely fine grinding necessary to liberate the gold values, closer control than that obtained by screens was found to be necessary, and Professor Haultain’s “Infrasizer” was successfully adopted as a plant-control and research device. A brief description of the instrument is found in “Fine Grinding Investigations at Lake Shore Mines” {Trans. 42, C.I.M. and M.} which incidentally is probably one of the most detailed studies to be found on the various aspects of particle size control in plant operation.
The Infrasizer was invented and developed by Prof. H. E. T. Haultain of the University of Toronto, is shown in Fig. 58.
In brief, it may be said that infrasizing is an air elutriation process carried on in a series of stainless-steel tubes whose diameters increase by the square root of 2.
Thus, the critical air velocity in each successive tube will be one-half of that in the immediately preceding tube. Each tube collects a fraction consisting of particles of equal settling rates in air. The settling rates of each successive fraction will be one-half the settling rate of the immediately preceding fraction. Assuming a material consisting of perfect spheres, all of one specific gravity, the size range collected in each successive tube will decrease by the square root of 2. However, in the actual sizing of ground products the particles will not be spherical or even cube-shaped but will vary from the extremes of flat lamellae and flakes to needlelike particles, with a fair percentage of particles of almost equidimensional shape. Not only will the particle shape depart from the ideal condition, but in addition there will probably be minerals of different specific gravity present in any given ore. Such heavier or lighter minerals will be sized according to their settling rates.
With regard to the meaning of particle size in measuring the fineness of a product, Dr. P. C. Carmen says: “Particle size of a nonspherical particle is not a term with a definite meaning unless it refers to a definite property of the particle, e.g., the diameter of a sphere with the same volume or with the same specific surface, etc. Only for a sphere are all these diameters identical. In the methods of measuring particle size, there are measured, respectively, a sieve aperture, a rate of free fall, and some arbitrary microscopic diameter, and the first step is to interpret these in terms of ‘particle size”. Dr. Carmen proceeds to point out the difficulties and fallacies arising when an attempt is made to interpret the measurements obtained with a microscope in terms of particle size.
To avoid confusion in the use of terms, a nominal micron size has been worked out for each infrasizer fraction. This nominal micron size may be defined as the “size” of irregularly shaped particles, of any specific gravity, which have the same settling rate in a column of air as do glass spheres of that diameter. As the specific gravity of glass (2.6) is almost the same as that of most siliceous ore {e.g., Lake Shore, 2.7), this nominal size is applicable.
In these investigations, all sizing results obtained with the Haultain infrasizer are reported in terms of the above nominal micron size. However, for reference, Table 34 has been drawn up, showing the nominal micron size of pyrite, tellurides, and gold referred to perfect spheres of pyrite, tellurides, and gold, respectively.
The infrasizer has three main uses:
- for sizing analysis,
- for gold-sulphur-size analyses, and
- for preparing samples for panning.
For sizing only, 100-gram samples are used and the separation takes 1 hr. 20 min. For the gold-sulphur-size analyses, between 400 and 3000 grams must be sized, depending on the grade of the sample. This, is done in 400- and 800-gram lots, taking 3½ and 6 hr., respectively. These gold-sulphur-size analyses form the basis for interpreting all the experimental results. From the percentage weight, gold and sulphur assays of the various, fractions, the gold and sulphur content, and percentage size distribution, are calculated.