Flotation Conditioning is often necessary as it is not sufficient merely to make the addition of the various reagents to the pulp and then to proceed with flotation. Intimate admixture is essential in order not only to bring about their even dissemination throughout the pulp but also to ensure that the millions of particles of varying nature shall each receive its proper degree of treatment. Moreover, sufficient air-bell contacts must be provided to promote flocculation of the minerals that it is desired to float. In general, opportunity must be afforded for the various complex physical reactions outlined in the preceding paragraphs to become effective, since the reagents are added in such relatively minute proportions that their effective action demands a multiplicity of contacts in order to establish the proper balance.
The time required to bring about this flotation equilibrium is termed the conditioning period, and it necessarily varies with the intensity of the agitation afforded to the pulp. A few minutes may suffice to condition the mixture in a mechanically operated flotation machine in which intense agitation is provided by means of a rapidly rotating impeller in a baffled chamber. On the other hand, the same ore may need a period ten times as long when the operation is carried out by comparatively slow stirring in a conditioning tank followed by flotation in a pneumatic cell without agitation. In the former type of machine the air is introduced into the mixing chamber and is subjected to the same intense churning action in the pulp as are the reagents ; in the pneumatic cell the air-bells are showered upward through the already prepared or conditioned pulp, which passes along the length of the cell in an even flow. It is evident, therefore, that the conditioning period depends largely on the type of machine employed for flotation.
Whatever the method of conditioning adopted, it is seldom that all the reagents are added at the same point. Frothing oils which are sparingly soluble in water, and some insoluble collecting oils, are often put into the feed end of the ball mill for ease in dissolving or emulsifying them. Those reagents which need a very long contact period with the pulp are also added at the same point. When the time of contact of some of the reagents cannot be controlled accurately enough by their addition to the ball mill, a conditioning tank is installed between the grinding and the flotation sections ; both the pulp and the reagents enter the tank in a continuous stream and the discharge level is so regulated as to give the exact period of contact necessary. If the pulp from the grinding section or conditioning tank has to be pumped to the flotation machine, it is common practice to add readily soluble frothing oils, such as cresol and pine oil, and quick-acting promoting reagents, such as xanthate, to the intake of the pump. This method of addition is specially useful when flotation is carried out in pneumatic machines in which no violent agitation takes place ; if machines of the agitation type are employed, the reagents can safely be put in at the head and at any point along the length of the machines themselves.
When a comparatively large quantity of a solid reagent such as soda ash is required in the circuit, the usual method of feeding it is by a miniature belt conveyor working under a storage bin ; sparingly soluble solids such as thio-carbanilide are also handled in this way. Very soluble chemicals such as the xanthates are made up into strong solutions and added by means of a special type of feeder which is described at the end of the chapter. Oils are fed to the circuit by the same method in their normal condition without dilution.
Modern chemical reagents require the hydrogen ion concentration (pH value) of the pulp to be controlled with considerable accuracy ; it is usual for determinations to be made at half-hourly or hourly intervals. While ordinary titrations for alkalinity or acidity with an indicator such as methyl-orange are often satisfactory enough, the more accurate method of determining the pH value by means of special indicators is becoming increasingly common.
Although the flotation reagents in common use are described in the pages that follow under the headings which indicate the purpose for which they are normally employed, it is seldom that they act solely in the manner indicated. Thus frothing oils function chiefly by reason of their partial solubility in water, the dissolved portion constituting the means of disseminating a cloud of air-bells through the pulp and stabilizing the froth at the surface, but their undissolved fractions remain emulsified and to this extent function as collectors. Even pure chemicals may perform a dual role ; xanthate, for instance, is generally classified as a promoter, although at times it undoubtedly behaves as an activator. Moreover, many reagents, whatever their ostensible functions, influence to some extent the surface-tension of water. Nevertheless their practical application is indicated with sufficient accuracy by the class under which they are described.
Preparing the Pulp (Slurry) for Flotation
Before the reagents are described in detail, it is advisable to consider the way in which the pulp is prepared in order to make the action of the reagents as effective as possible.
It is essential, in the first place, to grind the ore in water in the final stages of its reduction. The necessity for this is explained by the fact that, when broken in air, a freshly exposed ore surface is, for a while, in a condition of strain and exhibits the energy common to all “ strained ” areas. Such a surface, for a short period, is readily wetted by water, but this energy is quickly dissipated, owing to movements of the surface molecules in readjusting themselves to relieve strain, when the surfaces become more difficult to wet. When, however, the ore is broken under water, the strain-energy of the fresh surfaces is immediately relieved by its utilization in attractional reaction with the water molecules, and the surfaces are wetted to the full extent of the adhesion mutually possible. In flotation dry-crushed ore gives poor separations, and wet grinding is essential to secure effective wetting and sinking of the unwanted gangue particles.
After passing through the grinding section, the resulting pulp must conform to two conditions. Firstly, the particles of ore must be as nearly as possible of the size required for flotation. The general rule is that they should be reduced in the grinding machines to such a degree of fineness that the minerals which it is desired to concentrate are substantially free from gangue, but, since it is often possible to float particles of mineral which have not been completely liberated from the gangue, it may sometimes be advisable, for reasons of economy, to allow a certain proportion of such material to enter the concentrate. The latter will, of course, be permanently contaminated with the amount of gangue so floated, unless steps are taken subsequently to regrind and refloat it, a mode of procedure that is becoming increasingly common as a means of reducing the expense of grinding. It is not often possible, however, to allow mineral particles larger than 48 mesh to enter the flotation machines because they have too great a tendency to drop out of the froth ; only a few substances of a special nature, such as coal, graphite, and molybdenite, can be floated at a coarser mesh. This, therefore, determines the upper limit of grinding ; the lower limit does not depend on any factors affecting flotation but on the fact that grinding finer than 200 mesh is generally too costly to be economical. It should be remembered that, whatever the mesh of grinding, the pulp always contains all grades of particles from the maximum allowable size, as determined by the setting of the final classifiers, down to the finest slime.
Secondly, for reasons of economy, the pulp should be as dense as possible in order to keep the power and reagent consumption as low as possible. In practice a W/S ratio in excess of 4/1 is only used in exceptional cases ; for ores of normal specific gravity it lies between 3/1 and 4/1 mainly because the classifiers in the grinding section have ordinarily to be run at a dilution between these limits. The classification of ores of high specific gravity, however, can be carried out in denser pulps, and consequently their flotation can be, and often is, effected at W/S ratios between 1½/1 and 2/1. This applies particularly to operations such as the flotation of galena in preference to sphalerite in an ore with a pyrite gangue. The reagent cost of an operation of this sort is high, and, since the consumption of most of the reagents required is dependent on their degree of concentration in the water rather than on the amount of valuable minerals present in the ore, it pays to carry out flotation in as thick a pulp as possible, but the thickening must not be overdone. It is possible to reduce the proportion of water so much that, when the pulp enters the flotation machine, the bubbles are crowded too closely together and carry particles of gangue up into the froth by mechanical entanglement. It is evident, then, that the density of the pulp depends to a large extent on the work of the classifiers in the grinding section, and that, for reasons of economy, it should be as thick as is consistent with good flotation.