CHAPTER 9 PRACTICAL EFFECTS OF PARTICLE SIZE

Contents Introduction Recovery of fine particles Flotation Entrainment Combined effects Improving flotation of fines Recovery of intermediate sizes Recovery of coarse particles Selectivity & particle size Discussion

Introduction Particle size effects have been known for a long time but their importance is still under-rated in plant operation The size-by-size analysis of plant operation and laboratory tests are essential for an understanding of the overal flotation performance

Recovery-Size Plots

Recovery-Size Plots The plot may be divided into 3 regions Fine < 5 micron Difficult to float and separate Intermediate, in between Most floatable region Coarse > about 70 micron Flotation drops off depending on conditions The actual sizes separating the regions depend on the system and the conditions

Flotation of Fine Particles Flotation of fines is slow Theoretical studies have developed relationships between the flotation rate of fine particles and their size.   k  dn Where k is the flotation rate, d is the particle diameter and the power n is between 1.5 and 2 These calculations are based on dilute systems but the result of experiments in batch flotation cells support a value of n closer to 1.

Batch flotation of fine particles (excess collector)

Recovery by entrainment Entrainment is a very important way of transporting fine particles to the concentrate Non-selective receovery of particles in water recovered to float product. The more water that is recovered – the more fine particles there are that ‘float’ How is water recovered? Carried by bubbles (water layer on and wake behind the bubbles) ‘Crowding’ effect of bubble swarms pussing water into froth  

Water Carried by Bubble Water film on the surface Water in wake Bubble rising in water

Preferential Recovery of Fines Fine particles: same concentration inside and outside of wake Coarse particles: Settle out & Conc. inside wake < outside

Plot below is for < 5 micron particles Plot below is for < 5 micron particles. Slope of the curve changes with different sizes

Average Particle Size (µm) Recovery of quartz relative to water (%) Results for quartz are shown in table The entrainment mechanism becomes less important as size increases Average Particle Size (µm) Recovery of quartz relative to water (%) 40 4 29 18 20 35 14 45 9 59 3.5 72

Controlling Entrainment Use of cleaner stage(s) Froth washing Pulp dilution Gas rate Bank retention time Froth depth …

Cleaner Stage

Froth Washing

Froth Washing

Controlling Entrainment Use of cleaner stage(s) Froth washing Pulp dilution Gas rate Bank retention time Froth depth … Related to total Qg

Retention Time

Combined Flotation & Entrainment Slow true flotation rate of fine particles together with sometimes strong entrainment of both floatable and non-floatable minerals Common to most systems Accounts for much of the behaviour of fine particles

Sulphide Systems Effect of entrainment of quartz on chalcopyrite grade in batch flotation

Ways to improve flotation of fines Increase flotation time Increase particle size by aggregation - this includes selective flocculation, carrier flotation and oil agglomeration. Some of these have specific applications, but not a lot of progress has been made. Specific strong collectors. Once again there is not much evidence of success from this approach Improved equipment to assist bubble paticle collision Dissolved air flotation and other methods to produce very small bubbles aim at this The Microfloat system generates very small bubbles and introduces them to the flotation system Some interest is being shown in these new equipment types.

Recovery of intermediate particles The recovery here is genuine flotation The range of particle size over which good recovery is achieved varies, as shown below, depending on the mineral system and flotation conditions This is the region where flotation is very effective.  

Typical size recovery curves for a number of mineral & collector combinations

Recovery of coarse particles Recovery of coarse particles by flotation only No contribution from entrainment Sometimes they can float very well with recoveries to match or exceed intermediate range

Recovery of coarse particles

Recovery of coarse particles These results demonstrate the major characteristic of coarse particle flotation – the sensitivity of their response to collector addition and chemical environment The first signs of collector or acivator deficiency, an excess of depressant, or unfavourable pH conditions show first in a poor flotation response of the coarse particles.   It is thought that the degree of hydrophobicity required to promote a high level of floatability increases with the size of the particle.