Particle size effect on flotation process

There are many factors affecting the flotation process, among which the more important ones are:
particle size (grinding fineness), pulp density, chemical addition and adjustment, bubble and foam adjustment, pulp temperature, flotation procedure, water quality, etc.

Experience has proven that flotation process factors must be determined and selected according to the characteristics of the ore properties and through experimental studies in order to obtain the optimal technical and economic indicators.

1. Particle size effect on flotation

In order to ensure a high process index for flotation, it is important to study the effect of particle size on flotation and to determine the most suitable feed size (fineness) and other process conditions according to the nature of the ore. Flotation requires not only adequate monomeric dissociation of the minerals, but also a suitable feed size. Ore particles are too coarse, even if the minerals have been monomeric dissociation, because more than the bubble flotation capacity, often can not float. The upper limit of flotation size for each type of minerals is different, such as sulfide minerals are generally 0.2-0.25 mm, non-sulfide minerals for 0.25-0.3 mm, for some less dense non-metallic minerals such as coal, the upper limit of particle size can also be improved. However, the grinding size is too fine (such as less than 0.01 mm) is also detrimental to flotation. Practice has shown that there are differences in flotation behavior for various particle sizes(Refer Table 1)

The data in the table illustrate that different minerals have their own optimal particle size range for flotation. Both too coarse (>0.1 mm) and too fine (>0.006 mm) particle sizes are not conducive to flotation and recovery is reduced.

Timely testing of the changes in the fineness of the classification overflow can provide a basis for the grinding and classification operation. In the absence of automatic particle size measurement and automatic adjustment, the rapid sieve analysis method can generally be used for detection. If the fineness does not meet the requirements, the operating conditions of the grinding and classifying equipment should be changed in time, for example, adjusting the feed rate of the mill, the overflow concentration of the classifier, the grinding concentration, etc.

Timely check the particle size composition of the flotation concentrate and tailings can also be found in the changes in grinding fineness, such as increased loss of coarse particles in the tailings, the so-called “run coarse”, indicating that the grinding fineness is not enough; if the main loss of metal in the fine-grain level, it means that the grinding has been too much, should be appropriate coarse grinding and strengthening classification operations.

Both coarse and ultrafine grains (slimes) have many special physical and physicochemical properties, and their flotation behavior is different from that of normal size grains (0.001 < d < 0.1 mm), thus requiring special process conditions in the flotation process.

2. Process measures for coarse grain flotation

Under the premise of single dissociation of ore particles, coarse grinding flotation can save grinding cost and reduce beneficiation cost. When dealing with unevenly embedded ores and large porphyry copper ores, there is a tendency to flotation after coarse grinding under the premise of ensuring the recovery of coarse separation. However, as the coarser ore particles are heavier, they are not easily suspended in the flotation machine, and the chance of collision with air bubbles is reduced, and they are easily dislodged after attaching air bubbles due to high dislodging force. Therefore, the flotation effect of coarse grains is poor under the general process conditions. In order to improve the flotation of coarse grains, the following special process conditions can be used.

A. Selection and adjustment of flotation machine

Practice has proven that the strong turbulent motion of the pulp in the mechanical agitated flotation machine is the main factor that drives the ore particles off the bubble. Therefore, reducing the turbulence intensity of the pulp movement is a fundamental measure to ensure the flotation of coarse grains. To this end, measures can be taken on a case-by-case basis.

(1) Select special flotation machines suitable for flotation of coarse particles, such as ring shot flotation machine (China), Skinner (shinair) type flotation machine (Finland), froth sorting and fluidized flotation machine (former Soviet Union), etc..

(2) Improve and adjust the structure and operation of the conventional flotation machine, such as: appropriately reduce the depth of the tank (using shallow tank type) to shorten the floating distance of the mineralization bubbles to avoid the ore particles from falling off; add a grizzly screen above the impeller area to weaken the turbulent intensity of the pulp and keep the froth area smooth; increase the inflatable volume, form more large bubbles, which is conducive to the formation of a floating mass composed of bubbles and ore particles, and “arch lift” the coarse particles “float”; scraping bubble quickly and smoothly.

B. Increase the pulp density appropriately

C. Improve the chemical system

The purpose of choosing strong collecting agent and reasonably increasing the density of the agent is to enhance the solidification strength of minerals and air bubbles and speed up the floating speed, in addition, adding non-polar oil, such as diesel oil, kerosene and other auxiliary trapping agent can “consolidate” the three-phase contact periphery and enhance the solidification strength of minerals and air bubbles.

3. Process measures for fine particle flotation

Fine grains are usually mineral slimes less than 18 microns or less than 10 microns. Due to the small mass and large specific surface area of the fine-grained particles (sludge), this causes a series of special behaviors of the particles in the slurry and flotation process.

From the role of particles and particles, due to the significant enhancement of particle surface energy, under certain conditions, different mineral particles are prone to intercoagulation and the formation of non-selective aggregation, particles are easy to adhere to the surface of the coarse particles to form a mineral mud cover.

From the role of particles and media, particles have a large specific surface area and surface energy, therefore, has a high adsorption capacity of chemicals, adsorption selectivity is poor; surface solubility increases, so that the slurry “inevitable ion” increase; small mass is easy to be water mechanical entrainment and foam mechanical entrainment.

From the role of particles and air bubbles, due to contact efficiency and adhesion efficiency is reduced, so that the capture rate of air bubbles on the mineral particles decreased, while the generation of air bubbles of mineral sludge “armor” phenomenon, affecting the amount of air bubble transport.
All of the above actions are the main reasons for the slowing down of fine particle flotation, bad selectivity, lower recovery and significant deterioration of flotation index.

In order to mitigate and prevent the harmful effects of mud and to enhance fine-grained flotation, the following process measures are often used in recent flotation practice.

(1) Eliminate and prevent the interference of mineral sludge to the flotation process, the main measures are
-Desliming is a way to eradicate the influence of mineral mud. The most commonly used method is to separate the mud from a certain particle size with a hydrocyclone before flotation, or discard it or treat the mud and coarse sand separately, which is called “mud-sand separation”; for some easy-to-float mud, a small amount of frothing agent can be added before flotation to pre-flotation to remove it.

-Add mud dispersant to fully disperse the mud can eliminate the phenomenon of “mud coverage” and the harmful effect of non-selective mutual condensation between particles, commonly used mud dispersant are: water glass, sodium carbonate, sodium hexametaphosphate, etc.

-Batch dosing will be added to the trapping agent in sections and batches, not only to maintain the effective concentration of chemicals in the pulp, but also to improve selectivity.

-Reduce the pulp density of flotation, on the one hand, it can reduce the sludge pollution concentrate froth; on the other hand, it can also reduce the viscosity of the pulp.

(2) The use of particulate minerals with chemical adsorption or chelating effect of the trapping agent, in order to help improve the selectivity of the flotation process.

(3) The application of physical or chemical methods to increase the size of the appearance of particulate minerals, to improve the flotation rate and selectivity of the minerals to be sorted, the main new processes developed in recent times according to this principle are.

-Selective flocculation flotation uses flocculants to selectively flocculate particulate destination minerals or vein mud, which is then separated by flotation. This method has been used for flotation of fine-grained hematite ore.

-Carrier flotation uses mineral particles of general flotation grade as carriers, so that the fines cover the carrier, and then flotation with the carrier. The carrier can be of the same type of carrier (mineral) or of a different type of carrier (mineral).

-Agglomerate flotation, also known as emulsion flotation, in which fine-grained minerals are treated with a trapping agent and formed into an oily froth with ore under the action of neutral oil. This method has been used to sort fine-grained manganese ore, ilmenite, apatite, etc. The operating process conditions are divided into two categories: one is that the trapping agent and neutral oil are first formulated into an emulsion and added; the other is that in a high concentration slurry (70% solids), neutral oil and trapping agent are added successively with strong stirring.

(4) Reduce the bubble size to achieve microbubble flotation under certain conditions, reduce the bubble size, not only can increase the gas-liquid interface, while increasing the chances of particle collision and adhesion, which is conducive to the flotation of particulate minerals. The main processes are.

-Vacuum flotation using a pressure-reducing device, precipitating microbubbles from the solution of the vacuum flotation method, bubble particle size is generally 0.1-0.5 mm. Research has proved that the precipitation of microbubbles from water flotation of fine grains of heavy product stone, fluorite, quartz, etc. is effective. Other conditions are the same, with conventional flotation, the grade of barite concentrate is 54.4% and the recovery is 30.6%, while with vacuum flotation crystal level can be increased to 53.6%-63.6%, and the corresponding recovery is 52.9%-45.7%.

-Electrolytic flotation uses the method of electrolytic water to obtain micro bubbles, the general bubble particle size of 0.02-0.06 mm, used in flotation of fine-grained cassiterite, with electrolytic hydrogen bubble flotation alone, the coarse recovery is significantly higher than conventional flotation, from 35.5% to 79.5%, while the grade increased by 0.8%. In addition, in recent years, other new processes such as controlled dispersion flotation and branch flotation for iron ore and wolframite fine mud flotation have been carried out, all with good results.