Introduction on flotation separation
What is flotation separation
The flotation separation mainly refers to froth flotation, which is based on the difference in physical and chemical properties of the surface of minerals, using the buoyancy of bubbles in the ore pulp to achieve separation.
The basic process of flotation separation mainly includes: (1) Grind the ore finely to dissociate useful minerals from gangue minerals; (2) Adjust the ore pulp and add flotation agents; (3) Flotation processing; (4) Handle the product (do the filtering, concentrating, or drying).
Significance of flotation separation
1. It is the most widely used and most effective beneficiation method for the separation of fine-grained and particulate minerals;
2. It occupies an important position in various beneficiation methods, with a wide range of applications, and can process non-ferrous metal, non-metal and ferrous metal minerals;
3. Compared with other beneficiation methods, flotation separation is more efficient. Low-grade raw ore can be separated into a high-grade concentrate so that the mineral resources can be fully utilized. For the fine-grained disseminated ore, flotation separation is particularly effective. It solves many problems of the recovery of useful components from fine-grained mineral particles, thereby having a high recovery rate.
How to judge the mineral floatability
The criterion for judging the mineral floatability is mineral wettability.
Surface electrical property and floatability of minerals
The preferential adsorption, preferential dissociation, and lattice substitution of ions can charge the mineral surface in the ore pulp, which can affect the floatability of some minerals.
1. For the Oxide minerals and silicates, such as goethite, corundum and quartz, the surface electrical property will greatly affect the floatability. It lies in that these minerals will have electrostatic adsorption with collectors. The symbols of surface electrical property are related to the pH value of the ore pulp.
2. For the sulfide minerals, such as copper, lead and zinc, collected with xanthate, the surface electrical property has little or no effect on the floatability. That’s because the action of xanthate with minerals mainly depends on the chemical affinity rather than the electrostatic force.
3. In addition, for the non-sulfide minerals collected by oil collectors, the surface electrical property does not affect the floatability.
Adsorption forms of reagents on the mineral surface
- Molecular adsorption: The reagent is adsorbed on the mineral surface in the form of molecules;
- Ion adsorption: The reagent is adsorbed on the mineral surface in the form of ions;
- Ion exchange adsorption: The reagent is absorbed on the mineral surface when the certain ion in the ore pulp has an equivalent exchange with another ion of mineral surface that has the same charge sign;
- Electrical adsorption: The reagent is absorbed on the mineral surface by the electrostatic force;
- Semi micelle adsorption: when the mass fraction of collectors of a long hydrocarbon chain in the slurry is high, their ions or molecules are adsorbed on the mineral surface. Under the Van der Waals force, the non-polar group will associate with each other;
- Specific adsorption: The mineral surface has a special affinity for a certain component in the slurry to produce adsorption.
Effect of oxidation of mineral surface on floatability
Minerals are oxidized by air during storage, transportation, crushing and flotation. The oxidation of minerals, especially metal sulfide ore, can greatly affect the flotation.
Research and experiments have shown that some sulfide minerals will have a better floatability after being slightly oxidized within a certain limit. However, if they are over oxidized, the floatability will decrease.
To control the oxidation degree of minerals and adjust the floatability, some measures should be taken:
1. Adjust the speed of slurry agitation and control the flotation time. It has been proven that the speed of air agitation and duration are the important factors for controlling the oxidation degree of minerals;
2. Adjust the aeration of pulp tank and flotation machine;
3. Adjust the pH value of the ore pulp;
4. Add the oxidant or reductant to promote or inhibit the oxidation of mineral surface.
Selective flocculation can effectively separate the fine-grained minerals.
Its principle is to add the flocculant into the suspension that contains two or more compositions. The flocculant is selectively adsorbed on the surface of a certain mineral composition to promote its flocculation and sedimentation. The remaining compositions still maintain a stable dispersion state to achieve the purpose of separation.
Selective flocculation can be divided into four steps:
Step 1. Add the dispersant
The dispersant, such as water glass, can fully disperse the useful mineral compositions in the ore pulp to make them have no adhesion and connection with each other, thereby creating a better condition for the selective flocculation;
Step 2. Add the flocculant
The flocculant is selectively adsorbed on the surface of target minerals and meanwhile connects with other mineral particles to form larger, loose and porous flocs, which settles in the pulp.
Step 3. Stir the ore pulp
When stirring the ore pulp, please pay attention to the following points. Stirring is generally conducted in different stages with quick stirring first to fully disperse the reagents, and then full stirring to facilitate the selective flocculation of minerals; The ore pulp should be diluted to facilitate the cleanness of flocculant and reduce the adhesion of other minerals; The flocs should not be too large in case that the impurities are entrained.
Step 4. Settlement and separation
When the flocs reach a certain size, settlement and separation shall be carried out. Pay attention to the time and speed of settlement.
The speed of settlement must be slow to prevent impurities from being carried into the sediment. Sometimes the weak water flow can be used to release the impurities from the flocs.
The time of settlement should be appropriate. If the time is too short, the product will be purer, but will affect the recovery rate; if the time is too long, the impurities will also settle to affect the mineral quality. Besides, if the first flocculation fails to meet the product requirements, the flocculation process can be carried out again.
Three-phase system of flotation separation
The three-phase system of flotation includes the solid phase, liquid phase and gas phase. The solid phase refers to the finely ground mineral particles; the liquid phase refers to the water and solution; the gas phase refers to the dispersed bubbles.
Characteristics of the solid phase:
- There are often several useful minerals and gangues associated together, with so many minerals’ types and compositions. In terms of shape, the ore particles are square, columnar, flake, and irregular angular;
- It has a wide particle size range. The biggest particle size can reach 0.8 to 1 mm, while the smallest particle size is below 5 mm;
- It has quantities of ore particles with a large surface area. Each liter of the ore pulp can have tens of millions or even nearly 100 million fine particles, thus the surface area of the solid phase is large.
Characteristics of the liquid phase:
- The Water can interact with the solid and gas phases in the ore pulp, as the ore often contains various soluble salts that can be dissolved in water;
- The industrial natural water often contains some soluble salts;
- Oxygen, nitrogen and carbon dioxide are dissolved in the water. Therefore, the liquid phase is not simply water but is essentially a solution, which can greatly affect the flotation.
Characteristics of the gas phase:
- The air appears as the finely dispersed bubbles in the ore pulp, which can carry the mineral particles to float;
- The air can be repeatedly dissolved and precipitated in the pulp;
- The oxygen in the air can greatly affect mineral floatability.
Measures to increase the flotation speed
The flotation speed refers to the flotation time when a certain recovery rate or unit production capacity of the flotation machine is reached. Thus, by improving the recovery rate or shortening the flotation time, the flotation speed can be increased.
Measures should be taken to increase the flotation speed:
- 1Add the appropriate amount of reagents, especially the amount of foaming agent;
- 2Within an appropriate range, increase the speed of the impeller of the flotation machine and reduce the depth of groove to reduce the gap between the impeller and cover;
- 3Make the ore pulp pass through the flotation tank as soon as possible. The series tank is faster than the parallel tank.
- 4The selected tank must be in an appropriate size;
- 5The pulp concentration should be controlled in a proper range.
Flotation types and processes
- Positive flotation: The foams that float in the surface of pulp is the useful minerals, while the gangue minerals remain in the pulp;
- Reverse flotation: The foams that float in the surface of pulp are the gangue minerals, while the useful minerals remain in the pulp;
- Differential flotation: It separates the useful minerals to be a single concentrate in turn;
- Bulk flotation: It separates the useful minerals together, and then separates each of them in turn.
The flotation reagents are mainly divided into three types: collectors, regulators and foaming agents.
- Collectors are the reagents that aim to collect the target minerals by enhancing the hydrophobicity so that the floating mineral particles can adhere to the air bubbles.
- Regulators are the reagents that aim to change the properties of the mineral surface, adjust the interaction between minerals and collectors, and adjust the properties of pulp. The commonly used regulators mainly include inhibitors, activators and pH regulators;
- Foaming agents are the reagents that aim to produce a stable foam layer that can float minerals.
Xanthate is the collector of sulfide ore with a strong collecting ability. It is a yellow crystalline powder with an irritating effect. Because xanthate is easy to be soluble in water, and easy to be decomposed when it is heated, damp, and exposed to alkali or acid, it should be stored in a cool and dry place.
Its chemical name is hydrocarbyl dithiocarbonate, which includes ethyl xanthate, butyl xanthate, amyl xanthate, etc.