Froth flotation
Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in mineral processing, paper recycling and waste-water treatment industries. Historically this was first used in the mining industry, where it was one of the great enabling technologies of the 20th century. It has been described as "the single most important operation used for the recovery and upgrading of sulfide ores". The development of froth flotation has improved the recovery of valuable minerals, such as copper- and lead-bearing minerals. Along with mechanized mining, it has allowed the economic recovery of valuable metals from much lower-grade ore than previously possible.
Industries
Froth flotation is applied to a wide range of separations. An estimated one billion tons of materials are processed in this manner annually.Mineral processing
Froth flotation is a process for separating minerals from gangue by exploiting differences in their hydrophobicity. Hydrophobicity differences between valuable minerals and waste gangue are increased through the use of surfactants and wetting agents. The flotation process is used for the separation of a large range of sulfides, carbonates and oxides prior to further refinement. Phosphates and coal are also upgraded by flotation technology. "Grade-recovery curves" are tools for weighing the trade-off of producing a high grade of concentrate vs cost. These curves only compare the grade-recovery relations of a specific feed grade and feed rate.Waste water treatment
Froth flotation has been demonstrated for the removal of microplastics from waste water.The flotation process is also widely used in industrial waste water treatment plants, where it removes fats, oil, grease and suspended solids from waste water. These units are called dissolved air flotation units. In particular, dissolved air flotation units are used in removing oil from the wastewater effluents of oil refineries, petrochemical and chemical plants, natural gas processing plants and similar industrial facilities.
Principle of operation
The ore to be treated is ground into particles. In the idealized case, the individual minerals are physically separated, a process known as full liberation. The particle sizes are typically in the range 2–500 micrometers in diameter. For froth flotation, an aqueous slurry of the ground ore is treated with the frothing agent. An example is sodium ethyl xanthate as a collector in the flotation of galena to separate it from sphalerite. The polar part of xanthate anion attaches to the ore particles and the non-polar hydrocarbon part forms a hydrophobic layer. The particles are brought to the water surface by air bubbles. About 300 g/t of ore is required for efficient separation. With increasing length of the hydrocarbon chain in xanthates, the efficiency of the hydrophobic action increases, but the selectivity to ore type decreases. The chain is shortest in sodium ethyl xanthate that makes it highly selective to copper, nickel, lead, gold, and zinc ores. Aqueous solutions with pH = 7–11 are normally used in the process. This slurry of hydrophobic particles and hydrophilic particles is then introduced to tanks known as flotation cells that are aerated to produce bubbles. The hydrophobic particles attach to the air bubbles, which rise to the surface, forming a froth. The froth is skimmed from the cell, producing a concentrate of the target mineral.The minerals that do not float into the froth are referred to as the flotation tailings or flotation tails. These tailings may also be subjected to further stages of flotation to recover the valuable particles that did not float the first time. This is known as scavenging. The final tailings after scavenging are normally pumped for disposal as mine fill or to tailings disposal facilities for long-term storage.
Flotation is normally undertaken in several stages to maximize the recovery of the target mineral or minerals and the concentration of those minerals in the concentrate, while minimizing the energy input.
Flotation stages
The first stage is called roughing, which produces a rougher concentrate. The objective is to remove the maximum amount of the valuable mineral at as coarse a particle size as practical. Grinding costs energy. The goal is to release enough gangue from the valuable mineral to get a high recovery. Some concentrators use a preflotation step to remove low density impurities such as carbonaceous dust. The rougher concentrate is normally subjected to further stages of flotation to reject more of the undesirable minerals that also reported to the froth, in a process known as cleaning. The resulting material is often subject to further grinding. Regrinding is often undertaken in specialized regrind mills, such as the IsaMill. The rougher flotation step is often followed by a scavenger flotation step that is applied to the rougher tailings to further recover any of the target minerals.Science of flotation
To be effective on a given ore slurry, the collectors are chosen based upon their selective wetting of the types of particles to be separated. A good collector will adsorb, physically or chemically, with one of the types of particles. The wetting activity of a surfactant on a particle can in principle be quantified by measuring the contact angles of the liquid/bubble interface. Another important measure for attachment of bubbles to particles is induction time, the time required for the particle and bubble to rupture the thin film separating the particle and bubble. This rupturing is achieved by the surface forces between the particle and bubble.The mechanisms for the bubble-particle attachment is complex but is viewed as consisting of three steps: collision, attachment, and detachment. The collision is achieved by particles being within the collision tube of a bubble and this is affected by the velocity of the bubble and radius of the bubble. The collision tube corresponds to the region in which a particle will collide with the bubble, with the perimeter of the collision tube corresponding to the grazing trajectory.
The attachment of the particle to the bubble is controlled by the induction time of the particle and bubble. The particle and bubble need to bind and this occurs if the time in which the particle and bubble are in contact with each other is larger than the required induction time. This induction time is affected by the fluid viscosity, particle and bubble size and the forces between the particle and bubbles.
The detachment of a particle and bubble occurs when the force exerted by the surface tension is exceeded by shear forces and gravitational forces. These forces are complex and vary within the cell. High shear will be experienced close to the impeller of a mechanical flotation cell and mostly gravitational force in the collection and cleaning zone of a flotation column.
Significant issues of entrainment of fine particles occurs as these particles experience low collision efficiencies as well as sliming and degradation of the particle surfaces. Coarse particles show a low recovery of the valuable mineral due to the low liberation and high detachment efficiencies.
Flotation equipment
Flotation can be performed in rectangular or cylindrical mechanically agitated cells or tanks, flotation columns, Jameson Cells or deinking flotation machines. Classified by the method of air absorption manner, it is fair to state that two distinct groups of flotation equipment have arisen:pneumatic and mechanical machines. Generally pneumatic machines give a low-grade concentrate and little operating troubles.Mechanical cells use a large mixer and diffuser mechanism at the bottom of the mixing tank to introduce air and provide mixing action. Flotation columns use air spargers to introduce air at the bottom of a tall column while introducing slurry above. The countercurrent motion of the slurry flowing down and the air flowing up provides mixing action. Mechanical cells generally have a higher throughput rate, but produce material that is of lower quality, while flotation columns generally have a low throughput rate but produce higher quality material.
The Jameson cell uses neither impellers nor spargers, instead combining the slurry with air in a downcomer where high shear creates the turbulent conditions required for bubble particle contacting.
Chemicals of flotation
Collectors
For many ores, the collectors are anionic sulfur ligands. Particularly popular for sulfide minerals are xanthate salts, including potassium amyl xanthate, potassium isobutyl xanthate, potassium ethyl xanthate, sodium isobutyl xanthate, sodium isopropyl xanthate, sodium ethyl xanthate. Related collectors include sulfur-based ligands such as organodithiophosphates. Still other classes of collectors include the thiourea thiocarbanilide. Fatty acid carboxylates, alkyl sulfates, and alkyl sulfonates have also been used for oxide minerals.For some minerals, fatty amines are used as collectors.
Frothers
Additives are used to stabilize the foams. These include pine oil and various alcohols: methyl isobutyl carbinol, polyglycols, xylenol.Unstable foam would coalesce too quickly and particles would fall off their surface. On the other hand, too stable foam complicates transportation and dewatering of the concentrate formed.