Froth flotation Froth flotation is flotation is a process for selectively separating hydrophobic hydrophobic materials materials from hydrophilic hydrophilic.. This is used in several processing 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" ores".[! The development of froth froth flotation flotation improved the recovery of valuable minerals minerals,, such as copper and lead leadbearing bearing minerals. #long with mechani$ed mining, it allowed the economic recovery of valuable metals from much lo wer grade ore than before.
edit!! Principle of operation [edit %efore froth flotation can wor&, the ore to be treated is reduced to fine particles b y crushing and grinding 'a process &nown as comminution comminution(( so that the various minerals e)ist as physically separate grains. This process is &nown as liberation liberation.. The particle si$es are typically less than 0. mm '00 *m(, but sometimes si$es smaller than +0 *m are re-uired.[2! There is a tendency for the liberation si$e of the minerals to decrease over time as the ore bodies with coarse mineral grains that can be separated at larger si$es are depleted and replaced by ore bodies that were formerly considered too difficult. In the mining industry, the plants where flotation is underta&en to concentrate ore ore are are generally &nown as concentrators or mills mills.. or froth flotation, the ground ore is mi)ed with water to form a slurry and the desired mineral is rendered hydrophobic by the addition of a surfactant or collector chemical chemical 'although some mineral surfaces are naturally hydrophobic, [/! re-uiring little or no addition of collector(. The particular chemical depends on the nature of the mineral to be recovered and, perhaps, the natures of those that are not wanted. #s an e)ample, sodium ethyl )anthate '"1"( )anthate '"1"( may be added as a collector in the selective flotation of galena galena'lead 'lead sulfide( to separate it from sphalerite sphalerite '$inc '$inc sulfide(. This slurry 'more properly called the pulp the pulp(( of hydrophobic particles and hydrophilic particles is then introduced to tan&s &nown as flotation cells that cells that are aerated to produce bubbles. The hydrophobic particles attach to the air bubbles, which rise to the surface, forming a froth. The froth is removed from the cell, producing a concentrate '"con"( of the target mineral.
rothing agents, &nown as frothers, may be introduced to the pulp to promote the formation of a stable froth on top of the flotation cell. The minerals that do not float into the froth are referred to as the flotation tailings or flotation tails. These tailings may also be sub3ected to further stages of flotation to recover the valuable particles that did not float the first time. This is &nown as scavenging . The final tailings after scavenging are normally pumped for disposal as mine fill or to tailings disposal facilities for longterm storage. roth flotation efficiency is determined by a series of probabilities4 those of particlebubble contact, particlebubble attachment, transport between the pulp and the froth, and froth collection into the product launder .[5! In a conventional mechanicallyagitated cell, the void fraction 'i.e. volume occupied by air bubbles( is low '6 to 0 percent( and the bubble si$e is usually greater than mm.[6! This results in a relatively low interfacial area and a low probability of particlebubble contact. [6! 7onse-uently, several cells in series are re-uire d to increase the particle residence time, thus increasing the probability of particlebubble contact.[6! lotation is normally underta&en in several stages to ma)imi$e the recovery of the target mineral or minerals and the concentration of those minerals in the concentrate, while minimi$ing the energy input.
Science of flotation[edit! 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. # good collector will adsorb, physically or chemically, with one of the types of particles. This provides the thermodynamic re-uirement for the particles to bind to the surface of a bubble. The wetting activity of a surfactant on a particle can be -uantified b y measuring the contact angles that the li-uid8bubble interface ma&es with it. #nother important measure for attachment of bubbles to particles is induction time. The induction time is the time re-uired 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 bubbleparticle attachment is very comple) and consists 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 gra$ing tra3ectory. The attachment of the particle to the bubble is controlled by the induction time of the particle and bubble. The particle and bubble need to bind and this occurs if the time in which the particle and bubble are in contact with each other is larger than the re-uired induction time. This induction time is affected by the fluid viscosity, particle and bubble si$e and the forces between the particle and bubbles. The detachment of a particle and bubble occurs when the force e)erted by the surface tension is e)ceeded by shear forces and gravitational forces. These forces are comple) and vary within the cell. High shear will be e)perienced close to the impeller of a mechanical flotation cell and mostly gravitational force in the collection and cleaning $one of a flotation column. ignificant issues of entrainment of fine particles occurs as these particles e)perience low collision efficiencies as well as sliming and degradation of the particle surfaces. 7oarse particles show a low recovery of the valuable mineral due to the low liberation and high detachment efficiencies
Flotation equipment [edit!
9iagram of froth flotation cell. :umbered triangles show direction of stream flow. # mi)ture of ore and water called pulp [! enters the cell from a conditioner, and flows to the bottom of the cell. #ir [2! or nitrogen is passed down a vertical impeller where shearing forces brea& the air stream into small
bubbles. The mineral concentrate froth is collected from the top of the cell [/!, while the pulp [5! flows to another cell.
lotation can be performed in rectangular or cylindrical mechanically agitated cells or tan&s, flotation columns, ;ameson 7ells or dein&ing flotation machines. 7lassified by the method of air absorption manner, it is fair to state that two distinct groups of flotation e-uipments have arisen4pneumatic and mechanical machines.
Mechanics of flotation[edit!
The following steps are followed, following grinding to liberate the mineral particles4
. >eagent conditioning to achieve hydrophobic surface charges on the desired particles 2. 7ollection and upward transport by bubbles in an intimate contact with air or nitrogen /. ormation of a stable froth on the surface of the flotation cell 5. eparation of the mineral laden froth from the bath 'flotation cell( Simple flotation circuit for mineral concentration. :umbered triangles show direction of stream flow, ?arious flotation reagents are added to a mi)ture of ore and water 'called pulp( in a conditioning tan&. The flow rate and tan& si$e are designed to give the minerals enough time to be activated. The conditioner pulp [! is fed to a ban& of rougher cells which remove most of the desired minerals as a concentrate. The rougher pulp [2! passes to a ban& of scavenger cells where additional reagents may be added. The scavenger cell froth [/! is usually returned to the rougher cells for additional treatment, but in some cases may be sent to special cleaner cells. The scavenger pulp is usually barren enough to be discarded as tails. =ore comple) flotation circuits have several sets of cleaner and recleaner cells, and intermediate regrinding of pulp or concentrate.
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