Froth flotation is achieved by mixing chemicals known as collectors with the ore slurry. The collectors adsorb onto the surfaces of the particles of select minerals (usually the valuable mineral that is targeted for concentration), making these minerals hydrophobic. Air is passed through the slurry in a tank known as a flotation cell. The air is broken into tiny bubbles by various mechanisms (depending on the design of the flotation cell), and the now-hydrophobic minerals attach to the bubbles, rising with them to the surface of the flotation cell, where they form a froth. The froth flows over the top edge (or "lip") of the flotation cell and forms the flotation concentrate. Ideally, none of the unwanted mineral particles float, and they remain behind as the flotation tailings.
However, the selectivity of the collection mechanism is not perfect. Some unwanted ("gangue") minerals are also carried into the froth, largely by entrainment with the water rising with the bubbles. This is particularly the case for particles less than 10 μm in size. Some of the gangue particles follow the water between the bubbles as it drains back to the underlying pulp. This process can be assisted by the application of sufficient "wash water" to the froth to displace the water entrained with the bubbles and the fine gangue particles brought with them. Column flotation cells, invented in Canada by Boutin and Tremblay in 1961, grew increasingly popular in the 1980s and 1990s as a way of reducing entrainment of fine gangue particles during "cleaning" of flotation concentrates. With heights usually between 6 and 14 meters, they could have froth depths up to 2 m, providing more residence time than conventional cells and more stable froth surfaces that enable better froth washing.
Froth flotation efficiency is determined by a series of probabilities: those of particle–bubble contact, particle–bubble attachment, transport between the pulp and the froth, and froth collection into the product launder.
In a conventional mechanically agitated cell, the void fraction is low (5–10%) and the bubble size is large (2–3 mm), which results in a low interfacial area with a low probability of particle–bubble contact. In a conventional flotation column, the void fraction is similarly low, and so the probability of particle–bubble contact is increased by increasing the height of the column to provide greater residence time.
Traditionally, the ore slurry and the air are introduced separately to the flotation cell (see Figure 2). The Jameson Cell differs from this traditional approach by mixing the slurry with the air in the downcomers.
The slurry is introduced at the top of the downcomer as a jet that draws in air through a second pipe to form a stable two-phase mixture (see Figure 3). The plunging slurry jet shears and then entrains the air. The target minerals, with their collector-coated surfaces, attach to the bubbles and this mixture travels down the downcomer, driven by hydrostatic forces, before it is discharged into the tank portion of the Jameson Cell (see Figure 4). The downcomer is designed to provide high-intensity mixing of the air and the slurry to generate a dense foam of fine bubbles and maximise the contact between the target mineral particles and the bubbles. The probability of particle–bubble contact is "virtually 100%" with a slurry residence time in the downcomer of 5–10 seconds.
The high probability of particle–bubble contact, and subsequent short residence times (five to ten seconds in the downcomer, allows for a much more compact column design than conventional column flotation cells (see Figure 1). The fine nature of the bubbles (0.3 to 0.5 mm in diameter) gives them enhanced carrying capabilities for fine mineral particles. Fine bubbles also improve the separation of minerals, as they intensify the difference in the flotation kinetics of the valuable minerals from the gangue minerals, thus allowing higher grade concentrates to be produced.
The foam in the downcomer is about 50–60% air. Because of this, the pulp is distributed in the form of thin interfacial slurry films between the bubbles, providing an ideal environment for particle–bubble contact. Collection occurs by migration of the particles within the thin films, which are not much thicker than the diameter of the particles.
The best collection occurs when the volume of air roughly equals that of the injected slurry.
The Cell is operated by initially closing the air inlet at the top of the downcomer and feeding the flotation pulp in through the nozzle. The air in the downcomer is entrained in the pulp, creating a partial vacuum that draws pulp from the tank up into the downcomer. The pulp level quickly reaches the nozzle, which is at a level above that of the liquid level in the tank. This creates a hydrostatic head in the downcomer, meaning that the pressure inside the top of the downcomer is lower than the atmospheric pressure. When the inlet is opened, air is drawn into the top space of the downcomer by this lower pressure, where it too is entrained into the downcomer contents by the plunging jet. At the same time, a downward flow is established in the pulp in the downcomer that is sufficient to counter the buoyancy of the bubbles, and the aerated pulp discharges into the tank.
Once in the tank, the wider cross sectional area of the tank reduces the downward superficial velocity of the mixture, allowing mineral-laden bubbles to disengage from the liquid and rise to the surface as they would in a conventional cell, where they form the froth. The velocity of the mixture discharging into the tank, and the large density differential between it and the remainder of the pulp in the tank, results in recirculating fluid patterns that keep the particles in the tank in suspension without requiring mechanical agitation.
The purpose of the tank is simply for bubble–pulp separation, so the volume of the tank is small compared with alternative technologies.
The froth that forms at the top of the tank flows over its lip to be collected. This froth can be "washed" by a light flow of water, if desired. The bubbles flowing over the lip of the cell are smaller in diameter than those that flow over the lip of conventional flotation columns.
The non-floating tailings are discharged through a hole in the bottom of the tank.
The Cell has no moving parts and no requirement for compressed air or sparging mechanisms. This results in lower power consumption than the equivalent mechanical or column flotation cells. Maintenance costs are also lower because the only wearing part is the slurry lens used to create the jet in the downcomer.