FLOTATION PROCESS

Industrial flotation machines can be divided into four classes:

1. mechanical
2. pneumatic
3. froth separation
4. column
5. air-lift
6. matless

As pneumatic and froth separation devices are not commonly used in industry today, no further discussion about them will be given in this module. The mechanical machine is dearly the most common type of flotation machine currently used in industry, followed by the column machine which has recently experienced a rapid growth.

A mechanical machine consists of a mechanically driven impeller that disperses air into the agitated pulp. In normal practice this machine appears as a long tank-like vessel having a number of impellers in series. Mechanical machines can have open flow of pulp between the impellers or can be of cell-to-cell design with weirs between them. Below is a typical bank of flotation cells used in industrial practice.

The procedure by which air is introduced into a mechanical machine falls into two broad categories: self-aerating, where the machine uses the depression created by the impeller to induce air, and supercharged, where air is generated from an external blower. The incoming feed to the mechanical flotation machine is usually introduced in the lower portion of the machine. At the very below is shown a typical flotation cell of each air delivery type (Agitair & Denver)

Bank of flotation cells: schematic flotation process

The most rapidly growing class of flotation machine is the column machine, which is, as its name implies, a vessel having a large height-to-diameter ratio (from 5 to 20) in contrast to mechanical cells. This type of machine provides a counter-current flow of air bubbles and slurry with a long contact time and plenty of wash water. As might be expected, the major advantage of such a machine is the high separation grade that can be achieved, so that column cells are often used as a final concentrate cleaning step. Special care has to be exercised in the generation of fine air bubbles and the control of the feed rate to the column cell for such cells to be effective. Column cell use is often of limited value in the recovery of relatively coarse valuable particles; because of the long lifting distances involved, the bubbles can not carry large particles all the way to the top of the cell.

Whatever flotation machine design is chosen, it must perform a series of complicated functions. Some of the more obvious are:

• Good mixing of pulp. To be effective, a flotation machineshould maintain all particles uniformly in suspension within the pulp, including those of relatively high density anchor size. Good mixing of pulp is required for maximizing bubble-particle frequency.
• Appropriate aeration and dispersion of fine gas bubbles. An important requirement of any flotation machine is the ability to provide uniform aeration throughout as large a volume of the machine as is possible. In addition, the size distribution of the gas bubbles generated by the machine is also important, but experience has shown that the proper choice of frother type and dosage generally dominates the bubble size distributions being produced.
• Sufficient control of pulp agitation in the froth zone. As mentioned earlier, good mixing in the machine is important. However, equally important is that near and in the actual froth bed at the top of the machine, sufficiently smooth or quiescent pulp conditions must be maintained to ensure suspension of collector-coated particles
• Efficient mass flow mechanisms. It is also necessary with any flotation machinethat appropriate provisions be made for feeding pulp into the machine and also for the efficient transport of froth concentrate and tailing slurry away from the machine.

Advantages

The most important feature of these machines is the absence of a mixer propeller. Compressed air mixes and aerates the pulp from below. homogeneous distribution of air in the cell and

Care should be taken to ensure that the bubbles are small in size.

The pulp conditioned in the cell shown in the figure is pumped into the aeration reactor at high pressure. The pulp, which is ventilated in the aeration reactor, is fed into the separation cell as a jet. While the hydrophobic particles rise to the surface, the hydrophilic particles sink to the bottom and are taken from the bottom as waste.

The advantages of air flotation machines are:

• Suitable for selective flotation.
• Cell volume is less and investment cost is low.
• Energy cost per unit area is low.
• Since there are no moving parts, the wear cost is very low.