The mining industry has grown and developed over the years and has adapted itself to changing economic conditions. One of the changes has been the transition from cell-to-cell type to open flow type flotation machines. The high aerating open flow type flotation machine was developed specifically to meet modern requirements.

Principle of Operation

The flotation machine is the first mechanical open type machine to incorporate a vertical circulation of pulp, made possible by combining a “recirculation well” with the distinctive minerals top feed impeller. This arrangement provides positive vertical circulation of pulp similar to the action in a propeller agitator. Pulp from an intermediate zone of the cell is circulated down into the eye of the impeller, mixed with air, and diffused out over the entire bottom of the cell, creating a uniform upward current. This principle varies from most other mechanical open type cells which circulate pulp by lifting it up from the bottom of the cell into the centre of a rotating element.

Effective Aeration

The primary physical requirement of any flotation machine is to disperse finely divided air bubbles throughout the cell. In a machine, which induces its own air, maximum aeration rate is a function of impeller speed. Aeration rate and pulp mixing and circulation rates are inter-related.

Machines, which are designed to use an external source of air have a controlled aeration rate so that it can be maximized independently of impeller speed. The mechanism design is critical to ensure effective air-pulp mixing and dispersion within the cell.

The low pressure flotation machine requires air. An air pipe surrounding the impeller shaft leads directly to the centre of the impeller.

Immediately surrounding this air pipe is the pulp recirculation well which also leads to the open throat of the impeller. Air entering the cell is forced through the vertical curtain of re-circulating pulp and thus a positive air-pulp mixing occurs in the eye of the impeller. This mixture is then subjected to the impeller and is expelled against the diffuser blades, which further mix and shear the air and pulp.

The fluidizing action of vertical recirculation greatly minimises the dense sands zone at the base of the cell and this also gives reduced interference to the pulp-air mixture as it discharges from the diffuser.

The aeration efficiency of flotation machines is directly related to the volume of air and the volume of re-circulating pulp that is pumped through the impellers. Power is used effectively as it pumps, shears and re-circulates the air-pulp mixture.

Solids Suspension

In a cell-to-cell type flotation machine the sanding problem is minimised, as the pulp is 100% mixed in every cell as it passes through the feed pipe and impeller. In a conventional open-type machine, as the pulp progresses from one cell to the next, the coarser, heavier fractions tend to stratify or settle to the bottom of the cell.

The DR principle of vertical recirculation of pulp effectively minimises stratification and sanding.

If the pulp is circulated only in the bottom zone of the cell, solids may be kept in suspension, but additional solids from the upper area continue to settle and accumulate in the lower circulating zone until this zone stabilises at a much higher density. This stratification in the cell causes increased power requirements, accelerates part wear, brings about short-circuiting of pulp, limits the effective mixing of air and pulp in the upper area of the cell, and creates fluctuating operating conditions.

The flotation machine provides a fluidizing action to the pulp by vertical recirculation of pulp. Pulp from the upper zone of the flotation cell is brought down through the pulp recirculation well to the impeller, mixed with air and the pumped by the impeller into the cell creating a homogenous mixture in the tank. The action is possible only in flotation machines with “top feed”.

The flotation machine uses power to keep solids in suspension by vertical recirculation of pulp through the impeller. Power is thus used more efficiently than where agitation is caused by mere mechanical rotation.

Large volume flotation machines have been field proven. These machines have undergone fullscale plant tests and the results of these tests have justified the installation of large volume flotation machines throughout the world.

Flotation machines are now usually available in capacities up to 42.5 m3 (1500 cubic feet) per single mechanism.

Shallow Flotation in Deep Cells

The top vertical recirculation of pulp is ideal for handling all types of pulp containing coarse and abrasive solids or finely ground solids. In principle, the cell functions as a shallow flotation cell. As the air and pulp are hydraulically lifted to the top of the pulp recirculation well, the mineral laden air bubble has only to travel a short distance on its own to the froth overflow level.

Intense agitation and aeration occur in the mixing zone at the bottom of the cell. This mixing or vertical recirculation zone provides highly effective retention volume. It is within this portion of the flotation cell that intimate contact of the conditioned pulp and controlled air is made. The recirculating pulp to the top of the pulp recirculation lifts the mineralised air bubbles hydraulically well. Here a calm separation zone is created. It permits floatable minerals to separate from those, which will not float, and a mineralised froth is formed.

The mineralised air bubble has only a short travel on its own and the action is, in effect “shallow-cell flotation”.