Comparative Strengths and Weaknesses

Column cells are flotation devices that also act as three-phase settlers where particles move downwards in a hindered settling environment. Within the vessel there is a distribution of particle residence times dependent on settling velocity that may impact on the flotation of large particles. Impeller devices do not suffer from this effect to the same degree but do require higher energy input to suspend larger particles.

The low turbulence in columns means particles usually have low momentum, which in turn may reduce the probability of collection by passing bubbles. As a result, fine particle recovery may be hindered when compared to the capabilities of impeller-type designs.

The mechanism of particle-bubble collision in columns is different from intensive mixing devices such as impeller cells. Under the low intensity mixing caused only by a rising bubble swarm, particle drift from the liquid streamlines is caused mainly by gravity and inertial forces and also by interception, while in mechanical cells, according to many researchers, bubble-particle collision occurs at their relative movement within a turbulent vortex or at adjacent vortices. Also, as velocities of both bubble and particle during the attachment are slower under the quiescent conditions in a column, the contact time is generally higher. Therefore, probabilities of both collision and adhesion (components of attachment probability) are different to those in mechanical flotation processes.

The lower velocity gradient and less intensive shear forces in the vicinity of rising bubbles under low turbulent conditions in a column lead to reduced detachment probability. The latter is most important for improvement of recovery for coarse, heavy or weakly hydrophobic particles.

A column can support a deep froth bed and may use washwater to maintain a downward flow of water in all parts of the vessel. This essentially eliminates the entrainment of hydrophilic particles in the float product when the vessel is used for solid-solid separation. This property, along with the absence of stray flows of feed material to the float product by turbulence, means that column devices are normally superior to impeller-type machines for the selective separation of fine particles.

In immiscible liquid separation duties, columns do not emulsify the material like impeller devices.

The bubbles used in a column are usually generated within the size range that maximizes interfacial surface flux and collection intensity through the vessel. Dissolved air systems nucleate micrometer-sized bubbles on particles which require very low downward liquid velocities in large volume vessels to separate the bubble and water. Also, dissolved air systems cannot provide air hold-up higher than approximately 4-6%, due to limited gas solubility and lower flooding limits caused by the microbubbles. In mechanical cells, bubbles are usually generated by shear action of the impeller; thus, bubble size is dependent on both air flow rate and impeller rotation speed. As such, bubble size cannot be controlled independently of cell turbulence.

The height-to-diameter ratio of a column is significantly higher than the impeller-type machines. As a result, control and consistency of flow are more critical. The column requires much less floor space to operate.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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