(November 19th, 2021)

Mixing in the Minerals Processing Industries

By Richard K. Grenville, Director of Mixing Technology, Philadelphia Mixing Solutions, LLC an SPX Flow Brand (https://www.spxflow.com/)

For many operations in the minerals processing industries, the contacting of ore particles with a liquid is critical to the successful extraction of the desired metal product. This operation is carried out in large vessels in which an agitator provides energy to suspend the particles above the base of the vessel and to promote contacting of the liquid with them. There is a minimum speed at which the agitator must operate, often referred to as the “just suspension speed.” At this speed, no particles are stationary on the vessel base for an extended period. They’re constantly in contact with the liquid. When this mixing criterion has been met, the resistance to mass transfer between the particles and the liquid is diffusion through the liquid film around each particle. At lower speeds, when the particles can settle, the mass transfer rate depends on diffusion through the bed of settled particles which is much slower. Also, at speeds below the just suspended speed, particles will accumulate on the base, since they are not moving. Eventually the vessel will have to be drained so these particles can be cleaned out. This reduces the productivity of the plant and adds to operational costs.

While mixing can affect the rate of mass transfer between the liquid and the particles, the overall rate of an extraction process will also be determined by the diffusion within the ore particle. This is independent of mixing. The rate in this step of the process can be increased by grinding the particles to a smaller size, but that can make the particles more difficult to separate from the liquid after extraction, because their settling velocity will be lower.

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