Bubbling Innovation

As summarised by F.D Richardson in his 1971 lecture of the American Institute of Mining Engineers (AIME), “Droplets and bubbles are of great importance to the extractive metallurgist in his attempts to speed up processes…” Gas injection and bubble-making are used to accelerate high temperature processes, such as decarburisation in steel-making, as well as smelting and converting in sulfide processing. Gas dispersion is also used in more familiar processes, including a personal favourite of many engineers, beer making.

Spargers and the flotation process

In the area of extractive metallurgy mediated in aqueous environments, gas bubbles are also fundamental to separation processes and unit operations. The most important of these are froth flotation and hydrometallurgical leaching. As knowledge of the principles governing these separation processes has increased, more attention has been placed on the properties of bubbles and their populations – such as bubble concentration, stability, and size distribution. It is now recognised that the efficiency and speed of these extractive processes can be greatly improved and optimised by using engineering principles to tailor bubble properties.

Gas bubbles in extractive metallurgy

The class of equipment that creates and delivers bubbles into liquids is known as sparging systems. In the case of flotation, an ore is introduced into water and ground into a finely divided state to form a slurry. The grinding endpoint has been selected so that that an optimal amount of the ore particles are liberated, meaning that a significant fraction of the surface has an exposed hydrophobic species. In many cases, the ore surfaces are naturally hydrophobic, such as graphite. In other cases, hydrophobicity is achieved through the use of ‘collectors’, which are surface-active chemicals. These attach to exposed mineral surfaces and provide an outward facing hydrophobic tail that confers hydrophobicity. Once the particle surface has a significant hydrophobic character, it is ready to be combined with air bubbles introduced through a sparging system.

When the sparged air bubbles approach ore particles, they may collide, and if a bubble successfully collides with a hydrophobic species on a particle, the bubble may attach. This is based on surface chemistry and the thermodynamic preference of a hydrophobic surface to be in contact with air rather than water. When a bubble or bubbles have successfully attached to a particle in water, there may be enough buoyancy created by the bubbles to counteract the gravity acting on the particle and the ‘bubble-particle aggregate’ has become floatable. In the absence of other countervailing forces, such as excessive convection and froth layer drop-back, the bubble-particle aggregates will rise to the top of the flotation vessel and be recovered in a froth layer.

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