Mineral carbonation is the reaction of carbon dioxide with alkaline minerals such as calcium or magnesium oxide. Mineral carbonation is used for the industrial production of solid particles in the pharmaceutical, polymer and fertilizer industry as well as for carbon dioxide sequestration in alkaline materials. Particle treatment by power ultrasound has been found a successful means of process intensification resulting in higher carbonation conversion and faster reaction speed.

Mineral Carbonation: Process and Limitations

For carbonation, natural and waste materials are carbonated due to the presence of alkaline oxides, hydroxides or silicates in their composition. The carbonation process consists of the following reaction steps:

Steps of mineral carbonation

For the carbonation reaction, the particles must be available for the reagents. This means a high particle surface without passivating layers is required to improve the carbonation process.
The formation of an increasingly thick and dense carbonate layer surrounding the shrinking unreacted core of the solid particle creates three rate limiting steps:

• hydration of oxides/ silicates;
• leaching of cations; and
• diffusion to reaction zone.

To improve the carbonation process, these limitations have to overcome by a process assisting technology. High power ultrasound has been successfully applied as a process intensification technology enhancing the carbonation rate and the reaction speed.

Solution: Ultrasonic Carbonation

By the research group of the Katholieke Universiteit Leuven in Belgium, “ultrasound has been proven to be a potentially useful tool for intensification of mineral carbonation processes. Due to enhanced mixing, particle breakage and removal of calcium carbonate passivating layers it was possible to accelerate the reaction kinetics and achieve greater carbonation extent in shorter times. Moreover, in combination with magnesium ions in solution, ultrasound significantly enhances the synthesis of aragonite crystals, both by reducing the required concentration of magnesium and reducing the reaction temperature to near ambient conditions.”
[Santos et al. 2011, p.114]

Benefits at a glance:

• fine particle size distribution by ultrasonic mixing, deagglomeration & milling
• ultrasound removes passivating layers
• ultrasound enhances reaction kinetics
• ultrasound reduces basicity
• ultrasonic process intensification: higher yield, faster reaction

Ultrasonic effects on mineral carbonation. [Santos et al. 2013]

Ultrasonic Particle Treatment

Sonication is a powerful tool to treat particle slurries. Intense ultrasonic forces create mechanical vibration and strong cavitation in liquids. These high stress forces can break agglomerates and even primary particles, so that high-power/ low-frequency ultrasound is a reliable method for milling, deagglomeration and dispersing applications.

SEM pictures of calcium oxide initially (a) and after 10 minutes of sonication (b). [Santos et al. 2012]

The ultrasonic milling during the carbonation process of slurries creates small particles with large surface areas. Besides particle fragmenation, sonication also removes depositions from the particle surface, such as carbonated shells or depleted matrix layers that surround the unreacted particle core. By removing the passivating layers, diffusion limitations are reduced and unreacted material is exposed to the aqueous phase. Thereby, sonication can increase the carbonation conversion and the process kinetics – resulting in higher yields and a faster reaction.

Ultrasonic effects on particles [Santos et al. 2011]