Process routes-Pretreatment

A variety of pre-treatment options exist. The main ones are: size reduction, magnetic separation and thermal treatment. Their common goal is to increase the reaction rate by increasing the reactive surface available for carbonation.

Size reduction

In order to achieve a reasonable reaction rate the minerals have to be grinded. The reaction rate increases with the surface area. Among others, OConnor et al. examined the influence of the particle size of the conversion. These authors found that a reduction of particle size increased the conversion in their experiments from 10% to 90%. High-energy attrition grinding induces imperfections into the crystal lattice. This results in a higher  conversion than size reduction to the same diameter using 'normal' grinding. Attrition grinding, however, is energy intensive and difficult to conduct on a large scale.

Magnetic separation

The oxidation of iron (magnetite) slows down the carbonation of serpentine due to the formation of a layer of hematite on the mineral surface. To execute the process in a non-oxidising atmosphere complicates the process and increases the costs significantly. Magnetic separation of the iron compounds prior to the carbonation process resolves this complication. Furthermore, a potentially marketable iron ore byproduct is formed. When a combination of magnetic separation and thermal treatment is used, it is more effective first to conduct the magnetic treatment step.

Thermal treatment

Serpentine contains up to 13wt% chemically-bound water. By heating the serpentine to 600- 650C the water is removed and an open structure is created. This significantly improves the reaction kinetics owing to the increase reactive surface. For example heat-treatment of antigorite increased the surface areas from 8.5m2/g to 18.7m2/g. During attrition grinding small balls mixed with the sample are stirred in a chamber. The heating process can be further extended to higher temperatures in order to separate, for example, MgO from its matrix. Temperatures above 900C are needed for serpentine and even higher values for olivine:

The mineral porosity can also be increased by treatment with steam or supercritical water (T=385C, p=272atm).

O'Connor et al. have tried ultrasonic pre-treatment of olivine, but it failed to activate the mineral (O'Connor et al., 2001a). Furthermore, these authors have used chemical pre-treatment steps

xMgO ySiO2 zH2O (s) Τ xMgO (s) + ySiO2 (s) + zH2O (g)

Other pre-treatment options

O'Connor et al. have tried ultrasonic pre-treatment of olivine, but it failed to activate the mineral. Furthermore, these authors have used chemical pre-treatment steps combined with size reduction. Wet grinding in a caustic solution (1M NaOH, 1M NaCl) was found to be most effective, but still insufficient to achieve a reasonable reaction rate. Pre-treatment seems necessary in order to obtain a reasonable reaction rate in direct carbonation processes. This benefit should be balanced by the extra costs and energy consumption of the pre-treatment step. Thermal treatment in particular is very energy consuming and should, if possible, be avoided. Special attention should be paid to avoiding undesirable effects during pre-treatment. Gerdemann et al. reported surface oxidation when ground ore was allowed to stand in water after wet grinding resulting in an initiation period for carbonation of olivine of about 3 hours .


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