Extraction of boric acid from colemanite mineral by supercritical carbon dioxide

• B(OH)3 was extracted from colemanite mineral by supercritical CO2 in aqueous phase.
• 96.9% boric acid extraction efficiency obtained at 60 °C, for 2 h of reaction time.
• Supercritical CO2 is a good lixiviant for boron minerals and a substitute for H2SO4.
• A sustainable green process could be developed which can reduce CO2 emissions.

The use of H2SO4 in boric acid production from colemanite mineral has several problems, related to product impurities, corrosion and environmental discharge limits. To overcome these problems and to increase extraction efficiency of boric acid, heterogeneous reaction between colemanite and CO2 dissolved in H2O was studied at and above supercritical CO2 conditions. Supercritical conditions enhanced the extraction efficiency of boric acid from colemanite mineral, with 96.9% boric acid extraction efficiency being obtained from CO2–colemanite reaction at 60 °C, for 2 h of reaction time for particles in the range of +20–40 μm. A powder crystallized from filtrate of reaction was determined as H3BO3 and the solid formed at the end of reaction was characterized mostly as CaCO3 according to FTIR, XRD, TGA and SEM analyses. The use of supercritical CO2 as a leaching agent in colemanite does not only produce boric acid but also helps to reduce the amount of CO2 in the atmosphere. Based on these facts, supercritical CO2 as extractant makes this process green and sustainable for recovering boric acid from boron minerals.

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