Nuclear chemistry

Sodium

Sodium carbonate

Sulfuric acid

Biochemistry

Inorganic chemistry

Dithionite

Humic acid

Organic chemistry

Calcium carbonate

Sodium dithionite

Fertilizer

Sodium citrate

Aqueous solution

Enzyme

Medicine

Chemistry

Impurity

Pathology

Colloids and Surfaces A Physicochemical and Engineering Aspects

To widen applications of ground calcium carbonate (GCC), it is crucial to improve whiteness and remove the black-colored impurities (Fe, Mn and Zn) from GCC powders. Herein a new strategy termed as sodium dithionite-sodium citrate-sulfuric acid treatment (DCS) was developed to whiten natural GCC powders, and the whiteness was improved from 85% to 95.6%. The DCS strategy was adopted by washing GCC with aqueous solution of sodium dithionite, sodium citrate and sulfuric acid (Na2S2O4-C6H5Na3O7·2H2O-H2SO4) to remove the mineral impurities, and composites of CaSO4@CaCO3 (CaCO3 coated with CaSO4) were prepared in the final products. Optimal parameters were derived from orthogonal experiments (L16), and five factors were investigated. The use of H2SO4 was found to be essential in improving whiteness of GCC. For example, to whiten 250 g GCC which were dispersed in 250 mL water, the optimal parameters were found as dosages of H2SO4 (98% wt) 25.0 mL, Na2S2O4 2.0 g, C6H5Na3O7·2H2O 1.5 g, and the DCS-treatment time was 5.0 min at the temperature of 80ºC. The removal percentages of Fe, Mn and Zn were found to be 80%, 78% and 66% after DCS washing. The whitened GCC were characterized by different means. The coating of GCC powders by new formed CaSO4 layer was confirmed to play an important role in whitening GCC powders.

Whiteness improvement of ground calcium carbonate via sodium dithionite-sodium citrate-sulfuric acid treatment and mechanism study