The fabrication of high-whiteness nano CaCO3 by the preparation and whitening integration method with in situ coating of magnesium salt

Abstract

High whiteness nano CaCO₃ is widely used in toothpaste, paper, and wall paint which is greatly limited by the quality of limestone as a raw material. With the rapid consumption of high-whiteness nano CaCO₃, high-grade limestone with extra low iron content is becoming scarce, resulting in whiteness below the standard value of 92 degrees. It is important to develop high-whiteness nano CaCO₃ from natural lime ore to alleviate the shortage of high-quality lime ore. Herein, we proposed a new method to form a basic magnesium carbonate as a whitening agent, which is coated on the nano CaCO₃ surface by an in situ coating process via the precise addition of MgCl₂·6H₂O at the end of the carbonization reaction of nano CaCO₃. The strong electronegativity of Cl⁻ not only prevented colored impurity ions from entering the interior of nano-calcium carbonate but also enhanced the adsorption of Mg²⁺ on its surface. The synergistic effect between Mg²⁺ and Cl⁻ significantly improved the whiteness of nano CaCO₃ from 90.2 to 92.4 by sodium stearate activation, which can meet the requirements for rubber product fillers. The morphology of nano CaCO₃ did not change after the whitening process. Compared with 5 commercial nano CaCO₃ samples, the obtained sample exhibits regular cube morphology and superior whiteness, oil absorption value, specific surface area, and activation degree. Therefore, this simple integrated preparation method for whitening and enhancing the whiteness of nano CaCO₃, will promote their large-scale production and meet the market demand.