Calcium carbonate crystallization in the presence of polymeric additives.

摘要

Crystallization is one of the most widely used unit operations in the process industries. Impurities and/or additives greatly affect the crystallization process by alteration of nucleation, growth, and even phase transformation kinetics. This study is aimed at the design of additives with the intent of modifying a specific crystallization process.; Calcium carbonate is used industrially for a variety of applications, such as filler for plastic materials, rubber and paper. Precipitation of calcium carbonate on surfaces, also known as scaling, is a major problem for industrial equipment. Therefore, studying the crystallization of calcium carbonate is important from its inhibition as well as its control point of view.; Polycarboxylic acids are good chelating agents and greatly affect calcium carbonate crystallization. Maleimide was used as monomer for synthesis of polymaleimide using anionic and metal oxide-alcohol initiators. The hydrolysis of these polymaleimide resulted in polycarboxylic acid polymers. Proton NMR studies confirmed that the polymers synthesized using different initiators possessed different monomer linkages, with the percent C-N connected monomers lower in the case of metal oxide-alcohol type of initiators (about 40 percent) than for anionic initiation (about 80 percent). Gel permeation chromatography (GPC) was used for molecular weight determination of the polymers. Molecular weight of polymers made by metal oxide-alcohol initiators was about 11500 and was three times the molecular weight of polymers made by anionic polymerization. The effect of the resulting polymers used as additives in the crystallization of CaCO₃ was studied using a variety of techniques. Direct titration using a calcium selective electrode showed that the calcium chelating strength of the polymers was better than some of the commercial detergent building formulations currently in use such as Acusol® and polyaspartic acid. The kinetics of CaCO₃ crystallization were studied by nephelometry by monitoring crystal nucleation and growth rates. Polymaleimide synthesized by anionic polymerization was the most efficient inhibitor and exhibited a 59 percent growth rate inhibition at a concentration of 1.4 ppm. Raman spectroscopy was used for in situ monitoring of the effect of these additives on polymorph composition during the crystallization. Calcite is thermodynamically the most stable form of CaCO₃ and is formed predominantly during the industrial processes for manufacture of precipitated CaCO₃. Addition of 1.4 ppm of Acusol® and polyacrylic acid caused the vaterite form to crystallize out exclusively. XRD and SEM data were used to corroborate the Raman data and the results were within two percent of each other.