Calcium Carbonate Precipitation in Presence of Carboxylic Acids

Abstract: With a metal removal process based on precipitation aimed atthe purification of pulping effluent as a starting point, theinfluence of carboxylic acids on calcium carbonateprecipitation has been studied. The chosen carboxylic acids canact as more or less strong complexing agents for aqueouscalcium, just as many compounds found in pulping liquors.Several different experimental techniques were employed inorder to address, but also enable a distinction betweendifferent physical and chemical phenomena. Batch experimentswere used to investigate nucleation phenomena. Seeded crystalgrowth experiments with a constant thermodynamic driving forceapproach were conducted. The influence of process parameterssuch as temperature and pH was studied using semi-batchexperiments under pH-controlled as well as pH-drift conditions.The results were evaluated using a thermodynamic driving forceobtained from chemical speciation calculations.The results show that the chosen carboxylic acids have asignificant effect on almost every aspect of the precipitationof calcium carbonate. As a consequence they affect productproperties and significantly reduce yield. The batchexperiments revealed that the nucleation rate significantlydecreases in the presence of carboxylic acids at equal drivingforce. Yet, it varies from one acid to another. Crystal growthis also significantly retarded by the presence of carboxylicacids. The semi-batch experiments clearly showed that theability to control product properties diminishes in thepresence of the chosen carboxylic acids. The fact that theparticle shape is affected can be taken as evidence of theinteractions of the carboxylic acids with calcium carbonatebeing specific.The observed phenomena are interpreted as being the resultof the carboxylic acids interacting with the surface of thegrowing crystals of calcium carbonate and thereby blocking theaddition of further ions into the crystal lattice. Evidence ofthese interactions being specific was also found by linking thestructure of the crystal surfaces to features of the chosencarboxylic acids. It is also suggested that the carboxylicacids counteract processes leading to the formation ofpre-crystalline structures in the solution.A consequence of these findings for the envisioned metalremoval process is that it presents a great difficulty toovercome the problems related to the retardation ofprecipitation by the presence of carboxylic acids.Keywords:Calcium carbonate; Precipitation;Crystallization; Carboxylic Acids; Crystal Growth; Nucleation;Calcite; Aragonite; Kraft pulping; Non-Process Elements