The influence of impurities on crystallization kinetics - a case study on ammonium sulfate

The influence of impurities on the crystallization kinetics of ammonium sul fate was investigated. MSM PR experiments were conducted with the impuritie s aluminum sulfate and the azo dyes amaranth and fuchsine. Nucleation and g rowth rates as well as mean crystal sizes were related to the supersaturati on sigma and the width of the metastable zone. It was found that all impuri ty levels in the system reduce kinetic coefficients for crystal growth and suppress nucleation by adsorption on the crystal surfaces. An increase of s upersaturation and metastable zone width compensates for this reduction at low impurity concentrations and allows the growth of larger crystals compar ed to the pure system. At high impurity concentrations and increasing surfa ce coverage of the crystals, supersaturation rises faster than metastable z one width. causing an increase in nucleation rates and a higher fines conte nt in the product compared to the pure system. A similar interdependence be tween impurity concentration, crystal size and supersaturation was found fo r other systems not reported here. The observations made can be explained i n terms of adsorption equilibria of the impurities on the crystals. This se emingly general relationship allows the adjustment of crystal sizes in crys tallization processes by control of impurity concentrations. A second paper will discuss the changes in crystal morphology in greater depth (Kuch ct a l.. 2000). (C) 2000 Elsevier Science B.V. All rights reserved.