Filtration assisted crystallization technology: heterogenous seeds enable fast crystallization and easy filtration

FACT, which stands for Filtration Assisted Crystallization Technology, is a hybrid process based on heterogeneous crystallization and filtration. During the process development heterogeneous seeds are selected, which enable a combination of fast crystallization and easy filtration. Water softening is chosen to illustrate the principle and potential of FACT, but the FACT process can also be applied for the removal of heavy metals, phosphate and fluoride from process/waste water streams or the change of the shape and/or size of crystals/particles. Water softening is often applied in order to prevent scaling in heat exchangers. In the FACT-concept softening is achieved by the precipitation of calcium carbonate (CaCO3)on the surface of a small amount of heterogeneous seeds in a crystallizer. Next, a filter separates the softened water from the grown seeds, which are recycled to the crystallizer as a concentrated suspension. The heterogeneous seeds grow during the softening process and are bled off intermittently when they reached the required size. FACT is compact due to the fast crystallization and easy filtration and it can compete technically and economically with alternative softening technologies like the pellet reactor and ion exchange. The paper presents the results for the selection of heterogeneous seeds. The softening of ground water and a process water stream from a paper mill was investigated with a FACT pilot plant. The pilot plant results of the FACT process are compared technically and economically with other softening technique based on heterogeneous crystallization.