(57g) Filtration, Washing and Sizing of Multiple-Solute Crystal Products

Although crystallization has been used by a large number of industries as the final or an intermediate step in chemical processing, almost all research on the topic has addressed systems involving the formation of crystals of single solutes. Only a few applications and case studies have been developed for multi-species crystallization post-processing.

Crystallization generally produces a slurry with a variable solids density: i.e., mass of crystals per unit volume of either slurry or clear liquid. This slurry is recovered from the crystallizer and submitted to filtration or another solid-liquid separation process. Filtration produces two streams: one is of clear liquid known as filtrate and the other is wet solids containing crystals and interstitial mother liquor. The wet solids typically are washed to remove the interstitial solution, which improves product purity and prepares the crystals to be dried without the creation of agglomerates. Should information on the size distribution of the product crystals be desired, sieving are another technique can then be performed on the dry product.

Filtration, washing, and sizing are hence key steps in the post-crystallization process. The efficiency of these steps becomes more important when dealing with systems having complex crystal composition that mix species, sizes and shapes. In the work to be described, issues associated with processing slurries containing crystals of different species and, therefore, different shapes and sizes were addressed. Maintaining the purity of the crystalline product relative to a contaminant (cesium) in the mother liquor was a prime criterion in the process. Results obtained with a new laboratory filter design whose objective was to provide plug flow of wash liquor, are presented and compared to a multiple-stage slurrying process.

Along with the new design of the washing apparatus, a process was developed to provide a washing procedure that successfully complimented methods for determining crystal size distribution. Several chemical species, selected on the basis of volatility, impact on solubility, availability and safe characteristics were tested for filter cake washing. A procedure for sieving was developed for mixtures of multiple sodium salts and validated by polarized light microscopy. Validation of the sieving procedure was characterized by the qualitative and quantitative estimation of crystal breakage observed in the pan and crystal agglomerate in the larger sieve sizes. Complementarities between the sieving and microscopy methods represent a useful technique for multi-salt crystal size separation and crystalline species identification (form and shape).