Raman Spectroscopy Study on the Phase Transformation of Solids in Washing Powder Preparation Processes

Sodium sulphate and sodium carbonate are essential inorganic raw materials for the dry laundry industry. The composition of the final slurry mixture possesses a high process dependency, which has significant impact on the properties of the final granule product, such as particle size distribution, porosity, stability and liquid loading capacity. To obtain and design desirable properties requires a good understanding of the dissolution and crystallisation behaviour of both raw materials and their double salt Burkeite (Na4(CO3)0.61(SO4)1.39).
To perform this study, first, inorganic slurry calibration models were developed by applying an in-situ Raman spectroscopy technique that contributes in real-time monitoring of composition alteration of a multi-component system over a dissolution and crystallisation process. Based on the calibration model, a new methodology was developed for fast determining a detailed isothermal ternary phase diagram of the Na2CO3-H2O-Na2SO4 system. For Burkeite kinetic study, the detected metastable zone limits with respect to the phase equilibrium were correlated to the increased rate of supersaturation during dissolution of the Na2SO4. This novel strategy could be applied to other systems, offering a fundamental base for modelling nucleation rate and size control for industrial processing of co-crystals/double salts.
The impact of processing conditions on the sodium salt mixture demonstrates that increasing the system pH induces fast crystallisation and precipitates small crystals with larger inter- and intra-porosities. However, the addition of high pH sodium silicate solution in the system decreases the solubility of raw material and leads to less Burkeite formation, which was caused by the common ion effect and polymerisation of sodium silicates.
Overall, this research developed a feasible in-process monitoring method, which can simultaneously detect and analyse any composition and phase changes of the individual substance in a multi-component detergent slurry system. With the aid of this method, researchers were able to discover the fundamental cause of product failure, thereby sufficiently targeting, investigating and solving the problem, eventually achieving the desirable product by optimising the production procedure.