A novel scraped cooled wall crystallizer (SCWC-2) with 130 l capacity and total heat transfer area of 2.94 m2 m−3 crystallizer volume was specifically designed for eutectic freeze crystallization (EFC). It has been successfully tested for the recovery of Na2CO3 from an industrial solution. Compared with earlier designs this new crystallizer shows a high improvement in gravitational separation and ice scaling removal. High heat transfer fluxes between the heat exchanger and the solution (up to 10.5 kW m−2) were obtained and as a consequence high production rates were achieved.

Eutectic freeze crystallization was tested in a scaled up version of a scraped cooled wall crystallizer on an industrial aqueous sodium carbonate–sodium bicarbonate waste stream containing traces of molybdenum. A heat transfer rate of 5kWm−2 was maintained in the crystallizer. Sodium carbonate decahydrate with molybdenum content below 1ppm and pure ice were produced by continuous crystallization at −3.8°C by operating within the metastable zone width of bicarbonate. At −4.0°C bicarbonate started to co-crystallize. The bimodal size distribution of the mixture resulted in poor filtration and purity of the salt product.

Inline measurement of the supersaturation is important to support process control and for the evaluation of crystallization experiments. Especially, when different hydrates or polymorphs of the crystallized component occur, very precise measurement is required. For example MgSO4 center dot 7H(2)O-MgSO4 center dot 12H(2)O lines are quite close, and when operating under eutectic freeze crystallization (EFC) or cooling crystallization both salts might be produced. Therefore conductivity and refractive index inline measurement methods are investigated and compared for MgSO4 solution having concentration range between 16 and 22wt% and temperature range between 10 and -5 degrees C. Conductivity measurements are correlated to concentrations and temperature with the empirical Casteel-Amis equation and refractivity index measurements are correlated to concentrations and temperature with a developed empirical model. The metastable lines for ice and salt of MgSO4 system is drawn based on the measurements at the onset of crystallization upon cooling of several solution concentrations. In the working range of EFC, the relative supersaturations are calculated to be sigma(icemax) = 0.2 and sigma(saltmax) = 0.23: The accuracy for conductivity measurement in relation to the metastable zone width is roughly calculated to be around 20% and for refractive index measurements, 3%. (C) 2004 Elsevier B.V. All rights reserved.

During the eutectic freeze crystallization (EFC) of an industrial aqueous MgSO4 solution, ice and magnesium sulfate duodecahydrate (MgSO4 center dot 12H(2)O(s)) were crystallized simultaneously near the eutectic point. It was found that the crystallization was highly selective: although the industrial feed solution contained appreciable levels of inorganic impurities (typically: 320 mg/LCl, 410 mg/LCa2; 40 mg/LMn2+; 70 mg/L Na+ and 50 mg/LK+), the formed ice and salt crystals contained lower levels of impurities (typically: 17 mg/LCl-; 8 mg/LCa2+; 17 mg/LMn2+; and 5 mg/LNa). Also the ice was pure: only traces (typically: 20 mg/LSO42- and 5 mg/LMg2+) of magnesium and sulfate were found in the ice crystals.In this work the spatial distribution of the impurities in the crystals is investigated. Gradients of composition in solids are measured by laser ablation high-resolution inductively coupled plasma mass spectrometry (LA-HR-ICP-MS). A special cryogenic sampling cell (< -80 degrees C) for laser ablation has been constructed. The focused (5-10 mu m width) laser shoots at the frozen sample, scanning its surface. The vapor is fed to the mass spectrometer. In this way, the impurity concentration as a function of position in the crystal can be measured. The results of this method with MgSO4 center dot 12H(2)O and ice are presented. (C) 2004 Elsevier B.V. All rights reserved.

Eutectic freeze crystallization is a new separation process to produce pure ice and salt from concentrated salt solutions. For this new process, specialized crystallizers are being developed. A new, 2nd generation cooled disc column crystallizer (CDCC-2) with 150-l capacity and 5.6m2/m3 cooling area was designed and tested for MgSO4 crystallization. A heat flux of 1720–5750W/m2 was achieved at a temperature difference between coolant and crystallizer bulk solution of 3–6.5K and a residence time of 1–3h. Crystallization and gravitational separation of ice and salt are discussed.