Antonija Kaćunić, Marija Ćosić, Davor Rušić, Nenad Kuzmanić

The aim of this work was to experimentally investigate the influence of impeller off-bottom clearance on crystal growth kinetics of disodium tetraborate decahydrate (borax) in a dual-impeller batch cooling crystallizer. Examinations were carried out in a crystallizer of 15 dm3 with liquid height to crystallizer diameter ratio (H/dT) of 1.3. In this research, two pitched blade turbines (PBT) with diameter D/dT=0.33, were mounted on the same shaft with a constant spacing between impellers (s/D=1.0). Mixing was performed at the impeller speed which ensured the state of complete suspension in the system (N=NJS). The impeller off-bottom clearance (C/D) was varied from 0.2 to 1.3. Crystallization was carried out by a controlled cooling of the solution saturated at 30°C. Concentration of the solution was measured in line by potentiometric method, while the changes of crystal size over time were determined by optical microscope and image analysis software.

Experimental results indicate that impeller off-bottom clearance influences the change of crystal size over process time. But, regardless of the impeller off-bottom clearance, crystal growth is integration limited. The value of an order of crystal growth rate changes insignificantly with C/D ratio, while the crystal growth rate constant is affected by the change in hydrodynamics. Obtained results are a consequence of the hydrodynamic conditions in the dual-impeller crystallizer. In order to analyse the overall fluid flow pattern, in which an interaction of the flows generated by each of the impellers occurred, photographs of the flows were taken and simulations by VisiMix 2000 Turbulent were made. Taking into account the values of kinetic parameters of crystal growth, characteristic of obtained crystals, power consumption and the overall fluid flow pattern, it is possible to define the impact of impeller off-bottom clearance on borax crystal growth kinetics.

Antonija Čelan, Marija Ćosić, Nenad Kuzmanić

Experiments on crystallization kinetics of borax were conducted in 15 dm3 batch crystallizer equipped with two impellers of a different type mounted on the same shaft (PBT-SBT). Influence of hydrodynamic conditions in the crystallizer on supersaturation profile, metastable zone width, crystal growth mechanism and rate, as well as on properties of the final product was investigated. To gain a more complete insight into the hydrodynamics, which is highly influenced by impeller position, that is by impeller off-bottom clearance and impeller spacing, mixing time and specific fluid flow patterns were analyzed in detail. Results indicate that dual impeller position significantly affects the parameters of crystallization kinetics as well as the properties of the final product of crystallization.

Keywords: Crystallization, Impeller off-bottom clearance, Impeller spacing, Mixing, Dual impeller.

Tihomir Kosor, Barbara Nakić-Alfirević, Sandra Svilović

Glass-like behavior of the geopolymer samples have been investigated by determination of the influence of the geopolymerization reaction equilibrium on the types of SiO4 tetrahedra polymerization present. In order to manipulate geopolymerization reaction equilibrium, aluminum and sodium content in the samples was varied. Micro Raman spectroscopy has been used in order to analyze the dominant structure of the samples by evaluating the results against corresponding analysis of a related silicate material, both crystalline and amorphous, in order to determine the types and intensity of the SiO4 polymerization present. The obtained results suggest silicate glass-like behavior of investigated material concerning SiO4 tetrahedra depolymerization. The results also suggest the possibility of aluminum free geopolymerization what implies behavior other than that proposed by definition of geopolymers. Based on the obtained results, concept and calculation of the Geopolymer Depolymerization Index as an approximate measure of the content of the depolymerized SiO4 tetrahedra in the fly ash geopolymers using Raman spectroscopy have been proposed.

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Keywords: Geopolymer Depolymerization index; Glass; Quartz; Raman spectroscopy

Sandra Svilović , Mario Nikola Mužek, Ivona Nuić, Petra Vučenović

Taguchi's approach for designing the experiment was applied to determine the effect of selected process parameters on the sorption of copper ions onto different sorbents. The impact of solute concentration (c), temperature (T), mixing time (t) and sorbents type (S) on copper removal under batch conditions, was investigated. Optimum
experimental conditions and influence of controllable factors were determined using lager-the-better approach. The influence in descending order is c > S > t > T, for both removal and loading. Also, the impact of sorbents type and solute concentration on the process equilibrium were examined. The experimental data were modelled using
Jovanovic, Khan, Baudu, and Fritz-Schlünder isotherm models and their goodness-of-fit were compared. The Baudu isotherm model was the most useful in predicting the equilibrium of copper sorption on pure geopolymer. All isotherm models used were in a good agreement with experimental data obtained for the sorption studies on a modified geopolymer, and the goodness of fit of the selected isotherm models for the sorption of copper ions on zeolite NaX was in order: Fritz-Schlünder > Khan > Jovanovic > Baudu.

Marija Ćosić, Antonija Kaćunić, Nenad Kuzmanić

The effect of impeller blade angle on the nucleation and crystal growth kinetics of borax decahydrate has been investigated in detail. The importance of blade angle was studied in a laboratory scale batch cooling crystallizer equipped with a down-pumping pitched four-blade turbine. The blade angle to the horizontal axis was varied from 30° to 90°. Experiments were performed at constant impeller diameter, off-bottom clearance and impeller speed which ensured the state of complete suspension in examined systems. According to the Mersmann's criteria, nucleation took place by a heterogeneous nucleation mechanism but at different nucleation rates in all examined conditions.  Kinetics parameters of crystal growth were determined as well. Numerical values of order of crystal growth indicated that the growth of crystals was controlled by a combination of diffusion and surface-integration growth mechanism regardless of the impeller blade angle. Different values of crystal growth rate constants are a consequence of the fluid dynamics in the crystallizer caused by a different impeller blade angle. The transition from axial to radial flow due to blade angle inclination reflects on the crystal size distribution, agglomeration ratio and crystallization yield as well. In this work torque measurements were also performed to determine power consumption in mixing conditions examined.

Keywords: Mixing, impeller blade angle, nucleation, crystal growth, crystal size distribution, borax, crystal growth kinetic parameters

Mario Nikola Mužek, Sandra Svilović, Jelica Zelić

This study explored the cobalt ions removal efficiency from aqueous solutions by sorption process on geopolymer and zeolite NaX using batch experiments. The influence of initial solute concentration (4.284–11.937 mmol L-1) and various temperature (298, 308, 318 K) on the sorption process were examined at constant particle size (0.071–0.09 mm). Maximum amount of cobalt ions sorbed on zeolite (at 298 K) was 1.102 mmol L-1 and it was obtained at the highest initial concentration tested, unlike on geopolymer where maximum amount of cobalt ions sorbed was obtained at 9.998 mmol L-1 and was 0.895 mmol L-1. As the temperature increased, the maximum amount of cobalt ions sorbed also increased on both sorption materials used. An additional goal was to test the experimental kinetic data using several kinetic models: Lagergren pseudo-first order, Blanchard and Elovich models, using nonlinear regression technique. A kinetic study has shown that the best fit is achieved when the Blanchard model was applied suggesting that the sorption of cobalt ions on geopolymer and zeolite NaX is a second order reaction.

Keywords: Geopolymer, Zeolite NaX, Cobalt, Kinetics, FTIR, SEM