Purification of aqueous electrolyte solutions by air-cooled natural freezing

摘要

Freeze crystallization is a particular type of a purification method where the solventfreezes out, which constricts the volume of the solution, leaving thus behind a moreconcentrated solution. In the case of freezing an aqueous solution, water is the solventwhich crystallizes and can be separated from the concentrated solution by the virtue ofbuoyancy. In an ideal situation, freeze crystallization of an aqueous solution produces icecrystals that do not contain any of the impurities present in the original solution. As theprocess continues, the original solution becomes more concentrated and the freezingtemperature declines progressively.Freezing point depression (FPD) is of vital importance in characterising the freezingbehaviour of any solution. Due to this necessity, a new calculation method to predict FPDis presented in this work. In this method, designated ion-interaction parameters for thePitzer model are extracted from reliable FPD data found in the literature, other thancalorimetric data. The extracted parameters from FPD data are capable of predicting thefreezing point more accurately than those resulted from the calorimetric data. Thecalculation method is exemplified for numerous 1-1 and 1-2 types of electrolytes.Impurities in excess of the maximum recommended limits must be removed fromwastewater prior to discharge because of their persistent bio-accumulative anddetrimental nature. Natural freezing is suggested in the present work as a purificationtechnique to treat huge volumes of wastewater in a sustainable and energy-efficientmanner. The efficiency of freeze crystallization in the purification of wastewater byimitating natural freezing in a developed winter simulation with the provision of alteringwinter conditions is scrutinized in this thesis. Hence, natural freezing is simulatedexperimentally for ice crystallization from unsaturated aqueous Na2SO4 and NiSO4solutions to assess the feasibility of such a technique to be used to purify wastewaterscontaining electrolytes. This work presents a series of data in similitude of naturalfreezing of water from aqueous Na2SO4 and NiSO4 solutions in various concentrationsand freezing conditions. The influence of solution concentration and different freezingconditions, such as ambient temperature, freezing time and freezing rate, on the efficiency of the purification process is investigated by analysing the effective distributioncoefficient (K) of the solute between ice and the solution. The experimental resultsdemonstrate clearly that high purity ice can be obtained from slow freezing of the solutionwith the concentration typically found in industrial wastewater.During freeze crystallization, the diffusion of impurities from the solid-liquid interface tothe bulk of the solution, along with the growth mechanism of the solid phase play animportant role in determining the purity of the ice layer. Therefore, a calculation methodis introduced to estimate the concentration of the solution at the advancing ice–solutioninterface in terms of the limiting distribution coefficient (K*) from experimental K valuesat different growth conditions. The heat transfer -controlled growth rate of the ice limitedby the free convective heat transfer coefficient of air (hair) rather than the thermalconductivity of the ice (kice) and the heat transfer coefficient of the solution (hsol) wasfound to prevail over the mass transfer of rejected solute molecules from the ice–solution interface to the bulk solution of experimental interest. A simplified and robust model is developed to estimate the thickness and growth rate of the ice layer formed from solutions at different freezing conditions, and the model is validated with experimental results. In addition, inclusion formation within the ice matrix during freezing is investigated for various solution concentrations, both macroscopically and microscopically.