# Purification of aqueous electrolyte solutions by air-cooled natural freezing

##### Hasan, Mehdi (2016-10-19)

Väitöskirja

Hasan, Mehdi

19.10.2016

Lappeenranta University of Technology

Acta Universitatis Lappeenrantaensis

**Julkaisun pysyvä osoite on**

http://urn.fi/URN:ISBN:978-952-335-005-2

#### Tiivistelmä

Freeze crystallization is a particular type of a purification method where the solvent

freezes out, which constricts the volume of the solution, leaving thus behind a more

concentrated solution. In the case of freezing an aqueous solution, water is the solvent

which crystallizes and can be separated from the concentrated solution by the virtue of

buoyancy. In an ideal situation, freeze crystallization of an aqueous solution produces ice

crystals that do not contain any of the impurities present in the original solution. As the

process continues, the original solution becomes more concentrated and the freezing

temperature declines progressively.

Freezing point depression (FPD) is of vital importance in characterising the freezing

behaviour of any solution. Due to this necessity, a new calculation method to predict FPD

is presented in this work. In this method, designated ion-interaction parameters for the

Pitzer model are extracted from reliable FPD data found in the literature, other than

calorimetric data. The extracted parameters from FPD data are capable of predicting the

freezing point more accurately than those resulted from the calorimetric data. The

calculation method is exemplified for numerous 1-1 and 1-2 types of electrolytes.

Impurities in excess of the maximum recommended limits must be removed from

wastewater prior to discharge because of their persistent bio-accumulative and

detrimental nature. Natural freezing is suggested in the present work as a purification

technique to treat huge volumes of wastewater in a sustainable and energy-efficient

manner. The efficiency of freeze crystallization in the purification of wastewater by

imitating natural freezing in a developed winter simulation with the provision of altering

winter conditions is scrutinized in this thesis. Hence, natural freezing is simulated

experimentally for ice crystallization from unsaturated aqueous Na2SO4 and NiSO4

solutions to assess the feasibility of such a technique to be used to purify wastewaters

containing electrolytes. This work presents a series of data in similitude of natural

freezing of water from aqueous Na2SO4 and NiSO4 solutions in various concentrations

and freezing conditions. The influence of solution concentration and different freezing

conditions, such as ambient temperature, freezing time and freezing rate, on the efficiency of the purification process is investigated by analysing the effective distribution

coefficient (K) of the solute between ice and the solution. The experimental results

demonstrate clearly that high purity ice can be obtained from slow freezing of the solution

with the concentration typically found in industrial wastewater.

During freeze crystallization, the diffusion of impurities from the solid-liquid interface to

the bulk of the solution, along with the growth mechanism of the solid phase play an

important role in determining the purity of the ice layer. Therefore, a calculation method

is introduced to estimate the concentration of the solution at the advancing ice–solution

interface in terms of the limiting distribution coefficient (K*) from experimental K values

at different growth conditions. The heat transfer -controlled growth rate of the ice limited

by the free convective heat transfer coefficient of air (hair) rather than the thermal

conductivity of the ice (kice) and the heat transfer coefficient of the solution (hsol) was

found 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.

freezes out, which constricts the volume of the solution, leaving thus behind a more

concentrated solution. In the case of freezing an aqueous solution, water is the solvent

which crystallizes and can be separated from the concentrated solution by the virtue of

buoyancy. In an ideal situation, freeze crystallization of an aqueous solution produces ice

crystals that do not contain any of the impurities present in the original solution. As the

process continues, the original solution becomes more concentrated and the freezing

temperature declines progressively.

Freezing point depression (FPD) is of vital importance in characterising the freezing

behaviour of any solution. Due to this necessity, a new calculation method to predict FPD

is presented in this work. In this method, designated ion-interaction parameters for the

Pitzer model are extracted from reliable FPD data found in the literature, other than

calorimetric data. The extracted parameters from FPD data are capable of predicting the

freezing point more accurately than those resulted from the calorimetric data. The

calculation method is exemplified for numerous 1-1 and 1-2 types of electrolytes.

Impurities in excess of the maximum recommended limits must be removed from

wastewater prior to discharge because of their persistent bio-accumulative and

detrimental nature. Natural freezing is suggested in the present work as a purification

technique to treat huge volumes of wastewater in a sustainable and energy-efficient

manner. The efficiency of freeze crystallization in the purification of wastewater by

imitating natural freezing in a developed winter simulation with the provision of altering

winter conditions is scrutinized in this thesis. Hence, natural freezing is simulated

experimentally for ice crystallization from unsaturated aqueous Na2SO4 and NiSO4

solutions to assess the feasibility of such a technique to be used to purify wastewaters

containing electrolytes. This work presents a series of data in similitude of natural

freezing of water from aqueous Na2SO4 and NiSO4 solutions in various concentrations

and freezing conditions. The influence of solution concentration and different freezing

conditions, such as ambient temperature, freezing time and freezing rate, on the efficiency of the purification process is investigated by analysing the effective distribution

coefficient (K) of the solute between ice and the solution. The experimental results

demonstrate clearly that high purity ice can be obtained from slow freezing of the solution

with the concentration typically found in industrial wastewater.

During freeze crystallization, the diffusion of impurities from the solid-liquid interface to

the bulk of the solution, along with the growth mechanism of the solid phase play an

important role in determining the purity of the ice layer. Therefore, a calculation method

is introduced to estimate the concentration of the solution at the advancing ice–solution

interface in terms of the limiting distribution coefficient (K*) from experimental K values

at different growth conditions. The heat transfer -controlled growth rate of the ice limited

by the free convective heat transfer coefficient of air (hair) rather than the thermal

conductivity of the ice (kice) and the heat transfer coefficient of the solution (hsol) was

found 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.

##### Kokoelmat

- Väitöskirjat [872]