טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentPinhas Yosef
SubjectStudy of Water Hardness Removal by Electrochemical
Precipitation
DepartmentDepartment of Chemical Engineering
Supervisors Professor Emeritus Raphael Semiat
Professor Emeritus David Hasson
Full Thesis textFull thesis text - English Version


Abstract

The objective of this research was to examine the potential of the electrolytic technique for removing commonly scaling species encountered in water cooling systems and desalination plants. The main contributions of this work are as follows:

CaCO3 system.

Electrolytic removal of CaCO3 is based on the creation of a high pH environment around the cathode which induces precipitation of the CaCO3.

Modeling the kinetics of CaCO3 cathode deposition. A theoretical kinetic model was developed assuming that the deposition process was mass transfer control. Experimental measurements were in very good agreement with model results.

Characterization of CaCO3 deposition rate in the presence of anti-scalants.  A possible way to improve water recovery in brackish water RO desalination is to separate from the concentrate dissolved ionic species held in a supersaturated state by the presence of an anti-scalant and recycle part of the scale-free stream to the feed. A key factor in development of a high recovery process is effective CaCO3 precipitation in the presence of anti-scalants. This was successfully achieved by the electrolytic technique.

Electrolytic scale removal with sacrificial metallic anodes

Sacrificial metallic anodes were found to have two important advantages: objectionable anode chlorine generation is suppressed and dissolved cations of metals provide useful coagulation properties.

CaSO4 phenomena. Precipitation of CaSO4 with a dimensionally stable electrode was found to require continuous application of electrolytic current. Deposition on the anode is thought to be due to neutralization of the negative charged of colloidal nanoparticles. When a metallic anode was used, it was sufficient to apply the current only for the brief period of time. It is thought that the mechanism involved was a process of electro-coagulation of the colloidal nanoparticles present in an anti-scalant stabilized supersaturated solution.

Coprecipitation of CaSO4 and CaCO3. Precipitation of the CaSO4 is induced by brief application of the electric current. However, precipitation of the CaCO3 requires continuous application of the current since its precipitation is based on generation of alkaline environment on the cathode.

Silica removal. Experiments showed effective silica electrocoagulation similar to that observed with CaSO4.

Coprecipitation of CaSO4 , CaCO3 and SiO2. The electrolytic technique was found to be quite effective in precipitating a mixture of the above scaling species which is considered to be a difficult task.

In conclusion results of this study show that the little studied electrolytic scale technique holds considerable promise for expanding the rather limited scope of viable scale control techniques.