טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentEpshtein Alon
SubjectDevelopment and Investigation of a Novel Electrochemical
Membrane Processes for the Treatment of
Wastewater of Phosphoric Acid Industry
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Dr. Youri Gendel
Full Thesis textFull thesis text - English Version


Abstract

This study was dedicated to research and development of a novel process for treatment of phosphoric acid industry wastewater (PAIWW). PAIWW poses an environmental risk due to high concentrations of electrolytes, i.e. phosphate, fluoride, sodium, sulfate and chloride ions. Another serious problem is high acidity and low pH (1.5-2) of this wastewater that pose an additional complications to wastewater management. On the other hand, the PAIWW with its high concentration of phosphate (6600 mgP/l) can be an important source of phosphate in the production process. Existing solutions for wastewater treatment have insufficient efficiency of phosphorous recovery, result in low product purity and usually have high energy demand. An in-depth investigation of solution speciation was conducted via a geochemical software (PHREEQC) to investigate its composition at varied temperature, pH and ionic strength. It was concluded that a potential method for selective phosphate separation can be conducted via a two-step process that comprises (1) removal of fluoride ions, followed by (2) selective electrochemical separation of P species. For the first step of the novel process, several studies were reviewed and the most selective method for fluoride ions removal was found to be a heterogeneous reaction with SiO2-containing solids. Three SiO2 materials were investigated for F- removal: fly ash, quartz sand, and glass beads. The materials were analyzed for chemical composition and particle size distribution. The reaction of F- ions removal was studied using batch-mode experiments. Complete removal of fluoride was achieved using two subsequent operations: (1) reaction of wastewater with a mixture of SiO2-solids that comprised 10 %wt. fly ash and 35 %wt. quartz sand, and (2) additional treatment using 30 %wt. quartz sand. The heterogeneous reaction of fluoride and SiO2 resulted in a reduction of the pH value in the treated synthetic wastewater from pH 2.1 to 1.1. The remaining SiO2 solids were analyzed for surface area composition using the SEM-EDS technique which showed a substantial presence of fluorine on the surface of SiO2 materials. The second stage of the proposed process aimed at selective separation of phosphorous, was performed using the flow electrode capacitive deionization (FCDI) technique. The FCDI process was studied first for P separation from binary phosphate-chloride solutions at pH values between 1.2 to 2.3. The experiments were conducted using a single- and two-electrode FCDI configurations (5 %wt. activated charcoal powder slurry) at a constant voltage of 1.2V and 60 ml/min flow rate of both solution and slurry electrode. The results showed that lower pH resulted in lower desalination rates of phosphate which was attributed to lower dissociation ratio and higher concentration of neutral H3PO4 species. Consequently, the wastewater effluent from stage (1) with reduced pH value of 1.1 (after complete fluoride removal) was desalinated via the flow electrode capacitive deionization (FCDI) at similar conditions. First, approximately 75% of the phosphate in the wastewater effluent was retained while complete desalination of chloride and sulfate was achieved. Subsequently, during the optimization of the FCDI process (i.e. an increase in flow rate), phosphorus recovery of up to 90% was achieved.