Civil Engineering ETDs

Publication Date

Summer 7-11-2018

Abstract

Silica at high concentrations can precipitate and polymerize, forming scales on heat exchangers, boilers and turbines in industrial equipment, and on the feed side of the semi-permeable membranes in Reverse Osmosis (RO). Silica scale can cause decreased efficiency, increased treatment costs and, in some cases, irreversible damage. The removal of silica scale is challenging because it requires the handling of dangerous and hazardous chemicals. Therefore, much research has gone into the removal of soluble silica. The purpose of this research was to compare the overall effectiveness of silica removal in RO concentrate water with freshly precipitated Mg(OH)2 and Fe(OH)3,and calcined Hydrotalcite (HTC) by changing the design parameters adsorbent dose and pH. To complete this work, 15 experiments (12 batch experiments and 3 flow through experiments) were performed.

Initial batch studies investigated and compared the effects of changing the dose and on silica removal for freshly precipitated Mg(OH)2 and Fe(OH)3,and calcined HTC. The results showed that, for all materials, an increased dose at pH 10 led to increased silica removal. Then, using the three materials, the effect of pH was investigated on silica removal. When the pH was increased from 9 to 11, trends in silica removal varied for the three materials. Furthermore, batch studies were completed on the three materials to determine the sorption density and sorption kinetics onto the solids. The sorption densities were used to determine the most applicable isotherm (Freundlich or Langmuir) and identify isotherm parameters for the materials. All three materials fit the Freundlich isotherm model and based on isotherm parameters, the largest adsorption capacity was determined to be HTC and the most intense adsorption was determined to be Fe(OH)3. The sorption kinetics were examined for zero, first and second order kinetics to determine a rate constant for silica adsorption reactions. It was discovered that all three materials fit the second order kinetics models and the uptake rates were determined to be 3.0 X 10-4 mg/L×min for Mg(OH)2, 9.0 X 10-5 mg/L×min for Fe(OH)3 and 7.0 X 10-5 mg/L×min for HTC at various doses.

Using the results of the batch tests, a flow through system was constructed and used to examine the material’s capacities on a larger scale and determine if 70% silica removal can be maintained. The results showed that when the materials were compared, HTC could achieve the target percent silica removal at a lower dose than Mg(OH)2 and Fe(OH)3 but, all three materials could maintain silica removal on a large scale. This study provides important information for water treatment industries looking to remove soluble silica from water.

Keywords

Silica, Removal, Design, Parameters, Dose, pH

Document Type

Thesis

Level of Degree

Masters

Department Name

Civil Engineering

First Committee Member (Chair)

Dr. Kerry Howe

Second Committee Member

Dr. Bruce Thomson

Third Committee Member

Dr. José Manuel Cerrato

Fourth Committee Member

Dr. Patrick Brady

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