ABSTRACT

Silica is becoming the critical limiting factor in the reuse of water. Silica deposit is caused as a result of its polymerization, co precipitation with other minerals, precipitation with other multivalent ions, and biological activity in the water. Several of these processes may take place concurrently, making it difficult to predict equilibrium solubility. The solubility of amorphous silica is also dependent on many other factors such as, pH , temperature, particle size, particle hydration, and the presence of other ions, such as iron, aluminum, etc.. A hard silica scale is formed when calcium carbonate or other mineral precipitate provides a crystalline matrix in which silica can be entrapped.

INTRODUCTION

Fouling by silica scaling remains one of the key problems to be solved In geothermal energy utilization. The Electric Power Research Institute, in their report? on power plant water usage, has reported that, in 70% of power plants, water usage is limited by silica concentration. Recently, Amjad? et al reported that in many parts of the world including the western United States, Hawaii, Puerto Rico, Mexico, the Middle East, and Southeast Asia, recovery rates of reverse osmosis systems are limited by silica fouling. The reuse of water in many industrial plants , especially those operating at or designed for zero discharge, is also limited due to silica.

The potential for silica scaling, unlike most other mineral deposits, does not solely depend on the saturation level with respect to the mineral. There are other processes, such as polymerization, colloidal silica suspension, precipitation of silicate minerals, biological activity such as diatoms, and co precipitation of silica, all of which have a profound effect on the silica deposition; concurrent participation of many of these process makes the silica deposit control that much more challenging . To prevent silica based deposit, it is essential to control all these paths, simultaneously. This paper discusses various aspects of the solubility of amorphous silica and factors influencing silica deposition such as the effect of calcium carbonate precipitation on silica threshold level of soluble (monomeric) silica.

Experimental

A study was done to investigate the effect of other ions and co precipitation on the solubility of silica. Two different conditions were used. The study was carried out under stagnant conditions using two different conditions. The water matrix described in Table 1 was used for the initial studies. The flasks containing the appropriate amounts of these waters, other ions, and inhibitors were thermostated at 55 C for either 24 hours. At the end of incubation time, solutions were filtered through 0.2pm NuclerporeTM membranes and aliquots were drawn for analyses of various ions including silica. Hardness was measured using complexometric titrations. Silica was analyzed by forming a yellow silicomolybdate complex which was further reduced to produce molybdenum blue species. The intensity of the molybdenum blue color was measured using a spectrophotometer at 390 nm. A calibration curve was prepared with several standard solutions of silica prepared by using Na,SiF,.

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