Prevention of Scaling by Quick Removal of Metals from the Brine Via Adsorption

Abstract

Scaling (= mineral precipitation) in geothermal plants occurs due to oversaturation of salts caused by changes in the chemical equilibria during processing of thermal water. Since salts are composed of a cationic and an anionic part, uncontrolled precipitation can be avoided if the cation (often a heavy metal) is removed from solution by a sorption technique to concentrations below mineral saturation. Various sorption techniques are known from groundwater purification, but have never been applied in geothermal plants. For industrial use in such environments the adsorption material needs to be of low costs, temperature resistance, large sorption capacity at quick removal, and ideally it can be reused after cleaning/ regeneration. In this study three different sorption materials were tested to eventually develop a filter material that removes scaling-inducing elements below their saturation: Zeolite (highly porous alumosilicates), chitosan (a biopolymer obtained by deacetylation of chitin), and magnetite (Fe3O4; a magnetic mineral with high surface area). While zeolites and iron oxides could be added as seed material to the thermal water in a first step and be removed by particle filtration in a second step, the chitosan can be processed into fiber mats and integrated in a filter system directly. Mineral saturation was calculated for various conditions by PhreeqC to ensure that the metal removal process was not by mineral precipitation. The three sorption materials were tested at various salinities (0-2 M NaCl) in a batch attempt. In addition, a flow-through reactor was developed to investigate the ability of these materials for removing the scaling-relevant metals lead, barium, copper, and iron from geothermal brine under near-geothermal, flowing conditions.