The Metal Scavenging by Calcite Scaling During Utilization of Low Temperature Geothermal Waters. A Case Study in Selfoss Geothermal Area, SW Iceland

Abstract

Low-temperature geothermal fields (up to 150°C) are widely exploited in Iceland for district heating. One of the low-temperature fields is located within the Selfoss district heating, SW Iceland. The power plant utilizes two chemically distinct fluids originating from two aquifers located within the same basaltic geological formation. The first fluid endmember is rather dilute, whereas the second one is mineralized as it contains seawater component. To increase the hot water flow and meet the district heating needs, these fluids are mixed at the surface before reaching the distribution system. When used separately they do not cause calcite scaling in the surface pipes, however, when mixed at certain ratios – their chemical characteristics changes and carbonate minerals precipitate. This scaling material consists mainly of calcite carbonate mineral and its precipitation often results in complete blockage of the pipes. The precipitation of carbonate minerals immobilizes not only calcium but also other metals that fits into their crystallographic structure such as magnesium and iron. Previous studies showed that carbonate minerals have a potential to scavenge dissolved trace metals from the fluid such as Cd, Co, Cu, Mg, Mn, Sr, rare earth elements (REE) and the toxic metals As and Pb. Formation of the scales decreases the amount of dissolved metals in the hot water limiting their availability for future extraction. It also shortens the life time of the surface pipes delivering hot water to the households and therefore increases the costs of power production. Chemical analysis of scales collected from the pipelines will reveal the amount of trace metals immobilized from the fluid during the scaling process. Geochemical modelling using PHREEQC computer code will reveal extent of precipitation and metal loss from solution. These computer simulations will also result in assessing the best possible mixing ratios of the fluids to avoid scaling and therefore decreasing the efficiency of district heating.