PNNL

Abstract

Our industrial client (Geo40) has developed and deployed a process to remove silica from geothermal fluids and produce a high-margin specialty colloidal silica product comparable to those of market leaders. Geo40 now wishes to explore opportunities to extend their mineral extraction operations to other elements that are present in these brines. Geo40 has identified cesium (Cs) that is present in Ohaaki brines (pH ~8–8.5) at parts per million levels and could be sold to customers if it could be produced at an attractive price. With support from the Department of Energy’s (DOE’s) Geothermal Technologies Office, a simple and highly cost-effective magnetic nanofluid method for extraction of rare earth elements (REEs) from geothermal brine solutions has been developed and demonstrated at the laboratory bench scale at Pacific Northwest National Laboratory (PNNL). Core shell sorbent particles are produced using an iron oxide core particle, which is used to anchor and grow a surrounding adsorbent shell functionalized with a chelating ligand that selectively binds REEs. We extended PNNL’s work by exploring new sorbent shells that are highly selective for Cs. Uptake of Cs was measured as a function of exposure time by analyzing solution samples extracted from batch sorption tests. We screened and identified suitable sorbents that have a higher capacity for selective removal of Cs from Ohaaki brines in acidic and basic environments. The cobalt-containing Prussian Blue (Co-PB) tested using 5 ppm Cs with and without competing sodium (Na) ions exhibited the overall best performance toward Cs adsorption capacity reaching as high as 31.6 g/kg and 19.0 g/kg in the presence of competing Na ions. The Co-PB was subjected to repeated adsorption, stripping, and regeneration cycles to assess any degradation in sorbent capacity. We also conducted Cs desorption and Co-PB regeneration experiments using different stripping agents (nitric acid and potassium chloride) to determine which stripping agent is ideal. The techno-economic analysis revealed that the internal return rate (IRR) for extracting 90 percent of Cs from a geothermal brine containing 700 ppb of Cs using the magnetic nanoparticle adsorption technology was about 6.1 percent. The IRR will increase with a higher concentration of Cs and lower capital cost, and it is not sensitive to the adsorbent cost when the adsorbent can achieve a 6,000 h lifetime and higher than 99.9 percent magnetic retention rate.