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Energies 2021, 14, 6805 54 of 72 Some of these projects advanced to pilot-scale testing [31], but many were tested at the bench scale only. Hazen International was commissioned by the US Bureau of Mines to design metal recovery processes for Salton Sea geothermal brines [31,274,275,309]. The proposed pro- cesses included a primary step applying lime to precipitate iron hydroxides and then a second step where lime was used to precipitate mixed hydroxides of zinc, manganese, and lead [275]. Hazen International reportedly designed and operated a 15-gpm pilot plant, using post-flash brine from the San Diego Gas and Electric power plant at the Magmamax No. 1 well [31]. While their complete design included steps for silica, iron, manganese, and lithium recovery (Figure 22), the actual pilot plant used just one lime-precipitation step in which a co-mingled precipitate containing iron, manganese and zinc was obtained [31]. In the early 1980s, the US Bureau of Mines operated a pilot-scale metals recovery unit using post-flash geothermal brines from the Salton Sea KGRA [310]. The brine source was treated with lime to precipitate iron, manganese, lead, and zinc before reinjection of brine [310]. The process used 27 pounds of lime per 1000 gallons of brine and achieved a 95% to 99% removal of iron, manganese, zinc, and lead. It was observed that lower doses of lime were insufficient for complete precipitation of manganese, whereas higher doses of lime caused re-dissolution of precipitated lead [310]. The use of air oxidation was investigated and found to have either no effect or a negative effect on metals recovery [310]. The investigators concluded that the quality of the hydroxide precipitate was poor in terms of metal content and value and suggested that precipitation of metal-bearing sulfides would be a better approach to recover metals [310]. Subsequent studies by the US Bureau of Mines investigated sulfide precipitation using real brines [285]. Sulfide precipitation with H2S was able to remove over 99% of the zinc and lead in the brine so long as the pH was controlled with lime. Recycling the sulfide precipitates increased crystal size, resulting in a product that was easier to filter [285]. A typical sulfide precipitate contained 53% ZnS, 8% PbS, 24% MnS, and 15% FeS. Adding sulfide as FeS or CaS was not effective in precipitating zinc and lead, but zinc in the brine could be selectively removed with a strongly basic ion-exchange resin [285]. A proposed flowsheet for metals recovery included steps for silica removal, sulfide precipitation of metals, lime treatment, and lithium precipitation with aluminum (see discussion below) [285]. However, to our knowledge, sulfide precipitation was not applied or tested at the pilot scale. In the 1990s, Unocal developed a “Line Mine” process that was used to capture scaling minerals from brines prior to reinjection [311]. The process was operated at full-scale for several years and the potential to monetize the mineral deposits collected by the Line Mine process was investigated [272,311,317]. Although the collected mineral scales did contain precious and semi-precious metals, including silver, metal concentrations and amounts recovered were insufficient to support an economic commercial mineral recovery process [272,311,317]. CalEnergy Minerals operated a zinc metal manufacturing facility at its Elmore power plant in the early 2000s [287,313]. The demonstration plant produced 41,000 lbs. of high- grade zinc using an ion-exchange process [312]. After recovery of the zinc from the ion-exchange resin, the zinc was placed in electrolytic cells and the zinc was deposited on cathodes as zinc metal in a batch process over a 24 h period [312]. High-purity zinc deposits, reported to be a quarter of an inch thick, were scraped off of the electrodes and melted into ingots [312,314]. The facility was expected to produce an estimated 30,000 metric tons per year of zinc and had sales contracts with Cominco, a major zinc producer and broker [312,318]. The facility operated commercially for several years, but the venture was abandoned in 2004 as a result of not meeting production goals and a drop in commodity prices [313].PDF Image | Recovery of Lithium from Geothermal Brines
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