Design of a Symbiotic Device to Harvest Uranium from Seawater through the use of Shell Enclosures
At the current consumption rate, the global conventional reserves of uranium, 7.6 million tonnes, could be depleted in a little over a century. As reserves decrease, future uranium is expected to come from lower quality sites, resulting in higher extraction costs and even greater environmental impact. Fortunately, uranium is present in the ocean as uranyl ions at low concentrations of 3 ppb, which, over the total volume of the oceans, amounts to approximately 4.5 billion tonnes of uranium, nearly a thousand times that of conventional reserves. Methods to extract uranium from seawater have been studied since the early 1960's, when, at a time when uranium production was uncertain, post-World War II Britain desired a secure uranium supply. A recent review of uranium recovery technologies by found that uranium adsorbent by chelating polymers was the most promising in terms of adsorbent capacity, environmental footprint, and cost. In this technology, chelating polymers are deployed in seawater and remain submerged until the amount of captured uranium approaches the adsorption capacity. Then, an elution bath is used to strip metal ions, such as uranium, off the adsorbent polymer. A polymer may be immersed in a number of elution baths before it is regenerated by an alkali wash to free its functional groups, allowing for the reuse of the polymer. The output from the elution process undergoes purification and precipitation typical for mined uranium to produce yellowcake. Offshore systems for the extraction of uranium from seawater have been developed since the early 2000's. In these systems, the adsorbent is deployed and moored for extended periods of time, brought back to shore for the elution process, and redeployed afterward. The first system, developed by, used non-woven adsorbents that were immersed for 30 days in seawater. Due to the low mechanical strength of these fibers, they had to be incorporated into stacks composed of spacer nets and holders placed on large, heavy, floating frames, which resulted in a design that was too costly for implementation. More recently, buoyant braided adsorbents have been studied as a way to decrease the weight and cost of the system. In these systems, continuous polyethylene fibers are braided around a porous polypropylene float that can be made into long lengths. However, because this deployment scheme requires the adsorbent be brought back to shore for the elution process and redeployed afterward, this stand-alone, intermittent operation system has significant practical and economic deployment challenges and to date none of these systems have become economically viable. Detailed economic analysis by found that a major cost driver of seawater uranium extraction is the mooring and recovery of the adsorbent. Based on these results, first developed the development of a system that is attached to an offshore wind turbine and continuously takes the adsorbent from the ocean through an elution process and then returns it to the ocean. More recent designs by further build upon this idea. The integration of the design of a uranium harvesting system into an offshore wind turbine tower is pursued because the development of offshore wind or uranium harvesting by themselves bears a high capital cost for the structures, but if the mooring function can be shared, the overall cost for each will be lower. Furthermore, initial design analysis and prototyping of these symbiotic systems has proven they are technical feasible. (authors)