“The main advantage is that it works over a wide pH range of 5 to 11 compared to other direct lithium extraction methods,” said Palance, ORNL Corporate Fellow and co-author of the paper presenting these findings.・Paranthaman said in a media statement.
The patent-pending acid-free extraction process roasts mined minerals at 140 degrees Celsius, compared to traditional methods of roasting mined minerals at 250 degrees Celsius with acid or 800-1000 degrees Celsius without acid. It is done.
The technology works based on lithiation, where aluminum hydroxide powder extracts lithium ions from the solvent to form a stable layered double hydroxide (LDH) phase. Next, in delithiation, treatment with hot water causes the LDH to release lithium ions and regenerate the adsorbent. During relithiation, the adsorbent is reused to extract more lithium.
“This is the basis of the circular economy,” Palantaman said.
quick reaction
Aluminum hydroxide exists in four highly ordered polymorphs and one amorphous, or disordered, form.
It turns out that the shape plays a major role in the functionality of the adsorbent.
“Based on calorimetry, we found that amorphous aluminum hydroxide is the most unstable form of aluminum hydroxide and therefore highly reactive,” said Jayanti Kumar, co-author of the study. said. “This is the key to this method, increasing the lithium extraction capacity.”
Amorphous aluminum hydroxide is the least stable of the mineral forms and reacts spontaneously with lithium in brine leached from waste clay.
“It was only when we took measurements that we realized that the amorphous form is much less stable. That's why it's more reactive,” Kumar said. “It reacts very quickly compared to other foams for stability.”
Two steps to recover lithium
Kumar is optimizing a process in which the adsorbent selectively adsorbs lithium from liquids containing lithium, sodium, and potassium, forming LDH sulfate.
The researchers used scanning electron microscopy to characterize the morphology of aluminum hydroxide during lithiation. This is a charged, neutral layer containing atomic vacancies, or tiny holes. Lithium is absorbed at these sites. The size of these vacancies is key to the selectivity of aluminum hydroxide toward lithium, a positively charged ion or cation.
“That open space is so small that it can only accommodate lithium-sized cations,” Kumar said. “Sodium and potassium are cations with larger radii. Larger cations don't fit into the open space. But it's perfect for lithium.”
The lithium selectivity of amorphous aluminum hydroxide provides near-perfect efficiency. The process captured 37 milligrams of lithium per gram of recoverable sorbent in a single step. This is about five times as large as the crystalline form of aluminum hydroxide called gibbsite that was previously used for lithium extraction.
The first step in lithiation extracts 86% of the lithium in leachate or brine from a mine or oil field. Repassing the leachate through an amorphous aluminum hydroxide adsorbent recovers the remaining lithium. “We can completely recover the lithium in two steps,” Paranthaman said.
Greener process
Venkat Roy and Fu Zhao from Purdue University analyzed the life cycle benefits of a circular economy with direct lithium extraction. They compared the ORNL process to the standard method using sodium carbonate. They found that ORNL technology uses one-third less material and one-third less energy, resulting in fewer greenhouse gas emissions.
Next, the researchers hope to expand the process to extract more lithium and regenerate the adsorbent in a specific form. Here, the amorphous aluminum hydroxide adsorbent reacts with lithium and is then treated with hot water to remove the lithium and regenerate the adsorbent, resulting in the aluminum hydroxide polymorph becoming amorphous. The structure changes from this to a crystalline form called bayerite.
“The bayerite form is less reactive,” Kumar says. “The amorphous form reacts within three hours to remove all the lithium from the leachate, whereas the reaction requires longer time (18 hours) or more concentrated lithium. We need to find a route back to the amorphous phase, which we know is high.”
Scientists believe that if they succeed in optimizing the new process for extraction speed and efficiency, it could revolutionize the country's lithium supply. More than half of the world's onshore lithium reserves are located in areas with high concentrations of dissolved minerals, such as California's Salton Sea and the oil fields of Texas and Pennsylvania.
“There's really no lithium production happening in the country,” Paranthaman said. “Less than 2% of manufactured lithium comes from North America. If we could use the new ORNL process, there would be a variety of lithium sources across the United States. This adsorbent is so good that it It can also be used in ion battery solutions.
