Solid Phase Extraction
An Unparalleled Technology
Most solid materials that are used for absorbing and purifying metals place the metal binding group (or ligand) on the surface. The limitation to this method was discovered in 1924 by Otto Stern, who showed that a stagnant boundary layer of fluid exists around solid surfaces. This slows the exchange of metal ions on and off the surface. No matter how well the ligands are designed, their performance will still be slowed and limited by the Stern boundary layer at the surface. This is a problem for both polystyrene beads, which are the basis for Ion Exchange technology, and silica materials, which are the basis for technologies such as Molecular Recognition Technology (MRT).
Metals U.S. uses a cutting polymer ligand design (over a thousand different SPE binding materials have been designed to date), our essential advantage is that instead of attaching our ligands to the surface of our silica material, we construct an interstitial polymer “web” called the Spiderweb™. This web “delivers” the ligands far away from the surface into the active solution so that metal binding and exchange can occur nearly instantaneously. SPE material technology is protected by four US Patents, and is key to Metals U.S.’s market advantage.
The process advantages of our unique technology are profound:
High Throughput: Fast binding means high flowrates of metal solution through columns, leading to small columns processing large volumes. SPE columns have been shown to process a volume 20X their size in a minute (20 bed volumes/minute). This speed is unprecedented for any other metal binding technology. This provides SPE its unprecedented low capital equipment cost and reduces operating costs.
Rapid Metal Purification: Purification of metals requires that weaker binding metal impurities be displaced off the ligand by stronger binding target metals. Complete displacement needs to occur to provide a pure metal product on the resin. For example, in Rare Earth Element purification, SPE materials have been shown to purify Rare Earths directly out of mixed metal leach solutions. Other technogies require costly pre-treatment technologies before the resin can be used to purify the Rare Earth elements.
Easier Scalability: Because the units are so high throughput, a small unit can service very large operations. Thus, scaling issues for SPE technology are much less than with other technologies.
Longer Material Lifetimes: Most ligands are corrosive to silica materials, dramatically reducing lifetimes of silica based binding materials. By positioning the ligand binding groups in the polymer web far from the silica surface, and placing an additional chemical coating around the surface, SPE dramatically reduces corrosion issues, increases column lifetime, and decreases Operating Costs related to resin replacement.
Easier Recycling of Process Chemicals: In order to recycle process chemicals, impurities accumulated during the process need to be removed so that they don’t impede the effect of the chemical. Otherwise, the chemical and the accumulated waste need to be eliminated. SPE’s uniquely fast and efficient exchange properties are optimally suited to recycle process chemicals and therefore enable a Zero Discharge Process.
Advantages over Non-Solid Based Technologies
Metal purification methods based around binding solids, such as SPE and Ion Exchange have an essential processing advantage: the binding material is contained in a box. Impure metal solutions go in one end and come out the other end purified. This simplifies the process engineering. SPE’s unique Spiderweb overcomes the slow speed and exchange limitations of other solid binding materials. Here are a few other non-solid absorbent material methods, and a comparison with SPE:
Solvent Extraction: Instead of putting the metal binding group on a solid material, Solvent Extraction puts the binding groups in a liquid material, specifically an oil layer, which pulls target metals from water into oil. While the dissolved ligands bind metals quickly because they are in the liquid phase, it is well known that oil and water don’t readily mix, so the speed limitation on Solvent Extraction is related to getting the oil and water well mixed and then re-separated. Additionally, Solvent Extraction can only use ligands which are soluble in oil, limiting available ligand chemistries, and capital and operating costs for using the large expensive units, water contamination by the surfactants used in the process, and flammability of the oil materials all limit the method.
Biosorption: One method commonly used for environmental remediation is the use of cattail or bacterial or similar “ponds” to pass metal contaminated solution through. The metal then binds the biological materials rather like to an Ion Exchange or SPE material. However, while biological materials do have some metal binding capability, they are not designed for this purpose, so a very large amount of biological material is required to treat metal waste streams, and the metals are often even then not completely removed. Additional, there are also legacy issues of having a metal laden waste pond which must be maintained far into the future.