Iron Nanopowders
March 2021 - July 2021
This was a project to scale up nanopowder production from 100mL to 10L. This included optimizing the coating at the 1mL scale and developing the system necessary to produce the nanopowders at a 10L scale. I led this project as a part of a 7-month co-op, with the support of my full-time coworkers.
The iron nanopowder project was my biggest project during my 7-month co-op, in terms of both length and complexity. I was in charge of the scale-up of one of our production of nonzero valence iron (nZVI) from 100mL to 10L. These nanopowders are designed for use in magnetic inks and biomedical applications which require them to be magnetic and a very small particle size. Therefore, it is essential that they are coated in a protective substance before they are exposed to air so that they do not immediately oxidize. My job was to first optimize the chemical composition of this coating at the 1L scale and then design and fabricate the equipment to semi-automatically produce and coat the nanopowders at the production scale of10L.
One of the other engineers on my team had already developed the process for nanopowder formation and determined the optimal coating chemistry at the 100mL scale. After working on this project with her for several weeks, I wrote the standard operating procedure (SOP) for iron nanopowder production at the 100mL scale.
From there, I took over scaling up the process. The first step was to optimize the coating chemistry at the 1L scale. I made some adjustments to the formation setup to allow for this 10x scale-up, but for the most part, I was able to use the old equipment. Over the course of several months, I performed a series of experiments to determine the optimum coating chemistry at the 1L scale, evaluating them based on particle size and degree of oxidation. It was important that the coating be thick enough to eliminate oxidation, but thin enough to maintain an appropriate particle size.
Once the nanopowders were produced, dried, and weighed, I experimented with different ways to grind them that would be gentle enough not to disturb the thin coating, but aggressive enough to break up any agglomerations that formed during the drying process. Particle size was characterized and confirmed by passing the powders through a series of sieves with progressively finer mesh openings. In the photo on the left, samples from the experiments are organized by test date from top to bottom, and particle size from left to right. You can see how the nanopowders got progressively darker (indicating less oxidation) and finer as the chemistry was optimized. These later tests had the thinnest coating possible without allowing the nZVI to oxidize and therefore had the highest magnetic properties and smallest particle size possible with this coating.
When I had finished optimizing the coating chemistry at a 1L scale, I started designing the production system for the 10L scale. A key design constraint was that it needed to separate the nanopowders from the waste solution quickly and automatically so that they could be coated before they oxidized. If they were not separated well enough, the solution would ruin the coating. At the 1L and 100mL scales, this separation was done by hand, using neodymium magnets to pull the nZVI out of the solution. However, this process was far too labor-intensive to work on larger scales.
The nanopowder formation process was already well established at the 1L scale, and expanding it to the 10L scale only required some slight design changes, such as a new sonication method. Most components merely needed to be bigger. Sourcing them was fairly simple, although finding cost-effective chemically-compatible materials at this scale proved to be somewhat of a challenge, as it was smaller than most industrial production scales but too large for most lab equipment to handle. In the end, I was able to find everything I needed by getting creative and modifying consumer products when neccessary. What I couldn't find, I built from scratch, usually out of wood or 3D-printed plastic parts.
While designing the systems for nanopowder formation was relatively straightforward, figuring out how to separate them from the solution was anything but. Particles this small fouled most membranes, so a vacuum or pressure filter wouldn't work. They settled out of the solution too slowly to prevent oxidation, and a cyclone separator wasn't effective at this scale. After several failed experiments with the alternatives, magnetic separation appeared to be the best option, but 10L was far too much volume to be separated by hand the way it was at the 1L scale. So I began testing various methods of magnetic separation. First, I built a miniature drum magnetic separator, which went through several iterations and many tests but was ultimately unsuccessful.
When it became clear that the drum magnet wouldn't work, I pivoted to using a series of sluices lined with neodymium magnets. I made them out of a thin silicone membrane stretched over a sheet of magnets and supported by a 3D-printed plastic frame. The reaction solution flowed out of the reaction chamber and down the sluices, with the nanopowders sticking to the magnets at the bottoms of the sluices while the nonmagnetic solution continued on into a collection chamber. A pump then cycled the solution back through the system several times to pull out all the nanopowders, followed by a few rounds of a solvent to rinse away any remaining reaction solution. When the process was done, we pulled trays of neodymium magnets out from underneath the silicone sheets and rinsed nanopowders into a separate collection chamber to be coated and dried.
Above is a video that shows the magnetic separation during the first test of the system. To the right is a video that shows more of the setup during an earlier water test. My co-op ended before I got a chance to refine the design, but my first iteration prototype still impressed my supervisors--it achieved proper nanopowder formation and significant separation. I also documented my work thoroughly enough for someone else to easily pick up where I left off. This documentation included a detailed test grid for the chemical coating, sketches of the system design, SolidWorks models and printing instructions for the 3D-printed parts, a bill of materials, a standard operating procedure, and descriptions of proposed changes to the design based on what I had learned in the first few tests.