AETC is committed to using and developing high-temperature technologies (HTT). Our work with HTT mainly allows us to either synthesize new materials or refine mineral concentrates before conversion into value-added battery grades, among other functions.
One of the processes we use HTT for is treatment of flotation concentrates available from natural graphite mines. Typically, those concentrates come in at 89- wt.% C purity. However, even 99 wt.% C carbon precursor is not sufficiently pure for application in advanced battery systems. The remaining 1% of impurities is comprised of a number of battery poisons such as vanadium, iron, molybdenum, silica, nickel, and calcium, to name a few. In our labs, we have developed unique continuous high-temperature reactors that refine graphites to purities of 99.98 wt. % C or more. Some of our best products are as pure as 99.9999 wt. % C, which is even higher purity than the purity requirements for nuclear grade graphite. We offer these high-purity refinements to selected, ITAR/EAR-approved interested partners — reach out to us if this sounds like something you need.
Recovery of Valuable Byproducts
Often, the graphite minerals undergoing thermal purification in our labs contain valuable impurities. We can apply our HTT to recover these impurities, which could potentially be rare-earth minerals (REE) or precious metals. A deposit of rare-earth is considered to be good enough to mine if it has a combined 300 ppm (parts per million). However, mining usually involves blasting through hard rock like granite, and the REE resource owners quickly lose enthusiasm for the task when they realize how low of a yield the REE actually has.
However, we employ processes that make recovery much easier. When we heat treat graphite for refinement, impurities sublime and get carried into the wet scrubber where off-gas gets neutralized, to form gypsum byproduct in which the valuable impurities get trapped. As you can imagine, it is much easier to recover minerals and metals from soft gypsum than from solid granite or other rock. Our gypsum recovery methods are very lucrative; we estimate that just one of our production furnaces generates at least $3 million worth of rare-earth and precious metal byproduct annually. Although AETC is not in the business of rare-earth material recovery, we are open to working with such companies to help them select the proper concentrate of material to be run through our furnaces to create the valuable byproduct supply chain.
When using graphite in battery systems, it is important to coat the individual particles of the material. HTT can be used as a reactor for forming a structure of soft carbon on top of spherical graphite. These structures are often referred to as amorphous carbon coatings and are widely used in active materials of lithium ion batteries. The purpose of these coatings is to create a semi-permeable shell around graphite particles, which can let lithium ions through but prevents any other components of the battery electrolyte from entering into the graphite crystalline lattice. The furnaces that we operate for this process require slow agitation and extremely slow temperature ramp rates, such that the coatings that are being created around the graphite become virtually undetectable, sometimes invisible, and always nanoscale. Below is an example of AETC’s nanoscale coating, taken on a transmission electron microscope:
In this image, we can see an x-ray of the spherical graphite particle’s core, taken with atomic resolution. The inter-layer distance between graphene layers is 3.354Å. The outer coating is made of amorphous carbon, and the thickness does not exceed 10 nanometers. We can also see that this coating is highly uniform, a result of the advanced technology that we use in its formation.
We use heat to expand graphite particles in a fully continuous manner to improve conductivity of various active matrixes. In expansion, the material goes through a fascinating process of forming vermiform, accordion-shaped particulate structures referred to in the industry as graphite worms. When the worms are run through subsequent delamination processing, we create battery-grade expanded graphite. The material features levels of electrical conductivity in battery-active matrices or in conductive plastics that are at least one order of magnitude higher than traditional materials used for the same purpose. As a result of using higher-conductivity materials, battery manufacturers can use the same size load of conductive carbon as traditional materials with lower conductivity while then subjecting the battery to much higher drain rates. Alternatively, manufacturers can use a smaller load of conductive carbon, and fill the leftover space with more reactive material, creating batteries with longer run time. Ultimately, expanded graphite can be used to create either higher-energy or higher-power cells, and manufacturers can even build hybrid batteries which feature both high-energy and high-power with graphite worms incorporated into the design matrix.
We use activated carbon for a variety of processes and services, and HTT is used to produce this carbon in-house. Activated carbon can be used for the purposes of selective sorption of poisons from blood (medical sorbents), as scavenging technology for the absorption of gases coming off industry, or for selective sorption of oxygen used widely in gas-diffusion electrodes, such as in fuel cells or zinc air batteries. These reactors operate on the principle of fluidized bed technology. Fluidizing media in this case is a combusting natural gas and steam. Uses of these 2 fluidizing gases allow us to achieve a synergistic effect by making activated carbons more pure, more selective in absorption properties, higher in BET surface area, and lower in cost than commercially available counterparts.
HTT can even be used in the synthesis of cultured diamonds. In this process, we apply high temperature and high pressure simultaneously to resistibly heat a bed of pre-compacted graphite mixed with catalysts. In the matter of a few minutes, selected types of graphite in the presence of catalysts turns into a diamond phase. The process is followed by wet chemical separation, the purpose of which is to segregate superior diamonds from partially-conducted diamonds and unreacted graphite. We also converted certain grades of synthetic graphite and coal into diamonds. Particular success was achieved with subbituminous coal coming from a Navajo reservation. Those diamonds are shown in the SEM images below.
Read more about our work with synthetic diamonds here.