Removing Contaminants From Spent Precious Metal Bearing Catalysts
By Robert T. Jacobsen, Ph.D.
Sabin Metal Corp., East Hampton, NY, U.S.A.
“Pre-burning” prior to recovery/refining enhances sampling accuracy for higher returns of remaining precious metals
A general tutorial overview on recovery and refining remaining precious metals from spent hydrocarbon, petrochemical and chemical catalysts appeared in the September 2003 issue of this publication (Recovering spent catalysts – the profits and pitfalls, Kevin M. Beirne). That article was presented to help users of PGM-bearing catalysts (specifically platinum, palladium, rhodium, and ruthenium) obtain greater returns by carefully evaluating, selecting, and working with a well established precious metals refiner. By doing so, they could reduce – and in many cases, eliminate – many of the pitfalls associated with environmental issues at the processing facility; inaccurate materials sampling; improper recovery/refining procedures; and, many other common problems. The article pointed out that it is in the best interest of a precious metals user to have a clear understanding of how sampling processes are employed to evaluate the remaining precious metals content of large lots of spent catalyst materials. This is a critical issue since, ultimately, the value of the remaining precious metals which are returned to their owners is predicated upon meticulous and accurate sampling procedures. The article discussed three different sampling techniques, and also included information on sampling procedures for a typical lot of spent catalyst materials.
A key consideration in maximizing recovery of the remaining precious metals in spent catalysts concerns the removal of contaminants from the entire lot prior to the actual sampling procedures. While this was covered briefly in the previous article, it warrants further discussion here because of its importance to – and influence of – the overall recovery and refining process.
Removing contaminants is critical to maximizing return values
Precious metal bearing catalysts used for facilitating reactions in hydrocarbon processes are subject to harsh operating conditions during their lifetimes. In fact, in most cases they must be periodically “regenerated”, either while in operation in a continuous moving bed catalytic process, or removed and shipped off-site to eliminate contaminants accumulated during processing. Eventually, however, they lose their effectiveness to the level where they must be replaced by “fresh” catalyst materials.
Hydrocarbon processing catalysts that are in use for a number of years are typically contaminated by sulfur, carbon, moisture, and other unwanted elements depending upon how they are employed. Since most spent catalysts are subject to dry sampling procedures, to maximize sampling accuracy the organic materials and moisture in the spent catalysts must be removed to assure accurate evaluation of the remaining precious metals. This process is usually accomplished with an indirectly fired rotary kiln, and not only significantly enhances sampling accuracy to help assure maximum recovery of remaining precious metals, it can also help reduce overall refining costs when it is handled directly at the refiner’s facility. This is a key issue with regard to total cost of recovery and refining and, by inference, the overall profitability of a precious metals refining program. The typical rotary kiln removes the materials’ sulphur and carbon contents, usually at a rate of 300 to 1000 lbs. per hour.
Precious metal bearing catalysts exhibit high loss on ignition (LOI) characteristics which may be due to moisture and contaminants such as high sulfur and/or high carbon. Removal of moisture and contaminants is critical to the downstream sampling process, since the materials must first be free flowing to yield a final evaluation sample that is at least accurate to ± 1%. LOI is inherently less precise and less accurate than a settlement assay, and therefore must be as low as possible when samples are sent to the assay laboratory. Consequently, accurate LOI data is critical to eliminate possible weight changes while the sample is in transit to the laboratory. In order to arrive at an accurate settlement weight (the entire spent catalyst lot does not undergo the same sampling procedures as the assay sample), a separate LOI sample is necessary to maximize accuracy. Pre-burning has a significant affect on achieving this high degree of accuracy; if the high moisture content and other contaminants are not removed, a suitably accurate sample cannot be obtained by the refiner, thus eliminating the possibility of providing a fair and true return value to the customer (See Figures 1 & 2).
The contaminants in spent catalysts may also be removed by a multiple hearth furnace or fluidized bed furnace. Whatever the case, this first step, or “pre-burning,” is critical to the sampling process. As important – at least from a financial perspective – is where and how the contaminants are removed. Here, many catalyst users must first ship their large lots of spent catalysts (perhaps as much as 35,000-500,000 lbs.) to an independent facility where strip burning removes their hydrocarbon content such as benzene, and coke burning removes carbon. In addition, another furnace may be required for drying of fine particulates and other materials to eliminate moisture content.
More time, added costs for off-site contaminant removal
Added turnaround time and additional costs are the two main considerations associated with off-site strip and coke burning of spent catalyst materials. In other words, unless these capabilities are available at the refiner’s facility, catalyst users must pay substantial transportation charges for shipping to an independent, off-site facility. While there, it would not be uncommon for the material to remain up to a month for processing before it would again have to be shipped to the refiner to start the actual sampling, analyzing, recovery, and refining process. During this time the PGMs are unavailable to the catalyst user, and therefore new metal must be acquired at current market prices and lease rates. As an example, at todays platinum lease rate of 7% for a typical hydrocarbon catalyst containing 0.3% platinum, the finance cost per pound per week is U.S. $.045 a pound at the current platinum value of approximately $770/oz. For a typical shipment of 100,000- 200,000 lbs. this represents a cost of U.S. $4,500-9,000.
There is another, equally important advantage of having your refiner handle the “pre-burning” or contamination removal procedures in-house. That is, the control (perhaps “accountability” might be a better term) the refiner has over your specific catalyst lot, eliminating all possibility that your materials could be mixed in with unrelated materials from another organization. When that happens, there obviously is no way an accurate determination of its actual value could be calculated.
Clearly based on this information it is in your best interest to seek a precious metals refiner with complete in-house capabilities from beginning to end; from contamination/moisture removal of large spent catalyst lots through returning new metal or cash on a timely basis. However, sometimes that, in itself, may not even be enough to assure both highest possible sampling accuracy during the processing cycle, and peace of mind with regard to environmental issues. There are two reasons for this: first, the indirectly fired kiln used for the contamination removal, or “pre-burning” process must also incorporate extremely accurate, multiple temperature zones that must be programmed for the specific catalyst type and contaminants present; second, there still must be no harmful or unlawful atmospheric emissions from this process. The kiln should typically be part of an environmentally sound, complex system that incorporates downstream anti-pollution equipment such as scrubbers, baghouses, and afterburners to safeguard the environment and comply with applicable pollution control standards. Referring back to the September 2003 article, keep in mind that you may still be held liable – along with your refiner – for both unlawful atmospheric emissions as well as effluent discharges.