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Dec-2011

The science and art of sampling precious metals catalysts

When precious metal-bearing catalysts in hydrocarbon processing applications lose their efficacy, they must be recycled and replaced by fresh catalysts. The spent catalysts are then sent to a precious metals refiner to recover and refine the precious metals contained.

Kevin M Beirne
Sabin Metal Corporation
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Article Summary
These are commonly referred to as PGMs and typically include platinum, palladium, rhodium and ruthenium; rhenium, while not classified as a PGM, may also be present in hydrocarbon processing catalysts and must also be recovered because of its high value. Depending upon process volume, catalyst cycle time, accumulated contaminants in the catalyst and the precious metals content, it is likely that many thousands or hundreds of thousands of dollars of value could be recovered from a single spent catalyst lot. The question, simply, is this: how does one obtain the most value from spent catalysts?

Hydrocarbon and petrochem processors pay careful attention to values of the precious metals content in their catalysts, especially in this economy. With extraordinarily high values of many precious metals over the past few years, recovery of the metals’ values in spent catalyst lots is considered high priority. Because of these high values, precious metals catalyst users must make prudent decisions in the selection of their catalyst refining organisation and the policies and procedures associated with the recovery and refining process.

During these processes, precious metals refiners use a number of different methods to extract the highest possible quantity of precious metals from spent catalysts. Most of these techniques are based on well-founded scientific practices and principles; however, as in many complex scientific processes, “art” (in this example, insight gained from years of experience and expertise) can make a significant difference with regard to achieving maximum yields of precious metals.

Precious metals refiners use various techniques to recover PGMs from spent catalysts. However, all refiners must address key issues such as point-to-point transportation, materials documentation, recovery/refining efficiency, environmental compliance, process turnaround times and many more. While each of these functions is important in the overall recovery process, the sampling of spent catalysts is the most critical element of the entire process as it affects the determination quantity of precious metals returned.

The science of sampling spent catalysts will be discussed here clearly and concisely; the art of sampling, however, will become clear as each individual sampling procedure is described.

Homogeneity is the key to accurate sampling
At the start, it should be understood that homogeneity is the key to acquiring a representative sample from a spent catalyst lot. Without this characteristic, the precious metal content of the lot cannot be accurately determined. It should also be understood that spent hydrocarbon processing catalysts are not homogeneous. Even new catalysts on substrates (carriers) such as soluble and insoluble alumina, silica alumina, zeolite or carbon supports are not homogeneous masses; after years of exposure in the harsh catalytic reaction environment, spent catalysts are far from homogeneous. During the chemical process, catalysts accumulate a variety of contaminants such as sulphur, carbon, solvents and water.

In order to determine their precious metals content (and thus arrive at an accurate and realistic value based on the remaining quantity and quality of PGMs), the spent catalysts must first be “reduced”. This involves taking very large quantities of spent catalysts (mainly many tons) and reducing them to very small quantities (as little as a few grams), which represent virtually 100% of the multi-ton lot, after eliminating process contaminants. In the end, the sampling process enables the refiner to capture a representative sample of the overall spent catalyst lot. This is a critical consideration for the catalyst owner, since this procedure may not be customary at all precious metals refiners. Once this step is completed, the refiner can accurately determine the precious metals content of the entire lot with extraordinary accuracy.

Three different sampling techniques are used to determine precious metals content prior to the final recovery and refining processes. They include dry, melt and solution sampling. Each of these methods incorporates different techniques, and each offers specific advantages with regard to accurate determination of remaining precious metals. The method used depends on certain variables, which include the type of catalyst sampled, its estimated precious metals content and, most important, how the catalyst was used in the reaction process. Determining the most appropriate sampling method depends upon these and other factors; however, the expertise of the refiner is also a key part of the equation. This characteristic is often referred to as the “art” of sampling. After all is said and done, users of precious metal-bearing catalysts must look to the experience and reputation of their refiners.

Hydrocarbon processing catalysts are usually dry sampled
Due to their composition and chemistry, precious metal-bearing catalysts employed in hydrocarbon processes (to facilitate and/or speed chemical reactions) are usually sampled with dry sampling methods. Dry sampling is used when materials cannot be dissolved in a solution or are inappropriate to melt, either because of their structure or because of the cost associated with melting versus the possible return. Because it is difficult to achieve homogeneity in large spent catalyst lots, dry sampling is more complex and potentially less accurate than melt or solution sampling.

As sampling is also considered the most important part of the precious metals recovery and refining process, it must be viewed from the perspective of the refiner as well as the refiner’s customer. Clearly, the customer’s goal is to receive the highest possible value for the precious metals in their spent catalyst materials. The refiner, on the other hand, must not only consistently meet that goal for its customer, it must also provide detailed documentation of how this value was determined. The refiner and customer each have responsibilities that must be addressed in order to ensure a mutually beneficial relationship based on fair, straightforward business practices. Without this, there is no possibility that a precious metals refiner can retain their existing customer base, little possibility that it can continue to attract new customers and not much probability that it can remain in business over the long term.
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