Recovering and refining precious metals
Some factors to consider when choosing a route for recovering precious metals from catalyst.
BRADFORD COOK and ROBERT JACOBSEN
Sabin Metal Corporation
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Precious metals are perhaps among the most recycled commodities on Earth, mainly because their value usually makes it worthwhile to spend time and effort to recover them. The process of recovering and refining precious metals has been around for a long time, but even today it remains somewhat of a ‘hidden’ industry. This article aims to clear up some of the most common myths surrounding what is considered both a science and an art for recovering maximum value from spent precious metal-bearing catalysts.
When spent precious metal-bearing materials are reclaimed, there are really only two essentials to consider: accurate sampling and precise analysis of the entire catalyst lot. That is because there are still organisations (or even individuals) who will offer to pay you for the remaining precious metals in your spent process catalysts based upon some unknown (mythical?) and unproved premise instead of actual precious metal content and an accurate assay during the recovery and refining process. A catalyst owner must be aware of this kind of transaction; do your homework, and check your quotes and contracts carefully.
Another area that feeds this myth concerns the recovery and refining process itself: the methods and procedures that ultimately extract the remaining precious metals from spent catalyst lots.
Two methods for PGM recovery
Precious metals refiners typically use one of two methods to recover and refine platinum group metals (PGMs) from spent catalysts. PGMs include platinum (Pt), palladium (Pd), ruthenium (Ru), and rhodium (Rh). Rhenium (Re), which is not considered a PGM, is also present in many spent catalysts and is a valuable precious metal too. These refining methods are pyrometallurgical and hydrometallurgical technologies. There is a clear distinction between these technologies that affects the outcome with regard to capturing the highest possible amount of remaining precious metals in the spent catalyst lot, including Re, now worth about $3000/kg.
After a batch of spent precious metal-bearing catalysts is homogenised and a representative sample drawn, a series of laboratory instrument analysis procedures is conducted, commonly known as assaying. Sampling is a series of processes used by precious metals refiners to create a homogeneous mass from spent catalyst lots which are randomly sampled in order to determine the type and percentage of precious metals remaining in the entire lot. However, in cases where the lot size is large (as it usually is in the petroleum and petrochemical industries), sampling is accomplished from a moving stream (auto sampling).
Assaying ultimately enables the precious metals refiner and the catalyst owner to agree on the value of the recoverable precious metals contained in the spent catalyst. Once this is done, the actual refining – the processes that extract the precious metals by one of the two previously mentioned techniques – can begin.
Pyrometallurgical vs hydrometallurgical processing
What happens when spent catalysts contain a significant quantity of Re? Re is usually present in about a third of PGM-bearing hydrocarbon processing catalysts; for example, in combination with Pt for reforming naphthas into other desirable products. While all precious metals refiners are capable of recovering most of the Re content from spent process catalysts on soluble alumina carriers, until recently none has been able to recover virtually all of the Re content. There are many reasons for this, but the main reason concerns the inability to separate the remaining Re with a practical process for its recovery and subsequent refining. This is because refiners recover Re by dissolving their carriers (typically gamma-aluminum oxide) with strong caustic or acidic chemicals (the hydrometallurgical or ‘digesting’ process). While this process is capable of recovering the soluble PGMs and Re content in spent catalysts, an unknown portion of the desirable pay metals, sometimes as much as 20%, may remain behind due to the insolubility of their substrates or carriers. Insolubility occurs because the substrate may change phase as a result of overheating during years of operation, preventing dissolution, even with strong solvents.
A refiner who uses pyrometallurgical technology (for example, Sabin’s Pyro-Re process) can recover virtually all the Re content from spent catalyst lots (semi- regenerative and cyclic fixed bed), particularly from catalysts on substrates that cannot be dissolved with caustic chemicals. The Pyro-Re process also offers advantages with regard to maximising the return value of all precious metals in the catalyst lot, including PGMs.
Cheaper is not better
Cheapness usually proves to be false economy. Take, for example, a precious metals refiner’s recovery/refining reclamation contract that is five cents lower per kilo than the next lowest quote. The myth here lies in the fact that many refiners are middle men who simply broker materials out to third party vendors. In the precious metals refining industry, there are essentially three categories of refining organisations: full service refiners (those organisations that provide full in-house recovery and refining capabilities, including transport logistics to eliminate transshipping charges and delays for settlement returns); samplers/processors (those organisations which partially process materials, perhaps upgrading them somewhat and combining smaller lots into larger lots); and brokers (companies or individuals who simply buy and sell refining services and use off-site, third party refiners). We have seen some of these organisations promise returns as high as 99.99% of remaining precious metals. The question arising here is: 99.99% of what? We have seen this happen many times over the years, where major refineries award recovery/refining contracts on the basis of a few pennies per pound difference in processing fees. In fact, there are organisations that require the lowest bidder to get the business, no matter what the cost. While they are understandably trying to cut costs, this strategy has little to do with the real money involved; the actual returned value of precious metals in the spent catalysts.
This is another good reason why catalyst owners should consider working with a refiner who provides full in-house processing capabilities, beginning with storage and shipping arrangements and continuing through in-house sampling and assaying, with an open line for the catalyst owner or its representative to be present at any of stage of the process. Middle men simply add margin but no value. Many times, large lots of spent catalysts are sent to two or three different processing facilities, obviously adding costs and delays along the way. When this happens, settlements are delayed, and the catalyst owner may have to finance replacement metals for new catalyst to ensure a seamless process flow.
‘Quality’ precious metals recovery and refining requires skilled people, sophisticated equipment and many years’ experience to complete the full cycle and arrive at accurate (and agreed upon) samples that are used to determine actual precious metals content in a spent catalyst lot.
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