The Key Advantage
Bradford M. Cook, Sabin Metal Corp., USA, outlines how to enhance platinum group metals and rhenium returns from spent hydrocarbon and petrochemical process catalysts.
All hydrocarbon and petrochemical refiners believe they know the precise composition of their precious metal bearing process catalysts at the start of a campaign. But how many know their precise composition at the end of a campaign, after years of working under extreme conditions? Think about it: In addition to many common problematic factors found in spent catalysts (i.e., moisture, coke/carbon, benzene, excessive fines, and support media), there are often other extraneous elements present such as silica, iron and sulfur. While these troublesome traits and contents have little or no value themselves, they can have a significant affect on the total precious metals return value one receives from one’s refiner.
TWO DIFFERENT REFINING METHODS Precious metal refiners typically use one of two discrete methods to recover 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 catalysts. These refining methods are pyrometallurgical and hydrometallurgical technologies. After a batch of spent precious metal bearing catalysts is homogenised and a representative sample drawn1, a series of sophisticated laboratory instrument analysis procedures is conducted, commonly known as assaying.2 Assaying 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 can begin: the processes that extract the precious metals by one of the two previously mentioned techniques.
Baghouses and other pollution control systems represent state of the art air filtration technology, including recovery of precious metal particulates prior to atmospheric discharge.
There are fundamental differences between these two methods, but one of the key advantages of pyrometallurgical processing is its ability to fully recover the spent catalysts’ rhenium content, while a hydrometallurgical process can only recover the rhenium that is acid soluble.
What happens when spent catalysts contain a significant quantity of Re? This valuable metal is usually present in approximately a third of PGM bearing hydrocarbon processing catalysts; for example, in combination with platinum for reforming naphthas into other desirable products. While all precious metals refiners are capable of recovering most of the rhenium content from spent
Dual electric arc furnaces double pyrometallurgical processing throughput to help assure maximum recovery of remaining PGMs in spent catalysts, including rhenium.
process catalysts on soluble alumina carriers, until recently none has been able to recover virtually all of the rhenium content. There are many reasons for this, but the main reason concerns the inability to separate the remaining rhenium with a practical process for its recovery and subsequent refining. That is because most precious metals refiners recover rhenium 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 rhenium content in spent catalysts, an unknown portion of the desirable `pay metals’, sometimes as much as 20%, remain behind due to the insolubility of their substrates or carriers. That insolubility occurs because the substrates are hardened as a result of overheating during years of operation, preventing their dissolution, even with strong solvents.
Rhenium is a valuable, recoverable precious metal
The rising value of Re over the past few years has rendered this deficiency in precious metals recovery particularly significant. For example, catalyst grade ammonium perrhenate (typically the form in which Re is returned to catalyst manufacturers’ specifications) is currently valued at approximately US$ 3000/kg. With regard to this issue, precious metal refiners using hydrometallurgical techniques typically send these insolubles to third parties (smelters) to recover whatever precious metals remain. In this situation the catalyst owner is generally paid only for the acid soluble Re content and not necessarily the total Re content.
With pyrometallurgical recovery, however, the catalyst owner is paid based on the lot’s total Re content, since melting recovers the vast majority of the total Re content whereas digesting recovers only the acid soluble Re. As a catalyst owner, one should consider this a critical factor with regard to working with a precious metals refiner; settlement contracts and settlement documents will clearly state whether the precious metal values returned will be based on acid soluble Re content or total Re content.
A refiner that 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 offers significant advantages with regard to maximising return value that the catalyst owner is paid for total Re.
Understandably, it is hard to put a precise value on the Re in question when comparing these two technologies. Mainly this is due to the circumstances within the catalysts’ lifecycle: final coke and carbon content, the amount of heat they may have been exposed to during years of processing, and perhaps even additives that may have been used to extend their lifecycles. All of these factors have an affect with regard to final recovery at the precious metals refiner. However, when the spent catalyst lot is processed by hydrometallurgical techniques, these factors become more critical since they can interfere with the ‘digesting’ method, and its ability to capture the lot’s Re content. The trace elements that may or may not be present and the status of that catalyst substrate base can have a dramatic effect on how much precious metals will dissolve. Pyrometallurgical processing, however, eliminates these issues since the thermal reduction process has already removed the carbons to a certain level, and the pyrometallurgical process removes the rest. In other words, everything in an electric arc furnace melts. As a result, there is no need for processing of any insoluble or unmeltable material.
Adding a second electric arc furnace
To this end, Sabin Metal Corp. has recently added a second electric arc furnace (EAF) at its Williston, North Dakota, US refining facility. The new EAF allows the company to virtually double its pyrometallurgical processing throughput to help assure maximum recovery of remaining PGMs, and Re, in spent catalyst lots. The doubling of the smelting capacity by the addition of the new electric arc furnace, of course required doubling the capacity of the air cleaning system. Therefore a new multi million dollar, state of the art baghouse was also installed to supplement the company’s existing baghouse system. The new baghouse is designed for the special needs of the Pyro-Re® process while still being useful and fully compliant with environmental regulations as well as capture efficiency for the entire range of inorganic carrier catalyst including those that do not contain rhenium.
Responsibly recovering and refining precious metals requires a refiner to use well controlled processing methods that comply with all appropriate environmental regulatory agencies, throughout the world, with regard to reporting and managing of chemical and toxic emissions. In other words, a refiner must also adhere to all applicable environmental codes and standards covering effluent disposal and atmospheric emissions. Therefore, a properly equipped, and environmentally responsible, refiner will have the technology appropriate for pollution abatement, including afterburners, baghouses, web scrubbers, and liquid effluent neutralising equipment.
For example, any systems used for thermal oxidation must be able to combust organic contaminants completely. At Sabin, off gases resulting from thermal oxidation are channelled to its baghouse/scrubber system as previously discussed. Atmospheric discharges must be managed with pollution control systems that result in few or no pollutants being emitted before, during, and after the refining process. A refiner’s water treatment process should also minimise all causes of pollution.
A precious metal refiner should have approved status with all appropriate governing environmental agencies, and will generally provide copies of the required documentation stating as much. One thing to remember is that a refiner must be responsible for all its customers; the violation of a pollution control law while processing another customer’s materials can have legal implications for all of its customers.
As a catalyst owner, one should now fully understand the key advantage of pyrometallurgical technology with regard to recovering, and getting paid based on, total precious metals and rhenium content. Discussing these issues with your precious metals refiner will ensure that one achieves maximum returns on one’s investment: quickly, safely, and economically.
1. BEIRNE, K.M., ‘The science and art of sampling precious metal catalysts’, Hydrocarbon Engineering, December 2010. 2. JACOBSEN, R.T., ‘Rhenium: A hidden asset’, Hydrocarbon Engineering, June 2013.