Precious Metal Matters
Kevin M. Beirne Sabin Metal Corp., USA,
Examines how metal scavenging techniques help deliver added value when recovering PGMs from spent hydrocarbon processing catalysts.
The high value associated with hydrocarbon processing catalysts requires maximum recovery of their precious metal content when those catalysts have reached end of life. Catalysts play a key role in hydrocarbon processing. They help speed production processes and perform end-of-pipe reduction and, or elimination of atmospheric emissions of volatile organic compounds (VOCs) and other pollutants generated by hydrocarbon processes. Unfortunately catalysts do not last forever; they deactivate over time due to their harsh processing environment which contaminates them with carbon, sulfur, water, or other substances that reduce their efficacy to the point where they must be replaced. However, almost all of their precious metals content still remains and thus must be recovered and subsequently refined. Consequently, because of their high values, as well as constant price fluctuations year-to-year or even month-to-month, their users wisely view precious metals leasing, purchasing, and recovery management programmes with key emphasis on costs and returns.
The precious metals involved in hydrocarbon processing catalysts include platinum, palladium, ruthenium and rhodium (commonly referred to as PGMs) and are each valued in terms of high dollar amounts per tr. oz. (for example, current platinum value is US$1284 tr. Oz.; and palladium is now selling at approximately US$ 364 tr. oz.). Rhenium (not considered a PGM but nevertheless present in most spent catalysts) is valued at approximately US$3500/lb; many precious metals refiners are now able to recover a significantly high percentage of this metal. Obviously, the most efficient and effective spent catalyst PGM recovery process is the one that recovers the greatest amount of these PGMs in the shortest time possible. In selecting an industrial partner for reclaiming PGMs from spent catalytic materials, the refiner should meet a set of standards that result in high performance levels and environmentally responsible results. In order to accurately complete assays on incoming materials, for example, the refiner must maintain a well equipped analytical laboratory, employing advanced x-ray fluorescence equipment, atomic absorption (AA) and inductively coupled plasma (ICP) emission spectroscopy, and a variety of volumetric, gravimetric, and fire assay methods. These techniques have been approved by the New York Mercantile Exchange/ Commodities Exchange (NYMEX/COMEX).
Check for full capabilities
Figure 2. Rate of removal of palladium chloride bis triphenyl phosphine from solution using a silica based metal scavenger.
Maintaining these capabilities in-house rather than relying on outsourcing is strong evidence of a firm’s competence and responsibility in processing spent catalytic materials with a high return on PGMs. All reputable refiners will set goals for their customers to extract and return the highest possible value from precious metal bearing materials in the most cost effective manner, with many refiners allowing the PGM owners to be present when their spent catalyst materials are being evaluated. Another key consideration is the issue of transportation of spent catalysts, sometimes in lots as high as 200 000+ lbs. A ‘full service’ refiner would typically provide point-to-point transportation, including hazardous materials, virtually anywhere in the world, generally with no limit on the amount.
Most refiners use a wide variety of equipment and procedures to sample and process spent catalysts. These include rotary and crucible furnaces, kilns, roasters, thermal processors, pulverisers, granulators, screens, blenders, autosamplers, reactors, dissolvers, precipitators, electrolytic cells and filter presses. In addition to the equipment, skill and experience are required to achieve the highest percentage of precious metals recovered from spent hydrocarbon processing catalysts.
Speed saves money
Finding a precious metals refiner capable of processing spent hydrocarbon processing catalysts and recovering precious metals quickly can also be critical to the economic health of a hydrocarbon processing facility. For example, it may require three months to fabricate a new catalyst and three months to reclaim the precious metals from the old catalyst. A hydrocarbon processing plant may be forced to finance the precious metals needed for the interim catalyst during this recovery time. Since metals are often financed during the entire catalyst fabrication and reclamation period, PGM lease charges may represent a significant cost to a processing operation. Any amount of time that can be shaved off the PGM recovery period translates into controlled costs.
The sampling process
Figure 3. To produce statistically valid samples, a continuous catalyst sampling system generates homogeneous, consistent, and reproducible intermediate samples that represent, as accurately as possible, the entire lot of spent catalysts.
In order to recover precious metals from a hydrocarbon processing catalyst, a refiner must accurately determine how much of those metals are present in the materials at hand. Three different sampling techniques are employed for this purpose: dry, melt, and solution sampling. The idea in each case is to convert the scrap materials containing the precious metals into a homogenous mass in which all materials are evenly distributed so that measurements of small amounts yield accurate ratios of the precious metals to other material constituents.
Dry sampling, although the least accurate of the three methods, is used when it is impractical or too expensive to melt or place materials in solution. In order to achieve some level of homogeneity, materials for dry sampling are often ground into finer particles for analysis, with representative samples taken periodically. In melt sampling, a carrier metal such as copper is melted along with the precious metal bearing material. The molten metal that results is poured into ingots that are sampled at the beginning, middle, and end of the pour. Solution sampling is cost-effective and accurate, involving the formation of a homogenous dispersion of precious metals and other constituents from the scrap material. Multiple samples are taken from different parts of the solution to ensure accuracy in the measurements. In recovering PGMs from hydrocarbon processing catalysts, selecting the proper sampling approach is critical for achieving the highest yield.
Scavenging precious metals for added value
Figure 4. Pd(O) and Pd(II) solutions before and after metal scavenging.
Figure 5. To assure accurate assaying for remaining PGMs, an analytical laboratory uses advanced x-ray fluorescence equipment, atomic absorption (AA) and inductively coupled plasma (ICP) emission spectroscopy, as well as volumetric, gravimetric, and fire assay techniques.
Some PGMs may be lost during processing, well before their catalyst carriers lose their efficacy and are ready for the refining and recovery of their metals. In these instances, a process known as metal scavenging may be used to recover a significant amount of these precious metals. Scavenging occurs downstream of the actual catalytic reaction process, typically in solution phase, prior to the refining/recovery process. This metal scavenging process can be particularly valuable to recover and concentrate PGMs from dilute solutions which will be difficult to capture economically via other methods.
Metal scavengers are essentially materials that can adsorb metals from spent catalysts, in some form of solvent mixture, to speed and simplify the separation of PGMs from catalytic substrates. Scavengers should be selective, operating only on specific target metals. They should also exhibit fast kinetic reactions and be compatible with a broad range of solvents in order to work on a wide range of spent materials. They should provide high levels of activity on a metal or metals in a variety of oxidation states, and perform well over a wide range of temperatures and operating conditions. Finally, they should be mechanically and chemically stable to avoid unwanted reactions with trace elements or products that may remain in the scrap materials.
Studies have proved the effectiveness of metal scavengers on reclaiming certain PGMs. For example, in tests performed on metal complexes with as much as 1000 ppm palladium content, the optimum application of metal scavengers can reduce the post-treatment level to as low as 1 ppm in a relatively short period of time. (Figure 2) shows the rapid removal of palladium from solution using a silica based metal scavenger.
Much like the catalytic materials with which they interact, metal scavengers are generally attached to a framework or carrier material, such as carbon, silica, or alumina. A PGM catalyst user should consult with its precious metals refiner to choose a metal scavenger based on the precious metals to be reclaimed, the oxidation state of the metal, and other conditions, such as the type of solvent and operating temperature. For example, the choice of metal scavenger is often influenced by the structural nature of a catalytic material. If it has an acidic nature, metal scavengers with a basic nature should not be used. When the metal scavenger is in a solid form, it is usually applied in one of two ways. It can be added in powder form to the spent catalytic material which itself is dissolved in a solvent. The solid scavenger adsorbs the metal and is then isolated via filtration. Alternatively the metal catalyst solution is passed through a fixed bed of the solid scavenger, the metal being adsorbed onto the solid matrix.
The selection process for a precious metal scavenger is usually based on not only the particular metal to be scavenged, but what is known of its oxidation state or states. The metal in a spent catalyst can often be present in different oxidation states, meaning a broad spectrum scavenger (such as a silica based material) or sometimes, a mixture of scavengers will be required to satisfy a set of requirements. The target reclamation process temperature is also a consideration in selecting a scavenging process. Many scavenging processing steps can be operated within a reasonable time at room temperature. An increase in the solution temperature usually results in an increase in the rate of the process. Effective metal scavengers should withstand high temperatures (+150 to +200 °C) without degradation when it is necessary to accelerate the speed of a metal scavenging process.
Figure 6. Precious metals refiners must abide by scores of environmental laws to protect themselves and their customers. Exhaust air should be managed through pollution control systems (such as this baghouse/scrubber).
If added to a metal catalyst in a powder format, higher agitation rates enhance scavenging performance, although gentle to moderate rates are usually sufficient. In addition, the pH of the reaction mixture needs to be considered. At high pH, for example, the framework or carrier material of some metal scavengers can be degraded, while very low pH may degrade the performance of the metal scavenger itself. Other reactants present in the reaction when the metal scavenger is added should also be considered as part of the process. Needless to say, there should be no reaction between any remaining reactants and the metal scavenger.
Effective metal scavengers should be usable over a wide range of aqueous and organic solvents. Some scavengers have limitations. With good performing scavengers, the kinetics of removal should be unaffected by any change in solvent type and polarity.
When an optimum metal scavenger has been selected for a particular PGM, kinetics should be fast, with a reaction resulting in low levels of residual metal in mere seconds at room temperature. Usually, a strong indicator of the progress of a reaction is evidenced by the loss of colour from the solution (Figure 4). Often, an increase in temperature will bring an increase in activity for a metal scavenger, and this may be needed to maximize metal uptake to an economic level. Such a metal scavenger is ideal for use in short contact processes and leads to shorter batch process times with potentially significant cost and time savings.
A critical part of the selection process also involves determining the optimum amount of metal scavenger to be added to a PGM reclaiming reaction (in terms of the number of equivalents of the metal scavenging material). Together with the selection of operating temperature, scavenging time, and solvent used, this will determine the overall economics of the process.
A company specializing in the recovery of PGMs from spent hydrocarbon processing catalysts should also be environmentally aware and responsible. Precious metals refiners are governed by scores of environmental laws that, if violated, could result in substantial fines and legal fees not only to the refiner but to the originator of the material as well. Exhaust air should be managed through pollution control systems and any effluent from the reclaiming process should be contained within the facility, and/ or disposed of in an environmentally responsible manner.
In the US the environmental responsibilities of companies involved in hydrocarbon processing and the firms that process their catalysts are detailed in the Superfund Act, more formally known as the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA). The law, which was amended by the Superfund Amendments and Reauthorization Act (SARA) in 1986, created a tax on the chemical and petroleum industries and provided broad Federal authority to respond directly to releases or threatened releases of hazardous substances that may endanger public health or the environment. The Environmental Protection Agency (EPA) considers both the hydrocarbon processing company and the firm it selects for spent catalyst processing to be full partners in the eyes of the law. To be safe, a precious metals refiner should be ready and able to furnish copies of all detailed documentation relative to legal compliance.
The high costs of catalysts used in the hydrocarbon processing industry mandates efficient processing of spent materials and high yield recovery of the precious metals contained in those catalysts. It is a job best left to specialists with the capabilities, knowledge, and experience to perform all the process steps required to achieve maximum PGM yield in the shortest time possible. In recovering PGMs from spent hydrocarbon processing catalysts, time spent is money spent. Furthermore, ignorance of the environmental responsibilities on the part of either the metals refiner or their customer can be damaging. By using a reputable, reliable, and environmentally responsible ‘partner’ for the critical PGM recover process, numerous issues, including environmental impact, legal responsibilities, turnaround time for the return of precious metals, and even transport of spent and recovered materials, are effectively covered.