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Kevin M. Beirne, SABIN METAL CORP.
300 Pantigo Place, Suite 102
East Hampton, NY 11937, USA
Many catalysts used in chemical, petrochemical, and pharmaceutical process reactions contain valuable precious metals. When these catalysts lose their activity, their precious metals content must be recovered for economy and reuse in new catalytic materials. Given the value of those precious metals, they must be recovered and refined with the highest yields possible and in a timely manner. Accomplishing these tasks calls for a precious metals refiner with the right blend of equipment, skills, and experience. Finding such a partner requires an understanding of the key elements of how the process works specifically sampling, assaying, recovery, refining, and compliance with complex environmental laws.
Platinum Group Metals (PGMs) including platinum, palladium, rhodium and rhenium as well as gold and silver are often used with other materials as catalysts in chemical, petrochemical, and pharmaceutical production processes.
Over time, the activity of a PGM-based catalyst is reduced due to the effects of reactions and harsh environments and the catalyst must be replaced. A precious metals refiner processing spent chemical, petrochemical, and pharmaceutical catalysts (Figure 1) relies on a proven set of procedures to achieve high yields with fast turnaround times. Two of the most critical steps in the process include sampling and assaying lots of spent catalyst materials. Over the years precious metals refiners have generally optimized sampling and assaying to achieve the highest accuracy and precision possible.
The following text refers to typical sampling and assaying procedures for catalysts employed in chemical and petrochemical processes. These procedures differ somewhat for pharmaceutical processing catalysts, mainly because of the chemical composition of the substrates, or carriers as they are known. Please see the following inset for a full explanation.
Pharmaceutical manufacturers use a variety of precious-metal bearing catalysts to produce drugs and other products. Typical precious metal-bearing catalysts employed in pharmaceutical processes (Figure I) include heterogeneous palladium on carbon, platinum on carbon, and palladium on calcium carbonate, all of which are used to facilitate the hydrogenation of various intermediates. Because these catalysts are carbon-based they are sampled differently than ceramic-based catalysts typically used for chemical processing.
Spent PTA-based catalysts are contaminated with organic materials such as sulfur, carbon, moisture, and other unwanted elements. To assure accurate evaluation of their remaining precious metals, the spent catalysts are “pre-burned” to remove these and other contaminants and help provide free flowing properties. This process is critical to assure highest possible sampling accuracy which ultimately means highest possible return values for the PGMs in the catalyst.
Pre-burning to remove organic contaminants is typically done in a box furnace (Figure II) where the carbon-based catalyst is heated until its initial burn off is complete. From there, the catalysts is transferred to a cooling area where the “roasting” is completed and the remaining carbon can be reduced to as low as 1-2%. Burning virtually all the carbon and liquid from the entire catalyst lot is a key factor towards achieving highest possible sampling accuracy. Because of the importance of this process, the precious metals refiner should provide catalyst users with complete in-house “pre-burning” capabilities and services.
Once the contaminants have been removed from the entire lot, the spent catalysts are reduced in size, first by grinding into smaller and ever-finer particles, and then by passing the material through screens of successively finer mesh (Figure III). These procedures reduce successive sample lots of catalyst into multiple analytical samples, with each sample size getting smaller and smaller. Representative samples are packaged and sealed for the catalyst owner, the refiner, an umpire, and for reserves. This process is similar to that used for chemical/petrochemical catalysts described in the text.
SAMPLING CHEMICAL CATALYSTS
Typically, a precious metals refiner will assign a tracking number to incoming spent catalyst materials from a customer. Incoming chemical process catalysts are tested for carbon, hydrocarbon, and moisture content to verify that the catalyst materials pose no workplace hazard to the refiner and are free flowing, in order to determine the most appropriate sampling approach.
Sampling, as the name suggests, reduces a large batch of spent catalyst into smaller amounts suitable for accurate analysis. The goal of any materials sampling method is to maintain the relative amounts of component materials in the mix while reducing the amount of the material to a practical level which permits accurate determination of the remaining precious metals content in the catalyst. Catalysts used for chemical and petrochemical reactions are typically based on ceramic substrates such as alumina, silica or alumina silicate.
To provide an accurate determination of remaining precious metals in spent catalyst lots, representative samples of these catalysts must be obtained under accurate and repeatable conditions. Over time, process catalysts become contaminated by sulfur, carbon, volatile organics, moisture, and other unwanted elements. As a result, when the catalyst is removed from the process, it is usually moist and sticky, and it will not flow freely through automatic sampling equipment.
These contaminants must first be removed to assure the accurate sampling and analysis of the remaining precious metals. This is often accomplished by pre-burning the catalyst in an indirectly fired rotary kiln (Figure 2), a multiple- hearth furnace or a fluidized-bed furnace. Precious metal-bearing catalysts may exhibit high loss on ignition (LOI) as the contaminants are burned off. Thus, accurate LOI data are necessary to account for any weight changes from the time the material is received, during the pre-burning process, and while the sample is in transit to the laboratory.
Pre-burning may be performed at the refiner’s site or elsewhere. If it is performed by a third party, the spent catalyst – perhaps as much as 200,000 lb (ca. 90 t) – must be shipped to that facility, which may use strip burning to remove hydrocarbons, coke burning to remove carbon, and another furnace to dry fine particulates and remove moisture.
The reduced lot is then shipped to the refinery for recovery of the precious metals. Offsite pre-burning involves additional turnaround time of up to several weeks, as well as additional costs for transportation and for leasing replacement metal during the time the PGMs are unavailable to the catalyst user. Offsite pre-burning also adds additional costs for catalyst owners’ representatives who must account for their client’s materials. Therefore, when on-site pre-burning is conducted overall reclamation costs are significantly reduced.
Another important benefit of in-house pre-burning is the control the refiner has over the catalysts it processes. This eliminates the possibility that one catalyst user’s material will be mixed in with unrelated materials from another organization. When that happens, there is no way to accurately determine the actual value of either company’s materials.
A refiner should be equipped with high volume pre-burning capabilities for the material to be sampled, since highest possible sampling accuracy requires that contaminants be removed from the mix first. The effects of high LOI after pre-burning in an oxygen environment can account for significant weight reduction in the processed spent catalyst materials, so accurate measurements are important before and after any pre-burning steps, and samples must be hermetically sealed following pre-burning to mitigate weight gain from moisture absorption. Accurate LOI data is vital for minimizing measurement errors due to weight changes while a sample is in transit. In general, the highest accuracy results for LOI are determination when analysis is conducted as closely as possible to the sampling procedure. Consequently, it would be a prudent for the catalyst owner to select a refiner that handles in-house LOI determinations under the supervision of independent inspectors.
Dry sampling involves the use of mesh screens, vibratory feeders, and rotary samplers (Figure 3). A typical process begins by extracting two portions equal to 10% of an initial materials lot. Then, 10% samples of these portions are taken, resulting in two 1% samples of the initial lot. One of these 1% samples will be used for a loss-on-ignition (LOI) test that burns contaminants and further reduces the volume of the sample, while the other 1% sample is further subdivided to create smaller, laboratory-sized samples to be used for the determination of the precious metal content.
Accurate sampling of chemical/petrochemical catalysts involves tight process control, to ensure that each sample has the representative composition of the initial material lot. Materials should be weighed at every step of the sampling process to minimize the effects of atmospheric conditions, such as absorption of moisture, on any measurements performed on the samples. Because samples can gain or lose moisture with handling, laboratories handling samples should reheat samples for an assay at high temperatures (typically +900°C) to remove any absorbed moisture from the materials and then hermetically seal the sampled materials.
Once accurate samples are available, the precious metals refiner and the catalyst owner may assay the samples for their precious metals content independently. Ideally, their independent assays provide values in close agreement. If both assays are within prescribed tolerances their values can be averaged to arrive at an agreed-upon figure for valuation of the PGMs in the spent catalyst. In cases where the values of the two independent assays are far apart, a third sample may be sent to an independent “umpire” laboratory to determine a settlement amount. Standard industry practice requires that both parties – the catalyst owner and the refiner agree on how an independent umpire’s assay is used to arrive at a final settlement.
ASSAYING CHEMICAL PROCESS CATALYSTS
Repeatable, accurate assaying procedures are needed to determine the amount of precious metals contained in those samples of chemical catalyst materials. Most precious metal refiners’ laboratories perform assays in triplicate by more than one method to ensure the accuracy of PGM measurements. Assaying methods require specialized instruments, including x-ray fluorescence systems, which can identify the desired precious metals as well as any remaining contaminates which might degrade the accuracy of a precious- metals determination. X-ray fluorescence spectroscopy can semi-quantitatively analyze over 80 elements within a few minutes per sample. In addition, such equipment as atomic absorption (AA) and inductively coupled plasma (ICP) emission spectroscopy systems (Figure 4) can.
support accurate assaying methods along with classical volumetric and gravimetric assaying techniques. As with sampling, the type of spent catalyst materials being analyzed will dictate the type of assay procedure to be used.
After completion of sampling the precious metals can be recovered from the spent catalyst by mixing the material with fluxes and smelting in an electric arc furnace to separate the precious metals from its substrates. The precious metals are recovered in the form of a metal- bearing bullion that is further processed for final extraction and purification of the precious metals. The high grade precious metal minimum 99.95% pure can be sent to a catalyst manufacturer to fabricate new catalysts for chemical processing or the catalyst owners may request payment for the value of the precious metals, if desired.
Responsibly recovering and refining precious metals requires that a refiner uses well-controlled processes complying with applicable environmental agencies, such as the Environmental Protection Agency (EPA). In the U.S., such legislation as the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA), or “Superfund” and the Superfund Amendments and Re-Authorization Act (SARA) establish guidelines for reporting and managing chemical and toxic emissions. The Superfund Act makes it clear that a user of precious metal-bearing catalyst materials and its precious metals refiner are responsible for the materials and the processing of those materials in recovering PGMs. European countries often have even more stringent environmental regulations.
Similarly, the Resource Conservation and Recovery Act (RCRA) concerns the generation, storage, transportation, treatment, and disposal of solid and hazardous wastes. The Clean Air Act (CAA) and the Clean Water Act (CWA) are mandated by the EPA to set environmental standards for air and water, respectively.
Throughout the entire refining and recovery process, a refiner must adhere to all applicable environmental codes and standards with regard to effluent disposal and atmospheric emissions. A properly equipped refiner will feature the technology appropriate for pollution abatement, including afterburners, bag houses, wet scrubbers, and liquid effluent neutralizing equipment. As an example, any systems used for thermal oxidation should have the capability for complete combustion of organic contaminants. Any off-gases resulting from thermal oxidation should be channeled to a baghouse or scrubber system (Figure 5). A refiner’s water treatment process should minimize all causes of pollution. Any atmospheric discharge must be managed with pollution control systems that result in little or no pollutants being emitted before, during, and after the precious metals refining process. Any gases generated by the process should be passed through a scrubber system for environmental control.
A precious metals refiner should have approved status with all appropriate governing environmental agencies. A refiner in good standing will generally be more than happy to provide copies of the required documentation stating as much. One thing to remember is that a refiner must be responsible for all customers: even violation of a pollution control law while processing another customer’s materials can have legal implications for all of the refiner’s customers.
Spent chemical and petrochemical catalysts contain PGMs which must be recovered as efficiently as possible to return value to their owners. By partnering with a responsible precious metals refiner, your organization can not only obtain real value for the company in the form of recovered PGMs, it may also achieve these results in a manner that is legally and environmentally sound.