The Science And Art Of Sampling Precious Metals Catalysts.
December 1, 2012
Kevin M. Beirne, Sabin Metal Corp., USA, looks at the art and science of recovering precious metals from spent hydrocarbon processing 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. 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 organization 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.
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 sulfur, carbon, solvents, and water.
In order to determine their precious metals content (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.
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 refiners customer. Clearly, the customers goal is to receive the highest possible value for the precious metals in its 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 its 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.
First, incoming catalyst materials are inspected, weighed, assigned tracking numbers and stored. The assignment of tracking numbers is critical; a specific lot, from its time of arrival, is segregated from all other materials to eliminate the possibility of mixing with other lots during the sampling process.
Spent catalysts, when removed from a reactor, may contain excess moisture and require elimination of non-essential contaminants. These must be removed to assure the free flowing properties necessary for accurate sampling, while also permitting safe and effective operation of an electric arc furnace (EAF), which is used to ultimately recover the precious metals from the bulk catalyst lot (Figure 1). Eliminating contaminants from the lot is accomplished in a rotary kiln, which removes up to 25% sulfur content and up to 40% carbon (at a rate of 300 -1000lbs/hr). Contaminates may also be removed by a multiple hearth furnace or fluidized bed furnace. This first step, or pre-burning, is critical to the sampling process, and is best handled in house, at the refiners facility. There are two advantages here: first, it eliminates the possibility that the customers materials might be mixed in with unrelated materials; second, catalyst users can achieve substantial cost savings by eliminating trans shipment charges to independent, offsite regenerators of what are typically many tons of spent catalyst materials.
After processing in a rotary kiln, any materials containing large agglomerates (chunks) in the lot are crushed in a rod mill or hammer mill, and later subsequently blended in with the lot for further reduction. (Ultimately, the goal with all sampling procedures is to obtain material samples that accurately represent entire lots of spent catalysts.) The oversize fraction is weighed and the weights recorded.
After weighing/recording, spent catalysts are placed in a drum/bag dumper or flow bin receiver. From there the catalyst is fed into a bucket elevator via a vibratory feeder to a screening station on Sabins continuous catalyst sampling system (Figure 2). Each step in the sampling process is carefully supervised to help assure maximum returns of precious metals in the lot. Empty containers are inspected tor any remaining materials, and tare weights taken to determine net weights.
At this point, each catalyst lot is screened to achieve the following objectives:
- An oversize fraction representing tramp material and non-precious metal bearing support media to be excluded from lot sampling.
- An intermediate catalyst fraction, which will be separately sampled and weighed.
- A catalyst fines fraction (for most materials a screen size between 6 mesh and 40 mesh is appropriate), to be separately sampled and weighed.
The intermediate catalyst fraction is passed through a double cutting Knight rotary sampler (Figure 3), which simultaneously extracts two samples of approximately 10% of the spent catalyst lot. One of these samples is fed into a single cutting Knight rotary sampler, which extracts another 10% (approximately), which results in an approximately 1% (by weight) sample for further processing later as an assay sample. The second of the initial samples is fed into another single cutting Knight rotary sampler, which performs the same function as the first one; that sample, however, is used for moisture and loss on ignition (LOI) determination.
Both 1% intermediate samples are considered identical, and either one may be selected by the catalyst owner or its representative as the assay sample, with the remaining 1% sample considered the LOI sample. The bulk discharge from the samplers is loaded into a previously weighed (tare weight) container, which is weighed again; this procedure is carefully documented. Next, the intermediate fraction assay and LOI samples are weighed.
The 1% LOI fraction is then spear sampled with a dryer tube to obtain a 1 kg sample that is packaged in a sealed container and sent to the Sabin Metal West laboratory for LOI determination. The remaining 1% sample of the bulk catalyst is further size reduced with a ball mill to 100% passing 40 mesh and subsampled. This 40 mesh sample is again subsampled using a 24 tray rotary splitter. Sabin combines the number of trays necessary to produce a weight of approximately 0.5 kg. At this point, the 0.5 kg subsample is pulverized to 100% passing 100 mesh. That sample is sent to the laboratory to be ashed by igniting it to the appropriate LOI temperature and split into final samples using a rotary micro sampler yielding a minimum of four replicate samples.
The fines fraction is sampled directly from the original screen using a fines dedicated Knight sampler. This technique produces a sample representing 10% of the fines fraction by weight. This sample is passed through a secondary sampler to obtain a 10% fraction of the original split, resulting in a 1% fraction of the original fines. Next, the 1% fraction is spear sampled with a dryer tube to obtain a sample for LOI determination. The resultant LOI sample is ignited to the appropriate LOI temperature at Sabin Metal Wests laboratory and the data recorded.
The remainder of the 1% fraction is ground until 100% passes through 40 mesh. This material is then introduced into a carousel splitter where a 0.5 kg sample is collected. The 0.5 kg sample is ground to 100 mesh and ignited to the appropriate LOI temperature. At this point the 0.5 kg sample is further split on a micro sampler to produce a minimum of four samples for metal analysis.
The process of LOI determination involves heating (igniting) the catalyst in the presence of air to burn off volatile components and oxidizable materials such as carbon and sulfur. This allows the precise determination of a settlement weight to which the precious metal analysis (determined in the laboratory after the same burn off procedure) is applied. Therefore, this sample is taken from the catalyst in the same form, and at the same time, as when it is weighed. The container for the LOI sample is also hermetically sealed to avoid weight changes by either evaporation or moisture absorption during transit through the laboratory. Of special note. it is always prudent that catalyst owners (or their representatives) observe LOI determination procedures at the refiners facility.
All of the samples generated by the above procedures are packaged and sealed for the customer, the refiner, the umpire, and for reserves. The materials owner and the refiner usually assay the quality samples (on an ignited basis) independently. If these assays agree to within predetermined limits they are simply averaged to arrive at the payable settlement. If they do not agree, then the sealed umpire sample is sent to an independent laboratory (the umpire). The three resulting assays are used (again by an agreed upon procedure) to determine the settlement. Many times this procedure involves averaging the two closest assays or using the middle assay to determine the final settlement. The reserve samples (usually sealed by both the materials owner and the refiner) are held in reserve to cover any possible irregularities during the assaying procedures. When sampling procedures are completed, the bulk spent catalyst lot is blended with a mix of flux and a carrier metal such as copper or iron. The proportions in this mix are determined by the calculated concentration of recoverable precious metals in the lot and the desired slag chemistry, which takes into account its electrical conductivity, corrosivity, morphology, melting temperature, and other parameters.
Now the spent catalyst lot is finally loaded into an EAF for refining. The EAF helps maximize precious metals recovery, essentially producing two end products: molten precious metals and slag (the slag also contains trace amounts of precious metals and is also subsequently refined).
Finally, the molten precious metals from the EAF are poured into preheated graphite molds where they eventually cool into ingots weighing approximately 500 lbs each, which are removed for vault storage. Throughout the sampling procedure the refiner must adhere to all applicable environmental codes and standards with regard to effluent disposal and atmospheric emissions.
Therefore, an ideal sampling system would typically be enclosed for dust control and evacuated under a low volume flow into a dedicated baghouse. In addition to the obvious reasons for preventing atmospheric discharge of toxic and/or noxious fumes, the dust collected during this sampling process is also recovered and sampled separately, with its value returned to the catalyst owners, since its value may be substantial.
The process of sampling tons of contaminated spent catalyst materials is not only based upon science and technology, but also founded on judgment from many years’ of successful experience. Virtually every step in the refining and recovery process involves details that can be critical to the outcome. Because of its complexity, it is prudent that the catalyst owner work closely with a precious metals refiner during the sampling process. The catalyst owner should also consider the refiner’s policies and procedures with regard to applicable pollution control codes and standards compliance, hazardous materials shipping, conformance to the Basel Convention, anti money laundering requirements, and all other relevant issues concerning precious metals in one form or another. This knowledge will enable the catalyst owner to ‘partner’ with a precious metals refiner based on a relationship of mutual trust and fair treatment.