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Reclaiming Precious Metals from Spent Carbon Catalysts
Careful planning can help add profits and eliminate possible legal complications
By James Lewis, Technical Director, Sabin Metal Corp.
Many pharmaceutical, nutraceutical, and cosmetic production processes use precious metal-bearing catalysts to facilitate reactions. As an example, heterogeneous palladium on carbon, platinum on carbon, palladium on alumina, and palladium on calcium carbonate catalysts facilitate hydrogenation and other reactions of intermediates. Although these catalysts may represent a small portion of overall pharmaceutical manufacturing costs, they are by no means insignificant nor should they be perceived that way considering the value of the precious metals incorporated in the catalyst. (The precious metals used in pharmaceutical catalysts – typically platinum, palladium, and rhodium – are commonly referred to as “PGMs,” or Platinum Group Metals.)
At the end of a catalyst lot’s useful life, the value of its remaining precious metals could easily be worth in excess of many hundreds of thousands of dollars. With the astronomically high costs associated with developing, producing, and successfully marketing new pharmaceutical-based products, it is simply prudent management to look at ways to lower costs – and add profits – wherever possible. One often neglected approach concerns methods to maximize recovered value of the remaining precious metals in spent process catalysts.
All precious metals refiners are not the same
There are many organizations that recover and refine PGMs from spent catalysts. While they all essentially perform the same tasks, there are also considerable differences among them. Some of these differences can have a significant impact on your returns, your relationship with the refiner, and your risk of litigation through exposure to the EPA and/or other pollution control authorities. To that end it is in your best interest to learn as much as possible about any precious metals refiner you are considering. For instance, you should know how the organization processes spent catalysts and what equipment and procedures they use. Critical areas of interest include the refiners’ contamination removal processes, material sampling and assaying methods, environmental protection policies and systems; and – perhaps most important – the refiner’s reputation with its long term customers. All of these issues (and there are others as well which will be discussed later) will have an impact on the relationship you establish with your precious metal refiner. This paper will discuss those subjects – and others – to provide some basic guidelines for determining the right refiner for your application.
Risk of litigation
There are legal implications that you must be aware of when working with a precious metals refiner. They concern possible effluent or atmospheric discharges at the refiner’s facility, since violations are taken quite seriously by controlling organizations. Based on these concerns, choosing the wrong refiner could result in a costly mistake. The question is, how do you go about choosing the right refiner?
There are many criteria to consider when selecting a precious metals refiner. Before that discussion, however, consider the various methods used to help maximize returns for your spent precious metals catalysts. Essentially there are three critical factors under the refiner’s control which apply to virtually all precious metal bearing materials. These are sampling, assaying, and processing turnaround time.
Recovery and refining
The process of reclaiming remaining precious metals in spent catalysts is referred to as “recovery and refining.” Included in the process are many specialized procedures such as materials documentation, contamination removal, sampling, assaying, environmental considerations, and turnaround time. Each of these is independent; however, together they will have a dramatic affect on how much of the remaining PGMs are recovered, the speed at which they are recovered, and the value returned. Consequently, these procedures impact the overall costs associated with an organization’s precious metals management program. While each of these functions is important, sampling procedures for spent catalyst materials are perhaps the most critical elements with regard to recovering maximum remaining precious metals. As a user of these materials, it is in your best interest to understand how sampling is used to determine the precious metals content of spent catalysts, and, ultimately, the value of these metals which is returned to their owners.
Precious metals sampling
To accurately determine the amount of precious metals present in materials for recovery the spent catalysts must first be sampled. Three common sampling techniques used in precious metals sampling are dry sampling, melt sampling, and solution sampling. As each of these techniques offers specific advantages, determining the most appropriate sampling method depends upon the type of material being processed as well as its estimated precious metals content.
Some precious metal bearing materials can be sampled only by one of the three methods described; however, others may be processed by more than one method. In this case the method chosen will depend on variables such as: the estimated value of the precious metals content, the cost-effectiveness of using one method over another for highest possible returns, and practicality (a function of refining costs, materials value, and other factors). Because precious metal bearing catalysts are made with various substrates and in many sizes and configurations (powder, granulated, and extrudates, for example), determining the best sampling technique is crucial to recovering the most value from your spent catalyst.
Most catalysts used for pharmaceutical processing are carbon based (90% or greater carbon content), and therefore are best suited for dry sampling. Dry sampling is commonly used whenever materials cannot be dissolved in solution or are inappropriate to melt either because of their structure, or because of the cost associated with melting vs. the possible return.
Dry sampling
Because it is difficult to achieve homogeneity, dry sampling is more complex and potentially less precise than melt or solution sampling; in fact, this method requires more judgmental skills than the others. Because of this, it is essential that the material is properly prepared prior to dry sampling in order to reduce sampling error. This process typically begins with a thermal oxidation step used to eliminate contaminants and results in large quantities of spent catalysts (as much as many tons) being “reduced” into smaller quantities (as little as a few pounds). Once the contaminants are eliminated as completely as possible, the final step of proper preparation generally includes grinding large pieces into smaller and ever finer particles and passing the material through a predetermined sized screen. The goal is to reduce sampling error by eliminating contaminants and produce a material that is free flowing. This will help generate as precise a representative sample of the overall material lot as possible, and thus permit as accurate a determination as possible of the actual value of recoverable precious metals within the lot.
Following the preparation steps a proper sampling procedure would then allow the fine material from the screening step (oversize material is handled separately) to free-fall in a stream into a crosscut, timed automatic sampler. An ideal dry sampling system would be capable of drawing representative samples from free flowing catalyst according to the principles of Pitard[i] and Gy[ii] and the practices of Merks[iii]. Proper preparation (which will be expanded on later) and sampling will result in representative samples where sampling accuracy is typically ± 2%.
(Figure 0 – Typical dry sampling process)
Like most concise descriptions of complex procedures, this is easier said than done, since there are many processes, evaluations, equipment, and systems involved in the preparation and sampling process, and there are also vastly different sampling methods used depending upon circumstances. The refiner’s experience and expertise play a major role here, since some sampling procedures and their ultimate outcome can be affected by judgment. Therefore, it is critical that the catalyst user understands the preparation and sampling methods of the refiner to ensure that they properly minimize sampling error and variability. Because of this, a refiner should have detailed documented procedures describing their preparation and sampling protocols. An example of proper preparation and sampling procedures and what to look for follows.
Identification and tracking
A proper dry sampling procedure for spent pharmaceutical catalyst should include an organized identification and tracking system as well as approved preparation and sampling procedures based on sound sampling principles. In a typical precious metal refinery, incoming catalyst materials are inspected, weighed, assigned tracking numbers, and stored prior to sampling. The assignment of tracking numbers is critical; a specific lot – from its arrival at the loading dock – is segregated from all other materials at the refiner’s facility to eliminate the possibility of mixing with other lots.
Removing contaminants
Spent catalysts are contaminated with organic materials that must be removed to assure accurate evaluation of their remaining precious metals. Removal of those contaminants helps provide free flowing properties to help assure accurate sampling. The removal of organic contaminates is typically done in a quiescent box furnace where the carbon catalyst is placed in a tray which is subsequently placed in the furnace and heated until its initial burn off is complete. From there, the catalyst tray is transferred to a cooling area where the “roasting” is completed and the remaining carbon is 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 sample accuracy.
To illustrate the importance of removing contaminants before sampling, consider a 10,000 lb. spent catalyst lot that is 50% by weight liquid. Now assume that sampling the spent catalyst for contained liquid results in ± 2% relative accuracy and that during contaminant removal 95% off the weight of the spent catalyst (virtually all of the liquid and most of the carbon) is burned off or evaporated in the thermal oxidation step. A comparison of sampling error due to contained liquid using a contaminant removal procedure versus a sampling as received procedure follows.
1) Sampling with Pre-burning
Net weight received: 10,000 Lbs.
Afterburn weight: 500 Lbs.
Moisture content after pre-burn: 0.1% ± 2% relative
Settlement weight: 499.51 --- 499.49 Lbs.
Settlement Pd assay: (assuming 1% Pd as received) 20.02 %
Pd content 1458.27 --- 1458.33 t.o.
Pd accuracy: ± .002% ± 0.03 t.o.)
2) Sampling without Pre-burning
Net weight received: 10,000 Lbs.
Afterburn weight: 10,000 Lbs.
Liquid content as received: 50% ± 2% relative
Settlement weight: 4900 --- 5100 Lbs.
Settlement Pd assay: (assuming 1% Pd as received) 2 %
Pd content 1429.13 --- 1487.47 t.o.
Pd accuracy: ± 2.0% ± 29.17 t.o.)
It should be mentioned that variances are additive and that the above example only considers one source of variance, so total error would be greater for both procedures. While somewhat simplistic, this example shows the significance of limiting sampling error and demonstrates how contaminant removal assists in this. This, of course, translates into maximum possible returned value and reduced variability for the remaining precious metals in the lot.
Preparation and sampling
As previously stated, the goal with all sampling procedures is to obtain material samples that accurately represent entire lots of spent catalysts. Once the organic contaminates have been removed, it is necessary to further prepare the spent catalyst for dry sampling by creating a free-flowing powder where ample particles will be contained in a given sample (there is a direct relationship between particle size and sample size). To illustrate, after the burning and roasting process, the material is milled (if necessary) and screened through a 40 mesh (40M) screen. Any +40M material (oversize) that cannot be reduced to –40M should be inspected. Many times this material will include non-value material (referred to as “tramp”) such as filter cores – even nuts and bolts may be included. Tramp material should be cleaned of catalyst particulate, weighed, documented, and set aside. If there are any questions about value, the oversize material will have to be sampled separately (typically melt sampled) for value determination. After the milling and screening step, the -40M materials (fines) are subsequently reduced in a timed crosscut automatic sampler(s) to 1-lb. portions.
During this step it is important that the refiner have procedures or guidelines in place ensuring that during every pass through an automatic sampler, an adequate number of cross cuts are taken. These cross cuts should be taken at regular intervals during the passage of the entire lot through the sampler; they should also be well into the hundreds at a minimum to avoid the vagaries that the statistics of small numbers might introduce to the sample.
Depending on the specific automatic sampler used, multiple 1 lb. samples should be available at this step. One of these samples should be used for a moisture determination while another should be used for the analytical sample. Moisture determination involves heating the sample to slightly over the boiling point of water to drive off any contained moisture. This allows for the precise determination of a “dry settlement weight” to which the precious metal analysis (determined in the laboratory after the same evaporation procedure) is applied.
The next step is reducing the 1 lb. analytical sample into multiple samples. As the sample size is getting smaller, it is important to understand that the relationship of particle size to sample size affects the precision of the entire sample. Therefore, it is important at this stage to ensure that 100% of the 1 lb. analytical sample passes through a 100M screen. This is usually accomplished with a ring and puck mill. After 100% of the 1 lb. portion is ground to -100M it is able to be split into smaller samples.
The -100 mesh sample is subsequently split on a small rotary sampler into eight 60-gm. (approximately) laboratory sized samples. These are then packaged and sealed for the customer, the refiner, an umpire, and for reserves. The materials’ owner and the refiner usually assay the quality samples (on a dry basis) independently. If these assays are within predetermined limits they are simply averaged to arrive at a final settlement. If they do not agree, 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 case any samples are compromised (lost in the mail, spilled in the lab, etc.) or consumed before any value determination is agreed upon. When these sampling procedures are completed, the spent catalyst lot is ready for standard refining techniques.