Robert T. Jacobsen, Sabin Metal Corp., USA, argues that a thorough
A comprehensive article on recovering and refining precious metals from spent reaction catalysts appeared in the July 2003 issue of Hydrocarbon Engineering.1 This article discussed methods and equipment used by precious metals refiners for maximising returns of remaining precious metals in spent catalysts from hydrocarbon reaction processes. Most hydrocarbon/petrochemical processors employ fixed or moving bed catalytic reaction processes to facilitate hydrogenation of intermediates, and also to control and/or abate harmful or unlawful exhaust emissions for end of pipe applications.
Precious metals used in most catalytic reaction processes are referred to as platinum group metals (PGMs) and typically include platinum, palladium and rhodium. The previous article described in detail many of the key activities performed at a precious metals refining facility.
Specifically, issues such as sampling; assaying; processing turnaround time; documentation; and environmental concerns were covered. While each of these functions is important to the precious metals recovery and refining process, sampling procedures for spent catalyst materials are perhaps the most critical element with regard to maximising yields on remaining precious metals catalysts.
As a sequel to the July article, this article will provide more detail concerning specific sampling techniques and their influence on value received for spent precious metal catalysts. As a user of precious metal catalysts, it is in the reader’s best interest to have a clear understanding of how sampling processes help to determine precious metals content of spent catalyst materials, and ultimately the value that is returned to their owners.
Fundamentally, sampling spent catalysts containing precious metals can be compared to sampling any homogenous mass. The problem is that even new catalysts on substrates (carriers), such as soluble and insoluble alumina, silica alumina, zeolite, or carbon supports, are not homogenous masses. Indeed, after years of exposure to the extremely harsh catalytic reaction environment, spent catalysts are even further away from being considered homogenous, wherein ‘lies the rub’. If one imagines what occurs during the hydrogenation process, one can more easily understand that spent catalysts accumulate many different contaminants of various densities, among them sulfur, carbon, solvents and water.
In order to assure maximum (or as close as possible) accuracy in determining the quantity (and quality) of remaining PGMs in spent catalyst materials, the spent catalysts must first be ‘reduced’. This process basically involves working with large quantities of spent catalysts (many t) and ultimately ‘reducing’ them to smaller quantities (to as little as a few g), as well as eliminating all possible non essential contaminants. The goal is then to arrive at as precise a representative sample of the overall material lot as possible, in a material lot that is as homogenous as possible, so as to make as accurate a determination as possible of the actual value of recoverable precious metals in the lot. Clearly, this is easier said than done, as there are many processes, evaluations, equipment and systems involved in the sampling process. There are also vastly different sampling methods depending upon the circumstances. Then there is the matter of experience and expertise, as some sampling procedures, and their ultimate outcome, are solely affected by judgment. In any case, all of these steps must be accomplished to arrive at an accurate, final determination of value.
Three sampling procedures
In the previous article, three sampling techniques were discussed: dry, melt and solution sampling. Each of these incorporates a number of specific methods and equipment, and also 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. Because of their composition and chemistry, precious metals bearing catalysts are usually sampled with dry sampling processes. 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 vs the possible return. As it is difficult to achieve homogeneity, dry sampling is more complex, and potentially less precise, than melt or solution sampling; in fact, this method generally requires better judgment than the others. An ideal dry sampling system is capable of drawing representative samples from free flowing catalyst according to the principles of Pitard2 and Gy3 and the practices of Merks4 at a rate of 2000 – 3000 lb/hr.
As sampling is considered the most important procedure in the precious metals recovery and refining process, it must be viewed from the perspective of the refiner as well as the refiner’s customer. Clearly, the customer’s goal is to receive the maximum possible value for the remaining precious metals in their spent catalyst materials. The refiner, on the other hand, must not only consistently meet that goal for its customer; it must also provide the customer with 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 rewarding 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.
As previously mentioned, dry sampling procedures begin by converting large lots of spent catalysts materials (as much as many t) to as little as a few g, in a homogenous mass, to distribute molecules of precious metals and other constituents evenly. In essence, when the material cannot be broken down any further, the results of sampling (or reducing) the homogenous mass represent an accurate ratio of the precious metal content in the overall matrix.
What follows is a discussion of a typical sampling procedure for a lot of spent catalyst materials sent to a precious metals refiner. The process begins when incoming catalyst materials are inspected; weighed; assigned tracking numbers; and stored, prior to sampling. The assignment of tracking numbers is critical. From the beginning, this specific lot is segregated from all other materials at the refiner’s facility to eliminate any possibility of mixing with other lots at the facility.
Dry sampling is basically used whenever materials cannot be dissolved in solution, or when they are inappropriate to melt either because of their structure, or because of the cost associated with melting vs the possible return. As it is difficult to achieve homogeneity, dry sampling is tedious and complex. Most spent hydrocarbon process catalysts are sampled with this method. As previously mentioned, spent catalysts, when removed from a reactor, are often contaminated with organic materials that must be eliminated to ensure accurate evaluation of their remaining precious metals. These catalysts must first be processed to remove those contaminants to provide free flowing properties, which ensures accurate sampling and permits safe and efficient operation of the electric arc furnace (EAF), ultimately recovering the remaining precious metals.
Elimination of contaminants from the lot is accomplished in a rotary kiln, removing up to 25% of the sulfur content and up to 40% carbon (at a rate of 300 – 1000 lb/hr). Contaminants can also be removed by a multiple hearth furnace or fluidised bed furnace. This first step, or ‘preburning’, is critical to the sampling process, and is best handled inhouse at the refiner’s facility. There are two advantages to this: first, it eliminates any possibility that the customer’s materials are mixed in with unrelated materials; second, customers can achieve substantial cost savings by eliminating trans shipment charges to independent, offsite ‘regenerators’ of what are typically many t of spent catalyst materials. After processing in the rotary kiln, the materials containing large agglomerates (chunks) in the lot are crushed in a rod or hammer mill, and later subsequently blended in with the lot for further reduction.
The goal with all sampling procedures is to obtain material samples that represent entire lots of spent catalysts accurately. To that end, a continuous catalyst sampling system generates homogenous, consistent and reproducible intermediate samples, using a variety of equipment and techniques. For example, after preburning in the rotary kiln, a two deck Sweco screen separates ‘tramp’ or oversized material and fines, which are subsequently reduced from catalyst pellets in a Knight automatic sampler, before progressing to the next process. This next step is also critical. Knight samplers generate dual bulk samples of 10% each, followed by another 10% of the remainder. This essentially creates two 1% samples of the total original lot. At this point, one of these samples is split into portions of approximately 1 lb and retained in hermetically sealed aluminium cans for loss on ignition (LOI) determination. This process involves heating (igniting) the catalyst, in the presence of air, to ‘burn off’ volatile components and oxidisable materials such as carbon or 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. This sample is therefore taken from the catalyst in the same form as when it is weighed, and as close in time as possible. The container for the LOI sample must be hermetically sealed to avoid weight changes (either due to evaporation or to moisture absorption) during transit through the laboratory.
The other 1% sample (perhaps as large as several hundred lb) is ground in a batch ball mill crusher to 100% -40 mesh. From the ball mill crusher, the sample is then placed onto another, finer Sweco screen, which removes materials larger than -40 mesh for regrinding. As a result, the entire bulk sample of 1% of the batch is ground to -40 mesh before further splitting in another Knight sampler. If the sample material is not yet homogenous enough for highest possible accuracy, this is a critical process (perhaps the most important of the entire sampling procedure), as the relationship of particle size to sample size affects the precision of the entire sample.
After processing in the Knight sampler, the material, or ‘sample’, has been made substantially finer, but still is not homogenous enough for an accurate and final determination of the entire lot’s recoverable precious metals content. From here, a rotary carousel screener separates the -40 mesh sample into 24 individual bins or pans.
At this point, the material is ready for further reduction in a ‘ring and puck’ mill, which grinds an approximate 2 lb portion of the sample to 100% -100 mesh, which is subsequently split on a smaller rotary sampler into eight laboratory sized samples of approximately 150 g. These are then packaged and sealed for the customer, the refiner, the umpire, and for reserves. The materials’ owner and the refiner usually assay the quality samples independently on an ignited basis. If these assays fall within predetermined limits, they are simply averaged to arrive at the payable 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. This procedure frequently involves averaging the two closest assays, or using the middle assay to determine the final settlement. The ‘reserve’ samples (usually sealed by both the owner and the refiner) are held in reserve to cover any possible irregularities during the sampling procedures.
When sampling procedures are completed, the spent catalyst lot is blended with a mixture 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, corrosiveness, morphology, melting temperature and other parameters.
It is important to remember that throughout the sampling procedure, the refiner must adhere to all applicable environmental codes and standards with regard to effluent disposal and atmospheric emissions. An ideal sampling system is therefore typically enclosed for dust control, and evacuated into a dedicated baghouse under a low volume flow. 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, as it is often substantial.
The role of assaying
In conjunction with thorough and comprehensive spent catalyst sampling proceduÏres, accurate and repeatable assaying practices also play a major role in determining precise values of remaining precious metals. Once the final samples have been obtained, sophisticated instrumentation is used to measure their precise precious metals content. Among the equipment and methods used in a well equipped analytical laboratory is x-ray fluorescence (to determine the approximate grade of recoverable precious metals). This helps to fix the amount of copper to be added to the mix in order to obtain the desired bullion grade, and to provide information on the matrix or non precious metals constitution.
Other assay procedures employ atomic absorption (AA), inductively coupled plasma (ICP) emission spectroscopy, and classic volumetric, gravimetric and fire assay techniques.
When all appropriate assaying methods are combined, they provide the most thorough approach for determining precious metals content in spent catalyst materials; thus assuring highest possible returns. Just as in the sampling procedures, it is important (but not necessary) that the catalyst owner (i.e. the customer), or a representative, is present during these procedures, and that full and complete documentation is provided by the refiner for each sample lot.
After sampling and analysis, the value of the entire spent catalyst lot (or ‘reject’ as it is known after the ‘pre- burning’ step) is determined and agreed upon by the refiner and customer. Finally, the spent catalyst is loaded into an electric arc furnace (EAF) for refining. The EAF helps to maximise precious metals recovery, essentially producing two end products: molten precious metals and slag (which contains trace amounts of precious metals and is also subsequently refined). The molten precious metals from the EAF are poured into preheated graphite molds, where they eventually cool into ingots weighing approximately 500 lb each, and are removed for vault storage.
The key to obtaining maximum value (recovering all possible remaining precious metals) from spent catalysts is to focus on detail. The thoroughness and accuracy of the materials sampling process is most important, with assaying of the sample lots close behind. When seeking a precious metals refiner for spent catalyst materials, or working with one presently, it is essential to look into these areas carefully, and work closely with the refiner whenever possible. These steps, and the refiner’s overall policies with regard to applicable pollution codes and standards compliance, should bring the knowledge and confidence to select (or work with) the correct precious metals refiner. In any case, the relationship with the refiner must be viewed as a ‘partnership’, based upon mutual trust and fair treatment.
1. BEIRNE, K.M., ‘Choice of refinement’. Hydrocarbon Engineering, July 2003. pp. 39 – 44.
3. GY, P.M., ‘Sampling of Particulate Materials’. Elsevier Scientific Publishing Co., 1982.