Process Engineering: Get the most from spent catalysts

The process of recovering from precious metals from spent catlysts is a necessary move.  Making sure that you recover fair market value demands accurate sampling.

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Many plants employ fixed- or moving-bed catalytic reactions for hydrogenation of intermediates and for controlling or abating emissions. These operations rely on precious metal catalysts (PMCs) containing platinum group metals (PGMs), including platinum, palladium, ruthenium and rhodium.

PMCs, whether in the form of monolithic structures, beads, pellets, powders or extrudates, have finite useful lives. Catalyst lifetimes depend upon how and where the materials are used, but typically extend to five to six years. Afterward, the spent catalysts should be processed to recover their remaining precious metals.

The process of reclaiming the remaining precious metals is referred to as “recovery and refining.” It includes individual procedures, such as materials documentation, pre-burning, sampling and assaying. In addition, environmental considerations and turnaround time can markedly impact economics. Each of these is independent; together, however, they significantly affect how much remaining PGMs are recovered, their returned value, the speed at which they are reclaimed and environmental compliance.

While each of these functions is important, sampling procedures for spent catalyst materials are perhaps the most critical elements for achieving maximum return. As a user of the catalysts, it is in your best interest to understand how sampling helps determine the precious metals content of spent catalysts and, ultimately, the value of these metals that is returned to their owners.

The problem in sampling is that even new catalysts on substrates or carriers, such as soluble and insoluble alumina, silica-alumina, zeolite or carbon supports, are not homogenous masses. During use, the materials accumulate many contaminants of various densities, including sulfur, carbon, solvents and water. Therefore, spent catalysts are even less homogenous.

Sampling procedures
Three different techniques — dry sampling, melt sampling and solution sampling — are employed. Each uses different methods and equipment; each 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.

However, because of their composition and chemistry, PMCs usually undergo dry sampling. This method is used whenever materials cannot be put in solution or melted either because of their structure or the cost associated with melting versus the possible return. Since it is difficult to achieve homogeneity, dry sampling is more complex and potentially less precise than solution or melt sampling. This method requires more judgmental skills than the others do. An ideal dry system would be capable of drawing representative samples from free-flowing catalyst at a rate of 2,000 lb/hr to 3,000 lb/hr.

To determine the quantity and quality of remaining PGMs as accurately as possible, the spent catalysts first must be “reduced.” This involves taking large quantities, up to many tons, of the materials and reducing it into smaller quantities of as little as a few grams, while also eliminating contaminants. The goal is to achieve an accurate determination of the actual value of recoverable precious metals within the lot by getting a precise representative sample of the overall lot after it has been made homogenous. In essence, when the material cannot be broken down any further, the result of sampling (or reducing) the homogenous mass represents a highly accurate ratio of the precious metal content in the overall matrix.

Assuring accuracy
Dry sampling procedures are tedious and complex. However, before the actual sampling, the entire spent catalyst lot first must be “decontaminated” — contaminants that have accumulated over the years must be removed.

Contaminants are eliminated in a rotary kiln (or a multiple-hearth or fluidized-bed furnace), which removes up to 25% of the sulfur content and up to 40% of the carbon. This first step, or pre-burning, is critical to the sampling process and is best handled at the refiner’s facility. This is because doing so eliminates the possibility that the materials could mix with an unrelated lot and also affords the materials’ owners substantial cost savings by obviating trans-shipment charges to independent “regenerators” for what typically are many tons of spent catalyst.

After processing in the rotary kiln, materials containing large agglomerates are crushed and subsequently blended in with the lot for further reduction.

This provides homogenous, consistent and reproducible intermediate samples. One of these samples is divided into about 1-lb. portions and retained in hermetically sealed aluminum cans for loss on ignition (LOI) determination. This involves heating the catalyst in the presence of air to burn off any remaining volatile components and oxidizable materials, such as carbon or sulfur. The procedure permits the precise determination of a “settlement weight” through laboratory analysis.

The settlement
The materials’ owner and the refiner usually assay these small samples (on an ignited basis) independently. If these assays agree to within predetermined limits, they simply are averaged to arrive at a final settlement. If they do not agree, a sealed “umpire” sample is sent to an independent laboratory (i.e., 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. “Reserve” samples (usually sealed by both the materials’ owner and the refiner) are held back to cover any possible irregularities during the sampling procedures.

When sampling procedures are completed, the 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 chemistry of the slag, which takes into account its electrical conductivity, corrosivity, morphology, melting temperature and other parameters.

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