Refining Developments Special Report
Recover precious metals
Precious metal-bearing catalysts, particularly those containing platinum group metals (PGMs)—such as platinum, palladium, rhodium and ruthenium—play a vital role in the hydrocarbon processing industry, for speeding chemical reactions and for end-of-pipe emission control (Fig. 1).
For example, catalysts containing platinum are used in refining crude oil and producing various aromatic compounds. Catalytic reforming uses platinum on an alumina base to produce high-octane gasoline. Hydrocracking employs palladium and platinum impregnated on a silica-alumina base to break large molecules into smaller ones, while hydrotreating uses palladium on alumina to add hydrogen atoms to a molecule. In the platforming process, naphtha feedstock is processed over a platinum catalyst to produce reformate and hydrogen.
At the exhaust end of the process, platinum and palladium (and to a lesser extent rhodium and/or gold) are used to reduce or eliminate atmospheric emissions of volatile organic compounds (VOCs) and other pollutants.
Regardless of how or in what form they are used, most catalysts deactivate over time as a result of exposure to harsh processing conditions, and they must eventually be regenerated or replaced. Because the catalysts are very expensive, the valuable metals remaining at the end of the catalyst’s useful life are normally recovered.
Selecting a precious metals refiner. Businesses are continually seeking new ways to increase profits. While virtually everyone involved with precious metals is well informed regarding the benefits of reclaiming them, some areas may be overlooked. Working with a precious metals refiner, and understanding the technologies and procedures involved in the precious metals refining process, can help maximize returns and ultimately improve profitability.
Many factors should be considered when choosing a precious metals refiner. It is important to select a refiner that:
Financials PGMs are very expensive, and their prices can fluctuate significantly from year to year, and even within the same year (Table 1). Because of these high costs, maximizing the recovery of any PGMs remaining in spent catalysts is important. Precious metals can also be recovered from filter cakes, polishing filters, cloths and papers, floor sweepings and protective clothing.
In general, catalyst users do not actually purchase the precious metals in their catalysts, but rather lease the metals from a "pool account." A pool account is the physical location where metal from many owners and/or lessees is commingled and held. Users draw on this material as needed, and can request delivery of metals for incorporation into catalysts. They typically do not consume the leased metal, but merely use it in the fabrication of catalysts. Because much of the precious metal in catalysts can be reclaimed, users get their metal back after recovery and refining.
The decision whether to buy or lease precious metals is made based on the perception of prevailing lease rates and long-term trends. In addition, many users prefer not to own precious metals so as to keep their costs from appearing on the company’s balance sheet as inventory or a fixed asset.
Leasing PGMs can be viewed as a financial transaction, like borrowing money from a bank. The user borrows the material when required, and pays a leasing fee to do so — just as one takes out a loan and pays interest for the use of the money. Lease rates vary widely depending on supply, demand and other market factors.
Leasing works like this: Spent catalyst is removed from the process and sent to a precious metals refiner for reclamation. Unless this is done during a plant turnaround, fresh catalyst is charged to allow the process to continue operating. The user pays to lease the metal in this replacement catalyst, until the material recovered from the spent catalyst is returned to the customer’s pool for use in making new catalyst.
Leasing replacement metal can be expensive. Consider a shipment of 50,000 lb of catalyst containing 0.3% platinum, with platinum at $1,250/oz and a lease rate of 3%. Leasing the metal in this material would cost nearly $1,600/ week.
To minimize financing charges, working with a refiner that offers the fastest possible processing turnaround time is important. For the above example, if one refiner has a six-week turnaround time and another has a 12-week turnaround, working with the latter would cost nearly $10,000 more in leasing charges.
Shipping and handling included. Precious metals refiners have typically employed third-party logistics companies, freight forwarders, hazardous-waste transporters or specialized brokers to ship spent catalyst and refined metals to and from their facilities. This can add to the complexity and cost associated with catalyst reclamation.
Regulations governing the handling and transporting of materials, both domestically and internationally, have become increasingly complicated. Federal, state and local permits in both the exporting and importing countries as well as international permits are often needed. In the US, the Patriot Act requires HAZMAT carriers to perform new security functions. Ultimately, the catalyst owner is liable for its material, and must ensure that all parties handling those materials do so responsibly.
Some refiners have recently started handling transportation logistics in-house. This eliminates steps in the reclamation process. This approach is not only a convenience, but it also reduces costs and provides catalyst users with the peace of mind that comes with knowing their materials are handled in compliance with applicable regulations.
Environmental considerations. In addition to transportation regulations, catalyst recovery is subject to environmental laws. In the US, the Resource Conservation and Recovery Act and the Comprehensive Environmental Responsibility and Liability Act, or Superfund Act, impart "cradle-to-grave" responsibility—as well as future liability—on both the precious metals refiner and the catalyst user that is the source of the precious metals being recovered. Most European countries have similar, or stricter, laws.
If the refiner violates any environmental regulations or permits, regardless of which customer’s materials were being handled at the time of the violation, any of the refiner’s customers could incur liability, including high legal costs and heavy fines. Thus, it is important to examine the refiner’s environmental compliance status and how it handles solid, liquid and gaseous byproducts.
Ideally, a precious metals recovery plant should employ state-of-the-art pollution-control systems, such as afterburners, baghouses, scrubbers and process water evaporation. It should also not ship hazardous wastes offsite for treatment and/or disposal. If it does send wastes elsewhere, the waste hauler and waste receiver must also meet all applicable environmental standards. The catalyst user’s cradle-to-grave liability extends to ultimate disposal, even if that is handled by a subcontractor with which the user has had no direct dealings.
Catalyst users should request detailed documentation on environmental compliance from the refiner and from federal, state and local regulatory agencies. A review of air and water permits and periodic compliance reports will reveal any past violations and may be an indicator of future environmental performance. The refiner should also supply documentation that it qualifies as a bona fide precious metals refiner, as defined in the US Environmental Protection Agency’s Boiler and Industrial Furnace rule. Most precious metals refiners willingly provide copies of such paperwork.
Removing contaminants before sampling. Over time, hydrocarbon processing catalysts become contaminated by sulfur, carbon, volatile organics, moisture and other unwanted elements. As a result, when the catalyst is removed from the process unit, 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, a multiple-hearth furnace or a fluidized-bed furnace (Fig. 3). 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 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 Ib or more) 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. Overall reclamation costs are reduced when the refiner conducts the pre-burning onsite.
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.
Accurate sampling is critical. Three sampling techniques are used to determine the amount of precious metal remaining in spent catalyst—melt, solution and dry sampling. Some materials can be sampled by only one of these methods, while others may be processed by more than one technique. Determining the best sampling approach for a particular catalyst lot is essential to recovering the greatest possible value. That determination is based on factors such as the type of material being processed and its estimated precious metals content.
Sampling involves reducing large quantities of precious metal- bearing material (as much as many tons) into smaller batches (as little as a few grams). The material is converted into a mass that is as homogeneous as possible, so that molecules of precious metals and other constituents are evenly distributed. Samples are then extracted from different fractions of this homogeneous mass to obtain an accurate representation of the ratio of precious metal in the overall matrix.
In melt sampling, a carrier metal such as copper is melted with the precious metal-bearing material. The molten metal is poured into ingots, which are sampled at the beginning, middle and end of the pour. This technique has an extremely high degree of accuracy, with a tolerance as close as ±0.1% from sample to sample. Metal mesh pollution-abatement catalysts may be sampled in this manner.
Solution sampling is a cost-effective and extremely accurate method of determining the composition of precious metal-bearing solutions, such as homogeneous catalysts. It, too, involves obtaining a homogeneous dispersion of precious metals and other constituents at the molecular level and taking multiple samples from different parts of the solution for further analysis. Accuracy is comparable to that of melt sampling.
Due to their composition and chemistry, most hydrocarbon processing PGM catalysts are sampled by dry sampling, which is used for materials that cannot be dissolved in solution or are inappropriate to melt, either because of their structure or because the cost associated with melting exceeds the possible return. Because it is difficult to achieve homogeneity, dry sampling is more complex and potentially less precise than melt or solution sampling, and it requires more skill and judgment.
Materials to be dry sampled are homogenized by grinding large pieces into smaller and ever finer particles. When the material cannot be broken down any further, the homogeneous mass represents an accurate ratio of the precious metals content in the overall matrix. The reduced material is then allowed to free-fall in a stream into a cross-cut, timed automatic sampler, and representative samples are taken periodically. Accuracy is typically ±2%.
Analyzing the samples. Accurate and repeatable assaying procedures, some using sophisticated instrumentation, are employed to measure the precious metals content of materials being reclaimed. These analyses must produce highly accurate results with moderately high precision (± 1 %) at analyte levels on the order of 0.1%.
To do this, the refiner should have a well-equipped analytical laboratory with advanced X-ray fluorescence, atomic absorption spectroscopy and inductively coupled plasma (ICP) emission spectroscopy, as well as conventional volumetric, gravimetric and fire assay techniques (Fig. 4). The specific methods used are determined by the types of materials being processed. For instance, the matrix of the sample and the particular mix of analytes will determine such things as which analytical methods can and cannot be used, the collector metal used in fire assay or the wavelength(s) used in ICP analysis. When used together, these techniques provide the most thorough and precise approach for determining precious metals content in spent catalyst materials.
Recovery and refining. After sampling and analysis, the spent catalyst is blended with a mix of flux and a carrier metal such as copper or iron (Fig. 5). The proportions in the mix depend on the recoverable precious metals in the lot and other parameters.
This mix, called the charge mix, is then smelted, usually in an electric arc furnace or an induction furnace (Fig. 6). The furnace produces a two-layer molten pool. The top layer is slag, the reaction product of the catalyst’s support and the added fluxes. The bottom layer is collector metal in which the precious metals have been dissolved. After the slag is poured off, the remaining metal layer is poured into molds to make ingots.
The ingots are then sent to a conventional hydrometallurgical or electrorefining facility for separation and refining into market-grade precious metals. The slag is usually also processed further to recover trace metals.
Kevin M. Beirne is vice president/sales and marketing at Sabin Metal Corp. in East Hampton, New York. He has been in the precious metals industry for over four decades. In addition to his sales and marketing background, he has managed analytical, instrumentation and fire assay laboratories as well as precious metals refining and manufacturing organizations. Mr. Beirne attended Fairleigh Dickinson University and also completed many specialty courses for instrument analysis and marketing. He was a member of the American Electroplaters Society, Investment Recovery Association, and is past president of the International Precious Metal Institute.