Special report: clean fuels - How to choose a refiner for your precious metals catalyst
Most hydrocarbon and petrochemical processors operate precious metals recovery or asset recovery departments in one form or another. These are typically managed as independent profit centres, which, because of global economic uncertainties, have assumed more important roles in the past few years.
K M Beirne
Sabin Metal Corp
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Finding and working with the right refiner can make a significant difference in returns, thus enhancing profitability. There are more than a few unfortunate stories about an organisation’s selection of, and relationship with, its precious metals refiner. A relationship with a refiner may have significant legal implications if the refiner violates environmental regulations when processing spent catalyst.
PGMs and other precious metals
Precious metal-bearing catalysts are widely used for facilitating or accelerating hydrocarbon/petrochemical processes (where they “rearrange” hydrocarbons into specific molecules); for hydrocracking low-quality feedstock into higher-quality, more commercially useful products; and for controlling/abating harmful or unlawful volatile organic compounds and NO emissions.
Catalysts help reduce energy use in a wide range of other petrochemical/chemical manufacturing processes. For all these applications, the catalysts typically contain platinum, palladium, rhodium or ruthenium; these metals are commonly referred to as platinum group metals (PGMs). Hydrocarbon processing catalysts may contain two or more PGMs, and they may also include rhenium (another valuable precious metal) in addition to gold, silver or other metals. A variety of carriers, or supports, for these metals are also used, depending upon application; these typically include soluble or insoluble alumina, silica/alumina or zeolites (Figure 1).
Over the past few decades, worldwide demand for PGMs has increased significantly, mainly as a result of emerging economies. However, the volatility of global financial markets and geopolitical instability has also served to escalate costs for many precious metals. Precious metals costs represent only a small portion of the total processing/production dollar with regard to raw materials, equipment, personnel, transportation, etc. Nonetheless, they can still be significant when their market value is factored in, along with leasing costs for replacement metals, delays in transit and processing time for recovery and refining, and legal implications if the precious metals refiner commits an environmental infraction.
With the dynamics of costs, profits and possible legal problems, it is clearly in an operator’s best interest to work with a precious metals refining organisation that does the following:
1. Provides the highest possible returns for PGMs from spent catalysts
2. Provides rapid processing turnaround time
3. Complies with applicable environmental standards concerning process effluent disposal or atmospheric discharges at its refining facilities.
Choosing the wrong refiner can prove to be an expensive and troublesome mistake.
There are many criteria to consider when selecting a precious metals refiner. Mainly, the rules come down to specific areas that can be controlled and that apply to virtually all refiners that process spent catalyst with PGMs. These include the policies and procedures associated with the refiner’s sampling, assaying, processing and logistics arrangements. Each of these areas are briefly covered as a way of examining how to select and work with the right refiner to meet a plant’s specific requirements.
Precious metals sampling
To accurately determine the amount of precious metals present in materials for recovery, refiners typically use three different sampling techniques. These techniques are dry sampling, melt sampling and solution sampling. Each of these techniques offers discrete advantages; determining the most appropriate sampling method depends on the type of material being processed, as well as its estimated precious metals content.
In-house moisture and contaminants removal
To provide an accurate determination of remaining precious metals in spent catalyst lots, representative samples of these catalysts must be obtained under accurate and repeatable conditions. Over time, process catalysts become contaminated by sulphur, carbon, volatile organics, moisture and other unwanted elements. As a result, when the catalyst is removed from the process, it is usually moist and sticky, and it will not flow freely through automatic sampling equipment.
Contaminants in the catalysts must first be removed to ensure accurate sampling and analysis of the remaining precious metals. This process is accomplished with an indirectly fired rotary kiln (Figure 2), which not only greatly enhances sampling accuracy to ensure maximum recovery value of remaining precious metals, but also significantly reduces overall refining costs when handled directly at the refiner’s facility. This is a key issue with regard to the total cost of recovery and refining, and, by inference, to the overall profitability of a precious metals refining and recovery program. The typical rotary kiln will remove up to 25% of the materials’ sulphur content and up to 40% of the carbon content, usually at a rate of 300 lb/hr to 1,000 lb/hr.
Most contaminants associated with spent precious metal-bearing catalysts typically exhibit high loss on ignition (LOI), in addition to other contaminants previously mentioned. The removal of moisture and other contaminants is critical to the downstream sampling process. The reason for this is that the materials must be free flowing (with low LOI) initially to arrive at a final evaluation sample that is accurate to ±2%. Here, pre-burning can make a key difference; if the high moisture content and other contaminants are not removed, a suitably accurate sample cannot be obtained by the refiner, thus eliminating the possibility of providing a fair and true return value to the catalyst owner.
Contaminants in spent catalysts also may be removed by a multiple hearth furnace or fluidized-bed furnace. Whatever the case, this first step (i.e., pre-burning) is critical to the sampling process. Just as important least from a financial perspective is where and how the contaminants are removed. This is because many catalyst users must first ship large lots of spent catalysts (35,000 lb to 500,000 lb is a typical range) to an independent facility, where strip-burning removes their hydrocarbon content, and coke burning removes carbon. Another furnace may be required for the drying of fine particulates and other materials to eliminate moisture content.
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