Comparing Capital and Operating Costs for MRT and Legacy Technologies in PGM Separations
Emphasis on development and use of green chemistry and green engineering technologies is increasing as societal demand for a clean environment and reduced pollution grow and industry recognizes the capital expense (capex) and operating expense (opex) advantages associated with more efficient operations. A partial list of principles associated with these technologies [1,2] includes prevention of waste generation; design of processes and equipment to ensure that energy inputs and outputs are as inherently non-hazardous as possible; design of separation and purification processes to minimize energy consumption and materials use; design of products, processes, and systems to maximize mass, energy, space, and time efficiency; and design of material and energy inputs to be renewable rather than depleting. These and other principles inherent in green chemistry and green engineering processes are used increasingly in the chemical and pharmaceutical industries with positive results.
In the metallurgical processing industry, green chemistry and green engineering principles have been used in Molecular Recognition Technology (MRT) processes for the past 25 years for metal separations in numerous commercial installations [3-9]. These installations feature significantly reduced capex and opex costs compared to operations that feature traditional separation systems, such as solvent extraction (SX), ion exchange (IX), and precipitation (Legacy Technologies).
In this paper, benefits of using MRT for selective separations of individual platinum group metal (PGM, singular or plural) are presented. Capex and opex costs for MRT and SX are compared for separation and recovery of PGM (Pt, Pd, Rh, Ru, Ir) from virgin ore and from secondary sources. The argument is made that MRT processes have significantly lower capex and opex costs consistent with their use of green chemistry and green engineering principles. Furthermore, the ability of MRT processes to selectively separate and recover individual PGM from dilute waste solutions containing these metals at mg L-1 or lower levels is a great advantage of MRT over Legacy Technologies in preventing loss of these valuable metal resources as waste.
PGM have similar chemical properties and occur together in nature . Unique properties of PGM, such as high melting points, corrosion resistance, and remarkable catalytic qualities, have made them indispensable for many industrial applications that define our high technology society. The United States (U.S.) National Research Council has identified critical minerals and materials considered essential for industry and emerging technologies in the domestic civilian, government, and military economies . Several PGM, i.e., Pt, Pd, and Rh, are included. In 2010, the U.S. imported about 94 percent of the Pt and 58 percent of the Pd it consumed . Approximately 90% of global PGM production is from South Africa and Russia, areas where geopolitical concerns exist . A reliable domestic source of PGM from secondary sources is needed for U.S. national security. The leading use of PGM, especially Pt, Pd, and Rh, is in catalytic converters that decrease harmful emissions from automobiles [9-12]. Other uses of PGM include catalysts in chemical production and petroleum refining; electronic applications such as in computer hard disks to increase storage capacity, in multilayer ceramic capacitors, and in hybridized integrated circuits; in glass manufacturing; in jewelry; and in specialized laboratory equipment. Other important applications include Pt in the medical sector; Pt and Pd with other components as dental restorative materials; Ir as a corrosion-resistant component of spark plugs and jet engine turbine fan blades; Ru in hard disks; and Pt, Pd, and Rh as investments in the form of physical bars, coins, and other forms.
Recycled PGM from spent products are important as a secondary source of supply . In 2010, PGM recovered from recycled automobile catalytic converters, electrical products, and jewelry accounted for about 31 percent of the gross global supply for rhodium, 30 percent for platinum, and 25 percent for palladium. In 2010, about 34,000 kg of platinum, 41,000 kg of palladium, and 7,400 kg of rhodium were recycled from automobile catalytic converters, about 310 kg of platinum and 14,000 kg of palladium were recycled from electrical components, and about 23,000 kg of platinum and 2,600 kg of palladium were recycled from jewelry. Since 2006, the amount of palladium and platinum recovered annually from recycled sources has increased at a higher rate than the increase in primary capacity for these metals.
Demand for PGM continues to grow. Mooiman, et al.  have discussed current and emerging challenges confronting the mining industry in meeting global demand for PGM. These challenges include metal price volatility; decreasing grades and increasingly complex mineralogy of global PGM deposits; increasing metal production costs; increased requirements to properly dispose of deleterious by-products such as toxic metals; increasing need to deal with geopolitics, public perception, and environmental regulations in the mining region; maintenance of sustainable development in the mining region; and increased energy and water use as mining increases in complexity.
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Recycling provides a way to avoid these challenges and to provide a reliable domestic source of PGM. Excellent reviews of refining approaches involved in recycling PGM are available [8,9,13]. The bulk of PGM recycling (60% of Pt and 70% of Pd) originates from low-grade autocatalytic material where the PGM are confined to a single unit that can be easily separated from the vehicle. This unit is treated at integrated smelters where high PGM recovery rates approaching 100% are achieved [8,9,13,14]. Although recovery rates can be very high in these cases, the cost to achieve them varies considerably depending on the smelting and refining processes used. Costs include not only direct capital and operating costs, but environmental, health, and working capital costs, as detailed below. Few integrated smelters capable of separating and recovering PGM exist globally. Spent automotive catalytic converters that cannot reach such a smelter may be treated by less sophisticated means for their PGM content usually with lower recovery rates. These converters and most other spent products that contain PGM, usually in small amounts, ~ mg L-1, eventually go to landfills. The resulting loss of PGM is significant as shown by the statistic that only 25-50% of mined PGM is eventually recycled . Recycling rates for PGM in spent electronic and other products where they are present in low concentrations is <10% . The need for improved separation technologies capable of recovering PGM from low level spent secondary sources has been mentioned , but little has been done to develop them. MRT has a long history of using green chemistry and green engineering procedures for selectively separating and recovering individual PGM during processing of native ores and from secondary sources containing these metals, even at concentration levels of mg L-1 or lower [3-9]. Traditional separation systems become less efficient as metal concentration decreases making them ineffective in separations at low concentrations. Thus, MRT is valuable as a method of choice for the commercial separation of PGM from spent products containing these metals, a source that is presently largely discarded as waste.
Use of MRT for commercial separation and recovery of PGM has been summarized  and includes Pd recovery from native ore; Rh, Pd, and Pt recovery from spent catalyst and other low grade resources; Pt and Ru recovery from alloy scrap; and Ir separation from Rh and base metals. Green chemistry and green engineering principles that apply to MRT PGM separations are formulated in Table 1.
Table 1 – Green Chemistry and Green Engineering Principles Applied to MRT Platinum Group Metal Separations .
- No organic solvents (inherently flammable) are used in the separation process that operates at room temperature (~25oC) and pressure (~0.1 MPa).
- No contaminants are added to the process stream during the separation process.
- Wash and eluent solutions are as simple as possible while being compatible with the overall PGM refining plant operations. Washes and eluents used include H2O, HCl, NaCl, KCl, Na2SO3, (NH4)2SO3, and NH4HSO3.
- Target PGM are recovered in pure concentrated form from eluate solutions following elution from the column with a small amount of eluent.
- Target PGM in eluates are easily precipitated to final products using common reagents. For example, HCl, air, and H2O2 are used to form Pd yellow salt. Precipitated PGM compounds are collected by filtration. No contaminating or hazardous reagents are used.
- Impurity metals, such as Ag, Au, and base metals are selectively separated from the raffinate and recovered either for value or safe disposal.
- Selective separations of individual PGM is of critical importance since it simplifies the procedure, eliminates need for multiple stages, and avoids downstream use of azardous/contaminating chemicals for further separations.
- MRT processes can make a variety of individual and/or group separations of PGM from g L-1 to mg L-1 or lower concentration levels even with high concentrations of impurity metals present.
- Minimal hazardous waste is generated using the MRT process.
Several characteristics of the MRT system allow it to operate in accordance with green chemistry and green engineering principles [3-9]. First, MRT systems are simple in design and operation. The systems operate in a column mode, have small space requirements, do not use solvents or highly corrosive chemicals, employ highly metal selective SuperLig® products, and produce pure target metal products rapidly with small inventory times. Second, these SuperLig® products, consisting of pre-designed metal-selective organic ligands chemically bound to silica gel particles by a tether, are highly selective and have high affinity for individual target metals. The MRT process is distinct from IX in that it involves selective binding of the target metal by a proprietary SuperLig® product followed by release using an appropriate eluent. Ions are not exchanged in the MRT process. High metal selectivity and high metal affinity make possible separations of individual PGM at high purity in a single step. The resulting raffinate does not contain traces of the target PGM that would require further separation steps downstream. Metals in the raffinate can be separated and recovered for reuse or appropriate safe disposal resulting in minimal generation of waste. Third, elution of the target metal with a small amount of eluent allows the metal to be concentrated more than 100 times in the eluate. This concentration feature is particularly important in recovering metals present in solutions at concentrations of mg L-1 or lower, as in the case of PGM since many secondary streams contain PGM at these low concentration levels.
Benefits inherent in the MRT process (Table 1) provide numerous advantages over Legacy Technologies in space, time, labor, safety, reduced metal inventory time, lower maintenance, fewer process steps, lower waste generation, and reduced amounts of materials. These advantages are summarized in Table 2.
Table 2 – Advantages of MRT in PGM Refining Versus Legacy Technologies .
- Highly selective single-pass separations of 99 + % and product purities of 99.95% ‐ 99.99%.
- Minimization of platinosis and other health and safety risks due to the MRT system being self-contained, thereby greatly reducing worker exposure.
- Rapid recycling of target PGM by reduction of total processing time.
- Increased level of PGM recovery by reduction in processing losses to environment.
- Lower PGM refining cost (space, labor, materials) by eliminating and/or reducing use of process chemicals and number of process steps.
- Lower labor, construction, and maintenance costs by use of a much simpler system.
- Reduction in processing pipeline time, thus reducing metal financing costs and releasing metal earlier for sale.
- Rapid kinetics for metal complexation and release enabling on-line processing.
- Treatment of a wide variety of metal feed streams ranging from concentrated (g L-1) to dilute (mg L-1 or lower).
- Efficient production of a salt product that may be sold or reduced to market‐grade metal.
- Increased efficiency from use of a rapid and semi-continuous separation process that may be put on-line.
- Treatment of any solution volume.
- High loading and elution flow rates.
- Recovery of impurity metals from raffinate for reuse or proper disposal, thus minimizing waste generation.
- Reduction of PGM security risk as system is self‐contained, thus minimizing exposure.
- Reduction of capex of SuperLig® products by their regeneration for multi‐cycle use.
A summary of relative capex and opex costs for MRT and SX systems is given in Table 3. The economy of MRT as shown in Table 3 has been proven in the market place with commercial applications involving both base and precious metals [3-9].
Table 3 – Comparison of Molecular Recognition Technology and Solvent Extraction Capex and Opex Costs
|Item||Molecular Recognition Technology||Solvent Extraction|
|Spent chemical, solvent, waste water discard||Small||Large|
|Processing time||Rapid with low metal inventories and working capital||Slow with high metal inventories and working capital|
|Single pass recovery rates||High, >99%||Low, especially at low target metal concentrations|
|Environmental effect||Minimal||High unless controlled by increased capex costs|
|Waste generation||Minimal||High unless controlled by increased capex costs|
|SuperLig® (MRT)/ Extractant (SX)||Variable, cost dependent on SuperLig® used, but costs reduced by regeneration of resin multiple times||Variable, cost dependent on extractant used|
A typical commercial MRT system for Rh recovery from scrap sources is shown in Figure 1 .A perceived advantage of less selective Legacy Technologies over MRT is the low price of resins or reagents used in these technologies compared to the (usually) higher cost of SuperLig® products . However, the low cost of resins or reagents used is not indicative of the true costs incurred by IX and SX processes. As compared to MRT, negative externality costs associated with less selective technologies are paid for in more complex capital equipment systems; larger system footprints; limited or no resin reusability; extensive use of sub‐optimal and/or hazardous and flammable chemicals, such as solvents, that introduce substantial risk into the process and require complicated operational and environmental protocols; higher energy costs; lack of system flexibility to target specific, commercially important metals early in the flowsheet; higher number of separation stages; increased volumes of eluates, washes, and wastes; larger and more complex waste treatment systems; slower metal binding and release; incomplete and slow extraction and recovery of the target metal; complex pre‐ and post‐treatment regimens; lower metal purities; lower metal recoveries; higher metal losses; and longer retention of valuable metals in the process, resulting in high metal inventories and working capital costs. Low selectivity means that traces of impurity metals follow target metals, and this translates into the creation of multiple side streams that then need to be processed, resulting in higher costs, greater environmental liabilities and increased worker exposure. These factors, summarized in Table 3, must be assessed to determine the true cost effectiveness of a separations technology.
Comparison of opex and capex costs for MRT and SX in Table 3 suggests that these costs are much smaller in the case of MRT. Dollar amounts are not given in Table 3 because specific numbers are unique to each situation depending on, among other things, sources of feed material, depreciated capital, and operation protocols. However, the general concepts and conclusions in Table 3 are valid. It is apparent that MRT is much more economical because (1) many steps are deleted, (2) numbers of stages and treatment of side streams are reduced markedly as a result of high metal selectivity resulting in very high single pass recovery rates, (3) inventory or retention time of PGM in the process is much shorter, (4) space requirements are reduced significantly, (5) operating costs are lowered dramatically because no solvents are used, fewer and less corrosive chemicals are employed, and waste generation is minimal, and (6) capital expenses are much lower, since less space is required, the system used is much simpler and can be expanded on a modular basis, less labor is required, fewer health requirements are needed, and minimal pollution control is necessary.
The United Nations Bruntland Commission (formerly the World Commission on Environment and Development) defined sustainability as follows : “Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs.” Achievement of global metal sustainability requires increased use of highly selective and efficient green chemistry and green engineering processes, such as MRT, for separation, recovery and purification of PGM and other critical metals from a variety of spent materials, wherein these metals are present at very low levels making their recovery difficult using Legacy Technologies. The degree to which metal sustainability can be accomplished means preservation of valuable resources that otherwise would be discarded to the commons and become unrecoverable . It is imperative that green technologies, such as MRT, be developed and used more widely in global recycling and other metallurgical processes.
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Reed M. Izatt received a BS degree in Chemistry from Utah State University (1951) and a PhD degree in Chemistry with an Earth Sciences minor ... <Read more about Reed M. Izatt, PhD>