The recovery of platinum group metals from spent petrochemical catalysts has economic benefits by the protection of the supply chain of these valuable and rare metals, thereby helping to create a sustainable value chain in the petrochemical industry for catalysts.
The platinum group metals (PGMs) are used extensively in catalysts for the petrochemical industry owing to their excellent catalytic properties. Their high value and natural scarcity mean that high recycling rates not only have economic benefits but also environmental benefits, thereby generating a sustainable supply of these metals. Thermal plasma is a pyrometallurgical technique which is used extensively for the recovery of the PGMs from catalysts and recovery efficiencies of over 98% can be achieved. This paper outlines the use of thermal plasma in the recovery of these metals from petrochemical catalysts with a case study of Tetronics International's plasma facility recovering ruthenium from spent petrochemical catalysts.
Platinum Group Metals in the Petrochemical Industry
Despite their scarcity, the catalytic properties of the PGMs make them especially important for industrial use, particularly in petrochemical catalysts. The use of PGMs in petrochemical catalysts has been well established for decades and are used widely in reformation, hydrocracking and isomerisation processes.
Platinum and palladium are the most widely used PGMs in petrochemical catalysts although they are often used in combination with other PGMs and occasionally other metals such as rhenium and cobalt. These additions can enhance the stability of the catalysts; rhenium for example is added to Pt/AI203 catalysts to maintain the catalyst's activity under high level of coking as well as enhancing its selectivity in the reforming of naphtha.
Bifunctional catalysts such as Pt/Ca-Y-zeolite catalysts are used in the hydrocracking process. The Pt allows for the dehydrogenation of reactants to alkenes and the hydrogenation of olefins whilst the zeolite component acts as a Br0nsted acid site which cracks the feedstock into intermediates fordehydrogenation/hydrogenation.
Table 1: Examples of PGM containing petrochemical catalysts
The advantage of PGMs is that the reactants are adsorbed with a moderate strength compared to base metal catalysts. This means that the process reaction rates are favourable, whereas either a too strong or a too weak adsorption strength causes a slow reaction rate which is less commercially attractive. Therefore, although alternative non-PGM catalysts do exist, the PGM catalysts remain trusted and effective and are ultimately difficult to replace.
Examples of PGM bearing catalysts are shown in Table 1 and it can be seen that the typical loading of PGMs on catalysts ranges from 0.2 wt% to over 1 wt%. A loading of 0.3 wt% is around 400 times greater than the PGM concentration in their primary ores and the high value of these metals coupled with their natural scarcity mean that spent catalysts are an important secondary source of PGMs. This has stimulated high recycling rates of spent catalyst in the petrochemical industry, which operates as a semi-closed loop recycling market where the precious metals are recovered and reused and the only requirement for virgin metal is to cover losses from the system due to use and recycling losses (Figure 1).
PGM Recovery Processes
The recovery processes for these metals can be broadly divided into two major types: hydrometallurgical and pyrometallurgical routes.
The hydrometallurgical routes rely on wet chemistry extraction and precipitation techniques. For example the catalysts undergo a pre-treatment stage to dissolve the catalytic support and the then the noble metals can be leached using a suitable lixiviant such as aqua regia or sulphuric acid depending on the selectivity required or chemistry of the metals. The target metals are then precipitated as salts which can be then purified using a roasting process, if required.
Pyrometallurgical processes are thermally based treatment techniques where the catalysts are treated at high temperatures thereby melting the components. The ceramic-like supports of the catalysts form a slag material which has a lower density than the metallic components which are then able to move through and settle beneath the slag and be separated. The addition of a collector metal is often required due to the small particle size of the metallic particles which inhibits their settling rate through the slag. The collector metal forms larger particles which settle through the slag at a faster rate and absorb the target metals forming an alloy. This alloy is rich in the target metals and is then sent for subsequent refining, usually via hydrometallurgical routes.
Figure 1: The petrochemical semi-closed loop PGM recycling market
It is often practical to utilise a combination of pyro and hydrometallurgical routes to achieve maximum recovery efficiencies of the PGMs. The hydrometallurgical processes are often very sensitive to compositional variation in the feedstock and can experience difficulties in directly processing spent catalysts due to the high stability of the catalytic support. A particular issue is found with AI203 supports where the PGMs are originally seeded onto a Y-AI203 support, the metastable phase of alumina, which under the petrochemical process conditions can undergo a phase change to a-AI203 which is extremely difficult to dissolve and so results in significant recovery losses. Consequently hydrometallurgical recovery rates can vary from 85% to 95%.
Pyrometallurgical routes on the other hand are not so sensitive to variations in the composition of the feed material or the phase of the support and are able to treat a large volume of material at high throughputs. Difficulties in pyrometallurgical processes can arise when there are metallic components which have high vapour pressures or are easily oxidised meaning that they can be vaporised and the process requires off gas capture of these metals which can become quite complex. The pyrometallurgical processes often require the use of hydrometallurgical routes for the final refining of the alloy produced, which is often iron or copper based; however this is much more suitable for final refining than the original catalysts and so high recovery rates can be achieved.
This paper will now focus on the use of thermal plasma arc furnaces, which is one such pyrometallurgical technique used extensively in industry for the recovery of valuable metals from spent catalysts and other wastes and ores. These plasma processes are typically able to achieve high recovery efficiencies of PGMs from catalysts of over 98%.