Plasma Arc Recycling of Precious Metals

Ben Messenger looks at how plasma arc technology is helping recyclers to bridge the gap between supply and demand for a number of platinum group and other precious metals.

With one in four of all products manufactured requiring platinum group metals in some regard, it's unsurprising that demand for these costly yet highly useful metals has surged. Primary sources have struggled to keep pace and recycling is on the rise. WMW investigates how plasma arc technology is helping recyclers to bridge the gap between supply and demand.

by Ben Messenger

Having been known by Native Americans for some considerable time, platinum was first encountered by Europeans in Central American mines between Panama and Mexico. The Spanish colonialists named the metal 'platina', or 'little silver', and regarded it as nothing more than an undesirable impurity in the silver they mined.

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Fast forward half a millennia or so, and the value we attach to platinum could not be more different. When an industry measures its total global output in ounces (1 troy ounce is equal to approximately 31.1 grams) then supplies of the raw material are clearly scarce. But platinum and the rest of the Platinum Group Metals (PGMs) are highly valuable not only because of their low natural abundance and the often complex extraction processes required to refine them, but also because of a number of unique properties they possess.

The PGM group is made up of six metallic elements clustered together in the periodic table and consists of ruthenium, rhodium, palladium, osmium, iridium, and platinum.

Thanks to properties such as high resistance to chemical attack, excellent high temperature characteristics and stable electrical properties they have become increasingly desirable for industrial applications, including catalysts for the chemical and automotive industries. Indeed, according to the International Platinum Group Metals Association, PGMs are either contained in quarter of all goods manufactured today, or were essential to their manufacture.

Supply and demand

Figures published in the 2011 Platinum Review by Johnson Matthey - an international speciality chemicals company - show gross demand for platinum increased by 16% to 7.88 million ounces (oz) in 2010, with a global surplus in the supply chain of just 20,000 oz. At the same time recycling of platinum rose by almost a third, to 1.84 million oz.

According to Geoffrey Bond, Emeritus Professor at Brunel University, while platinum is a particularly good catalyst for reactions where selectivity is not so important, such as in the catalytic combustion of hydrocarbons, some others PGMs, especially palladium, are equally useful. Figures from the 2011 Platinum Review back this, showing that gross demand for palladium increased by 23% in 2010 to 9.63 million oz, its highest ever level.

One of the main drivers for this increase is the growing demand from the automotive industry, which upped its demand by 35% to 5.45 million oz. This was partly due to higher global production in the light duty passenger vehicle sector, together with tightening emissions legislation.

Despite a 29% increase in open loop recycling through 2010 to 1.85 million oz - the majority from end-of-life autocatalysts - the palladium market was in an overall deficit of some 490,000 oz. Meanwhile, global supplies of palladium from primary sources rose by just 189,700 oz in 2010 to 7.29 million oz.

Clearly then recycling is playing a rapidly growing role in the supply chain for PGMs, and recovering these highly valued metals from the waste stream will become increasingly important - and profitable.

High value recycling

One company hoping to benefit from the increasing need to recover PGMs from wastes is Swindon, UK based plasma arc technology developer, Tetronics, which supplies direct current plasma arc systems that recover precious metals including gold and silver as well as PGMs.

Speaking to WMW, Dr. David Deegan, environmental technical director at Tetronics explains that catalysts from the automotive, petrochemical and pharmaceutical industries offer a prime waste stream for the recovery of PGMs, and that nearly every car scrapped in the developed world contains 2 - 3 grams of PGM.

"Every car on the road has a brick of platinum containing ore which is typically two to three orders of magnitude richer than the ore that is mined," says Deegan.

Autocatalysts consist of a metal can containing a honeycomb type ceramic base. In operation exhaust gases from the vehicle pass through the honeycomb, which is coated with a very fine layer of PGMs. When sent for recycling using Tetronics' plasma arc technology the catalyst is first de-canned and the honeycomb ceramic part is crushed into a powder to be fed into the furnace. Tetronics claims to recover in excess of 98% of the PGMs present in the catalyst, which it traps in a 'collector metal'. The process can be equally applied to the recycling of e-waste.

"Typically e-waste material is shredded and then pre-incinerated to drive off all the organics, and also to drive off things which can reduce the technical recovery rate. The remaining char is what would go into our furnace," says Deegan.

Robotically controlled plasma arc

The furnace itself is similar to a steel making furnace, but on a very much smaller scale. For a plant that will process two million catalysts per year the internal diameter is around 1.2 metres and the whole plant can be fitted into around 700 square metres.

As part of the process the pre-treated catalyst material or e-waste is blended with a number of ingredients, including fluxing agents which help the ceramic melt at a reasonable temperature and reduce energy consumption. A collector metal is also added, usually iron or copper. Typically the 'charge' for the furnace is >80% waste and a < 20% mixture of these ingredients. Before the charge is introduced the furnace is preheated with the plasma to a temperature of around 1600°C.

The plasma itself runs from a plasma torch, which is a water cooled metallic device that uses electrical energy to ionise gas and create a plasma arc. A plasma arc is a very intense, flexible heat source with a high temperature capability. "The torch is the cathode and the actual material undergoing treatment is the anode. The arc runs from the torch to the molten material. At high temperatures even rock becomes electrica conductive," adds Deegan.

The process is controlled by a robot which holds the torch outside the furnace and manipulates it by pivoting it via a ball seal connection.

"To visualise the arc you can make an analogy with Darth Vader. If you imagine his light sabre, the torch is the hand held part and the arc is the column of light that extends away from it. The plasma arc from our torches are very much like that. But in the process we actually manipulate the torch so it moves following a circular path within the furnace distributing the heat and the arc extends from the torch and moves with it," explains the technical director.

When processing autocatalyst the plasma arc requires around 450 kW of electrical power and in addition to heat generates a lot of light, particularly UV, which catalyses the destruction of toxic organics such as PCBs, dioxins and furans. But according to Deegan one of the key benefits of the technology is that it's also very controllable, with exceptional turn down ratios allowing for flexibility to adjust the process to suit the waste stream.

The material as it sits in the furnace forms a molten lava, but does not bubble and remains stable. That material reacts so that droplets of collector metal are formed which percolate through the molten bath of material taking the valuable metals into solution. Making another vivid analogy Deegan explains that "it's almost like a high temperature washing process, where liquid metal is used as the detergent".

Output

The process works in a continuous manner, with periodic interruptions, so normally the material addition to the furnace is equal to the speed with which material exits the process on a mass flow basis, while maintaining a certain residency time. The valuable PGMs, gold and silver are periodically collected, mixed in a solution with the iron or copper collector metal.

To extract and purify the individual precious metals, the collector metal needs to undergo final hydrometallurgical refining. In this process the collector metal is dissolved in acid, and subsequent treatments then cause the discreet components to precipitate.

The main product by mass coming out of the process is a material that Tetronics refer to as Plasmarok®, which from the treatment of catalysts consists mainly of the melted ceramic honeycomb. This output has been endorsed by the UK Environment Agency as a product based on its low environmental impact and its reuse credentials.

Economics and the future

The company's customers fall broadly into two categories. The first collect and refine the precious metal and then market the collector metal on the metal exchanges. But the second group, which includes companies such as BASF and Solar, are not just interested in the financial return. They want the precious metals for closed loop recycling back into their own products.

Tetronics has already installed plants across the world, with two operating in the U.S., two in South Korea, operated by BASF, as well as one in Taiwan and is also currently building one in China and one in Japan.

"It is very economically attractive at a small scale because it's technically efficient," says Deegan, also noting that a single tonne of autocatalysts is worth upto $80,000 and that the two million catalysts which a standard size plant could handle per year would equate to around 2000 tonnes. With a growing number of waste catalysts likely to enter the recycling circuit in the coming years, and demand for precious metals looking likely to continue to rise, the company is confident of strong interest in its technology for the recycling of this waste stream.

However, Deegan says that it is also keen to exploit the growing resolve to recover a higher proportion of the valuable materials being discarded in e-waste, as well as exploring the possibility of recovering rare earths.

"E-waste is a wholly under utilised stream, especially for the recovery of some precious metals. There's mass recycling, but there should also be value recycling. That's where we would see the application of our technology extending in the future," concludes Deegan.

Ben Messenger is managing editor of Waste Management World
e-mail: benm@pennwell.com


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