Metal Recycling - Coping with Complexity

As products become increasingly complex, so too does metal recycling. For this reason recent report by the UNEP's International Resource Panel has called for a 'Product-Centric' approach.

The increasing complexity of products is making the 'metal-centric' approach to recycling increasingly obselete. Instead there needs to be a shift to 'product-centric' approach

Products are becoming increasingly complex. It is now possible for any one metal to be found alongside almost any other, or any number of other materials in a single, difficult to dismantle product. For this reason a recent report by the United Nations Environment Programme's International Resource Panel has called for a far more sophisticated 'Product-Centric' approach to recycling.

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Metal Recycling - Coping with Complexity

If populations in emerging economies adopt similar technologies and lifestyles as currently found in OECD countries, it is estimated that the amount of metal needed would be three to nine times larger than all of the metal currently used in the world.

If long-term growth trends in population and prosperity are factored in, the global stock of metals in-use by 2050 could be equivalent to five to 10 times today's level - supplies permitting. And as societies and technologies are changing, the demand for some metals is growing much faster than for others.

On average, the metal stocks used in more developed countries equate to between ten and fifteen tonnes per person. Of that five metals - iron, aluminium, copper, zinc, and manganese - make up more than 98%.

Despite the vast available reserves of several industrially important metals, it is clear that the growing world population cannot continue to consume metals at the current rate of western industrialised society, without going far beyond what is likely to be sustainable. For instance, global steel production is estimated to produce the equivalent of 3.6 billion tonnes of CO2 and altogether, metal production represents about 8% of global energy consumption.

As global demand continues to rise, more low-quality ores for many metals are increasingly being mined, leading to increased energy use and thus rising GHG emissions, even with improved extraction methods. Today, depending on the metal concerned, around three times as much material needs to be moved for the same ore extraction as a century ago. Therefore, future metal production must not only focus on ore mining, but also on recycling and the 'urban mine', the stock of metals in use above ground.

However, the 'geology' of the urban mine is complex and unpredictable, making economic predictions difficult.

Increased complexity

Until recently metal recycling was relatively straightforward. As most products themselves were fairly simple, it concentrated on specific metals, following the so-called 'Material-Centric' (MMC) approach.

However, over recent decades products have become increasingly complex. A much wider variety of metals and other materials are in common use, and any single product can contain dozens of them. For recyclers, this has led to a situation where trying to recover one material can often destroy or scatter another. It is clear then, that a 'Product-Centric' approach is needed, where recycling targets the specific components of a product, devising ways to separate and recover them.

For example the increasingly complex composition of End-of-Life Vehicles (ELVs) now includes a multitude of interlinked materials: commodity materials (pure metal, alloys and compounds, such as steel, copper, aluminium, zinc and nickel), plastics, rubber, and scarce elements identified as 'critical' for the future economy.

ELV recycling now commonly has to deal with over 50 elements. This has significant implications for metal purity and makes the recovery of metals and materials increasingly difficult. It also leads to the intertwining of different metal cycles. ELVs are not unique in this regard; electronic and other waste streams show a similar trend.

While on the face of it different appliances may contain similar suites of functional materials, depending on the product and the combinations of materials, the recovery of metals may be different due to chemistry, concentration and metallurgical processes being incompatible. Physical and metallurgical recycling technologies and processes exist for the separation of many metals, but each process flowsheet will deal differently with metal mixes. Therefore, forced recycling-rate quotas, especially for the minor metals are a fallacy, and the focus should instead be on maximising recovery of the elements.

Limiting Factors

The reality is that many products contain several metals, as well as their alloys and compounds. To tackle this, several 'recycling chains' are necessary from End-of- Life (EoL) product to metal. This creates a multi-dimensional system, whose level of complexity must be clear to stakeholders and policy makers.

In the recycling process, EoL products are usually broken into small pieces, and a first attempt is made to sort the different mixed materials. However, this is generally only partly successful. For example, materials that are attached to each other for functional purposes will often stay close together. Therefore, most metals enter metallurgical processing as a mix, often rather a complex mix.

When such metals and their compounds have compatible thermodynamic and physical properties, the metallurgical processing technology used will succeed in economically separating them. If not, mixed alloys, sludges, slimes and slags are produced, wasting the contained resources and creating an additional dumping or storage cost. The degree to which these metals can be separated, thus affects the economics of recycling.

For example, because of their chemical and thermodynamic properties steel recycling can cope only to a certain extent with copper, tin and antimony in the input streams. The removal of these elements during the production of high-quality steel poses a formidable challenge.

Commonly, the only way to cope is to dilute them into concentrations that are tolerated by alloy requirements. Sorting helps, but the steel scrap has to be de-tinned and de-galvanized before being fed into the smelter. Plastics containing antimony flame retardant must also be removed to alleviate this problem.

However, complex post-consumer scrap such as WEEE can contain 50+ elements at the same time, many of which cannot be dealt with during steel making and thus cannot be recycled. While non-ferrous metallurgy can cope with these elements, as the name suggests, the iron will then be lost as FeO in the slag.

Policy drivers

Many of the world's existing recycling policies have grown out of environmental policies, and are often still under the control of environmental ministries. While this reflects the potential environmental benefits from increased recycling, it can also obscure the fact that recycling is primarily an economic industrial activity.

Waste and recycling policies directly affect the cost of recycling processes as well as the cost of alternatives such as waste disposal. These policies also influence the availability and composition of waste streams for recycling. Trade restrictions, taxation, labour regulation and energy costs also play their part.

The balance of costs and benefits from these policies determine whether recycling is more or less profitable than the alternatives, or even to what extent individual substances are recovered from complex products.

However, government policy can have a very significant positive or negative impact on increasing recycling. It can influence the economics of any part of the recycling chain, changing the economic viability of the whole chain, or of any part of it, as well as provide the incentives and means for stakeholders to exchange information and cooperate.

Recommendations

Defining the system boundaries for which targets are stipulated is of critical importance. Establishing weight-based, product-recycling rates for all individual trace and critical elements is impossible. Therefore, policy should focus instead on well developed, Best Available Technology (BAT) Carrier Metal-recovery systems. A BAT infrastructure, once in place, will operate by itself to maximise the recovery of all critical elements with an economic incentive.

In addition, priorities have to be set for different metals, such as base metals, special metals, critical-technology metals, etc. This further highlights the dilemma of defining recycling targets for metals that are present in small quantities in products.

Targets which go beyond what is thermodynamically possible are likely to fail and might lead to excessive energy consumption. Policy makers can set appropriate targets from a life cycle perspective and by drawing on the expertise and tools available within the recycling industry.

Policy makers can be further aided in this with the adoption of a Product-Centric view to help understand the tradeoffs between achieving high recycling targets and natural-resource depletion. Including a Product-Centric view of recycling into the discussion requires thorough rethinking of policies to ensure that resource efficiency is maximised.

Designers too must take up the challenge. Optimal recycling can only succeed through increased physics-based 'Design for Recycling' or 'Design for Sustainability'. Here, product design is based on, or at least cognisant of, recycling BAT. This can be aided through the use of tools such as computer modelling which capture the physics of recycling.

However, none of the recycling stages can be optimised in isolation. Optimisation of the system requires participants to take a wider view and communicate and work with other stakeholders in the system. Policy needs to create the conditions that facilitate cooperation, for example, between the product manufacturer and the recycling operator. The goal must be to optimise the whole system, not just parts of it.

The article was taken from a report by the United Nations Environment Programme (2013) entitled: Metal Recycling: Opportunities, Limits, Infrastructure. The report was produced from the working group on the Global Metal Flows to the International Resource Panel, including Reuter, M. A.; Hudson, C.; van Schaik, A.; Heiskanen, K.; Meskers, C.; Hagelüken, C.

Alloys

The alloying of metals is done to achieve better strength, workability and for better welding, to name a few reasons. Different alloys improve different properties. An alloy typically contains a major component and several other components in much smaller concentrations. Not all alloys of the same main components will be compatible from a recycling point of view, as is shown by the three examples below:

Steel: There are about 5000 carbon-steel alloys, from simple construction steel to ultra- high-strength steel, in addition to steels for high-
temperature and high-wear uses. Stainless steels also come in six major alloy groups.

Aluminium: Aluminium is alloyed into seven main wrought alloy groups with varying properties, suitable for making such different products as airplane
frames, beverage cans, engines, electrical cables and foils. Aluminium is also used in several types of cast-alloy grades.

Copper: Alloys of copper fall in eight main groups, the most common ones being with zinc (brass) and tin (bronze), and together as gunmetal. Hoever, copper
alloys have moved from the age-old bronze and brass to now include more than four hundred different alloys, with different alloying metals creating
different properties of the Cu-alloys.