MSW slag: optimized recovery of metals

Every ton of waste incinerated in the waste incineration plant (MWIP) produces around 200 kilograms of solid residue, the slag. It consists largely of minerals, but up to 20 percent of it is easily recyclable and valuable metals and glass. This is no longer a secret: since 2013, slag processing for the recovery of metals has been mandatory in Switzerland. However, it is anything but trivial to separate the metals from the slag in as good a quality and quantity as possible. This is precisely the goal of the slag processing plant operated by ZAV Recycling AG at the Kehrichtverwertung Zürcher Oberland (KEZO) site in Hinwil. Compared to conventional processing of wet-discharged slag, it exclusively processes dry-discharged slag. But what does it mean for the environment when metals from the MWIP slag are returned to the cycle? A life cycle assessment study by ETH Zurich has examined this in detail for the first time.

Circulation instead of landfill 

Two mechanisms are decisive for the reduction of the environmental impact (environmental credit): On the one hand, slag processing ensures that the remaining - and landfilled - residual slag is depleted of metals. As a result, in the long run, fewer heavy metals are leached from the landfill into the groundwater. In the article, we refer to this as "reducing landfill emissions." On the other hand, primary produced metals can be substituted by the recovered metals, in the following referred to as "substitution of primary metals". Since the primary production of metals is usually associated with massively greater environmental impacts, such substitution reduces the overall environmental impact. The prerequisite for this is that primary metals and not other secondary metals are substituted, which can be assumed in the current situation of the world market (IRP 2019).

Survey the material flows 

The LCA is based on material flow data for the metals iron, stainless steel, aluminum, copper, lead, silver and gold. Other recovered metals such as zinc, tin or palladium were not taken into account due to a lack of reliable data. One must be aware: The waste mass flows that pass through a MWIP are heterogeneous and vary both seasonally and temporally. For this very reason, representative material flow data are elementary for the ecological evaluation. They were collected in a measurement campaign in 2017 with a high degree of representativeness and detail:

• What quantities of which metals are recovered and in what quality?
• Which primary metals do they replace and how do they have to be processed for this purpose?
• What impact does this have on the composition of the residual slag?

If the metal flows are expanded with life cycle assessment data, the environmental impact of reprocessing can be evaluated. In this context, the data situation allows a fraction-specific analysis of the metals in order to quantify the contribution of the individual metal fractions to the overall environmental credit. Likewise, the quality of the individual fractions could be taken into account with respect to the substitution effect. The better the quality of the metal fraction separated in the slag processing, the lower the further metal losses and expenses due to subsequent recycling processes - ergo, the greater the environmental credit. The ecological potential always means the environmental credit due to the recovery of the mentioned metals compared to the hypothetical scenario without metal recovery, i.e. with direct slag landfilling. In addition to the impact on climate change (in CO2-eq), eco- and human-toxic effects as well as resource consumption are considered as impact categories.

Realizing the potential 

Overall, metal recovery in Hinwil provides an environmental credit of around 780 kg CO2-eq per ton of dry slag. So what does this mean exactly and how is it made up? First: Compared to the substitution of primary metals, the reduction of landfill emissions is negligible in the case of a considered time horizon of emissions of 1000 years. In this case, the substitution of primary metals by the recovered metals contributes between 75 and 99 percent to the total environmental credit, depending on the impact category considered. A little more light is shed on the contributions of the individual grain size fractions and metals to the substitution credit. In terms of mass, the majority of coarse iron separated with a first magnet dominates, accounting for just under half of the recovered metals. From an ecological perspective, however, a different picture emerges: the metal fractions in the lower grain size range between 0.3 and 12 mm play a central role ecologically despite their low mass share of around 15 percent and, depending on the impact category, contribute between 29 (climate change) and 64 percent (resource consumption) to the total environmental credit of primary metal substitution (see Fig. 1). The heavy nonferrous metals (NF metals) copper, lead, silver and gold are associated with particularly high substitution credits because their primary production causes specifically high environmental impacts. For this reason, it makes particular sense from an environmental perspective to recover these metals. In the case of climate change, aluminum is an additional factor, as primary production causes a good ten times the greenhouse gas emissions compared to secondary production. Since nonferrous metals are often enclosed in sinter slag and located in the deep grain size spectrum below 12 mm, the slag must be broken down to very small grain sizes in order to exploit this potential. Thus, if only the iron were recovered, one would miss out on between 73 (climate change) and 93 percent (resource consumption) of the ecological potential. (see Fig. 1)

Where is the limit?

In the measurement campaign, the residual slag fractions were also sampled. In addition to the mineral content, this contains the so-called residual metals, which were not removed by the processing. Metals that can potentially be recovered if the process is improved. And that is the goal of the operators: "It should be possible to increase the recyclable yield of the climate-relevant metals by 20 percent," says Daniel Böni, managing director of KEZO and the ZAR Foundation. Based on the data, the residual metals can be differentiated into free residual metals and residual metals enclosed by sintered slag. According to this classification, the theoretical limits of metal recovery in Hinwil were additionally investigated in two steps:

• (i) the maximum potential with the current plant configuration by additionally recovering all free residual metals.
• (ii) total residual metal potential by completely breaking down the slag and completely exposing all metals.

The additional energy requirements of the processing plant were not elicited for these cases, but the potentials nevertheless provide a vivid insight into the still hidden ecological potential. Specifically, the additional recovery of all free residual metals would increase the environmental credit by 10 (climate change) to 44 percent (resource consumption), and in the case of complete exposure theoretically by as much as 24 to 89 percent. This is due in particular to the fact that fine-grained nonferrous metals are more abundant in the residual slag. It can therefore be assumed that an increase in the mass yield by a few percentage points contains a significantly greater ecological potential.

A considerable reduction in greenhouse gas emissions 

The fact that optimized metal recovery from the slag brings considerable added ecological value is also evident in the context of the overall environmental balance sheet of an MSWI plant: The processing of the slag and recovery of the metals saves a total of around 140 kg CO2-eq per ton of thermally recycled municipal waste, which accounts for almost two thirds of the credit from the total energy recovery from incineration. If all the slag produced annually in Switzerland were processed at the same metal recovery rate as in the plant studied, this would result in total annual savings of just under 560 kilotons of CO2-eq. By way of comparison, this is roughly equivalent to the annual greenhouse gas emissions of the population of Thun (42,600 inhabitants), including gray emissions. The additional savings compared to the
conventional processing of wet slag could not be calculated to date, since representative material flow data for the metals of these plants are still lacking.

Already today, the reality of the Hinwil plant lies somewhere between the results of the study and the plus-20-percent target. The motto is clear: even metals that are not collected separately and end up in the waste incineration plant should be recovered as efficiently as possible and in high quality. The operators of the processing plant in Hinwil are therefore far from satisfied and see potential for further optimization of the thermorecycling process. The data are on the operators' side: from an ecological point of view, the operation is a success story. ■

Source Notes
Publication: Mehr J., Haupt M., Skutan S., Morf L., Adrianto L.R., Weibel G., Hellweg S. (2020). The environmental performance of enhanced metal recovery from dry municipal solid waste incineration bottom ash. Waste Management, 119, 330-341. https://doi.org/10.1016/j.wasman.2020.09.001

Other sources:
IRP 2019. Global Resources Outlook 2019: Natural Resources for the Future We Want. A Report of the International Resource Panel. United Nations Environment Programme. Nairobi, Kenya.