Mining waste

Materials World magazine
,
4 Aug 2014

Landfills around the world are potential goldmines – at least in terms of the metals and materials that could be recovered from them. But there are barriers to overcome before they can be exploited on a commercial scale. Guy Richards reports.

Official figures show that the UK produces around 290Mt of waste each year, of which roughly half goes to landfill. At this rate, we will have run out of landfill space by 2018. And while finding new sites could be an option, it is hardly a sustainable solution. Meanwhile, in many developed economies there is a growing realisation that the value of the materials and metals buried in landfill could be high – and can only rise as resources become more scarce – as is the re-use value of the land given over to them. It is for these reasons that landfill mining and reclamation (LFMR) is gaining credence.

LFMR is not a new idea. The first such project was set up in Israel in the 1950s, not to recover metals but to obtain soil amendment materials to improve fertiliser levels in citrus groves in the Tel Aviv area. Since then, there have been about 60 projects around the world, although many have been pilot tests or for research, and even then many others were for purposes such as mitigating pollution or reclaiming landfill capacity rather than materials recovery.

A range of barriers is restricting widespread adoption of LFMR. A good illustration of those barriers is contained in a report produced in 2013 by sustainability consultancy Ricardo-AEA for the Scottish Government’s Zero Waste Scotland (ZWS) department into the feasibility of LFMR in the country. Although the report was focused on Scotland, Dr Adam Read, Ricardo-AEA’s Practice Director for Resource Efficiency and Waste Management, and one of the report’s authors, says the barriers to LFMR in Scotland would apply to most other nations.

One issue is the difficulty in understanding the composition of landfill waste. Records for many older landfills, particularly those from before the 1970s, are either non-existent or incomplete, and if the composition is not well understood then it is hard to evaluate the risk and cost of a project. The report says the likely composition could be assessed by reviewing the findings of studies carried out at similar landfills, but even then intrusive investigation and analysis of a potential site – bringing with it further costs – would be needed before a project could be deemed viable.

Another issue is the quality of some of the materials that could be recovered, such as plastics and construction and demolition waste, which is likely to be poor when compared with their counterparts from fresher sources as they will probably be contaminated with soil and other materials, making their separation much harder. However, metals should be the primary and most simple materials to recover.

On this point of recovery, the report warns that while materials could be excavated from a landfill with conventional machinery and associated plant, using advanced separation technologies such as eddy-current separators, induction sorting and infrared sensors to sort waste on a large scale is likely to create unexpected problems with separation efficiency, breakdowns and so on.

As Dr Read explains, ‘This kind of equipment is well proven within the waste industry, but not so for excavated landfill waste, which will typically be far denser, wetter, more variable, more entwined and compressed, and therefore much harder to process using advanced waste sorting technology. So there will be increased wear and tear, which would decrease the viability of landfill mining projects.

‘However, there is intense interest from a range of organisations in LFMR approaches and applications, and it is entirely possible that technologies will develop and that at some point in the future, the practice of LFMR will become more prevalent. Sorting technologies in materials recovery facilities are much more advanced than those applied 15 years ago, and could well prove important for the viability of LFMR in the future.’

Other barriers include the environmental and social issues, given LFMR’s potential for uncovering hazardous waste, releasing landfill gases and odours, and generating dust. These can be mitigated, of course, but the costs of doing so can be prohibitive. On the other hand, potential sources of pollution would be removed, and a viable project would create jobs.

There are also regulatory issues and the need to make the economic case for a project, both of which would need to be considered on a country-bycountry and a case-by-case basis. Dr Read says, ‘The regulatory regimes and material markets may be more or less of an issue from one country to another. [On the economics side] countries with high population densities and limited landfill void will have high land prices, making LFMR more viable, while in countries with manufacturing economies, the local industries will value the buried materials more than elsewhere.’

Read points out that many of the issues are likely to apply in relatively equal measure from country to country within the EU, because the regulatory and policy frameworks are similar and the economic situation is relatively consistent at present. The report concludes that LFMR in Scotland could be feasible, although its viability would depend on factors such as whether it would increase the value of the land and the potential for generating energy – this latter element being a key part of the Remo project.

As it is, ZWS has no current plans to carry out any further research or work programmes on this. A spokesperson for ZWS explained, ‘The findings of this research do not justify us taking a strategic approach to pursuing the systematic recovery of resources from Scotland’s landfills at the present time’.

It is the same picture across the UK, and Ricardo-AEA concedes that there may not be a viable economic case for LFMR on a commercial scale in Britain, or anywhere else for another 15 years, even though it could prove financially worthwhile before then in specific landfills and given the right circumstances. The technology is not sufficiently developed, although if one proposed strand of research is anything to go by then the case for LMFR could reach a tipping point sooner than predicted.

Overcoming obstacles
At the moment, many of the barriers to LMFR at the moment centre on the initial need to excavate the waste, but a collaborative project announced in May 2014 between Cardiff University, the University of Warwick and the University of the West of England will look at ways to recover valuable elements used in a range of applications by leaching them out of the landfill.

This in-situ or heap approach, as used in the mining industry for recovering metals such as uranium and copper, relies on biogeochemical interactions often enhanced by a liquid medium called a lixiviant to recover the desired metal. But as one of the project’s researchers, Dr Peter Cleall at Cardiff University, explains, ‘In our approach we will consider typical mining lixiviants, novel lixiviants based on metal scavenging polymers and ionic liquids, and microbiological approaches for leaching and recovery of elements from key UK wastes’.

As well as examining which lixiviants can be used, the three-year project will also seek to understand and control flow behaviour through various industrial and municipal waste media, not just in landfills but in all repositories, including those containing incinerator and fuel ash. Dr Cleall admits that there are tradeoffs with the in-situ approach, such as a lower rate of recovery, but says the costs, environmental impact and energy use may be far lower.

Not only will less energy be used, but the research also aims to use leaching to extract methane from the lignin component of wood and paper in some repositories. Cleall explains, ‘Lignin is slow to degrade, and lignocellulose compounds are responsible for much of the long-term methane production in landfills. This long tail of production is problematic because the gas concentrations it contains are not high enough to combust through engines or flaring, so it escapes to the atmosphere. However, biotechnological manipulation of in-situ lignocellulose degradation using microbial/enzyme-enhanced lixiviants could increase overall methane yield by making degradation-resistant lignin bioavailable, and prevent methane escaping to the atmosphere.’

It is such research, along with the growing need to husband dwindling resources and lessen the impact on the environment, which will bolster the case for harvesting the riches buried in the world’s landfills. The reality of LFMR may still lie in the future, but that future may now not be so far off.

The Remo enhanced mining project
The Remo project, near Hasselt in Belgium (shown right), is widely regarded as being key to demonstrating the viability of landfill mining in Europe. It is also a showcase for the concept of enhanced landfill mining, developed partly by Remo’s operator Group Machiels, which according to Dr Adam Read of sustainability consultancy Ricardo-AEA differs from ordinary landfill mining in that it tends to include the production of a refuse-derived fuel stream as well as metals and materials recovery.

The site contains about 1Mt of municipal solid waste (MSW) and industrial waste (IW) put there since the 1970s. When initial commissioning is complete, by the end of 2016, the project will begin to mine 200,000tpa, about half of which will go to generating electricity using gas plasma technology for 200,000 households. Of the materials portion, 16% will be directly reclaimed for re-use and 22% reclaimed after further treatment.

Research into the composition of the waste shows that the MSW consists of 34–50% (depending on the age of the waste) paper, textile, plastic and wood, a soil-like fraction of 41–45%, and an average of 10±6% of stones, 1.3±0.8% of glass and 2.8±1% of metals (all w/w). The figures for the IW, going by shredder-type waste only, are lower – 23±8% for paper, textile, plastic and wood, 64±16% for the soil-like material (containing elevated concentrations of heavy metals including copper, chromium, nickel and zinc) and an average of 10±10% of stones, 0.05±0.04% of glass and 3.2±3% of metals. The IW data is to be confirmed with further research.

The €300m project will employ 800 people over its 20-year life and is designed to operate with a 15% profit margin. Eventually, the site will be remediated as a sustainable nature park.