An Integrated Materials Valorisation Scheme for Enhanced Landfill Mining

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An integrated materials valor isation scheme for EnhancedLandfill Mining

Tom VAN GERVEN1, Daneel GEYSEN2, Yiannis PONTIKES2, zlem CIZER3,

Gilles MERTENS

4

, Jan ELSEN

4

, Koen VAN BALEN

3

, Peter Tom JONES

2

, BartBLANPAIN21Department of Chemical Engineering, K.U.Leuven, 3001 Leuven, Belgium2Department of Metallurgy and Materials Engineering, K.U.Leuven, 3001 Leuven, Belgium3Department of Civil Engineering, K.U.Leuven, 3001 Leuven, Belgium4Department of Earth and Environmental Sciences, K.U.Leuven, 3001 Leuven, [email protected], [email protected],[email protected], [email protected],[email protected], [email protected],[email protected], [email protected],[email protected]

AbstractThis contribution gives an overview of how materials valorisation fits into the overall schemeof Enhanced Landfill Mining and shows its relations with energy recovery, separation ofmaterial streams and carbon sequestration. As such there is a close link with thecontributions of Lieve Helsen and Anouk Bosmans; Mieke Quaghebeur et al. and BenLaenen and Peter Van Tongeren, elsewhere in these Conference Proceedings. In additionsome novel technological methods and considerations will be highlighted, which underlineELFM as a holistic and innovative route for materials recycling. Finally, two recent initiativeswill be briefly described as they illustrate the approach that is needed to cope with thisinterdisciplinary challenge.

Materials valorisation as part of ELFMELFM consists of two essential technological components. The first one is energyvalorisation, a.k.a. Waste-to-Energy, WtE. This is in fact the part that has been realised inmany landfills where landfill mining is performed. Indeed landfill mining is often not more thanmethane recovery from the landfill gases and subsequent conversion into electricity. It isclear that WtE can be much more than only methane recovery (see the contribution ofHelsen and Bosmans. in this book). A fully fledged WtE gives rise to energy production(electricity, syngas) with solid residues, CO2 and heat as by-products. The other componentis materials valorisation, a.k.a. Waste-to-Materials or WtM, leading to new materials that can

be re-introduced in the economic cycle. In recent years this has been attracting more andmore attention in scientific literature1-4 and through commercial trials. Examples of the latterare presented in the annual Global Landfill Mining Conference. However, this Conferencealso shows the vulnerability of the landfill mining concept to the economic context. The 2ndGlobal Landfill Mining Conference, which was due in October 2009, was postponed to 2010because of the collapse in global commodity and waste material prices. This vulnerabilityunderlines the need for an integrated view on landfill mining with optimal valorisation ofstreams. The word optimal refers in this respect to 1) achieving near 100% recycling, and 2)recycling in as-high-value-as-possible options. This double goal often requires a balancedtrade-off. Therefore, in ELFM the two basic components (WtE and WtM) need to besupported by state-of-the art separation technology and decision tools. The latter include a.o.Life Cycle Analysis and Life Cycle Costing and have to be flexible to account for a dynamic

economic and societal environment.


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Figure 1 shows the relation between WtM, WtE and the separation/decision stage. Whilesome of the materials can immediately serve as new resources after separation, the majoritywill have to be treated in the WtM section. WtE inevitably produces solid residues that willflow into the WtM. Also a (small) part of other outputs of WtE, CO2 and heat, can be used inthe WtM section. The majority, however, has to find other outlets, such as undergroundinjection of CO

2, use both heat and CO

2in greenhouses for the production of vegetables,

and possibly in future the chemical reduction of CO2 to fuels and/or materials.

Figure 1: Overall scheme of ELFM

Novel components of a materials valorisation scheme for ELFMIn this section we will take a closer look at possible components of the WtM plant. The focuswill be on technologies and tools that are being investigated in the domain of solid wasterecycling, but are new in the context of landfill mining. The discussed topics are a selectionand constitute by no means an exhaustive or closed set of options.

Alternative raw materials for cement

Production of Portland cement, an essential building material for the construction industryworldwide, is a significant source of energy-related and process-related CO2 emissions. Thesuggested figures for cements industry footprint vary between 5 to 7% of the globalanthropogenic CO2 emissions.

5 Process-related CO2 emissions result from the dissociation

of limestone (CaCO3), the primary raw material for clinker production, to calcium oxide(CaO), an intermediate product in the clinkering process, and to CO2. The final product istypically heated to a temperature in the region of 1450C and the energy requirement isbetween 3,2 106 kJ /t and 3,5 106 kJ /t for dry kiln systems with 4/5-stage preheating.6

As part of the transition to low-carbon closed loop economies, the development of newcement types from alternative CaO-rich input streams with low-energy requirements is thusbecoming of utmost importance to reduce the CO2 emissions from clinker productionconsiderably. Other strategies are also being investigated, namely decreasing the clinker tocement ratio (by introducing Supplementary Cementitious Materials) or producing non-Ordinary Portland Cements. In the latter direction, considerable work is being done in thedirection of reactive belite-rich cements.

landfill

separation

& decision

WtEWtMsolid

residue

new

materials

syngas,

electricity

CO2,

heat

CCS,

greenhouses


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Metallurgical slags, such as steel slags and stainless steel slags, are typically rich in CaO (upto 70 wt%) and 2CaO.SiO2(- or -dicalcium silicate, up to 80 wt%). This renders them intomaterials with a high potential to form cementitious systems and thus would allow thetransformation of these waste residues into products.7-8 This approach is the currentresearch domain of the authors in several projects embedded within the SMaRT-Pro

Industrial Knowledge Platform on waste valorisation, where the complex chain from high-temperature processes to final products, including a reverse engineering approach (see inthe following section), is taken into account. It is currently being investigated if stainless steelslags can be transformed into a new cement type or can be used as additives along withPortland cement, forming blended cements. Alternatively, it will be investigated if these slagscan be used as the primary input materials in clinker production with the target of producinglow-energy and low-CO2 belite-rich binders. Probably, the most interesting opportunity arisesfrom the ability to alter the chemistry of the slags at high temperatures and to adapt thecooling process. These actions can drastically modify the mineralogy of the final slag andthus produce a better material for subsequent use.

In the context of Enhanced Landfill Mining in which various waste streams are considered,

the separation of these metallurgical slags and the improvement of the waste properties interms of heavy metals leaching and reactivity are the major challenges in formingcementitious systems. Cement today is also made by combining different wastes as a sourceof energy. There as well, heavy metal content is a topic requiring much attention. The heavymetal content and heavy metal availability of the different ELFM materials should beconsidered. Cement is known to be able to reduce the availability of several metals. Thework at K.U.Leuven will envisage cement making from ELFM materials with low or noenvironmental concerns in its application.

Alkal i-activated binders out of waste

Propelled by the same concerns as in the previous section, new materials are being

investigated to replace cement. Alkali-activated binders, produced by alkali-activated silica-and alumina-containing materials such as kaolinitic clays, metakaolin or industrial waste (flyash and blast furnace slag), seem to have a high potential9-10. The concept of alkali activatedbinders was first developed in 1940s with the work of Purdon on blast furnace slag and alkalicombinations. Major contribution came from Davidovits who developed and patented abinder obtained from alkali-activated metakaolin called geopolymer that is comprised of anamorphous to semi-crystalline, silico-aluminate polymeric network.11 This was then followedby the developments of alkali-activated (pozzolanic) cements and geopolymeric cements,including ordinary Portland cement and geopolymers in the composition. Alkali-activatedcements show lower porosity, higher early strength and higher durability (i.e. higherresistance against corrosive environments) than Portland cement due to the formation ofzeolitic compounds (sodium alumino-silicate hydrates). These cements are also much better

in stabilisation/immobilisation of hazardous and radioactive wastes.12 Alkali-activatedcements from blast furnace slag, phosphorous slag, nickel and copper slag were fullystandardised in the former Soviet Union.

Alkali-activated cements are mainly produced in two ways13: (1) activation of calcium andsilica rich materials, such as the activation of blast furnace slag under relatively moderatealkaline conditions, giving the main reaction products of calcium silicate hydrate (C-S-H),being similar to the one produced in cement hydration; (2) activation of materials with lowCaO and high silica and alumina content, such as metakaolin or type F fly ash, using solid orliquid alkaline activators under high alkalinity and high temperatures (60 to 200C), formingalkaline inorganic polymers of zeolitic compounds. Another option is alkali-activated blendedcements with low Portland cement clinker (< 30%) and with high mineral additions (> 70%),giving a composition rich in silica, calcium and alumina. The exact reaction mechanism ofalkali activation is not yet understood thoroughly because it depends on the composition of


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the primary materials and on the alkaline activator. However, a three-step model ofdissolution of raw materials, transportation or orientation and hardening via polycondensationof the reaction products is suggested to occur.

Waste residues from high temperature processes such as waste incineration, ferrous- andnon-ferrous metallurgy are characterised by their oxidic nature. Some have high silicacontent whereas others have high alkali content. Due to the high temperature at which thesematerials are produced and as a result of the fast cooling, the siliceous phases can be glassyin nature and thus easily soluble in alkali, i.e. they are reactive. That particular characteristicmakes them interesting raw materials for alkali activation. In this way, the leaching problemof the elevated heavy metal content in these materials is tackled by applying them as binder,making their reuse possible. So far, investigations on alkali-activated binders include reactiveprimary materials such as metakaolin and fly ash. Alkali activation of less consistent (in termsof composition) waste materials will not give the same reactivity and final product quality. It isalso argued that parameters related to alkali activation of metakaolin cannot be applied towaste materials because not all silica and alumina are reactive. A particular interest hasrecently arisen in this topic.13 In-depth research is required in this promising domain with the

main goal of transforming these waste residues into alternative binders having performancecomparable with or even higher than Portland cement.

At the Remo landfill site (see the contribution of Yves Tielemans and Patrick Laevers in thisbook), MSW bottom ashes, fly ashes, APC residues, non-ferrous slags and stainless steelslags are present. Besides, the new plasma technology introduced seems also promising indelivering a final product, with high silica content and highly glassy, i.e. reactive, that couldproduce geopolymers.14This wide array of wastes necessitates innovative research becausealkali-activation studies have been focused on blast furnace slag and fly ash from electricpower plants (from coal combustion). It will be investigated how an alkali-activated bindercan be made from ELFM materials and which ELFM residues are suitable. Moreover, it willbe investigated how the reactivity of the materials can be enhanced, e.g. by

mechanochemical activation through milling. Because the use of pure alkaline activators isexpensive, low cost alkali waste streams will be studied. Alkali activation can be used tomake monolithic materials such as bricks or concrete type blocks.

Mineral carbonation of alkaline materials

Mineral carbonation is the reaction of alkaline minerals with atmospheric carbon dioxide toform carbonate minerals and is one of the major processes in the long-term global carboncycle.15 The process is being investigated in the domain of solid waste management forseveral reasons. First, it is the most important reaction in the ageing process of alkalinewaste materials and, therefore, it is relevant to know what the effect is of mineral carbonationon the properties of the waste. In many cases carbonation appears to have beneficial effects,

and then it becomes interesting to accelerate mineral carbonation. Secondly, carbonation ofselectively separated waste streams may generate pure carbonate materials with high-valueapplications. Finally, mineral carbonation also implies CO2 sequestration.

Many ashes from municipal solid waste incineration as well as slags from metallurgicalprocesses contain high amounts of calcium and magnesium, either in oxidic phases or in theform of silicates, aluminates, etc. These phases are prone to carbonation, with the freeoxides being the most reactive. These materials also contain troublesome heavy metals suchas Cu, Pb, Zn, Cr, As, etc. It has been shown that leaching of several of these metals suchas Cu, Pb and Zn decreases by carbonation.16-17 For other elements such as the oxyanion-forming Cr, Sb and Mo, carbonation can be steered towards an increase or decrease ofleaching16,18 (Figure 2). In addition, the compressive strength of these materials seems toincrease dramatically by the effects of carbonation. Values of several (tens of) MPa have


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been reported.19 In the context of Enhanced Landfill Mining the improvement of wasteproperties is probably the major driver for the application of this technology.

In particular waste streams the amount of Ca and/or Mg may be so high that it becomesworthwhile to investigate the separation of these cations followed by carbonation in view ofthe production of granulated calcium carbonate (GCC), precipitated calcium carbonate (PCC)or the Mg counterparts.20 These high-value products are used as a supplemental ingredientin pharmaceutical, food and fertilizer

Figure 2: Effect of accelerated carbonation on leaching of Cu (left) and Cr (right) from MSWIbottom ash16. The different data points at the same carbonation time indicate differentprocessing conditions.

industry, and as filler or coating agent in the polymer, paper and paint industry. To achievecommercial application, however, much improvement on the separation process needs to bemade.

With increasing concern for climate change and rising CO2 prices sequestration will gainimportance in the next decades. The CO2 sequestration potential of mineral carbonation isundoubtedly low (typically one or so percent of the CO2 emitted by waste incinerators couldbe sequestered in the residues it produces; higher values may be possible in themetallurgical sector). However, it can be seen as a first and easy step for a company inlowering its CO2 emissions. The biggest hurdle in achieving maximum sequestration is theslow and incomplete reaction due to the fact that is a surface-related process. The finer thealkaline material, the more efficient carbonation will proceed. Therefore, investigation isfocusing on improving conversion while avoiding energy-intensive milling as much aspossible.21

Recovery of scarce elements

A rather new topic that is arising is the scarcity of several of the elements present in theperiodic table. The situation is described in the excellent position paper of Dodson et al.22 Itappears that while many carbon-low technologies are developed, these are all using thesame rare and precious elements. And in contrast to biological materials, the availability ofthese materials is finite. Availability is also not only a technological story, there is also a geo-political aspect to it. Mineral reserves are located in certain areas which appear to becontrolled by an astonishing small amount of actors. China for example now provides morethan 95% of the worlds supply of rare earth metals.

Landfills can be an interesting source for these scarce metals. In J apan it appears that moregold and silver has been accumulated in goods and waste than there is currently present in

the reserves of the richest resource possessing country for each of these elements.23 Alsohuge amounts of indium, tin, tantalum, platinum and lithium have been collected. Three times

0,5

5,0

1 10 100

leaching(mg/kgdrymatter)

carbonation time (h)

Cu

0,1

1,0

10,0

1 10 100

leaching(mg/kgdrymatter)

carbonation time (h)

Cr


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more gold, silver and indium are contained in J apans waste than the world uses in a year.When comparing the concentration in different waste streams ending up in landfills with themetal content in ores (Figure 3), it becomes incomprehensible that countries are willing toship their waste abroad and even pay for it. Exact information on concentration in Europeanlandfills remains scarce.

Although the total concentration of scarce elements in ore and solid residues can becomparable, an important drawback of the solid residues is that in most cases the desiredelements are present in many different mineral phases and occur together with many otherbut similar (from a chemical point of view) elements. This makes it difficult

Figure 3: Total concentration of selected metals in residues of thermally treated municipalsolid waste and comparison with concentration in crude ore (Halada et al.24)

to extract them both completely and selectively. However, many investigations are beingconducted on (selective) hydrothermal, pyrometallurgical and biotechnological extraction ofscarce elements from specific waste streams. More work on the recovery potential fromlandfills is desirable.

Hot-stage engineering for a better product

A part of the materials input in the WtM section comes from a thermal process (WtE). In this

case it can be useful to study the possibilities of hot-stage engineering, which reflects every

activity performed in the hot (even liquid) stage of the slag production with the aim of steering

the properties of the cold, solidified product to a desirable direction. Although not extensively

practised, a number of case studies illustrate the potential of hot-stage engineering and how

it could redefine established operations. A review paper has been published recently.25

Hot-stage engineering can involve: a) additions during the molten state of the slag for

reduction and separation of a metallic phase or stabilisation of minerals (and complementary

additions to secure the dissolution of the materials added) and b) selection of appropriate

cooling paths to deliver the desirable product.

Several stabilising agents have been investigated to deal with the volume instability of

stainless steel decarburisation slag caused by the to transformation of C2S (2CaO.SiO2)leading to disintegration and dust formation. The option of inhibiting this transformation was

0,1

1

10

100

1000

10000

100000

Zn Pb Cu Sn Sb Ag Bi Ga Ge In Pd Te Tl

Concentr

atoin(ppm)

Element

Incineration fly ash

Gasification fly ash

Crude ore


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first elaborated in 1986 by Seki and co-workers26, who developed a borate-based stabiliser

for stainless steel decarburisation slag. Typical boron minerals are kemite (Na2B4O7.4H2O),

colemanite (2CaO.3B2O3.5H2O) and borax (Na2B4O7.10H2O) whereas lately, boron-

containing glazing powders27 with promising results were also used. Adding a relatively large

amount of silica is an alternative way to avoid C2S. This was proven on a laboratory scale bySakamoto28, who stabilised a stainless steel decarburisation slag with 12 wt% of waste

glass, containing 70-75 wt% SiO2. However, an additional slag treatment process is probably

required in order to dissolve the waste glass. This is also the case in the treatment of volume

instability caused by expansive hydration of free CaO and MgO to Ca(OH)2 and Mg(OH)2.

Kuhn et al.29-30 developed a process where additives are introduced in order for free CaO

and MgO to react towards a stable matrix of calcium silicates and ferrites. This can be

achieved by the addition of SiO2-containing materials, such as quartz sand, glass cullet and

spent foundry sands. The treatment with quartz sand offers the advantage of higher SiO2

content per mass of additives and is not introducing other components that can cause side

reactions. Oxygen is required for the treatment process in order to supply additional heat by

the oxidation of FeO to dissolve the added sand totally and to keep the slag liquid. In Figure

4 a schematic drawing of the process is presented.

Besides obtaining a cold product with the required technical properties, hot-stage additions

can also help in reducing the release of troublesome components by leaching. Mudersbach

et al.31 show that it is possible to improve both the technical stability of Electric Arc Furnace

(EAF) slag and the immobilisation of chromium in the slag by additions of materials which

decrease the basicity and favour the formation of spinel type phases during solidification.

Even if the slag contains high chromium contents, the leaching of chromium can besuppressed. The authors developed the so-called factor sp to empirically describe the

expected chromium content based on the slag composition, showing that there is a

correlation between the spinel factor and the actually measured chromium leaching levels

(which seem to confirm that spinel

Figure 4: Scheme of the process for dissolving a large quantity of SiO2 (~10 wt%) in carbonsteelmaking slags.

slag pot

slag

SiO2O2


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behaves, in practice, as a stable phase with respect to chromium leaching). Based on thisrelation, the authors propose different types of additions which should mitigate any chromiumleaching problems from EAF slags

Finally, rapid cooling has been shown to result in a glassy slag, potentially encapsulating partof the leachable elements in the matrix and thus lowering the total leaching ability.Experiments showed, however, that glass formation in water granulated steel slag samples isinsufficient to prevent leaching.32 The differences between the original slowly cooled andgranulated slag samples were low. Further development is required.

An interdiscipl inary approach towards materials valorisationIn many optimisation schemes for valorisation materials recycling is only one of the essentialbuilding blocks. As shown before energy recovery is often another one, and decision toolsare required to choose between them. Other important building blocks pay attention to thenon-technological issues: economics, legislation, multi-actor processes, etc. Coping withsuch a complex web of interactions requires an interdisciplinary consortium to bring forth

robust solutions. SMaRT-Pro and CR are two such consortia that are (co-)coordinated bythe K.U.Leuven.

The SMaRT-Pro Industrial Knowledge Platform

This Platform is funded by the Industrial Research Fund of the Flemish Government. Itsmission is to develop promising fundamental research on the sustainable valorisation of high-temperature and other inorganic residues into application-oriented proof-of-concept. Threeresearch institutions (K.U.Leuven, HUB and KHBO) bring together expertise in the domainsof chemical engineering and technology, metallurgical and materials engineering, buildingmaterials and technology, geology and applied mineralogy, economy, psychology and law.

They collaborate with industry, investment funds, governmental bodies and civil society

actors on specific topics such as mineral carbonation, sorbent synthesis, constructionapplications with waste, novel cement formulations incorporating waste.a1

CR, international collaboration for resources recovery and recycling

The Center for Resource Recovery and Recycling has a wider scope than the previousconsortium because it aims at both inorganic and organic materials. It consists of severaluniversities in the US and, for the time being, one European university: K.U.Leuven.Partnering industrial companies can tailor project definitions to their needs and share theircosts on a pre-competitive basis. The goal is to incorporate the whole chain of relevantactors from initial product design through manufacture and end-of-life disposition to closing ofloops.a2

ConclusionsA holistic approach is needed in order to achieve robust solutions in the landfill miningconcept: materials valorisation is an essential building block, but it is only one. Others includeenergy recovery, decision tools and non-technological components. Within the Waste-to-Materials context, novel technologies should be taken into account and integrated with state-of-the-art separation methods, achieving both maximum recycling and high-value targeting.Only with an interdisciplinary approach Enhanced Landfill Mining can emerge as a corner-

a1More information can be found on http://www.smartpro2.eu.a2 More information can be found on http://www.wpi.edu/academics/Research/CR3/

index.html


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stone of the waste management system, helping to pave the way for the transition toSustainable Materials Management.

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An Integrated Materials Valorisation Scheme for Enhanced Landfill Mining