Circular economy potential of inorganic polymers from bauxite residues and other slags: An LCA study

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NTNU Norwegian University of Science and Technology

Master ’s thesis

Circular economy potential of inorganic polymers from bauxite residues and other slags: An LCA study

Master’s thesis in MSc in Industrial Ecology Supervisor: Johan Berg Pettersen

July 2020

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I thank my advisor Johan, who always kept contact with me and provided guidance in times of pandemic. I send my warmest hugs to my friends around the planet and to the cucumber gang, which made this year unforgettable. Finally, I thank my family, who

always sent me more love and support that I could ever ask for.

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Summary

The present study aims at discussing a new possible model of utilisation of bauxite residue, the waste produced from the extraction of alumina from its ore, and other metallurgical slags in the European Union, taking into consideration technological, economic, and environmental factor. The model aims at providing the EU construction sector with a more sustainable, and possible better performing alternative to ordinary Portland cement: alkali activated materials, inorganic polymers, and geopolymers produced from the byproducts of the metallurgical industries in the region. Full-scale utilisation of the resources was set as a goal of the new system, and assumptions were formulated from the results of the literature review.

In order to build a supply model and measure the environmental performance of the new system, a combined approach and different methodologies were used. In subsequent steps, the system of production of bauxite residue-derived geopolymer binder was modelled and mass-balanced, and then used to build a life cycle assessment study. The resource base of BR and copper slag was quantified and mapped, and so was the consumption of OPC in the EU. The economic impact of transport was kept in mind when setting the conditions of the supply model, which was built with a GIS software. Finally, data were elaborated to give a final assessment of the circular economy potential of the new system proposed, in terms of potential supply, substitution of OPC, and avoided deposition of BR.

The study argues that developments in this technology and its upscaling would bring net environmental benefits to the region, and underlines the scale of availability of the waste materials that could be used as inputs. In particular BR, who currently is a envi- ronmentally hazardous material and a cost for the aluminium industry, could become a valuable resource if a more circular model and the principles of industrial symbiosis were applied.

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List of Tables

2.1 Typical weight fractions for the main components of BR. . . 8

2.2 Typical phases present in BR. . . 9

2.3 Typical weight fractions for the main components of copper slag. . . 13

3.1 Composition of the BR-derived activating solution. . . 26

3.2 Average costs per ton-km for heavy duty vehicles (HDV) in Europe for 2005. 35 3.3 Data sources for OPC consumption in the EU. . . 36

4.1 . . . 44

4.2 Consumption of OPC in the EU27. . . 47

4.3 Top 10 local administrative units in the EU27 by consumption of OPC, calculated using population data. . . 48

4.4 BR production per plant and substitution of OPC over the respective supply area. . . 49

4.5 OPC displaced by BR-derived GP by country. . . 49

4.6 Copper slag production per plant and substitution of OPC over the respective supply area. . . 51

4.7 OPC displaced by copper slag derived GP by country. . . 51

4.8 Impacts of the full scale utilisation of BR for the production of GP in the EU27. . . 53

5.1 Distance of transport at which the break-even point between the impacts of GP production and those of its transport is reached. . . 59

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List of Figures

1.1 Waste management hierarchy from the EU Waste Framework Directive. . 2 2.1 An amphirole used in red mud farming.Source: Svendsen (2019). . . 10 2.2 3D-printed GP house in Russia. . . 16 2.3 Industrial process for the production of IP from BR.Adapted from Hertel

et al. (2016) . . . 20 3.1 Four phases of LCAAdapted from: ISO (2006a). . . 24 3.2 Structure of the LCA study. . . 27 3.3 MFA of aluminium production in the EU for the year 2013. All values

expressed inktAl.Source: Passarini et al. (2018) . . . 32 3.4 MFA of copper production in the EU for the year 2013. All values ex-

pressed inktCu.Source: Passarini et al. (2018) . . . 33 3.5 Local administrative divisions in the EU27 as they appear in the degree of

urbanisation (DGURBA) layer. . . 37 3.6 Custom azimuthal equidistant projection used as the custom CRS in the

study. . . 39 4.1 MFA of the BR-derived geopolymer precursor and activating solution sys-

tem, normalised for 1,000kgof precursor. . . 45 4.2 Relative impacts of processes on the selected impact categories, calculated

from the LCIA characterisation. . . 45 4.3 Impacts of processes on the selected impact categories, as found in the

LCIA normalisation. . . 46 4.4 Classification of average per capita consumption of OPC by country for

the EU27. . . 47 4.5 Potential areas of supply of BR-derived GP in the EU27 from active alu-

mina plants. . . 49 4.6 Potential supply of Cu slag-derived AAMs in the EU27. . . 50 4.7 Map of the local substitution of OPC by BR-derived GP and Cu slag-

derived IP in the EU. . . 52

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6.1 . . . 74 6.2 . . . 74 6.3 . . . 75

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Abbreviations

AAM = Alkali-Activated Material

APOS = Allocation at Point of Substitution BAT = Best Available Techniques BFS = Iron Blast Furnace Slag BOD5 = Biological Oxygen Demand BOF = Basic Oxygen Furnace

BR = Bauxite Residue

BREF = Carbon Footprint CAPEX = Capital Expenditures

CF = Carbon Footprint

COD = Chemical Oxygen Demand

DGURBA = Degree of Urbanisation DOC = Dissolved Organic Carbon EAF = Electric Arc Furnace

ELCe = Extraordinary Leuven Cement

EU = European Union

GBFS = Granulated Blast Furnace Slag GGBFS = Ground Granulated Blast Furnace Slag GIS = Geographic Information System

GISCO = Geographic Information System of the Commission

GP = Geopolymer

IP = Inorganic Polymer

JRC = Joint Research Centre LCA = Life Cycle Assessment LCI = Life Cycle Inventory LCIA = Life Cycle Impact Analysis MFA = Material Flow Analysis

NORM = Naturally Occurring Radioactive Materials

OECD = Organisation for Economic Co-operation and Development OPC = Ordinary Portland Cement

OPEX = Operating Expenses REE = Rare-Earth Elements

SFS(-S/-C) = Steel Furnace Slag (from carbon/stainless steel production) TOC = Total Organic Carbon

USGS = United States Geological Survey VOC = Volatile Organic Compound

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Chapter 1 Introduction

1.1 Background

Due to its desirable properties and abundance, aluminium finds application in many sectors and plays an important role in the modern world. Aluminium is currently the second metal, after steel, by volumes of production, which are greater than that of all other non- ferrous metals combined (Kvande, 2015). Global demand for aluminium has been increasing steadily for the past 20 years, and it is projected to keep increasing in the next decades to satisfy the demand in transportation, packaging, construction, and more sectors (World Aluminium, 2020). In the context of anthropogenic climate change and environmental degradation, which represent an existential threat to life on Earth, it is of paramount importance to achieve environmental sustainability in large material cycles (Rockstr¨om et al., 2009; Arrow et al., 1995; IPCC, 2014). These include the aluminium cycle, in which the production of primary aluminium plays a crucial role in terms of environmental impacts.

Primary aluminium has consistently made up around 68% of the global supply in the past 20 years, the rest being provided by recycled, or secondary, aluminum. More than 99% of primary aluminium is produced from bauxite, a sedimentary rock containing 40- 60% aluminum oxide, also called alumina. Alumina is extracted from bauxite through the Bayer process, and then transformed into aluminium metal through the Hall–H´eroult process. Both processes are highly energy- and CO₂-intensive, and produce large amounts of waste materials, posing major challenges to the sustainability of the global aluminium cycle (World Aluminium, 2020; Kvande, 2015).

The Bayer process discards most of the initial ore, which becomes bauxite residues (BR, often referred to as”red muds”), traditionally leaving the alumina production plant as a highly alkaline sludge with a high content of iron oxide, responsible for its red hue, and a number of critical metals. More than 150 Mt of BR are generated every year globally, and 3-4 Gt are estimated to make up the existing global stockpile. In the EU alone, around 2 Mt of primary aluminium are produced every year (European Aluminium, 2020). Due to its high volumes of production, caustic nature, and high water content, BR has always represented a problem in terms of waste management. Alongside the environmental risks

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they pose, as illustrated by the Ajka alumina plant accident in 2010 and at least other 20 recent red mud disasters (Boily, 2012), traditional disposal methods pose challenges in terms of land occupation and forgone opportunities for material recovery (Balomenos, 2018).

From an industrial ecology perspective, the optimization of resource flows is crucial, and a flow as large and as rich in raw materials as the wasted BR represents a valuable resource, rather than a problematic industrial waste. The theme of material recovery is becoming central for companies operating within the EU, as EU policy is undergoing a shift towards a circular economy and EU legislation currently includes many principles of industrial ecology.

The Waste Framework Directive (European Parliament and Council of European Union, 2008), amended by Directive 2018/851 (European Parliament and Council of European Union, 2018), requires that waste legislation and policy of the EU Member States shall apply as a priority order the waste management hierarchy shown inFig. 1.1.

Figure 1.1:Waste management hierarchy from the EU Waste Framework Directive.

The 2015 Circular Economy Action Plan, as explained in Communication 2015/0614 (European Commission, 2015), and the 2018 Circular Economy Package, encourage circularity in production systems. In particular, the Action Plan sets out to encourage and support industrial symbiosis, which allows for the waste or by-products of one industry to become inputs for another. In an effort to do so, the Commission clarified rules on byproducts and end-of-waste status, and supported Research and Innovation Projects relevant to the Circular Economy Strategy through the Horizon 2020 (H2020) programme.

Furthermore, the Communication states that the Commission”will include guidance on best waste management and resource efficiency practices in industrial sectors in Best Available Techniques reference documents (BREFs)”(European Commission, 2015). BREFs were first introduced into EU legislation with the Industrial Emissions Directive (Euro- pean Parliament and Council of European Union, 2010), and include a section, called Best Available Technology (BAT) Conclusions, which sets out legally binding requirements for industrial activities: such BAT Conclusions must be integrated into installation permits within 4 years of their publishing.

As a consequence, companies in the primary aluminium production sector in the EU are increasingly adopting BATs for the treatment and disposal of BR. BAT-treated BR is characterised by lower moisture and alkalinity (lower pH), which make its transportation and disposal safer and cheaper, and present greater opportunities for utilisation (Balomenos, 2018). Technologies for the utilisation of BR exist and have proved successful, but their use is currently very limited and the almost totality of BR generated, both globally and in

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1.2 Research goal and questions the EU, is deposited.

The extraction of minerals and elements from BR is possible, but it only allows for the recovery of a fraction of the mass of BR produced. In order to fulfil the EU policy on waste management, applications that consume large quantities of BR must be be adopted. In this perspective, the most promising techniques involve the utilisation of BR in”road construction, cement manufacture, capping materials, and refractory replacement”(Cusano et al., 2017).

This study will cover a process that allows for 100% utilisation of BR in the production of geopolymers, which are inorganic binders that can substitute cement. This utilisation pathway allows for greater volumes of utilisation of the material, as opposed to similar applications that mainly use BR and metallurgical slags as additives to the raw meal for clinker. Other metallurgical slags can also be converted to GP/IP/AAM with processes similar to that of the BR. Such slag-derived materials represent an opportunity in the quest for sustainability in the construction sector, as the use of locally-available waste as raw inputs combined with well-formulated mix designs can provide local sources of cement substitutes in every area in which metallurgical plants are found Provis (2018). This has important implications when considering the circular economy potential of the process, as its EU-wide application could effectively integrate primary metal production and the construction sector with a better sustainability profile.

Of these metallurgical slags, the present study takes into consideration those that are produced from the main metals in the EU, namely aluminium, as already discussed, copper, and iron. These three metals are the most prominent in terms of volume of production and consumption, and will provide a good start for illustrating the circular economy potential of AAM technologies.

1.2 Research goal and questions

As a general goal, this project aims at exploring and discussing what role could this and similar technologies for the creation of AAMs from metallurgical slags play in the quest for a circular economy in the EU.

In particular, the study aims at answering four research questions:

1. To assess the environmental hotspots of bauxite residue-derived geopolymer production, and its positive environmental impacts due to the substitution of OPC and avoided treatment and deposition of red mud;

2. To quantify and map the availability of resources (bauxite residue and metallurgical slags) and the consumption of ordinary Portland cement (OPC) in the EU;

3. To model the potential full-scale supply of inorganic polymer binders derived from bauxite residue and metallurgical slags as a substitute of OPC in the EU, taking into account economic limitations;

4. To assess the regional circular economy potential of such inorganic polymers, by combining their modelled supply and environmental performance.

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The first research question requires a material flow analysis (MFA) of the production process to define the mass balances involved, which are then used in a life cycle assessment (LCA) study of the production process from cradle to gate, the foregone treatment and landfilling of BR, and the displaced production of OPC. The second and third questions combine data gathering from different sources, to create a data-set of BR and slag generation and OPC consumption, and the use of a GIS software to map the potential supply and demand of inorganic polymers. The fourth question is answered by combining the results of the previous parts.

Transport plays a central role in the supply of construction materials, due to their large volumes of consumption. For this reason, the sensitivity of the transport factor was tested from different perspectives, trying to analyse the break-even point in terms of environmental and economic sustainability.

1.3 Structure

TheBasic Theory and Literature Reviewis divided in five sections. The first one refers to bauxite residue, which is the central material considered in the current study and covers all relevant aspects, regarding: production; typical characteristics and composition; traditional waste management practices in the EU along with the best available techniques;

and a focus on potential utilisation pathways. In the second section the same aspects are covered for copper and ferrous slags, but less in depth, as the modelling and discussions were also focused on BR. Alkali activated materials are then introduced in the third section, presenting their potential application and materials that can provide good precursors, with a focus on BR and metallurgical slag. Some general considerations on the economic sustainability of the process are also included. The fourth section brings the focus to the technology for BR-derived geopolymers on which the environmental assessment was performed, and its proof of concept in literature was presented along with its real-world application (existing projects based on this technology). Finally, a literature review on the existing studies on environmental sustainability of similar technologies was conducted and presented in section five.

Methodologycontains a detailed description of the methods and procedures used to answer the research questions, including intermediate results, sum input matrices, and calculations. Analytical solutions from simple calculations are included, and presented again in the results. The environmental assessment methodology used, the LCA method, was first presented in general terms referring to the ISO standards respected in the study.

The methods of the environmental assessment of the chosen production system were then presented in the second section, including the system definition, mass balancing, and set- tings of the LCA study. The third section covers how the resource base was quantified and how geospatial data were retrieved, referring to BR and copper slag generation in the EU, whereas the fourth section includes the quantification and georeferencing of OPC consumption. The fourth section also includes all assumptions made for modelling the conversion of the mapped resources into IP, their supply to the local construction sectors, and the subsequent displacement of OPC. Finally, this section presents the GIS modelling procedures, and the subsequent elaboration of the outputs of the GIS model.

TheResults and Analysischapter presents the results for the first part of the study,

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1.3 Structure including the demand of OPC and the MFA of BR-derived GP production, which are used as inputs for the LCA and GIS models. The results of these two models are presented separately, followed by the results reached by combining the two approaches. The section on the LCA results includes the mass-balanced production system, the structure of the LCA, the complete inventories, and the results of the impact assessment. In the mapping section, the results on OPC consumption are first presented, followed by the mapping of GP and IP supply and substitution to OPC, presented both separately and combined. The last results refer to the combined approach.

TheDiscussionincludes remarks on the basic assumptions of the study, the sources of uncertainty, the interpretation step of the LCA and discussion of the results of the GIS and combined LCA-GIS results. In this LCA interpretation section, the choice of impact categories is briefly discussed, followed by the interpretation of the LCIA results, other observations, and a discussion on the role of transport in the overall sustainability of the process. Relevant aspects of the mapping exercise are discussed, as well as the results of the modelling. Finally, the combined approach is presented, focusing on its magnitude and applicability.

Inconclusions, the study is wrapped up and the possible direction of future research is outlined, focusing on the aspects on which the author believes larger margins of improvement are possible.

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Chapter 2 Basic Theory and Literature Review

2.1 Bauxite residue

Bauxite residue (BR), or red mud, is the main byproduct of the Bayer process, a hydrometallurgical process used to extract alumina (Al₂O₃) from bauxite ore. Bauxite is a sedimentary rock comprised of alumina, silica (SiO₂), iron oxide (Fe₂O₃), titanium oxide (TiO₂) and a number of other compounds, some containing REE and other critical metals and, depending upon the quality of the ore, between 1.9 and 3.6tof bauxite is required to produce 1 tonne of alumina (Pontikes, 2005). The Bayer process consists of 2 main steps (Habashi, 1995), namely:

1. The digestion and pressure leaching of bauxite with a concentrated sodium hydroxide (NaOH) solution to obtain a sodium aluminate (Na[Al(OH)₄]) solution (green liquor);

2. The precipitation of pure aluminum hydroxide from this solution by seeding with fine crystals of aluminium hydroxide (Al(OH)₃).

After the pressure leaching, the pregnant liquor, whereas residues are separated and treated, thus becoming BR, a waste slurry containing all the undissolved components of bauxite, some unrecovered green liquor, rich in sodium hydroxide, and other supplementary elements added during the bauxite refining process, which include Ca and alkali.

After the first step of the Bayer process, most of the alumina and silica are in solution in the green liquor, which is separated and proceeds to the second step. The residue underflow is washed in counter-current decantation washer trains in order to recover sodium aluminate and sodium hydroxide. Sodium aluminate is the desired byproduct at this stage of the process, and part of what was precipitated from the saturated solution is thus recovered and sent back to production. Sodium hydroxide represent a major cost of the production process, and plants aim at maximizing its recovery, reaching an industry standard of ¿96%

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of the material recovered. The washed BR is then further treated and optimised to meet the requirements of the planned disposal techniques. Usually, the solid content is increased by sending the underflow to a thickener or a filter (Dentoni et al., 2014; Power et al., 2011).

It is estimated that for 1 t of alumina, 0.9-1.5 t of BR dry matter are generated (Balomenos, 2018; Dentoni et al., 2014). These high waste generation rates, combined with the sustained levels of production of alumina in the EU, suggest very high output levels, in the order of the millions of tonnes per year.

2.1.1 BR characterisation

The BR slurry has a high moisture content, as the solid composition ranges between 8-36%

by weight. It is characterised by slow settling, as its fine median particle size is 5-10µm.

Mixed with the silty mud there is a coarse fraction with particles larger than 100µmand with a generally high content of silica, sometimes called sand residue. BR is thixotropic, a property which describes higher viscosity under static conditions, and a more fluid- like behaviour when subjected to movement or stress (Hammond, 2014; Evans, 2016).

Furthermore, the sodium hydroxide and soluble sodium compounds (sodium aluminate and sodium carbonate) remaining in the slurry after the pressure leaching gives BR its high pH of 9.2–12.8 with an average value of 11.3, making it strongly alkaline (Gr¨afe et al., 2011). All these characteristics are crucial in the waste management of BR.

BR usually contains varying quantities of alumina (Al₂O₃), iron oxide (Fe₂O₃), quartz (SiO₂), titanium dioxide (TiO₂), calcium oxide (CaO), and sodium oxide (Na₂O). Smaller fractions of vanadium pentoxide (V₂O₅), manganese oxide (MnO), and value elements including Rare-Earth Elements (REE), are also found in low concentrations. These elements include Ga, Sc, Nb, Li, V, Rb, and Zr (Lima et al., 2017; Sutar et al., 2014). Finally, naturally occurring radioactive materials (NORM) like Uranium and Thorium as well as heavy metals and other toxic elements are found in trace concentrations (World Aluminium and European Aluminium, 2015). The general composition of BR, as found in Khairul et al.

(2019), is presented inTable 2.1. The actual composition varies according to the quality of the bauxite ore, the location of extraction, and the different parameters of the Bayer process. However, the main elements are generally found in a number of mineral phases, presented inTable 2.2(World Aluminium and European Aluminium, 2015).

Composition Weight %

Fe₂O₃ 30-60

Al₂O₃ 10-20

SiO₂ 3-50

Na₂O 2-10

CaO 2-8

TiO₂ (trace)-25

Table 2.1:Typical weight fractions for the main components of BR.

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2.1 Bauxite residue

Sodalite (3 Na₂O · 3 Al₂O₃· 6 SiO₂· Na₂SO₄) 04 - 40

Goethite (FeOOH) 10 - 30

Hematite (Fe₂O₃) 10 - 30

Magnetite (Fe₃O₄) 0 - 8

Silica (SiO₂) crystalline and amorphous 3 - 20 Calcium aluminate (3 CaO · Al₂O₃· 6 H₂O) 2 - 20

Boehmite (AlOOH) 0 - 20

Titanium dioxide (TiO₂) anatase and rutile 2 - 15 Muscovite (K₂O · 3 Al₂O₃· 6 SiO₂· 2 H₂O) 0 - 15

Calcite (CaCO₃) 2 - 20

Kaolinite (Al₂O₃· 2 SiO₂· 2 H₂O) 0 - 5

Gibbsite (Al(OH)₃) 0 - 5

Perovskite (CaTiO₃) 0 - 12

Cancrinite (Na₆[Al₆Si₆O₂₄] · 2 CaCO₃) 0 - 50

Diaspore (AlOOH) 0 - 5

Table 2.2:Typical phases present in BR.

2.1.2 BR waste management overview

Waste management strategies for BR vary greatly over time and location, as different treatment, disposal, and utilisation techniques are chosen depending on economic and technological factors, as well as environmental regulations and corporate policies. In general, the waste management model of BR is fundamentally linear, as the waste in almost all cases undergoes minimal treatment and leaves the production system to be disposed of through deposition. Some general trends of improvement in BR management strategies can be identified at a global level. These trends, however, are more consistent and homogeneous when considering the EU alone, due to the harmonised legal framework, common policies, and integrated markets.

According the the European List of Waste, BR is classified as a non-hazardous waste, with the possibility of becoming inert waste if the moisture content is reduced to levels that make the residue leachability absent or negligible. In both cases the residue is expected to be landfilled according to Directive 1999/31/EC (Council of European Union, 1999), with differences regarding leaching control and prevention measures, or utilised if possible.

Furthermore, the introduction of legally binding BREFs for companies operating in the EU established a number of best available techniques (BAT), that need to be gradually implemented by the alumina production industry as a whole.

As found in the BREF for the Non-Ferrous Metals Industries (Cusano et al., 2017), all alumina refineries ”should aim to minimise the alkalinity of the residue disposed of and maximise the solids content,” although the technical recommendations need to be tailored specifically for each plant, depending on its location, technologies used, resource availability, and any other factors. In case of proximity to a plant with high carbon emission (e.g., an ammonia production plant or an aluminium smelter), the BR alkalinity can be re-

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duced through industrial symbiosis by bubbling CO₂through the BR, with the co-benefit of reducing fossil carbon emissions to the atmosphere. Seawater, water, or steam can be also used for lowering the pH, depending on availability (Sutar et al., 2014).

Dewatering is crucial for achieving minimisation of bauxite residue storage, and can be achieved with plate and frame filters that can increase the solids content above 70%, and produce semi-dry cakes - the best available form of BR for deposition. In the global picture, whereas filtration is used in some plants, the majority of refineries use thickeners, which can be either deep cone thickeners or superthickeners (Power et al., 2011). Mud farming can achieve a solid content in the range of 60-65% (World Aluminium and Euro- pean Aluminium, 2015), and is the BAT treatment after high-moisture sludge leaves the plant, as in the case of refineries that rely on pumping technologies for the transport of BR.

The moisture is progressively squeezed out of the mud with heavy machinery, in particular amphiroles (amphibious vehicles with large rollers, as inFigure 2.1). By ploughing the surface layers of the red mud, amphiroles also increase the area of interface between the sludge and air, in turn encouraging carbonation of the BR with the benefits of reducing the alkalinity of the BR and sequestering CO₂from the atmosphere (Svendsen, 2019).

Figure 2.1:An amphirole used in red mud farming.Source: Svendsen (2019)

The BREF mentions some utilisation techniques that proved successful, in particular by using BR as a raw material for ”road construction, cement manufacture, capping materials, and refractory replacement.” However, the BREF notes that ”only a very small proportion of the bauxite residues produced is currently used,” thus recognising that none of these technologies has found large scale applications, and that the BR management model in the EU (and globally) is still far from circularity. In the following paragraphs, the most widespread techniques, their environmental impacts and their presence in the EU will be briefly discussed.

The most traditional disposal techniques, used since the dawn of the industrial implementation of the Bayer process in the 1890s, are marine discharge and lagooning. Marine discharge, or in some cases river discharge, is traditionally the cheapest and easiest option.

As most of the bauxite ore is extracted in the tropics, alumina plants are traditionally built on the coast, where large shipments of the raw material can arrive and be transferred. BR

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2.1 Bauxite residue requires minimal to no dewatering, as it is pumped as a slurry directly into the sea with no land occupation, minimal infrastructure, and very low capital and operating costs. Due to the obvious environmental concerns, including the destruction of coastal seabeds, dis- ruption of marine ecosystems, and toxic metals pollution, this practice has been gradually abandoned throughout the years, becoming a ”last resort” technology for the industry, and currently accommodating BR from less than 2-3% of the global production of alumina (Power et al., 2011). In the EU, the ALTEO alumina plant in Gardanne (France) was the last to stop marine discharge in 2015.

Lagooning consists in depositing BR slurry with low solid content, usually around 18-22% (World Aluminium and European Aluminium, 2015), into land basins. It is the simplest land-based storing technique, but it involves several environmental risks which need to be addressed in the design and management of the ponds, and leaves behind the possibility of an indefinite legacy. Disaster risks are the most compelling, as failures in the containment structures can result in disasters like the Ajka Timf¨oldgy´ar (Hungary) accident in 2010 and many more, as found in Boily (2012). There are risks associated to soil and groundwater contamination from leakage, which can be mitigated by introducing artificial liners and leachate control systems. A general tendency of BR treatment, also concurring to the mitigation of environmental risks, is the reduction of the BR alkalinity by maximising the recovery of sodium hydroxide or by adding seawater or mineral acids.

The AOS plant in Stade (Germany) is the only active plant to perform lagooning in the EU, and its improved BR washing techniques result in extremely low caustic liquor losses (Aluminium Oxid Stade GmbH (AOS), 2020). Many legacy sites with large quantities of BR slurry are still present in the EU, however.

Dry stacking and dry disposal are more recent techniques which generally require higher investments and more advanced technologies, and whereas some plants were already using them in the 1940s, it was only in the 1970s that they emerged as a response to increasingly stringent environmental regulations (Dentoni et al., 2014). In dry stacking, the BR is thickened to a paste with 48-55% solid content, which can be pumped, due to its thixotropic nature, and deposited into thin layers where it is allowed to dry and lose decant liquor, until it reaches a final dry density around 70% solid content and an increased shear strength (Power et al., 2011). This technique allows for decreased environmental risks relative to containment structures failure and liquor leakage, but the problem of dust pollution arises and needs to be mitigated, according to the BREF, with water sprinklers in the short-term, and with specific capping provisions after the decommissioning of the landfill.

Another problem is posed by a greater difficulty in re-vegetation, due to the increased compactness of the stored residue. Most refineries in the EU and most large refineries globally are currently using dry stacking as their main form of deposition.

Finally, dry disposal techniques are given higher priority in the BREF and are currently considered the BAT for BR treatment. The solid content of the BR leaving the plant must be increased to levels above 65% through additional mechanical treatment, and in particular filtration, as thickeners cannot provide sufficient dewatering. As an example, Aluminium of Greece (AoG) achieves 72% solid content with filter presses. The dry, non-thixotropic BR cakes can then be transported more safely and economically than their wet or semi-dry counterparts to relatively simple containment facilities, requiring minimal additional treatment. The environmental risks associated to this technique are lower,

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especially due to the structural stability of the stored material and the absence of caustic liquor, and dust control provisions can be put in place when needed. Most importantly, dry BR cakes are suitable for utilisation, as they can become a raw material or a substrate in a number of applications.

2.1.3 BR utilisation

For the main waste management technologies described above, the price of disposal of BR is high, reaching 1-2% of the price of alumina (Tsakiridis et al., 2004). The costs, together with the high global volumes of production and some desirable physico-chemical properties of BR, brought the industry and academia to research extensively potential strategies for its utilisation (Sutar et al., 2014). According to Shinomiya et al. (2015), 734 patents on BR utilisation were obtained between 1964 and 2008, 33% of which concern civil construction applications. Whereas a variety of applications have proven to be techni- cally feasible and successful in achieving net environmental benefits and improving the environmental performance of the industry, in many cases BR substitutes cheap, abundant materials, making BR inferior in terms of costs and risks (World Aluminium and European Aluminium, 2015). A limited amount of BR is currently utilised worldwide in a variety of applications, including recovery of components, production of building materials, use as a filling material, and applications in pollution control.

BR can contain variable fractions of value elements and critical metals, as mentioned insubsection 2.1.1. Given the high market prices and limited supply of these elements, their extraction from BR can prove to have high economical and strategical significance, along with desirable environmental impacts on global supply chains. BR can also be a secondary ore for the extraction of iron and alumina, as these metals are present in high concentrations, and recovery rates of respectively 86% and 88% have been reached (Sutar et al., 2014). Alumina extraction techniques can also be used for the removal and recovery of sodium compounds, as they consume sodium hydroxide and can result in concentrations of Na₂₀ in the treated BR below 0.3% (Wang et al., 2019). This has the co-benefit of transforming BR into an inert and almost neutral waste material, simplifying its disposal and increasing its possible number of applications.

In the construction sector, BR can find a applications: by exploiting its clay-like behaviour, as in the production of bricks, mortars, and ceramics; as an additive in concrete or cement production, depending on its hydraulic and pozzolanic indications; as a component in the production of geopolymers, as discussed insection 2.3(Sutar et al., 2014).

As an inert material, BR has been successfully used in road construction as filler in bituminous mixtures or in pavement base layers, often resulting in performances that meet national guidelines or that are superior to conventional materials (Lima et al., 2017). BR can also be used in landfill surface covers as a capping material, or as a geological liner to limit leachate or increase drainage (depending on the way the material is treated) in different types of landfills, including BR landfills.

Finally, BR can be used for pollution control, as it is shown that the material has the capability of absorbing a number of heavy metal ions from (Sutar et al., 2014; Ciccu et al., 2003). Sutar et al. (2014) reviews a number of perspective applications in wastewater treatment, soil remediation, and removal of SO₂from waste gasses.

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2.2 Metallurgical slags

The other 2 top produced metallurgical residues in the EU that can be used in the production of non-cementitious alkali activated inorganic binders are the slags generated during the production of copper, iron and steel.

2.2.1 Copper slags

According to the European Copper Institute (European Copper Institute, 2020), the production of semi-fabricated copper and copper alloy products in the EU in the year 2016 was 4.4Mt. Due to the significant recycling rates in the material cycle of copper in the EU, which makes it possible for recycled copper to satisfy around half of the regional demand, it must be noted that the generation of slag must be attributed only to primary copper production. Primary copper is extracted from its ores mostly through the pyrometallurgi- cal route, which includes roasting, smelting, converting, refining, and electrorefining processes. The alternative route, which includes hydrometallurgical processes, is responsible for only 20% of the global primary copper production (Cusano et al., 2017).

As opposed to the Bayer process, however, primary copper smelters usually include some copper scrap in the input flow. Since smelters generally accept medium- to low- grade copper scrap (Schlesinger et al., 2011), the scrap input is also responsible for the generation of slag.

The majority of the slag occurs in the smelting and converting steps, and and average 2.2tof slag are generated per tonne of primary copper (Gorai et al., 2003). This slag contains residues from the ore, as well as fluxes added during the process. As opposed to BR, which comes from hydrometallurgical processes, copper slag from pyrometallurgy does not contain moisture, as it leaves the production process in its liquid phase at high temperatures, and is subsequently water quenched or air cooled. Its composition, as found in Gorai et al. (2003) and presented inTable 2.3, is similar to that of dry BR, the main differences being lower levels of alumina, presence of copper, and no sodium hydroxide.

Fe 30-40

SiO₂ 35-40

Al₂O₃ ≤10

CaO ≤10

Cu 0.5–2.1

Table 2.3:Typical weight fractions for the main components of copper slag.

Copper slag is easier to manage than BR as it does not present its high moisture contents, high alkalinity, and presence of potentially carcinogenic titanium oxide and NORMs. Furthermore, copper slags generally have physico-mechanical properties that make it suited for a number of applications, and similar to BAT BR. The phases contained in copper slags depend mostly on the cooling conditions, giving to the material properties

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that are similar to basalt when crystalline phases prevail, usually resulting from air cooling, and to obsidian in its amorphous phase, obtained by water quenching (Gorai et al., 2003). The main mineral phase found in copper slags is fayalite, due to the nature of the smelting and converting processes (Mihailova and Mehandjiev, 2010). Thus, copper slag resulting from these processes can also be called fayalitic slag.

Current waste management of options of this slag are metal recovery, utilisation, and deposition. According to the BREF for the Non-Ferrous Metals Industries (Cusano et al., 2017), ”several facilities in the non-ferrous metals industries have demonstrated that there is a market in which they are able to sell slag for further beneficial use” (p.94). For this reason, copper slag currently finds a number of utilisation techniques that allow material substitution and avoid landfilling.

In its amorphous phase, the slag is granulated to produce abrasives, but can also be used as an additive to concrete due to its pozzolanic properties. Slower cooling can produce fayalitic slags suited for civil engineering applications, as a filling material in road construction or a replacement for aggregates, the last one being only possible when the amount of leachable metal compounds is low (Cusano et al., 2017). If the content of calcium oxide is high enough, or after treatment with sodium hydroxide, the slag presents cementitious properties and can be used as partial or full replacement in OPC (Gorai et al., 2003).

Finally, fayalitic slags can be mixed with or substituted to BR in the production of AAMs, as illustrated by the case of ELCe (Pontikes, 2019a).

2.2.2 Ferrous slags

Of the three metals that were planned to be covered in this study, ferrous metals (iron and steel) are the ones with the largest volumes of production in the EU. As a reference, production of crude steel in the EU for the year 2018 was of 155.24Mt(The European Steel Association (EUROFER), 2019).

The production of steel follows 2 routes. Molten iron is produced in a blast furnace (BF) from iron ore in the reducing presence of heated air, coke, and limestone, and it is then exothermically refined, together with recycled steel and in the presence of fluxes, coke, and oxygen in a basic oxygen furnace (BOF), producing what is called BOF or crude steel. In the other production process, scrap or direct reduced iron are smelted in an electric arc furnace (EAF), and EAF steel is thus produced. These processes yield 3 different types of slags, respectively iron blast furnace slag (BFS), EAF slag, and steel furnace slag (SFS), further divided into SFS-C if generated in the production of carbon steel, or SFS-S in the case of stainless steel (Heidrich et al., 2017; Euroslag, 2020).

BFS is formed in a blast furnace with molten iron from iron ore in the reducing presence of heated air, coke, and limestone. Once removed from the furnace, the cooling technique and further processing of the slag produce different types of marketable byproduct, with different properties and field of application. Air-quenched slag has time to so- lidify and build crystalline structures, whereas rapid quenching produces granulated BFS (GBFS), which can then be ground using traditional cement clinker grinding technologies.

This forms ground granulated BFS (GGBFS) (Heidrich et al., 2017).

Due to its volumes of production, properties, and composition BFS can be considered as the most convenient precursor for slag-based AAMs (Nazari and Sanjayan, 2015). As a

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2.3 Alkali activated materials, inorganic polymers, and geopolymers matter of fact, along with fly ash, BFS is the most popular choices as main raw materials in the production of AAMs, and many studies were conducted last decade exploring its mode of application (Ismail et al., 2014; Bakharev et al., 1999).

According to the”BREF on Iron and Steel Production”(Remus et al., 2012), slags from the different processes of the steel sector have technical and chemical properties that make them suitable for applications in civil and hydraulic engineering as well as in agri- culture. In these and other sectors, ferrous slag has large number of potential applications and a high global rate of utilization, above 80% for most OECD countries (Sofili´c et al., 2012). Utilisation of iron and steel slags (ISS) is of critical economic importance to the iron and steel industry, as well as to the supply chain participants, which have invested, researched, developed, and promoted ISS into various end use markets (Heidrich et al., 2017).

In particular, 81.40% of BFS produced in the EU in 2018 was used in in cement or concrete, whereas most of the rest was applied to road construction Euroslag (2020). This figure shows that BFS already has mature markets in which it is traded, and mature technologies for the production of traditional cementitious binders. The utilisation of ferrous slag as a geopolymer precursor would mean to shift most slag away from the mature sector of traditional binders, hardly improving the circularity of the sector. For this reason, ferrous slags were finally excluded from the study, and left for future research as addressed in chapter 6.

2.3 Alkali activated materials, inorganic polymers, and geopolymers

Some waste materials are already widely used as supplementary cementitious materials into traditional cements. These include fly ash from coal energy plants and ground granulated blast furnace slag from iron production, which are among the most abundantly available residual resources that are suited for this application, as well as burnt shale, limestone, and silica fume. As the consumption of coal in the EU decreases, however, driven by environmental regulation towards climate change mitigation, the supply of fly ash is also reduced and new materials are being explored and studied. Different technologies are being explored to diversify the incorporated waste materials, and increase their content in the binder, so that the use of other raw materials (usually more expensive and with higher environmental impacts) can be reduced (Ujaczki et al., 2019). Alkali activation is one promising technology, which can make use of different residual resources and use them as core ingredients to produce a binder that can substitute traditional cements.

Alkali activation is a generic term used to denote the reaction between a solid precursor and an alkali activator to produce a hardened binder (Provis, 2018). In this study, we will adhere to the definitions of alkali activated materials, inorganic polymers, and geopolymers found in van Deventer et al. (2010). Alkali activated materials (AAM) are a broad set of materials that are produced through alkali activation of a silicate precursor. Inorganic polymers (IP) are a subset of AAM that makes use of a smaller set of precursors to create a polymer structure through alkali activation. In particular, these precursors can contain Al, Si, Ca, and Fe, and include iron-silicate precursors, such as slags from copper pro-

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duction. Finally, geopolymers (GP) are a subset of the IP, and utilise only aluminosilicate precursors, such as metakaolin, and an alkaline solution (mainly alkali-hydroxide/silicate) (Davidovits, 1991). For this reason, BR-derived AAM will be called GP, whereas AAMs produced from copper or ferrous slags will be IP.

Depending on their characteristics, AAM, IP, and even more GP can be used as inorganic binders and replace traditional cementitious binders. According to Jamieson et al.

(2017a), ”the primary function of a geopolymer is to act as a cementitious binder and replace OPC in concrete manufacture or provide complementary products.” The current study only include those AAM that can effectively replace ordinary Portland cement (OPC), which is the industry standard for cementitious binders, with similar compressive and flexural strength. Of these AAM, only IP are modelled, and whereas copper and ferrous slag produce IP, BR is said to produce GP as according to the definition above. For this reason, the denomination used from this point on will be ”ferrous/copper slag-derived IP” and ”BR-derived GP.”

AAM, and in particular GP, have proven to have desirable properties. According to Singh and Middendorf (2020), compressive strength of GP cement paste was found in many instances to be much higher as compared to that of OPC paste, and the material usually presents fire and heat resistance and anticorrosive properties. Its versatility is greater than for cementitious binders, as specific mix designs can be used for special applications including water-repellent coatings, ignifuge construction elements, and high-performance binders (Dudnikova et al., 2019). One example is provided by a project carried out by the Apis Cor and PIK companies in the Moscow region, Russian Federation, where a residen- tial house was 3D-printed using only GP (seeFigure 2.2)

Figure 2.2:3D-printed GP house in Russia.

One selling point of all AAM is the flexibility of inputs: different raw (and waste) materials can be used to produce solid precursors. The mix design of AAM can be chosen according to the availability of materials and the desired properties of the product. Choosing inputs among locally and readily available materials, and meeting specific requirements of

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2.4 Technology for the formation of geopolymer binders from bauxite residue the demand are two aspects that can optimize the marketability and sustainability profile of AAM, as argued in Peys et al. (2018). Fly ash is the most common component in AAM, along with different metallurgical slags.

One valid method to determine the suitability of different metallurgical slags as precursors for AAM can be chemometrics. As found in Simonsen et al. (2020), screening a broad range of minerals generates large and complex data-sets regarding their chemical composition, mineralogical content, and physical characteristics, which are best handled with the chemometrics method. This study, in particular, considers 13 samples of mine tailings and sets to assess their potential as precursors for cement substitutes. Five of the samples were chemically suited as precursors, three of which also proved to be physically suited, and the most suitable proved to be those with smaller particle size and higher content of oxides. The study suggests that the other tailings could also become successful precursors if opportunely treated. Pre-treatment of suitable slags follows fundamentally the same two processes, namely cooling the material to a reactive state, and finely grinding it (Provis, 2018).

As illustrated by Hertel and Pontikes (2020), projects within the EU aim at upscaling the production of AAM from metallurgical slag by achieving technical and economic feasibility, while maintaining net environmental benefits. One proof of concept study, namely Hertel et al. (2016), outlines the basic technology for the transformation of BR into a reactive precursor, and the subsequent production of GP, in two EU projects. The pilot plant scale project KIC KAVA RECOVERhttps://recover.technology/

proposes to achieve full-scale production of IP derived from BR and Cu slag, and the H2020 IA RemovAl projecthttps://www.removal-project.com/encompasses up-scaling the production of high performance GP binders and lightweight concrete from BR.

Sodium silicate has a major impact also on the economic sustainability of the GP production process. As an example, in Tempest et al. (2015) sodium silicate contributes to over 72% of the cost of raw materials to produce geopolymer cement concrete from fly ash.

Other raw materials are inexpensive at the source, but their cost grows considerably with transport and depends largely on the scale of production. In the case of Dudnikova et al. (2019), fly ash is priced at 2e/tat the production plant, but supply of 1tof fly ash for lab-scale production of GP costs 733 e/t. The marginal cost decreases as the scale of production increases, as no intermediaries are needed and the material can be hauled with full trucks, reaching 42e/tat industrial scale. Although this figure is considerably lower, it must be noted that transport represents over 95% of the total cost, showing the importance of sourcing materials locally.

2.4 Technology for the formation of geopolymer binders from bauxite residue

Where adequately treated, BR can provide the aluminosilicate precursor needed for the formation of GP. Several studies show lab-scale production of GP that contain mostly BR in their precursor and display satisfactory to high performances in terms of strength, often

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coupled with other desirable properties.

In Dimas et al. (2009), BR and metakaolin were mixed in a mass ratio of 85:15 as raw material for the solid precursor, and later activated with an alkaline solution containing sodium and silicon. The cured GP presented high compressive strength (up to 20.5MPa), very low water absorption, satisfactory apparent density, and excellent fire resistance.

Also in He et al. (2013) a maximum compressive strength of 20.5 MPa has been reached for a GP containing BR and rice husk ash in its solid precursor in a mass ratio of 67:33. Rice husk ash, produced from the industrial combustion of rice husks for energy production, contains mostly silica (¿90-95 wt%, and mostly in its amorphous phase), some carbon, and smaller quantities of alkali metals. The presence of amorphous silica in the precursor justifies the absence of silicon in the activating solution, which only contains sodium hydroxide. Microstructure analysis of the cured samples showed that the resulting GP is a composite, containing pure GP binder and inactive fillers, mostly representing crystalline phases in the parent materials.

In Hairi et al. (2015), compressive strengths of 58MPawere reported for a mix of thermally dehydroxylated (500 °C for 24h) BR and silica fume in a mass ratio of 83:17 and a sodium silicate activating solution, at a L/S ratio of 0.5. Compressive strengths of up to 44 MPawere still achieved without the need for thermal treatments. Note that, whereas He et al. (2013) only use sodium hydroxide in the activator, Hairi et al. (2015) exclusively use sodium silicate. The choice depends on the reactivity of the silicon added to the precursor.

Ke et al. (2015) shows the possibility of creating one-part BR-derived GP binders with compressive strengths up to 10 MPa. The advantage of this technology is that the dry binder does not need an activator, but can just be mixed with water and aggregates needed for the desired mixture. Using silica fumes to improve the SiO₂/Al₂O₃ molar ratio, in a mass ratio BR to silica fumes of 75:25, and with the addition of 0.5 wt% of sodium lignosulphonate as dispersant to reduce the water content of the mixture, the resulting one-part GP binder reached a maximum compressive strength of 31.5MPa.

Excluded the one-part mixes, in order to be activated and for a GP, the precursor needs to be mixed with a precise quantity of activating solution, which must be strongly alkaline and contain soluble silicates. Some studies show that the alkalinity and composition of BR liquor can provide the basis for an the activating solution.

Good results have been achieved utilising Bayer process liquor and fly ash to manufacture GP. In this formulation, the Bayer liquor substitutes the activator mix by providing sodium hydroxide, and the fly ash provides the aluminium and the silicon needed in the precursor. Compressive strengths up to 40MPahave been reached by Nazari and San- jayan (2017), and above 30MPaby Jamieson et al. (2017b), with a maximum compressive strength of 45MPawith ambient curing, which was achieved with the addition of a Ca source. In both cases resulting in values of embodied energy that are much lower than for OPC.

From these examples, and more reviewed in Hertel and Pontikes (2020), it is clear that BR has potential to be utilised both as an activator and a precursor in the production of geopolymers. Arnout et al. (2017) consider the possibility of adding BR slurry in all its components in a hybrid binder system. Finally, Hertel et al. (2016) consider the possibility of treating the aqueous and solid phases of BR separately, and thus creating an industrial

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2.4 Technology for the formation of geopolymer binders from bauxite residue system that exploits all the GP potential of BR slurry. This technology is known as ”single- precursor GP technology” because the BR serves as the sole raw material for both the solid precursor and the activator.

2.4.1 Selected BR-derived geopolymer technology

The technology presented in Hertel et al. (2016), and in particular its potential industrial application, formed a basis for the present study for a number of reason: its 100% utilisation rate of BR, its independence from other industrial sectors, its low requirement from additional material inputs, and its possible integration into alumina refineries. Being this a thermal process, it comes with the main disadvantage of high energy requirements, which increase costs and environmental impacts (Hertel and Pontikes, 2020).

The experiment was conducted using 3 different initial blends of BAT-BR sourced from the Aluminium of Greece (AoG) plant in Agios Nikolaos, Greece, a carbon source, and a silica source. The blends were thoroughly mixed, fired for 1hat 1100Cin absence of oxygen, and subsequently air-quenched and milled. Thus 3 different GP precursors were formed, which were subsequently activated in an alkali solution and poured in moulds to cure for 72hat 60C. The cured binders were then analysed and tested for compressive and flexural strength. Note that the curing conditions seem to yield in only 3 days the final strength of the product, which was tested to be approximately the same for a sample cured over 28 days at ambient temperature.

The addition of carbon and silica results in GP with improved homogeneity, minor pores and cavities, a denser microstructure, and a well-developed amorphous binder ma- trix. Of the 3 mixtures, the one containing both carbon and silica also performed the best in the strength tests, with a compressive strength of 43.5MPaand a flexural strength of 9.8MPa. The initial blend for this IP was of 88.56% dw BR, 10.00% dw silica, and 1.44%

dw carbon.

The study then suggests a possible in-house industrial implementation of the process, to be integrated in alumina refineries. This aspect is important, as it is expected to have a major impact on the sustainability of the process, both economical, as discussed in??, and environmental, as discussed in section 2.5. The process, illustrated in Figure 2.3 as adapted for the present study, proposes to generate at an industrial scale an IP with properties similar to the ones presented above, but which uses mostly locally available resources:

the basis for the activating solution is provided by the alkaline liquor contained in the BR slurry, rich in sodium, which can be improved by removing water or adding other alkalis or waterglass; the carbon and silica sources can be chosen among those locally available, e.g., lignite and sand. Finally, there is the possibility of recovering iron from the precursor by magnetic separation.

In addition to the alkaline liquor extracted from the BR, part of the spent sodium hydroxide solution reclaimed to the Bayer process could instead be used. It was argued in section 2.1 that increasing the recovery of spent sodium hydroxide solution improves the economic efficiency of the refinery, but it must be noted that many impurities are returned to the system in this way. As argued by Jamieson et al. (2017a), utilising this alkaline solution in the production of IP and substituting it in the Bayer process with new sodium hydroxide concentrate solution would prove to be an effective impurities removal method.

Although this solution could appear as a decrease in the circularity of the Bayer process,

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Figure 2.3:Industrial process for the production of IP from BR.Adapted from Hertel et al. (2016)

it would represent a good example of industrial symbiosis and a more efficient utilisation of resources.

The geopolymer mortar produced with this technology has high compressive and flexural strengths. One of the binders based on a similar technology is the Exceptional Leuven Cement, or ELCe, developed by KU Leuven and presented in Pontikes (2019b), which appears to be superior to OPC from a number of perspectives, including strength values, fields of application, environmental profile, and material efficiency (Pontikes, 2019a; Peys, 2019). To the knowledge of the author, the largest scales of application for this technology in the EU can be found within: the EU H2020MSCA-ETN REDMUDproject; the Euro- pean Commission funded Innovation ActionRemovAL, in which at least 10tof BR will be processed in the Rio Tinto Pilot plant in France; and the experience with ELCe from KU Leuven.

2.5 Review of previous studies on environmental impacts

Few studies on the environmental impacts of the production of single-precursor GP binders from BR have been conducted, as the technology is relatively new and has not reached significant scales of production outside the laboratory (Hertel and Pontikes, 2020; Wang,

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2.5 Review of previous studies on environmental impacts 2019). Most studies refer to GP that use BR or caustic liquor from BR as one of the raw materials, or GP with different mixes. Nevertheless, some general considerations can still be extracted from such studies.

Habert and Ouellet-Plamondon (2016) showed that environmental studies on the sustainability of AAM show contrasting results, and whereas they mostly agree that their impacts on global warming are smaller than those of traditional materials, the compar- isons for other technologies is less straightforward. In the same study it is shown that the production of GP concrete has a higher environmental impact in other impact categories than global warming due to the production of sodium silicate solution. This is important for the current study, since the selected technology for GP binder also uses sodium silicate as an additive.

According to (McLellan et al., 2011), transport has a great weight on the overall sustainability of the process. In the study it is argued that it is not possible to make a simple sustainability comparison on the use of OPC and GP, since the impact of reagent transport and the source of energy and technology used to produce the reagents vary substantially across studies. Whereas OPC is a mature technology with optimised large-scale industrial processes, GP is only produced at small scales in an sector where economic and environmental returns to scale are increasing (Dudnikova et al., 2019). It must also be considered that OPC already has established supply chains for material inputs, energy, and fuel, whereas GP needs to find cheaper, closer, more sustainable sources of materials.

In Jamieson et al. (2017a), GP mortar for the manufacture of artificial aggregate is taken into analysis. The GP in the study does not include dry BR in the solid precursor, but instead uses the alkaline liquor from fresh BR in the production of the activator.

Whereas the study does not analyse multiple impact categories, it still yields a results for the embodied energy of the binder, which represents under a specific assumption only 6%

of the embodied energy of dry OPC. The assumption consists in considering the sodium hydroxide in the alkaline liquor as a waste product, and thus ignoring its embodied energy.

This assumption is made based on the fact that, whereas recycling of the caustic liquor is currently the industry standard in the EU (and in most of the world), a more linear model could provide benefits to the Bayer process by avoiding the accumulation of impurities, and avoiding expensive purification processes that might be needed otherwise. In this sense, the use of new sodium hydroxide solution could be desirable for the alumina plant, and thus the caustic liquor would become a waste product with no embodied energy (all allocated to the alumina produced).

One study from Yao et al. (2019) performs an LCA of a ready-to-use lightweight porous concrete produced using waste materials, of which BR made up the 27.83% by weight. As well as presenting the characteristics than were sometimes better than those of traditional lightweight porous concretes, this materials performed better for all environmental impact categories, except for particulate matter formation, fossil depletion being the category with the greatest environmental savings. Nikbin et al. (2017) also analy- ses a technology for the production of lightweight concrete containing BR, with the main differences that this version included higher proportions of BR and showed inferior mechanical properties. The comparison of the traditional and BR-containing concretes, however, proved so disproportionately favourable for the latter that the break-even point was reached only when transport was set to 1kmfor the traditional cement and 800kmfor the

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BR cement.

Joyce et al. (2018), finally, provides the most solid basis of the present LCA study, as it conducts an anticipatory LCA on high-BR content IP paving blocks, modelling the production at industrial scale. Only production is included in the system boundaries. The production of the solid precursor material stands out as the most significant environmental hotspot in all but one impact categories, whereas the consumption of alkali silicates (e.g., sodium silicate) is the most significant process only for resource depletion. From the sensitivity analysis, it appears that transport is a sensitive parameter, and that 300 km of distance have a very significant impact on the overall sustainability of the process.

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Chapter 3 Methodology

The use of BR- and copper slag-derived IP as an alternative to cementitious binders in the EU construction sector, after being discussed in its technical aspects in chapter 2, was more deeply analysed with the use of different methodologies in three subsequent steps.

The environmental performance of this potential substitution was first analysed through an LCA study, which included one specific technology for the utilisation of BR in the formation of GP binders. The availability of resources was then researched and compiled, identifying both aggregate figures for the EU and local data estimating the BR and slag generation of individual plants. Finally, a sustainable system of supply of these materials was modelled with the use of a GIS software, focusing on the economic sustainability of hauling materials across the EU.

3.1 Life Cycle Assessment methodology

Life Cycle Assessment (LCA) is a tool that allows to qualify and quantify the environmental performance of a product, a process, or a system. The core concept of this tool is to yield a holistic picture of the environmental impacts of the object of analysis, including all direct and indirect emissions and resource consumption relative to the different stages of its life cycle. The overall principles, frameworks, requirements, and guidelines are provided by ISO 14040 and ISO 14044 (ISO, 2006a,b). As illustrated inFigure 3.1, an LCA study is divided in four subsequent but interdependent steps, which are carried out in an iterative process. The double arrows show that the results from one step can provide a feedback to other steps, which are in turn modified in an effort to increase the consistency and quality of the study. The complete methodology is available consulting the ISO standards above

Circular economy potential of inorganic polymers from bauxite residues and other slags: An LCA study

Master ’s thesis