Near-zero-waste processing of low-grade, complex primary ores and secondary raw materials in Europe: technology development trends

With an increasing number of low-grade primary ores starting to be cog-effectively mined, we are at the verge of mining a myriad of low-grade primary and secondary mineral materials. At the same time, mining practices and mineral waste recycling are both evolving towards sustainable near-zero-waste processing of low-grade resources within a circular economy that requires a shift in business models, policies and improvements in process technologies. This review discusses the evolution towards low-grade primary ore and secondary raw material mining that will allow for sufficient supply of critical raw materials as well as base metals. Seven low-grade ores, including primary (Greek and Polish laterites) and secondary (fayalitic slags, jarosite and goethite sludges, zincrich waste treatment sludge and chromium-rich neutralisation sludge) raw materials are discussed as typical examples for Europe. In order to treat diverse and complex low-grade ores efficiently, the use of a new metallurgical systems toolbox is proposed, which is populated with existing and innovative unit operations: (i) mineral processing, (ii) metal extraction, (iii) metal recovery and (iv) matrix valorisation. Several promising novel techniques are under development for these four unit-operations. From an economical and environmental point of view, such processes must be fitted into new (circular) business models, whereby impacts and costs are divided over the entire value chain. Currently, low-grade secondary raw material processing is only economic and environmentally beneficial when the mineral residues can be valorised and landfill costs are avoided and/or incentives for waste processing can be taken into account

Alkali-activation of Fe-RICH slag using mixed Na-K solutions

Inorganic polymers (IPs) are typically obtained by alkali-activation of inorganic precursor materials, often residues from other industries such as fly ash, ground granulated blast furnace slag or non-ferrous metallurgy (NFM) slag.1 The majority of inorganic polymers has been created by activation with alkali-silicate Solutions, where the alkalis are usually either sodium (Na) or potassium (K). However, research on the combination of both Na- and K-silicate activators is scarce, and mainly focused on aluminosilicate precursors whereas the NFM slags are richer in iron.2 The effect of using mixed-alkali Solutions is still unclear and seen to vary in literature. Duxson et al.3 reported an increased compressive strength for metakaolin-based geopolymers actjvated with mixed-alkali Solutions with respect to data obtained when single-alkali Solutions were used, and attributed this to a further unspecified mixed-alkali effect (MAE). A strength increase of mixed-alkali-activation with respect to sodium-based activation of fly ash was seen by Leong et al.A at elevated temperatures (60°C). Flowever, at room temperature decreased strength was reported. Phair and van Deventer5 also observed a negative interaction of both alkalis, leading to decreased strengths for mixed-alkali activated fly ash with respect to pure-alkali activated samples. In another study, Chuah et al.6 reported a strength decrease going from Na- to K-activated fly ash, vyith mixed-cation alkali-activation yielding intermediate strengths. The effect of activation with mixed Na/K-solutions is still unclear, and possibly depends on the precursor. Furthermore, alkali-activation of iron-rich precursors, such as NFM slags, has only been scarcely investigated and even more so with respect to mixed-alkali Solutions. Therefore, this work aims to investigate the mixed-alkalieffect by activation of a synthetic NFM-like slag with both pure Na- and K-silicate Solutions as well as mixed Na/K-silicate Solutions

Potassium-rich biomass ashes as activators in metakaolin-based inorganic polymers

The use of biomass ashes as an alkaline activator in the synthesis of metakaolin-based inorganic polymers was investigated in the present work. Maize stalk and maize cob ashes reached the highest pH after mixing with water, 13–14, and contained the largest amount of potassium, 30–32 wt.% K₂O. Of these two, the maize cob ashes showed a higher reactivity and reaction extent when mixed with water and metakaolin calcined at an optimized temperature of 700 °C. A maximum reaction enthalpy of −372 J/g was reached with a mixture with an ash to metakaolin mass ratio of 0.9. In attenuated total reflection Fourier-transformed infrared spectroscopy, the wavenumber shift of the Si–O–T (T: Si, Al) stretching band upon activation proved to be linearly related to the compressive strength of pressed samples, cured with all surfaces exposed at 80 °C or 60 °C for 48 h. A maximum strength of 27MPa and a wavenumber shift of the Si–O–T stretching band of 52 cm‾¹ were observed for samples with a maize cob ash to metakaolin mass ratio of 0.9 cured with an open surface at 80 °C. This shift and increase in strength is explained by a larger reaction extent, which was also observed using calorimetric techniques, and confirmed by electron probe micro-analysis. The curing conditions were altered to optimize the microstructure. The temperature was lowered to 60 °C, the samples were wrapped in plastic to avoid water evaporation, or the samples were subjected to a pre-cure of 24 h at 20 °C before curing at 80 °C. The latter resulted in the maximum compressive strength of 40 MPa.publisher: Elsevier articletitle: Potassium-rich biomass ashes as activators in metakaolin-based inorganic polymers journaltitle: Applied Clay Science articlelink: http://dx.doi.org/10.1016/j.clay.2015.11.003 content_type: article copyright: Copyright © 2015 Elsevier B.V. All rights reserved.status: publishe

Detoxified spent pot lining from aluminum production as (alumino-)silicate source for composite cement and autoclaved aerated concrete

New sources of supplementary cementitious materials (SCMs) are needed to meet the future demand. A potential new source of SCM is spent pot lining, a residue from aluminum production. The present work showed that the refined aluminosilicate part of spent pot lining (SPL) has a moderate chemical reactivity in a cementitious system measured in the R3 calorimetry test, comparable to commercially used coal fly ash. The reaction of SPL led to the consumption of Ca(OH)2 in a cement paste beyond 7 days after mixing. At 28 and 90 days a significant contribution to strength development was therefore observed, reaching a relative strength, which is similar to composite cements with coal fly ash. At early age a retardation of the cement hydration is caused by the SPL, which should most likely be associated with the presence of trace amounts of NH3. The spent pot lining is also investigated as silica source for autoclaved aerated concrete blocks. The replacement of quartz by spent pot lining did not show an adverse effect on the strength-density relation of the lightweight blocks up to 50 wt% quartz substitution. Overall, spent pot lining can be used in small replacement volumes (30 wt%) as SCM or as replacement of quartz (50 wt%) in autoclaved aerated concrete blocks

Biomass ash as alkaline activator in inorganic polymer synthesis

status: publishe

Performance of Fe-Rich Alkali-Activated Materials in Na₂SO₄ Solution: Role of MgO/(MgO + CaO) in the Slag

To enable the usage of non-ferrous metallurgy slags in alkali-activated materials (AAMs), the influence of the chemical composition of slags on durability must be better understood. In this work, two slags were synthesized with different MgO/(MgO + CaO) weight ratios to investigate the effect on the sulfate resistance (Na2SO4) of AAMs. Experimental results suggested that a higher MgO/(MgO + CaO) ratio does not lead to higher strength, but the trend of the mass change and compressive strength change of two AAMs is quite similar upon Na2SO4 exposure for 24 weeks. The leaching of elements (Na, Al, and Si) during Na2SO4 exposure is more significant in the early stage, while Ca leaching is more pronounced in the late stage

Detoxified Spent Pot Lining from Aluminum Production as (Alumino-)Silicate Source for Composite Cement and AutoClaved Aerated Concrete

New sources of supplementary cementitious materials (SCMs) are needed to meet the future demand. A potential new source of SCM is spent pot lining, a residue from aluminum production. The present work showed that the refined aluminosilicate part of spent pot lining (SPL) has a moderate chemical reactivity in a cementitious system measured in the R3 calorimetry test, comparable to commercially used coal fly ash. The reaction of SPL led to the consumption of Ca(OH)2 in a cement paste beyond 7 days after mixing. At 28 and 90 days a significant contribution to strength development was therefore observed, reaching a relative strength, which is similar to composite cements with coal fly ash. At early age a retardation of the cement hydration is caused by the SPL, which should most likely be associated with the presence of trace amounts of NH3. The spent pot lining is also investigated as silica source for autoclaved aerated concrete blocks. The replacement of quartz by spent pot lining did not show an adverse effect on the strength-density relation of the lightweight blocks up to 50 wt% quartz substitution. Overall, spent pot lining can be used in small replacement volumes (30 wt%) as SCM or as replacement of quartz (50 wt%) in autoclaved aerated concrete blocks

Impact of the solidification path of FeOx-SiO2 slags on the resultant inorganic polymers

status: publishe

Influence of CaO/FeO ratio on the formation mechanism and properties of alkali-activated Fe-rich slags

In the search for CO2 mitigating cement technologies, alkali-activated Fe-rich slags present a possible alternative. The influence of the chemical composition of the synthetic Fe-silicate slag on the reaction mechanisms is assessed by varying the CaO/FeO molar ratio. The alkali-activated Fe-rich slags consist of trioctahedral layers that are shown to be hydroxylated and in contact with silicates, in a similar way to phyllosilicate minerals. The formation of this phyllosilicate-like structure is hindered during the first days for samples with high CaO/FeO ratios, because of the partial incorporation of Ca in the trioctahedral layers. At later ages, the samples with higher CaO/FeO ratios gain in reaction extent, to exceed the reaction extent of low CaO/FeO ratios at 28 days and beyond. The increase in compressive strength is even more pronounced than the effect on the reaction extent, which underlines the importance of Ca on the inherent strength of the binding phases

A proposal for a 100% use of bauxite residue: The process, results on the novel Fe-rich binder and how this can take place within the alumina refinery

status: publishe