Metallurgical Slags

Environmental Geochemistry and Resource Potential

Nadine M. Piatak, Vojtch Ettler, Nadine M. Piatak, Vojtch Ettler

• Publisher: Royal Society of Chemistry
• Year: 2021
• ISBN:
  9781839164583
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Environmental Science

Metallurgical slags are generated as a by-product of smelting during ironmaking, steelmaking, and the production of ferroalloys and non-ferrous metals. The formation conditions result in complex chemical and mineralogical characteristics unique to slags alone. Historically slags have been discarded as a waste product and, through release of potentially toxic trace elements, represent a hazard to the environment and human health. However, increasingly we are realizing the resource potential of what was previously thought of as waste, thus reducing the environmental impact and taking a step closer to a circular economy.

This book is a definitive reference on the environmental geochemistry and resource potential of metallurgical slags by summarizing processes for the generation of slags, describing their chemical and mineralogical characteristics, outlining the fundamental geochemistry that propels slag weathering, and illustrating the utilization of slags. Particular attention is given to the value of slags in modern society as they are widely used as construction materials in civil engineering, and as an irreplaceable filter in sequestering excess nutrients, pathogens, metal and/or organic contaminants, and even greenhouse gases. The latest developments on recovering residual valuable metals in slags, including new techniques for extracting by-product elements needed for green and frontier technologies, are revealed. This book is essential reading for environmental geochemists, geologists, metallurgists, mining and civil engineers, waste and resource managers, and all those interested and inspired by a circular economy and minimizing our environmental footprint on planet Earth.

CHAPTER 1

Introduction: Metallurgical Slags – Environmental Liability or Valuable Resource?

NADINE M. PIATAK^*a AND VOJTĔCH ETTLER^b

^a US Geological Survey, 954 National Center, Reston, Virginia 20192, USA, ^b Institute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 00 Prague 2, Czech Republic
^*E-mail: [email protected]

Slags are important by-products generated by ferrous and non-ferrous pyrometallurgical operations, with hundreds of millions of tonnes generated globally each year. Depending on the chemical and mineralogical compositions of slags, they may be disposed of as waste, which can then weather and release contaminants into the environment with the potential to impact the ecosystem and humans. Alternately, slags can find use as raw materials with numerous applications in civil engineering or environmental technologies. Furthermore, residual metals, either those targeted for extraction or those that co-occur in the ore or furnace feed and partition into the slag, can be recovered for value. With the ultimate goal of a sustainable environment and a circular economy, the research on slags that is presented in this book will lead us to a better understanding of the environmental consequences of slag disposed of as waste and motivate us to find value in it.

1.1 Slag Production

Slags are by-products generated by the pyrometallurgical processing of both ferrous and non-ferrous metal ores and secondary resources (recycling). They are formed during smelting from the reaction of gangue minerals in the furnace feed and fluxes and, possibly, reductants intentionally added during the smelting process. Fluxes combine with the impurities to form a slag melt that is immiscible with the metal and/or alloy phase targeted for recovery. Slags can be crystalline or glass-like depending on their composition and cooling rates and are usually silica-rich. Several major categories of slags exist, including: ferrous slags generated from the manufacture of iron (Fe) and steel products; ferroalloy slags formed from the production of bulk ferroalloys (e.g., FeCr, FeMn, FeNi, and FeSi); and base metal slags formed from the recovery of metals such as Cu, Ni, Pb, and Zn, commonly from sulfidic ores. Other less commonly reported slags, some that belong in the base metal slag category, include slags generated from the extraction of Mn, platinum-group metals, and Sn from natural ores, or the recovery of metals from recyclable products, such as Pb–acid batteries, Al scrap, and other post-consumer waste, including batteries, magnets, catalytic converters, and computer circuit boards (see Chapter 2 for technological details).

Hundreds of millions of tonnes (t) of slags are produced annually worldwide in numerous pyrometallurgical processes in our industrialized society. Iron blast furnaces (BF) generate approximately 0.25 to 0.30 t of slag per t of crude iron for typical ore grades, with higher amounts generated for low ore grades (up to 1.0 to 1.2 t of slag per t of crude iron).¹ Steel furnaces commonly produce approximately 0.2 t of slag per t of steel, with some being returned to the furnace to recover entrained metal.¹ Based on iron and steel production, van Oss¹ estimated that between 295 and 354 Mt of BF slag and 169 to 254 Mt of steel slag were generated in 2017; the USA contributed 6 to 7 Mt of BF slag and 8 to 12 Mt of steel slag to global slag generation that year.¹ Similarly, Tripathy et al.² stated that approximately 430 Mt of ferrous slag from BFs were produced worldwide in 2016, with 87% being produced in Asia-Pacific, 7% in Europe, 3% each in North America and Latin America, and 1% in the Middle East and Africa, collectively. The most recent European statistics for 2018 indicate that 19.2 Mt of BF slag and 15.7 Mt of slag from steel production were produced in Europe.³ For non-ferrous metal smelting industries, the production of slag per amount of metal produced is rather variable, dependent on the technology used. For example, Goonan⁴ compiled data on material flows in 30 large-scale Cu smelters around the world and found that the amount of slag produced was in the range 0.65–5.14 t per 1 t of produced Cu, with a weighted average of 1.91 t (Figure 1.1). Given the fact that Cu production accounted for 20.4 Mt in 2018,⁵ it can be estimated that approximately 39 Mt of Cu slag were generated in that year. Pan et al.⁶ suggested that Pb slag produced in 2016 likely exceeded 5.5 Mt based on global Pb production of 11.1 Mt from both primary Pb ores and secondary resources. The amounts of slags generated from the production of base metals and other metals and alloys vary depending on the smelting conditions and how much metal is in production each year. However, overall, it is clear that ironmaking and steelmaking generate the largest volumes of slags, in the order of hundreds of millions of tonnes per year, whereas the amount of base metal slags generated, although not trivial in quantity, is an order of a magnitude lower than that for ferrous slags.

Figure 1.1Copper slag production by different smelting technologies (tonnes of slag per 1 tonne of produced Cu) (compiled data for 30 large-scale smelters from ref. 4).

Depending on their chemical and mineralogical compositions, slags are either considered as waste, and many slags resulting from non-ferrous metallurgy contain appreciable amounts of potentially toxic elements [PTEs] (see Chapters 3, 4, 5, and 6 of this book), or as raw materials, which can find applications in civil engineering and environmental technologies (mainly ferrous slags, see Chapters 7 and 8) and/or can be used for metal recovery (mainly non-ferrous slags, see Chapter 9).

1.2 Literature Summary and Perspective

Scientific interest in slags has been increasing steadily over the last few decades with thousands of abstracts and manuscripts published annually over the last few years on slag research. The abundant scientific literature on slags can generally be divided into research on the environmental effects of slags or on the reuse, potential reuse, and value of slags. Several valuable review manuscripts have been published, typically focusing on one slag type and from one or limited perspectives. For example, Potysz et al.⁷ provided an excellent review of Cu slags, highlighting their chemical and phase composition, environmental issues related to their disposal, and approaches for recovering valuable residual metals. Additionally, other insightful reviews include Pan et al.⁶ on Pb slag generation, characteristics, and utilization; Naidu et al.⁸ on uses of basic oxygen furnace slag, a type of steel slag; and Yildirim and Prezzi⁹ on chemical, mineralogical, and morphological characteristics of all steel slag types. Special issues dedicated to slag research include “Metallurgical slags” in the journal Minerals¹⁰ and an upcoming issue in Applied Science on “Utilization of steel furnace slag in cementitious composites.” Currently, conferences and workshops on slags draw international attention and provide a venue for the newest research to be shared and advanced. Many books have also focused on principles of extractive metallurgy and contain some information about metallurgical slags.^11,15 Several books or chapters in books are dedicated to particular aspects of slag use, such as in construction,^16,18 or to evaluating the ecosystem impacts of mining waste, including slags.^19,20 Some reviews on slags have attempted a more holistic approach by including various aspects of metallurgical slags, two of which, by Piatak et al.²¹ and Piatak,²² included discussions of the characteristics of ferrous and base metals slags, their environmental impacts, and utilization potential. These reviews provided motivation for the creation of this book, which provides a comprehensive and detailed overview of how metallurgical slags are generated, their fate in the environment, and their numerous beneficial uses – all captured in one central publication.

Interestingly, published studies on base metal slags and ferrous slags generally have very different focuses, which influenced the perspectives of the chapters included in this book. The majority of research on ferrous slags has been on the physical and mechanical properties owing to their wide use in construction and cement, as well as their environmental applications. A search for the types of ferrous slag [e.g., iron slag, steel slag, basic oxygen furnace (BOF) slag, electric arc furnace slag, BF slag, ladle slag] in the titles of abstracts and citations in the Scopus database²³ gives a query result of over 5000 documents for 2010 to 2020. A small fraction (around one in ten) of those documents are in the subject areas of Environmental Science or Earth and Planetary Science with the vast majority being in the Engineering and Material Science fields. These results imply that there is a major research focus on the physical and mechanical properties of this material for reuse, as highlighted in Chapters 7 and 8, and less of a focus on the environmental consequences of its disposal. As for the subject of weathering studies (discussed in Chapter 4), steel slags are intentionally weathered or activated before use in construction to hydrate the lime (CaO) so it does not expand and create instability or swelling when exposed to the atmosphere in the final product. The generation of secondary phases, also discussed in Chapter 4, such as portlandite (Ca(OH)₂) and calcite (CaCO₃), is intentional and studies have focused on how to facilitate and expedite it. Additionally, environmental applications of ferrous slags, such as in treating nutrient-rich or metal-rich wastewater or agricultural runoff, neutralizing acid mine drainage, or sequestering carbon dioxide gas, cause the dissolution of phases and precipitati...

Table of contents

• Cover
• Halftitle
• Series
• Title
• Copyright
• Preface
• Contents
• Chapter 1 Introduction: Metallurgical Slags – Environmental Liability or Valuable Resource
• Chapter 2 Metallurgical Overview and Production of Slags
• Chapter 3 Geochemistry and Mineralogy of Slags
• Chapter 4 Weathering of Slags
• Chapter 5 Slag Leaching Properties and Release of Contaminants
• Chapter 6 Environmental Impact of Slag Particulates
• Chapter 7 Diverse Applications of Slags in the Construction Industry
• Chapter 8 Environmental Applications of Slag
• Chapter 9 Secondary Metal Recovery from Slags
• Subject Index