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Introduction

STEELMAKING AND ELECTRIC ARC FURNACE SLAG

Steelmaking is the process of producing steel from iron ore or steel scrap. Steels are alloys with compositions that contain a variety of elements, depending on the intended use of the steel. For example, stainless steel contains high quantities of chromium and nickel, while tool steels can contain tungsten, molybdenum, cobalt, and vanadium. The majority of steels produced are termed carbon steels, which contain quantities of carbon, manganese, silicon, and aluminum in addition to smaller quantities of other alloying elements. In 2022, approximately 75 percent of the steel produced in the United States was from the electric arc furnace (EAF) process, in which an electric current is used to produce an arc that melts scrap steel or other iron-containing materials.

During the processing or production of a liquid metal, a layer of liquid metal oxide is formed that is lower in density than the liquid metal and floats on top of it. In addition, during processing of a liquid metal, solid oxide materials that are used to form the liquid metal are also less dense and collect on top of the liquid metal. Thus, the slag found on top of a liquid metal can contain both solid and liquid oxides. It is preferred that during the processing of a liquid metal the slag is fully liquid, as it will be poured into a container for treatment, disposal, or recycling, and other oxides are often added during processing to ensure the slag is fully liquid at the liquid metal processing temperature.

The majority of slags are formed during the processing of steels due to the high production quantities of steel worldwide. These slags are termed ferrous slags. Other slags are termed non-ferrous slags to indicate a metal other than steel was processed. Ferrous slag chemistries are determined by the type of process used to form the liquid steel. For example, during scrap remelting using an EAF, the EAF slag is predominantly formed by oxidation during melting and thus contains significant quantities of iron oxide, manganese oxide, and silicon dioxide with other oxides such as lime and magnesia that are added to form a liquid slag pool of defined chemistry and volume. Another slag that is common in steelmaking is a slag that is purposely added to the top of the container used to transport liquid steel (a ladle), and this is termed a ladle slag. Slags formed during reduction of iron ore are also defined by their process name—for example, as the blast furnace is commonly used for high production quantities of liquid iron, the slags formed are termed blast furnace slags. In this report, processing the EAF slag and the ladle slag will be discussed.

For the EAF steelmaking process the composition, intended function, and quantity of slag depend upon which type of steel is being produced and the various oxides added during the process. Thus, slag chemistry and properties are dependent on the specifics of the EAF operation. When the molten slag cools, it solidifies into a nonmetallic rock-like material and can be crushed into different sizes for various applications, often instead of natural rock fragments. The steel slag produced during the scrap-melting stage of steel production is referred to as furnace slag or EAF slag, which is the major source of steel slag that is sold by slag processors for two main types of slag uses: encapsulated (bound), such as use as an aggregate in concrete, and unencapsulated (unbound), such as use as loose ground cover for landscaping.

STUDY BACKGROUND

The U.S. Environmental Protection Agency (EPA) considers EAF slag to be a type of nonhazardous secondary material from the industrial sector that has the potential to be used beneficially. According to EPA, beneficial use involves the substitution of secondary materials, either as generated or following additional processing, for virgin raw materials in a product in a way that provides a functional benefit, meets product specifications, and does not pose concerns to human health or the environment (EPA, 2016). Potential benefits include reduced costs, preservation of natural resources, reduced air and water pollution

from extraction activities, reduced greenhouse gas emissions, and reduced use of landfilling. In general, encapsulated beneficial uses are those where the secondary material is bound in a solid matrix that minimizes mobilization into the surrounding environment. Unencapsulated beneficial uses are those where the secondary material is used in a loose or unbound particulate or sludge form and involve the direct placement of the secondary material on the land (EPA, 2016).

EPA does not currently regulate the use of EAF slag. State, tribal, and territorial regulatory agencies have the authority (often referred to as beneficial use determination) to determine whether to allow a given beneficial use of a secondary material. Regarding unencapsulated uses of EAF slag (the focus of this report), some states classify EAF slag as a product, with no restrictions on uses. Other states have determined that measures are needed to mitigate risks to children and others, such as not allowing slag to be used in residential settings.

A main impetus for this study arose during EPA’s Superfund remediation activities at Pueblo, Colorado to address lead and arsenic contamination from the past activities of a lead smelter. While in the process of cleaning up residential properties, EPA conducted routine assessments of EAF slag in the yards that was used as landscaping material.¹ Testing of EAF slag, which had been produced by a nearby EAF steel plant, revealed that it contained elevated concentrations of manganese and hexavalent chromium.² This discovery led to concerns on the part of EPA about possible hazards of applying unencapsulated EAF slag in residential yards.

COMMITTEE’S STATEMENT OF TASK AND APPROACH

Given concerns about potential health effects of unencapsulated EAF slag, EPA asked the National Academies of Sciences, Engineering, and Medicine (the National Academies) for advice regarding the human health risks associated with the unencapsulated EAF slag. In response, the National Academies convened the ad hoc Committee on Electric Arc Furnace Slag: Understanding Human Health Risks from Unencapsulated Uses under the aegis of the Board on Environmental Studies and Toxicology of the Division on Earth and Life Studies.

The committee was asked to consider various aspects related to human health risks, including the potential release of toxic constituents from applied EAF slag over time, human exposure to those constituents, advances in understanding the toxicology of selected constituents, subgroup characteristics associated with the highest exposure sensitivities, and other important factors that may lead to elevated health risks for the general population people or for some groups. The committee was also asked to identify research needed to fill important information gaps. The full statement of task is provided in Box 1-1. The committee focused on chromium and manganese as selected components likely to be present in EAF slag at concentrations relevant to human health risk assessment. Manganese has posed high potential concern in various EAF slag risk assessments. Hexavalent chromium is a slag constituent for which exposure is associated with a theoretical excess cancer risk and has also been a focus of EAF slag risk assessments.

The committee held 11 meetings, including information-gathering sessions to hear presentations from EPA officials, academic researchers, and industry representatives. In addition to its analysis of the peer-reviewed literature, the committee received written materials provided by EPA and the National Slag Association as well as reports issued by government agencies other than EPA and nongovernmental organizations.

Consistent with the committee’s statement of task, this report covers the potential exposures and risks associated with the end use of unencapsulated slag from EAF steelmaking facilities. The committee focused on exposures and health risk associated with the use of EAF slag for residential applications. The committee was not asked to compare the life-cycle risks and benefits of EAF slag with those of using natural aggregate.

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¹ See https://cumulis.epa.gov/supercpad/cursites/csitinfo.cfm?id=0802700.

² See https://www.epa.gov/smm/electric-arc-furnace-eaf-slag.

ORGANIZATION OF THE REPORT

Chapter 2 discusses the EAF steelmaking process, the production of slag, factors that affect variation in slag composition, various uses of unencapsulated EAF slag, and states’ regulations of slag. Chapter 3 discusses chemical, physical, and geochemical properties of EAF slag and the environmental dynamics of the constituents of slag in situ. Chapter 4 discusses various dimensions of human exposure to unencapsulated slag. Chapter 5 discusses toxicological consideration of selected slag components, primarily chromium and manganese. Chapter 6 focuses on multiple stressors (chemical and nonchemical)

in disadvantaged communities that are likely to exacerbate health risks of EAF slag constituents. Based on the limited amount of data available, Chapter 7 discusses the committee’s overall conclusions about health risks associated with unencapsulated EAF slag, including factors that may lead to the highest risks. The chapter also identifies research needed to address important information gaps regarding exposure and toxicity of EAF slag constituents.