**2. Ironmaking and steelmaking slags and their use**

Every year more than 40 million tonnes of iron and steel slags are produced in Europe. The production of iron and steel slag in Europe in 2008 was of 45.6 million tonnes1 and decreased in 2009 and 2010 only due to the steel production slow down caused by the economic crisis. The main compounds contained into the slags are calcium, silicon, magnesium, aluminium and iron oxides.

Slags are generated in all stages of steel production and the following four different kinds of slags from different routes can be identified: BF slag, BOF slag, EAF slag and Secondary Steelmaking Slag. The slagging agents with fluxes, such as limestone, dolomite and silica sand, are added into BF or steelmaking furnaces in order to remove impurities from ore, scrap and other ferrous charges during smelting. The slag formation is the result of a complex series of physical and chemical reactions between the non-metallic charge (lime, dolomite, fluxes), the energy sources (coke, oxygen, etc.) and refractory bricks. Because of the high temperatures (about 1500°C) during their generation, slags do not contain any organic substances. The slags protect the metal bath from oxygen and maintain temperature through a kind of lid formation. Due to the fact that slags are lighter than the liquid metal, they float and may be easily removed. Slag is generated in a parallel route of the main processes of hot metal production in ironmaking and steelmaking and therefore the slag generation process is considered as a part of the whole steel production process (EC, 2001).

The uses of different types of slags in Europe are shown in Figure 3. Ironmaking and steelmaking slags are used in different ways with high added value. While steelmaking slags are mainly used for road construction, hydraulic engineering, and fertilizer, BF slag is mainly used for cement production2.

<sup>1</sup> Source: European Slag Association, EUROSLAG

<sup>2</sup> Source: European Slag Association, EUROSLAG

fertiliser and as liming agent, its potential use as amending material for soils, and by paying attention also to different technologies and methodologies aiming to improve the quality of the slag, in order to increase and make progress in its use in agriculture. On one hand, studies based on the use of slag in agriculture will be considered, which treat the use of steel slags for amending acid soils and as source of important factors and growing agents (not only calcium and magnesium compounds, but also other elements such as silicon, providing important beneficial effects for some crops and increasing the plant yields) and its use as Fe source for reducing Fe chlorosis in different crops. Moreover, investigations will be described concerning the heavy metals contained into the slags and their behaviour on the soil, in order to evaluate possible harmful effects after slag application for agricultural purposes and to avoid their possible negative environmental impacts, as well as the use of steel slags for metal stabilisation in contaminated soils. On the other hand, investigations focused on the obtaining a slag with high phosphorus content to be used as fertiliser (together with other slags with a low content in phosphorus to be recycled inside the

Every year more than 40 million tonnes of iron and steel slags are produced in Europe. The production of iron and steel slag in Europe in 2008 was of 45.6 million tonnes1 and decreased in 2009 and 2010 only due to the steel production slow down caused by the economic crisis. The main compounds contained into the slags are calcium, silicon,

Slags are generated in all stages of steel production and the following four different kinds of slags from different routes can be identified: BF slag, BOF slag, EAF slag and Secondary Steelmaking Slag. The slagging agents with fluxes, such as limestone, dolomite and silica sand, are added into BF or steelmaking furnaces in order to remove impurities from ore, scrap and other ferrous charges during smelting. The slag formation is the result of a complex series of physical and chemical reactions between the non-metallic charge (lime, dolomite, fluxes), the energy sources (coke, oxygen, etc.) and refractory bricks. Because of the high temperatures (about 1500°C) during their generation, slags do not contain any organic substances. The slags protect the metal bath from oxygen and maintain temperature through a kind of lid formation. Due to the fact that slags are lighter than the liquid metal, they float and may be easily removed. Slag is generated in a parallel route of the main processes of hot metal production in ironmaking and steelmaking and therefore the slag generation process is considered as a part of the whole steel production process

The uses of different types of slags in Europe are shown in Figure 3. Ironmaking and steelmaking slags are used in different ways with high added value. While steelmaking slags are mainly used for road construction, hydraulic engineering, and fertilizer, BF slag is

steelmaking process) will be discussed.

magnesium, aluminium and iron oxides.

mainly used for cement production2.

1 Source: European Slag Association, EUROSLAG 2 Source: European Slag Association, EUROSLAG

(EC, 2001).

**2. Ironmaking and steelmaking slags and their use** 

Fig. 3. The uses of different types of slags in Europe.

The marketed BF slag can be subdivided into three main types, depending how they are cooled:


Steelmaking slags include slags from BOF and EAF. Since at this stage the steel production processes vary, depending on the steel being made, the slags chemical properties change as well. This results in a more difficult use of steel slags compared to the BF slag. They are discharged to a cooling yard or to a slag ladle and they are naturally cooled with moderate sprinkling. After crushing, sieving and removal of magnetic matter, they achieve granularity appropriate to different applications. Because of their lime contents they expand in reaction with water. After this expansion they are stabilised by "natural ageing" for long periods outdoors in natural rainfall and other weather or "steam ageing", through high-temperature vapour.

Steelmaking slag deriving from BOF process (using the Linz-Donawitz (LD) converter) comes from the pig iron refining process, which converts molten pig iron and steel scraps into high quality steel. Most slags from steel plant derive from this process, with an average of 150-200 kg of slag generated per tonne of steel produced. X-ray diffraction studies have shown that the major phases present in LD slag are dicalcium ferrite, calcium alluminate

Possible Uses of Steelmaking Slag in Agriculture: An Overview 341

The main issue concerning the use of ferrous slags is the question whether it is a waste or a by-product. In order to market them the better way is to consider them by-products because the term "waste" indicates a material to be deposited instead to be used (Kobesen, 2009).

The Waste Framework Directive (WFD) (2006/12/EC) is the most important document governing the use of slag. Until recently, slag was considered as waste, but, after many years of discussion, the WFD has been amended by adding the term by-product (2008/98/EC). On one hand, the WFD provides the main concepts and definitions concerning the waste management; on the other hand, it sets the principles of waste management. Moreover it clarifies the definition of waste by introducing definitions of by-product and *end-of-waste* status, thanks also to the work of steel industry in supporting the EU Commission. The most

• Article 5, which concerns by-products and provides the following conditions to meet in order a by-product is not considered a waste: direct use of a by-product, without any further processing other than normal industrial practice; by-product production as an integral part of a production process; any further use has to be lawful and certain; the by-product use has to be consistent to the principles of the EU waste policy, such as

• Article 6, End-of-waste status, regulates when a substance is classified as waste, ceases to be waste. The article 6 of the WFD defines the end-of-waste (EOW) criteria and what requirements have to be met. These criteria are developed in accordance with the

3. The substance or object fulfils the technical requirements and meets the existing

4. The use of the substance or object will not lead to overall adverse environmental or

The EOW criteria shall embrace limit values for pollutants where necessary and shall take into account the possible adverse environmental effects of the substance or object (Eloneva

The Kyoto Protocol (Conference of the Parties, 1997), drawn in 1997 and come into force since 2005, regulates and decreases CO2 emissions down to the 1990 CO2 emission levels,

The use of slag in different fields of application, for example the granulated BF slag (GBFS) as substitute for cement and for clinker in cement and the steelmaking slag for soil conditioning, can allow the reduction of the tonnages of CO2 emission. Furthermore, in order to reduce the CO2 emissions from the ironmaking and steelmaking processes, some studies have been carried out to sequestrate CO2 in slag, through the free lime (CaO) and the

The BREF document (EC, 2001) provides the best available techniques concerning environment, health and safety for iron and steel industry. It has been recently revised (EC,

• Article 3, which provides the definition of waste and hazardous waste; • Article 4, which describes in what order wastes are to be discarded;

1. The substance or object is commonly used for specific purposes

important articles for the slag use are, as follows:

environment and human health protection.

following four conditions:

2. A market or demand exists

human health impact.

et al., 2010).

legislation and standards

because of its key role in the global warming.

di- and tricalciumsilicates carbonation (Abassapour et al., 2004).

and wüstite, but it contains also some reactive mineral phases, such as 2CaO. SiO2, 3CaO. SiO2 and free CaO e MgO (Das et al., 2007).

The EAF slag utilisation is quite similar to the one of BOF slag, but, due to its lower lime content, EAF slag is very stable and can be used in asphalt without any problems.

Secondary Steelmaking Slag disintegrates into a powder due to instability of the dicalcium silicate, causing an increase in dust emissions to the environment. Some studies have successfully been carried out in order to reduce this phenomenon and to make this slag suitable for use and valuable (Rozman et al., 2007), (Branca et al., 2009).

Recently, due to a better understanding of the slag formation mechanisms and of the overall BF process, a large number of different slags has been designed, and it is also currently possible to control, optimise and minimise slags production. This progress applies also to BOF slag. The silicon, phosphorous and sulphur removal, before the refining into the basic oxygen furnace, has led to the reduction of tap-to-tap time, to the costs decrease, to the reduction of the amount of slag produced and to the production of higher quality steels. Moreover, through the vacuum degassing process, after decarburisation in the BOF, the hydrogen and nitrogen contents have been reduced. The ladle treatments produced significant reduction in impurities in steel and the use of selected slags minimised the formation and modification of *inclusions* (usually they are non-metallic particles contained in steel, that, depending on their number, their size and their distribution, can have a detrimental effect to mechanical properties of steel), with the result of improving mechanical properties of steel (Dippenaar, 2004).

Some slags are internally used in steelmaking furnaces or in sinter plants, while about 50% of this kind of slag is used outside the plant in the construction sector, mainly for road construction, as an aggregate in bituminous pavements, as a binding agent in base courses, for strengthening the subsoil and for soil conditioning. Free lime, after separation, can be used as fertiliser, in cement and concrete production, for waste water treatment and in coastal marine blocks. In soil conditioning slags are efficient in soil neutralisation. In addition, the siliceous liming materials improve soil structure and reduce fungal infections. Blast furnace slag can be used also in agriculture because of its high sorption capacity of phosphorus, which remains into the available form for the plants. Negative effects, resulting from steel slags use, could derive from their heavy metal concentrations, but such metals tend to bound to the slag matrix and thus they are not available for plants. All these factors contribute to underline positive effects of using slag as liming materials, that lead to better yield of the crops, soil protection and reduction of natural resources consumption (R. Hiltunen & A. Hiltunen, 2004).

#### **2.1 The main legislation about the use of slags**

Over the past decades the by-products recovery and reuse have significantly increased also because of the stringent legislation for environmental protection, that differs all over the world. In Europe, slag is mainly bound to the Waste Framework Directive (WFD), but the main legislation concerning the use of slag includes other laws, as follows: the Kyoto protocol, the Reference Document of Best Available Techniques, Harmonisation Committees TC 351 Dangerous Substances and TC 154 Aggregates, the REACH directive.

The EAF slag utilisation is quite similar to the one of BOF slag, but, due to its lower lime

Secondary Steelmaking Slag disintegrates into a powder due to instability of the dicalcium silicate, causing an increase in dust emissions to the environment. Some studies have successfully been carried out in order to reduce this phenomenon and to make this slag

Recently, due to a better understanding of the slag formation mechanisms and of the overall BF process, a large number of different slags has been designed, and it is also currently possible to control, optimise and minimise slags production. This progress applies also to BOF slag. The silicon, phosphorous and sulphur removal, before the refining into the basic oxygen furnace, has led to the reduction of tap-to-tap time, to the costs decrease, to the reduction of the amount of slag produced and to the production of higher quality steels. Moreover, through the vacuum degassing process, after decarburisation in the BOF, the hydrogen and nitrogen contents have been reduced. The ladle treatments produced significant reduction in impurities in steel and the use of selected slags minimised the formation and modification of *inclusions* (usually they are non-metallic particles contained in steel, that, depending on their number, their size and their distribution, can have a detrimental effect to mechanical properties of steel), with the result of improving

Some slags are internally used in steelmaking furnaces or in sinter plants, while about 50% of this kind of slag is used outside the plant in the construction sector, mainly for road construction, as an aggregate in bituminous pavements, as a binding agent in base courses, for strengthening the subsoil and for soil conditioning. Free lime, after separation, can be used as fertiliser, in cement and concrete production, for waste water treatment and in coastal marine blocks. In soil conditioning slags are efficient in soil neutralisation. In addition, the siliceous liming materials improve soil structure and reduce fungal infections. Blast furnace slag can be used also in agriculture because of its high sorption capacity of phosphorus, which remains into the available form for the plants. Negative effects, resulting from steel slags use, could derive from their heavy metal concentrations, but such metals tend to bound to the slag matrix and thus they are not available for plants. All these factors contribute to underline positive effects of using slag as liming materials, that lead to better yield of the crops, soil protection and reduction of natural resources consumption (R.

Over the past decades the by-products recovery and reuse have significantly increased also because of the stringent legislation for environmental protection, that differs all over the world. In Europe, slag is mainly bound to the Waste Framework Directive (WFD), but the main legislation concerning the use of slag includes other laws, as follows: the Kyoto protocol, the Reference Document of Best Available Techniques, Harmonisation Committees

TC 351 Dangerous Substances and TC 154 Aggregates, the REACH directive.

SiO2, 3CaO.

SiO2

and wüstite, but it contains also some reactive mineral phases, such as 2CaO.

suitable for use and valuable (Rozman et al., 2007), (Branca et al., 2009).

content, EAF slag is very stable and can be used in asphalt without any problems.

and free CaO e MgO (Das et al., 2007).

mechanical properties of steel (Dippenaar, 2004).

**2.1 The main legislation about the use of slags** 

Hiltunen & A. Hiltunen, 2004).

The main issue concerning the use of ferrous slags is the question whether it is a waste or a by-product. In order to market them the better way is to consider them by-products because the term "waste" indicates a material to be deposited instead to be used (Kobesen, 2009).

The Waste Framework Directive (WFD) (2006/12/EC) is the most important document governing the use of slag. Until recently, slag was considered as waste, but, after many years of discussion, the WFD has been amended by adding the term by-product (2008/98/EC). On one hand, the WFD provides the main concepts and definitions concerning the waste management; on the other hand, it sets the principles of waste management. Moreover it clarifies the definition of waste by introducing definitions of by-product and *end-of-waste* status, thanks also to the work of steel industry in supporting the EU Commission. The most important articles for the slag use are, as follows:

	- 1. The substance or object is commonly used for specific purposes
	- 2. A market or demand exists
	- 3. The substance or object fulfils the technical requirements and meets the existing legislation and standards
	- 4. The use of the substance or object will not lead to overall adverse environmental or human health impact.

The EOW criteria shall embrace limit values for pollutants where necessary and shall take into account the possible adverse environmental effects of the substance or object (Eloneva et al., 2010).

The Kyoto Protocol (Conference of the Parties, 1997), drawn in 1997 and come into force since 2005, regulates and decreases CO2 emissions down to the 1990 CO2 emission levels, because of its key role in the global warming.

The use of slag in different fields of application, for example the granulated BF slag (GBFS) as substitute for cement and for clinker in cement and the steelmaking slag for soil conditioning, can allow the reduction of the tonnages of CO2 emission. Furthermore, in order to reduce the CO2 emissions from the ironmaking and steelmaking processes, some studies have been carried out to sequestrate CO2 in slag, through the free lime (CaO) and the di- and tricalciumsilicates carbonation (Abassapour et al., 2004).

The BREF document (EC, 2001) provides the best available techniques concerning environment, health and safety for iron and steel industry. It has been recently revised (EC,

Possible Uses of Steelmaking Slag in Agriculture: An Overview 343

LD slag can be used in different fields of application such as fertilizer, soil conditioners, recovery of metal values. Because of its hard characteristics it is also used as aggregates for road construction, for the base and sub-base layer in road construction and for hydraulic engineering structures. On this subject tests have been carried out in order to assess technical properties. In particular the volume stability, which is the key aspect for using steel slags as a construction material, has been evaluated, by comparing the behaviour of slag under practical conditions, such as in road constructions; on the other hand, the assessment of environmental compatibility of aggregates as building material has been tested through leaching tests in order to continuously control quality (Motz & Geiseler,

Since 1880 steelmaking slag from Basic-Bessemer or Thomas process has been used as a phosphatic fertiliser, but also the current LD slag composition (mainly containing CaO, MgO, SiO2, Mn and other valuable micronutrients, such as copper, zinc, boron and cobalt) makes it suitable as liming materials. On one hand, calcium and magnesium compounds, because of their basicity, improve soil pH; on the other hand, they are also plant nutrients and stabilisers for soil aggregates. Physical treatments of slag as well as its mineral

Silicate has a special bond in the slag minerals and it is useful for plant nutrition and soil quality. In fact silicate provides beneficial effects on plant health and soil structure, increase the phosphate mobility in the soil and the efficiency of phosphate fertilisation (Rex, 2002).

Although the by-products recycling has always been a commitment of the steel industry, the growth of steel production in recent years has pressed the sector for increasing their use in a more effective way, in order to achieve a sustainable steel production. Even though steelmaking slags are continuously studied in order to improve their recycling, there are some limiting factors for their use. In particular a small amount of slags is used as fertiliser in agriculture and this use depends on the market situations. Due to the low market value of fertilisers, the long distances transportation is a limiting factor. In addition natural lime stone fertilisers are in competition to the slag use. Therefore the development of new markets for the slag, in order to ensure its utilisation in the future, is required. In this respect the steel industry is committed to minimize the amount of slag which has to be deposited,

Until the eighties steel was produced via the Thomas-Bessemer process, through the open hearth furnaces. The resulting slag containing phosphate has been used as fertiliser for about 70 years. The current steelmaking process is based on the Basic Oxygen Steelmaking process, where a basic slag is produced in the Linz-Donawitz converter. The LD slag contains about 1-3 wt% of P2O5, which is too low to be used as phosphate fertilizer, but, at the same time, it is too high to be used in the BF or recycled in the sinter plants.

composition influence the solubility and plant availability of the nutrients.

**3.1 The use of steelmaking slags as fertilisers and as liming agents** 

by improving its use through the increase of its properties (Drissen et al., 200).

2. TS-2, describing a dynamic surface leaching test (DSLT);

**3. The use of converter slag** 

2001).

3. TS-3, containing information on horizontal up-flow percolation test.

2009) and, among other things, it gives the guidelines for EU Member States about the best available techniques for producing, treating, processing and using slag.

The new European Regulation No 1907/2006 for Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH), adopted by the European Parliament and the European Council in December 2006, came into force on 1st June, 2007 (EC, 2006). This is not a directive, but a regulation, which replaces some national regulations and directives with a single system. It aims at: providing a high level of human health and environmental protection; ensuring that people are responsible to understand and manage the risks linked with the use of chemical substances that they put on the market; consolidating innovation and competitiveness of the EU chemical industry; encouraging the implementation of alternative methods for evaluating of the hazards of substances; promoting a free circulation of substances on the internal market while enhancing competitiveness and innovation (Kobesen, 2010). The registration concerns only products or by-products, while wastes are excluded from registration obligation. The regulation comprises new substances (substances which are put on the market after 18th September 1981) and phase-in-substances (substances which have been put on the market before 18th September 1981). Phase-insubstances are already registered in the so-called EINECS register (European Inventory of Existing Commercial Chemical Substances), whereas new substances will be registered in the ELINCS register (European List of Notified Chemical Substances).

Since the steel industry has been committed to clarify that iron and steel slag is produced and sold as by-product but not as waste, it was clear that ferrous slag had to be registered under REACH as a substance before 1 December 2010. In this respect the FEhS-Institute initiated the formation of a Consortium "Ferrous Slag", open to all European producers of iron and steel slag, in order to make a joint registration (Bialucha et al., 2011).

With regard to the Harmonisation Committees, the Technical Group (TG) 13, within the Technical Committee (TC) 154 concerning aggregates, deals with Dangerous Substances, by producing standards about release of some dangerous substances from aggregates. These include natural aggregates, ironmaking and steelmaking slags, defined of manufactured lightweight aggregates and recycled aggregate from material previously used in construction. The standards have to be compared with geologically similar deposits and Regulated Dangerous Substances (RDS) released are compared and identified. For slags the relevant RDS are, as follows: mineral oil, metals like V, Cr, Zn, Pb, Mo, As, Hg, Cd, other inorganic substances such as chlorides and sulphates (Kobesen, 2009).

Member states used different methods of investigation, which have to be harmonised. For example, the objectives of the Technical Committe 351 ("Construction Products – Assessment of release of dangerous substances") WG 1 are to:


The following three technical specifications have been proposed (Bialucha et al., 2011):

1. TS-1, describing principles for selecting appropriate leaching tests for a specific product;

