**3. The use of converter slag**

342 Material Recycling – Trends and Perspectives

2009) and, among other things, it gives the guidelines for EU Member States about the best

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

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

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

Member states used different methods of investigation, which have to be harmonised. For example, the objectives of the Technical Committe 351 ("Construction Products –

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;

available techniques for producing, treating, processing and using slag.

the ELINCS register (European List of Notified Chemical Substances).

inorganic substances such as chlorides and sulphates (Kobesen, 2009).

Assessment of release of dangerous substances") WG 1 are to:

• enable product TC to select the appropriate test;

• determine the release performance; • ensure methods are scientifically sound; • be relevant to CE marking of the product.

iron and steel slag, in order to make a joint registration (Bialucha et al., 2011).

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, 2001).

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 composition influence the solubility and plant availability of the nutrients.

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).

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

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

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.

Possible Uses of Steelmaking Slag in Agriculture: An Overview 345

because of the higher original pH of the slag. In addition they have supplied Ca and Mg to

The use of alkaline slag for amending acid soil and improving plant growth has been analysed in a recent study carried out in Iran (Ali & Shahram, 2007). After the application of increasing amount of slag, the soil pH proportionally increases. Moreover, at pH values between 7.4 and 8.5, the Fe availability decreases, while at higher pH values it increases; on the other hand, the P and Mn availability proportionally increases. The greenhouse studies have shown that the slag application (1% and 2% (w/w)) in tea garden soil and (0.5, 1 and 2%) in rice field soil leads to the increase of plant yield and the P and Mn uptake; an increase of Fe and K uptake has been detected in rice field, a decrease of K uptake in tea garden has

The same results have been achieved after the application of basic slag in acid sulphate soils in an incubation study. The investigation aimed at assessing the ability of basic slag to neutralize acid and its effectiveness on the solubility of basic cations in the soils, in order to achieve a sustainable use of acid sulphate soils in coastal areas of Bangladesh (Shamim et al., 2008). A wide range of processes can lead to the addition of acid cations and the removal of basic ones in the soils. The acid water, penetrating into the ground, through the leaching process, tends to increase the acidification of the soil, except if bases are compensated by different sources, such as atmospheric deposition. When acid cations are in soil solution, they tend to replace basic cations. This can also affect the metals and metalloids mobility in the groundwater, with the result that this can be a threat to groundwater and the health of aquatic and terrestrial ecosystems. Incubation experiments showed that basic slag increases soil pH, mainly due to its neutralizing effect that releases basic elements in the acid sulphate solution. This process reaches the highest value after 180 days of incubation under saturated moisture condition, probably due to slow releasing of basic ions from basic slag. In addition the application of basic slag increases K, Ca and Mg in soils, although, in some cases, over the course of time, a small decrease of this trend has been observed, probably because of the

After oxygen, silicon (Si) is the most abundant element in the earth's crust. Along with some other elements that are not considered essential, under particular agro-climatic conditions, it can increase the crop yields by promoting some physiological processes. Silicon sources for agricultural purposes must display some important features, such as high soluble Si content, low cost, availability for plants, balanced ratios and amounts of Ca and Mg, increase of phosphate mobility, suitable physical properties, easy application, and absence of heavy

In order to have an effective use of Si fertilization in agronomic practices, an adequate knowledge of physical and chemical characteristics of Si sources and of the rates and methodologies for applying them are needed. A number of field and greenhouse studies have demonstrated that the use Si soil amendments increases crop production and quality. The application of Si fertiliser has beneficial effects on both rice and sugarcane. Although the mechanism of response of sugarcane to Si fertilization is not yet well understood, some studies have shown that the yield increase of the sugarcane may be associated with different

soil and, consequently, their concentrations have increased in tea leaves.

been observed, while Fe uptake has not been changed.

formation of insoluble compounds of Ca and Mg.

metals.

**3.2 Steelmaking slags as a silicon source for plant** 

Nevertheless the LD slag contains high levels of lime (CaO) and MgO that make it a potential liming agent, may improve soil pH and can be used as plant nutrients. Particularly free lime, which is one the main slag constituents, can partially dissolve by reacting with water to produce calcium hydroxide, Ca(OH)2, as shown in Eq. (1):

$$\text{CaO} + \text{H}\_2\text{O} \rightarrow \text{Ca(OH)}\_2\tag{1}$$

The calcium hydroxide dissolves into Ca2+ and OH-, resulting in a pH increase.

All these factors characterizing this material, can allow to recycle an industrial residue and to improve the fertility of acid soils.

On this subject some studies have been performed in several countries. Among these a three-years research carried out in the Basque Country of northern Spain, by using LD converter slag on pasture land, has produced significant results (Besga et al., 1996). The comparative study analyzed soil modifications produced by LD slag and those produced by traditional liming agents. In particular, the influence on soil pH, soil Ca and Mg content, the percentage of Al saturation and the yields have been taken into consideration in this study. The achieved results concern different aspects that have been considered in it. Firstly, increasing rates of LD slag application have increased the soil pH linearly, that, as a consequence, has led to Al solubility decrease; this has allowed the P absorption, due the changing of insoluble forms to soluble ones. In addition the Ca and Mg soil contents increase, resulting in an increased yield, while the consequence of the pH has reduced the toxic effects of Mn (e.g. on the white clover). On the other hand, the Cd, Cr and Ni monitoring has shown that, after LD slag application, there was not heavy metals accumulation in the soil.

A research project that was funded by the Research Fund for Coal and Steel (RFCS) programme (Kühn et al., 2006), led to relevant results about the liming and fertilising effects of fine grained iron and steel slags, such as BF slag, LD slag or ladle slag, compared to other liming materials, such as burnt lime or carbonate limestone, in field trials investigations as well as in greenhouse pot experiments. The research aimed to investigate the fertilising effects on both the soil and the plants. These investigations have been carried out in arable land (Germany, Austria and Spain), in green land meadows (Germany and Spain), and in forestry (Spain). Furthermore the behavior of trace elements, such as Ca, Mg, P, Cr, V, Zn and Pb, in soils and plants has been investigated, in particular, the behavior of heavy metals, especially of Cr and V, their mobility and their bonds in the soil.

Investigations carried out on soil and yield have proven that the yields of experimental crops, in long term experiments by using iron and steel slags, were higher than those achieved after using different liming materials; on the other hand, pH has increased in the same way as a result of the use of both kind of materials. The use of basic slag in soil tests has produced the same fertilising effects, while the results of P content and soil pH has been higher, compared with super phosphate or rock phosphate use.

The comparison between the converter slag and the converter sludge applications to soils and tea plants, conducted in northern Iran, produced effects on soil properties and on the tea plant nutrient concentrations (Jamali, S. F. K. et al., 2006). Results have shown that converter slag application increased soil pH more than converter sludge treatment, probably

Nevertheless the LD slag contains high levels of lime (CaO) and MgO that make it a potential liming agent, may improve soil pH and can be used as plant nutrients. Particularly free lime, which is one the main slag constituents, can partially dissolve by reacting with

All these factors characterizing this material, can allow to recycle an industrial residue and

On this subject some studies have been performed in several countries. Among these a three-years research carried out in the Basque Country of northern Spain, by using LD converter slag on pasture land, has produced significant results (Besga et al., 1996). The comparative study analyzed soil modifications produced by LD slag and those produced by traditional liming agents. In particular, the influence on soil pH, soil Ca and Mg content, the percentage of Al saturation and the yields have been taken into consideration in this study. The achieved results concern different aspects that have been considered in it. Firstly, increasing rates of LD slag application have increased the soil pH linearly, that, as a consequence, has led to Al solubility decrease; this has allowed the P absorption, due the changing of insoluble forms to soluble ones. In addition the Ca and Mg soil contents increase, resulting in an increased yield, while the consequence of the pH has reduced the toxic effects of Mn (e.g. on the white clover). On the other hand, the Cd, Cr and Ni monitoring has shown that, after LD slag application, there was not heavy metals

A research project that was funded by the Research Fund for Coal and Steel (RFCS) programme (Kühn et al., 2006), led to relevant results about the liming and fertilising effects of fine grained iron and steel slags, such as BF slag, LD slag or ladle slag, compared to other liming materials, such as burnt lime or carbonate limestone, in field trials investigations as well as in greenhouse pot experiments. The research aimed to investigate the fertilising effects on both the soil and the plants. These investigations have been carried out in arable land (Germany, Austria and Spain), in green land meadows (Germany and Spain), and in forestry (Spain). Furthermore the behavior of trace elements, such as Ca, Mg, P, Cr, V, Zn and Pb, in soils and plants has been investigated, in particular, the behavior of heavy metals,

Investigations carried out on soil and yield have proven that the yields of experimental crops, in long term experiments by using iron and steel slags, were higher than those achieved after using different liming materials; on the other hand, pH has increased in the same way as a result of the use of both kind of materials. The use of basic slag in soil tests has produced the same fertilising effects, while the results of P content and soil pH has been

The comparison between the converter slag and the converter sludge applications to soils and tea plants, conducted in northern Iran, produced effects on soil properties and on the tea plant nutrient concentrations (Jamali, S. F. K. et al., 2006). Results have shown that converter slag application increased soil pH more than converter sludge treatment, probably

CaO + H2O → Ca(OH)2 (1)

water to produce calcium hydroxide, Ca(OH)2, as shown in Eq. (1):

especially of Cr and V, their mobility and their bonds in the soil.

higher, compared with super phosphate or rock phosphate use.

to improve the fertility of acid soils.

accumulation in the soil.

The calcium hydroxide dissolves into Ca2+ and OH-, resulting in a pH increase.

because of the higher original pH of the slag. In addition they have supplied Ca and Mg to soil and, consequently, their concentrations have increased in tea leaves.

The use of alkaline slag for amending acid soil and improving plant growth has been analysed in a recent study carried out in Iran (Ali & Shahram, 2007). After the application of increasing amount of slag, the soil pH proportionally increases. Moreover, at pH values between 7.4 and 8.5, the Fe availability decreases, while at higher pH values it increases; on the other hand, the P and Mn availability proportionally increases. The greenhouse studies have shown that the slag application (1% and 2% (w/w)) in tea garden soil and (0.5, 1 and 2%) in rice field soil leads to the increase of plant yield and the P and Mn uptake; an increase of Fe and K uptake has been detected in rice field, a decrease of K uptake in tea garden has been observed, while Fe uptake has not been changed.

The same results have been achieved after the application of basic slag in acid sulphate soils in an incubation study. The investigation aimed at assessing the ability of basic slag to neutralize acid and its effectiveness on the solubility of basic cations in the soils, in order to achieve a sustainable use of acid sulphate soils in coastal areas of Bangladesh (Shamim et al., 2008). A wide range of processes can lead to the addition of acid cations and the removal of basic ones in the soils. The acid water, penetrating into the ground, through the leaching process, tends to increase the acidification of the soil, except if bases are compensated by different sources, such as atmospheric deposition. When acid cations are in soil solution, they tend to replace basic cations. This can also affect the metals and metalloids mobility in the groundwater, with the result that this can be a threat to groundwater and the health of aquatic and terrestrial ecosystems. Incubation experiments showed that basic slag increases soil pH, mainly due to its neutralizing effect that releases basic elements in the acid sulphate solution. This process reaches the highest value after 180 days of incubation under saturated moisture condition, probably due to slow releasing of basic ions from basic slag. In addition the application of basic slag increases K, Ca and Mg in soils, although, in some cases, over the course of time, a small decrease of this trend has been observed, probably because of the formation of insoluble compounds of Ca and Mg.

#### **3.2 Steelmaking slags as a silicon source for plant**

After oxygen, silicon (Si) is the most abundant element in the earth's crust. Along with some other elements that are not considered essential, under particular agro-climatic conditions, it can increase the crop yields by promoting some physiological processes. Silicon sources for agricultural purposes must display some important features, such as high soluble Si content, low cost, availability for plants, balanced ratios and amounts of Ca and Mg, increase of phosphate mobility, suitable physical properties, easy application, and absence of heavy metals.

In order to have an effective use of Si fertilization in agronomic practices, an adequate knowledge of physical and chemical characteristics of Si sources and of the rates and methodologies for applying them are needed. A number of field and greenhouse studies have demonstrated that the use Si soil amendments increases crop production and quality. The application of Si fertiliser has beneficial effects on both rice and sugarcane. Although the mechanism of response of sugarcane to Si fertilization is not yet well understood, some studies have shown that the yield increase of the sugarcane may be associated with different

Possible Uses of Steelmaking Slag in Agriculture: An Overview 347

As described above, since the silicon fertilisation has been turned out to be beneficial for plant growth, such as rice and sugar cane, the identification the most promising and potential available Si sources to plant has been studied. In particular, in a greenhouse experiment several Si sources have been evaluated in order to test their ability to supply Si to rice crops. Among different Si-rich materials, metallurgical slags have been evaluated, because the high temperatures used in ironmaking and steelmaking processes release Si from crystalline form to reactive and as consequence more soluble forms, with the result to supply it to plants (Pereira et al., 2004). In the comparative study differences between silicon sources in relation to Si uptake have been observed. Furthermore steel slags (LD, AOD, electric, and stainless steel furnaces) have shown higher Si availability than BF slag, and differences depending on the type of steel produced and on the type of furnace used to

On the other hand, recent studies have shown that Si concentration is negatively related to As content in straw and polished rice, that is Si in the soil available for plant reduces the

Some investigations about the addition of steel slags in contaminated soils have been carried out. The stabilization technique is based on the incorporation of amendments, in order to minimize metals and metalloids, such as As, Cr, Cu, Pb, Cd and Zn that can be found in contaminated soils at wood treatment plants. In particular, when the copper sulphate and chromate copper arsenate are used to protect wood from insects and fungi, they can cause the soil phytotoxicity. While the As can be stabilized by sorption on Fe oxides and also by the formation of amorphous Fe (III) arsenates, the Cr immobilization takes place through Cr reduction from Cr (VI), which is mobile and toxic, to Cr (III), which is stable. The Cu stability in soil is pH dependent, because its mobility increases with decreasing pH. Carbonates, phosphates and clays can reduce the mobility and availability of Cu in soil. The proposed mechanism consists in precipitation of Cu carbonates and oxy-hydroxides, ion exchange and formation of ternary cation–anion complexes on the surface of Fe and Al oxyhydroxides. While Pb can be stabilized by using phosphorus-containing amendments, that reduce the Pb mobility, Zn can be immobilized in soil by using phosphorus amendments and clays (Kumpiene et al., 2008, as cited in Negim et al., 2009). To this aim some chemical and mineralogical agents, such as industrial by-products have been applied. For instance, the use of alkaline materials, organic matters, phosphates, alumina-silicates and basic slag has been shown to limit the accumulation of Cu in plants cultivated in Cu-contaminated

Among recent studies, the use of slag with basic properties into a Cu-contaminated soil has led to relevant results in soil composition (Negim et al., 2010). Because of its Ca and P content, the basic slag, on one hand, is a fertiliser, as it improves the physico-chemical properties of the soil and by increasing plant growth; on the other hand, it is a liming material, as it increases the precipitation and sorption of metals such as Cu. For this reason the investigation concerned the effects on soil pH, soil conductivity, plant growth and chemical composition of bean plants (*Phaseolus vulgaris*) in pot experiments, by mixing soil with increasing basic slag addition rates, from 0% to 4%, in controlled conditions. This material affects the soil solution composition through acid-base, precipitation and sorption

**4. Use of steel slags for metal stabilization in contaminated soils** 

produce steel. Phosphate slags provided the highest Si uptake.

uptake of As (Bodgan & Schenk, 2009).

soils.

factors, such as Al, Mn and Fe toxicity alleviation, increased P availability, reduced lodging, improved leaf and stalk erectness, freeze resistance and improvement in plant water economy. Furthermore the Si accumulation protects plants from certain diseases, such as a resistance to biotic and abiotic stresses (Savant et al., 1999). The use of Si in the plants can help the plants against pathogen attacks (Motz & Geiseler, 2001).

Fig. 4. Production of potassium silicate fertilizer from steelmaking slag (Takahashi, 2002).

The use of silica (SiO2) as a fertiliser increases the rice resistance to diseases and vermin. For this purpose the slag produced in the hot metal desiliconization process, which contains mainly silica, has been taken into consideration in order to develop a potassium silicate fertiliser. This is an example of a new steelmaking process developed by NKK (Nippon Kokan Kabushiki Kaisha) and referred as ZSP (Zero Slag Process), focused on the reduction of the amount of generated slag and also to the stabilization of the composition of slag generated through hot metal pre-treatment (Takahashi, 2002). This fertiliser, developed by adding potassium to the desiliconization slag, dissolves with difficulty in the water and slowly dissolves in the weak citric acid released by plant roots. The potassium contained into this fertiliser is slowly released and is effectively adsorbed by plant. The process consists in the desiliconisation process of the hot metal and subsequently the potassium carbonate (K2CO3) is continuously added into the ladle containing the hot metal (Fig. 4). Then the uniformly melted slag, that is recovered from the hot metal ladle, is solidified by cooling and crushed into a granular form. This fertilizer has been demonstrated to be as effective as other commercial potassium silicate fertilisers and combined potassium chloride-calcium silicate fertilisers when it was applied to some vegetables, in particular to rice. Its marketing started in December 2001 in Japan.

factors, such as Al, Mn and Fe toxicity alleviation, increased P availability, reduced lodging, improved leaf and stalk erectness, freeze resistance and improvement in plant water economy. Furthermore the Si accumulation protects plants from certain diseases, such as a resistance to biotic and abiotic stresses (Savant et al., 1999). The use of Si in the plants can

Fig. 4. Production of potassium silicate fertilizer from steelmaking slag (Takahashi, 2002).

rice. Its marketing started in December 2001 in Japan.

The use of silica (SiO2) as a fertiliser increases the rice resistance to diseases and vermin. For this purpose the slag produced in the hot metal desiliconization process, which contains mainly silica, has been taken into consideration in order to develop a potassium silicate fertiliser. This is an example of a new steelmaking process developed by NKK (Nippon Kokan Kabushiki Kaisha) and referred as ZSP (Zero Slag Process), focused on the reduction of the amount of generated slag and also to the stabilization of the composition of slag generated through hot metal pre-treatment (Takahashi, 2002). This fertiliser, developed by adding potassium to the desiliconization slag, dissolves with difficulty in the water and slowly dissolves in the weak citric acid released by plant roots. The potassium contained into this fertiliser is slowly released and is effectively adsorbed by plant. The process consists in the desiliconisation process of the hot metal and subsequently the potassium carbonate (K2CO3) is continuously added into the ladle containing the hot metal (Fig. 4). Then the uniformly melted slag, that is recovered from the hot metal ladle, is solidified by cooling and crushed into a granular form. This fertilizer has been demonstrated to be as effective as other commercial potassium silicate fertilisers and combined potassium chloride-calcium silicate fertilisers when it was applied to some vegetables, in particular to

help the plants against pathogen attacks (Motz & Geiseler, 2001).

As described above, since the silicon fertilisation has been turned out to be beneficial for plant growth, such as rice and sugar cane, the identification the most promising and potential available Si sources to plant has been studied. In particular, in a greenhouse experiment several Si sources have been evaluated in order to test their ability to supply Si to rice crops. Among different Si-rich materials, metallurgical slags have been evaluated, because the high temperatures used in ironmaking and steelmaking processes release Si from crystalline form to reactive and as consequence more soluble forms, with the result to supply it to plants (Pereira et al., 2004). In the comparative study differences between silicon sources in relation to Si uptake have been observed. Furthermore steel slags (LD, AOD, electric, and stainless steel furnaces) have shown higher Si availability than BF slag, and differences depending on the type of steel produced and on the type of furnace used to produce steel. Phosphate slags provided the highest Si uptake.

On the other hand, recent studies have shown that Si concentration is negatively related to As content in straw and polished rice, that is Si in the soil available for plant reduces the uptake of As (Bodgan & Schenk, 2009).
