Production and utilisation of iron and steelmaking slag in Japan and the application of steelmaking slag for the recovery of paddy fields damaged by Tsunami

Abstract

In Japan, about 40 million tonnes of slag is generated annually during the iron and steelmaking process. This large amount of byproduct warrants investigation on its disposal. In this study, the production, chemical characteristics, and use of iron and steel slag in Japan, especially converter slag, have been reviewed. Then, the necessity for sodium removal and nutrient supply for the recovery of the paddy fields damaged by the Tsunami, which resulted from the great east Japan earthquake, has been explained. Since converter slag can serve the above two requirements simultaneously, owing to its chemical properties, investigations on efficient utilisation of converter slag have been carried out. With the perspective of both agriculture and engineering science, the positive effects of converter slag on desalting, pH improvement, and supply of nutrients such as silicon have been found.

Keywords

Iron and steel slag Converter slag Slag utilisation Paddy Fertiliser Soil improvement

Survey of iron and steel slag production and utilisation in Japan

Steel is an indispensable material for our society. Consequently, the steel industry contributes greatly to other industries as well as to the world economy; in fact, the annual steel consumption per capita is considered a sign of the development level of a nation.

However, in recent years, because of stricter environmental requirements, the iron and steel industry faces a tough situation, since it is a large resource consumer and waste generator, one clear path is the more effective use of its most abundant byproduct: iron and steel slag.

In general, iron and steel slag includes the gangue separated from the iron ore after reduction, and the artificially added oxide flux for removing impurities during refining (^{Tsukihashi, 2011}). Iron and steel slag is an inevitable byproduct of iron and steel production.

Slag production and classification in Japan

In Japan, almost 100 million tonnes of steel is produced every year, and along with this production about 40 million tonnes of various slags are generated (^{Nippon Slag Association, 2010}).

Figure 1 (^{Nippon Slag Association, 2010}) shows the typical iron and steelmaking process and slag generation in Japan. For ironmaking, about 300 kg of blast furnace slag is generated in producing 1 tonne of iron. For steelmaking, about 100 to 150 kg/t steel converter slag is discharged, which contains not only the converter slag but also the slags generated during hot metal pretreatment. When scrap is used as the main raw material for steelmaking, about 120 kg/t steel of electric arc furnace slag is generated.

Figure 1

Iron and steelmaking process and slag generation in Japan

Normally, blast furnace slag is either quenched by water or slowly cooled in air; the quenched slag is referred to as granulated blast furnace slag. Recently, the ratio of the granulated blast furnace slag to the total blast furnace slag has been found to be 80% (^{Nakagawa, 2011}). Generally, converter slag and electric arc furnace slag are slowly cooled at slag yard in air.

The main chemical compositions of classified iron and steel slag are listed in Table 1 (^{Nippon Slag Association, 2010}). For all kinds of slags, the main components are CaO, SiO₂, Al₂O₃, MgO, and iron oxides. However, because the slag compositions are controlled to achieve different metallurgical functions (composition control of hot metal and molten steel, impurity removal, etc.) and high-temperature physicochemical characteristics (melting temperature, viscosity, wetting condition, etc.), the variation of composition is wide due to different refining purposes and causes difficulties in utilisation since consideration must be given to every kind of slag.

Table 1

Typical chemical compositions of slag in iron and steel industry (mass-%)

Component	Type
	Blast furnace slag (water quench, air cooling)	Converter slag	Electric arc furnace slag
	Blast furnace slag (water quench, air cooling)	Converter slag	Oxidizing slag	Reducing slag
CaO	41·7	45·8	22·8	55·1
SiO₂	33·8	11·0	12·1	18·8
Total iron	0·4	17·4	29·5	0·3
MgO	7·4	6·5	4·8	7·3
Al₂O₃	13·4	1·9	6·8	16·5
S	0·8	0·06	0·2	0·4
P₂O₅	<0·1	1·7	0·3	0·1
MnO	0·3	5·3	7·9	1·0

Figure 2 (^{Nippon Slag Association, 2013}) shows the utilisation of blast furnace slag, converter slag, and electric arc furnace slag in Japan in 2012. Almost 70·4% of blast furnace slag was used to produce cement. Compared to common cement, blast furnace cement has a stronger latent hydraulic property and can reduce nearly 40% of the CO₂ emission during cement production (^{Nakagawa, 2011}). In addition, since blast furnace slag has not been sent to landfills, as shown in Fig. 2, the utilisation ratio of blast furnace slag can be considered as 100% in Japan.

Figure 2

Utilisation of iron and steel slag in Japan in 2012

Both converter slag and electric arc furnace slag are mainly utilised for road and civil construction, ground improvement, and reuse within the steel plant. Since the production of converter slag in 2012 (10·9 million tonne) is 3·7 times greater than that of the electric arc furnace slag (2·9 million tonne) (^{Nippon Slag Association, 2013}), the utilisation of converter slag is more important.

According to Table 1, both the CaO/SiO₂ ratio and the total iron content are very high for converter slag; a small amount of phosphorus and magnesium oxide is also present. This variable composition makes for more diverse utilisation of converter slag; nevertheless, it has been mainly used for road and civil construction, as shown in Fig. 2. In order to better understand the properties of converter slag before discussing its use, more attention must be paid on the production of converter slag.

Production of converter slag

In Japan, before decarburisation of pig iron, hot metal pretreatment for removing Si, P, and S is widely used. This hot metal pretreatment is often performed in a torpedo, ladle, or converter depending on the conditions in each steel factory. Therefore, as mentioned, the converter slag in Japan does not only mean the slag generated from the converter, but also includes hot metal pretreatment slags.

Slag compositions are quite different depending on the specific refining purposes during hot metal pretreatment and converter refining, which is summarised in the CaO–SiO₂–FeO _x ternary system in Fig. 3 (^{Kitamura et al., 2012}). The differences among these slags can be distinguished by the comparison of the iron oxide concentration and the CaO/SiO₂ ratio.

Figure 3

Variations on slag compositions for different refining processes during steelmaking

Except for desulphurisation, which is carried out in a reducing atmosphere, the slags after desiliconisation, dephosphorisation, and decarburisation contain iron oxides.

The CaO/SiO₂ ratio is often used as the basis for determining the amount of lime required; lime plays the main role in controlling the slag amount. Therefore, in order to shorten the refining time and decrease the slag amount for the dephosphorisation and decarburisation processes, in some cases silicon in pig iron is removed first (^{Kawabata et al., 2002}). However, a high CaO/SiO₂ ratio for a traditional desiliconisation process is not required, as shown in Fig. 3.

For dephosphorisation slag, a high CaO/SiO₂ ratio is often required, and the variation of the composition regions of dephosphorisation slag, shown in Fig. 3, refers to different dephosphorisation conditions adopted in steel plants. In normal practice, hot metal dephosphorisation uses 2CaO·SiO₂ saturated slag at 1623–1723 K, 2CaO·SiO₂–3CaO·P₂O₅ solid solutions forms during dephosphorisation (^{Fix et al., 1969}; ^{Ito et al., 1982}; ^{Ogawa et al., 2001}; ^{Tsukihashi, 2009}; ^{Shimauchi et al., 2009}; ^{Pahlevani et al., 2010}; ^{Kitamura et al., 2009}; ^{Hamano et al., 2004}; ^{Hamano et al., 2006}; ^{Saito et al., 2009}; ^{Miyamoto et al., 2009}), and the ratio between 2CaO·SiO₂ and 3CaO·P₂O₅ in solid solution changes with the variation of liquid-phase composition during refining (^{Gao et al., 2012}, ^2013a, ^b, ^c).

For the decarburisation slag, a higher CaO/SiO₂ ratio is required, even after the hot metal dephosphorisation to suppress the phosphorus recovery during the process. Thus, solid CaO and 2CaO·SiO₂ coexist in the slag after decarburisation. In many cases, the decarburisation slag is reused in the dephosphorisation or sintering process, as the content of phosphorus in this slag is very low (^{Ogawa et al., 2001}).

The CaO/SiO₂ ratio is extremely high in the desulphurisation slag because of the low SiO₂ content, as desulphurisation is a reduction reaction (^{Hara et al., 1986}). After refining, the desulphurisation slag can be reused in the sintering process (^{Kumakura, 2012}).

The dephosphorisation and decarburisation slag consists of solid solutions and liquid phase in the molten state, and the decomposition of liquid phase to various mineralogical solid phases happens when these slags are slowly cooled in air (^{Yildirim and Prezzi, 2011}).

In addition, owing to the requirement of a high CaO/SiO₂ ratio and the shortened refining time, the added lime (CaO) cannot be totally dissolved in molten slag, thus free lime is present in the dephosphorisation and decarburisation slag after cooling. Furthermore, since this free lime reacts with moisture in the air leading to volume expansion, the slag cannot be directly utilised without treatment.

On the other hand, as previously mentioned, the presence of some valuable elements (e.g., Fe, P, and Mn) provides more functional utilisation options for converter slag.

Utilisation of converter slag

No matter the application, converter slag used in an open environment must satisfy the standards (JISK0058, JISA5015, etc.) legislated by the ^{Japanese Industrial Standards Committee (JISC, 2009}, ²⁰¹³). In addition, before utilisation, normally the slag is crushed within the steel plant to recover the metallic iron within the steel plants in order to retrieve iron and also to reduce the impurities of slag when used as earthwork material (^{Horii et al., 2012}).

Conventional utilisation of converter slag

Typical uses of converter slag outside the steel plant are mainly for road material and civil construction (see Fig. 2).

In the case of road material, the converter slag is often applied to construct the sub-base for the road (^{Nakagawa, 2011}), but before applying steelmaking slag, the volume expansion because of the presence of free-CaO and free-MgO must be eliminated. Therefore, slag aging was developed in Japan (^{Sasaki et al., 1982}). The principle of slag aging is to stabilise the free-CaO and free-MgO into calcium or magnesia hydroxide. The typical method for slag aging is to react the converter slag with water steam at 373 K for 2–4 days (^{Nishinohara et al., 2013}). Subsequently, the utilisation of converter slag for road material becomes acceptable, and moreover the advantages such as high strength and wear resistance due to the iron content highlight the application value. In 2002, converter slag was declared as an eco-friendly good on which priority should be placed in procurement for both sub-base material and aggregate material for asphalt concrete, according to the Law Concerning the Promotion of Procurement of the Eco-Friendly Goods and Services by the State and Other Entities (Law on Promoting Green Purchasing), enacted by the Ministry of the Environment of Japan (^{Nakagawa, 2011}). In the case of utilisation for civil construction, converter slag is used for building temporary roads and levelling ground.

In addition to these major utilisation methods, one example for the direct application of the chemical characteristic of converter slag is the utilisation for fertilisers and soil improvement materials. However, the utilisation ratio for both converter slag and blast furnace slag to the total slag production is low (0·5% in 2010; ^{Tsutsumi and Kitano, 2012}).

According to Table 1, since half of the converter slag is composed of alkali elements, especially calcium, the soil pH can be adjusted. In addition, the converter slag has higher soil pH improving ability compared with other normal fertilisers made with magnesia lime or calcium hydroxide, because of the rapid dissolution of free lime and long-term dissolution of solid solutions containing silicon and calcium. In the case of paddy field, silicon is the most important nutrient element for paddy growth, while iron acts to suppress hydrogen sulphide formation. In addition, phosphorus and manganese, as important nutrient elements, are also contained in converter slag.

Similar to the case for road material and civil construction, in some cases, treatments for converter slag are needed before being utilised as fertiliser, such as steam aging of slag after crushing and metallic iron recovery in order to prevent spontaneous decay during deposition (^{Hirano and Minex Co. Ltd, 2006}), etc.

Frontier utilisation of converter slag

Despite the above common applications with their long history of use, other new utilisation methods for converter slag based on its chemical characteristics have been investigated and adopted.

One is the innovative utilisation of steelmaking slag in marine environment for recovering phytoplankton to absorb CO₂ (^{Hino, 2003}; ^{Yamamoto, 2003}). In conjunction with this utilisation, in order to suppress pH increase after slag is introduced in sea water, the carbonation treatment for stabilising steelmaking slag has been applied (^{Miki et al., 2009}). The investigation of this marine utilisation of steelmaking slag is still in progress and is focused on a more effective nutrients supply for plankton (^{Yamamoto et al., 2003}, ²⁰¹¹; ^{Futatsuka et al., 2003}; ^{Arita et al., 2003}; ^{Yokoyama et al., 2010}; ^{Fujimoto et al., 2011}; ^{Zhang et al., 2012}). Other utilisation purposes in marine environment, such as preventing the eutrophication of phosphorus and suppressing the activity of sulphide-reducing bacteria, have also been investigated (^{Hayashi et al., 2012a}, ^b).

Additionally, with the aim of extracting valuable elements, efforts have also been made on the recovery of Mn and P from slag (^{Matsube-Yokoyama et al., 2009}; ^{Teratoko et al., 2012}; ^{Maruoka et al., 2013}; ^{Kim et al., 2011}, ²⁰¹²)

After the great east Japan earthquake and Tsunami in 2011, the requirement for reconstruction, such as roads and ports, as well as damaged farmland recovery, has initiated recent attention on the utilisation of the iron and steel slag. In particular, the value of converter slag used as a fertiliser and soil-improving agent on the recovery of damaged paddy fields has been considered and evaluated more intensively.

Measures to recover the paddy field damaged by Tsunami

Paddy fields after the Tsunami

As shown in Fig. 4 (GSI, 2011; Ministry of Agriculture, Forestry and Fisheries of Japan ^{[MAFF], 2011}), about 23 600 ha (2360 km²) of cultivated land along the northeastern coast was immersed by Tsunami (^{MAFF, 2011}), caused by the great east Japan earthquake on 3 March 2011. After the sea water receded, thick sludge remained on the land surface. This means a large area of fertile granary, as we so remembered, is gone.

Figure 4

Cultivated land damage in Miyagi area after Tsunami caused by the great east Japan earthquake on 11 March 2011

Table 2 lists the concentration differences between sea and river water in Japan. For all the main components, the concentrations in sea water are much higher than that of river water (^{Kitamura et al., 2012}). Therefore, after the Tsunami, the chemical property of the soil was severely damaged, not only by sea water but also the marine sludge.

Table 2

Comparison on the main chemical compositions of sea and river water in Japan

Composition	Sea water¹/mg L⁻¹	River water²/mg L⁻¹
Na	10 556	6·7
Mg	1 272	1·9
Ca	400	8·8
K	380	1·2
Cl⁻	18 980	5·8
	2 649	10·6
	140	31·0

1: Average composition of sea water; 2: average composition of river water.

In the damaged paddy fields, the electrical conductivity (EC) values of soil are often used to appraise the salted state (^{MAFF, 2011}), Fig. 5 shows the effect of increased EC of the soil on paddy growth (^{Nakata, 2011}). It is obvious that the productivity of the paddy suffers as the EC of the soil increases. The following reasons are considered for this:

Figure 5

Paddy growth on salted soil

With increase in osmotic stress, the absorption of moisture from roots is suppressed.

As excessive sodium surrounds the root, the absorption of other nutrient elements, e.g. potassium ion, becomes difficult and causes many physiological disorder.

III

By the enrichment of sodium and loss of calcium, potassium etc., the aggregated structure of clay particles in the soil is destroyed.

The first two reasons can be attributed to the excessive but water dissolvable sodium, while the last one is triggered by the excessive sodium absorbed by clay particles. The detail of the damage on the soil due to this excessive absorbed sodium can be explained as follows: normally, as expressed in the left side of Fig. 6, many cations absorb on the negatively charged clay particle, and the aggregated structure is formed with good balance of water, soil, and air. However, excessive sodium cations destroy the aggregated structure as shown in the right side of Fig. 6; thus, the balance of water, soil, and air become worse and the soil structure becomes dense (^{MAFF, 2011}). Then, the drainage characteristics and breathability decrease (^{MAFF, 2011}).

Figure 6

Paddy soil degeneration after Tsunami

Paddy field desalting by calcium supply

The chemical state of the soil after the Tsunami is summarised in Fig. 7 (^{Kitamura et al., 2012}; ^{Kanno et al., 2014}; ^{Akai et al., 2012}). The sodium content is much higher than normal levels. Although the water dissolvable sodium can be washed out by rain, the content of sodium is still higher than normal. The sodium absorbed by the soil cannot be simply removed by irrigation or rainfall. Instead, the addition of calcium ions holds the key. As shown in Fig. 8 (^{Kitamura et al., 2012}), after the addition of calcium ions to the paddy soil, the sodium ions could be exchanged and finally washed away by rainfall. The operation for desalting of paddy fields recommend by the Ministry of Agriculture, Forestry and Fisheries (MAFF) of Japan is introduced in Fig. 9 (^{MAFF, 2011}), in which the requirement for calcareous material is specified.

Figure 7

Chemical status of damaged soil

Figure 8

Desalting process by feeding calcium ion

Figure 9

Desalting operation recommended by MAFF of Japan

In order to feed calcium to the paddy soil, calcareous materials such as gypsum and calcium carbonate were considered first. However, the paddy soil became poor in nutrients especially silicate, as they have also been washed out by the desalting treatment through drainage after flooding and breaking up the soil (^{Kitamura et al., 2012}). Therefore, a material that provides nutrient elements along with calcium becomes a better choice.

Slag fertiliser could serve both the above purposes at the same time (Kato et al. 1997; Kato and Owa et al. 1997). The expected characteristics of converter slag fertiliser on the recovery of damaged paddy fields are summarised in Table 3 (^{Gao et al., 2013a}.^b, ^c).

Table 3

Expected characteristic of fertiliser made by converter slag on paddy field recovery

Main functional components	Content/mass-%	Effect on paddy field
CaO	50	Enhancement of exchanging eluviation of Na and adjustment of concentration balance in basic elements.
		Adjustment of pH by the supplement of basic element.
Iron oxides	20	Restraint on formation of H₂S by the supplement of easily reducible iron oxides.
SiO₂	15	Improvement of rice growth by the alleviation of excess Na damage through the supplement of silicate.
P₂O₅	1	Normal nutrient elements.
MgO	4	Normal nutrient elements.
MnO	3	Normal nutrient elements.

Necessity of Fe supply

The damaged paddy soil also suffers from acidification by the formation of hydrogen sulphide (H₂S), due to sludge accumulation. As shown in Fig. 10, the sludge left by the Tsunami contains FeS₂, which is stable under sea water. However, it is easily oxidised by air and forms sulphate ( ). Then, by the role of sulphate-reducing bacteria, the formation of H₂S, which acidifies the soil and causes the detrimental damage to the paddy field, is predictable. To stabilise the formed H₂S, the addition of iron ions is effective as it forms stable FeS. Since the converter slag fertiliser contains iron as shown in Table 1 and which is dissolvable in soil solution (^{Nozoe et al., 1999}, ²⁰⁰¹), the release from soil acidification can be expected by applying converter slag fertiliser.

Figure 10

Restraint on the generation of hydrogen sulphide

Application of converter slag on damaged paddy field for recovery

In 2012, the Iron and Steel Institute of Japan (ISIJ) Innovative Program for Advanced Technology was established with the representative of current author Shin-Ya Kitamura. The purpose is to recover the paddy fields damaged by the Tsunami after the great earth quake in northeastern Japan using steelmaking slag. In this project, the cooperation among researchers majoring in either agriculture or engineering has been established.

For agricultural research, the effects of commercially available converter slag fertilisers (lawful slag fertiliser of sufficient amount of plant required elements and environmental bearable heavy metal release, and also satisfy the open environment standards mentioned in section 2) on both desalting and soil improvement during paddy growth will be confirmed. This will be achieved by the following: (1) comparison of paddy growth condition with and without converter slag applied during both pot cultivation and the cultivation in an experimental plot and (2) evaluation on practical application of converter slag on damaged paddy fields. In this study, the results obtained by applying commercially available slag fertiliser to actual damaged paddy field will be introduced.

The damaged paddy field selected in this study is located in the lower reaches of the Kitakami River where the sea water flowed backward in the river because of the Tsunami. Several decimetres of sea water covered the field as well as several centimetres of sludge. The desalting treatment was carried out in 2011 following the recommended operation shown in Fig. 9, and then the converter slag fertiliser was applied at the rates of 200 and 400 g m⁻². This applied amount was determined by the standard value according to the fertiliser maker. As shown in Fig. 11, the paddy grew well after the converter slag fertiliser was applied. Within this paddy field, the existence of slag promotes the paddy growth during sprouting and maturity season, and no apparent obstacle was found. After harvest, as shown in Fig. 12, an increase of about 8% on the yield of rice compared to that without slag application was confirmed, and this increase extends to another 4% when the applied slag amount was doubled. The increase in the yield of rice is attributed to the following: (1) improvement of soil pH by slag application, which promotes transformation of nitrogenous organics into inorganic compounds and (2) supply of silicate from slag.

Figure 11

Attempt on paddy cropping in salted soil with steelmaking slag applied

Figure 12

Effect of converter slag fertiliser on the soil pH

Figure 13 shows the pH variation of the paddy soil with and without the application of converter slag fertiliser during paddy growth. The pH was measured by inserting an pH probe in the plough layer within the entire cultivation period. The improvement of soil pH by converter slag fertiliser is obvious, and this improvement lasts throughout the cultivation period. This can be explained by the short-time alkali release by free lime and the long-term release by the solid solutions containing calcium and silicon mentioned previously.

Figure 13

Effect of converter slag fertiliser on the supplement of hydrated silica

Figure 14 shows that there is an increase in silicate after the converter slag fertiliser was applied. The rapid and slow decrease in silicate concentration in the figure corresponds to the different growing periods of the paddy. The soil solution for silicate analysis was sampled at 7 cm depth from soil surface by suction through a porous cup, and the silicate content in the soil solution was analysed by chemical method.

Figure 14

Effect of converter slag fertiliser on silicate concentration in soil solution

On the other hand, the task for engineering research is to provide a more effective slag fertiliser for both desalting and soil improvement, with the consideration of refining ability for steelmaking process. This indicates that an interdisciplinary perspective for both engineering and agricultural science is required, and a newly evolved steelmaking slag, which can be used as a multifunctional soil improvement material while maintaining good refining ability, can be expected. Therefore, the desire for slag valorisation for steel plants, desalting, and soil improvement needs to be simultaneously satisfied. First, the surveys and statistics on both the composition and the mineral phases of the commercially available converter slag fertilisers were conducted. This was necessary because of the differences in both the chemical compositions as mentioned in Fig. 3 and the mineral phases shown in Fig. 15. Then, the dissolution behaviours of converter slags as well as the mineral phases in the paddy field environment will be clarified. In this case, the dissolution behaviour under both an oxidising and reducing atmosphere will be paid attention, since the redox potential changes widely in the paddy field environment (^{Husson, 2013}), leading to the variation of the solubility of some valence variable elements such as Fe. Based on the above engineering research, the role of converter slag fertiliser will be evaluated and the optimisation for more effective fertiliser will be achieved.

Figure 15

Morphology for rapidly and slowly quenched slag

Conclusion

The production, chemical characteristics, and recent utilisation of iron and steel slag in Japan have been reviewed. Specifically, the details on the production and utilisation methods for converter slag were introduced. Moreover, the necessities for the recovery of paddy fields damaged by the Tsunami after the great east Japan earthquake, through sodium removal and nutrient supply, were explained. In such circumstances, converter slag can serve the above two necessities simultaneously because of its chemical property, but more concerns need to be addressed before actual application. Therefore, research has been established among researchers in agricultural science and engineering science, under the ISIJ Innovative Program for Advanced Technology. By the efforts of this collaboration, attempts based on laboratory scale experiments and actual cultivation have been made. Positive effects of converter slag fertiliser on desalting, pH improvement, and nutrients supply, such as silicon, have been found, on normal and damaged paddy fields. Investigations are under way to provide a more effective converter slag fertiliser and soil improver that still maintains good refining ability for the steelmaking process.

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