**6. Recovery of phosphorus from steelmaking slags**

Phosphorus is an essential element not only for plants and animals but also, along with nitrogen (N) and potassium (K), for fertilisers production. Nevertheless P has a detrimental effect on steel, by reducing the low-temperature toughness of iron and steel products. For this reason during the hot metal pre-treatment and in the steelmaking processes the dephosphorisation process takes place. The result is that most of the phosphorus in hot metal, originating from raw materials (e.g. iron ore and coal), is removed through the steelmaking slag.

The requirement of reducing environmental impact and disposal costs of slags has led to study and develop different strategies in order to internally recycle them. The slag recycling in steel production processes allows to realize a waste-free steelmaking process as well as the consumption reduction of iron and lime.

For its high content of lime (CaO) the LD slag can be used as fluxing material, replacing limestone, with the result that iron and steelmaking costs are reduced. Nevertheless free CaO reduces the effective use of LD slag, because this compound makes low hydraulic properties of slag. However the slag recycling in internal processes is limited, due to its high amount of S and P (about 1-3% of P2O5 content in the converter slag) that negatively affect them. These values are too high for using it into sinter process (where fine grain raw material is processed into coarse grained iron ore sinter for charging the BF), BF and in predephosphorisation process; on the other hand they are too low for slag use as phosphatic fertiliser. Therefore some studies have been carried out in order to remove phosphorus from steelmaking slag. One of them concerns a waste-free steelmaking process in which the slag is recycled within the steelmaking process and a slag containing high phosphorous is produced and it is suitable as fertiliser (Li et al., 1995). This study has been carried out as a process modelling, by computer simulations from the thermodynamic perspective and mass balance, and results indicate the possibility to develop this process inside steelworks. The proposed steelmaking route mainly consists in a de-siliconisation process in the De-Si furnace of hot metal produced in the blast furnace and in a de-phosphorisation process in the De-P furnace before the refining into the conventional converter. The process is outlined as follows and shown in Figure 5:

Possible Uses of Steelmaking Slag in Agriculture: An Overview 351

to save natural resources) and the production of slag with a high P content, that will be

Among different studies concerning phosphorus recovery from steelmaking slag, a research conducted in Japan, based on a method using a strong magnetic field, has been implemented for the separation and recovery of crystalline phases containing P from

The phases of hot metal pretreatment slag, resulting by the composition analysis, can be

• *phase B*, that is composed mainly of Ca and Si, with a few percentage of P and hardly

Thus the pretreatment slag can be subdivided in general into two groups: the crystalline phases containing P but not Fe (*phase A* and *phase B*) and the phases containing Fe but not P. Through measurements of magnetic properties of phases components, different behaviours in a magnetic field have been found. This has allowed to magnetically separate them, by

The P recovery has resulted in obtaining a raw material for the production of fertiliser in the chemical industry. On the other hand the slag obtained after P recovery is used as building material, for roads construction and can be recycled in sintering process. Therefore the expected results of the magnetic separation method will have positive effects on environment, by reducing the generation of slag and CO2 emissions and by reducing natural resources exploitation and landfill consumption. Moreover, the implementation of this technology can allow the use of iron ore with a higher P content, which is cheaper, and thus

According to its environmental policy, the European targets concern the environment preservation, protection and its quality improvement, the human health protection and the efficient use of natural resources, by adopting measures for handling environmental issues on global and local level. The use of slags has a crucial significance as regard the environmental aspects. The main problem concerning the utilization of steel slags in

Heavy metals are broadly distributed in the Earth's crust and some of their chemical forms can be a potential risk to biosphere, in particular to the water life, because of their solubility. Their bioavailability depends on the plants ability to uptake them from soil and water, due to the secretion by plants roots of chelators compounds; furthermore many heavy metals are transported by sulphur ligands, such as glutathione, and organic acids. Moreover some heavy metals are insoluble and they often interact with soil particles, and therefore they are

• *phase A*, where P is concentrated at 10% or more and hardly any Fe content;

applying a strong magnetic field of several teslas to the crushed slag.

**7. Environmental concerns about the slags use in agriculture** 

agriculture consists of the possible leaching of heavy metals.

available for fertilising use.

subdivided, as follows:

any Fe;

steelmaking slag (Yokoyama et al., 2007).

• *phase C*, which is composed closely to pure FeO; • *phase D*, that is composed of CaO-SiO2- FeO.

it will produce substantial economic benefits.

not available to plants (Babula, P., et al., 2008).


Fig. 5. Proposed method of converter slag utilization (Li et al., 1995).

The simulation results of the proposed process show that it is possible to develop a wastefree steelmaking process, through the internal slag recycling. The slag obtained in the desiliconisation furnace can be used into the BF and/or into the sinter plant, due to its low content in phosphorus ((mass% P) = 0.48). The slag resulting from the de-phosphorisation furnace, with high P content (about 10%), can be used as fertiliser. The simulation results show that steel with low P content could be produced and that the slag generated in each unit could be completely recycled. The energy required for regenerator reactions is supplied by electric power and the total energy required is 130 kWh/t; the lime consumption is about one half of the current process. The proposed method of converter slag utilisation could allow the whole slag recycling (by thus reducing its environmental impact and contributing

• The whole slag produced into the converter (at 1923K), is totally returned to the Pre-De-

• The whole slag from the Pre-De-P furnace (at 1623K) is transferred to the Regenerator; • The Regenerator (at 1873K) contains the carbon-saturated hot metal and the most of the

• The hot metal containing phosphorous is transferred from the regenerator to the De-P-II unit (at 1623 K), which contains hot metal and synthetic slag. The phosphorous is transferred from the hot metal to this slag, by obtaining the final slag, which contains

• A part of dephosphorised slag from the regenerator is transferred to the Pre-De-P

• The slag from the De-Si furnace (at 1623K) can be used in the sinter plant or in the blast

P furnace, where the hot metal is dephosphorised;

phosphorous is transferred from the slag to the hot metal;

more than 10% of P and thus it can be used as fertilizer;

Fig. 5. Proposed method of converter slag utilization (Li et al., 1995).

The simulation results of the proposed process show that it is possible to develop a wastefree steelmaking process, through the internal slag recycling. The slag obtained in the desiliconisation furnace can be used into the BF and/or into the sinter plant, due to its low content in phosphorus ((mass% P) = 0.48). The slag resulting from the de-phosphorisation furnace, with high P content (about 10%), can be used as fertiliser. The simulation results show that steel with low P content could be produced and that the slag generated in each unit could be completely recycled. The energy required for regenerator reactions is supplied by electric power and the total energy required is 130 kWh/t; the lime consumption is about one half of the current process. The proposed method of converter slag utilisation could allow the whole slag recycling (by thus reducing its environmental impact and contributing

furnace and the remainder to the De-Si furnace;

furnace.

to save natural resources) and the production of slag with a high P content, that will be available for fertilising use.

Among different studies concerning phosphorus recovery from steelmaking slag, a research conducted in Japan, based on a method using a strong magnetic field, has been implemented for the separation and recovery of crystalline phases containing P from steelmaking slag (Yokoyama et al., 2007).

The phases of hot metal pretreatment slag, resulting by the composition analysis, can be subdivided, as follows:


Thus the pretreatment slag can be subdivided in general into two groups: the crystalline phases containing P but not Fe (*phase A* and *phase B*) and the phases containing Fe but not P. Through measurements of magnetic properties of phases components, different behaviours in a magnetic field have been found. This has allowed to magnetically separate them, by applying a strong magnetic field of several teslas to the crushed slag.

The P recovery has resulted in obtaining a raw material for the production of fertiliser in the chemical industry. On the other hand the slag obtained after P recovery is used as building material, for roads construction and can be recycled in sintering process. Therefore the expected results of the magnetic separation method will have positive effects on environment, by reducing the generation of slag and CO2 emissions and by reducing natural resources exploitation and landfill consumption. Moreover, the implementation of this technology can allow the use of iron ore with a higher P content, which is cheaper, and thus it will produce substantial economic benefits.
