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

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 soils.

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

Possible Uses of Steelmaking Slag in Agriculture: An Overview 349

during the incubation time the pH decreased. This can be due, according to some previous studies, either to the precipitation of the free carbonates as calcium carbonate (Abassapour et al., 2004) or to the hydrolysis of Fe3+ in the soil (Rodriguez et al., 1994). The decrease of soil pH probably results from the decomposition of organic matter applied and subsequent organic acids and CO2 release as well as the buffering ability of the calcareous soils. The observed yield increase in these soils may be due to the some nutrients availability as a

The converter slag application has proportionally increased ammonium bicarbonatediethylenetriamine pentaacetic acid (AB-DTPA) extractable Fe, although in some incubation soils Fe extractable decreased, maybe due to the temporary fixation of iron by organic matter. Further in the pot experiment converter slag has been shown to be very effective in

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

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

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

consequence of pH increase.

steelmaking slag.

correction of Fe chlorosis in calcareous soils.

the consumption reduction of iron and lime.

as follows and shown in Figure 5:

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

reactions. On the other hand, the foliar concentration can be influenced by soil solution changes, competitions for root uptake and root-to-shoot transfer. In particular the soil pH has increased from 5.6 to 9.8, and the soil conductivity has proportionally risen from 0.14 mS cm-1 to 0.82 mS cm-1 by applying increasing rates of basic slag, probably due to the basic slag composition, particularly to the Ca content. Furthermore the foliar Cu concentration has probably caused a phytotoxic effect in plants grown in Cu-contaminated soil. After the basic slag addition at 1% rate the bean growth, along with the decrease of foliar Cu concentration, has been observed. Moreover while the Ca foliar concentration has increased after applying increasing rates of basic slag, the foliar P concentration has not been improved. These results suggested that the use of basic slag at 1% addition rate is effective as a liming material but, is not effective as P fertiliser. Furthermore, the basic slag addition in contaminated soil does not increase the foliar concentrations and accumulations for Cd, Cr, and Zn.
