3.1 Chemical characteristic of raw steel slags.
Table 2 Compositions of steel slag sample (%)
Composition
|
CaO
|
Fe2O3
|
SiO2
|
MgO
|
MnO
|
Al2O3
|
P2O5
|
V2O5
|
Cr2O3
|
Content
|
41.82
|
25.05
|
11.89
|
6.61
|
5.48
|
2.18
|
1.59
|
0.73
|
0.69
|
The main chemical compositions of the BOF steel slag were shown in Table 2. Except for the listed components in Table 2, there were some coexisted minor constituents in the slag samples, like ZnO, SrO, MoO3, KO, CuO. These substance proportions were very small and made negligible contribution to the ANC of steel slag, therefore, only the main components concentrations of Ca, Fe, Si, Mg, Al, Cr and Mn in the filter liquor were determined, other minor components were not considered in the study.
The XRD components of steel slag sample were shown in Fig.1 (a). It is known the slags are heterogeneous materials consisting of a mixture of crystalline phases. The analysis of the diffraction lines revealed that the major constituent phases in steel slag matrix were Ca2SiO4 (calcium silicate), Ca3SiO5 (tricalcium silicate), CaFe2O4 (calcium ferrite),CaO (lime) and RO phase, however, the f-CaO diffraction peak was not prominent.
The SEM image of slag samples and the magnified SEM image were shown in Fig 1(b) and Fig. 1(c). It can be seen that the slag particles had a loose amorphous microstructure.
The alkalinity releasing results of slag samples were shown in Fig. 1(d). The alkalinity concentration was close to stable after 60 min. For the mentioned L/S ratio (1:100), the stable alkalinity was in a range of 100-110 mg/L and the corresponding pH of the filtrate was 10.5.
Calcium containing components in BOF slags were mainly originated from flux materials during the steel making process, like lime, limestone. (Zvimba et al. 2017). The unreacted limestone was transformed to free CaO and was left in slag, while the reacted lime stone were transfer to dicalcium silicate (C2S) and tricalcium silicate (C3S), and calcium ferrite et al. (Manchisi et al. 2020). For this reason, the percentage of the free CaO in slags was limited. Therefore, the f-CaO diffraction peak in XRD was not prominent in this sample. The alkalinity releasing tests results verified that the alkalinity concentrations in solution was abundant, which was in consistence with observation in the previous study (Goetz and Riefler 2014a). Meanwhile, the enlarged SEM image confirmed the loose amorphous microstructure of slag, which had positive influence on the metal removal by providing big surface area and inner channels (Yang et al. 2021).
3.2 Steel slag ANC determination
The ANC results titrated by simulated AMD was shown in Figure 2 (a). With the simulated AMD amount increasing, the pH value of the solution decreased to 2.15 gradually. When reaching the endpoint, the total titrated simulate AMD amount was 115ml for 1 g slag. And it was about 40 ml when the pH decreased to the neutral rang 7.07. The relationship between pH and simulated AMD amount was basically with the linear law in the whole process, the square R was 0.94. To know the influence of particle size on the ANC, slag samples of 20, 50 and 100 mesh were also titrated shown in Fig. 2 (b). The results indicated that the particles size of the slag had significant influence on the ANC. The total titrated amounts of simulated AMD were 8 ml, 14 ml and 45 ml for steel slag particles of 20, 50 and 100 mesh. It means the ANC increased dramatically when the particle size became small. Particles size distribution was an important designed parameter in practice. The recommended size of slag was less than 3mm in SLB (Alizadeh and Naseri 2014). However, the fine particle slags in SLB were more likely to precipitate on the bottom of the beds, and to increase the running resistance (Alizadeh and Naseri 2014 , Iakovleva et al. 2015), especially for the passive treatment applications. Therefore, for slag applications, it is necessary to optimize the steel slag particle size distribution according to the ANC demand and the geographical environment of remediation site, so as to balance the advantages and disadvantages of the fine particle size.
3.3 Slag leaching behavior characteristics
It was reported that the f-CaO and other Ca-containing components in BOF slag samples would react with acid firstly, because they are easily dissolved in acidic solution (Mulopo et al. 2012). But the dissolution behaviors of other components in samples were unknown yet. Based on the ANC results, the intermediate process of the titration were sample when the accumulative amount were 20 %, 40%, 60% and 80% of the total amount at the endpoint, i.e. the intermediate amount were 23ml, 46ml, 68ml and 92ml. The procedure was consistence with the ANC determination mentioned above. The experimental results were shown in Fig. 3. It was found that the leaching ratios of all the tested elements in solution were increased with the AMD amount increasing, but each element exhibited distinctive characteristics.
Ca content in the slag was 41.82 % calculated as oxide. In leaching results, its leaching ratio was increased firstly and then decreased. The maximum value was 28.73 % when the titration amount was 46mL, and the corresponding concentration was 9.20 mg/l. Both concentration curves and leaching ratio curves had similar variation trends.
For Mg and Mn containing constitutes, their concentrations in solution increased directly with the increase of the titrated simulated AMD amount. Mg had the maximum leaching ratio of 56.75 %. However, the percentages of MgO and MnO in steel slag were relatively few. At the titration endpoint, the final concentrations of element Mg and Mn in solution were 2.03 mg/l and 0.06 mg/l, which had minor influence on the the slag alkalinity.
For elements Al, Fe and Cr, the leaching ratios were close to zero till the titrated AMD was more than 69 ml, particularly for Al and Fe. The pH variation result in Fig. 3 indicated that for the solution titrated 60 ml simulated AMD, the solution pH was 7. In other words, when the titrated AMD amount was less than 60 ml, the solution was alkaline. Thus, Al and Fe were likely to precipitate as hydroxide in alkaline solution. Elements concentration in Fig. 3(b) also showed that the concentration of Al, Fe, and Cr were very 0.31 mg/l, 2.14 mg/l and 1.06 mg/l at the endpoint.