Advantages of slag waste for soil acidity correction

Structural characterization of slag samples

The FTIR spectra of granulated blast furnace slag (sample 1), landfilled waste slag (sample 2) and the combination of 50% granulated blast furnace slag + 50% landfilled waste slag (sample 3) are shown in Fig. 1 .

By analyzing the spectrum (detailed figure) in the range from 700 to 1100 cm⁻¹, it is seen that there are obvious absorption peaks in the spectra of all the slag samples. The granulated blast furnace slag shows the characteristic absorption bands at 3640, 1418, 980, 944, 861, 753 and 710 cm⁻¹. The band at 3640 cm⁻¹ is attributed to the stretching vibration of the hydroxyl group from the weakly absorbed water molecules on the surface of the slag²⁴. The characteristic absorption bands at 1418, 861 and 710 cm⁻¹ are attributed respectively to the asymmetric stretching mode and to the bending mode of the carbonate group and to the band at 980 cm⁻¹ are attributable to the stretching vibrations of Si–O²⁵. The band at 944 and 752 cm⁻¹ represent the internal vibration of [SiO₄]⁴⁻ and [AlO₄]⁵⁻ tetrahedral and arises from the Si(Al)–O-antisymmetric stretching vibration²⁶.

The different vibrational modes of the waste slag sample can be observed in the FTIR spectrum. Absorption bands shown are at 1418, 873, 712, 667 and 419 cm⁻¹. The peak at 1418 cm⁻¹ is attributed to the asymmetric stretching mode and bending mode of the carbonate group. The calcite phase is confirmed by characteristic peaks at 712 cm⁻¹ (ʋ₂ CO out-of-plane bending vibration₃⁻² ions) and 873 cm⁻¹ peak (ʋ₂ split the bending vibrations in the CO plane₃⁻² ion²⁷. The calcium aluminate phase is identified by a characteristic peak at 419 cm⁻¹²⁸. Peak around 667 cm⁻¹ is described as an absorption band for different M–O (metal oxide) such as Al–O, Fe–O, Mg–O, etc.²⁹.

In the case of a combination of 50% granulated blast furnace slag and 50% landfill residual slag, the intensity of the absorption peaks is lower compared to sample 1 and sample 2 slag. The characteristic absorption peaks (978 and 753 cm⁻¹) that correspond to characteristic peaks of sample 1 are offset from sample 1, attributed to Si–O stretching vibration and antisymmetric Si(Al)–O stretching vibration, respectively, can provide important evidence of the chemical interaction between Sample 1 and Sample 2. The decrease in the intensity of the bands appearing at 875 and 709 cm⁻¹ can be attributed to CO vibration overlap₃⁻² calcite phase ion.

Figure 2 shows the SEM micrographs of the slag samples (sample 1 to 3). One can see the characteristic morphology – the sizes and shapes of the slag samples.

At higher magnifications, the surface can be observed to be rough and uneven, and jagged formations resembling rounded grains can be noticed. The slag samples show aggregated particles with an average diameter of a few microns. Moreover, in these rounded formations, one can see different morphologies like spheres, rods, planks specific to each compound/phase of the metallurgical slag.

Figure 3 illustrates the EDX elemental analysis of granulated blast furnace slag (sample 1), waste slag dumped in a landfill (sample 2) and a combination of 50% granulated blast furnace slag + 50% waste dumped in a landfill (sample 3).

It can be observed that the predominant elements in the examined area are found in carbon, oxygen, calcium and iron, confirming the FTIR spectra.

Figure 4 shows the EDX spectra of slag samples recorded on different selected spot areas, to get more information about the elemental composition of specific areas. Because all the slag samples tested have similar element contents.

The selected point areas are highlighted as follows: the spherical structure is in yellow line and the plank structure is in green line for all the slag samples analyzed. In the case of sample 1 for both structures, the values ​​of the chemical elements present are similar and silicon has a higher value at the spherical structure which can be correlated with the presence of silica (SiO₂). The higher calcium content reveals that Sample 1 is a blast furnace slag dominated by calcium and silicon compositions. In the case of the slag dumped in a landfill (sample 2), the carbon content increases for both structures and certain chemical elements like titanium, barium, manganese do not appear in the EDX spectra and the explanation of this phenomenon is that the slag has been dumped in landfill for over 30 years. It can be observed for the combination of 50% granulated blast furnace slag + 50% landfill waste slag (sample 3) that the values ​​of all chemical elements for the spherical and plank structure lie between the first two samples, confirming the FTIR spectra regarding the chemical interaction between sample 1.

The XRD patterns of the slag samples with the identified phases are shown in Fig. 5. Sample 1 shows minor peaks of free CaO and MgO, which can be harmful and cause a reduction in strength. The phases and the amorphous content of the granulated blast furnace slag of sample 1 are generally in accordance with the literature³⁰. Slag sample 3 consists of crystalline phase – Ca₂mg₂SiO_sevenCalifornia₂Fe₂AlO₅CaCO₃ and CaO as observed by XRD analysis. From the point of view of phase thermal equilibrium relationships, the identified compounds form an isomorphic series of melilite specific to basic metallurgical slags.

Table 1 shows the values ​​expressed in ppm of chemical element detected in the slag samples (samples 1, 2 and 3).

The results show a large amount of calcium in all three slag samples. Moreover, the detected elements such as Fe, Al, Mg and Si are consistent with the XRD spectra.

Physico-chemical characterization of soil-slag mixtures

The chemical composition of the main elements that make up the soil, soil slag and slag samples were determined by XRF. The values ​​expressed in ppm of chemical elements are presented in table 2. In the case of a soil sample, the content of the main constituents is iron, titanium, manganese and potentially toxic elements (PTE) such as arsenic, zinc, copper and cobalt. . For soil-slag 1 with a soil:slag weight ratio (1:1), one can observe the disappearance of potentially toxic elements (PTEs) found in the soil sample and the decrease in the zinc concentration value. As the slag weight ratio increases to 3 (soil-slag sample 2), the principal component values ​​increase with the slag sample values, but in the case of soil-slag sample 3 where the weight ratio soil is larger (3 ) the presence of cobalt can be observed. Based on these XRF results, we can say that there is a removal of potentially toxic elements in contaminated soils by applying slag in a greater proportion.

Using a pH meter, CONSORT C 533, important parameters of soil and slag solutions were measured such as: pH, conductivity and salinity as shown in Table 3. The data presented in Table 3 suggest that the sampled soil has pH = 5.2 corresponding to a moderately acidic soil, which does not maintain high fertility and is not able to provide favorable conditions for crops. In addition, soil pH has an important influence on soil fertility, decreases the availability of essential elements and the activity of soil microorganisms that can determine calcium and magnesium deficiency in plants and decreases the availability phosphorus. The pH value of the slag solution (12.5) corresponds to a strongly basic character which is beneficial in the process of improving acidic soils and the presence of this type of slag also supports the improvement of soil characteristics. For soil-slag samples, the pH value increases with increasing slag weight ratio and the resulting soil-slag mixtures can be classified as weakly alkaline soils.

The data presented in Table 4 shows that soil moisture is greater and decreases in soil-slag samples with the addition of slag content. The values ​​of the total soil-slag porosity are between 40 and 50% and depend on the density and bulk density of the soil being influenced by the mineralogical composition, the content of organic matter and the degree of compaction and loosening of the soil. , the crystal structure structure of soil minerals.

Considering the structural and morphological characterization of the studied slag samples, we propose a recipe for blast furnace slag and landfill waste slag according to the Waste Directive 2008/98/EC regarding the strategic objective of the of a complete elimination of the elimination. of waste. Galati steel plant slag dump contains a huge amount of unused slag waste which can be mixed with granulated blast furnace slag, to save natural resources used as raw materials in the metallurgical technological process.

The presence of Ca²⁺ in the composition of slag can maintain high alkalinity in the soil for a long time in the natural environment. Alkaline soil pH can contribute to a decrease in the available concentration of heavy metals by reducing metal mobility and binding metals into more stable moieties. One of the objectives of this research is to improve the quality of the environment by using the mixture between two different slags on agricultural lands and reintroducing them into the agricultural center, especially in acidic soils. Acidic soils are characterized by an acidic pH that has spread in recent years due to excessive fertilizer or overly aggressive work.³¹. The production is significantly influenced and the treatment of acidic soils is usually done using a series of natural materials (lime, dolomite), the consumption being approx. 20 t/hectare depending on the acidity of the soil and the nature of the plants grown on the respective surfaces.

Our research consists in improving the characteristics and qualities of acidic soils and contributing to their reintroduction into the agricultural circuit by transforming waste into a new environmentally friendly material, the mixture of blast furnace slag and waste slag dumped.