Chemical fixation of toxic metals in stainless steel pickling residue by Na₂S∙xH₂O, FeSO₄∙6H₂O and phosphoric acid for beneficial uses

Abstract

The leaching concentrations of different metals in stainless steel pickling residue (SSPR) were determined and the toxic metals were treated using Na₂S∙xH₂O, FeSO₄∙6H₂O, and phosphoric acid. A modified European Community Bureau of Reference (BCR) sequential extraction was used to identify the speciation of the concerned metals. Results showed that SSPR contains a large amount of Ca (58.41%), Fe (29.44%), Cr (3.83%), Ni (2.94%), Mn (2.82%) and some of Al, Cu, Mg, Zn. Among them, Cr and Ni were the most toxic metals in SSPR, thus the raw SSPR falls into hazardous waste category due to the leaching amount of Cr. In addition, the leached Cr was identified as Cr⁶⁺ (MgCrO₄) in the waste. BCR test revealed that risk assessment code (RAC) of Cr and Ni were 33.29% and 61.7%, indicating they posed “high” and “very high” risk to the environment, respectively. After fixing by Na₂S∙xH₂O and FeSO₄∙6H₂O, the leaching concentrations of Cr and Ni were less than 1.5 and 0.5 mg/L, respectively. After fixing by Na₂S∙xH₂O and FeSO₄∙6H₂O the treated SSPR can be safely reused as roadbed materials, concrete and cement aggregates. This study provides a useful implication in treatment and beneficial reuse of heavy metal-containing hazardous wastes.

Introduction

Stainless steel consists of a group of metals with high percentage of iron (Fe), chromium (Cr), nickel (Ni) and manganese (Mn). The stainless steel is famous for its corrosion resistance which mainly depends on the amount of Cr. According to the International Stainless Steel Forum (ISSF), the minimum content of Cr in crude stainless steel should no less than 10% (ISSF, 2012). During stainless steel production, pickling is an important process that usually employed to improve the surface quality of the stainless steel. In pickling process, aggressive mixed acids (i.e., nitric acid and hydrofluoric acid) were used to remove the scale layers from the steel surface and thus, pig iron powder as well as various heavy metals dissolved into pickling liquors (Shi, 2015). The resulted pickling liquors were then precipitated using lime (CaO) or calcium hydroxide (Ca(OH)₂) and produced a large amount of sludge. The sludge was then dewatered and finally concerted to residue with a moisture content of 50%–60%. The toxicity characteristic of the residue exceeded the national hazardous waste standards due to the leaching concentrations of Cr and Ni (Zhang et al., 2015, Su et al., 2019a). However, the disposal of stainless steel pickling residue (SSPR) in the hazardous waste landfill is inexecutable due to the high disposal cost and limited landfill space. Therefore, the SSPR had to be temporarily stored on sites in China (NBS, 2013). This provisional solution has the potential of contaminating the sites with heavy metals that could leach out from the SSPR through contacting with rain and surface water. Base on above concerns, there is an acute need to treat and dispose of SSPR.

As have been reported in previous publications, stabilization/solidification (S/S) was commonly used to treat heavy metal-containing solid wastes (Karayannis et al., 2017). Portland cement, fly ash, lime, and thermoplastic materials were widely used as binders (Hu, 2005, Li et al., 2011, Bie et al., 2016). In the S/S process, the mobility of the heavy metals was minimized and the engineering properties of wastes were improved (Wang et al., 2018, Su et al., 2019b). Nevertheless, S/S with above-mentioned binders introduce a large amount of binder that results in a significant increase in the volume and weight of the wastes. This is unacceptable due to the rapid urbanization and increase in the land price. Research indicated that thermal treatment (i.e., incineration, melting and combustion) was an attractive option for waste sludge treatment due to its energy recovery and advantage of reduction of solid waste amount by weight and volume (Zhang et al., 2008, Hu et al., 2013). However, this SSPR was produced from chemical precipitation of industrial pickling liquor that there were almost no organic matters in it. Consequently, SSPR cannot be treated by thermal treatment solely. In this case, Zhang et al. (2017) investigated co-combustion of bituminous coal and pickling sludge in a drop-tube furnace. Their results revealed that heavy metals in flue gas met the national standard, however, the leaching toxicity of the bottom ash exceeded the thresholds for landfill disposal. In addition, harmful off-gases such as SO₂, NO_x, HF and HCl were detected in the emissions (Zhang et al., 2016).

By comparison, chemical fixation enables to immobilize heavy metals in solid wastes without pre-treatment which makes it cost-efficient, land-saving and thus a promising technology (Jiang et al., 2004, Quina et al., 2014, Wang et al., 2015). Hu (2005) studied the fixation of Pb in municipal solid waste incineration (MSWI) ash using ferrous/ferric solution and stated that when MSWI ash was treated with 1.6 mol/L ferrous/ferric sulfate solution, the formation of MFe₂O₄ and Ca₃Fe₉O₁₇ could reduce the leaching concentration of Pb significantly. Moreover, it has been proved that metals in air pollution control (APC) residue could be effectively fixed by soluble phosphates and sodium carbonate (Quina et al., 2010). If the toxic metals in the SSPR can be fixed by chemical fixation, it will provide a new approach to deal with the final disposal of SSPR. Therefore, the objective of this paper was to study the chemical fixation of toxic metals in SSPR. Three chemicals: sodium sulfide hydrate (Na₂S∙xH₂O), ferrous sulfate heptahydrate (FeSO₄∙6H₂O), and phosphoric acid (H₃PO₄) were employed to disclose whether precipitation or reduction working on the treatment of metals in SSPR. The leaching amount of different metals was determined using different toxicity characteristic leaching procedures. A modified European Community Bureau of Reference (BCR) sequential leaching method was conducted to recognize the environmental risk and existing forms of metals in the samples before and after fixation.