Coprocessing of Stainless-Steel Pickling Sludge with Laterite Ore via Rotary Kiln-Electric Furnace Route: Enhanced Desulfurization and Metal Recovery

Highlights

• Coprocessing

• of SSPS with laterite ore via the RKEF route is a promising way.

• The pre-reduction temperature should be higher than 850 °C for sufficient desulfurization.

• Laterite ore enhanced the desulfurization of SSPS by forming CaMgSi₂O₆ and SO₂.

• Components of CaO and CaF₂ from the SSPS served as fluxes for slag-forming.

Abstract

Stainless-steel pickling sludge (SSPS) is a hazardous solid waste, which mainly contains CaSO₄, CaF₂ and a high level of metals such as Fe, Cr and Ni. The high-content of sulfur in the SSPS hinders its adequate treatment and deteriorates the quality of hot ferronickel when the SSPS was used as a feed to the rotary kiln-electrical furnace process (RKEF). Thus, the desulfurization behaviors of the SSPS, including thermodynamics, phase transformation and off-gas emission characteristics during pre-reduction were primarily investigated in this work. Pre-reduction temperature and carbon content were essential for the desulfurization, and the reduction temperature should be higher than 850 °C for sufficient sulfur removal. The presence of laterite ore enhanced the desulfurization of SSPS, and on the other hand, the presence of SSPS facilitated the subsequent reduction smelting. Under optimal conditions, almost all of the sulfur within SSPS could be removed by pre-reduction, and 94.72% Fe, 81.90% Ni and 46.14% Cr were recovered by reduction smelting.

Introduction

Stainless steel is widely used and its output of China was 26.7 million tons in 2018, ranking first in the world. Stainless-steel strips need surface pickling to improve its corrosion resistance and appearance using acids such as H₂SO₄, HNO₃ and HF (Li et al., 2005; Li et al., 2006). Consequently, stainless-steel pickling sludge (SSPS) is generated from the neutralization and precipitation of pickling acidic wastewater, which mainly contains calcium sulfate, calcium fluoride and a significant amount of Cr and Ni (Ito et al., 1998; Li and Celis, 2003; Zhang et al., 2011). It is a hazardous waste, with a great threat to human health and the environment (Zhang and Hong, 2011).

The yield of SSPS is commonly about 30–50 kg/per ton of stainless steel (Li et al., 2011; Zhang et al., 2017). For the treatment of SSPS, the landfill disposal after solidification/stabilization could be of relatively low cost (Singhal et al., 2008; Yang et al., 2016; Zhang et al., 2016), but the problem of secondary environmental pollution might still exist (Su et al., 2019). The hydrometallurgical processes are performed to recycle valuable metals by acid leaching (Ji et al., 2017; Zhang et al., 2011). It is reported that leaching efficiency of iron, nickel and chromium could exceed 99.8%, 99.3% and 99.2%, respectively (Liu et al., 2017; Wang and Zeng, 2004). Nevertheless, the hydrometallurgical process will produce a large quantity of wastewater and cause secondary pollution as well.

Pyrometallurgical processes such as smelting reduction (Hara et al., 2000; Hasegawa et al., 1998; Money et al., 2000; Tang et al., 2018) and direct reduction (Ma et al., 2005; Wu et al., 2019; Yoshikawa et al., 2008) are commonly preferential in terms of simple process flow and high efficiency. The rotary kiln-electric furnace (RKEF) process is one of the most efficient ferronickel production technologic route, where the feed is usually pre-reduced in a rotary kiln at 800–950 °C followed by reduction smelting at 1500–1550 °C in an electric furnace for the separation of alloy from slag (Li et al., 2015; Li et al., 2012; Rao et al., 2016a, b). Given this, for the existing ferronickel production plants, the RKEF process can be expected to be adopted to recycle valuable metals from SSPS. Nevertheless, once an abundant amount of sulfur within the SSPS enters into the reduction smelting procedure (the second high-temperature stage of RKEF process), the excessive sulfur would deteriorate the quality of steel (Li et al., 2014). Hence, the desulfurization in the pre-reduction procedure (the first high-temperature stage of RKEF process) is the key to the high-efficient utilization of SSPS.

CaSO₄ itself is difficult to decompose in an oxidizing atmosphere or inert atmosphere, while the presence of reducing agent could lower the starting decomposition temperature of CaSO₄ (Van Der Merwe et al., 1999; Zheng et al., 2013). However, the products of reductive decomposition of CaSO₄ might be CaO or CaS which are dependent on the reducing temperature and the reducing potential (molar ratio of CO to CO₂) (Sohn and Kim, 2002), while the formation of CaS is unfavorable for the desulfurization of CaSO_4. It has been reported that the CaSO₄ is tend to convert into CaO at a low reducing potential, and the proper temperature for the sulfur removal form the pickling sludge is 800–1000 °C (Li et al., 2013; Li et al., 2018). Moreover, additives such as Fe₂O₃, SiO₂, Al₂O₃ are found to be able to promote the desulfurization of CaSO₄ and impede the formation of CaS (Gruncharov et al., 1988; Mihara et al., 2007; Yan et al., 2013).

In light of the enhanced desulfurization of CaSO₄ by Fe₂O₃, SiO₂ or Al₂O₃, which could be provided by laterite ore, the coprocessing of SSPS with laterite ore in the RKEF process was put forward and validated in this work for the first time. As desulfurization is essential to the utilization of SSPS by the RKEF process, therefore the desulfurization behaviors of the SSPS including reaction thermodynamics, phase transformation and off-gas emission characteristics during pre-reduction were primarily investigated. Furthermore, reduction smelting of the pre-reduced calcines was conducted for metal recovery.