Leaching behavior of valuable metals from nickel smelter slag

Abstract

With the rapid development of energy materials, an increasing amount of nickel is needed worldwide and the demand keeps increasing year by year. Simultaneously, efficient utilization of nickel process waste such as slag is receiving more attention globally. Discarded smelter slags can also pose challenges of potential environmental damage or waste resources. Consequently, recycling valuable metals from metallurgical processing slags by eco-friendly methods is of increasing importance as a research topic on a global scale.

In this thesis, nickel smelter slag was investigated as a potential resource of valuable metals. Copper, cobalt, and nickel in the slag not only substituted iron atoms in fayalite but also existed in the metal sulfide. A comparison was made between the leaching behavior of valuable metals in direct acid leaching and combined leaching in alkaline and acid solutions. The effect of parameters on leaching behavior was learnt through analytical techniques such as scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), atomic absorption spectroscopy (AAS), X-ray fluorescence (XRF), and X-ray diffraction (XRD).

The results suggested that high-concentration alkaline leaching was efficient for fayalite disaggregation. The best extraction results in direct acid leaching (43% of copper, 70% of cobalt, and 66% of nickel) were achieved when nickel slag with 3 mm diameter was leached in 2 mol sulfuric acid at 70℃ for 4 hours. In combined leaching, the same nickel slag was leached in 4 mol sodium hydroxide first for 2.5 hours. The residue from alkaline leaching was further leached in 2 mol sulfuric acid for 5.5 hours, resulting in extraction levels of 89% for copper, 100% for cobalt and 98% for nickel. Gravity separation and magnetic separation were not found to be reliable for improving the grade of valuable metals in this nickel slag and they were not recommended.

Some challenges were observed such as decreased the filtration performance due to silica gel formation. This could be mitigated by using the appropriate filtering temperature (~ 70℃). Also, the use of hydrogen peroxide was shown to decrease the extraction of the target metals. It can be speculated that the releasing silicate formed silica gel, which also prevented the diffusion of metallic ions into the solution. However, further experiments and analysis are needed to clarify the mechanism.

Overall, this study compares the effect of direct acid leaching and combined leaching on copper, nickel, and cobalt extraction with different parameters. Combined leaching was shown to dissolve the fayalite phase thoroughly and enhance the extraction of target metals in more efficient manner than direct acid leaching. This finding may pave the way toward valuable metals recovery from currently underutilized industrial slag.