Recovery of valuable metals from electroplating sludge with reducing additives via vitrification

• Electroplating sludge has high levels of toxic heavy metals.
• Vitrification with additives transformed electroplating sludge into ingot and slag.
• The ingot can serve as raw material or an additive for the steel making industry.
• The slag can be reused as building materials or as road paving.

In this study, vitrification was applied to treat Ni–Cu electroplating sludge. The sludge was mixed with additives (limestone:cullet = 4:6) and then heated to 1450 °C. The cooled product could be separated into slag and ingot. An atomic absorption spectrometer was used to determine the metal levels of specimens and toxicity characteristic leaching procedure (TCLP) tests, whereas the crystalline and surface characteristics were examined using quantitative X-ray diffraction (XRD) analysis and scanning electron microscopy, respectively. With a glassy structure, the slag was mainly composed of Ca, Si, and Mg. The TCLP results of slags met the Taiwan regulated standards, suggesting that slag can be used for recycling purposes. With the aid of additives, the crystalline phase of slag was transformed form CaMgSiO4 into CsSiO3. The ingots were mainly composed of Ni (563,000–693,800 mg/kg), Cu (79,900–87,400 mg/kg), and Fe (35,000–43,600 mg/kg) (target metals) due the gravity separation during vitrification. At appropriate additives/sludge ratios (>0.2), >95% of target metals gathered in the ingot as a recoverable form (Ni–Fe alloy). The high Ni level of slag suggests that the ingot can be used as the raw materials for smelters or the additives for steel making. Therefore, the vitrification approach of this study is a promising technology to recover valuable metals from Ni–Cu electroplating sludge.