Arabian Journal of Chemistry (Mar 2021)
High-performance Al separation and Zn recovery from a simulated hazardous sludge
Abstract
Zn2+ is a heavy metal ion, and hazardous sludge from the electroplating and alloy industry is rich in Zn and impure Al and Ca. Such sludge is commonly recycled by dissolution in strong acid and then selective extraction to recycle Zn2+ by a special extraction reagent. In this process, impurity Al3+ is dissolved and then participates in the extraction of Zn2+, so Al3+ should be removed first. Here, a new strategy was reported for the effective removal of Al3+ and recovery of Zn2+ from a simulated Al/Zn-bearing sludge via an improved acid solution–precipitation route. The sludge was simulated by coagulating Zn-bearing waste water (290 mg/L Zn2+) with 600 mg/L polyaluminium chloride. The sludge was dissolved in sulphuric acid and nitric acid to form an acidic solution with Al3+ and Zn2+ concentrations of 3.2 g/L and 5.7 g/L, respectively. The solution was treated directly by hydrothermal method at 270 °C, in which 55% Al3+ was precipitated as boehmite and Al hydroxide. After the addition of 0.2 mL of ethylene glycol, the removal rate of Al3+ dramatically increased to 99.8%, with Zn2+ loss of 1.5%. The residual Zn2+ was 5620 mg/L in the treated solution and further directly precipitated by adjusting the pH of the solution to pH 7.5 with NaOH. The precipitated Zn2+ was in the form of simonkolleite with ZnO content of 63.1%, with Al content of only 0.8%. In the hydrothermal precipitation, the removal rate of Al3+ increased with the temperature and extension of reaction time. Al3+ was hydrolysed and precipitated as aluminium oxonium sulphate hydroxide and then recrystallised in boehmite form. Then, H+ was generated and consumed in the redox reaction of nitrate and ethylene glycol, accelerating the Al3+ precipitation. This method provides a way to efficiently separate Al3+ from a Zn-bearing solution and can be applied in the recycling of Al/Zn-bearing sludge.
