Removal and recovery of sulphate from RO rejects tannery wastewater by double-jacketed agitated crystallizer via gypsum crystallization

[Display omitted] •Double-jacketed agitated crystallizer is an emerging technique of zero-liquid discharge.•∼90 % of sulphate has been removed and recovered as gypsum from tannery RO reject.•XRD results of recovered CaSO4 are 95 % identical to commercial gypsum.•Process temperature favors the irreversible reaction.•Nucleation and growth rate occur at room temperature as an inverse trend. This present study has assessed the resource recovery of tannery RO reject wastewater using crystallization in a double-jacketed agitated crystallization reactor under batch performance. The main objectives of this process were to remove sulphate from tannery RO reject wastewater and to recover the product as gypsum crystals. In the primary stage of initiator in crystallization, the CaCl2 initiator has better results in recovering the gypsum crystals involving chemical reactions. SEM, XRD, FTIR, and optical microscope characterize the product gypsum crystals. XRD results revealed that the uninformative size of gypsum crystals ranged between 54.61 and 61.86 nm, respectively. The experimental investigation on pH (4–12), temperature (30–180 °C), and reaction time (30–150 min) was performed. The maximum removal of sulphates from tannery RO reject wastewater was found to be 90–94 % under ideal pH 7, 30 °C for 30 min of reaction time, and 85–90 % gypsum crystal recovery, respectively. Moreover, gypsum cum Plaster of Paris was also investigated since interlocking structure and intercrystallite interactions can improve mechanical strength. It can be utilized for construction materials and ceramic mouldings for real-time applications. It could be concluded that the crystallization study helps in combating the harmful effects caused by occupying landfill sites and environmental damages.