Technology for recycling still residues from dehalogenation to produce commercial zinc compounds

The study describes a method for recycling the still residue from the synthesis of hexafluoro-1,3-butadiene (HFBD) to produce zinc phosphate in the form of Zn 3 (PO 4 ) 2 ·2H 2 O, which is used as a component in anti-corrosion pigment materials. The still residue (“heavy liquid”) is preliminarily subjected to deep vacuum distillation (residual pressure 30 Pa, final temperature 160 °C) to recover volatile solventsnamely, isopropanol and dimethylformamide (DMF). The remaining residue is a concentrated solution of ZnCl 2 (about 70 wt. %) containing approximately 10 g/dm 3 of iron in the form of Fe(II) and Fe(III), as well as colored organic impurities of unidentified composition. According to the proposed process, the vacuum distillation residue is diluted with water at a ratio of 1 : 2, filtered to remove suspended solids, acidified to pH 2 by the addition of concentrated HCl, and treated oxidatively with H 2 O 2 at 70 °C. Fe(III) is removed by extraction with a 30 % solution of Cyanex 272 in an aliphatic diluent, and the colored impurities are removed by adsorption onto BAU-1 grade activated carbon. An alternative method for removing Fe(III) and part of the colored impurities involves precipitating zinc in the form of (ZnOH) 2 CO 3 using a 10 % Na 2 CO 3 solution. Final clarification is also carried out using BAU-1 activated carbon. The purified, clear ZnCl 2 solution is then subjected to a twostep precipitation process to obtain zinc phosphate. The resulting precipitate is filtered, thoroughly washed with water, dried, and ground. The study showed that after drying at 100–105 °C, the resulting powder corresponds to the composition Zn 3 (PO 4 ) 2 ·2H 2 O. The content of regulated impurities falls within acceptable limits, and the properties of the material meet the requirements for pigment-grade substances. A comparison of the obtained zinc phosphate with a commercially available sample of pigment-grade zinc phosphate was conducted. It was established that the proposed technology yields 580 g of zinc phosphate dihydrate per 1 kg of initial raw material.