Coregulation of crystal growth and hydroxylation induces the conversion of phosphorus-containing sludge into multifunctional hydroxyapatite nano flame retardant

Abstract

The urgent demand for phosphorus flame retardants in the flame-retardant industry and the shortage of industrial phosphorus resources present supply-demand imbalance. Recovering phosphorus from phosphorus-containing sludge in the form of hydroxyapatite (HAP) and using it for flame retardancy can alleviate supply-demand imbalance as well as the problem of phosphorus contamination of sludge. In this study, phosphoric acid etching sludge was converted into a multifunctional HAP flame retardant through the synergistic regulation of crystal growth and surface hydroxylation. The thermal stability of HAP was enhanced by modulating the Ca/P molar ratio and crystallinity during crystal growth. The mechanical strength of HAP was enhanced by regulating the morphology and size during crystal growth. The generation of ∼200 nm rod-shaped HAP underwent a dissolution regrowth process, and CTAB was the main structure-directing agent. Hydroxylation of the crystal surface was achieved by hydrothermal-calcination reaction, which enhanced the compatibility of HAP with the substrate and improved the tensile mechanical strength of the substrate. At 10 wt%, the vertical combustion level of HC-HAP was increased from V-2 to V-1 and the limiting oxygen index (LOI) was increased from 22.0% to 26.5%, compared with the addition of etching sludge. This study provides a new idea for the resourceful secondary utilization of industrial phosphorus-containing sludge and the development of multifunctional inorganic phosphorus-based flame retardants.