Recovery of iron from iron-rich pickling sludge for preparing P-doped polyferric chloride coagulant.

The pickling sludge produced from rolling process contains a large amount of Fe, Ca, Al as well as other metals. If these metals can be extracted and used, it will promote the recycling of pickling sludge. Herein, we proposed a two-step extraction method to extract Fe ions out from the pickling sludge, and then the extracted Fe was oxidized by HO and prepared into Fe-containing coagulant in the presence of NaHPO as stabilizer. The three main factors that affect the color removal efficiency and COD removal efficiency are identified as P:Fe ratio, HO adding amount, and curing time. Results show that the optimal preparation conditions are: P:Fe = 0.05, HO amount = 115%, and the curing time = 6 d, at which the color and COD removal efficiency reached 96.25% and 65.91%, respectively. The assessment of toxicity of the PPFC indicated that the content of harmful substances meets the thresholds in Chinese national standard GB141591-2016. The findings in this study are expected to provide new implications in treating different kinds of Fe-containing sludge.