Modified chemical mineralization-alkali neutralization technology: Mineralization behavior at high iron concentrations and its application in sulfur acid spent pickling solution

•Mineralization-neutralization technology was used to treat spent pickling solution.•Gelation phenomena appeared in high Fe solutions with large SO42−/Fe2+ molar ratio.•Dilution and precipitation with Ba or Ca salts could optimize mineralization condition.•Above 90% of iron and most of ions were eliminated during mineralization period.•The resultant secondary iron mineral had a high adsorption capacity toward arsenate. Mineralization coupled with neutralization is a dual-function technology for disposing acidic iron-rich waters, which can recover the valuable iron in the form of secondary mineral and concurrently purify the wastewater. In this study, a modified technology for treating high Fe wastewater (sulfur acid spent pickling liquor, 62 g Fe/L) was proposed based on the specific investigation of the mineralization behaviors in Fe concentration range of 20–70 g/L. Results showed that high SO42−/Fe2+ molar ratio (> 2.0) tended to trigger gelation phenomena at Fe concentrations above 30 g/L. Fe specie distribution suggested that the insufficient polymerization among Fe-OH complexes might be responsible for the gelation phenomena, since the strong Fe-SO4 coordination almost completely suppressed the Fex(OH)y(3x-y)+ form (a general terms of Fe3+ hydrolysates and their polymers). Modified mineralization strategies were proposed, including pretreatment with dilution or BaCl2/CaCl2 precipitation, of which CaCl2 pretreatment was a versatile and low-cost method. Following CaCl2 pretreatment, chemical mineralization converted above 90% of iron into secondary mineral, which therefore drastically reduced the alkali consumption (from 164.2 g/L to 1.4 g/L) and sludge yield (from 328.1 g/L to 2.4 g/L) in subsequent neutralization treatment. The resultant mineral was identified as schwertmannite, and exhibited efficient adsorption capacity toward arsenite (364.2 mg/g). The modified chemical mineralization-alkaline neutralization is a cost-effective technology for the treatment of the acidic iron-rich waters. In practical applications, several regulating strategies should be further explored to improve the mineral purity, and the mineralization conditions must be optimized according to the Fe and SO42− concentrations in wastewater. [Display omitted]