Red gypsum from the TiO2 sulfate process: a critical review of recovery flowsheets and carbonation.

Red gypsum (RG) is the calcium-sulfate residue precipitated when the weak-acid stream of sulfate-route titanium dioxide manufacture is neutralised with lime or limestone. This critical review of 47 documents connects the provenance and variability of RG to the design and environmental performance of its treatment flowsheets. Across 20 feedstock records from twelve sites, RG spans 23.1-40.3 wt% CaO, 27.0-47.0 wt% SO3, 3.5-29.0 wt% Fe2O3 and 0.8-7.6 wt% TiO2; site-level and pooled medians agree within one percentage point, so the ranges, not a typical composition, are the design specification. Iron, a co-precipitated, poorly crystalline hydroxide, varies by factors of 1.7-2.1 between campaigns at one plant. Purification, phase transformation, integrated waste-acid recovery, thermal reduction, carbonation and direct use are compared on harmonised reagent, water, energy, secondary-stream and technology-readiness bases. Within the reviewed corpus, routes using the plant's own waste acid give the most selective iron removal and the only purified gypsum tested against a product standard. Indirect carbonation yields 93-99% pure carbonate and gross uptakes of 173-176 kg CO2 per tonne, about 74% of the corpus-median stoichiometric capacity, but reagent inventories of 0.4-8.9 t and a sulfate-salt co-product of 0.4-0.8 t per tonne make reagent circularity, not chemistry, the binding constraint; non-selective leaching transfers iron, titanium, thorium and uranium into the carbonate. No reviewed study reports a net CO2 balance, a life-cycle assessment or operation beyond laboratory scale, and seven of eight construction-type studies omit leaching or radiological testing. A conditional route-selection framework with explicit criteria and confidence levels is proposed.