In this paper, recent advances made in the atmospheric scorodite process originally developed at McGill and later commercialized by Ecometales in Chile are presented. The original process allowed for crystallization of well-grown scorodite solids via supersaturation control and seeding regulated by stepwise adjustment of pH with lime. As a result, the produced scorodite was obtained mixed with gypsum reducing the %As content in the disposed solids from 30% down to ~10%. Motivated to make clean scorodite (free of gypsum) for compact disposal (near 30% As content), a new generation of atmospheric scorodite process has been developed in which supersaturation is controlled by iron dissolution/leaching. In other words, the process is controlled by the rate of iron dissolution. This has been demonstrated with goethite, magnetite, and iron(III) oxyhydroxide but other iron sources like hematite or metallic iron may be used too. The process involves the addition of the iron (hydro)oxide solids to acidic solution of As(V) in the absence of lime. Essentially, the iron solids act both as a source of iron but also as a neutralizer. In this chapter, laboratory batch test results are reported along a conceptual flowsheet that integrates oxidation of As(III) with SO2/O2 and scorodite production by iron dissolution.