The advantages of generating stable and compact spinel-type precipitates from aqueous solutions at ambient temperature, instead of the amorphous and voluminous hydroxides obtained by conventional treatment of effluents are supported by experimental evidence. As a first step, the study of magnetite formation by controlled aerial oxidation of ferrous solutions at ambient temperature was considered. The parameters studied were: aerial oxidation intensity, precipitation pH, SO2-4 concentration, initial Fe2+ concentration and aging of the sludges. The results suggest a close relation between precipitation pH and the kinetics of conversion of intermediate green rust-II into γ-FeOOH and/or Fe3O4. A precipitation pH between 10 and 11 and a well-controlled oxidant environment achieved by a moderate aeration of the solutions promoted suitable physical and chemical stability characteristics of quickly generated magnetite at ambient temperature. Furthermore, the minimum concentration of ferrous ions conducive to magnetite formation will be limited by the preexistent amount of dissolved oxygen in the starting solutions. Generated information was related to a laboratory-scale simulation of the 'Recycle Process', but applied to the generation and densification of magnetic sludges. The stability conditions of iron precipitates allowed a better understanding of those transformations that occur in the stages of this process: aerated alkaline precipitation, solid/liquid separation, chemical conditioning of settled solids and their recycling to the precipitation stage. © 1998 Elsevier Science B.V. All rights reserved.