Mineral transformations and textural evolution during roasting of bog iron ores

The processes occurring during roasting of bog iron ores were characterized using TG-DTG-DTA-QMS, XRD, FTIR and specific surface analysis. Removal of physically adsorbed water is followed by dehydroxylation of iron oxyhydroxides and oxidation of organic matter at 200-600 A degrees C. The main product of the transformations is disordered nanocrystalline (proto)hematite or hematite/maghemite mixture, depending on organic matter content and heating conditions. The conversion of iron oxyhydroxides to hematite occurs at temperatures different than those reported for pure compounds. At higher temperatures, protohematite undergoes recrystallization to the stoichiometric hematite, and manganese oxides are partially reduced. At 1000 A degrees C, the roasting products consist of hematite and cristobalite together with Mn-Fe spinels if the initial ore contained Mn oxides. The admixtures of various secondary silicates were encountered as well. Low- to moderate-temperature roasting slightly affects the specific surface area and lowers volume of micropores. The high-temperature transformations lead to decrease in the specific surface area and to the destruction of porous texture of the bog iron ores. Although the general course of the processes during roasting was similar in all the samples, some of their details as well as mineralogy and properties of the products are highly dependent on the composition of the initial material.