Abstract
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 °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 °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.
Highlights
Iron oxides and oxyhydroxides are important Fe carriers in the Earth’s crust, especially in near-surface environments [1], and the only iron ores used at present
Our study showed that a series of chemical reactions and physical transformations occur during heating of the bog iron ores under oxidizing conditions
Physically adsorbed surface water is removed followed by the continuous release of structural water and hydroxyls from ferrihydrite
Summary
Iron oxides and oxyhydroxides are important Fe carriers in the Earth’s crust, especially in near-surface environments [1], and the only iron ores used at present. Some of the (oxyhydr)oxides, those containing water molecules and/or hydroxyl groups, are transformed at elevated temperatures. The overall course of the processes is widely known, the effects of various crystal-chemical and chemical factors on the transformation mechanisms and properties of the products are still under vigorous debate [1, 3]. This type of research focuses usually on monomineral synthetic samples [e.g., 4–10] and does not fully reflect the complexity of the processes during thermal transformations of natural
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