The flash smelting process is used widely around the world to extract primary copper. Smelting of sulphidic copper ores produces flue dust containing SO2-rich exhaust gas, causing corrosion problems in the gas cleaning equipment of the process, especially in the heat recovery boiler. The aim of this thesis was to understand the corrosion mechanisms of the boiler steels and to determine what kind of steel would provide the best corrosion protection. Especially the flue dust causes corrosion problems due to formation of dust accretions on the boiler walls. Within these presence heavy metal chlorides results in formation of molten salt deposits causing rapid corrosion.
In order to understand the corrosion behaviour of the boiler steels, conditions of the heat recovery boiler were simulated in laboratory. Corrosion reactions of carbon steel and three high alloy steels (AISI 304, AISI 316 and Sanicro 28) were studied, focusing on corrosion protection properties of chromium, molybdenum and nickel alloying. In addition, thermogravimetric measurements of oxidation behaviour of a stainless steel (TP347H) with two different grain sizes were studied to determine the effect of grain size on the quality and formation of the steel's protective oxide scale. Melting of the dust accretions was identified as the initiator of series of corrosion reactions between the steels and the environment inside the heat recovery boiler. To verify the possibility of formation of molten salt deposits, phase diagrams of few of the possible chloride systems present in the dust accretions were studied experimentally by equilibration-quenching, scanning electron microscopy and energy-dispersive spectrometry in order to evaluate the melting behaviour of these chlorides. Different binaries of the CuCl-CuCl2-ZnCl2-PbCl2-FeCl3-FeCl2-system were optimized using the CALPHAD-method.
In this thesis, a description of the sequence of corrosion reactions is given between steel and the chemical environment of the heat recovery boiler. Understanding the corrosion mechanism of the boiler should help to improve its lifetime by selection of suitable steels or by adjusting the operational parameters. Thus, the effectiveness of copper production can be improved.
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- corrosion, molten salts, copper flash smelting, heat recovery boiler