The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.

Sulphation of Cuprous and Cupric Oxide Dusts and Heterogeneous Copper Matte Particles in Simulated Flash Smelting Heat Recovery Boiler Conditions

Tiina Ranki-Kilpinen

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Materials Science and Rock Engineering for public examination and debate in Auditorium V1 at Helsinki University of Technology (Espoo, Finland) on the 28th of April, 2004, at 12 o'clock noon.

Dissertation in PDF format (ISBN 951-22-7022-6)   [4829 KB]
Dissertation is also available in print (ISBN 951-22-7021-8)


Copper smelting with the Outokumpu flash smelting process generates significant amounts of SO2-rich off-gas and flue dust. From the smelting unit, gases with a dust load are directed into a heat recovery boiler (also known as a waste heat boiler). In the radiation section temperature decreases, sulphates become thermodynamically stable, and the sulphation of oxidic dust particles commences. Releasing heat may lead to an increase in particle temperatures, softening of the sulphated particles, and the formation of dust accretions on the heat transfer surfaces. Decreased heat transfer efficiency and blockages of the gas flow paths may cause severe operational problems. To maintain stable boiler operation, sulphation behaviour has to be well understood, but only scant published data concerning dust sulphation reactions is available. The objective of this work was to gain basic knowledge of the sulphation behaviour of dust components to ascertain that boiler design and operation can be carried out so that sulphate formation takes place in a controlled manner.

The reactions of synthetic Cu2O and CuO (mainly 37-53 µm) and a partially oxidised copper matte were studied experimentally with the aim of arriving at a better understanding of dust sulphation in industrial heat recovery boilers. The parameters in the laboratory-scale experiments were gas composition (20-60 vol-% SO2, 2.5-10 vol-% O2), temperature (560-660 °C), reaction time, and particle size. Standard chemical analysis and scanning electron microscopy with EDS were utilised when examining the samples.

Sulphate formation was found to be sensitive to gas composition and temperature. Also particle size and surface morphology have significant effects on the sulphation rate. On the basis of the experimental results the temperature range for effective sulphation of pure cuprous oxide is narrow; the optimal sulphate formation temperature lies between 580-640 °C, depending on the gas composition. An increase in oxygen concentration expands the favourable temperature range and lowers the most optimal sulphate formation temperature; on the contrary an increase in sulphur dioxide concentration raises the favourable sulphation temperature.

On the basis of the present experiments pure cupric oxide behaves like cuprous oxide, but the conversion degrees are slightly lower and there is not such a clear enhance in the sulphation rate at a certain temperature.

Fine, heterogeneous partially oxidised matte reacts significantly faster compared to synthetic oxides. The reason for more effective sulphation is suggested to be the smaller particle size and more detailed morphology (larger specific surface area).

In the heat recovery boiler dust particles must have a sufficient residence time in the gas phase at a correct temperature range to allow the dust particles to reach complete conversion in the radiation section before they enter the boiler convection section and come into contact with the convection tube banks. Enough oxygen has to be supplied to the appropriate zone to ensure effective sulphation at the right place. Also, mixing of the oxygen must be efficient.

Keywords: sulphation, cuprous oxide, cupric oxide, flue dust, flash smelting, heat recovery boiler

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© 2004 Helsinki University of Technology

Last update 2011-05-26