Thermal conductivity and characteristics of copper flash smelting flue dust accretions

Abstract

In Copper Flash Smelting, flue dust can cause severe problems in the gas handling system, which typically consists of a heat recovery boiler and electrostatic precipitator. If the process is not operated properly, the flue dust may form accretions on the boiler wall, reducing boiler heat transfer and subjecting the surfaces to corrosion. Flue dust accretions on heat recovery boiler tube walls have a reducing effect on the heat transfer efficiency of the boiler. No previous data on the thermal conductivity of metallurgical dust accretion layer have been published and it is the main focus of this work to provide these data. This work also focuses on the physical and chemical characterisation of the accretions and understanding their formation mechanism. The results can be used in the dimensioning of metallurgical heat recovery boilers and they also provide accurate input data for process models. Flue dust accretions, loose process flue dust, and their pure chemical components were characterised and their thermal transportation properties were determined through experimental research. Flue dust particles were found to consist of two layers: oxides of copper and iron in the core of the particles and sulphate layers on the surface. The sulphation reactions take place in the atmosphere of the heat recovery boiler and are believed to participate in the accretion formation mechanisms. Sulphate bridges found between the flue dust particles in the accretions suggest that the sulphate phase is the binding component that results in the formation of accretions. Dust accretions consist of several layers possessing varying particle sizes, densities, and thermal properties. The initial deposition of fine particles takes place through thermophoresis or condensation, after which larger particles are able to deposit by sedimentation or inertial impaction. Sufficient time and temperature exposure results in the sintering and condensation of the layer structure and species migration and re-crystallisation, resulting in higher density and thermal conductivity of the layer. The thermal diffusivities and thermal conductivities of flue dust accretions, particulate samples of process dust, and pure chemical sulphates were determined. All the samples can be regarded as effective thermal insulators with thermal conductivity values of less than 2 W/mK for fused deposit and less than 0.7 W/mK for porous samples. The thermal diffusivity values for all samples were less than 0.005 cm²/s. Thermal diffusivity values show a decreasing temperature dependence, but an increase in the temperature dependence of the specific heat capacities results in slightly increased values for the thermal conductivity of the flue dust accretions. The particulate samples show slight decreasing temperature dependence for the thermal conductivity values, which is typical of crystalline structures. All the samples have thermal transportation values of the same order of magnitude and also resemble the values of the literature data on similar fouling layers found in other processes, indicating that chemical composition may not significantly affect the thermal properties of fouling layers. Porosity may be regarded as a fairly good indicator of the thermal transport efficiency of these types of materials, but the material microstructure must also be considered.