The present disclosure is directed to an improved direct smelting process for improving the efficiency of energy/heat recovery from hot smelter offgas for the purpose of steam raising and power generation. In examples, the formation of alkali sulfate accretions in the temperature range 500-1000 C. are removed by introducing a mechanical sweep-cleaning system to an offgas duct of the smelter. The sweep-cleaning system comprises at least one central rotating shaft with chains, with each chain optionally having an accretion-removal member at its distal end. When rotated, the chains lift and the accretion-removal member sweeps the walls of the duct to remove the accretions.