Coal-fired power plants with semi-dry flue gas desulfurization produce several metric tons per day of residues with 10-18%w/w of hemi-hydrated calcium sulfite. The rest of the 90-80%w/w of such residues contains silica, aluminium-silicates and calcium/magnesium carbonates, sulfates and hydroxides. This material could be added to cement, but sulfites degrade the cement quality and lead to costs of landfill disposal. To test upgrading desulfurization residues to turn it into an acceptable cement feedstock, a pilot plant was built to oxidize residues with hot air converting sulfites to sulfates. This pilot comprehends a fluidized bed reactor, an air heater, a cyclone and a heat recovery exchanger. Its operation showed that residues react favorably under fluidization. The effect of independent variables, residence time and temperature, was investigated and sulfite conversions up to 89.4% were observed. With statistical treatment of pilot experimental data, response surface of conversion of sulfites to sulfates was developed, allowing to estimate the effect of independent coordinates on conversion and the optimal oxidation conditions. Experimental data and model predictions showed agreement leading to low estimated variance. (C) 2017 Elsevier Ltd. All rights reserved.