Precipitation of calcium sulfate dihydrate, gypsum, from (i) a pure sulfuric acid/lime suspension and (ii) a waste sulfuric acid/lime suspension in a continuous pilot plant in the temperature range from 40 degrees C to 80 degrees C was studied. It was observed that in the case of waste sulfuric acid with a high content of Mg2+ and Fe2+ ions, several hours after the beginning of the precipitation, partial dissolution of the product and modification of the crystals from needle- and plate-like to agglomerated structures occurred. It is suggested that the secondary changes occur due to the increased concentration of Mg2+ and Fe2+ ions in the reactor. Below 60 degrees C, and above 70 degrees C plate-like and needle-like single crystals respectively were formed. The mean size (d(50)) of the crystals was found to increase with increasing temperature and decrease with the initial concentration of lime. Gypsum produced between 70 degrees C and 80 degrees C is suitable for further processing for construction plaster. In a precipitation process with pure sulfuric acid only single perfect needle-like crystals occurred. (c) 2005 Society of Chemical Industry.