Heat treating a catalyst precursor in a rotary kiln through which a gas stream is passed comprises recirculating at least part of the gas stream. Independent claims are also included for the following: (1) a rotary kiln apparatus comprising a circulating gas compressor (13), an off-gas compressor (17), a pressure reducer (15), a fresh gas supply (20, 21), a heatable rotating tube (1) and a gas recycle line; (2) a reactor with a nest of 5000-40000 contact tubes packed with a catalyst produced as above comprising a mixed oxide of molybdenum (Mo), vanadium (V) and niobium (Nb) and/or tungsten (W), where the Mo content is 20-80 mole%, the Mo/V molar ratio is 1-15:1, the Mo/(W+Nb) molar ratio is 80:1 to 1:4, and the arithmetic mean activity for partial oxidation of acrolein to acrylic acid for a random sample of 12 contact tubes differs from the highest or lowest activity by no more than 8degreesC; and (3) a reactor with a nest of 5000-40000 contact tubes packed with a catalyst produced as above comprising a mixed oxide of Mo, V and tellurium and/or antimony and niobium, tantalum, tungsten, titanium, aluminum, zirconium, chromium, manganese, gallium, iron, ruthenium, cobalt, rhodium, nickel, palladium, platinum, lanthanum, bismuth, boron, cerium, tin, zinc, silicon, sodium, lithium, potassium, magnesium, silver, gold and/or indium, where the arithmetic mean activity for partial oxidation of acrolein to acrylic acid for a random sample of 12 contact tubes differs from the highest or lowest activity by no more than 8degreesC.