A bench rotary kiln was used to produce pure YBa2Cu3O6+x by continuous calcination of spray-roasted precursor powder using a countercurrent air flow to avoid a reaction between evolved CO2 and the product. Product agglomeration caused a spread in the velocity distribution, requiring the use of extra calcination for a pure product. High-purity YBa2Cu3O6.6 was obtained at 940-degrees-C for 0.9 vol % solid loading, an average residence time of 75 min, and Ntheta (N = rotation rate, theta = inclination) between 3 and 4.5 rpm deg. However, at a residence time of 77 min, increased product agglomeration at 2.5 rpm deg caused incomplete conversion to YBa2Cu3O6.6. Increasing the average residence time above 75 min increased the oxygen content of the product, with pure orthorhombic YBa2Cu3O6.9 forming at 160 min. Due to reaction and temperature induced changes in particle size and shape, existing correlations cannot predict the dependence of the average solid residence time on the operating conditions.