The semi-industrial plant at Kalna was constructed to separate ore extracted from Mt. Stara Planina. In 1960, it was operating with a capacity to process between 23 to 50 tons of ore per day. During the 1960s, the plant conducted "large-scale" uranium ore processing experiments to become familiar with the technologies and processes, and to obtain necessary information to design industrial scale facilities. In particular, the plant investigated alkaline leaching and filtration methods to obtain a 70% UO2 end product. In the general vicinity of the plant are facilities for semi-industrial research into precipitation of uranium from solution through hydrogen reduction. The current status of these facilities and the pilot plant are unknown, however they are presumed to be shut-down.

Source:
Slobodan Nakicenovic, Nuclear Energy in Yugoslavia, (Beograde: Export Press, 1961), pp. 88, 89.