While Kroll experimented in 1937 with the magnesium-based reduction process of titanium chloride he realised that the same process could be used for producing pure malleable zirconium from zirconium chloride. He expressed this clearly in his second German patent application for the production of malleable titanium (K148,168, filed on 7 October 1937)  in the following terms:
Selbstverständlich können auch andere Metalle durch das erfindungsgemässe Verfahren aus ihren Chloriden reduziert werden, wenn nur ihre Chloride sich dem Magnesium gegenüber ähnlich verhalten wie Titanchlorid.
In 1937 Kroll also disclosed his method of producing malleable zirconium using calcium as reducing agent in a scientific publication.  A year later he managed to use the magnesium- based reduction process on zirconium chloride.
“On July 13, 1938 the first zirconium reductions were made with a similar equipment, provided with a ZrCl4 evaporator.”
He applied for a corresponding patent in Luxembourg, but did not file a patent application in any other country, or, if he did, no equivalent patent was granted outside of Luxembourg.
One author  states that Kroll obtained substantial financial rewards for his work on producing malleable zirconium but it is not clear from Kroll’s patent portfolio which essential patents he held to justify collecting royalties from the emerging zirconium industry in the 1940s and 1950s.
 the patent never issued
 Zeitschrift für anorganische und allgemeine Chemie, Band 234, 1937, pages 42-50
 Robert Stumper, d’Lëtzeburger Land, No 15, 13 April 1973, page 4