In 1922 Kroll filed a patent application in Germany (and later in France) for desulferising iron and steel.
In the year 1923 Kroll set up his laboratory in Luxembourg (Bel-Air). He filed the following patent application:
Kroll also applied in 1923 for a patent for magnesium-base alloys containing cerium: 
“I also developed then a magnesium-base alloy with high creep strength at medium temperatures which contained up to 6 per cent cerium or “Mischmetall” and I proposed it to the I. G. Farbenindustrie. The company produced the alloy and the good creep properties were confirmed. However, the firm refused to go ahead with production because of the high cost of cerium-Mischmetall.”
Kroll gives a reference to a German patent application, but no corresponding granted patent can be located.
 “Germ. Pat. Appl. B 11,558 VI/40 b, publ. June 18, 1925” according to Kroll.