The Kroll process was invented in 1940 by William J. Kroll in Luxembourg.  After moving to the United States, Kroll further developed the method for the production of zirconium. 
Many methods had been applied to the production of titanium metal, beginning with a report in 1887 by Nilsen and Pettersen using sodium, which was optimized  into the commercial Hunter process. In the 1920s van Arkel had described the thermal decomposition of titanium tetraiodide to give highly pure titanium. Titanium tetrachloride was found to reduce with hydrogen at high temperatures to give hydrides that can be thermally processed to the pure metal. With this background, Kroll developed both new reductants and new apparatus for the reduction of titanium tetrachloride. Its high reactivity toward trace amounts of water and other metal oxides presented challenges. Significant success came with the use of calcium as a reductant, but the resulting mixture still contained significant oxide impurities. Major success using magnesium at 1000 °C using a molybdenum clad reactor, as reported to the Electrochemical Society in Ottawa. Krolls titanium was highly ductile reflecting its high purity. The Kroll process displaced the Hunter process and continues to be the dominant technology for the production of titanium metal, as well as driving the majority of the worlds production of magnesium metal.