Hagemann was employed by the
Øresund cryolite factory () in 1864. In 1865, when he had completed his exams, sent him to the US to oversee the deliveries of
cryolite to the
Pennsylvania Salt Manufacturing Company. In early 1856, he travelled to the US to provide technical support in connection with the first deliveries of cryolite from
Greenland. With inspiration from
David Alter's nearby production site, Hagemann began to work on improving methods to manufacture and purify
bromine from
salt wells. He obtained several patents and in the spring of 1868 established his first plant in
Pomeroy, Ohio. During a visit to Europe in the summer of 1869,
C. F. Tietgen convinced Hagemann to purchase the Øresund cryolite factory in a partnership with Vilhelm Jørgensen (1844–1925). Hagemann then sold his bromine manufacturing activities in the US and settled permanently in Denmark. Hagemann and Vilhelm Jørgensen also constructed for Count Mogens Frijs and operated it on a ten-year lease. They also established in
Småland, Sweden. When
De Danske Sukkerfabrikker ('The Danish Sugar Factories') were founded in 1872, Tietgen selected Hagemann for the position as chief chemist (Danish: ) in the new company. Hagemann oversaw the construction of the company's first manufacturing site, , in
Odense, which began operations in 1873–1874. He was then sent to the
Danish West Indies to reorganize the sugar cane industry. started operations in 1878 and the last technical challenges were solved when Hagemann visited St. Croix for the third time in 1879. Hagemann, who was appointed to chief technical officer in 1882, was also responsible for planning the new sugar manufacturing plants in
Nakskov (1881),
Assens (1883) and
Stege (1883). In 1894, Hagemann purchased the sugar plantation
La Grange on
Saint Croix and immediately began to modernize its operations. He also acquired the sugar plantations
Prosperity,
William,
Wheel of Fortune and
Punch. Hagemann retired from the position as chief technical officer of in 1897 but then served as chairman of the company from 1898 until his death. Hagemann had also joined the board of
Burmeister & Wain in 1893 and became its chairman in 1898. The company was then reorganized and outdated machinery was modernized. He was also involved in the establishment of the first Danish steel works and introduced a new focus on
diesel engines. ==Other pursuits==