Former potash mine at Lehrte-Ilten, closed in 1994, with the listed Schacht Bergmannssegen headframe surviving on a site where K+S continues to manufacture fertiliser from imported raw materials and the underground workings have been flooded since 1998.
The Kalibergwerk Bergmannssegen-Hugo, located between the towns of Lehrte and Sehnde in the Region Hannover, Niedersachsen, was the last active potash mine in the Sarstedt–Sehnde–Lehrte salt stock district, finally closed in 1994 after 85 years of production. The enterprise originated in two separate shaft installations: the Gewerkschaft Hugo, founded 23 October 1899 to Gothaischem Berggesetz, which began shaft-sinking in December 1907 and commenced production in 1909; and the Gewerkschaft Bergmannssegen, which began sinking from 25 November 1912, reached 610 metres final depth, and commenced production in 1913. Both workings exploited the Staßfurt horizon Zechstein sylvinite and Hartsalz at 400 to 520 metres.
The first underground connection between the two shafts was driven in 1947. In 1983 the adjacent Kaliwerk Friedrichshall at Sehnde was also connected underground. From 1935, the Wehrmacht commandeered the Erichssegen shaft complex in the immediate vicinity for a Heeresnebenmunitionsanstalt, and a forced-labour camp for approximately 500 workers was established.
In 1987 the ESTA electrostatic separation plant was commissioned at Hugo. After the Kaliwerk Friedrichshall was acquired from the Kalichemie AG in 1981 and integrated, the combined Verbundbergwerk Bergmannssegen-Hugo ceased all potash production and raw salt processing in 1994. The underground workings have been flooded with brine and freshwater since 1998.
The Schacht Bergmannssegen Förderturm with its Schachthalle and Fördergerüst are listed monuments (Baudenkmal) of historical, scientific, and urban significance. The K+S plant continues to produce approximately 50 different fertiliser products from imported raw materials on the surface site.
The Gewerkschaft Hugo was founded under Gothaischem Berggesetz on 23 October 1899. Preparatory work on the shaft site at Ilten, where a plot of 23 Morgen had been acquired in the Gemarkung Ilten, began in December 1907. At approximately 182 metres depth the sinking team encountered solid rock salt, and the shaft reached 520 metres by 1906 — the first level at which production was possible. Production commenced in 1909. In 1912, 130 miners were already employed underground. By 1929 the works had obtained its own harbour on the newly opened Mittellandkanal. An iron Förderturm was erected above the shaft, together with a Kesselhaus, an electrical generating station in brick construction, a works railway connection, and a provisional Chlorkaliumfabrik producing 100 tonnes of fertiliser per day.
The Gewerkschaft Bergmannssegen, based in Lehrte, commenced shaft-sinking on 25 November 1912. The shaft reached its final depth of 610 metres. Production commenced in 1913. The Zweischachtverordnung was satisfied by a later underground breakthrough with the Gewerkschaft Hugo. The mine's Kali-Syndikat Beteiligungsziffer stood at 1.6378 Tausendstel in 1918 and 2.6837 Tausendstel from 1 April 1927; from 1 October 1934 the Syndikatsquote was 2.6048 Tausendstel. The Schacht Hugo's Kali-Syndikat quota was 2.6837 Tausendstel from 1 April 1927. Additional nearby shaft installations — the Gewerkschaft Erichssegen and the Gewerkschaft Königsburg / Ottoshall — were sunk before 1916 but never produced potash, serving instead as ventilation shafts. The Gewerkschaft Hohenfels at Wehmingen was another neighbouring operation in the same salt stock.
From 1935 the Wehrmacht's Wehrwirtschaftsführung identified the Kaliwerke Hugo, Bergmannssegen, and Erichssegen as suitable for conversion to underground munitions storage. A Heeresnebenmunitionsanstalt (MUNA Sehnde, also called Munitionsanstalt Ilten) was established primarily at the Schacht Erichssegen; up to 500 forced labourers lived and worked at the camp. The Erichssegen shaft building dating from this period still stands. After 1945 the British Army occupied the Kaliwerk site until 1966; the Bundeswehr used the facility from 1966 to 1973. The potash headframe at Schacht Hugo could not be restored and was demolished in 1984. In 1985 the underground workings were flooded — i.e. the former MUNA workings specifically.
The first Streckendurchschlag connecting the underground roadways of the Schacht Hugo and the Schacht Bergmannssegen was achieved in 1947, formally creating the Verbundbergwerk. In 1956 the Thomaskali factory at Bergmannssegen was taken into service. In 1981 the neighbouring Kaliwerk Friedrichshall at Sehnde — formerly belonging to the Kalichemie AG — was acquired and its underground workings were connected by a further Durchschlag with the Verbundbergwerk in 1983, expanding the combined field. In 1987 a Selektiver Abbau (ESTA) electrostatic separation plant was commissioned at Schacht Hugo. The mine's Wintershall AG group connection brought it within the Kali und Salz GmbH on 1 July 1970 and subsequently into the K+S group.
In 1994 all potash salt mining and raw salt processing was permanently ended at the Verbundbergwerk Bergmannssegen-Hugo. The site at Bergmannssegen has since been reoriented as a fertiliser-blending and Mehrnährstoffdünger production facility using imported raw materials; approximately 50 different products are manufactured and the site employs around 150 workers. From 1995, roll and press granulation with imported raw materials was taken into operation; from 2005 the site was strategically repositioned as a multi-nutrient fertiliser production and logistics centre. From 1998 the underground workings were systematically flooded with a mixture of brine, salt solutions, and water from the Mittellandkanal.
The Förderturm with Schachthalle and Fördergerüst of the Schacht Bergmannssegen is a listed Baudenkmal, of historical, scientific, and urban significance per the Niedersächsisches Denkmalschutzgesetz. A seilscheibe and Lore are also preserved at the site.