Active potash mine shaft at Mühlwärts near Unterbreizbach, sunk 1955–1963 to 829 metres, currently serving as the manriding and material shaft of the K+S Verbundwerk Werra.
Schacht II of the Gewerkschaft Sachsen-Weimar, now designated Unterbreizbach II or Schacht Mühlwärts, is the only potash shaft sunk in Thuringia after the Second World War. The decision to sink a second shaft to complement the existing Schacht I at Unterbreizbach was taken in 1953. VEB Schachtbau Nordhausen commenced sinking on 9 June 1955 at the hamlet of Mühlwärts, in the Sünna district between the villages of Buttlar and Sünna on the Bundesstraße 84.
Severe water inflows in the Plattendolomit zone between 607 and 626 metres hampered progress. The shaft reached the first production level on 11 March 1962 and its final depth of 829 metres on 11 December 1963. It was commissioned on 30 April 1964.
The shaft served as the manriding shaft, carrying workers to and from the underground workings. A seismic event in 1975 caused surface damage in Sünna and affected the underground workings; a second geologic event in 1989 impacted the Merkers field and the ventilation connection with Unterbreizbach. Since German reunification and integration into the K+S Verbundwerk Werra in 1997, Schacht II has continued to function as the manriding and material shaft for the active Unterbreizbach mine, which remains in production as part of the Verbundwerk Werra complex.
The origins of the Sachsen-Weimar potash enterprise at Unterbreizbach lie in 1897, when exploratory boring rights were granted by the Großherzoglich Sächsisch Staatsministerium. The Kalibohrgesellschaft Sachsen-Weimar was founded in Eisenach in the same year. Boring Sachsen-Weimar 1 encountered the upper Hessen seam at 705 metres and the Thüringen seam at 775 metres the following year.
In December 1899 the drilling company was converted into the Gewerkschaft Sachsen-Weimar. The decision to sink a shaft was taken on 9 December 1899, but financial difficulties delayed the start. Preliminary shaft works began on 4 July 1905 and sinking commenced on 21 August 1905.
Significant water inflows were encountered up to 125 metres depth; the zone between 59 and 67 metres yielded flows of up to 4.5 cubic metres per minute. Water ingress was contained by 8 July 1906, after which sinking proceeded in dry ground. The shaft reached 362 metres by the end of 1900 and 584 metres by the end of 1907.
To manage anticipated water inflows in the Plattendolomit zone at 529–550 metres, ten boreholes for cementation were drilled around the shaft and two Thomson pumping devices were installed between October and November 1907. The base of the Plattendolomit was reached at 540.5 metres on 1 January 1908. The first rock salt was intersected at 602 metres on 9 February 1903 and the first potash seam at 709.5 metres on 28 May 1909.
The seam, 3.5 metres thick and characterised as a sylvinite (Hartsalz, Sylvinit) layer, displayed an average potash content of 18 to 19 percent, rising in the best sections to 45 percent. Shaft fittings were completed on 21 November 1909 and the operation, which had opened provisionally on 28 June 1910, was formally resumed on 7 February 1910 after the headframe and shaft buildings were finished. The final shaft depth was 728.5 metres.
Regular production began at an initial rate of 500 tonnes per day. From 1914 onwards repeated production interruptions occurred as a result of the First World War and its aftermath. In 1921 the Wintershall AG acquired the works.
A rail connection, the Grubenanschlussbahn, was opened to the works on 22 June 1909. In 1911 a permit was granted for a chloropotassium and sulphate processing plant, which was subsequently built and operated at the site. In 1942 the existing headframe at Schacht I was replaced by the structure that remains standing today.
The site was occupied by American troops in April 1945 following brief combat actions in the vicinity. After the war the works fell within the Soviet occupation zone; in 1952 the enterprise was converted into a Volkseigener Betrieb (VEB) and renamed in 1953 as VEB Kaliwerk Marx-Engels. In 1953 the decision was taken to sink a second shaft, Unterbreizbach II, to remedy the single-shaft constraint that had existed since production began.
VEB Schachtbau Nordhausen was commissioned for the work, and sinking began on 9 June 1955 near the hamlet of Mühlwärts in the Sünna district. Major water inflows in the Plattendolomit zone at 607 to 626 metres delayed progress considerably. The first production level was reached on 11 March 1962 and the final depth of 829.4 metres was achieved on 11 December 1963.
Schacht II was commissioned on 30 April 1964 as the manriding shaft. In 1970 the Unterbreizbach works were grouped with the Merkers and Dorndorf/Springen works under the VEB Kalibetrieb Werra. On 23 June 1975 a mining-induced seismic event of magnitude 5.2 struck the area; surface damage was caused in Sünna and the underground Ostfeld of the second Thüringen seam was affected.
In 1989 a geologic event in the Westfeld of the Merkers mine affected the underground ventilation connection with Unterbreizbach. Following the political changes of 1989–1990, the Mitteldeutsche Kali AG took over the Thüringen operations in 1990. With the Kali merger of 1993 the site became part of Kali und Salz GmbH, and in 1997 Unterbreizbach was integrated into the newly formed Verbundwerk Werra, jointly with the Hessian mines Wintershall and Hattorf.
Schacht I continues as the principal hoisting shaft and production centre. Schacht II at Mühlwärts serves as the manriding and material shaft, used for personnel transport and for lowering large equipment into the mine. On 1 October 2013 a CO2 outburst following a production blast at 900 metres depth killed three miners underground.