Analyse und Optimierung der Versatzeigenschaften im Scheelit-Bergbau Mittersill

Translated title of the contribution: Evaluation and optimization of cemented paste backfill properties in the scheelite mine Mittersill

Maria Walkner

Research output: ThesisMaster's Thesis

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This Master thesis was created in cooperation with Wolfram Bergbau und Hütten AG in Mittersill (Salzburg, Austria). It is assigned to analyse and optimize the properties of cemented paste backfill (CPB) produced in the scheelite mine in Mittersill. The findings show why the CPB does not develop strength despite of a binder content of 20 wt%. Measures on how to achieve a backfill product with measurable strength are stated. The minimum uniaxial compressive strength (UCS) is 0.4 MPa according to permit obligations. The CPB is produced from dewatered pulp containing tailings from scheelite flotation and 20 wt% of binder. From the continuous mixer it is transported to the cavity. The binder consists of 20 wt% cement (CEM II B-S 42.5 N) and 80 wt% of a filter ash from the Salzburger Reststoffgesellschaft. The water/binder mass ratio is up to 2.5 depending on fresh water addition. The corresponding water/cement ratio is 12.5. Due to a considerable content of fines (grain size <20 μm) in tailings and filter ash the water demand to ensure pumpability is high. The high water/cement ratio is one possible reason for the insufficient strength. Furthermore the process reagents used in flotation and dewatering can interact with the binder components and interfere with hardening. The hardening behaviour was examined according to EN ONORM 196-3. The results show that the process reagents do not influence the hardening of the binder negatively. However the filter ash contains anhydrite (CaSO4) which hydrates and precipitates as gypsum (CaSO4·2 H2O). As a result the workability time is short and the binder cannot develop high strength. Therefore and due to the increased water demand binders based on filter ash and cement are not appropriate for CPB. Trials with 5 wt% cement (CEM II B-S 42.5 N) show that the water demand to obtain a pumpable paste fill can be reduced and a water/cement ratio of below 6.0 is accomplished. These paste fill mixtures achieve a UCS of more than 1 MPa after 28 days. An additional decrease of the water/cement ratio by adding a superplasticizer or the use of CEM I 52.5 R do not enhance the strength. Furthermore the application of cement as a sole binder improves the bleeding behaviour of CPB in comparison to the binder mixture of cement and filter ash. Trials with a binder mixture containing the same amount of slag instead of filter ash show that the affinity to bleeding can be decreased compared to CPB mixtures with cement as sole binder. However, the water demand is higher and the strength is lower. The conducted trials demonstrate that the paste fill properties can be improved with a binder content of 5 wt% cement. All requirements concerning pumpability and strength can be satisfied. Due to the reduction of the binder content in the paste fill product more tailings can be backfilled.
Translated title of the contributionEvaluation and optimization of cemented paste backfill properties in the scheelite mine Mittersill
Original languageGerman
  • Harmuth, Harald, Supervisor (internal)
Award date7 Apr 2017
Publication statusPublished - 2017

Bibliographical note

embargoed until 05-03-2022


  • cemented paste backfill
  • backfill
  • binder
  • cement
  • filterash

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