Backfilling in the permafrost: predicting pressure loss and temperature distribution along the paste backfill pipeline system

Abstract

Underground mine backfilling of permafrost open stopes with cemented paste backfill (CPB) has many challenges to overcome, including keeping the CPB unfrozen during pipeline transportation at low pumping costs and ensuring the required strength development under sub-zero temperature conditions. However, CPB strength development depends strongly on the evolution of its internal curing temperature that, in turn, is controlled by the CPB placement temperature (initial) in the stope and by the permafrost boundary temperature. Hence, it is relevant to predict the required pumping pressure and distribution of the CPB temperature along the pipeline by considering the internal and external heat exchanges and the thermo-rheological behaviour of the CPB. The objective of this paper is to simulate the CPB pipe flow in a full-scale pipeline distribution network anticipated for a mine located in the permafrost region of the north of Canada. For that purpose, CPB mixtures were prepared at 10°C (initial temperature expected in the backfill plant) in the laboratory with 5% of type HE Portland cement at a slump height of about 17.8 cm (corresponding to a solid concentration of about 76.3 %). The numerical simulations were performed using the non-isothermal pipe flow model of COMSOL Multiphysics ® 5.2 considering a distribution network of approximately 1,600 m long that includes an outdoor section of approximately 294 m exposed to air circulating at a speed of 1 m/s and to an extreme temperature of -50°C. The permafrost temperature was taken to be -5°C. and heated (at +2°C) underground openings were studied. A CPB flow velocity of 1.04 m/s and a pipe diameter of 0.1463 m were considered, as proposed by the owner of the mine project. Results indicate that the total pumping pressure is about 12 MPa (heated and unheated stopes) and the pressure gradient ( / ) distribution along the horizontal pipeline sections is about 13 kPa/m. A decrease in the CPB temperature was observed in the surface section, followed by an increase in the underground sections. This temperature increase can be explained by the predominance of heat transfer by convection rather than radial conduction of the heat generated by viscous dissipation. The effect of a thermal insulation of the pipeline section at the surface was also studied for comparison purposes.