Study On Rheological Behavior And Pipe Flow Resistance Of Paste Backfill

In recent years, paste backfill technology has become a substitute for underground mining because of its advantages in ground support, operation cost and environmental conservation. As an important part, pipeline transportation can influence the running and efficiency of the filling system directly, thus making it necessary to study pipe flow resistance which is theoretically important for system design and process optimization. Comparing to traditional filling slurry, the flow characteristics of paste are more complex due to its flocculated structure. For researches on structural flow characteristics, the rheology theory seems to be more effective than traditional solid-liquid two-phase flow theory. However, the former one is not fully-developed as a result of time deficiency. In conclusion, although paste backfill technology has been adopted in some mines, the lack of a clear understanding of pipeline flow movement has largely restricted the advance of this technology.Focusing on the paste interior structure, some experimental studies and theoretical analysis were carried out in terms of material properties, paste microstructure, rheological behavior and pipe flow resistance, and finally the accurate calculation of pipe flow resistance was achieved. The main research contents are as follows:(1) A method has been developed for quantitatively characterizing the microstructure of paste, and how material properties and the external shear influence it was revealed. By analyzing the material composition, the value of Cv/Cvm was presented to measure the microstructure integrity. An empirical model of Cvm was obtained based on the experimental data. With environmental scanning electron microscopy, computer image processing and fractal theory, a method has been established to quantitatively describe the microstructure and morphology of paste that has considered how the microstructure changes under shearing.(2) A rheological model was developed to better describe the viscoelastic behavior of paste, and how the material properties influence rheological parameters was figured out. According to structural dynamics, a thixotropy-yield model was established to comprehensively describe the viscoelastic behavior of paste, which was discovered in paste flowing tests. An approach for the determination of rheological parameters was proposed based on the "same structure concept", contributing to the understanding of the influence mechanism of concentration, grain composition and cement content. Besides, two prediction models were obtained for estimating yield stress and plastic viscosity respectively.(3) Some experiments were firstly taken for studying wall slip of paste flow, and the effect of local slip on pipe flow was revealed. A basic equation about paste slip flow in pipeline was deduced and a multi-diameter experimental platform was established. The effects of concentration and grain composition on slip velocity and the thickness of slip layer were studied. The formation mechanism of wall slip in different working conditions was analyzed, an index of slip contribution rate (DR) was presented and the effect of wall slip on pipe flow was revealed.(4) Some simulation experiments of pipeline transportation in different conditions were carried out and the calculation method of pipe flow resistance was deduced. Based on the pilot-scale looping pipe test system, some simulation tests were conducted in different working conditions such as long-distance pipeline transport and stop-restart pumping. The influences of time, flow rate on pipe pressure drop were studied. A mathematical model was built to describe how pipe pressure drops with time. The influences of concentration, cement dosage on pipe flow resistance were studied, and the combined action of rheological properties and wall slip on pipe pressure drop was analyzed. Therefore, a model for computing friction loss of paste in pipelines was constructed. After analyzing paste properties and how pipe flow resistance changes in restart process, a calculation method of start-up pressure was finally deduced.(5) A general process for designing pipeline transportation system was formed and a proposal on reducing resistance was developed based on grading optimization. With some practical paste backfill projects, some problems of the pipeline transportation system like technological parameters design, pipeline layout, equipment selection were analyzed, thus a general process of pipe transportation system design was presented. For unclassified-tailings paste, a preliminary concept was put forward to lower pipe resistance by adding coarse particles and a reasonable proportion was then determined according to the gradation theory. This result was verified by several rheological tests.