Pressure-quantity characteristic curves for a mature panel cave

Panel caving is an underground bulk mining method that uses gravitational force to mine massive, steeply dipping and deep-seated ore deposits at lower operating cost. To control dust and gas emissions entering the production level during the extraction of ore from the draw points in a panel cave, relatively negative pressure is created in the cave by installing an exhaust fan in the exhaust drift located on top of the cave. As caving is a dynamic process, designing an effective ventilation system is challenging; therefore, estimating the airflow resistance offered by the broken rock inside the cave is critical. This study investigates the effect of changes in the bulk porosity of the broken rock on the cave airflow resistance using the computational fluid dynamics (CFD) approach. The results show an inverse relationship between the cave airflow resistance and the bulk cave porosity. The pressure-quantity characteristic curves presented in this paper provide valuable information for sizing and operating an exhaust fan used for a mature panel cave ventilation. Background An optimally designed panel cave mining method can have the lowest operating cost compared with other underground mining methods, subject to keeping ore dilution under checked limits [1,2]. Cave mining methods demand a robust ventilation system to support the high production rates of future super caves. This research study contributes to the knowledge base of panel cave mine ventilation. It investigates a less explored concept of cave airflow resistance that plays a vital role in sizing and operating the exhaust fan used for ventilating a mature panel cave in terms of airflow and pressure requirements with respect to the cave size and caved material properties (bulk porosity). Method This study considered a panel cave continuum model with cave dimensions of 375 m × 256 m × 356 m (height × length × width) and nine production drifts, three undercut inlet ducts, eight undercut drifts and eight exhaust drifts. The model also contained 94 drawbells, broken rock region and uncaved in situ rock inside the cave. These regions were simulated as porous zones. The isometric, side and front views of the panel cave model are shown in Fig. 1, and a summary of the boundary conditions applied to the panel cave model is presented in Table 1. The simulation study considered a steady-state, incom pressible and turbulent airflow through the panel cave model. The standard k-ε turbulence model was used for the study to consider the effects of turbulence. Results For a given porosity of the cave and airflow in the production drifts, the pressure difference across the cave was calculated and then plotted against the airflow quantity flowing through the cave (Fig. 2). For example, five different air quantities were simulated for a bulk cave porosity of 35 percent. Therefore, for six different bulk cave porosities, a total of 30 simulations were performed to develop the pressure-quantity (P-Q) characteristic curves for a mature panel cave mine. From Fig. 2, the relationship between the pressure difference, P (Pa), across the cave and the airflow resistance, R (Nsa/mb) and the air quantity supplied, Q (m3/s) can be summarized by ΔP = RQ c, where 1.8 ≤ a ≤ 1.9, 7.4 ≤ b ≤ 7.7 and 1.8 ≤ c ≤1.9. The equation suggests that the flow inside the cave is neither fully laminar nor fully turbulent. To validate the CFD framework used for the full-scale mature panel cave, we conducted an experimental and CFD simulation study for a reduced cave model (1:100), shown in Fig. 3. From the reduced CFD cave model simulations and experimental analyses, it was concluded that the reduction in porosity and particle size of caved materials increased cave airflow resistance while the existing undercut openings decreased the airflow rate through production drifts, increased the overall airflow through the cave system, reduced the overall cave resistance and increased the exponent n value in the P-Q equation. The additional fan increased cave airflow resistance and decreased the exponent n value [3]. Fig. 1 (a) Isometric view and (b) side and front views of the panel cave model. Fig. 2 Pressure-quantity characteristic curves. Conclusion The scope of work for this paper is to develop pressurequantity characteristic curves for a mature panel cave. The novelty of the paper is the investigation of the panel cave mine ventilation under various cave conditions. This study is valid for a mature panel cave (with no air gap) where the cave is connected to the older working areas, or an exhaust drift located on top of the cave. This study reveals that porosity plays a vital role in changing the resistance offered by the broken rock to the airflow leaking into the cave. The airflow resistance increases as the porosity of the broken rock pile decreases. The resistance of the panel cave changes dynamically with the bulk porosity of the broken rock. The CFD analysis of the scaled experimental model provided valuable information critical for the success of the experimental studies. No significant change in the airflow resistance of the cave was observed beyond the upper limit of 50 percent cave porosity. Fig. 3 (a) Isometric view and (b) top view of the scaled model. Selected references Laubscher D (1994) Cave mining-the state of the art. J South Afr Inst Min Metall 94(10):279–293 Trueman R, Castro R, Halim A (2008) Study of multiple draw-zone interaction in block caving mines by means of a large 3D physical model. Int J Rock Mech Min Sci 45(7):1044–1051 Pan Y, Tukkaraja P, Jayaraman Sridharan S, Jha A (2021) Investigation of airflow characteristics under parallel fan conditions in a block cave mine. CIM J 12(4):169–178

Pressure-quantity characteristic curves for a mature panel cave

Authored by:
R. Bhargava, Y. Pan, S.J. Sridharan, P. Tukkaraja, V.V.S. Vytla

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