Stockpiles are widely used but may experience different mass transport related phenomena resulting in solids flow problems and safety risks. J&J Engineers have utilised a simulation technique to model the flow of liquids within a porous media to help determine the final moisture retained in the bulk material after gravity drainage.
stockpiles vary in shape
(mainly flat, conical or prismatic) and size (1000 up to 500,000 tons). They can be uncovered (exposed to the environment) or partially/fully covered. They are formed by overhead conveyors such as stackers, shuttles, or t¬¬¬rippers, or by dump trucks, front-end loaders, or bulldozers. Reclaim is usually done by gravity via various types of hoppers and feeders or gates housed in concrete tunnels beneath the pile [1], or mechanically using scrapers, bucket wheel excavators or front-end loaders. Figures 1 and 2 show typical stockpile geometries and a bucket wheel reclaimer often used in industry.
In mining, stockpiles separate the discontinuous mine operations from the plant that must operate continuously. In exporting ports, they provide surge capacity while waiting for ship loading. In many quarry operations the sand is excavated, washed, classified by sizes, and then stockpiled for natural drying. This process saves energy and prevents the risk of instabilities in the base of the piles. Dewatering wet and saturated sand stockpiles by gravity drainage and solar evaporation allows for the reduction of the sand’s moisture content down to four per cent to five per cent after several days at rest depending on the permeability, particle size, shape, and pile height [2].
Issues due to excess water in the material
Rainfall onto an open (uncovered) stockpile can either run off the pile surface or will seep into the pile body. The infiltrated water can evaporate, drain naturally by gravity, or remain trapped within the material as free moisture. To minimise infiltration that may cause higher transportation costs, handling, flooding and/or flushing problems, compacting the surface of ore and coal stockpiles with bulldozers improves and increases the potential for surface runoff. The runoff water can then be collected in sumps and ponds for proper water management and/or treatment [3]. Also, the excess water usually contains a significant quantity of fine particles flowing in suspension (mudflows) that might contaminate rivers and oceans. Figure 3 shows an example of stockpile mudflows that can occur during an intense rain event.
Liquefaction problems are created by the flow of water in stored materials. It refers to the unwanted change of state of the material when its shear strength is reduced to near zero and the material behaves like a liquid [4]. Several large bulk carriers have sunk or capsized due to the liquefaction of the cargo (mainly iron ore fines and nickel ores) when the cargo slides or shifts from side-to-side.
The rapid movement of material, resulting from flowslides failures that have occurred with stockpiled coals and other ores have also been attributed to liquefaction of saturated and relatively loose material, where shearing is accompanied by generation of excess pore water pressure with a resultant loss of strength. Investigations have revealed that some of these stockpile failures were preceded by rainfall infiltration and/or downward migration of moisture present in the material at placement [5, 6].
This liquefaction phenomenon may also occur on belt conveyors or in open trucks and railcars, leading to unsafe operation and accidents.
Material characterisation
It’s important that material characterisation is performed on the handled material to understand whether water will free drain or be retained. The results from the test work will inform the simulation and analysis.
Different tests are available to determine the material characteristics related with stockpile drainage and seepage. These tests are particle size and size distribution, particle density, compressibility (or bulk density as function of consolidating pressure), moisture content, bed porosity, saturation, as well as the water retention and hydraulic conductivity properties of the material.
The hydraulic conductivity depends not only on material properties such as porosity, particle size, density and shape, but also on fluid density and viscosity, and on the consolidation pressure of the sample. Many of these tests follow well-known ASTM and/or ISO standard procedures. Others require skilled technicians to operate scale models or special equipment to “mimic” the real-life conditions as no standards have yet been developed.
Analysis of water infiltration and runoff of iron ore stockpiles
A mining company was experiencing severe stockpile stability issues of some of its prismatic-shape iron ore product stockpiles during the winter season. This was resulting in damage to multiple stockyard machines (i.e., stacker, reclaimers, conveyors, etc.), safety issues, and production limitations. An example of the slope stability issues is shown in Figure 4.
The stockpile failures were attributed to water saturation and pore pressure development in the bottom layers of the pile preceded by significant rainfall events. Contributing to the stockpile failures, the stockpile erosion and sediment transport (mudflows) to the vicinity of the stockpiles toe during the intense rainfalls was blocking the natural drainage of the stockyard, causing ponding near the stockpile toes.
J&J was contracted to investigate water drainage and seepage in the stockpiles (refer to Figure 5), considering rainfall events (i.e., intensity and duration) recorded by an onsite weather station. Similarly, to the above example, to model the flow of water or seepage within the stockpile, COMSOL Multiphysics software was utilised using the Richard’s equation model for flow in variably saturated porous media. The hydraulic and other physical properties measured for several samples of the iron ore material were utilised in the analysis.
Rainfall can either infiltrate the stockpile or directly run off the surface of the pile. The relationship between stockpile run off and infiltration is dependent on factors such as degree of saturation at the surface, particle size, stockpile surface texture, rainfall intensity and duration, moisture content, and the degree of compaction of the surface. For this study, we considered that rainfall infiltrated the stockpile depending on the degree of saturation near the stockpile surface, the rainfall intensity and duration. No other additional resistance to water infiltration was considered. A daily rainfall was assumed for the analysis (repeated for five consecutive days), a constant rainfall intensity was also considered.
The results of the simulation conducted by J&J engineers are illustrated in Figure 6. A good drainage system was compared to a poor drainage (or impervious stock pad), taking into consideration the rainfall events from the nearby weather station. The colour legends illustrate the effective degree of saturation.
Given the high hydraulic conductivity of the ore, for the cases of poor drainage at the base of the pile, the initial water content of the ore in addition to the infiltrating water from the heavy and consecutive rainfall events can quickly mound at the bottom of the stockpiles; a water table inside the stockpile of approximately 4 m high formed during storage. The amount of water accumulating at the bottom of the pile was significantly reduced when a good drainage system at the bottom of the pile was considered. The analysis indicated that forming the stockpile with a certain initial moisture saturation was effective in limiting excessive accumulation of water at the bottom of the pile.
Conclusions
COMSOL Multiphysics finite element analysis was conducted to estimate the drainage and seepage in an iron ore stockpile, utilising well recognised equations to describe the flow of fluids in unsaturated porous media. The outcome of the presented example illustrates the effective use of the COMSOL simulation tool to analyse and predict drainage and seepage in stockpiles and provide the necessary inputs to a commercial installation to make corrective actions as necessary.
References
Cabrejos F. and Goodwill D., “Tunnel Reclaim from Ore Stockpiles”, Bulk Solids Handling, Vol. 16, No. 3 (July/Spt. 1996), pp. 393-400.
Hashemi E., “Experimental Study for Enhanced Drainage from Sand Stockpiles”, M.Eng. thesis, School of Civil, Environment and Chemical Engineering, RMIT University, Australia, August 2011.
Campbell Q., Le Roux M. and Espag C., “Coal Product Moisture Control Using Stockpiles”, XVIII International Coal Preparation Congress, Rusia, 2016, pp.747-752.
DNV, Bulk Cargo Liquefaction – Guideline for Design and Operation of Vessels with Bulk Cargo that may Liquefy, Norway, 2015.
Eckersley J.D., “Flowslides in Stockpiled Coal”, Eng. Geology, 22 (1985) 13-22.
Davies P., Zargarbashi S., and McQueen L., “Flow failure in coal stockpiles – how to reduce risks”, ACG, Slope Stability proc. 2013.