IX. Effect Of Layers With Small Rock Particles Formed Due To Equipment Traffic

     

Matthew Otwinowski

Non-linear waste rock modelling

IX. EFFECTS OF LAYERS WITH SMALL ROCK PARTICLES 
FORMED DUE TO EQUIPMENT TRAFFIC

In Section V we have shown that if one assumes the fractal dimension d_f=2.5 for the particle size distribution function, the rock reactivity Rate_surf increases by a factor of 5.44 when the maximum size of particles is reduced from 50 cm to 10 cm. We use this result to analyse the large scale affect due to the meso-scale nonhomogeneity of waste rock properties. We have performed numerical simulations for piles with layers of small particles. Figs. 9.1 and 9.2 present results obtained by inserting a small-particle layer into a subcritical pile of height L=10 and σ=0.5 (pile A10.0 in Table IV). We present two scenarios. Pile A10.0L1 has a layer of thickness 0.5 m and reactivity σ=2.72. Pile A10.0L1 contains a layer of thickness 0.5 m and reactivity σ=5.44 (see Figs. 9.1 and 9.2). The results summarized in Table IV show that a single layer of fine rock particles constituting about 5% of the total waste rock mass can increase the acid generation rates by as much as 300%. When the layer absorbs water, however, the oxidation rates will be limited not by the transport of oxygen in the gas phase but by the much slower transport of oxygen in water. Therefore, in wet periods, the oxidation rates may become slower than indicated by our numerical results which do not assume the water saturation affect.

Pile A10.L1 has about a 50% faster average acid generation rate than pile A10.0 without a fine-particle layer. By raising temperature everywhere in the pile, the layers of small particles accelerate oxidation rates in the whole pile volume.

Pile A10.L2 has about a 250% faster average acid generation rate than pile A10.0 without a fine-particle layer.

The fine-particle layer in pile A10.0L2 is in a supercritical state. This is evident from plots in Fig. 9.2. Temperature distribution and acid generation rates are very nonuniform along the layer.

The small-particle regions may generate acid faster than it can be locally neutralized if the distribution of neutralizing rock is not adjusted to the method of pile construction.

Fig. 9.1. Distribution of temperature and sulphate generation rates inside pile A10.0L1 which contains a layer of fine particles with a maximum diameter five times smaller than in the rest of the pile. Layer thickness is equal to 0.5 m; rock reactivity, σ=2.72 m^-1 in the layer is 5.44 times greater than σ=0.5 m^-1 outside the layer. A fractal particle size distribution with d_f=2.5 is assumed.

Fig. 9.2. Distribution of temperature and sulphate generation rates inside pile A10.0L2 which contains very fine particles with a maximum diameter eight times smaller than in the rest of the pile. Layer thickness is equal to 0.5 m; rock reactivity σ=5.44 m^-1 in the layer is 10.88 times greater than σ=0.5 m^-1 outside the layer. A fractal particle size distribution with d_f=2.5 is assumed.