VIII. Effectiveness Of Impermeable Covers

     

Matthew Otwinowski

Non-linear waste rock modelling

VIII. EFFECTIVENESS OF IMPERMEABLE COVERS

In Table IV, the results for four different piles with covers and impermeable lining of different thickness are presented. The diffusion coefficient inside the covers is assumed to be 1.0x10^-8 m²/s. In each case the cover and lining thickness is selected so that the same values of oxygen concentration, Y_s1=Y_s2=1.17 mol/m³, at the cover - waste rock interface and at the lining - waste rock interface are produced. Covers are responsible for a significant reduction of oxygen concentration to a value of 1/8 of the normal atmospheric value. From examples summarized in Table IV it is evident that in order to generate the same reduction in oxygen concentration, when the pile height increases linearly (subsequent values of L are equal to 10, 15, 20 and 30 metres), the cover thickness has to increase in a nonlinear way (subsequent values of cover thickness d are 0.45, 0.85, 1.20 and 1.50 metre, respectively).

A very important feature of our results is the indication that the critical size effect and thermodynamic catastrophes can be eliminated by covers of sufficiently low permeability.

In Fig. 8.1, pile A30.0C has a completely different spatial distribution of oxidation rates than in the lower piles. When covers of sufficiently low permeability are used, one can create a situation in which the average acid generation rate can be decreased by increasing the pile height. By limiting the transport of oxygen through the boundaries one can create the situation in which the pile height can be increased infinitely. A cover breakdown will have, however, dramatic consequences in high piles. The risk factor increases with the pile height.

Numerical results for piles of different size, different cover permeability 

and different fine particle content

TABLE IV. The symbols in the first column refer to the pile lateral dimensions and height in metres; diffusion coefficient is equal to 10^-8 m²/s in covers of variable thickness d. All the piles have horizontal dimensions s1=100 m, s2=90 m. The value of the reactive surface area σ per unit volume is equal to 0.5 m^-1. Piles A10.0L1 and A10.0L2 contain layers of thickness 0.5 m with σ=2.72 m^-1 and σ=5.44 m^-1, respectively.

Fig. 8.1. Sulphate generation rates in piles (from left to right) A10.0C, A20.0C and A30.0C. Other results are listed in Table IV.