Abstract
Traditional comminution methods based on mechanical breakage are energy inefficient and thereby incur high costs. Comminution practices can be enhanced e.g. by thermal pretreatment which induces cracks and thus facilitates the mechanical breakage.
This study aims at modelling numerically thermal shock in rocks caused by rapid application of heat flux (e.g. by means of a flame torch or a furnace), as a treatment prior to conventional fracturing. The rock constitutive model employs a (strong) embedded discontinuity finite element formulation to describe cracks in rock material. A staggered approach is employed to develop the algorithm for the solution of the uncoupled thermo-mechanical problem of thermal shock induced cracking of rock.
The model performance is tested first in numerical simulations of single, rapid application of heat flux on numerical idealized granite-like heterogeneous rock specimens. Second, the effect of this thermal pretreatment is evaluated by modelling conventional uniaxial compressive tests on heat-shock pretreated numerical rock specimens. Moreover, special emphasis is on the influence of initial crack distribution on the compressive strength of the specimens at the end of the heat shock pretreatment and the mechanical tests.
The simulations results show that the thermal shock pretreatment weaken the specimen uniaxial compressive strength by about 45%. However, considerably lower weakening effect is obtained by thermal shock treatment of the specimens with randomly oriented initial crack population. This means that thermal shock pretreatment is an effective method to enhance mechanical comminution for high-quality intact rocks.
Mechanical fracturing of rocks, like crushing and grinding, is an energy intensive process. For this reason, alternative breakage methods based on usage of non-mechanical agents to improve and/or replace the mechanical breakage are being searched at present time (Somani et al., 2017).
Thermal shock pretreatment is a method for weakening rocks by applying a high intensity external heat flux at the rock surface. Rapid heat flux application induces a high thermal gradient, which consequently causes the build-up of a compressive stress state in a thin layer of rock adjacent to the surface. This initiates new cracks, due to mismatching thermal and elastic properties of the constituent minerals, and it propagates further the favorably oriented preexisting cracks, e.g. by the wing crack mechanism. Consequently, depending on flux intensity and heating time, cracking can occur inside the rock body or locally, in proximity to the heated surface (spallation phenomenon).
