Why do large grains absorb the small ones in the re-crystallization process?

Answer 1

Because smaller grains have larger surface area per grain volume, thus their surface energy per volume is higher than that for the larger grains, thus consuming smaller grain lowers surface energy and the energy of the system in general

Answer 2

During a crystallization process, larger crystals grow at the expense of smaller crystals/particles, which dissolve. This phenomenon is called Ostwald ripening, and it is due to the balance of two different types of energies that are involved in the formation of crystals in solution:

1. surface energy: "unfavorable" energy that is due to the fact that the ions/atoms at the surface of a crystal are not able to form as many bonds as the ions/atoms that are in the bulk of the crystal (i.e., closer to the center of the crystal, farther away from the surface). For example, in the bulk of NaCl crystals, each Na⁺ cation is surrounded by 6 Cl^- anions (we say that the coordination number of Na⁺ in NaCl is 6, meaning that each Na⁺ is bonded to 6 "neighbors"). However, at the crystal surface, each Na⁺ ion will not be able to have 6 neighbors because the crystal lattice is abruptly interrupted at the surface. So, a Na⁺ ion at the surface might only be bonded to 3, 4, or 5 Cl^- ions, which makes this Na⁺ less stable.
2. bulk energy: "favorable" energy that is a result of the bonds that are formed among ions/atoms in the bulk of the crystal. Continuing with the NaCl example, large NaCl particles are more energetically favorable since more Na⁺ ions are bonded to 6 Cl^- neighbors and fewer Na⁺ ions are at the unfavorable surface.

As the system tries to lower its overall energy, ions/atoms on the surface of a small particle (energetically unfavorable, with only 3, 4, or 5 bonded neighbors) will tend to detach from the particle, causing the particle to dissolve (the detached ions/atoms diffuse into the solution). As more small particles dissolve, the concentration of free ions/atoms in solution increases and these will tend to attach to the larger particles.

In summary: to decrease the overall energy of the system (solution plus crystals) by decreasing surface energy and increasing bulk energy, smaller particles will shrink/dissolve, while larger particles grow, and the average particle size will increase. This phenomenon is called Ostwald ripening.