Copper dispersed in porous low-grade ore pellets 0.2 cm in diameter is leached with 4-M H2SO4. The copper dissolves quickly, but diffuses slowly out of the pellets. Because the ore is low grade, the porosity can be assumed constant, and the copper concentration will be low in the acid outside of the pellets. Estimate how long it will take to remove eighty percent of the copper if the effective diffusion coefficient of the copper is 2.5 106 cm2/sec.