Effect of Acidic Solution on Mechanical Properties and Mesoscopic Structure of Hard Sandstone

Abstract

The present study envisaged the effects of leaching solution on the mechanical properties of stope surrounding rock in in situ blasting leaching of sandstone type uranium deposit. Experiments were performed using uniaxial compression tests and SEM observation of sandstone samples after soaking in the H2SO4 solution with pH 1, 2, 3 and 4 for 5, 10, 20, 30 and 40 days. The results showed that at pH 1, the solution levels increased slowly and steadily all the time, and for other pH solutions the levels increased rapidly in the first 5 days with a slow subsequent increase. The pH 2 solution showed reddish brown coloration in varying degrees, since it provided good conditions for Fe(OH)3 precipitation. It is suggested that the pH value of leaching solution should be controlled within 1.5–2.0 during the regular leaching period of ore pile, and then adjusted to 2.0–2.5 after the leaching process based on the forming conditions and uranium-adsorbing characteristic of the Fe(OH)3 colloid. Furthermore, with the increase in the solution pH, the mass change rate of the sandstone sample decreased. The uniaxial compressive strength and elasticity modulus of sandstone samples decreased with an increase in the soaking time and a decrease in the pH. After soaking in acid solution (pH < 4) for 40 days, the uniaxial compressive strength of sandstone decreased by about 50%, which needs to be considered in the design of pillar size. Compared to the samples in the natural state, the failure sound of sandstone treated with acidic solution weakened, accompanied by more debris and cracks, and the failure mode changed from brittle to less brittle. SEM analysis showed that, at the same soaking time, the stronger acid corrosion resulted in a looser surface structure and the larger diameter of the corrosion pore in the sandstone sample. After soaking in the same solution, there was no significant difference in the surface structure and pore diameter of the sandstone sample with an increase of soaking time.