Abstract:  The caved ores by non-pillar sublevel caving method have directly contact with the overburden rock，which makes the ore loss and dilution. The cover layer is extremely important for ensuring the normal operation of the underground. The unreasonable formation of cover layer will increase the loss of ore. According to the grain size and density of the cover rocks，the experiment is carried out based on the central composite design，and the second-order response prediction model for the influence of grain size and density on the rock mixing rate is established. The experimental results are analyzed by variance analysis and significance test. The test results show that the significance of the prediction model is high，and the experimental value fit well with the prediction value. The influence of the interaction both particle size and density on the mixing rate was analyzed by drawing the three-dimensional response surface. The experimental results showed that the influence of size and density of the cover layer rocks on the rock mixing rate was nonlinear. When the density of rock particles ranged from（3.05~3.46）×10³ kg/m³，increasing the size of rock particles would help to reduce rock mixing.

Key words: Non-pillar sublevel caving method, Cover layer, Grain size, Density, Central composite design, Response surface, Rock mixing rate