The residual rich SiO2 slag was obtained for magnesium was leached from the boron mud by sulfuric acid. The SiO2 was leached from the residual slag by sodium hydroxide solution. The effect of the leaching temperature and the leaching time, the ratio of the sodium hydroxide solution to slag and the concentration of the sodium hydroxide solution on the leaching efficiency of SiO2 in the residual slag were also studied. The reasonable technical condition as follow : the leaching temperature is 130°C, the leaching time is 6 min, the concentration of the sodium hydroxide solution is 19mol·L-1, the ratio of the sodium hydroxide solution to slag is 2.5:1. The SiO2 leached rate can be stabilized about 85% under this condition. Silicon dioxide was produced by double carbonating the sodium silicate solution and its percentage purity is above 99%.