The SiO2 concentrate after extracted of aluminum from clay from Saneheong-Hadong, Korea, was leached with NaOH at 25-100 degreesC under atmospheric pressure. About 80% of the silicon in the concentrate was extracted at 25 degreesC within 30 minutes. The molar SiO2/Na2O of the sodium silicate solution thus obtained ranged from 2.5 to 3.1, The silicate solution contains 0.11 wt.% of aluminum and 67 ppm of iron as impurity, Out of 150 g of the concentrate charged, about 100 g was dissolved and the balance was undissolved. The filterability of the resulting slurry was studied in a pressure filter with varying pressures and temperatures. By analysis of the undissolved solid, the formation of insoluble sodium compounds during NaOH leaching is Pound to be negligible.