Acid leaching purification and neutron activation analysis of high purity silicas

The effects of acids on the removal of impurity from 2N grade silica have been studied using five leaching acids: 0.2 M-oxalic acid (pH 1.5 and 2.5), c-aqua regia, 2.5%-HCl/HF, and 1%-HNO3/HF. The presence of 39 impurities in the 2N grade silica and the reference material (RM, 5N grade silica) were investigated by neutron activation analysis (NAA), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS) methods. Major impurities of the 2N silica were Al, K, Fe, Na, Ti, Ca, Mg and P. The fractions of the eight major impurities were 99.2% and 90.9% of total impurity in the 2N and RM silica, respectively. Among the leaching acids, almost all of the major impurities were removed effectively by the 2.5% HCl/HF leaching acid. All the major impurities, except for phosphorous, as well as 21 minor and trace impurities could be determined by the NAA.