Quote Originally Posted by gainer View Post
The highest grade I have seen from Rio Tinto specs is NF, a pharmaceutical grade, which lists only heavy metals at <20 ppm.
It's interesting that NF grade only is concerned with heavy metals (as quoted my Pat), and not any of the other substances that the other grades are spec'ed for, like chloride or insoluble matter. It really does point out the differences in the grading system for chemicals and how one should consider these things when chosing a particular grade for a particular use.