Kirk, you spelled out "mule crap" in your previous post. I just abbreviated that.
Potassium tetraborate is about 25 times more soluble in cold water than the sodium. By the difference in solubilities we could get a first solution that removed 95% of the soluble impurities in sodium tetraborate at a cost of about 5 grams of borax decahydrate. We pitch that, make another solution in cold water from the sediment removing 95% of remaining impurities and leaving about 90 grams. The soluble impurities are down to about 2 ppm. If that is not good enough, the process could be repeated using another 5 grams and leaving a fraction of a ppm.
I doubt you would have any difficulty getting Pharmaceutical grade from Rose Hill. IIRC, its about $2 a pound in 50 pound bags.
If you can filter instead of decanting, you could probably get 98% of impurities out at 1 pass.