The thorium nitrate solution from the dissolver will be about 9 Af in nitric acid. To obtain satisfactory decontamination of thorium from fission-product protactinium, ruthenium, and zirconium-niobium, it was found necessary to remove all of the nitric acid from the solution and make the solution around 0.15 Af acid-deficient in nitrate ion by converting a fraction of the A1(N03)3 to a water-soluble basic nitrate. This also converts the readily hydrolyzed nitrates of these fission products to basic nitrates that are less extractable than the species present in the acid dissolver solution.

In the initial development of the Thorex process [S9], the feed was made acid-deficient by evaporation until the boiling point reached 155°C. Trouble was experienced with corrosion and with precipitation of solids. The procedure finally adopted [R2] is shown in Fig. 10.20. The dissolver solution is evaporated until its boiling point reaches 135°C, at which point about 70 percent of the original volume has been evaporated and the nitric acid concentration is down to 3 M. Further stripping at constant volume and a constant temperature of 135°C is carried out by adding water and boiling off aqueous nitric acid until the solution is 0.2 M acid-deficient. The solution is finally diluted with water to make it around 0.15 M acid-deficient.

Prior to solvent extraction, the solution is treated with 0.02 M NaHS03 at 55°C for 1 h to convert ruthenium to a less extractable form.

If the irradiated thorium to be processed still contains appreciable protactinium activity from 27-day 233Pa, 97 percent of this can be removed and recovered by adsorption on unfired, porous Vycor glass [G10], At a flow rate of 1.57 ml/(min-cm2), columns containing 100-120 mesh Vycor, water-cooled to prevent boiling, could be loaded with 3.1 g protactinium per kg glass with only 3 percent break-through. Washing with eight volumes of 10 M HN03, 0.1 M A1(N03)3 removed most of the uranium and thorium. Then, elution with 0.5 M oxalic acid recovered 98.5 percent of the protactinium at an average concentration of 1.46 g/liter.