lab scale process according to example 1 was modified to permit semi batch operation whereby the residual slurry of unreacted iron remaining after removal of the supernatent das fe 3 o 4 is reutilized the reduction reactor discharge apparatus is adjusted to leave a residual slurry fe u00b0 volume of about 550 ml after removal of the supernatent liquid of example 1 to the 550 ml agitated residue in the reducer is charged 98 gm of iron powder heating is resumed and 1 0 gram of h 2 so 4 and 0 25 grams of acetic acid are added the ph should be less than 6 5 6 5 8 and the orp should again be within the range 130 150 mv at 90 u00b0 100 u00b0 c the dns batch prepared as in example 1 and contained in dns vessel a is then added to the reducing reactor b and the reduction procedure is completed over a series of 10 semi batches yields of substantial pure das in excess of 96 yield were obtained into a 2000 gallon slurry vessel fitted with an agitator and a bottom discharge was pumped a 17 wt wt slurry of dns 2 299 lbs in 11 569 lbs of water to this was added 12 gal 167 lbs of 93 h 2 so 4 or an equivalent amount of oleum the ph of the resulting slurry was in the range 1 5 to 2 0 the slurry was ready for feeding to the reducing reactor below a 3000 gal stainless reactor fitted with a steam heating jacket a reflux column an agitator and sampling.