Precipitation of arsenic in the Fe(II)-NH3-(NH4)(2)SO4-O-2 system

Abstract

In our earlier studies [R.P. Das, S.A. Anand, Precipitation of iron oxides from ammonia-ammonium sulphate solutions, Hydrometallurgy 38 (1995) 165-173; R.P. Das, S.A. Anand, Thermogravimetric and XRD studies on iron compounds precipitated from Fe(II)-SO2-NH3-O-2 system. Int. J. Miner. Process. 48 (1996) 159-168] precipitation of Fe(II) in ammoniacal medium in the presence of oxygen and characterisation of iron precipitates thus obtained have been reported. The present work describes the studies on arsenic precipitation in the Fe(II)-NH3-(NH4)(2)SO4-O-2 system. The various parameters changed were initial arsenic concentration, temperature and oxygen partial pressure. With the increase in initial arsenic concentration, percentage precipitation of both iron and arsenic decreased. Precipitation of arsenic increased with the increase in temperature from 343 to 373 K. Increase in pO(2) from 70 to 140 kPa also resulted in increase of arsenic precipitation from 82% to 93% at 15 min precipitation time. At 45 min similar to 98% As precipitated. The XRD patterns of the compounds showed that at low As concentration magnetite was the only crystalline phase and with the increase in initial arsenic concentration the magnetite phase disappeared. Amorphous precipitates were obtained for initial arsenic concentration greater than or equal to 0.94 g l(-1). The % weight of As in the As-Fe precipitate increased with the increase in initial As but no compound could be identified from the XRD patterns even when the precipitate contained 9 wt.% arsenic. The % weight loss for the As-Fe precipitates obtained with varying amounts of initial arsenic passed through a maximum at arsenic concentration of 0.94 g l(-1). The DTA trace of low (3.0%) and high arsenic (9%) containing precipitate were different in nature indicating different types of compound formation. (C) 1999 Elsevier Science B.V. All rights reserved.