Sludge densification by iron (III) hydroxide precipitation

Cordelia Selomulya, Wee Yong Gan, Rose Amal Lydia, Geoff Tapsell

Research output: Contribution to conferencePaperpeer-review


Iron is a substantial component in acid mine drainage (AMD), a potential by-product in the mining industry. Conventional neutralization is a common approach to treat AMD, although the generation of voluminous sludge creates a major disposal problem. Solid density can be improved by recycling the sludge, as existing solids may provide the surface area for precipitation. Slower rate of precipitation could facilitate the growth of agglomerates. The mechanisms of iron sludge densification are not fully understood, mainly because of the complex nature of iron chemistry, and the variety of amorphous, polymeric oxides that could be formed. Experiments were carried out to observe the parameters influencing the properties of sludge formed through iron hydroxide precipitation. The solid content could be improved by adjusting the environment in which the reaction occurred. In the precipitation of iron hydroxide from the neutralization of iron chloride with calcium hydroxide, the method of alkali addition was found to be a factor for improving sludge density. A prolonged mixing of existing solids and Fe(III) ions at specific pH environment (between 3.5 and 4.5) favoured the growth of large agglomerates. This was achieved by injecting the alkali slurry in unequal proportions within the dosing period, so that the suspension pH would increase gradually. Precipitates with plate-like structures were produced under this condition, and were of higher density than needle-like precipitates formed under less effective dosing procedures. Their structures possibly helped ease the release of entrapped water between solids during shearing, resulting in sludge with higher solid content. The presence of sulfate ions influenced sludge densification, as well as the crystalline phases of iron(III) precipitates.

Original languageEnglish
Publication statusPublished - 1 Dec 2005
Externally publishedYes
Event18th Annual American Filtration and Separations Society Conference and Exposition, AFS 2005 - Atlanta, United States of America
Duration: 10 Apr 200513 Apr 2005


Conference18th Annual American Filtration and Separations Society Conference and Exposition, AFS 2005
Country/TerritoryUnited States of America


  • Densification
  • Iron(III) hydroxide
  • Neutralization
  • PH
  • Precipitation
  • Sludge recycle

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