The traditional approach for metals removal is Hydroxide Precipitation where the solubility for the metals changes to insoluble (precipitation) in the form of a metal Hydroxide as the pH increases.
The common pH range for this is between 10-11 pH depending on the metal. Lime (calcium hydroxide or CAOH2), caustic (sodium hydroxide or NAOH) or magnesium hydroxide (MgOH2) are generally the chemicals of choice as the hydroxide ion source. The process uses a rapid mix to add the hydroxide source and usually an anionic flocculant to help as a settling aid and sludge conditioner.
The problems with this technology are:
- Large volumes of sludge production
- Inability of the metal hydroxide to become sufficiently insoluble if chelating or surfactants are in the water
- Unstable treated metal levels especially if the influent is highly variable.
The quantity of sludge is directly proportional to the amount of hydroxide source added. This can range from 500 to 3,000 ppm by weight and yields 4 times this weight in sludge addition assuming a 25% dry sludge cake.
The dewatered sludge is then classified by its leachable metals following the EPA TCLP/STLC testing protocols where hydroxide sludge most often leach their heavy metals and are therefore classified as hazardous.
Additional problems are in the form of…click here to continue reading Floccin™ for Metals Removal Beats Hydroxide Precipitation Hands Down…
The traditional approach for metals removal is Hydroxide Precipitation where the solubility for the metals changes to insoluble (precipitation) in the form of a metal Hydroxide as the pH increases.
The common pH range for this is between 10-11 pH depending on the metal. Lime (calcium hydroxide or CAOH2), caustic (sodium hydroxide or NAOH) or magnesium hydroxide (MgOH2) are generally the chemicals of choice as the hydroxide ion source. The process uses a rapid mix to add the hydroxide source and usually an anionic flocculant to help as a settling aid and sludge conditioner.
The problems with this technology are:
- Large volumes of sludge production
- Inability of the metal hydroxide to become sufficiently insoluble if chelating or surfactants are in the water
- Unstable treated metal levels especially if the influent is highly variable.
The quantity of sludge is directly proportional to the amount of hydroxide source added. This can range from 500 to 3,000 ppm by weight and yields 4 times this weight in sludge addition assuming a 25% dry sludge cake.
The dewatered sludge is then classified by its leachable metals following the EPA TCLP/STLC testing protocols where hydroxide sludge most often leach their heavy metals and are therefore classified as hazardous.
Additional problem are in the form of