Replacing caustic soda with magnesium hydroxide for metal ion precipitation

Challenge

In this particular operation, an acidic wastewater of pH 1.7 containing primarily copper, zinc, and nickel was initially pH-adjusted with 50 lb bags of magnesium oxide (MgO), before the addition of caustic soda (50% NaOH) in order to drive the pH up to ~11.

This was done for the metal ions to be precipitated and separated from the final effluent stream, in the form of insoluble metal hydroxides.

Once the pH was properly adjusted, coagulant and flocculant polymers were added to aid in separation and sludge dewatering.

During a technical sales visit, IER introduced the wastewater treatment operator to the possibility of replacing heavy MgO bags and hazardous caustic soda with IER’s nonhazardous AMALGAM-60 in this complex metal-ion precipitation application.

Additionally, he was interested in determining whether there could be chemical cost savings by replacing the use of ~1000 gallons/month of 50% NaOH with the use of AMALGAM-60.

AMALGAM-60 (60% Mg(OH)₂) has a higher neutralizing value per pound than any other alkaline additive.

This typically translates into a significant reduction in chemical usage when compared with caustic soda, while being significantly safer for operators to handle and more nutritive for the microorganisms being maintained – making it the most cost-effective option.

Solution

IER takes pride in being a technically credible wastewater treatment company, not simply a magnesium hydroxide supplier. For over 20 years, IER has worked with plant engineers and operators to offer the most cost-effective solutions for their specific needs.

An initial bench testing showed that about 750 gal/month of AMALGAM-60 could be used to replace 1000 gal/month of 50% NaOH, resulting in a cost savings of ~$1400/month.

A full-scale evaluation was arranged to determine if the bench testing results could be duplicated in large-scale. IER provided and installed an agitated storage tank and metering pump required for the on-line test, along with the associated feed line for AMALGAM-60 product delivery into the pH adjustment tank.

Hydroxide Precipitation Using Magnesium Hydroxide

The industrial standard method for removing contaminating metal ions from a wastewater stream is by raising the pH to a level that causes the formation of insoluble metal hydroxide precipitates. Below is a graph showing the solubility of common heavy metal ions versus pH.

Note that the Y-axis is exponential, showing that changes in pH from 7 to 8 to 9 will result in the solubility of Zinc (Zn) to fall from about 100 mg/L to 10 mg/L to 1 mg/L, respectively. Metal ions of Nickel (Ni) and Cadmium (Cd) require even higher pHs in order to minimize the solubility and cause precipitation.

A unique characteristic of Magnesium Hydroxide is that it provides two different pH environments within the same solution matrix. Most obviously, as Mg(OH)2 dissolves in water it releases free hydroxide ions (OH-) that neutralize any acidity in the water, driving an increase in the bulk water pH. Less obviously, any micron-sized particles of Mg(OH)₂ that have yet to dissolve possess a pH within their micro-environment that will be in the range of 9 to 11, providing a nucleation site for those metal ions that form insoluble hydroxides at higher pH values.

It is believed that this dual function of magnesium hydroxide allows for effective removal of a full range of heavy metal ions even at bulk water pH values far lower than expected, especially for ions such as nickel. Therefore, though a two-step pH adjustment approach may be needed using caustic soda in order to effectively remove Chromium at pH 7.5 and Nickel at pH 10, the simple maintenance of a pH of 7.5 using Mg(OH)₂ provides effective removal of both heavy metal contaminants.

During the trial, AMALGAM-60 performed very well in adjusting pH of the effluent stream to about 8.3 while maintaining the heavy metal concentrations well below the required permit limits.

A side benefit was that the precipitated solids settled more densely (more compact) and the liquid fraction was much clearer (less turbid) when using AMALGAM-60 as compared to caustic soda, indicating that AMALGAM-60 was aiding the polymer to give superior coagulation properties despite feeding at a reduced dose as compared to 50% NaOH.

In order to test the effect of an overdose, a 500 mL sample of acidic wastewater was dosed with AMALGAM-60 to raise the pH to 6.9. At that time, we then injected an additional dose equivalent to 12 times above that which was needed for treatment.

After 30 minutes of stirring the pH had stabilized below a value of 9, illustrating that even a gross overdose of AMALGAM-60 will not exceed their pH regulatory requirement of 10.

This was done because it was common for them to accidentally over-feed caustic, requiring them to back titrate with another hazardous chemical – sulfuric acid.