Magnesium hydroxide is the least expensive and most powerful alkaline chemical treatment available on the market for pH control applications. At the same time, it is by far the safest and most gentle to use. By replacing the sodium in caustic soda or soda ash with the magnesium ion, the treated wastewater is transformed from having to deal with a detrimental form of salinity (Na+) to the benefit of a macronutrient (Mg2+). Mg2+ is the core element in chlorophyll that drives photosynthesis.
Controlling the pH of industrial wastewater is important to prevent harm to the wastewater collection/transmittance infrastructure by minimising the corrosive effect of gaseous acidic contaminants (such as H₂S) or, if applied to farm land, to prevent harm to the irrigated crops. Once the wastewater enters a treatment plant, the control pf pH is essential to optimise aerobic and anaerobic microorganism activity. In this article, we explain how magnesium hydroxide can cost-effectively replace caustic soda and lime in different wastewater treatment applications.
Disclaimer: This experiment was not performed in a controlled lab environment and is routinely performed safely as a benchtop demonstration to show that magnesium hydroxide provides more CaCO3 equivalent alkalinity on an equal weight basis when compared to caustic soda, which means less chemical consumption and can result in significant cost savings. Magnesium hydroxide is non-hazardous and non-corrosive which makes handling safer and easier.
While Mg(OH)2 is typically more expensive on a “price per unit” basis, it is significantly less expensive in use. Every 1.0 kg of 50% NaOH can be replaced by 0.6 kg of 60% Mg(OH)2 to provide the same number of moles of hydroxide (OH–) for pH neutralization.
When an alkaline compound dissolves in water it produces hydroxide ions, OH–. These ions are responsible for the alkalinity of the solution.
Because Magnesium Hydroxide Mg(OH)2 provides two hydroxides for alkalinity compared to one with NaOH caustic soda, the usage rate for Mg(OH)2 is much less than that required for NaOH.
Unlike caustic soda, Magnesium Hydroxide is non-hazardous and non-corrosive, which makes it easier and safer to handle. Spill is not reportable.
Because magnesium is a macronutrient – it is the core element of chlorophyll (makes everything green!) – it is also beneficial to the environment while sodium is a salinity hazard.
Magnesium also has coagulation properties that sodium does not. This can result in reduced sludge volume and reduced sludge hauling fees.
The wastewater stream for many industrial processes can be quite unpredictable and inconsistent. Dramatic changes in pH can have adverse effects on the settleability of suspended solids. To stabilise the pH, the wastewater is commonly neutralised with alkalis. The nature of the alkali selected can have a dramatic impact on the ability to settle suspended solids. Caustic soda typically provides no settling benefit and often results in gelatinous sludge that does not dewater easily. With lime (Ca(OH)2) there is typically a significant increase in sludge volume due to the formation of calcium minerals, like calcium carbonate (CaCO3).
In contrast, magnesium hydroxide assists with solids settling, providing a more compact sludge with a boost in alkaline pH to improve subsequent secondary treatment and anaerobic digestion performance.