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All about IER’s ALKA-Mag+
The biggest benefit of IER’s ALKA-Mag+ (60% Magnesium Hydroxide) is that it is completely nonhazardous, which is a huge feather in the cap for an Environmental Manager interested in making the switch from caustic soda.
In addition, the dose required for ALKA-Mag+ to provide a certain pH boost to a wastewater stream is far lower than that for 50% NaOH. There are two reasons for this:
- ALKA-Mag+ contains 60% total active molecules versus 50% in caustic soda
- Magnesium hydroxide releases two moles of OH– from a single molecule of Mg(OH)2, while NaOH only releases one
Calculating the stoichiometric number of moles of hydroxide that can be released from each product results in a projected usage rate reduction of 0.6 lbs of 60% Mg(OH)2 for every 1.0 lb of 50% NaOH. The following equations show how the moles of OH– released from 100 lbs of 50% NaOH and from 60 lbs of 60% Mg(OH)2 are nearly identical:
Moles of OH– from 100 lbs of 50% NaOH:
(100 lbs) x (50 lbs NaOH/100 lbs) x (453.5 g/lb) x (1 mole NaOH/40 g) x (1 mole OH–/mole NaOH) = 567 mole OH–
Moles of OH– from 60 lbs of 60% Mg(OH)2:
(60 lbs) x (60 lbs Mg(OH)2/100 lbs) x (453.5 g/lb) x (1 mole Mg(OH)2/58 g) x (2 mole OH–/mole Mg(OH)2) = 563 mole OH–
This 40% reduction in chemical usage can result in dramatic cost savings when switching from caustic soda to magnesium hydroxide.
Likewise, when comparing 25% NaOH with 60% Mg(OH)2, the calculation shows that every 1.0 lb of 25% NaOH can be replaced with 0.3 lbs of ALKA-Mag+ to provide the same number of moles of OH– (as a side note, both 25% NaOH and ALKA-Mag+ freeze at the same temperature as water).
500 gallon ALKA-Mag+ feed System
Vinegar Test Data
Magnesium Hydroxide vs Caustic Soda in a customer trial
We were once challenged by a potential customer about these usage estimations. The customer runs a plant that made condiments in the Midwest, having a large amount of vinegar on-site as a raw material in their products.
They performed an experiment that involved diluting vinegar 20-to-1 with city water and filling two 2000 mL flasks. They then proceeded to add incremental doses of 50% NaOH into one of the flasks and 60% Mg(OH)2 into the other, while stirring. The data provided in the table demonstrates that the expected 1.0-to-0.6 ratio is accurate. For instance, after adding 5 mL of 50% NaOH the pH had climbed to 4.92, while it only took 3 mL of 60% Mg(OH)2 to achieve roughly the same pH. Likewise, it took a dose of 6.5 mL of 50% NaOH to achieve roughly the same pH as 4 mL of 60% Mg(OH)2.
This demonstration of the actual 40% reduction in chemical use resulted in chemical cost savings that when coupled with the dramatic safety benefit resulted in this customer rapidly making the switch. That was about 15 years ago, and they remain a happy customer to date.
This same customer also realized another pretreatment benefit of magnesium hydroxide that they were not expecting. When using caustic soda, whenever the plant would release a large volume of concentrated acids into the wastewater stream, the pH in their flow equalization tank (EQ Tank) would drop precipitously. This is because caustic soda provides little to no buffering benefit.
However, after switching to magnesium hydroxide, the wastewater pH in the EQ Tank was only marginally decreased after a similar release of acids from the plant, allowing them to remain within their compliance range. This strong buffering effect was also seen when an operator accidentally overfed magnesium hydroxide into the EQ Tank. Instead of the pH shooting up over 12, as would have happened with NaOH, the pH climbed only into the mid-8s, which was still within their compliance range.
So, if magnesium hydroxide is so perfect then why isn’t everyone using it? This brings us to the primary negative aspect of magnesium hydroxide. It is a slurry that is unstable and prone to settling over time – like old paint in a can. It needs to be well mixed in storage in order to feed reliably through a small metering pump. Because past chemical suppliers did not work with their customers to develop reliable magnesium hydroxide storage and feed systems, this product has an unfortunate history of being “impossible to feed” or “constantly plugging”. It’s hard for an operator to enjoy the benefits of this inexpensive and nonhazardous product if they can’t feed it!
Conclusion: How IER Water can help you
Therefore, in order for an industry to have a smooth and successful transition from caustic soda to magnesium hydroxide, they will need to figure everything out on their own…or, select a magnesium hydroxide supplier that provides technical support in the design and installation of a robust and reliable storage and feed system, as well as on-site service to make sure it performs optimally. Since reliable feed is the primary challenge with magnesium hydroxide, IER has made it our responsibility to provide our customers with robust and reliable agitated storage and feed systems.
For instance, as a first step toward the transition away from caustic soda, we provide customers with a trial storage tank ranging from 150 to 1500 gallons that, as shown in the photo, can be simply set into place to demonstrate performance without being intrusive to the existing wastewater pretreatment process. In this way, the customer can focus their attention on the actual performance comparison of magnesium hydroxide versus sodium hydroxide, rather than having to fight with the feed of a new, unknown slurry product.
Another difference between NaOH and Mg(OH)2 is how rapidly they react to neutralize acids. As mentioned previously, caustic soda requires only a few seconds to boost a wastewater pH from 4 to 7, while it may require 10 minutes for magnesium hydroxide to do the same. Therefore, consideration might need to go into finding a dosing location upstream of the caustic feed point in order to allow enough time for magnesium hydroxide to do its job. Alternatively, we’ve worked with customers that have redirected their wastewater flows into a large holding tank prior to discharge, allowing the necessary time for Mg(OH)2 to do its job. In such cases, the costs to add a “detention time” holding tank, along with the agitated magnesium hydroxide storage and feed system, were still lower than that of continuing to feed caustic soda because of the reduction in chemical usage as discussed previously.
In summary, there are numerous benefits to transitioning from the use of caustic soda to magnesium hydroxide when managing a wastewater pretreatment process. The most obvious is safety. However, process stability from the increased buffering strength of magnesium hydroxide is also a tremendous operational asset. In order to enjoy these benefits, care must be taken to design and install proper storage and feed system. The 40% reduction in chemical usage is typically enough to cover the costs of installing the storage and feed system with an ROI of less than one year. Then, in year two and onward the cost savings can be dramatic.
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Interested in finding out more about the properties of magnesium hydroxide in wastewater treatment? Click the links below to read the next articles in this series:
Part Two: Wastewater Collection System
Part Three: Wastewater Treatment Plants
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