MagOx: Magnesium Oxide Relationship with Magnesium Hydroxide

John Van Wingerden \ June 30, 2022

It is very common to hear someone in the wastewater treatment industry refer to something called “MagOx” as an alkaline additive they use to control pH or low alkalinity conditions.  Often they are simply using this term as a nickname when talking about either Magnesium Hydroxide or Magnesium Oxide, which is the chemical precursor to Magnesium Hydroxide, but which has very different properties.  So, when looking to obtain an additive that will provide the most cost-effective pH control performance for your wastewater treatment application, it is important to understand the relationship between Magnesium Oxide and Magnesium Hydroxide, and how not all products are equal.

First of all, the chemistry of Magnesium Oxide (MgO) and Magnesium Hydroxide (Mg(OH)2) is based on the Magnesium di-cation (Mg2+).  Magnesium is an essential macronutrient and is the core element in chlorophyll – the stuff that makes plants green and drives photosynthesis.  While MgO and Mg(OH)2 are very similar, knowing the differences in the manufacturing processes for each will help to understand the physical and chemical differences – which is very important in the selection process of the chemical best suited for your application.

Magnesium Oxide (MgO) is typically obtained from the calcination (heating) of Magnesite ore (Magnesium Carbonate or MgCO3) in much the same way as Quicklime (CaO) is formed from Limestone (Calcium Carbonate or CaCO3).  By changing the temperature and speed of how fast the MgCO3 passes through the heat zone, one can control the structure, porosity, and reactivity of the resulting MgO particles.  You may ask why this matters?  The more porous the particulate structure, having more available surface area, the more reactive the MgO.  The ability to control structure and porosity can result in the development of a wide range of MgO products with diverse properties for numerous industrial applications.

The addition of water to dry MgO powder results in its conversion to Mg(OH)2.  Therefore, MgO is always purchased as a dry powder or granular material, while Magnesium Hydroxide is most regularly obtained as a liquid.  So, when someone talks about feeding MagOx into water from a pallet of bags or a one-ton bulk bag, the product being used could very well be powdered Magnesium Oxide.  However, if someone talks about feeding a liquid MagOx product into water from an IBC tote or storage tank, it is most likely a Magnesium Hydroxide product.

There are three ways to manufacture Magnesium Hydroxide, which all result in a liquid slurry.  We have mentioned the first method of adding MgO to water and stirring – a process known as slaking, in which energy is released into the water from the breaking of the Mg-O bonds.  This reaction is so powerful that after minutes or hours, depending on the volume of the reaction and the reactivity of the MgO, the water temperature rises from ambient to boiling!  After the reaction is completed and the temperature has returned to ambient, the conversion from MgO to Mg(OH)2 is complete, resulting in a creamy, easy-flowing slurry product.  The advantage of this process is that by controlling the nature of the MgO starting material, one can control the resulting settling stability and reactivity properties of the resulting Magnesium Hydroxide product.  This is the process used by Calix and IER to develop Magnesium Hydroxide products having improved settling stability and dramatically higher rates of reactivity.

Some may wonder why to go through the slaking process to make Mg(OH)2 when you could simply add MgO straight into your wastewater stream.  The problem is that by adding small amounts of MgO into wastewater, the conversion (slaking) reaction does not fully occur.  Instead, the MgO particles can become hydrated on the outer shell, with the less reactive MgO remaining within the core of the particle.  This results in a much more settling unstable product that is difficult to feed and can cause plugging.  Secondly, the reactivity of the MgO particles is typically lower than Mg(OH)2, requiring a much higher dose in order to achieve the same pH or alkalinity increase.

 

After the slaking process, magnesium hydroxide being pumped for quality testing.

Magnesium Oxide being fed into reactor to start slaking process.

Driver checking tanker levels before delivery of Magnesium Hydroxide to customer.

Performing QC testing before filling tanker with Magnesium Hydroxide for delivery to customer.

The other two processes of manufacturing Magnesium Hydroxide do not have a direct relationship with Magnesium Oxide.  The first of these is the mining of a natural Magnesium Hydroxide ore called Brucite, which is ground into a fine powder and added to water to make a slurry.  The limitation with this process is that one does not have control over the structure, porosity, and reactivity of the resulting product.  It is exactly as Mother nature provides.  Since Magnesium Hydroxide is not the most thermodynamically stable form of magnesium, the fact that it can be mined as an ore in relatively rare locations on Earth suggests that it is quite unreactive.  If it were reactive, it would not have survived for thousands of years waiting to be mined.  This is shown when testing the much lower reactivity of Brucite-containing Mg(OH)2 products versus Magnesium Hydroxide products made using calcined MgO.

The other manufacturing process involves precipitating Mg(OH)2 from a salt brine solution rich in Mg(Cl)2 and Ca(Cl)2.  This brine is treated with lime which raises the pH.  As Mg(OH)2 is less soluble than Ca(OH)2, it is the first thing that precipitates from solution.  This precipitate goes through proprietary steps to make the product stable for shipping and feeding.  As with the Brucite process you do not have control over the molecular structure.  Therefore, though the product can be made to be highly stable to settling, it is a less reactive product.

Unfortunately, all of the chemistries mentioned above are loosely referred to as MagOx, even though Magnesium Oxide is simply a building block for one type of Magnesium Hydroxide.  Again, IER and Calix focus our manufacturing processes on the conversion of different types of MgO into Mg(OH)2, employing different particle sizes and porosities, and calcining at low temperatures so as to generate the most reactive Magnesium Hydroxide products available in the industry – which equates to needing less product to control your wastewater pH.  So when looking for “MagOx”, the goal should be to obtain the product that will give me the best value for the money, while still possessing the necessary stability for reliable ease of feed.  A Magnesium Hydroxide derived from Magnesium Oxide is best suited for the control of pH and alkalinity, as well as for odor/corrosion/FOG reduction in wastewater collection systems.

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