Magnesium Hydroxide Slurry Storage and Handling: A Complete Guide to Preventing Settling

When a storage tank of magnesium hydroxide slurry sits idle over a long weekend, the suspended particles begin their slow migration toward the bottom. By Monday morning, the bottom third of the tank can be a dense, paste-like layer that standard pumps cannot pick up. This scenario happens in wastewater plants, FGD scrubbers, and chemical dosing systems every year — and it is almost always avoidable.

The short version: magnesium hydroxide slurry is an excellent, low-corrosion source of alkalinity, but only if you store it, agitate it, and deliver it correctly. In the sections below, we walk through the storage tank, agitation requirements, piping choices, and equipment selection so you can avoid the settling trap.

What Is Magnesium Hydroxide Slurry?

Magnesium hydroxide slurry is a non-carbonate alkaline suspension made by grinding or precipitating Mg(OH)₂ in water. It typically ships at 56–58% solids by weight, and it is used in acid neutralization, heavy-metal precipitation, flue-gas desulfurization, and pH adjustment in municipal and industrial wastewater.

Because Mg(OH)₂ has low solubility and a relatively low pH of about 10.5, it is far less corrosive to workers and equipment than caustic soda or hydrated lime. Yet on an equal-alkalinity basis, 1 gallon of magnesium hydroxide slurry replaces about 2.85 gallons of 34% hydrated lime slurry, according to Magnesia Specialties. That compact alkalinity makes it an efficient choice for high-volume alkaline dosing.

The compact alkalinity of Mg(OH)₂ is a real cost advantage, but it comes with a trade-off: the material is a suspension that requires constant attention. Without it, the slurry behaves less like a chemical and more like a solid that only looks like a liquid.

Why Storage and Agitation Are Non-Negotiable

Magnesium hydroxide slurry is unstable by nature. Its particles have a higher density than water, and they settle steadily when the flow stops. A production disturbance or weekend shutdown can create a settled layer that is extremely difficult to resuspend. In extreme cases, the layer hardens into a crust that has to be manually broken out.

The most common mitigation is continuous agitation through a properly sized mixer. For large storage tanks, a magnetic side-entry mixer keeps fluid rotating without the seal leakage typical of shaft penetration. It is also easy to retrofit on existing tanks.

TOPC001 CMC Series Magnetic Side-Entry Mixer for Zero-Leakage AgitationThis side-entry mixer uses a magnetic coupling with static sealing to achieve permanent zero leakage, making it suitable for hazardous or toxic media. Its low power consumption makes it an efficient choice for large storage tanks requiring continuous agitation.View Product →

When choosing an agitation system, the goal is to maintain a uniform suspension at the lowest practical energy input. A low-pitch impeller that moves the entire tank volume is better than a high-shear blade that only stirs a localized zone. The chart below shows why Mg(OH)₂ slurry is so attractive compared with lime.

Selecting the Right Agitator for a Slurry Tank

There is no universal agitator for magnesium hydroxide slurry. The right choice depends on tank diameter, liquid level, solids concentration, and how quickly the slurry is withdrawn. We always recommend starting with a CFD-based mixing analysis to model flow velocities and shear rates before committing to a motor size or impeller diameter. That type of engineering review is part of our normal proposal process — you can see how we approach it on our mixing technology page.

For storage tanks in the 1,000–10,000 gallon range, a top-entry mixer with a marine-style propeller or hydrofoil impeller is usually sufficient. In smaller tanks, a direct-drive motor with a variable frequency drive gives operators the flexibility to adjust speed as the slurry level drops.

TOPA004 Top-Entry Mixer with Online Mechanical Seal ReplacementThis top-entry mixer ranges from 0.37 to 45 kW and features an original impeller design that boosts discharge volume. Its online seal replacement capability simplifies maintenance, making it practical for production reactors handling continuously fed slurry.View Product →

For production reactors that continuously feed slurry into a process, the impeller must be sized not only for suspension but also for rapid blending. In those cases, the tank often needs baffles to convert swirl into vertical flow, particularly if the slurry is fed through a bottom nozzle.

Piping, Valves, and Pumping: Preventing Plugged Lines

The shortest distance between the storage tank and the injection point is not always the easiest path. Magnesium hydroxide slurry needs pipe runs that are as short as possible, with large-radius elbows and no pockets where solids can drop out. Here are the rules we recommend to every plant engineer:

• Use full-port ball valves or knife-gate valves in slurry lines; standard globe valves create dead zones.
• Install a recirculation loop that keeps the slurry velocity above 1 m/s (3 ft/s) in the header pipe.
• Choose pumps that tolerate abrasive solids. Progressing cavity pumps are ideal for 56–58% slurry; peristaltic or hose pumps also work well. Avoid high-speed centrifugal pumps.
• Place drain valves at every low point, and flush lines with clean water before prolonged shutdowns.

Slurry that is allowed to cool and thicken in a dead leg can block a 2-inch line in less than an hour. A simple preventive measure is to schedule a 5-minute recirculation cycle two or three times a day, even when no dosing is occurring.

Unloading and Receiving: What to Check

When a delivery of magnesium hydroxide slurry arrives, do not simply connect the hose and pump it in. A few minutes of inspection can prevent weeks of operating problems.

1. Verify the certificate of analysis to confirm the solids content and particle size distribution.
2. Sample the slurry before unloading. If a clear liquid layer sits on top and the lower layer is stiffer than expected, the tank may have settled during transport.
3. Start the storage tank agitator before offloading and keep it running throughout the transfer.
4. After transfer, check the tank level and re-sample to confirm the average solids content has not shifted.

Some plants also perform a simple “pour test”: a well-suspended slurry should form a uniform ribbon when poured from a cup, not separate into water and lumps. It is a quick low-tech check that catches most early signs of separation.

Maintenance and Common Failure Modes

Most magnesium hydroxide slurry emergencies trace back to one of three components: the mixer, the pump, or the tank drain. A worn impeller loses its pumping capacity slowly, so operators often mistake the result for a thickening problem when the real issue is a damaged blade.

Inspect the agitator every three months if the plant runs continuously. Check blade thickness, especially at the leading edge, and compare vibration levels with baseline readings. A well-maintained slurry system can run for decades without a major failure.

Cost and Procurement: Getting the Value Equation Right

Pricing for magnesium hydroxide slurry varies with solids content, distance from the production site, and delivery volume. A price quoted on a per-gallon basis is misleading unless the solids content is stated. Two suppliers can look identical until you convert both quotes to a cost-per-unit-of-alkalinity basis.

Garrison Minerals notes that their customers typically receive a 56–58% solids slurry that is only a few days old. Freshness matters because aged slurry can thicken, and the extra handling cost can erase a purchasing discount. When comparing vendors, ask for a recent sample and measure viscosity at your expected storage temperature.

One practical tip: use the alkalinity factor when comparing prices. If magnesium hydroxide slurry costs $0.50 per gallon, and 34% lime slurry costs $0.20 per gallon, the lime price is not actually lower — 2.85 gallons of lime carry the same alkalinity, so the equal-alkalinity cost for lime is $0.57 per gallon. This simple calculation prevents buyers from overpaying for the cheaper-looking reagent.

Frequently Asked Questions about Magnesium Hydroxide Slurry

For more practical operating tips, see our frequently asked questions.