Magnesium Hydroxide: Slurry Rheology; Buffering and the Settling That Plugs a Dosing Line

A plant switches from caustic soda to magnesium hydroxide to stop overshooting pH, and three weeks later the dosing pump is blocked by a settled layer nobody saw forming. The chemistry worked; the handling did not.

This note is for plant engineers and buyers responsible for effluent neutralisation. It covers how magnesium hydroxide behaves differently from caustic, why the slurry is the difficult part, and what to check before a delivery is accepted.

Magnesium hydroxide buffers where metals stay insoluble, while slurry handling decides whether the line survives

The material dissolves only as the pH falls, so it stops acting once the target is reached. That self-limiting behaviour is the reason it is chosen over caustic soda.

It also means the reaction is slower and depends on particle size and mixing, so a dosing system designed for a soluble alkali will not transfer directly.

Most project failures are not chemical. They are a pump, a tank or a line that was sized for a clear liquid and is now handling a suspension.

Powder and slurry are different products with different handling

Powder is cheaper per tonne of active material and requires a make-down step on site, with dust control and a wetting system that actually works.

Slurry arrives ready to dose and carries water you are paying freight for, and it has a shelf life measured in weeks rather than months.

Choosing between them is a decision about labour and capital, not about the chemistry, and it should be made before the storage tank is built.

The buffering ceiling is the point of the product, not a limitation

Magnesium hydroxide will not easily take a liquor above about pH nine, which is where most metal hydroxides are least soluble.

For a plant fighting an over-dose problem, that ceiling is the feature. For a plant that needs a high pH for a different reason, it is the wrong reagent.

Writing the target pH into the specification makes this obvious early, instead of during commissioning.

Particle size and reactivity set how fast neutralisation actually happens

A fine slurry reacts quickly and stays suspended longer. A coarse one settles faster and dissolves slowly enough to leave the pH lagging behind the dose.

Reactivity is measurable. A simple acid titration under defined mixing gives a curve you can compare between suppliers.

Plants that skip that measurement discover the difference as an inconsistent effluent result, and usually blame the control loop rather than the material.

Settling and stratification are the failure mode in every storage tank

Magnesium hydroxide slurry is thixotropic. It forms a gel structure at rest and thins when agitated, which is helpful in the line and unhelpful in the tank.

A tank without continuous or programmed agitation develops a dense layer at the bottom that no normal pump will move.

The practical fixes are a recirculation loop, a low-shear agitator and a tank shape without dead corners. All of them are cheap before installation and expensive afterwards.

Dosing control needs a different strategy from caustic

Because the reaction depends on dissolution, a pH controller tuned for caustic will oscillate. The loop reacts to a measurement that lags the addition.

Feed-forward on flow and load, with pH trimming the result, is more stable than feedback alone on this reagent.

The control change costs engineering time rather than capital, and it is the single biggest reason retrofits succeed or fail.

Metals precipitation produces sludge that has to be dewatered

Heavy metals come out as hydroxides, and the volume and settling behaviour of that sludge decide the dewatering equipment downstream.

Magnesium hydroxide tends to produce a denser sludge than caustic in many applications, which is a benefit in the filter press.

It also adds magnesium to the water, which matters if the discharge has a limit on total hardness or on magnesium specifically.

Compared with caustic soda, the comparison is not only price per tonne

Caustic is sold on concentration and magnesium hydroxide on solids, so the two prices are not directly comparable without converting to equivalents.

Caustic carries a sulphate load when neutralised and a serious handling hazard. Magnesium hydroxide removes both problems, at a higher cost per equivalent.

For many plants the safety case alone justifies the difference, and the calculation should include the cost of the caustic handling regime rather than the reagent price.

Scale and passivation appear where the slurry dries out

Deposits form at seals, at the discharge of a pump and anywhere the slurry sits still long enough to dry.

Flushing on shutdown and avoiding dead legs removes most of it. The rest is a maintenance schedule rather than a design change.

A line that is never flushed will eventually plug, and the plug will arrive during a discharge event rather than during a quiet shift.

Incoming checks that catch a bad delivery before it reaches the tank

Solids content, viscosity at a stated shear rate, reactivity by titration and chloride level cover most of the risk.

Chloride matters because it accumulates in a closed loop and attacks stainless steel. A supplier change is the usual place it appears.

The water treatment chemical tender checklist is a reasonable template for turning those four tests into acceptance criteria.

Where the material is used decides what supply looks like

Effluent treatment is the largest use, followed by flame retardants and pharmaceutical applications that require a much tighter specification.

Those grades are not interchangeable. A flame retardant grade bought against an effluent specification will carry a purity the plant is paying for and cannot use.

The magnesium hydroxide range is wide enough that grade selection deserves its own conversation, separate from price.

Transport and unloading are where the product is most often compromised

Slurry is heavy, and a tanker that has been sitting in traffic arrives partially settled. The receiving tank then inherits the problem the driver did not mention.

Agitating the tanker before discharge, or specifying a delivery that includes recirculation, is a contractual detail worth writing down.

Powder avoids this entirely and brings its own issues, mainly dust and the wetting step, which is where most make-down systems underperform.

Dilution and make-down decide the quality of a powder installation

Powder that is dumped into a tank of water without adequate shear forms lumps that hydrate on the outside and stay dry inside.

Those lumps behave as coarse particles for weeks, dissolving slowly and giving a reactivity result that looks like a supplier problem.

A proper eductor or a high-shear mixer solves it. A hose and a paddle does not, and the difference appears in the effluent result rather than in the tank.

FAQ: questions buyers ask

Why does magnesium hydroxide stop raising the pH?

Because it dissolves only as the pH falls, and once the liquor reaches the equilibrium point around pH nine there is little driving force left. That ceiling is useful for metals precipitation and unsuitable where a higher pH is required.

Can a caustic dosing system be reused?

The pump and line usually need changing, and the control loop certainly does. A suspension behaves differently from a solution, and a pH controller tuned for caustic will oscillate on this reagent.

How long can slurry be stored?

Weeks rather than months, and only with agitation. Settled material is difficult to re-suspend and the layer at the bottom of a tank can harden beyond what a normal pump will move.

Is the sludge easier to dewater than with caustic?

Often yes, because the precipitate tends to be denser. The result depends on the metals present and on the filter equipment, so it should be tested rather than assumed.

What is the most common cause of a failed conversion?

Handling, not chemistry. A tank without agitation, a pump sized for clear liquid, or a control loop carried over from caustic will each produce a failure that looks like a product problem.