Magnesium Hydroxide Storage — Mg(OH)2 Slurry Tank Selection
Magnesium Hydroxide Storage — Mg(OH)2 Slurry Tank System Selection
Magnesium hydroxide (Mg(OH)2, CAS 1309-42-8) is a white powder with very low aqueous solubility (0.0012% at 20°C) that is commercially supplied as a 58 to 62% solids aqueous slurry with fine particle size (0.5 to 5 micron) designed to remain suspended with moderate agitation. The slurry form at specific gravity 1.55 to 1.60 is the dominant commercial format for wastewater pH control, flue-gas desulfurization, and acid-mine-drainage treatment applications. The dry powder ships as 50-lb bags, supersacks, and rail-car lots for flame-retardant-compounding, cosmetic, pharmaceutical, and refractory use. This page consolidates resin-level compatibility, regulatory hazard communication, storage protocol, and field-handling reality for specifying a magnesium hydroxide slurry tank that holds the product safely across a 15-to-20-year service life.
The six sections below work from chemistry and material compatibility through storage protocol, operator FAQs, and supply-chain reality. Ratings reference Premier Magnesia, Martin Marietta Magnesia Specialties, and Sibelco technical bulletins. Regulatory citations point to NSF/ANSI 60 water-treatment certification, AWWA B202 alkalinity chemicals for water, EPA 40 CFR 423 steam-electric power plant effluent guidelines (FGD context), FDA 21 CFR 184.1428 GRAS, and USP Milk of Magnesia monograph.
1. Material Compatibility Matrix
Magnesium hydroxide slurry is mildly alkaline (supernatant pH 10 to 10.5) and contains suspended Mg(OH)2 particles that deposit on wetted surfaces when agitation is lost. The chemistry is gentle on most engineering materials — this is the single largest operational advantage driving its adoption as an alternative to NaOH for pH adjustment. No violent reactions, no eye or skin burns, no heat of dissolution; the pH ceiling at 10.5 is self-limiting and cannot exceed that regardless of overdose. The key operational concerns are slurry handling (pump wear, pipe abrasion) and settling (tank and pipe deposit management when agitation fails).
| Material | Slurry 58–62% solids | Dilute 5–15% solids dosing | Dry powder | Notes |
|---|---|---|---|---|
| HDPE (1.5 SG) | A | A | A | Day tank and IBC standard; slurry abrasion on pump suction wears over years |
| XLPE (1.9 SG) | A | A | A | Bulk-tank slurry standard at 2,000–50,000 gal; 1.9 SG for slurry-mass engineering margin |
| Polypropylene | A | A | A | Elevated-temperature FGD duty to 180°F |
| PVDF (Kynar) | A | A | A | Dosing valve seats and high-temp piping |
| FRP vinyl ester | A | A | — | Large-volume bulk option for FGD duty at power plants |
| PVC / CPVC | A | A | A | Dosing standard; CPVC for hot FGD service to 180°F |
| 316L stainless | A | A | A | Pump impeller and valve standard; no corrosion at pH 10 |
| 304 stainless | A | A | A | Acceptable for non-critical slurry service |
| Carbon steel (lined) | A | A | A | FGD reaction-tank with epoxy or rubber lining is standard |
| Carbon steel (bare) | B | B | A | Slow alkaline passivation; decade-plus life with minor scaling |
| Aluminum | C | C | A | Slow alkaline attack at pH 10+; avoid long-term |
| Galvanized steel | C | C | A | Slow zinc attack; avoid slurry service |
| Copper / brass | A | A | A | Stable at pH 10; historical cooling-water-treatment use |
| Concrete | A | A | A | No attack; borate-alternative pH buffer |
| EPDM elastomer | A | A | — | Standard slurry-service gasket; annual replacement due to abrasion |
| Viton (FKM) | A | A | — | Pump o-ring standard |
| Natural rubber (NR) / neoprene | A | A | — | Pipe lining and pump casing abrasion-resistant standard |
The matrix covers ambient through 180°F slurry operation (flue-gas desulfurization reaction-tank service). The critical slurry-service consideration is abrasion rather than chemical attack: slurry-service pumps are typically rubber-lined centrifugal or PVDF magnetic-drive, piping is rubber-lined carbon steel or XLPE with abrasion-resistant lining at elbows and tees. Below 25°F, slurry viscosity rises sharply; heat tracing and continuous agitation are standard in freeze-prone installations.
2. Real-World Industrial Use Cases
Wastewater pH Adjustment (Safer NaOH Alternative). The single largest growth use of Mg(OH)2 slurry is as a replacement for sodium hydroxide in industrial and municipal wastewater pH adjustment. The operational advantages over NaOH drive the substitution: self-limiting pH ceiling at 10.5 prevents overshoot (in contrast to NaOH which can drive pH to 13+), eye-and-skin safety profile eliminates the severe chemical-burn hazard, and the by-product MgSO4, MgCl2, or Mg(NO3)2 salt from neutralization is often more biodegradable than the corresponding Na salts. A medium-scale industrial wastewater plant treating 500,000 to 5,000,000 gallons per day of acidic process wastewater consumes 20,000 to 200,000 lb/month of Mg(OH)2 slurry in day-tank-to-neutralization dosing service. Bulk tank storage is XLPE at 10,000 to 25,000-gal capacity with continuous agitation.
Flue-Gas Desulfurization (FGD) at Coal-Fired Power Plants. Wet-FGD scrubbers at coal-fired power plants use Mg(OH)2 or Ca(OH)2 slurry as the alkali absorbent to capture SO2 from combustion flue gas. Mg(OH)2 FGD produces soluble MgSO4 byproduct (versus Ca(OH)2 FGD that produces gypsum solid), with pros and cons — Mg-FGD has lower slurry densities and faster reaction kinetics, but requires liquor discharge or cycling to avoid sulfate accumulation. Large Mg-FGD installations at 500+ MW power plants consume 30,000 to 100,000 lb of Mg(OH)2 per day. Storage is 200,000+ gal FRP vinyl-ester or rubber-lined carbon-steel reaction tanks with continuous mechanical agitation. EPA 40 CFR 423 governs power-plant effluent from the FGD wastewater stream.
Acid-Mine-Drainage Treatment. Abandoned coal mines and metal mines in the Appalachian and Western US produce acid mine drainage (AMD) that requires neutralization before discharge to meet state water-quality standards. Mg(OH)2 is preferred over Ca(OH)2 at some AMD sites because the resulting mag-sulfate sludge is more tractable for disposal than gypsum, and because the slurry handles better in automated dosing at remote unmanned treatment stations. Annual US AMD neutralization consumes 50,000,000+ lb of alkali chemistry across hundreds of treatment sites; Mg(OH)2 represents roughly 10 to 20% of that alkali supply.
Flame-Retardant Polymer Additive. Dry Mg(OH)2 powder at fine particle size (3 to 10 micron) is incorporated at 40 to 65 weight-percent into halogen-free flame-retardant polyolefin compounds for wire-and-cable jacketing, automotive interior parts, and construction-industry sheathing. The thermal decomposition of Mg(OH)2 at 350°C releases water that cools the polymer and forms a magnesium-oxide char barrier. Halogen-free FR wire is UL VW-1 and IEC 60332 compliant without the brominated-flame-retardant environmental concerns of legacy formulations. Global Mg(OH)2 FR consumption exceeds 500,000,000 lb/year and is growing at 5 to 8% CAGR.
Pharmaceutical Antacid (Milk of Magnesia). USP Milk of Magnesia is 8% Mg(OH)2 suspension used as an antacid and osmotic laxative. The pharmaceutical-industry consumption is volumetrically smaller than the industrial applications but requires USP-grade precursor with tight heavy-metal and microbial specifications. Bayer (Phillips' Milk of Magnesia) and several private-label brands source USP-grade product from qualified suppliers.
Dietary Magnesium Supplement and Food Ingredient. FDA 21 CFR 184.1428 lists magnesium hydroxide as GRAS for specific food-ingredient applications at up to 0.2% in flour and baked goods as a pH adjustment agent, and as a dietary-magnesium source in mineral supplements. Food-industry consumption is a fraction of total demand but again requires tight specification.
Refractory and Ceramic Precursor. Calcination of Mg(OH)2 at 800 to 1,200°C produces reactive MgO used in refractory brick (steel-industry ladle linings and cement-industry kiln linings) and in specialty ceramics. This is primarily a dry-powder-handled industry with pneumatic conveyance to rotary calcining kilns.
3. Regulatory Hazard Communication
OSHA and GHS Classification. Magnesium hydroxide carries the GHS classification H315 (causes skin irritation) at concentrated slurry exposure; no other hazard classifications apply at typical commercial concentrations. OSHA has no specific Mg(OH)2 PEL; the general particulates-not-otherwise-classified PEL of 15 mg/m3 total and 5 mg/m3 respirable applies to dry-powder handling. ACGIH TLV-TWA is 10 mg/m3 inhalable total particulate. Skin and eye protection for routine handling are minimal: safety glasses and nitrile gloves are adequate for slurry transfer operations. This safety profile is the single largest operational driver of the chemistry's growth versus NaOH, which requires full chemical-splash PPE protocols.
NFPA 704 Diamond. Magnesium hydroxide rates NFPA Health 1, Flammability 0, Instability 0, no special flag. The chemistry is essentially benign in a workplace-safety and fire-protection framework.
DOT and Shipping. Mg(OH)2 powder and slurry are not DOT-regulated. Domestic rail, truck, and marine shipments carry no hazmat placarding at any concentration. Tank truck delivery of slurry follows standard food-grade or industrial-grade hauler protocols.
EPA CERCLA and EPCRA. Mg(OH)2 is not CERCLA-listed and carries no reportable quantity. EPCRA Tier II reporting applies at the 500-lb aggregate-site threshold; SARA 313 TRI does not apply.
NSF/ANSI 60 and AWWA B202. Magnesium hydroxide for drinking-water treatment is NSF/ANSI 60 certified by specific manufacturers (Premier Magnesia, Martin Marietta, Sibelco). AWWA B202 governs quality requirements for alkalis used in water treatment. Drinking-water coagulation and pH adjustment with Mg(OH)2 requires the certified product; non-certified industrial grades are not acceptable.
EPA 40 CFR 423 Power-Plant Effluent. Coal-fired power plants operating Mg-FGD produce FGD wastewater subject to 40 CFR 423 effluent guidelines for steam-electric power generation. Regulated parameters include total dissolved solids, chloride, nitrate, mercury, and selenium in the FGD blowdown stream. Total alkalinity and magnesium are not directly regulated parameters but contribute to TDS loading.
FDA 21 CFR 184.1428 (GRAS). Magnesium hydroxide is GRAS as a direct food ingredient and antacid active pharmaceutical ingredient. USP monograph governs pharmaceutical-grade quality (heavy metals below 10 ppm, arsenic below 2 ppm, soluble salts below 1%). FCC (Food Chemicals Codex) governs food-grade quality with similar specifications.
4. Storage Protocol and Field Handling
Bulk Slurry Tank Configuration. The industry-standard bulk Mg(OH)2 slurry tank is a 1.9-SG XLPE or FRP vinyl-ester vertical closed-top tank at 5,000 to 50,000-gal capacity with integral top-entry mechanical agitator sized for continuous-duty slurry suspension. Agitator power density is typically 1 to 2 horsepower per 1,000 gal of tank volume; impeller design is pitched-blade turbine or hydrofoil optimized for slurry suspension without excessive shear. Positioning is in secondary containment per EPA SPCC; neutral floor coating (bare concrete or acid-brick) is acceptable because slurry leaks are mildly alkaline and do not aggressively attack concrete.
Fittings, Piping, and Valve Selection. Manway gaskets are EPDM with 316L stainless hardware. Fill connections use 3-inch or 4-inch Camlock with EPDM gasket; the Camlock is the industry-standard for slurry delivery. Transfer piping from bulk tank to day tank uses rubber-lined carbon steel or HDPE at 2- to 4-inch diameter with bolted flange joints; all elbows and tees are specified as long-radius or wear-resistant ceramic-lined to extend service life. Pump selection is rubber-lined centrifugal (Goulds XHD, Warman SHR, or equivalent) or PVDF magnetic-drive for smaller flows.
Day Tank Configuration. Day tanks at 500 to 5,000-gal HDPE with integral slow-speed paddle agitator serve the process dosing point. Recirculation from day-tank to bulk-tank keeps slurry homogeneous and prevents stratification during low-demand periods. Metering pumps for dosing are peristaltic (hose rupture is the primary failure mode, requiring routine hose replacement) or PVDF-diaphragm for longer service life.
Dry Powder Storage. Dry Mg(OH)2 powder is stable at warehouse conditions; storage in sealed polyethylene-lined fiber drums, supersacks, or bulk rail-car-to-silo is straightforward. Silo storage at flame-retardant compounding plants is typical: 20 to 100-ton silos with dust-collector baghouse and pneumatic conveyance to the compounding line. Ambient-humidity control is not required; the product is not hygroscopic and does not cake significantly.
Maintenance and Turnaround. Slurry-service bulk tanks receive annual inspection of agitator gearbox oil, impeller wear, and pump internal condition; agitator failure is the critical failure mode because slurry settling in a failed-agitator tank over 24 hours creates a cemented-hard layer that requires vacuum-truck removal. Agitator gear oil sampling quarterly catches gearbox failure indicators early. Slurry-service pumps receive rubber-lining inspection annually with replacement at 25% wear; bearings and seal surfaces every 5,000 to 10,000 operating hours.
5. Operator FAQs
Why is Mg(OH)2 gaining market share against NaOH for pH adjustment? Self-limiting pH ceiling at 10.5 is the single biggest operational advantage. NaOH can overshoot pH 13 easily with dosing-pump failure; Mg(OH)2 cannot. The worker-safety profile is also much better: no chemical-splash-burn hazard, no violent dissolution heat. Total cost of ownership including PPE, training, and incident-response capability for NaOH often exceeds total cost of ownership for Mg(OH)2 at equivalent effective alkali pound delivery.
Why does my Mg(OH)2 slurry harden at the tank bottom if the agitator fails? Fine-particle magnesium hydroxide settles rapidly (Stokes-law sedimentation rate of order centimeters per minute for 3-micron particles). Once settled, the slurry compacts from 62% solids to 75%+ solids by squeezing water out, forming a cement-like layer. Recovery from a day-long agitator failure typically requires vacuum-truck removal and restart with fresh slurry delivery. Agitator uptime is the single most critical operational parameter.
Can I use Mg(OH)2 slurry in place of lime (Ca(OH)2) for acid-mine-drainage treatment? Yes, and with several operational advantages: lower sludge volume because Mg-sulfate is more soluble than gypsum, better settleability because the reaction product remains fluid rather than cementing, and gentler pH overshoot characteristics. Cost per pound of alkali is higher for Mg(OH)2 than for Ca(OH)2; the trade-off analysis depends on site-specific sludge-disposal economics and sulfate limitations.
What is the slurry freeze point? Approximately 30°F for 60% solids slurry; below that the water phase begins to freeze and the slurry becomes unworkable. Heat tracing at 6 to 10 W/ft on tank walls plus insulation maintains above 45°F in northern US climates. Agitator heat also contributes to maintaining temperature; agitator-running tanks rarely see freeze issues above 20°F ambient.
Why does the slurry develop a supernatant water layer on top? Even with active agitation, fine-particle Mg(OH)2 stratifies slightly with lower solids at the top and higher solids at the bottom after long periods of steady state. This is cosmetic if the pump intake is properly positioned at mid-depth; draw-from-top pumps will see lower-solids slurry and underdose downstream. Design dosing pump suction at 40 to 60% of tank depth.
Shelf life of slurry in a stirred XLPE tank? 6 to 12 months with continuous or periodic agitation. The chemistry does not degrade, but biological growth (algae, bacteria) can occur at pH-10 supernatant interface with daylight exposure. Covered or closed tanks with no daylight exposure extend useful shelf life to 12+ months. Dry powder is stable indefinitely.
Can I directly dose dry Mg(OH)2 powder into wastewater for pH adjustment? Yes, but slurry dosing is operationally more reliable. Dry-powder-metering systems require bin-level controls, feeder-screw accuracy, and settled-powder bridging management. Slurry dosing with metering pumps is typically easier to automate and calibrate. Exception: remote unmanned treatment stations occasionally use dry powder for simpler logistics.
6. Field Operations Addendum
Vendor Cadence and Supply Chain. Primary North American magnesium hydroxide manufacturers are Premier Magnesia (Weston MD and regional plants), Martin Marietta Magnesia Specialties (Baltimore MD), and Sibelco (US operations). Imported product comes from Korea Magnesium Chemicals, Kyowa (JP), and European Premium Minerals. Delivered US pricing in 2026 runs $0.35 to $0.50 per pound of 60% solids slurry in tanker-truck loads (equivalent to $0.60 to $0.85 per lb dry basis), with dry-powder pricing at $0.70 to $1.20 per lb reflecting handling premium. NSF/ANSI 60 drinking-water-grade product commands 20 to 30% premium over industrial grade.
Dosing Control and Process Automation. pH-adjustment dosing of Mg(OH)2 slurry uses feedforward flow-pacing combined with feedback from an in-line pH sensor at the discharge mixing point; typical target is pH 7.5 to 8.5 for industrial wastewater. The self-limiting pH ceiling simplifies overdose protection: even with total pump-failure maximum-stroke dosing, pH cannot exceed 10.5. FGD dosing at power plants uses feedback from SO2 concentration in the clean-gas stack and from slurry pH in the absorber tower; control bandwidth is wider than pH-adjustment service because the power-plant operating profile varies with dispatch.
Operational Cost Comparison vs NaOH. Net alkali equivalence: 1 lb of dry Mg(OH)2 provides 1.37 lb-equivalent NaOH alkali. At Mg(OH)2 slurry pricing of $0.40 per lb of slurry (60% solids = $0.67 per lb dry = $0.49 per lb NaOH equivalent), compared against 50% NaOH pricing of $0.35 per lb of 50% solution ($0.70 per lb dry = $0.70 per lb NaOH equivalent), the magnesium hydroxide is typically 10 to 30% lower cost per pound of effective alkali. Adding reduced PPE and training overhead, plus safer spill-response implications, total cost-of-ownership favors Mg(OH)2 strongly for most industrial-wastewater service.
Related Chemistries: Alkaline + Magnesium Chemistry
Related to:
- Magnesium Sulfate (MgSO4) — Mg neutral-pH chemistry
- Magnesium Chloride (MgCl2) — Mg de-icer chemistry
- Calcium Hydroxide (slaked lime) — Ca alkali counterpart
- Sodium Hydroxide (NaOH) — Stronger Na alkali
Related Hub Pillars
For broader chemistry context, see the OneSource Plastics high-traffic chemical-compatibility hub pillars: