Calcium Hydroxide Storage — Ca(OH)2 Slaked Lime Slurry Tank Selection
Calcium Hydroxide Storage — Ca(OH)2 Slaked Lime Slurry Tank Selection
Calcium hydroxide (Ca(OH)2, CAS 1305-62-0) is a white powder produced by hydrating quicklime (CaO) with a stoichiometric water quantity, commercially supplied as dry hydrated lime powder in 50-lb bags, supersacks, and rail-car lots, or as milk-of-lime slurry at 25 to 35% solids. The saturated solution (“limewater”) carries only 1.5 g/L dissolved Ca(OH)2 because of low solubility, so bulk storage is always slurry or dry powder — never saturated solution. The chemistry is the workhorse alkali of the US water-treatment, wastewater-treatment, flue-gas-desulfurization, and kraft-pulp industries, with global annual production exceeding 140,000,000 tonnes. This page consolidates resin-level compatibility, regulatory hazard communication, storage protocol, and field-handling reality for specifying a slaked-lime 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. Compatibility ratings reference National Lime Association handbook, Lhoist, Carmeuse, Mississippi Lime, and Graymont technical bulletins. Regulatory citations point to NSF/ANSI 60 drinking-water, AWWA B202 water-treatment alkalis, EPA 40 CFR 503 lime-stabilization biosolids, FDA 21 CFR 184.1205 GRAS, and NFPA 400 caustic solids.
1. Material Compatibility Matrix
Calcium hydroxide slurry is strongly alkaline with saturated pH 12.4 — the natural pH ceiling for Ca(OH)2 chemistry. This is higher than Mg(OH)2 (pH 10.5) but lower than NaOH (pH 13+). At pH 12.4 the chemistry causticizes aluminum, zinc, and galvanized coatings; polyolefins, FRP, concrete, stainless, and carbon steel all resist. The operational concerns are abrasion from suspended solids, settling when agitation fails, and the general skin-and-eye caustic hazard that all high-pH alkalis share. Unlike NaOH, Ca(OH)2 does not generate significant dissolution heat and does not cause immediate deep-tissue burns.
| Material | Milk-of-lime 25–35% | Dilute 5–15% dosing | Dry powder | Notes |
|---|---|---|---|---|
| HDPE (1.5 SG) | A | A | A | Day-tank standard; slurry abrasion reduces pump life over years |
| XLPE (1.9 SG) | A | A | A | Bulk-tank slurry standard at 5,000–50,000 gal |
| Polypropylene | A | A | A | Elevated-temperature FGD and biosolids-stabilization duty to 180°F |
| PVDF (Kynar) | A | A | A | Premium dosing valve and high-temp piping |
| FRP vinyl ester | A | A | — | Large-volume power-plant FGD bulk at 100,000+ gal |
| PVC / CPVC | A | A | A | Dosing piping standard; CPVC for hot FGD service to 180°F |
| 316L stainless | A | A | A | Pump impeller and valve standard; no corrosion at pH 12 |
| 304 stainless | A | A | A | Acceptable for non-critical slurry service |
| Carbon steel (bare) | A | A | A | Passivates in alkaline service; decade-plus life typical |
| Carbon steel (lined) | A | A | A | Rubber-lined or coal-tar-epoxy lining for abrasion protection |
| Aluminum | NR | NR | NR | Rapid alkaline attack at pH 12; never specified |
| Galvanized steel | NR | NR | NR | Zinc stripped; never in service |
| Copper / brass | B | A | A | Slow attack at concentrated; OK in dilute dosing |
| Concrete | A | A | A | Ca(OH)2 is a cement hydration product; self-consistent |
| EPDM elastomer | A | A | — | Standard gasket; annual replacement at abrasive slurry manway |
| Viton (FKM) | A | A | — | Pump o-ring standard |
| Natural rubber (NR) / neoprene | A | A | — | Pump casing and pipe-lining abrasion-resistant standard |
The matrix covers ambient through 180°F slurry operation. Dry-hydrate powder service extends to elevated temperatures (FGD dry injection at 300 to 600°F flue-gas temperature, with the Ca(OH)2 particles reacting on-the-fly with SO2). Below 32°F, slurry freezes and heat tracing at 6 to 10 W/ft plus insulation is standard in freeze-prone installations.
2. Real-World Industrial Use Cases
Municipal Drinking-Water Lime Softening. The dominant US drinking-water softening technology is conventional lime-soda softening, where Ca(OH)2 slurry is added to raw water to precipitate calcium carbonate (CaCO3) and magnesium hydroxide (Mg(OH)2) from the dissolved hardness. A medium-scale hard-water utility serving 100,000 people consumes 100,000 to 500,000 lb/month of Ca(OH)2 slurry at doses of 50 to 150 mg/L raw-water lime feed. Bulk storage is 30,000 to 100,000-gal XLPE or concrete clarifier-effluent tanks with mechanical agitation. Lime-softening plants are concentrated in the US Midwest, Texas, and Florida where calcium-hardness source water is regional; softer-water regions (Pacific Northwest, New England) rarely operate lime softening.
Drinking-Water Corrosion Control. Post-softening or as a standalone corrosion-control dose, calcium hydroxide at 5 to 20 mg/L raises finished-water pH from source-water low pH (typically 6.5) to the corrosion-control target (7.5 to 8.5). The elevated pH protects distribution-system pipe from aggressive-water attack and reduces lead-and-copper leaching from service lines under the EPA Lead and Copper Rule. Utilities that do not softening-treat still use calcium hydroxide for this corrosion-control-only duty.
Industrial and Municipal Wastewater pH Adjustment. Wastewater plants treating acidic industrial streams (mining, metal finishing, food processing, pulp-and-paper effluent) use calcium hydroxide as the primary alkali for pH elevation to the 7 to 8 discharge range. The chemistry is cheaper per pound of effective alkali than NaOH and Mg(OH)2; the trade-off is 3× to 5× higher sludge volume from the calcium-sulfate, calcium-carbonate, and metal-hydroxide precipitate generated in the process. Medium-scale industrial wastewater plants consume 50,000 to 500,000 lb/month of Ca(OH)2.
Flue-Gas Desulfurization (FGD). Coal-fired power plants use calcium-based FGD absorbents at two scales: wet-FGD uses Ca(OH)2 or CaCO3 slurry in spray-tower absorbers producing gypsum (CaSO4·2H2O) byproduct; dry-FGD uses Ca(OH)2 powder injected into the flue gas for in-flight SO2 capture producing dry calcium-sulfite-and-sulfate byproduct for disposal. Large coal-plant FGD installations consume 500,000 to 5,000,000 lb/day of calcium alkali. EPA 40 CFR 423 steam-electric effluent guidelines govern FGD wastewater discharge.
Biosolids Stabilization. EPA 40 CFR 503 permits lime stabilization of sewage sludge to produce Class A or Class B biosolids suitable for land application. Ca(OH)2 at 15 to 30% by weight of dry sludge raises pH above 12 and holds for at least 2 hours (Class B) or 30 minutes at 70°C (Class A), destroying pathogens and making the biosolids legally land-appliable. Municipal biosolids programs consume 20,000,000+ lb/year of Ca(OH)2 across US utility installations.
Acid-Mine-Drainage Treatment. AMD from abandoned coal and metal mines is the largest single use of Ca(OH)2 in environmental remediation. Annual US AMD neutralization consumes 50,000,000+ lb of calcium alkali at treatment stations across Appalachia, the Rocky Mountain West, and regional coal basins. Ca(OH)2 is preferred over Mg(OH)2 for AMD at lower-cost sites; Mg(OH)2 is preferred where sludge disposal economics favor the more-soluble magnesium byproduct.
Kraft-Mill Causticizing. The kraft-pulp industry uses quicklime (CaO) in the recausticizing stage of the kraft chemical recovery cycle: green liquor (sodium carbonate + sulfide) reacts with Ca(OH)2 to regenerate white liquor (sodium hydroxide + sulfide) per the reaction Ca(OH)2 + Na2CO3 → 2 NaOH + CaCO3. The lime-mud CaCO3 is calcined back to quicklime and recycled. A medium-scale kraft mill (1,000 tons/day pulp) circulates 100,000 to 300,000 lb/day of calcium across the cycle. This is a self-contained regeneration loop rather than a net consumption.
Cement, Mortar, and Construction. Masonry mortar, plaster, and lime-based stucco use Ca(OH)2 as the primary binder. Global construction industry consumption of hydrated lime is large-volume commodity with pricing driven by regional lime-kiln capacity and freight economics.
Animal Feed Calcium Supplement. FDA 21 CFR 184.1205 lists Ca(OH)2 as GRAS for use as a direct food-ingredient and animal-feed calcium source at regulated levels. Livestock feed mills consume 10,000,000+ lb/year across US operations. Food-grade processing of corn tortillas traditionally uses Ca(OH)2 in the nixtamalization step that softens corn kernels for masa flour production.
3. Regulatory Hazard Communication
OSHA and GHS Classification. Calcium hydroxide carries GHS classifications H315 (causes skin irritation), H318 (causes serious eye damage), and H335 (may cause respiratory irritation). The H318 eye-damage classification is the most operationally consequential: Ca(OH)2 dust or slurry in eye contact can produce corneal opacification similar to NaOH exposure, though clinical outcomes tend to be less severe because the pH-12.4 ceiling is lower than NaOH-induced pH-13 burns. OSHA PEL for total dust is 15 mg/m3 8-hour TWA; respirable fraction 5 mg/m3. ACGIH TLV-TWA is 5 mg/m3 inhalable total particulate. Eye protection (chemical-splash goggles, not safety glasses) is mandatory for all slurry-handling and dry-powder-handling operations.
NFPA 704 Diamond. Calcium hydroxide rates NFPA Health 3, Flammability 0, Instability 0, no special hazard flag. The Health 3 rating reflects the severe eye corrosivity and the dust-inhalation respiratory-irritation hazard.
DOT and Shipping. Calcium hydroxide solid is not DOT-regulated for domestic ground transport. Calcium hydroxide slurry at commercial concentration is similarly not DOT-regulated. International marine shipment may classify under Class 8 at higher concentrations; domestic supply chain is hazmat-free.
EPA CERCLA and EPCRA. Calcium hydroxide is not CERCLA-listed and carries no reportable quantity. EPCRA Tier II reporting applies at the 500-lb aggregate-site threshold in most states; SARA 313 TRI does not apply.
NSF/ANSI 60 and AWWA B202. Calcium hydroxide for drinking-water treatment is NSF/ANSI 60 certified by multiple manufacturers (Lhoist, Carmeuse, Graymont, Mississippi Lime). AWWA B202 governs quality for water-treatment-grade alkali chemicals. Both certifications are nonnegotiable for municipal water-utility purchase.
EPA 40 CFR 503 Biosolids. Lime stabilization of sewage sludge under 40 CFR 503.32 requires raising and holding pH above 12 for a specified time interval (Class A: pH >= 12 for 30 minutes at 70°C; Class B: pH >= 12 for 2 hours then pH >= 11.5 for 22 hours). The regulation specifies monitoring, record-keeping, and third-party-validation protocols that govern municipal biosolids programs.
FDA 21 CFR 184.1205 (GRAS). Calcium hydroxide is GRAS as a direct food-ingredient at specified maximum levels. Food-grade (FCC) specification requires heavy-metal limits below 10 ppm, arsenic below 3 ppm, and microbial limits consistent with food-ingredient standards. USP specification applies to pharmaceutical-grade product.
4. Storage Protocol and Field Handling
Bulk Slurry Tank Configuration. The industry-standard bulk Ca(OH)2 slurry tank is a 1.9-SG XLPE or FRP vinyl-ester vertical closed-top tank at 10,000 to 100,000-gal capacity, often exceeding this scale at large power-plant FGD installations (where concrete reaction tanks with rubber or coal-tar-epoxy lining handle the 250,000+ gal capacity). Agitators are top-entry mechanical at 1 to 2 HP per 1,000 gal, continuously running to prevent settling. Secondary containment per EPA SPCC is standard.
Dry-Hydrate Silo Storage. Many large calcium-hydroxide users receive dry hydrated lime in bulk rail-car or tanker-truck lots and store in 50 to 500-ton covered carbon-steel silos with dust-collector baghouse venting. Pneumatic conveyance from the silo to the batch dissolver or the direct-injection point is standard. Silo-wall thickness inspection every 10 years and vent-bag replacement annually are typical maintenance items.
Slaker / Dissolver Operation. Some large water-treatment plants and kraft mills receive quicklime (CaO) rather than hydrated lime, and operate on-site slakers that hydrate CaO to Ca(OH)2 with stoichiometric water plus heat management. Slakers are specialty equipment (paste slakers, ball-mill slakers, detention slakers) that control the highly-exothermic hydration reaction (1170 kJ/kg CaO) to produce a usable slurry without boiling or thermal damage. Slaker operation is a skilled-operator duty.
Day Tank and Dosing. Day tanks at 1,000 to 10,000-gal HDPE with paddle agitator serve dosing pumps feeding the process injection point. Peristaltic pumps or rubber-lined centrifugal pumps handle the slurry. Dosing piping is rubber-lined carbon steel or HDPE at 2 to 4 inch diameter with wear-resistant elbow inserts. Automatic pH feedback control dosing is standard at water-treatment and wastewater applications; manual dose-set operation with periodic lab-titration verification is standard at kraft mills and biosolids plants.
Maintenance and Turnaround. Slurry-service tanks receive annual agitator-gearbox inspection and interior visual for accumulated scale (minor CaCO3 deposition on walls is normal and does not require removal until substantial thickness develops over years). Agitator failure for 24+ hours typically creates a cemented slurry layer requiring vacuum-truck cleanout and fresh product refill. Dry-hydrate silos receive interior visual every 5 years and exterior carbon-steel thickness survey every 10 years.
5. Operator FAQs
Ca(OH)2 vs Mg(OH)2 for wastewater pH adjustment? Ca(OH)2 provides higher pH ceiling (12.4 vs 10.5) and lower cost per pound of effective alkali ($0.15-0.25/lb vs $0.40-0.50/lb). Mg(OH)2 provides self-limiting safety (no overshoot possible), 3× to 5× lower sludge volume, safer worker profile (no severe eye-burn hazard), and handles more tractably in automated dosing. Site-specific trade-off between raw-chemistry cost and total-cost-of-ownership including sludge disposal governs the choice.
Why does my milk-of-lime slurry cake at the tank bottom? Identical mechanism to Mg(OH)2: fine particles settle rapidly, compact from 25-35% solids to 50%+ solids once settled, and cement within hours to days depending on temperature. Agitator uptime is the critical operational parameter. Vacuum-truck cleanout after prolonged agitator failure is standard recovery procedure.
Quicklime vs hydrated lime — which should I receive? Quicklime (CaO) is roughly 30 to 40% cheaper per pound of effective alkali, but requires on-site slaking equipment with skilled operation. Hydrated lime (Ca(OH)2) is more expensive per pound but requires no slaking infrastructure and handles as direct-addition slurry. Large-scale water utilities and kraft mills typically operate slakers; smaller industrial users buy hydrated lime directly.
Why does lime softening generate so much sludge? Lime-softening removes calcium and magnesium hardness by precipitating CaCO3 and Mg(OH)2; a medium-hardness water (200 mg/L as CaCO3) generates roughly 400 lb of lime-softening sludge per million gallons treated. The sludge is managed by thickening, dewatering, and land-application or landfill disposal under state residuals-management regulations.
Can I use Ca(OH)2 for pH adjustment in a fish-farm or aquaculture application? Yes, with dilution and careful dosing to avoid pH excursion above the target 7.5 to 8.0. Agricultural lime (agricultural-grade Ca(OH)2 or CaCO3) is commonly used for pond-water pH and alkalinity management in aquaculture. Fish tolerate pH 12.4 slurry only if diluted at the feed point to the point that local pH does not exceed 9.
Slurry freeze point? 31 to 32°F for typical milk-of-lime. Heat tracing at 6 to 10 W/ft plus insulation protects against winter freeze in northern climates. Dry-hydrate silos have no freeze issue at any climate.
Shelf life of slurry in stirred XLPE? 3 to 6 months. Ca(OH)2 slowly absorbs atmospheric CO2 at the liquid surface to form CaCO3 scum, which reduces effective alkali. Closed-tank operation with minimized headspace air extends shelf life; annual slurry replacement is a common practice to ensure consistent chemistry.
6. Field Operations Addendum
Vendor Cadence and Supply Chain. Primary North American calcium hydroxide and quicklime producers are Lhoist North America (Fort Worth TX and regional plants), Carmeuse Lime & Stone (Pittsburgh PA), Graymont Lime (Richmond BC plus US operations), Mississippi Lime (St. Louis MO), and Martin Marietta. Delivered US pricing in 2026 runs $0.12 to $0.20 per pound of dry hydrated lime in bulk rail-car or supersack, $0.08 to $0.15 per pound of quicklime (CaO) in bulk, and $0.20 to $0.35 per pound of 25 to 30% milk-of-lime slurry in tanker-truck delivery (equivalent to $0.70 to $1.20 per lb dry basis when accounting for the water). Pricing is regional — lime-kiln freight economics dominate, so prices near kiln locations (Midwest, Texas, Virginia/West Virginia) are significantly lower than in remote markets.
Dosing Control and Process Automation. Lime-softening plant dosing uses feedforward raw-water flow-pacing combined with feedback from a settled-water calcium-and-magnesium analyzer, maintaining finished-water hardness at a target like 100 mg/L as CaCO3. Corrosion-control-only dosing uses pH feedback at finished-water pH 7.8 to 8.5. Wastewater-neutralization dosing uses pH feedback at discharge pH 7.0 to 8.5. FGD dosing at power plants uses feedback from clean-gas SO2 concentration and absorber-tower slurry pH; control bandwidth is wider than water-treatment service because of dispatch-driven power-plant operating profile.
Sludge Handling Economics. The single largest operational cost difference between calcium and magnesium alkali chemistries is sludge disposal. A typical industrial wastewater plant treating acidic-process stream generates 3 to 5× more sludge volume with Ca(OH)2 than with Mg(OH)2; at $0.10 to $0.30 per pound of sludge disposal cost, that translates to $5,000 to $25,000 per month differential for medium-volume operations. Total-cost-of-ownership analysis including raw-chemistry cost plus sludge-disposal plus PPE-and-training overhead is site-specific; the optimum chemistry varies by local economics.
Related Chemistries in the Lime + Calcium Chemistry Cluster
Related chemistries in the lime + calcium-chemistry cluster (shared slaking + water-treatment + steelmaking applications):
- Calcium Carbonate (CaCO3) — Limestone precursor via calcination
- Calcium Oxide (CaO, quicklime) — Dry form pre-slaking
- Magnesium Hydroxide (Mg(OH)2) — Alkaline alternative with self-limiting pH
Related Hub Pillars
For broader chemistry context, see the OneSource Plastics high-traffic chemical-compatibility hub pillars: