Lime Slurry (Mining Flotation) Storage — Ca(OH)2 Slurry Tank for Flotation pH Modification
Lime Slurry (Mining Flotation) Storage — Ca(OH)2 Slurry Tank Selection for Flotation pH Modification
Hydrated lime calcium hydroxide (Ca(OH)2, CAS 1305-62-0) slurry at typical 15-25% solids concentration is the most-tonnage flotation reagent in the global mining industry by far. A major copper porphyry concentrator at 50,000+ tpd ore feed consumes 200-1,000 tonnes per month of hydrated-lime slurry — orders of magnitude more than any other single flotation reagent. The chemistry's role is pH modification of the flotation pulp from natural mine-water pH 6-8 up to the selective-flotation operating window of 9.5-11.5. Within this pH window, careful pH control governs sulfide-mineral selectivity: pyrite (FeS2, the dominant sulfide impurity in most copper, gold, and lead-zinc ore bodies) floats below pH 9 and depresses above pH 10; chalcopyrite (CuFeS2) floats well across the pH 9-11 window; sphalerite (ZnS) floats only above pH 10 after copper-sulfate activation; arsenopyrite floats well at pH 8-9 but depresses sharply above pH 10. Lime at the rougher conditioner box is therefore the single most powerful selectivity lever in the metallurgist's toolkit. At most concentrators, lime is delivered as quicklime (CaO) and slaked at the on-site slaker into hydrated-lime slurry; smaller operations receive pre-slaked Ca(OH)2 slurry by tank-truck. Slurry is dosed at 0.5-3 kg/t of ore via slurry pump or open-launder gravity feed to the conditioner box, with target pH measured continuously at the rougher-cell first cell. The hazards of lime slurry are skin and eye corrosivity (H314), CO2-absorption-induced scale buildup, and (for quicklime slaking) significant exothermic heat release. This pillar covers honest material compatibility, real producer landscape, OSHA / MSHA hazard communication, slaker and storage system specification, and field handling reality for a lime-slurry flotation-pH installation.
Regulatory citations point to OSHA 29 CFR 1910.1200 GHS classification, OSHA PEL for calcium hydroxide 5 mg/m3 8-hour TWA, ACGIH TLV-TWA Ca(OH)2 5 mg/m3 8-hour, MSHA 30 CFR 56/57 Subpart D mine air contaminant control, EPA NPDES Multi-Sector General Permit (MSGP) Sector G for metal-mining stormwater (no direct lime restriction; pH compliance for tailings discharge), and DOT non-regulated transport for hydrated-lime slurry (UN 1910 calcium oxide for quicklime is Class 8 PG III).
1. Material Compatibility Matrix
Hydrated-lime slurry is highly alkaline (pH 12-12.5 saturated), abrasive (calcium hydroxide solid particles in suspension wear pump impellers and pipe walls), and CO2-absorbing (forms calcium-carbonate scale on any air-exposed surface). Material selection is constrained by tolerance of high alkalinity, tolerance of slurry abrasion, tolerance of carbonate-scale buildup, and elimination of aluminum / galvanized / zinc / amphoteric metals which dissolve in caustic.
| Material | 15-25% slurry | Notes |
|---|---|---|
| HDPE / XLPE | A | Standard for slurry storage and slaker supply tanks; abrasion-resistant |
| Polypropylene | A | Standard for fittings, valves, pump heads |
| FRP vinyl ester | A | Standard for larger 5,000-30,000 gal slurry storage |
| PVC / CPVC | A | Standard for low-pressure piping; abrasion limits flow velocity to 1-1.5 m/s |
| HDPE-lined steel | A | Premium for high-pressure slurry pump headers |
| 316L stainless | A | Acceptable; abrasion resistance is the limiting factor |
| Carbon steel | A | ACCEPTABLE for clean lime slurry (alkaline + reducing); rubber-lined for abrasion |
| Aluminum | NR | Caustic dissolution; never |
| Galvanized steel | NR | Zinc dissolves in caustic; never |
| Copper / brass | B | Acceptable for short service; carbonate scale builds rapidly |
| Natural rubber lining | A | Standard abrasion-resistant lining for slurry-pump volutes and pipe spools |
| EPDM | A | Preferred elastomer for gaskets and pump diaphragms |
| Buna-N (Nitrile) | C | Caustic attack; avoid |
For the dominant flotation-pH-modifier use case at 15-25% Ca(OH)2 slurry, opaque HDPE rotomolded slurry storage tanks (5,000-30,000 gallons) with PP fittings, rubber-lined slurry pumps with EPDM elastomers, and rubber-lined or HDPE pipe spools for the slurry distribution system are the standard. At very large concentrator scale, FRP vinyl-ester slurry tanks (30,000-200,000 gallons) replace HDPE for size and structural-bracing reasons. Aluminum and galvanized-steel materials are absolutely excluded from any lime-slurry wetted-contact surface.
2. Real-World Industrial Use Cases
Primary Copper Porphyry Flotation pH Control (Dominant Mining Use). Lime slurry is the universal pH-modifier at primary copper porphyry concentrators, operating in the pH 10-11.5 range to depress pyrite and float chalcopyrite/bornite/chalcocite cleanly. Dosing 1-3 kg/t of ore. Annual global lime tonnage in mining flotation alone is 5-10 million tonnes.
Pb-Zn Selective Flotation pH Control. Lead-zinc flotation circuits use lime to elevate pH from natural mine-water pH 6-8 up to pH 8.5-9 for the lead rougher (sphalerite depressed by pH and ZnSO4) and pH 10.5-11.5 for the zinc rougher (after copper-sulfate activation). Red Dog (Alaska), Mount Isa (Australia), Cannington (Australia), Penasquito (Mexico), Trepca (Kosovo), San Cristobal (Bolivia). Lime dosing 0.5-2 kg/t.
Gold Sulfide Flotation pH Control. Refractory gold concentrators floating gold-bearing sulfide for downstream pressure-oxidation or roasting use lime at 0.5-1.5 kg/t to deliver the optimum pH for arsenopyrite-pyrite-pentlandite-bearing-gold-sulfide rougher pull. Witwatersrand (South Africa), high-grade gold belt (Nevada).
Cyanide Heap-Leach pH Control (Adjacent Use). Cyanide heap-leach pad pH-modification (pH 10.5-11 to prevent HCN gas evolution from CN--protonation) is the second-largest mining-industry lime use after flotation. Heap-leach lime is typically applied as agglomeration drum addition at 1-3 kg/t prior to stacking on the pad; in-situ pH monitoring in the leach solution feeds make-up lime additions. Operations: high-grade gold belt, Round Mountain, Cripple Creek, Veladero (Argentina), Pueblo Viejo (Dominican Republic).
Tailings Pond Discharge pH Compliance. Mining-site tailings-pond water-treatment for NPDES discharge frequently uses lime addition to neutralize acid mine drainage from sulfide oxidation and to precipitate dissolved metals. Adjacent industrial use to flotation, often integrated through the same lime-slaker station.
Acid Mine Drainage Treatment. Closure-stage and operating-stage acid mine drainage from sulfide-bearing waste rock is typically treated with lime addition to a pH 9-10 endpoint, precipitating Fe(OH)3, Al(OH)3, and other metal hydroxides. The treated water meets NPDES discharge limits before release. Hundreds of US mining sites under closure operate this chemistry continuously.
3. Regulatory Hazard Communication
OSHA / GHS Classification. Hydrated lime Ca(OH)2 carries GHS classifications H315 (causes skin irritation), H318 (causes serious eye damage), H335 (may cause respiratory irritation). The H318 eye-damage classification is the dominant safety message: lime-slurry splash to the eye can cause severe corneal injury or blindness. Quicklime CaO carries the same classifications plus H402 (harmful to aquatic life) for the slaking-event water-temperature excursion. OSHA PEL 5 mg/m3 8-hour TWA for calcium hydroxide and ACGIH TLV-TWA 5 mg/m3 apply to lime dust at the slaker bag-tip, the hydrated-lime stockpile, and any unconfined slurry-tank ventilation point.
NFPA 704 Diamond. Hydrated lime rates Health 2, Flammability 0, Instability 0 with no special hazard flag. Quicklime rates Health 3, Flammability 0, Instability 1 (water reactive) with W (water-reactive) special hazard flag for the slaking-event heat release. Storage building classification per IFC Chapter 50 is corrosive-solid for hydrated lime; water-reactive solid for quicklime.
DOT and Shipping. Hydrated lime slurry is non-regulated for transport (no UN number, no Class). Quicklime ships under UN 1910 (Calcium oxide), Hazard Class 8, Packing Group III. Bulk pneumatic-truck and rail-car shipping of dry quicklime is the dominant delivery mode at major mine sites.
MSHA 30 CFR 56/57 Mining Compliance. Surface metal/nonmetal mines (CFR 56) and underground metal/nonmetal mines (CFR 57) must control mine-air contaminants per Subpart D. Lime dust at the slaker, the hydrated-lime stockpile, and the truck-discharge area is the relevant exposure pathway; bag-house dust collectors at every transfer point and respiratory protection at the slaker bay are standard practice.
EPA NPDES MSGP Sector G + State pH Discharge Limits. Metal-mining stormwater discharges fall under EPA NPDES Multi-Sector General Permit Sector G. Most state-delegated NPDES programs set discharge pH in the 6.0-9.0 range; lime-treated tailings water at pH 9-10 from the flotation circuit must be retained in tailings impoundment for natural CO2-absorption-driven pH drift down to the discharge range, OR re-acidified with CO2 sparging or sulfuric-acid addition before discharge.
4. Storage System Specification
Quicklime Bulk Storage (Major Mine Sites). Quicklime CaO is delivered to major mine sites by pneumatic tanker truck (25-30 tonnes per load) or rail-car (90-100 tonnes per car) directly into a vertical-cylinder steel silo (150-1,000 tonnes capacity). Silo construction is carbon steel with rubber-lined or stainless-steel discharge cone, vibrating bin discharger, and rotary-airlock or screw conveyor to the slaker. Pneumatic-conveyor return-loop dust collection (bag house) on the silo top vent is mandatory. Inventory turnover targeted at 14-30 days; quicklime absorbs CO2 and atmospheric moisture slowly losing reactive CaO content over time.
Slaker Design. The slaker is the heart of the mine-site lime system. Detention slakers (most common at mining scale) feed quicklime into water at controlled rate, retain the slurry for 10-15 minutes at design temperature 95-99 °C from the slaking exothermic reaction (CaO + H2O = Ca(OH)2 + 1,140 kJ/kg), then discharge slaked-lime slurry to the holding tank. Slaker construction: typically rubber-lined carbon steel or 316L stainless. Slaker effective sizing: 1 tonne/hr CaO produces approximately 1.32 tonne/hr Ca(OH)2 at theoretical yield. Major mine sites operate 5-30 tph slakers continuously.
Slurry Storage Tank. Slaked Ca(OH)2 slurry at 15-30% solids is stored in a 10,000-100,000-gallon HDPE rotomolded or FRP vinyl-ester agitated storage tank. Continuous agitation prevents settling and re-cementation. Tank fittings: 4-6-inch top fill from slaker, 4-6-inch bottom outlet to slurry distribution pump, 24-30-inch top manway, vented headspace. Tank inventory turnover targeted at less than 7 days to limit CO2-absorption-driven calcium-carbonate scale.
Pre-Slaked Slurry Tank-Truck Delivery (Smaller Operations). Operations consuming less than 30 tonnes/month of hydrated lime typically receive pre-slaked Ca(OH)2 slurry by tank-truck (5,000-7,500-gallon load at 25-30% solids). Storage in HDPE or FRP slurry tanks of 10,000-30,000 gallons capacity with continuous agitation. Inventory turnover 14-30 days.
Pump Selection. Slurry centrifugal pumps (rubber-lined volute, EPDM impeller / wear-resistant elastomer impeller, slurry-rated mechanical seals) are standard for lime-slurry distribution. Warman, Metso, KSB, Goulds, and Schurco have lime-slurry-rated configurations. Progressive-cavity pumps with EPDM stators are common for the slaker-to-storage-tank transfer. Diaphragm metering pumps (PTFE diaphragm, EPDM check valves, PP head) for small dosing duties to small auxiliary streams.
5. Field Handling Reality
The Slaker Reality. The lime slaker is the dominant mine-site engineering item for any new flotation-circuit project. Improper slaker design or operation produces under-slaked lime (low Ca(OH)2 yield, high CaO grit content destroying downstream pumps), over-slaked lime (heat damage, lower reactivity, scale formation), or grit-overload conditions (slaker plugging, downtime). The exothermic slaking reaction (1,140 kJ/kg CaO heat release) requires careful water-quenching design with thermocouple feedback control and emergency-water spray for runaway-temperature events. Major-miner operations have lost slaker buildings to fire from over-temperature events; the mitigation is rigorous engineering and standard-operating-procedure discipline.
The Calcium-Carbonate Scale Reality. Atmospheric CO2 absorbs into Ca(OH)2 slurry forming calcium carbonate (CaCO3) scale. Open-top slurry tanks build scale at the air-water interface in days; covered tanks with inert-gas-blanket build scale in months. Pump impellers, pipe walls, and dosing-line nozzles all suffer scale buildup over time. Mine-site operations include scheduled acid-cleaning (citric or hydrochloric acid CIP) of dosing lines on monthly cadence and full slurry-tank cleanout on annual cadence.
The Eye-Damage Reality. Lime-slurry splash to the unprotected eye causes severe corneal injury or blindness within seconds via OH--driven saponification of corneal proteins. The defense is rigorous chemical-splash goggle / face-shield use at every dosing-station, slaker-discharge, and tank-cleaning operation, plus eye-wash within 10-second reach. Mine-site safety records globally include lime-eye injuries; the corrective action at every responsible operation is mandatory full-face PPE at lime operations.
The Quicklime Heat Reality. Quicklime contact with water generates significant heat (1,140 kJ/kg CaO). Skin contact with damp quicklime — from sweat, rain, or splashed water on contaminated work clothes — produces severe thermal-plus-caustic burns. The combined hazard is more severe than either alone. PPE for quicklime handling includes dust-tight goggles, full-face dust respirator, dust-tight Tyvek suits with sealed cuffs, butyl rubber gloves under leather over-gloves, and water-rinse stations at every operating area. Slaker-bay and silo-discharge zones require posting and PPE-required signage.
PPE. Lime-slurry operations require chemical-splash goggles, full-face shield, butyl rubber gloves, Tyvek or equivalent disposable coveralls (replace at end of shift), and steel-toe boots with rubber overshoe boots to prevent caustic contact through leather. Eye-wash and emergency shower at every lime-handling area are mandatory. Slaker and quicklime-discharge operations additionally require NIOSH-approved dust respirator (P100 cartridge).
Related Chemistries in the Alkaline & Caustic Cluster
Related chemistries in the alkaline & caustic cluster (lime / hydroxide / oxide pH-modifier + acid-neutralization + flotation pH-modifier chemistry):
- Calcium Hydroxide (Slaked Lime) — Hydrated-lime parent chemistry
- Calcium Oxide (Quicklime / CaO) — Burnt-lime precursor chemistry
- Sodium Hydroxide (NaOH) — Strong-alkali companion chemistry
- Magnesium Hydroxide (Mg(OH)2) — Mild-alkali companion chemistry
- Calcium Carbonate (Limestone) — Carbonate-form sister chemistry
Related Hub Pillars
For broader chemistry context, see the OneSource Plastics high-traffic chemical-compatibility hub pillars: