Lime Slaking Systems

Lime slaking systems convert quicklime (CaO) into lime slurry by controlled mixing with water, producing calcium hydroxide [Ca(OH)2] for pH adjustment and alkalinity control in municipal treatment processes. The exothermic reaction generates temperatures up to 200°F, requiring careful water-to-lime ratios and detention time management. Properly designed systems achieve 95-98% conversion efficiency while producing consistent slurry concentrations of 5-15% solids. The primary trade-off involves balancing reaction completeness against equipment wear from the highly abrasive slurry and managing the significant heat generation that can damage pumps and piping if not properly controlled.

• Water Treatment pH Adjustment: Lime slakers produce calcium hydroxide slurry for pH correction in softening plants and corrosion control. Systems typically feed 2-15% slurry to rapid mix basins or distribution systems. Selected over caustic soda for cost savings at larger plants (>5 MGD) where bulk lime handling justifies capital investment
• Wastewater Treatment Alkalinity Addition: Used in biological nutrient removal plants where low influent alkalinity limits nitrification. Slakers feed lime slurry to aeration basins or primary clarifiers, maintaining 100-150 mg/L alkalinity as CaCO3. Preferred over sodium bicarbonate for operational cost reduction in plants >10 MGD
• Biosolids Stabilization: Lime slaking supports Class A biosolids production through pH elevation to >12 for pathogen destruction. Systems feed concentrated slurry (15-25%) directly to dewatered cake or liquid sludge streams. Essential for plants pursuing land application programs without thermal treatment
• Phosphorus Precipitation: Enhanced biological phosphorus removal plants use lime for chemical backup during process upsets. Slakers provide rapid response capability for meeting <1.0 mg/L effluent phosphorus limits when biology fails

Daily Operations: Operators monitor slaking temperature, slurry density, and feed rates hourly. Typical adjustments include lime feed rate based on process demand and dilution water flow for target 8-15% slurry consistency. Daily lime consumption ranges 200-2000 pounds depending on plant size and application. Temperature control prevents poor slaking quality and equipment fouling.

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Maintenance: Weekly grit removal and monthly agitator inspection required. Quarterly slaking chamber cleanout removes scale buildup using dilute acid wash. Operators need confined space training for tank entry and respiratory protection for lime dust exposure. Lime feeders require daily lubrication and weekly belt/chain inspection. Typical slaker rebuilds occur every 5-7 years with proper maintenance.

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Troubleshooting: Poor slaking quality indicates incorrect water temperature or lime/water ratios - adjust dilution water flow first. Excessive grit carryover suggests inadequate detention time or worn classifiers. High slurry temperatures indicate need for increased dilution water flow or reduced lime feed rate.

• Slaking Chamber: Stainless steel vessel where quicklime contacts water in controlled ratios (typically 1:3 to 1:4). Sizes range 50-500 gallons for municipal applications. Internal baffles ensure complete hydration while managing exothermic reaction temperatures <180°F
• Lime Feeder: Gravimetric or volumetric systems delivering 50-2000 lb/hr quicklime. Screw conveyors with variable speed drives provide accurate feed control. Dust collection systems mandatory for indoor installations per OSHA requirements
• Detention Tank: Mild steel tanks with agitation storing 2-8 hours slurry production. Typical sizes 1000-10,000 gallons with 1-3 HP mixers. Prevents settling while allowing grit removal through bottom drains
• Dilution Water System: Provides temperature-controlled water (60-80°F optimal) through flow control valves and temperature sensors. Prevents flash heating that creates poor slaking and equipment scaling
• Grit Removal: Classifying tanks or hydrocyclones removing unreacted lime particles >100 mesh. Critical for downstream pump protection and process consistency

• Lime Feed Rate: 50-5,000 lbs/day (0.025-2.5 tons/day) based on alkalinity demand and treatment capacity. Size for 125-150% of average demand
• Water-to-Lime Ratio: 3:1 to 4:1 by weight for optimal slaking. Higher ratios (5:1-6:1) reduce heat generation but increase slurry volume
• Slaking Temperature: Target 160-180°F during reaction. Temperatures below 140°F indicate poor slaking; above 200°F risks equipment damage
• Detention Time: 15-30 minutes in slaking chamber for complete hydration. Shorter times result in gritty slurry; longer times allow settling
• Slurry Concentration: 8-12% by weight lime solids typical for municipal applications. Higher concentrations increase pumping requirements
• Water Supply Pressure: 30-60 psi minimum for proper atomization and mixing. Low pressure causes poor slaking efficiency
• Grit Removal: Size grit classifier for 2-5% of total lime feed rate to handle impurities and incompletely slaked particles
• Agitation Requirements: 0.5-1.0 HP per 1,000 gallons slaker volume for adequate mixing without excessive wear
• Storage Capacity: 4-8 hours of slurry storage downstream of slaker, with continuous gentle agitation to prevent settling

• What lime quality and feed rate drives slaker sizing? Quicklime with 85-95% CaO content requires different water ratios than hydrated lime blends. Feed rates above 1,000 lbs/day typically justify continuous slakers over batch units. Underestimating feed rate results in incomplete slaking and downstream scaling; oversizing wastes energy and increases maintenance
• Should the system use paste or detention slaking? Paste slakers (3:1 ratio) work well for feeds under 500 lbs/day but require careful water control. Detention slakers (4:1+ ratio) handle variable loads better but need larger tanks and higher-capacity pumps. Wrong choice affects reliability and operating flexibility
• What level of automation is required? Manual systems work for small plants (<2 MGD) with dedicated operators. Automated feed control becomes essential above 500 lbs/day lime usage due to process sensitivity. Insufficient automation leads to inconsistent slurry quality and potential equipment damage from temperature excursions
• How will grit and unreacted lime be handled? Plants using lower-grade lime need robust grit removal and slurry screening. Without proper grit handling, downstream pumps and injection points experience accelerated wear and frequent plugging

• Division 40-48: Process Integration
• 40 31 33 - Water Treatment Lime Slaking Equipment (Primary)
• 40 31 00 - Water Treatment Equipment (General)
• Note: Some specifications place lime slaking under chemical feed systems (40 32 00) depending on project organization

• Material/Equipment Verification: Verify 316SS construction for all wetted parts, Confirm agitator materials and bearing specifications, Check electrical classifications for dust environments
• Installation Requirements: Overhead crane access for agitator removal, Adequate ventilation for dust control (minimum 6 ACH), Concrete pads sized for thermal expansion
• Field Challenges: Quicklime delivery coordination with startup, Water quality verification before commissioning, Dust collection system integration
• Coordination Issues: Chemical feed system interfaces, SCADA integration timing, Lead times: 16-20 weeks typical for engineered systems

• Carmeuse - Model CSS-4000 paste slaker (2-8 TPD capacity) dominates smaller municipal market
• Graymont - GS-Series detention slakers popular for 10+ MGD plants
• Chemtrade - CT-2500 ball mill slakers for high-capacity applications
• Walker Process - WP-Slaker systems integrated with clarifier packages
• All maintain strong municipal references and field service networks

• Hydrated lime feed systems - Lower capital cost ($75K vs $200K), simpler operation, but 40% higher chemical costs and storage challenges
• Liquid caustic soda - Eliminates slaking entirely, costs 3-4x more but provides operational simplicity for plants under 5 MGD
• Magnesium hydroxide slurry - Premium option at 5-6x lime cost, justified only for specific pH control applications requiring precise dosing

Establish relationships with multiple quicklime suppliers early - single-source dependency creates vulnerability during shortages. Size detention chambers 25% larger than calculated minimum to handle lime quality variations. Specify removable agitator assemblies even on smaller units; bearing replacement in-place is extremely difficult. Consider lime storage silos with 7-day minimum capacity to buffer delivery disruptions.