Alkalis — also called bases or caustic solutions — represent approximately 4% of industrial pump applications. While generally less aggressive than mineral acids, concentrated alkalis are highly corrosive to organic tissue, attack certain metals (notably aluminium and its alloys), and can cause stress-corrosion cracking in sensitised stainless steel. Hot caustic solutions are among the most hazardous liquids encountered in process industries.
Common Industrial Alkalis
- Sodium hydroxide (NaOH / caustic soda): The most widely used industrial alkali — 50% solution (700 g/L) is the standard commercial grade; freezes at 12 °C; highly exothermic when diluted; violently reactive with acids
- Potassium hydroxide (KOH / caustic potash): Similar to NaOH but more soluble and used in electrolysis, batteries and soap manufacture
- Sodium carbonate (soda ash, Na₂CO₃): Moderate alkalinity (pH 11 in solution); widely used in water softening, glass manufacture and detergents
- Ammonia solution (NH₃·H₂O): Volatile — releases ammonia gas above the liquid surface; strong odour; affects copper alloys severely
- Lime slurry (Ca(OH)₂): Low solubility, high abrasivity — a slurry rather than a true solution; causes scale deposition on pump surfaces
- Sodium hypochlorite (NaOCl / bleach): Strongly oxidising alkali; decomposes at elevated temperature; attacks most organic materials and carbon steel
Key Physical Properties
- Density: 50% NaOH solution has a density of 1.52 g/cm³ — significantly denser than water; affects pump head and power calculations
- Viscosity: 50% NaOH at 20 °C has viscosity of approximately 80 cSt; falls rapidly with temperature; standard centrifugal pumps require correction above 50 cSt
- Crystallisation temperature: 50% NaOH crystallises at 12 °C — heat tracing or dilution required in cold climates
- Vapour hazard: Ammonia solutions are volatile — shaft seals must prevent vapour release; sealless pumps strongly recommended
Corrosion and Attack Mechanisms
- Aluminium and its alloys: Dissolve rapidly in dilute NaOH — must never be used for caustic service
- Austenitic stainless steel: Susceptible to stress-corrosion cracking (SCC) in hot concentrated NaOH above 60 °C — avoid sensitised or welded stainless in hot caustic
- Copper alloys: Attacked by ammonia solutions forming copper-ammine complexes; not suitable for any ammonia duty
- Organic gaskets and seals: NaOH attacks natural rubber and some synthetic elastomers — EPDM and Viton have good resistance
- Scale deposition: Lime slurry and sodium carbonate deposit scale on impeller and casing surfaces — increases maintenance frequency
Pump Type Selection
- Dilute to moderate caustic (below 25%): Centrifugal pump in stainless steel 316L or polypropylene — the standard solution for water treatment and CIP service
- Concentrated caustic (50% NaOH): Centrifugal with cast iron or carbon steel casing (compatible with concentrated caustic); magnetic drive sealless preferred
- Hot concentrated caustic (above 60 °C): Carbon steel centrifugal or gear pump — avoid stainless; apply heat tracing to suction piping
- Ammonia solution: Sealless centrifugal (magnetic drive) — prevent ammonia vapour release to atmosphere
- Lime slurry: Progressive cavity or diaphragm pump — lime is abrasive and causes rapid wear in tight-clearance centrifugal designs
Materials of Construction
- Carbon steel: Surprisingly good resistance to concentrated NaOH — the preferred material above 35% concentration
- Stainless steel 316L: Suitable for dilute caustic at ambient temperature; avoid in hot concentrated caustic (SCC risk)
- Nickel and nickel alloys (Alloy 200, Monel): Excellent for hot concentrated caustic — used in evaporator service
- Polypropylene (PP) and PVDF: For dilute alkalis at moderate temperature; cost-effective alternative to stainless
- EPDM: Standard elastomer for caustic service — good resistance to NaOH, KOH and dilute hypochlorite
- Avoid: Aluminium, copper alloys, tin, zinc and their alloys in any alkali service
Applications
- Water treatment — pH correction and lime dosing for softening
- Pulp and paper — white liquor (NaOH + Na₂S) cooking liquor circulation in kraft pulping
- Food industry — CIP caustic cleaning circuits for dairy, beverage and food plant
- Textile manufacturing — mercerising and scouring of cotton with NaOH
- Chemical synthesis — alkali as reagent or neutralising agent in batch reactors
- Soap and detergent manufacture — saponification with NaOH or KOH
- Wastewater neutralisation — pH adjustment of acid effluent before discharge