Scaling Salts
The Blueprint of Water frames scale as a thermodynamic negotiation that reverses when operating conditions crowd ions together. Chapter 8 catalogs five scaling salts that matter most in industrial water systems, along with the levers that govern each.
| Salt | Formula | Solubility behavior | What triggers it | How it is controlled |
|---|---|---|---|---|
| Calcium carbonate | CaCO₃ | Low solubility, exhibits inverse solubility (precipitates more at high temperature). Dissolves reluctantly, precipitates eagerly when conditions shift. | High cycles (concentration), high pH (carbonate becomes available), high temperature. Cooling towers concentrate calcium and alkalinity while CO₂ stripping raises pH. Worst at hot, high-flux surfaces. | Softening, RO or DI, pH and alkalinity control, acid treatment, scale inhibitors. Tracked with LSI, RSI, and PSI. |
| Calcium sulfate | CaSO₄ | Much more soluble than CaCO₃, so often overlooked at low cycles. Once formed, deposits are dense, hard, and resistant to acid cleaning. | Calcium and sulfate concentration at high cycles. pH is a secondary lever. Systems using sulfuric acid for pH control build large sulfate loads. | RO or DI, softening, inhibitors, cycle control. Prevention is far easier than removal. |
| Silica (amorphous) | SiO₂ | Exists as neutral silicic acid, Si(OH)₄, so it adds little conductivity and can concentrate quietly. Practical limit around 120 to 180 mg/L. Polymerizes into glassy deposits that are extraordinarily hard to remove. | High cycles (concentration), low temperature favor amorphous silica. Once the practical limit is exceeded it polymerizes. Standard CaCO₃ indices do not apply. | RO or DI, lime softening, magnesium precipitation routes, direct silica analysis for monitoring. |
| Calcium phosphate | Ca₃(PO₄)₂ | Low solubility. Forms when orthophosphate reacts with calcium, especially at elevated pH. | Elevated pH and temperature (concentration also a lever). Often caused by phosphate release or overfeed: polyphosphates hydrolyze to orthophosphate, or phosphonates degrade under oxidizing or thermal stress. | Manage phosphate and phosphonate dosing to avoid overfeed and degradation. Occurs in phosphate-treated cooling towers. |
| Magnesium silicate | MgSiO₃ | Low solubility. In real systems deposits are usually mixed and hydrated rather than pure crystal. Glassy, adherent, difficult to remove. | High pH and high temperature with magnesium and silica present together (concentration also a lever). | Maintain hardness and silica control. Risk highest in high-pH, high-cycle, silica-rich systems and boilers where control has failed. |
Related concepts
Cycles of Concentration · Supersaturation · Gibbs Free Energy · Blowdown
Part of The Blueprint of Water