Particle separation: Chemical precipitation TVM 4145 Vannrenseprosesser / unit processes Particle separation: Chemical precipitation Prof. TorOve Leiknes torove.leiknes@ntnu.no

Definition:

General expression: aA + bB  cC + dD A+ + B-  AB(s) Equilibrium: aA + bB  cC + dD Solubility: A+ + B-  AB(s) AxBy(s)  xAy+ + yBx- Precipitation with metals (M) the following general equation applies: MAx(S)  Mx+ + xA- For A- = OH-

Example: excess lime process Removal of hardness (Ca+2, Mg+2) Carbonate equations with addition of excess lime (for pH increase) and resulting precipitation:

Classical process configuration for excess-lime / soda ash softening: 8 sludge 1. Dosage and mixing of lime 2. Precipitation of CaCO3 and Mg(HO)2 3. Dosage and mixing of CO2 and soda ash 4. Precipitation of CaCO3 5. pH adjustment and precipitation of residual CaCO3

Carbonate system determines the waters stability: Measured with respect to the carbonate systems equilibrium – expressed as Langlier’s saturation index Rearranged: Index: Positive – precipitation of CaCO3 Negative – dissolution of CaCO3

Chemical precipitation of inorganic compounds: Iron (Fe) and manganese (Mn) Problem: precipitation / sludging colouring of water growth of “iron bacteria” odour and taste issues Methods for removal: chemical precipitation oxidation and precipitation ion exchange biological oxidation Drinking water standards (NOR):

Reduced form – Fe2+, Mn2+ Oxidation by: oxygen (air) ozone potassium permanganate chlorine dioxide Iron removal: (Ksp = 4.10-38, 25C) Manganese removal: not so readily oxidized by oxygen (air) precipitation is more pH dependent

Basics of iron oxidation: Stoichiometric balance: 1 mg Fe/L  0.14 mg/L O2 requires 0.04 mg/L H+ produced 1.80 mg/L alk as CaCO3 used Kinetics of oxidation: Where: concentration is in mol/L pO2 is partial pressure in atm k=8.1013 L2/(min)(atm)(mol)2 at 25C Pseudo 1.order expression commonly used: K = 1.68.10-15 / [H+] min-1 at PO2 = 0.21 atm

Phosphate (P) Coagulation and precipitation by: - aluminum salts : AI2(S04)3.14-18H20 - ferric salts : Fe(CL)3, Fe(S04)3 Stoichiometric balance Al: Al2(SO4)3 ·18H2O + PO43-  2AlPO4 + 3SO42- + 18H2O