Lime & soda ash · HCl neutralisation · salt top-up
1 BATCH DETAILS
2 SPENT BRINE — BEFORE TREATMENT sets the lime & soda ash dose
Analysis of the spent regeneration brine
Reagents & target excess
Magnesium only precipitates as Mg(OH)₂ above about pH 10.5, so some lime excess is necessary. Every mg/L of excess you carry past that point comes back as acid demand in Step 3.
3 FILTRATE AFTER FILTER PRESS — HCl DOSE the main answer
Titrate the filtrate (this is what decides the acid dose)
Acid & allowances
Alkalinity breakdown of this batch
Titration curve — pH against HCl added
Where the line is flat the water is buffered and pH barely moves; where it falls away, a small extra addition swings the pH a long way. That cliff is why batches get overshot.
4 SALT TOP-UP FOR REGENERATION
Chloride added by the HCl dose is counted automatically. Enter either chloride or NaCl — chloride is used when both are given.
5 WHY THE ACID DEMAND MOVES BATCH TO BATCH
⚠ Safety — dosing concentrated HCl into alkaline brine
Add the acid slowly with the agitator running. Carbonate releases CO₂ and the tank will foam — a fast dose can boil over.
The reaction is exothermic. Watch the tank temperature on large doses.
Always add acid into water, never water into acid.
Goggles, face shield, PVC/neoprene gloves and apron. Eye-wash within reach.
Never let hypochlorite or bleach reach this tank — with HCl it produces chlorine gas.
