Hydrated lime (calcium hydroxide)
The cheapest alkalinity you can buy and the most troublesome to dose. On a large plant with a maintainer, it saves real money. On a small plant without one, it becomes an abandoned slurry system with a blocked line.
Hydrated lime, Ca(OH)₂, is supplied as a 90–95% purity white powder and dosed as a 5–15% slurry. One milligram per litre provides 1.35 mg/L of alkalinity as CaCO₃ — more per kilogram than caustic soda and at roughly a third to a half the cost.
It is used for alkalinity replacement, pH lift, heavy metal hydroxide precipitation and lime softening. It is not a solution: it is a suspension, and every design decision downstream follows from that.
Specification
| Parameter | Water treatment grade | Industrial grade |
|---|---|---|
| Ca(OH)₂ content | 92–96% | 88–93% |
| Available CaO | 70–73% | 66–70% |
| Magnesium as MgO | ≤ 1.5% | ≤ 3.0% |
| Acid insolubles | ≤ 1.0% | ≤ 2.5% |
| Fineness, through 90 µm | ≥ 95% | ≥ 90% |
| Bulk density | 0.4–0.6 kg/L | 0.4–0.6 kg/L |
| Solubility at 20 °C | 1.7 g/L | 1.7 g/L |
Dose calculation
| Duty | Typical dose as Ca(OH)₂ | Notes |
|---|---|---|
| Alkalinity replacement after alum | 0.37 × alum dose | 200 mg/L alum needs ≈ 74 mg/L lime |
| Alkalinity replacement after ferric chloride | 0.67 × FeCl₃ dose | 150 mg/L FeCl₃ needs ≈ 100 mg/L lime |
| pH lift to 9.5 for metal precipitation | 100–400 mg/L | Highly stream-dependent; titrate |
| Lime softening, carbonate hardness | 1.35 × carbonate hardness as CaCO₃ | Plus 30–50 mg/L excess |
| Sludge stabilisation to pH 12 | 15–30% on dry solids | Must hold pH 12 for 2 hours |
| Sulphide and odour control | Raise to pH 9.0–9.5 | Shifts H₂S to non-volatile HS⁻ |
The slurry problem
Calcium hydroxide dissolves to only 1.7 g/L. Every lime dosing system therefore handles a suspension of undissolved solid, and the consequences are consistent across every plant that has one:
- The slurry settles the moment agitation stops. Keep the make-up tank stirred continuously, size the recirculation loop for at least 1.5 m/s, and fit a flushing connection at every dead leg.
- Lime scales on contact with dissolved CO₂ and with any carbonate hardness. Calcium carbonate builds inside dosing lines and injection quills, and it does so faster in warm water. Plan for periodic acid cleaning — dilute hydrochloric or sulphamic — as a routine maintenance task, not an emergency.
- Use hose or flexible line for the last section into the injection point, so a blockage can be flexed loose rather than dismantled.
- Dose through a full-bore injection quill, never a small orifice. Lime will find any restriction and build on it.
- Slurry strength of 5–10% is the practical band. Above 15% the slurry becomes difficult to pump and abrasive to the pump internals.
Where the site cannot commit to that maintenance, caustic soda costs more per kilogram of alkalinity and is worth every cent of the difference.
Handling and safety
- Alkaline burn risk. Saturated lime water sits at pH 12.4. Wet lime on skin causes burns that develop slowly — the same delayed-onset pattern as caustic, and often ignored for the same reason.
- Dust. Lime dust is strongly irritant to eyes and respiratory tract. Use P2 respiratory protection and local extraction at the bag tip point.
- Storage: dry, sealed, off the floor. Lime absorbs atmospheric CO₂ and converts slowly to calcium carbonate, which is inert. In humid coastal African conditions an opened bag loses meaningful activity within weeks.
- Materials: carbon steel, stainless, HDPE and rubber-lined pipe all acceptable. The engineering issue is abrasion and scaling, not corrosion — specify pump internals for abrasive duty.
- Shelf life: 6–12 months sealed; assume less in high humidity.
Frequently asked questions
Lime or caustic soda?
Lime costs roughly a third to a half as much per unit of alkalinity and adds calcium that increases floc density — a real benefit on a flotation or settling duty. Against that, it needs a slurry make-up and recirculation system, it scales lines, and it demands maintenance the plant must actually perform. Below about 200 kg/day of alkalinity demand, caustic is normally the better economics once the maintenance burden is priced in. Above that, lime pays.
Quicklime or hydrated lime?
Quicklime, CaO, is cheaper per unit of alkalinity and carries less water, but it must be slaked on site — an exothermic reaction requiring a proper slaker, and a hazard if done badly. Hydrated lime is pre-slaked and ready to make into slurry. For all but the largest municipal plants, hydrated lime is the correct specification.
Why does my lime dosing line keep blocking?
Three usual causes, in order: the slurry is settling because recirculation velocity is too low or agitation stops between batches; calcium carbonate is scaling the line because dissolved CO₂ or carbonate hardness is reacting with the lime; or the lime is too coarse and settling in horizontal runs. Fix the velocity first, schedule acid cleaning second, and specify the fineness on the next order.
Does lime help the flotation itself?
Sometimes, and more than caustic does. Calcium ions increase floc density and can improve floc strength on some streams, and lime contributes fine particulate that acts as a weighting agent. On a dissolved air flotation duty that cuts both ways — denser floc is harder to float. Where lime is used ahead of a DAF, check on the bench that it has not made the float sink.