Engineering reference for sizing a hydrated lime or quicklime dosing package — capacity selection, slurry concentration choice, batch vs continuous preparation, and storage silo selection. Real numbers, honest limits.
Capacity, Concentration, Preparation Method, Storage
A lime dosing system is sized around four interlocking decisions. Capacity sets the feeder and pump duty; slurry concentration sets the make-up tank volume and the dosing pump turndown; preparation method (batch or continuous) sets the operator workload and the redundancy model; storage silo size sets the delivery cadence and the bulk-handling infrastructure. Get one wrong and the whole package under- or over-performs.
This page covers each in detail. Use the four hero tiles above to jump to a section, or read straight through.
How to Size the Feeder & Pump Duty for the Demand
Lime dosing demand is calculated from the alkalinity demand of the water being treated — not picked from a catalogue. The 5 to 5,000 kg/hr range covers everything from a small pre-treatment dose on a 20 m³/h industrial effluent up to bulk dosing on a 50,000 m³/day mineral processing plant. Knowing where the duty sits within that range governs feeder type, pump selection and redundancy.
| Application | Typical lime demand | Calc note |
|---|---|---|
| pH neutralisation from pH 3 to pH 7 (sulphuric acid) | 0.1–0.3 kg Ca(OH)2 / m³ per pH unit | Driven by alkalinity demand from titration |
| Acid mine drainage (AMD) | 1–5 kg/m³ | Pre-titrate sample; depends on sulphate & metal load |
| Metals precipitation (Fe, Cu, Zn, Ni) | 2.5–4× stoichiometric Ca(OH)2:metal | Mole ratio plus pH-buffering excess |
| Sludge stabilisation (pH 12 for 2 hr) | 200–300 kg/dry t sludge | For Class A biosolids by lime addition |
| Phosphorus precipitation | 1.5–3 mol Ca:P | Plus pH-raise demand |
| Drinking-water softening (lime-soda) | 40–80 mg/L CaO equivalent | Per mg/L of carbonate hardness removed |
A volumetric screw feeder gives 10:1 turndown; a gravimetric loss-in-weight feeder gives 20:1 with closed-loop mass control. Duty/standby on the feeders is mandatory above 500 kg/hr — an unexpected feeder trip on a single-line package immediately puts the plant in non-compliance because the downstream pH starts falling within minutes.
Sizing rule: never spec a feeder at its 100% rated point. Buy the next size up and run it at 60–80% — this preserves turndown headroom for low-demand periods and reduces vibration-driven wear on the screw seal.
At very high dosing rates the limit is no longer the feeder — it is the slaker (for quicklime) or the slurry mixer (for hydrated lime), the make-up tank residence time, and the delivery silo unloading rate. Talk to our engineers early on these duties; a 5,000 kg/hr package is a meaningful piece of infrastructure with its own civil and crane requirements.
Related pages: pH correction overview · pH neutralisation calculator · chemical dosing calculator · back to lime dosing
Pumpability vs Storage Volume vs Settling Risk
Slurry concentration is the most consequential design choice in a lime package because it cascades through tank sizing, pump selection, pipe specification and settling-control strategy. There is no “best” concentration — only the right concentration for the duty.
| Slurry concentration | Common use | Why pick it | Watchouts |
|---|---|---|---|
| 5–10 % w/w | Small / accurate dosing | Easy to keep in suspension; low scour velocity; minimal pipe scaling | Large make-up volumes; more dilution water demand |
| 10–15 % w/w (default) | Most industrial dosing | Best balance of pumpability and storage | Standard build; few traps |
| 15–20 % w/w | Mid-large dosing duties | Reduces make-up tank size; less dilution water | Needs maintained agitation; pipe-bend abrasion |
| 20–25 % w/w | Bulk dosing | Compact infrastructure; reduces footprint | Tight pump selection; peristaltic or progressive cavity |
| 25–30 % w/w | Very-high-throughput; some quicklime | Minimum dilution water; maximum solids transport | Approaches the pumpable limit of milk of lime; settles fast if agitation lost |
Above 30 % w/w “milk of lime” behaves as a paste rather than a slurry. Yield stress climbs sharply, peristaltic pumps stall, and a single hour of failed agitation leaves a hard cake at the tank bottom that requires a high-pressure water jet (and sometimes a chipping hammer) to recover. 30 % is the practical upper limit for continuous operation; some batch processes go higher but only with active heating and continuous high-shear mixing.
Pipe-loop design: any slurry pipe run longer than 5 m wants a recirculation loop back to the make-up tank so flow keeps moving when downstream dosing is paused. Without a loop, the line settles between dose events and blocks within hours.
Hydrated lime Ca(OH)2 is supplied as a fine dry powder. It disperses readily in water to form a stable milk of lime up to ~30 % w/w. Quicklime CaO, by contrast, reacts exothermically with water (~1,160 kJ per kg CaO) to form Ca(OH)2 in situ. That reaction is fast and energetic; a dedicated slaker is needed (see next section).
Watch for — grit. Quicklime is supplied with a residual fraction of inert grit and unburned limestone. The grit settles in any low-velocity zone of the slurry circuit. Always include a grit trap at the slaker outlet, or accept a more frequent tank clean-out schedule.
Related pages: progressive cavity pump · diaphragm pump · chemical dosing pump · chemical dosing hub
Operator Workload & Redundancy Model
Lime can be prepared as a slurry in either of two regimes. Batch — make up a tank, age it, dose from it. Continuous — meter dry lime and dilution water in real time at a slaker or in-line mixer. The choice cascades through tank sizing, operator workload, redundancy, and the response time of the dosing loop.
| Element | Typical value | Notes |
|---|---|---|
| Make-up tank volume | 1–10 m³ | Sized for 4–24 hours of dosing demand at design rate |
| Number of tanks | 2 (duty / standby) | One tank being prepared while the other dispenses |
| Agitator type | Top-entry propeller or marine impeller | 50–200 rpm; sized for full off-bottom suspension |
| Preparation time | 15–60 minutes | Includes dry-lime addition, water make-up, mix-and-age |
| Best for | Small / medium duties (< 500 kg/hr) | Operator-friendly; minimal capital for slaker |
Small footprint, simple operator interface, no slaker capital cost, easy to switch to a different lime grade or chemistry without recommissioning. The default choice for industrial pre-treatment, small WWTP, and pilot trials.
| Element | Typical value | Notes |
|---|---|---|
| Inline slaker (quicklime) | Paste, ball-mill or detention slaker | Paste slaker most common for < 2,000 kg/hr; ball mill above |
| Reaction tank residence | 10–30 minutes | Allows full hydration of CaO to Ca(OH)2 |
| Slaking temperature | 80–95 °C | Exothermic reaction generates ~1,160 kJ/kg CaO |
| Water-to-lime ratio | 2.5:1 to 6:1 by weight | Higher ratio → faster heat dissipation, lower reaction completion |
| Best for | Bulk duties (> 500 kg/hr) | Removes batch-tank inventory; matches steady-state demand |
Bulk dosing duties where batch make-up tanks would exceed 10 m³, or where the cost-per-kg of quicklime delivered as bulk powder is materially lower than the equivalent hydrated-lime supply contract. Justified above 500–1,000 kg/hr depending on lime scope differential.
| Aspect | Batch (hydrated lime) | Continuous (quicklime + slaker) |
|---|---|---|
| Lime feedstock | Ca(OH)2 — hydrated, ready to disperse | CaO — quicklime, requires slaking |
| Cost per kg as Ca(OH)2 | Higher (vendor has slaked it) | Lower (you slake it on site) |
| Capital cost | Low — tanks + mixer + feeder | Higher — add slaker + reaction tank + grit handling |
| Operator workload | Periodic batch make-up | Continuous monitoring of slaker |
| Safety | Powder handling, dust generation | Powder handling + hot reaction tank + steam vent |
| Inventory | 1–10 m³ ready-to-use slurry | Minimal in-process inventory |
| Crossover scale | 500–1,000 kg/hr — above which continuous becomes efficient | |
Steam venting from slakers: the exothermic reaction releases significant steam. A vent fan, condenser and weak-acid neutralising scrubber on the slaker vent are mandatory for indoor installations. We provide these as part of the slaker package, never as an afterthought.
Related pages: polymer preparation station · chemical dosing systems · pH correction · rapid mixer
Bulk Powder Handling & Delivery Cadence
Bulk lime is supplied as a fine dry powder. The storage silo size dictates the delivery cadence (tanker every fortnight vs every month vs every quarter), the bulk-handling civil cost, and the dust-control infrastructure that has to surround it. Get the silo wrong and either deliveries become unmanageable, or capital is locked up in unused storage.
Silo volume = 1.5 × (average daily consumption) × (delivery interval in days). The 1.5 multiplier is the safety factor for delayed deliveries and demand spikes. Example: a plant using 800 kg/hr at 12 hr/day = 9.6 t/day. At 25-day delivery interval that is 240 t lime ≈ 360 m³ bulk volume. Round up to 400 m³ — which is two 200 m³ silos in this case for redundancy.
| Component | Function | Notes |
|---|---|---|
| Cylindrical body | Bulk storage | Carbon steel painted or food-grade epoxy for drinking-water duty |
| Cone hopper | Mass-flow discharge | Minimum 60° wall angle for lime; steeper for sticky grades |
| Aeration boards | Fluidise the lime at the cone walls | Compressed air at 0.5–1.0 bar; prevents bridging |
| Vibrating discharger | Backup to break stubborn bridges | Electromechanical bin vibrator on cone exterior |
| Bin venting filter | Dust collection during pneumatic fill | Reverse-pulse jet bag filter, ~25 m²/m³ silo volume |
| Pressure/vacuum relief | Prevent over-pressurisation during fill, vacuum on discharge | Mandatory — over-pressurisation has destroyed silos with weak roof seams |
| High & low level sensors | Tanker call-out and feeder low-level interlock | Rotating-paddle, capacitance, or radar |
| Sampling port | Quality verification of incoming deliveries | Manual sampling at delivery into a pre-labelled sample bag |
| Earthing | Dissipate static from pneumatic fill | Tanker bonded to silo before fill; silo earthed to grade |
Silo over-pressurisation is the single most common cause of catastrophic silo failure. A tanker over-pressurises the silo if the bin vent is plugged or the bag-filter pulse-jet has failed. Roof seams burst, dust cloud surrounds the site, occasionally with a personnel fatality. Mandatory protections: PRV (pressure relief valve), differential-pressure transmitter on the bag filter with low-DP alarm, written procedure that the tanker driver must verify bag-filter pulse-jet operation before connecting.
For small duties under 100 kg/hr, an IBC (intermediate bulk container, ~1 t) or 25 kg bag stations may be a better fit than a fixed silo — lower Capital expenditure, no civil works, easier site mobility. Trade-off is more frequent ordering, manual handling effort, and dust control during transfer. IBC stations also work well for low-frequency / standby duties (emergency dosing during plant upset events).
Related pages: storage tanks overview · chemical tanks · lime dosing equipment · chemical dosing systems
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