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Material

Hydrated Lime

Answer in brief

Hydrated lime is cohesive, light and caustic, and it rewards every design shortcut with a blocked outlet.

It bridges over small openings, coats walls, and reacts with carbon dioxide and moisture from the air it is stored with.

  • Give it a generously sized outlet and a mechanical discharge aid.
  • Keep storage air dry and the silo sealed.
  • Protect operators, because the dust is alkaline and damages eyes and skin.

Reviewed October 21, 2026 · Updated November 10, 2026

Handling challenges

Hydrated lime is cheap to buy and expensive to handle badly.

It is one of the most cohesive bulk powders in common industrial use, and plants that size its silo like a sand silo find out quickly.

Flow behavior

Low bulk density, fine particles and a plate-like shape give the powder high internal friction and a strong tendency to arch.

  • Outlet diameters derived from measured flow function, usually much larger than intuition suggests.
  • Mechanical discharge aids such as bin activators or screw dischargers rather than vibration alone.
  • Steep, smooth hopper walls with a liner selected from measured wall friction.

Air cannon and vibration retrofits are common, and they are usually evidence that the outlet was undersized from the start.

Carbonation and set

Calcium hydroxide reacts with carbon dioxide and water vapour, and the product is a hard deposit on silo walls and in the filter.

Sealed silos, dried venting air and a filter cleaning regime that actually removes the cake keep this from becoming a demolition job.

Feeding into a process

Flue gas treatment and water treatment both need a stable feed rate, and a cohesive powder makes that harder than the controller suggests.

Loss-in-weight feeding with a properly conditioned inlet, short residence time and flexible connections that do not transmit vibration gives repeatable dosing.

Operator protection

Alkaline dust causes eye and skin injury and irritates airways, so enclosure, local extraction and personal protection are specified together rather than one substituting for another.

What to establish before buying equipment

  • Flow function and wall friction measured on the delivered grade.
  • Bulk density range from aerated delivery to consolidated storage.
  • Expected storage time, which drives the carbonation case.
  • Required dosing accuracy and turndown at the process.

Storage and conditioning

Hydrated lime is fine, light, cohesive and chemically active, which makes it one of the harder mineral powders to store.

It reacts with carbon dioxide and moisture from the air, so an unsealed silo slowly converts the surface of the bed into a crust of carbonate.

  • Seal the vessel and filter the vent air; the reaction is driven by ordinary building air.
  • Size the outlet for the consolidated condition, which arrives quickly.
  • Expect wall build-up and design access for removing it.

What this means for equipment

Cohesion and the caustic nature of the product set the design.

A plain cone will bridge. Aerated or live-bottom discharge is normal, and the conveying air has to be dry for chemical reasons, not only to prevent caking.

  • Aerated discharge and a generous outlet.
  • Dry, filtered conveying air with a stated dew point.
  • Closed transfer: this dust is caustic and should not be in the room.

Safety and regulatory

Hydrated lime is strongly alkaline and that is the main hazard.

It burns skin and eyes, and the damage from an eye exposure develops over time rather than immediately. The dust is also fine enough to stay airborne for a long while.

  • Closed handling first, capture at the source second.
  • Eye protection as a hard rule wherever the material is open, with eyewash available.
  • Closed handling of filter dust, which is the finest and most reactive fraction.

Material behavior

Flowability
Poor, cohesive
Abrasiveness
Low to moderate
Cohesiveness
High; arches over small outlets
Moisture behavior
Reacts with moisture and carbon dioxide, forming hard deposits
Contamination risk
Caustic dust exposure for eyes, skin and airways

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Frequently asked questions

Why does hydrated lime crust in a silo?

It reacts with carbon dioxide and moisture from ordinary air. An unsealed vent lets the bed surface carbonate into a crust.

What makes the dust dangerous?

It is strongly alkaline. Skin and especially eye exposure cause burns that develop over time, so eye protection and eyewash are not optional.

Can it be conveyed with ordinary plant air?

Not reliably. The air needs a stated dew point, because moisture drives both the caking and the chemical reaction.

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