Alumina Conveying System Design: Alumina Storage & Transport, Lime Milk Preparation and Coal Handling

An alumina plant is not one conveying problem — it is three. Fine alumina powder must be received, stored and distributed without dusting or contamination; quicklime must be slaked into lime milk and dosed accurately into the process liquor circuit; and coal must be delivered to the gas station that fires the calcination kilns. Each stream has its own material behaviour, its own safety envelope and its own equipment logic. This article walks through the conveying scope Tianyi Machinery engineered and supplied for an alumina project exported to Indonesia — covering alumina storage and transport, lime milk preparation and coal handling for the gas station, with the en-masse (buried scraper) conveyors, bucket elevators and auxiliary equipment that tie them together.

1. Three Conveying Streams in One Alumina Plant

An alumina (aluminium oxide, Al₂O₃) plant combines three material-handling duties that serve very different parts of the process. Our scope on this Indonesian project covered all three:

• Alumina storage and transport (氧化铝储运): receiving product-grade alumina into silos and distributing it on to the downstream consumers, with dust-free, contamination-free handling throughout.
• Lime milk preparation (石灰乳制备): receiving and slaking quicklime to produce milk of lime — the alkaline reagent used in the process liquor circuit — including lime feeding, slaking and dosing.
• Coal handling for the gas station (燃气站输煤): receiving, sizing and metering coal into the gas producers that supply fuel gas to the calcination kilns.

All three duties draw on the same equipment families — en-masse chain conveyors, bucket elevators, screw conveyors, feeders, dust collectors and valves — but the selection parameters behind them differ completely. Treating them as one generic "conveying system" is the most common and most expensive mistake in an alumina plant.

2. Why Alumina, Lime and Coal Cannot Share One Design

Before choosing a single machine, compare the three materials side by side. The differences below drive every downstream decision — casing design, chain speed, sealing class, liner material and explosion rating.

Alumina is the most counter-intuitive of the three. It is strongly aeratable: once air is entrained, the powder behaves almost like a liquid and "flushes" through openings that a designer assumed were sealed. It is also friable — high-speed handling grinds the coarse fraction into fines, and excess fines reduce efficiency downstream. That is why alumina is conveyed at low chain speed in a fully enclosed casing, with no exposed mild-steel wear surfaces that could contaminate the product.

Quicklime is caustic, dusty and reacts vigorously and exothermically with water, so every metre of its conveying path must be both dust-tight and moisture-tight. Coal brings a different hazard again: its dust is combustible, so the conveying line in the gas station area must be gas-tight and explosion-protected rather than merely closed.

3. Process 1 — Alumina Storage and Transport

The alumina handling line receives product-grade powder, stores it in silos and distributes it to the consumers. The typical flow is:

Receiving point → vertical lift → silo storage → horizontal distribution → metered transfer to the consumer.

• Bucket elevators lift alumina from the ground-level receiving point to the top of the storage silo. They are the natural choice wherever the conveying path needs height rather than distance.
• En-masse (buried scraper) chain conveyors form the horizontal distribution backbone along the silo row. Because one machine can carry multiple discharge gates, a single conveyor can serve several silos and downstream consumers without a separate line for each.
• Screw conveyors handle the short, fully enclosed, metered transfers where the material only has to travel a few metres to the process feed point.
• Dust collectors are fitted to silo vents and every transfer point, and valves and level instruments complete the line.

4. Process 2 — Lime Milk Preparation

Lime milk — milk of lime, a Ca(OH)₂ suspension — is produced by slaking quicklime (CaO) with a controlled amount of water. The reaction is strongly exothermic, so the plant's job is not simply to mix lime and water but to feed both at exactly the rate the slaker can absorb. The conveying scope therefore sits upstream of the slaker:

Quicklime silo → enclosed feeding → slaker (controlled water addition) → dilution and temperature control → milk-of-lime storage → dosing pumps.

• Screw conveyor or en-masse conveyor feeds quicklime from the silo outlet to the slaker. Both are fully enclosed, which contains the caustic dust and stops atmospheric moisture from reaching the lime.
• Bucket elevators lift quicklime to the slaker feed hopper when the slaker is mounted at height.
• Bin vibrators, air cannons and correctly angled chutes prevent bridging — quicklime is cohesive and will arch over an outlet if the geometry is wrong.
• Dust collectors at the silo and at the slaker feed point keep the working area within exposure limits.

5. Process 3 — Coal Handling for the Gas Station

The gas station (coal gas producer plant) supplies fuel gas to the calcination kilns, and it is fed by its own coal handling line:

Coal yard receiving → crushing → screening → metered feeding to the gas producers.

• Belt conveyors (TD75 / DTII) receive and transfer coal at the yard, where capacity and distance are the governing factors.
• Crushers and screens reduce the coal to the size band the gas producers require and remove the fractions that would upset gasification.
• Gas-tight en-masse chain conveyors (TMSF series) transfer coal through the enclosed sections of the gas station, where combustible dust rules out any open conveyor.
• Bucket elevators lift coal to the gas producer feed bunkers.
• Weigh feeders meter coal into each gas producer so that gas output can be controlled.
• Dust collectors serve every transfer point, together with explosion venting, anti-static belts and an earthing scheme for the whole coal line.

6. Core Machine: The En-Masse (Buried Scraper) Conveyor

The en-masse chain conveyor — also called a buried scraper or drag chain conveyor — is the workhorse of all three streams on this project. A continuous chain carrying scraper flights runs inside a closed rectangular casing; the material is dragged forward "en masse" by the internal friction between particles rather than being carried on top of a moving surface.

That principle produces a combination of properties that no other single machine offers:

• Fully enclosed: nothing escapes to the atmosphere and nothing enters from it — essential for alumina purity, caustic lime dust and combustible coal dust alike.
• Multi-directional in one machine: horizontal, inclined and vertical sections can be combined into L, Z and U layouts, replacing several separate conveyors and transfer towers.
• Multiple inlets and outlets: ideal for distributing alumina along a silo row from a single line.
• Gentle, low-speed handling: the material moves as a coherent mass, which minimises attrition, degradation and dust generation.
• Compact footprint: the casing is small relative to the tonnage moved, which matters on a congested plant layout.

Tianyi builds the family in the standard MS (horizontal), MC (vertical) and MZ (combined) configurations, plus two special executions used where an alumina plant demands them:

When specifying one, confirm at minimum: required capacity in t/h, chain speed, casing cross-section, material of construction, wear-liner type, required degree of sealing, and whether explosion protection applies.

7. Core Machine: The Bucket Elevator

The bucket elevator handles every vertical move on the project — alumina into the silo, quicklime up to the slaker, and coal up to the gas producer bunkers. Tianyi supplies three drive and traction families, and the correct choice depends on duty, height and hazard:

• TDG (steel cord belt): the high-capacity, high-lift option, capable of lifts up to roughly 80 m. For combustible dust duties a gas-tight, explosion-proof execution with an anti-static belt is available — see the TDG gas-tight explosion-proof bucket elevator.
• NE (plate chain): suited to heavy, abrasive or hot duty where a belt would be vulnerable.
• TH (ring chain): a robust, cost-effective option for moderate duties.

For alumina, select a centrifugal-discharge design with well-spaced buckets and a smooth, lined casing so that the product is not crushed or abraded on the way up. For quicklime, prioritise moisture-tight covers and corrosion-resistant contact surfaces. For coal in the gas station, the elevator must be gas-tight and explosion-protected. Key selection data: capacity, lift height, bucket type and spacing, discharge method, traction element, and explosion rating.

8. Auxiliary Equipment That Makes the Line Work

No conveying system is complete without the components around it. On this project the auxiliary scope included:

• Dust collectors (pulse-jet bag filters): fitted at silo vents, transfer towers and the slaker feed point. Dust control is an environmental requirement and a product-recovery measure — captured alumina is returned to the process.
• Valves and dampers: rotary airlock valves, slide gates, flap dampers and diverter valves control flow, isolate sections for maintenance and provide the air seal at silo inlets and outlets.
• Feeders: screw feeders for lime and coal metering, weigh feeders where gravimetric accuracy is required.
• Steel silos and bins: with aeration pads for alumina, level gauges, bin vibrators or air cannons for lime, and pressure/vacuum relief valves for every powder silo.
• Spare parts: chains, scraper flights, buckets, wear liners, bearings and drive components, supplied as a first-fill package.

9. Equipment Selection Matrix by Process

10. Design Notes for a Tropical Island Site

An alumina project on an Indonesian site adds environmental and logistical constraints that a European or Chinese inland plant does not face. The following points shaped our design:

• High humidity and rainfall: seal every casing and use moisture-tight covers where lime is handled. Protect carbon steel with hot-dip galvanising or a suitable coating system, use stainless steel for wet-contact parts, and specify IP55 or better for motors and instruments with anti-condensation heaters.
• Wet, sticky coal: tropical coal arrives with high surface moisture and will bridge in chutes and bunkers. Use steeper chute angles than the laboratory angle of repose suggests, line the chutes with low-friction wear plate, and fit bin vibrators or air cannons.
• Caking alumina: keep the alumina circuit sealed and dry, avoid stagnant pockets, and provide aeration at silo outlets so the powder cannot consolidate.
• Island logistics: size and split the equipment into modules that fit standard containers, pre-assemble and match-mark in the factory, and ship with erection drawings so the site team can rebuild the line without re-engineering it.
• Spare-parts strategy: local sourcing is slow on an island site, so ship a first-fill of chains, scraper flights, buckets, wear liners and bearings with the original equipment.