Screw Ship Unloaders for Cement and Fly Ash Handling Efficiency at Ports

Two figures decide whether a cement or fly ash berth pays for itself: the rated capacity of the unloader and the deadweight of the ships it must serve. Hangzhou Aotuo Mechanical and Electrical Co., Ltd. (AOTUO) builds screw ship unloaders rated to 3,000 t/h for vessels up to 200,000 DWT, and the company drafted JC/T 2575, the Chinese industry standard for bulk cement screw unloaders. Everything between those numbers is a question of flowability, dust and berth time.

Where Berth Productivity Is Actually Lost

Berth productivity is lost to three things long before it is lost to rated capacity: powder that will not flow steadily, dust that has to be contained, and minutes spent repositioning between hatch squares.

Cement and fly ash are powders rather than granules. Ordinary Portland cement is typically shipped at a bulk density of about 1.4 t/m3, while dry fly ash sits near 0.85 t/m3 and begins to behave like a liquid the moment a grab or an air stream aerates it. Powder that fluidises surges into any opening offered to it. Powder that compacts bridges over the same opening. Both reactions cost berth hours, and both are predictable from a cargo analysis instead of being discovered on the day of discharge.

Dust is the third constraint, and its cost is measurable rather than theoretical. A grab working an open hold is an open material system: the bucket releases cargo from height, air moves through the falling stream, and the finest fraction of the parcel leaves the quay as airborne dust. That fraction was mined, ground and shipped, and it never reaches the silo.

How a Screw Ship Unloader Moves Powdered Cargo

There is no pneumatic air circuit and no grab anywhere in the material path. The vertical screw is a positive-displacement conveyor: cement or ash enters at the open intake at the bottom of the casing, is lifted by the rotating screw, and cannot escape sideways because the casing encloses it along its full length. AOTUO has designed and manufactured these machines for more than 20 years, holds more than 100 patents, design registrations and software copyrights, and was among the first Chinese manufacturers to localise the bulk cement screw unloader.

The energy case follows from the same architecture. Published engineering work on mechanical ship unloaders records that screw-principle machines hold substantial energy advantages over other cement handling systems, because the drive motors move the material directly rather than using compressed air as an intermediate transport medium.

Throughput, Energy and Turnaround: Where the Gain Shows Up

The gain from screw unloading appears in three places: a continuous discharge curve instead of a cyclic one, a lower specific energy per tonne than pneumatic conveying, and no clean-up shift after the last hold.

The last row is where terminal budgets usually go wrong. Grab systems look inexpensive at the equipment line and expensive at the operating line, because cargo handled twice and every cubic metre of extracted dust carry a cost that repeats at every call.

Dust Control Is a Cargo-Recovery Metric

Cargo lost as dust is revenue lost, and the arithmetic is simple: on a 30,000 t parcel, a one percent release is 300 t of product that was shipped, unloaded and never received.

Enclosed screw conveying and negative-pressure collection solve the problem from both ends. Enclosure prevents dust from forming, because the material is never in free fall in open air. Collection at the belt transfer, the receiving hopper and the silo inlet captures what does form. AOTUO supplies dust collectors, air slides and belt conveyors as elements of the same system as the unloader, sized against the unloader's rated output rather than added afterwards.

Matching the Installation Type to Berth Geometry and Cargo

The screw geometry is largely shared across the range. The mounting and travel system decides which berths a machine can serve and how much of each hold it can reach.

The rail mobile cement screw unloader is the configuration most often specified for a dedicated cement quay, because it keeps the discharge point fixed over the receiving hopper while covering the full berth length.

ATXL-B / RMS-200-1500 Rail Mobile Screw Ship UnloaderRail-mounted screw ship unloader for dedicated cement quays, offering 200–1500 t/h capacity and enclosed conveying to shore systems.View Product →

Reference installations include the Port of Maoming and the Port of Huizhou in Guangdong Province, projects in Brisbane and Sydney in Australia, and the Port of Mariveles in the Philippines.

Fly Ash: The Cargo That Punishes Generic Equipment

Dry fly ash is roughly 40 percent lighter per cubic metre than ordinary Portland cement, and that single difference changes intake geometry, screw pitch, motor sizing and the handling rate the machine can hold.

A screw unloader handles fly ash well when the feed rate is controlled and the intake is designed for a light, aeratable powder. The failure mode to design against is over-aeration: if the ash is fluidised too aggressively at the intake, it fills the casing faster than the screw can clear it and the machine stalls. Moisture creates the opposite problem, because damp ash compacts and bridges over the intake.

AOTUO ATSL-T Tire-Type Multi-Material Screw Ship UnloaderTire-mounted screw unloader for dry bulk terminals, handling fly ash, clinker, coal and grain at 400–1,000 t/h without rail infrastructure.View Product →

What to Write Into the Unloader Specification

Six figures determine whether a screw unloader reaches its rated capacity in service rather than on paper.

Capacity commitments are verified in service rather than in a datasheet. AOTUO's 2 MTPA cement terminal project in the Philippines entered operation with unloader, conveying and dust control delivered as one system.

A terminal that sustains 1,000 t/h through sealed transfer points spends less per tonne on energy, dust extraction and clean-up than a terminal that reaches the same peak rate in grab cycles and pays the difference at every subsequent call.

Practical Questions from Terminal Operators