Cullet and glass batch: abrasive, precise, dusty

A glass furnace is fed by a recipe, not by a material. The batch house weighs several raw materials to tight tolerances, mixes them, wets them and delivers them to the furnace in a state that has to be the same every hour. Most are abrasive; one of them cakes in humid air; the sand carries a respirable dust hazard; and the recycled glass that makes up a growing share of the charge arrives sharp, variable and contaminated. What follows is how each component handles, where the batch house wears and segregates, and what a cullet line has to do.

What goes into the batch, and how does each part handle?

The behaviours in the table are directions, not design values. Bulk density, moisture, particle size and flow behaviour vary between sources and grades, and a batch house is sized on the materials it actually receives, measured, rather than on the ones the recipe names.

How does a batch house work?

Reception

Bulk road or rail delivery, unloaded pneumatically or mechanically into silos; sand may arrive damp and need drying before it will weigh and flow consistently.

Storage

One silo or bin per component. Sand and limestone flow; soda ash and fine additives need hoppers drawn for cohesive powders — see the pillar note on bridging and ratholing.

Weighing

Each component is fed into a weigh hopper on load cells — one cumulative hopper or a scale per material — to a tolerance the glass chemistry sets. Feeder cut-off, tare drift and residue in the hopper are the accuracy questions.

Mixing

A batch mixer homogenises the weighed components; too short leaves streaks in the glass, too long segregates and makes dust.

Wetting

A small percentage of water is added to bind the fines, limit dust and reduce segregation between the mixer and the furnace.

Charging

The wet batch, with cullet mixed in or added alongside, travels by belt or elevator to the furnace hoppers and is fed by a batch charger at the doghouse.

What wears, what segregates, what dusts?

Three questions decide most of the equipment. Wear: sand and cullet cut steel, so every chute, hopper cone, screw and elevator boot is lined and belt covers are chosen for abrasion; a liner is a wear part with a replacement route, not a finish. Segregation: the batch is a blend of materials with different sizes and densities, and every drop, every pile and every centrifugal elevator head separates them a little — which is why continuous-discharge elevators are the usual choice for mixed batch (the note on discharge types explains why), why drop heights are kept low, and why the pillar note on transfer point design applies with unusual force. Dust: silica dust is a health matter, soda ash dust a corrosion and housekeeping matter, and both are made at open transfers.

• Belt covers and cleaners — damp sand builds a layer on the belt that ordinary cleaners smear; the note on carry-back and belt cleaners sets out the choice.
• Enclosed transfers with extraction — for any sand or soda ash transfer, with the hood where the dust is made rather than where it is convenient.
• Wetted batch — the water is the dust control between mixer and furnace; a wet batch dropped from height still segregates, but it does not dust.

What is different about a cullet line?

Cullet is glass, but it is not batch: it arrives as a mixed, dirty, sharp solid, and the line's job is to turn it into a clean, sized, dry material before it meets the batch. Internal cullet from the plant's own rejects is predictable; external, post-consumer cullet is not.

• Crushing to a size the charger and the furnace accept, in a machine that tolerates the contaminants not yet removed.
• Contaminant removal — magnets for ferrous metal, eddy-current separation for non-ferrous, and optical or mechanical sorting for ceramics, stones and porcelain, which do not melt and end up as defects in the glass.
• Screening to remove fines and organic debris, which carry moisture and burn in the furnace.
• Storage and reclaim in bunkers or silos that tolerate a sharp, abrasive material of variable moisture; dry cullet flows, wet cullet with fines does not.
• Conveying by belt or vibrating feeder rather than screw wherever possible; shards wedge between flight and trough.

What to check on an existing batch house

• Scale calibration and tare drift on every weigh hopper, and residue on the hopper walls after discharge.
• Feeder cut-off — how much material is in flight when the feeder stops, and whether the control allows for it.
• Soda ash silo — bridging record, moisture ingress, condition of the cone and feeder; whether the vent filter keeps the silo dry.
• Mixer — liner and paddle wear, residue build-up, mixing time against what the glass quality data shows.
• Batch water — metering and distribution; dry streaks in the batch mean dust downstream.
• Liners — thickness at the impact points of chutes and hopper cones; belt cover wear pattern and cleaner condition.
• Silica dust controls — hood positions, extraction, enclosure integrity and housekeeping, against the plant's exposure assessment.
• Cullet contaminants — what reaches the furnace hoppers; magnet cleaning routine; screen condition; wear in the crusher.

Much of a batch house is standard handling equipment working to unusual tolerances, and the difference lies in the specification of liners, feeders, sealing and access. Our equipment, components and spares work covers the wear parts, feeders and transfers a batch house consumes; the glass duties in the Flow Finder list the equipment types involved.