That's the "vis de reprise" bit, anyway. When you have piles of things like sand, gravel, or possibly grain, "reprise" refers to taking the material from the bottom (and centre) by means of a conveyor belt (in a tunnel beneath the material), and is known as "reclaim".

My guess is that the "boîtard" is the "tunnel" at the bottom of the silo, leading from the outside edge to the centre of the silo where the grain will be scooped into the screw conveyor. (low confidence on this one, pending verification)

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Note added at 3 hrs (2007-08-09 12:34:53 GMT)
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SCREW CONVEYOR HOPPER

Seeing that these silos have external screw conveyors, I conclude that the "boitard" is not a tunnel inside the silo but a hopper at the bottom that feeds the conveyor.

Boitard, n. m. (Moulins) Pièce en bois ou en fonte scellée dans l'oeillard* de la meule dormante*.
Oeillard, n. m. (Moulins) Ouverture centrale de la meule*.
http://www.rempart.com/?IDINFO=15319

So it does indeed appear to be the system for evacuating the flour from the millstone (in this case) and consequently the grain from the silo in your case.

Pictures of (small) silos with and without "boitard de reprise par vis":
http://images.google.com/imgres?imgurl=http://www.beiser-se....

SCREW CONVEYOR HOPPER Options - S. Howes
S. Howes offers several standard inlet hopper styles for screw conveyors. Hoppers are available in various capacities ranging from stub inlets which easily mate to other equipment for continuous feed, to large holding capacities. All hoppers come standard with bolted or lift-out safety grating, and/or custom covers with customer-specific inlets. For more difficult materials, hoppers can be outfitted with agitators to aid material flow. Agitators are available in spiked, twin spiked, and spiral ribbon styles. Heating or cooling jackets are also available. All hoppers are built to customer specifications and requirements.
http://www.powderandbulk.com/videos/view_video.php?viewkey=4...
You can even watch the video


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Note added at 3 hrs (2007-08-09 12:39:42 GMT)
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Reclaimer
A reclaim belt or RECLAIM HOPPER is a part of the
coal handling equipment to transport coal from the
strange pile into the plant

It is loaded into a RECLAIM HOPPER by endloaders and emptied onto a conveyor .... The Standards Association of Australia in a document entitled "Glossary of ...
law.ato.gov.au/atolaw/view.htm?docid='JUD/35FCR371/00003'

Premcor shall propose measures to control coke dust when loading the RECLAIM HOPPER within ...
www.dnrec.state.de.us/DNREC2000/premcor/Phase II Technical ...

From the RECLAIM HOPPER, a belt conveys the material past a magnetic ...
www.allbusiness.com/furniture-related/office-furniture-incl...

for the coal pile RECLAIM HOPPER, belt feeder and transfer to Conveyor CC03. All of this equipment is located underground, ...
www.usda.gov/rus/water/ees/pdf/SME FEIS Vol II.pdf

transported to a feeder where it was metered into the RECLAIM HOPPER. ...
www.osti.gov/bridge/servlets/purl/828064-qvVX0W/native/8280...