Centrifugation

The objective is division. Crystals and mother liquor have entirely different destinations — one is the factory’s product, the other is either boiled again for more sugar or sold as molasses — and as long as they remain mixed neither can go anywhere. Centrifugation separates them, and does so quickly enough to keep pace with a continuously operating factory, which gravity settling could not.

The principle combines two mechanisms. The dominant one is size exclusion: the basket wall is a screen, and the crystals are larger than its openings while the liquor is not, so the liquor passes and the crystals are retained. Centrifugal force supplies the driving pressure that makes this happen fast, and it is very much larger than gravity could provide, which is why a viscous liquor can be driven through a bed of crystals at all. The separation is therefore a filtration accelerated by centrifugal force rather than a density separation as such.

Separation alone leaves each crystal coated in a film of the liquor it grew in, and that film carries the colour and the impurities the crystal itself excluded. Removing some of that film — washing the crystal bed while it spins — raises the purity of the sugar leaving. But every bit of washing dissolves a little of the crystal surface and returns that sugar to the liquor, so purity is bought by putting sugar back into the stream it just came out of. That trade is a genuine one and it is settled by the specification the sugar is sold against.

Raw sugar

The retained crystals, discharged still carrying a residual film of mother liquor. The factory’s primary product, sold to refineries as a traded intermediate.

Molasses

The mother liquor driven through the screen. Where it still holds recoverable sugar it returns to the boiling house; where it does not, it leaves as final molasses — a co-product sold into feed and fermentation.

The distinction between liquor that goes back and liquor that leaves is worth drawing, because the same equipment produces both. A factory boils and centrifuges in successive stages: liquor separated from one stage is boiled again to crystallise more sugar, and centrifuged again in turn. Only when a stage’s liquor can no longer economically yield sugar does it leave the factory as final molasses. So molasses is not simply what the centrifugal throws off — it is what the last centrifugal throws off, after the factory has taken every crystallisation it is worth taking.

Neither output is waste and neither is incidental. Raw sugar is the reason the factory exists; molasses is a commodity with buyers in animal feed and in fermentation, where its sugar content is exactly what is wanted.

Centrifugation determines the purity and colour of the sugar leaving, through how completely the mother-liquor film is removed. It cannot improve on the crystals it receives: sugar that crystallised poorly separates poorly, and no amount of spinning fixes an ill-formed crystal. Its influence is over the film, not the solid.

The losses are characteristic and they pull against each other. Crystals fine enough to pass the screen go into the liquor and are lost from the sugar stream — recovered if that liquor is reboiled, gone if it is final molasses. Washing to raise purity dissolves crystal surface and sends that sugar the same way. And liquor left behind on inadequately purged sugar carries impurity forward that the refinery must then remove. Every one of these is a trade rather than a defect.

• Fine crystals passing the screen into the liquor
• Crystal surface dissolved by washing, returning sugar to the liquor
• Sucrose leaving in final molasses once further recovery is uneconomic
• Residual liquor film on under-purged sugar, carrying colour and impurity to the refinery
• Spillage and equipment holdup

Compared with the boiling house alongside it, centrifugation is a modest consumer — its demand is electrical, for spinning, rather than thermal. In a cane factory that electricity is largely generated on site from bagasse. Its water use is confined to washing, and the wash water does not leave as effluent: it dissolves sugar and returns with the liquor into the process, so the water carries value back rather than away.