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Affination - blue

Topics: Industry   Refining   Process   Affination  
By Bento, Luis San Miguel
Posted on 2006-09-18    Last edited on 2013-02-08

The first operation of cane sugar refining,  when using  raw sugar, of normal quality, is Affination. In this operation, the syrup layer covering raw sugar crystals is removed. This operation is carried out in two steps. Initially, affination magma, with approximately 92º brix, is produced by adding affination syrup (initially water) to raw sugar, in special minglers with slow agitation and heating to 45ºC. Contact time and temperature must be such that softening of the original syrup layer is achieved. To avoid sucrose inversion, lime may be added to the syrup in order to maitain a pH higher than 7,2 in the syrup separated during the subsequent centrifugation.

Esquema Afinacao 2008_1

Simplified scheme of Affination

In the second step, affination magma is centrifuged in batch high speed machines (in some refineries continuals centrifuges are used ; batch machines are more efficient on impurity separation point of vue). The batchs centrifuges have a basket with a stainless steel screen, rotating around a vertical axis. Due to the "centrifugal force", magma fed into the basket is distributed upwards being pressed to the screen. Syrup pass through the screen holes, and crystals remain on the screen. After syrup separation, sugar crystals are washed with hot water injected through nozzles distributed vertically along the basket. The wash water is mixed with the syrup, forming the affination syrup. After washing, centrifugation continues to remove excess of water from the crystals. After a period at high speed, centrifugal slow down and a discharge blade (plough) is dislocated along the screen, vertically from top to bottom, dislocating the sugar that is discharged out of the basket bottom. This sugar (affined sugar) is melted with water, or sweet water, in vessels provided with agitation and heating systems using steam at 1kg/cm2. The heating temperature must not exceed 85ºC and contact time must be of 20 minutes or less to avoid sugar destruction and colour formation. The resulting sugar solution (affination liquor) must have a slight alkaline pH, between 7.2 and 8.0, and a solids concentration between 62º and 68º brix. Before next step, Clarification, affined liquor is screened to remove solid particles.

Dissolved solids (brix) in affination liquor vary according to the clarification system and equipment used and with the qualtity of raw sugar. Filtration is a crucial step to decide brix value. The objective is to achive a liquor with high solids content (brix) as possible. Some raw sugars, with high polysaccharides, are difficult to filter and liquor brix must be lowered. Alternatively enzymes to hydrolyze polysaccharides can be used.

Affination syrup, resulting from magma centrifugation, is stored and heated to around 76ºC, before being mixed with raw sugar to form affination magma. Excess of affination syrups is treated in the Recovery section.

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