How Cane Sugar is Refined - Decolourisation
Granular activated carbon is the modern equivalent of "bone char", a carbon granule made from animal bones. Today's carbon is made by specially processing mineral carbon to give a granule which is highly active but also very robust: it can withstand the mechanical abrasion that results from transporting it around the plant.
The carbon is used in the process in very large columns, perhaps 10 or more metres high. The sugar liquor, at about 65% dry solids, is pumped through 2 columns in series. Because of limitations in distributing the liquor across the width of large columns it is quite normal to split the total liquor flow into three or more parallel streams, each of which passes through a pair of columns. The first column of the pair has been in use for some time while the second column is fresher. When the carbon in the first column reaches is practical limit of absorption, that column is switched out of line, the second column becomes the first column and a column with fresh carbon becomes the second column. In a typical refinery with say 3 streams of liquor, a column will come off line every three days so any one column has a life of 18 days of which 9 are hard working in the first column position.
Decolourisation with granular activated carbon typically achieves 90% effectiveness: a 1200 colour liquor entering the system will depart at about 120 colour.