Lynne Wong's PhD thesis

clarified juice, increased sucrose losses in filter cake and viscosity of massecuites which
are difficult to exhaust (Anon., 1977).
Field soil entering the factory reduces the overall plant capacity and efficiency, increases
the cost per ton cane crushed and results in extensive wear to knives, mill rolls and
conveyors; poor calorific value of bagasse, poor settling in clarifiers with a high mud
volume and large volumes of filter cake and high sucrose losses in filter cake and molasses
(Smits and Blunt, 1976; Muller et al., 1982; Kent et al., 1999).
The trend in the decline of cane quality delivered to the mills has resulted in an intolerable
amount of extraneous material especially soil which, if left unremoved, causes increased
maintenance and replacement costs due to heavy wear and tear of factory equipment and
more or less serious difficulties of processing. Cane washing experiments effected by
Vignes (1980) showed that process difficulties could be minimized, however, a certain
amount of sugar was lost in the process.
In South Africa, a measure of soil in cane was introduced in mid-1970 (Lionnet and
Wagener, 1976; Brokensha and Mellet, 1977). It involved the ashing of a prepared cane
sample in a furnace. As it had been found that ash content in clean cane averaged 0.6%,
this value would be deducted from the ash content found in the prepared cane to give the
soil content in cane. Industrial values reported were 1.51% in 1991/92, 1.74% in 1993/94
(Lionnet, 1992a, 1994), and 1.67% in 2004 ranging from 1.17 to 3.08 for the eleven
factories which supplied the data (Anon., 2005b).
In Australia, Atherton et al. (1992) reported that the natural gamma-ray technique can be
used to monitor the soil content of sugar cane. Subsequently, a soil monitor based on
natural radioactivity has been developed for the on-stream monitoring of the soil content of
sugar cane on a moving conveyor belt (Mathew et al., 1994). It consists of a gamma-ray
detector and associated electronics, a belt-weigher and a personal computer. The soil
content of cane is computed from the average gamma-ray activity of a rake, the specific
gamma-ray activity of the soil and the conveyor load by using a calibration equation.
Subsequently, a commercial soil monitor was developed to measure soil in cane to within
1% (Olson et al., 1999). The predictions of soil in cane in one factory from 1994 to 1998
were: 1.7, 1.6, 1.8, 2.1 and 2.0%, respectively.
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