The separation of contaminants from the scouring liquor can be described by Stoke’s law.

           d² (Δρ).ng
V = ———————
                18μ

Where

V = rate of separation
d = diameter of particle
Δρ = difference in density between particle and scour liquor
g = acceleration due to gravity
n = multiplier determined by recovery device (n=1 for gravity separation)
μ = viscosity of scour liquor

The equation shows that the rate of separation will be increased by

• increasing the size of a particle
• increasing the difference in density between the particle and the scour liquor
• increasing the centrifugal force (n)
• reducing the distance that the particle has to travel before it is collected
• reducing the viscosity of the liquor

There are three main differences between the separation of dirt and wool wax.

First, dirt removal is a solid/liquid separation whereas wool wax removal is a liquid/liquid separation.

Second, the densities of the dirt and wool wax are respectively higher than and lower than that of the scour liquor.

Third, the density difference between the scour liquor and the dirt is considerably higher than the difference between the wool wax and the scour liquor.

Wool wax undergoes autoxidation while on the fleece and in storage. Wool wax removed from the fleece can be separated into two fractions, the unoxidised component from the base of the wool fibre and the oxidised component from the tip of the fibre. The unoxidised fraction is generally easy to recover using centrifugal techniques whereas the oxidised fraction forms very stable emulsions which are not recoverable in this way. The oxidised wool wax interacts with water, detergent and the other fibre contaminants to form complex particles having a density very similar to the scouring liquor.

This makes it very difficult to completely clarify a scouring liquor (it can be done in the laboratory using ultracentrifuges where n = 18,000 for 30 minutes).

Industrial recovery efficiencies generally noted will be in the region of 30% to 50% of the total wax on the greasy fibre.

A number of different devices are used in wool scouring for contaminant recovery.