Wool processing

Scouring

Raw wool shorn from sheep is washed in hot, soapy water to remove dirt, grease and other impurities such as sweat. The wool is scoured in a series of bowls of hot scouring liquor, followed by cold and hot rinses. The scouring water is normally about 65ºC, which is hot enough to dissolve the wax (i.e. lanolin), and detergent is added to help remove the dirt from the fibres and to emulsify the wax so it doesn’t stick back onto them.

As the wool enters each bowl, it is dunked under the surface to wet it thoroughly with the liquor in that bowl. A special set of harrows drags the wool through the liquor. When the wool reaches the other side of the bowl, it is lifted up into a pair of rollers that squeeze all the liquor out of it. The wool is then dropped into the next bowl, where the process is repeated.

The suint (i.e. sweat) dissolves quickly in the first bowl. The wax and dirt particles are steadily removed through a combination of mechanical agitation and gravity, and by the rollers between the bowls.

As the wool moves through the bowls it becomes cleaner and moves into cleaner liquor. Normally there are four hot scour bowls of detergent mixed with water.

The natural oil in the wool – the lanolin – is collected from the scouring liquor, refined, and used in soaps, face creams, ointments, shampoo and other products.

The next stage is to rinse the wool to remove the detergent and get rid of the remaining solids. The first rinse is normally done with copious quantities of cold water, followed by squeezing and a final rinse in very hot water.

Added chemical treatments

Chemical treatments are sometimes added to scour or rinse bowls:

• Sodium metabisulphite is sometimes used as a bleach to reduce the yellowness of average and poor coloured wools.
• Hydrogen peroxide can be used as a bleach to further brighten good coloured wools;
• Insect resistant (i.e. mothproofing) chemicals are sometimes added.
• Organic acids such as ascetic, formic or sulphamic acid can be added to adjust the pH of the wool to a pH7.
• A bacteriostat may be added to sanitise with wool when it is going to be used in mattresses, duvets or underlays.

Drying

Drying is a crucial part of scouring. Once the wool has been squeezed for the final time, it might still hold 50% water (also know as 50% regain). The scourer’s clients will normally want the wool dried to a precise figure, usually 16% or 17%.

The wool is dried by hot air in a large chamber. It is monitored by a sophisticated sensing system that ensures it is dried to exactly the right regain.

The Wool Research Organisation of New Zealand (WRONZ) has developed much of the scouring technology used in New Zealand, which is generally considered to be the best and most cost-effective in the world.