10. THE COOLING AND DRYING OF SOAP

Once saponification has been completed, the soap obtained - crude or neat - has to be turned into marketable soap. This is the cooling and drying stage, which gives the end product the required consistency and hardness. Household soap generally contains 25% water (75% fatty acids) whereas toilet soap undergoes forced drying until it contains only 14% or even 12% water (86 to 88% fatty acids).

Crude soap produces a lower-quality, inexpensive soap. This is why it does not usually undergo any drying other than air drying, which is carried out over several days in cooling frames with detachable walls to facilitate frame removal.

Forced cooling and drying are generally used to finish neat soap. Two methods are widely used:

® cooling by passing the soap coming from the kettle or saponification reactor through a series of cooled cylinders; the solidified soap is then removed from the final cylinder in the form of chips which are dried in a chamber or a hot air tunnel;

® vacuum spray drying of the liquid soap in an expansion chamber. This alternative allows the liquid soap to be cooled and dried simultaneously until the desired fatty acid concentration has been obtained. Generally speaking, it is tending to supersede other drying methods in modern soap factories.

Forced cooling and drying radically reduce the production cycles. However, they are a costly stage which remains optional. Their relevance depends on the availability and cost of cooling water and energy, and on the possible effect of these costs on the selling prices, bearing in mind the type of soap being marketed and the target clientele.

Depending on the method chosen, toilet soaps require repeated drying in order to meet quality standards. In modern soap factories, household soap generally follows the same circuit as toilet soap, but the drying time is shortened.