The soap that bobz mentions is of very low quality and is mainly soaps of shorter chain fatty acids. The soap manufacturers I worked for had good “green” credentials even though it was several decades before the green movement became de rigueur or even known in manufacturing.
That being the case and not wishing to contaminate the environment with tonnes of waste, the nigre waste was treated with ferric chloride to form short chain ferric soaps which separated as an unpleasant orange coloured, sticky, smelly paste. This stuff had a relatively high value in being sold to steel manufacturers as an additive to the mix in making high quality specialist industrial steels.
The residual water after this process comprised sodium chloride and a small amount of glycerin which was recycled into the brine used in the soap making.