Where soapstock comes from
Crude oil contains free fatty acids, phospholipids and pigments. In alkali neutralisation, the oil is treated with an alkaline solution: the alkali binds the free fatty acids into soaps, and impurities and some of the neutral oil settle out with them. This sediment is soapstock — the name comes from soap stock, a base material for soap.
What it consists of
- fatty acid soaps;
- neutral oil carried over from the oil being refined;
- water — up to half of the mass;
- phospholipids, pigments and other impurities.
Soapstock has a paste-like consistency, and its colour ranges from light to dark brown. It is alkaline, with a pH of about 9–10.
Key parameters
| Parameter | What it shows | Typical value |
|---|---|---|
| Total fat content | how much fatty matter can be recovered — this drives the price | from 40% |
| Moisture | the more water, the less useful substance per tonne | up to 50% |
| Non-fat impurities | ballast that will have to be separated | up to 3% |
| pH | residual alkali after refining | 9–10 |
When comparing offers, calculate the price per tonne of total fat, not per tonne of soapstock: a lot with 50% fat can turn out better value than a cheaper lot with 40%.
Where it is used
- Fatty acids. Soapstock is split with sulphuric acid to produce fatty acids — this is the main processing route.
- Soap making. Raw material for laundry soap.
- Biofuel. A source of fat for biodiesel.
- Lubricants and feed fats — after processing.
Transport and storage
Soapstock is shipped in bulk in rail tank cars and tank trucks. It thickens in the cold, so in winter it needs tank cars with heating or a steam jacket, and it has to be heated on site before unloading. Store it in closed tanks: exposed to air, soapstock oxidises and loses quality.