Soapstock
What soapstock is and where it comes from
Soapstock is a product of one specific refining step: alkali neutralization. In chemical (alkali) refining, a controlled dose of caustic soda is stirred into the crude or degummed oil to neutralize its free fatty acids. The caustic and the free fatty acids react to form soaps (sodium salts of the fatty acids), which are heavier than the oil and insoluble in it. When the mixture is spun in a centrifuge, this soapy phase separates out and is drawn off — carrying with it not just the soaps but also some neutral oil it entrains, leftover gums, pigments and trace metals, and water. That separated, dense, dark, soapy stream is soapstock. It is, in effect, the concentrated "everything the neutralizing step removed," and its volume is one measure of the refining loss.
What soapstock contains
Soapstock is a complex mixture, not a single substance. Its main constituent is the soaps formed from the free fatty acids and caustic, but it also contains entrained neutral oil (oil that was carried out along with the soaps and represents a real yield loss), residual phosphatides and colour bodies pulled from the oil, some trace metals, and a significant amount of water. The exact make-up varies with the oil, its free-fatty-acid level and how the neutralization was run — a higher-FFA crude produces more soapstock. Because it holds entrained neutral oil, minimising soapstock (without under-neutralizing) is part of controlling refining loss: an over-strong caustic dose destroys more neutral oil into extra soapstock, which is why the dose is matched to the measured acid value of the feed oil.
Acidulation: turning soapstock into acid oil
Soapstock is rarely used as-is; it is usually processed further. The most common step is acidulation: treating the soapstock with a mineral acid (typically sulphuric acid) splits the soaps back into free fatty acids, releasing an oily layer known as "acid oil" (or acidulated soapstock) that separates from the acidic water. Acid oil is a marketable product — a high-free-fatty-acid oily material — while the acidic aqueous layer must be treated as effluent. This conversion turns an awkward, soapy, alkaline stream into a more stable, saleable oil, which is why acidulation plants are a common adjunct to chemical refineries. The economics of a refinery therefore include not just the finished oil but the value recovered from soapstock as acid oil, against the cost of acidulation and effluent treatment.
What soapstock and acid oil are used for
Soapstock and the acid oil made from it have real downstream uses. Acid oil is used in animal feed as an energy ingredient, in the oleochemical industry as a source of fatty acids for soaps, surfactants and other products, and has been explored as a feedstock for biodiesel and other technical applications. Historically soapstock also fed directly into soap manufacture, which is fitting given that the soaps in it are the very thing soap-making sets out to create. The point is that this by-product stream is recovered and monetised rather than discarded — a well-run chemical refinery treats soapstock as a co-product with value, which improves overall economics and reduces waste, provided the acidulation and effluent handling are done properly.
Soapstock, effluent and the physical-refining contrast
Soapstock is also an environmental and handling consideration. It is alkaline, oily and, after acidulation, generates an acidic effluent that must be neutralized and treated before discharge — so managing soapstock responsibly is part of running a chemical refinery cleanly. This is one of the reasons the industry weighs physical versus chemical refining. Physical refining removes the free fatty acids by steam stripping during deodorization rather than by caustic, so it produces no soapstock at all — instead the fatty acids leave as a deodorizer distillate. Physical refining thus avoids soapstock and its effluent, and can mean lower neutral-oil loss, which is attractive for suitable (well-degummed, higher-FFA) oils like palm. So the presence or absence of soapstock is one of the clearest practical differences between the two refining routes.
Quick facts
| Attribute | Value |
|---|---|
| Type | Refining by-product |
| From | Alkali (chemical) neutralization |
| Main content | Soaps + entrained oil, gums, colour, water |
| Processed to | Acid oil (by acidulation) |
| Uses | Animal feed, oleochemicals, soap, biodiesel feedstock |
| Physical refining | Produces none (FFA leaves as distillate instead) |
Related pages
NeutralizationPhysical vs chemical refiningCaustic sodaRefining process
Related terms
Oilseed cake · Residual oil · Miscella
FAQ
What is soapstock in oil refining?
Soapstock is the heavy by-product of alkali neutralization: when caustic soda removes free fatty acids from crude oil, it forms soaps that — together with some entrained oil, gums, colour and water — separate out as a dense phase. It is a recoverable stream, not simply waste.
What is soapstock used for?
Soapstock is usually acidulated into 'acid oil', which is used in animal feed, as a fatty-acid source in the oleochemical industry, in soap-making, and as a feedstock for biodiesel and other technical uses. Recovering value from it is part of a chemical refinery's economics.
What is the difference between soapstock and acid oil?
Soapstock is the raw, alkaline, soapy by-product from neutralization; acid oil is what you get after acidulating soapstock with a mineral acid, which splits the soaps back into free fatty acids and yields a more stable, saleable high-FFA oil.
Does physical refining produce soapstock?
No. Physical refining removes free fatty acids by steam stripping during deodorization rather than with caustic soda, so it produces no soapstock; the fatty acids leave instead as a deodorizer distillate. Only chemical (alkali) refining produces soapstock.