Tall oil, also calledliquid rosin ortallol, is a viscous yellow-black odorous liquid obtained as aby-product of thekraft process ofwood pulp manufacture when pulping mainlyconiferous trees.[1][2] The name originated as an anglicization of theSwedishtallolja ('pine oil').[3] Tall oil is the third largest chemical by-product in a kraft mill afterlignin andhemicellulose; the yield of crude tall oil from the process is in the range of 30–50 kg / ton pulp.[4] It may contribute to 1.0–1.5% of the mill's revenue if not used internally.[citation needed]
In the kraft process, highalkalinity and temperature convert the esters and carboxylic acids inrosin into soluble sodiumsoaps oflignin, rosin, andfatty acids. The spent cooking liquor is called weakblack liquor and is about 15% dry content. The black liquor is concentrated in amultiple effect evaporator and after the first stage the black liquor is about 20–30%. At this stage it is called intermediate liquor. Normally the soaps start tofloat in the storage tank for the weak or intermediate liquors and areskimmed off and collected. A good soap skimming operation reduces the soap content of the black liquor down to 0.2–0.4% w/w of the dry residue. The collected soap is calledraw rosin soap orrosinate. The raw rosin soap is then allowed tosettle or iscentrifuged to release as much as possible of the entrained black liquor. The soap goes then to theacidulator where it is heated and acidified withsulfuric acid to producecrude tall oil (CTO).[citation needed]
The soap skimming and acidulator operation can be improved by addition offlocculants. A flocculant will shorten the separation time and give a cleaner soap with lower viscosity. This makes the acidulator run more smoothly as well.[citation needed]
Most pines give a soap yield of 5–25 kg/ton pulp, whileScots pine gives 20–50 kg/ton. Scots pine grown in northern Scandinavia give a yield of even more than 50 kg/ton. Globally about 2 mill ton/year of CTO are refined.[2]
The composition of crude tall oil varies a great deal, depending on the type of wood used. A common quality measure for tall oil isacid number. With purepines it is possible to have acid numbers in the range 160–165, while mills using a mix ofsoftwoods andhardwoods might give acid numbers in the range of 125–135.[2]
Normally crude tall oil containsrosins, which containsresin acids (mainlyabietic acid and itsisomers),fatty acids (mainlypalmitic acid,oleic acid andlinoleic acid) andfatty alcohols,unsaponifiable sterols (5–10%), somesterols, and otheralkylhydrocarbon derivates.[3]
Fractional distillation of the crude tall oil produces[5]pitch,tall oil rosin with a high purity in abietic acid,Tall Oil Fatty Acid (TOFA) consisting of non-saturated acids (mainly oleic acid and linoleic acid), andDistilled Tall Oil (DTO) consisting of a mix of heavier fatty acids and abietic acid.
The tall oil rosin finds use as a component ofadhesives,rubbers, andinks, and as anemulsifier. The pitch is used as abinder in cement, an adhesive, and anemulsifier forasphalt.[6]
TOFA is a low-cost andvegan alternative totallow fatty acids for production ofsoaps andlubricants. When esterified withpentaerythritol, it is used as a compound ofadhesives and oil-basedvarnishes. When TOFA is dimerized todimer acid it is used for makingpolyamide andepoxy resin.
DTO is used for makingalkyd resins, which improve the hardness, drying time, and water resistance of paints and varnishes.
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