CRUDE TALL OIL RECOVERY
Tall oil is one of the largest source of fatty materials used in industrial processes in the
United States. Tall oil and tall oil derivatives are used in surface coatings, plasticizer,
adhesives, asphalt products, liquid soaps, emulsifiers in the manufacture of synthetic
rubber, paper sizing, and printing inks.
The soft woods containing the necessary rosin acids and fatty acids required for the
production of crude tall oil are primarily of the Pinus genus.
Tall oil fatty acids offer several distinctive advantages over alternate sources of fatty acids
derived from animal or vegetable oils. Tall oil acids contain few highly unsaturated acids
and thus possess better oxidative and color stability. Hence, products made from tall oil
acids can provide better color and viscosity stability than products based on acids derived
from animal or vegetable oils. Because tall oil is a by-product of the paper industry, the
price and supply are regarded by users as more stable than acids derived from
agricultural crops. Finally, tall oil acids are available in the free acid form, where as other
oils must be hydrolyzed to liberate free fatty acids.
Typical Tall Oil Analysis
The consumption and amount of recoverable tall oil will depend on several factors.
Variations in composition will occur depending on the time of year or growing cycle of the
tree, the particular species involved, aging of the wood, the amount of heartwood, pulping
conditions, and the efficiency of the system used to recover the tall oil. Chemically, tall
oil is a natural mixture of rosin acids related to abietic acid and of fatty acids related to
oleic acid, together with some non-acidic compounds.
Typical Composition of Crude Tall Oil
Specific Gravity 0.95 - 1.24
Acid Number 100 - 175
Saponification Number 120 - 180
Iodine Number 140 - 150
Fatty Acids, % 20 - 60
Rosin Acids, % 20 - 65
Flash Point, degrees F. 350 - 400
Recovery of Tall Oil Soaps from Sulfate Liquors
In the sulfate or kraft cooking process, the high alkalinity and temperature in the digester
produces soluble sodium soaps of lignin, rosin and fatty acids originally present as esters.
The soaps remain with the black liquor that is separated from the pulp during the washing
operation. When black liquor cools or its total solids content is increased by evaporation,
the soap rises to the surface because of its lower density and is rendered insoluble by
the presence of water-soluble salts. The rosinate is skimmed mechanically from the
surface of the 20 to 30% black liquor. The skimmed liquor is then returned to the next
evaporator effect. Removal of the soap at this intermediate black liquor solids
concentration improves evaporator operation in the later effects.
The recovered sodium rosinate or soap is allowed to settle in storage tanks, or in the
case of a continuous system, the soap is centrifuged, to remove as much of the entrained
liquor as possible. The choice of a batch or continuous system depends mainly on the
amount of tall oil available and the mode of operation desired by the mill.
The soap is acidified with sulfuric acid and heated to convert the rosinate to free rosin
and fatty acids. The crude tall oil is then separated from the sodium sulfate solution
formed during acidification from the lignin and returned to the black liquor system.
Normally, the crude tall oil is then shipped to a fractional distillation plant where the
various components are separated to obtain the maximum value of each fraction.
Process and Technology - Batch System
After pulping, the black liquor from the pulp washers contains the fatty acids and rosin
acids in the wood which have been converted to soap by reaction with the strong alkali.
The black liquor is pumped to the multiple effect evaporators for concentration. During
evaporation the soap separates as frothy curd, known as "skimmings". These skimmings
are usually taken off from the evaporators at the third and fourth stage where the black
liquor concentration reaches 20 to 30% total solids. A typical analysis of these skimmings
is as follows:
Typical Analysis of Black Liquor Skimmings
Density - lbs/gal 7 - 8.4
Moisture, % 30 - 42
Total Solids, % 58 - 70
Lignin & Pulp, % 0.8 - 25
Tall Oil, % 48 - 60
Total Alkali as NaOH, % 8 - 9.2
Free Alkali as NaOH, % 0.5 - 1.0
This material is acidulated to form tall oil and the resultant oil is recovered by gravity
settling. Approximately two pounds of soap will yield one pound of crude tall oil.
Flow diagram CL-003843 illustrates the flow of material in the process.
The soap skimmings are pumped to storage tanks (4). These tanks act as surge tanks
and also as settling tanks for removal of excess black liquor from the soap skimmings,
thus reducing the amount of acid required for acidulation. The black liquor is periodically
drawn off and pumped back to the evaporators by means of the black liquor return pump
(1). Soap pump (5) conveys the soap to the tall oil acidulation reactor (6) which is
equipped with an agitator (7). Sulfuric acid or chlorine dioxide system spent acid from
storage tank (9) is metered into the reactor by means of an acid pump (8). The mixture
is heated and agitated in the reactor to convert the sodium soap into free rosin acids and
fatty acids. The tall oil is decanted from the reactor by pump (10) to the wet tall oil
storage (11) then to dry tall oil storage (12). Tall oil storages (11 & 12) allow for further
removal of any remaining acidulation acid water.
Finally, the oil is removed from the system by the tall oil product pump (13). Returning
to the acidulation reactor (6), the acid water solution and accompanying precipitated lignin
"heel" are mixed with white liquor or caustic by means of an inline mixer (25) and
transferred to the lignin reactor (15) by means of the lignin transfer pump (14).
In the lignin reactor the acid waste is neutralized and the lignin heel re-solubilized and
returned to the black liquor system by the neutralized liquor pump (18). Since up to ten
percent of the tall oil can remain with the lignin heel, the lignin reactor also reclaims this
tall oil, which has been converted back to a soap. The soap is transferred back to soap
storage (4) by means of a soap pump (17).
During this reversion process, any solids formed (mainly calcium precipitates) are
removed from the system by a centrifugal cleaner (20).
White liquor or caustic from storage (21) is delivered to the reactor by the caustic pump
(22). Some caustic is diverted first through the vent scrubber (23). Gas fan (24) and
scrubber insures that any hydrogen sulfide formed in the acidulation reactor or lignin
reactor are vented to the scrubber and not emitted to the atmosphere.
The system described above is effluent free, the only product being tall oil. All other
streams are retained in the kraft recovery cycle.
Batch Crude Tall Oil Plant