Page 599 - Materials Chemistry, Second Edition

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570 Waste Management Practices: Municipal, Hazardous, and Industrial
first subjected to filtering, heating, and settling to remove water and large solids (Bryant, 1989;
Arner, 1992). A vacuum is then created within a column of used oil, which strips out organic con-
taminants. The oil is subsequently treated with hydrogen, which bonds with certain contaminants
that subsequently settle. Finally, the heavier lubricating oil is separated from the lighter fuel oil.
With the re-refining process the production of hazardous waste is minimized and residuals can be
burned as a fuel or used to produce asphalt (EERC, 1988).
Re-refining possesses several advantages. First, unlike other recycling options, re-refining
allows used oil to be reused over and over again. In addition, re-refining used oil into lubricating oil
is simpler and less expensive than refining crude oil into lubricating oil. It saves energy; approxi-
mately 50 to 85% less energy is needed to re-refine used oil than to refine virgin oil into lubricat-
ing oil (Byrne et al., 1989). A disadvantage of re-refining is that it is more complicated than other
recycling options. Another disadvantage is that only a few re-refiners are currently in operation in
the United States. As a result, costs of transporting used oil to a re-refiner may be substantial.

19.8.2 SLIPSTREAMING
With slipstreaming, small amounts of used oil (approximately 1% of the feed) are mixed with crude
oil and introduced into the standard refining process (Arner, 1992). The used oil does not require
pretreatment before it is mixed with the crude oil because the refining process removes contami-
nants that might impair the quality of the final product.
The major advantage of slipstreaming is that the heat or lubrication value of the used oil is real-
ized without complicated processing methods. Slipstreamed used oil could serve as a base for sev-
eral petroleum products including fuel oil, gasoline, and lubricating oil. In addition, slipstreaming
poses no greater environmental risk than refining crude oil. Used oil channeled into the refining
process after distillation or catalytic cracking is exempt from the Used Oil Management Standards
provided that it is on-specification used oil (see below)(U.S. EPA, 1994).

19.8.3 PROCESSING

Processing involves treating used oil to improve its fuel characteristics. Relevant contaminants that
must be removed include water, sediment, and ash (e.g., metals). Once removed, the quality of the
used oil is similar to that of virgin fuel oil (Mueller, 1989; Arner, 1992). Physical treatment meth-
ods such as settling, filtering, and centrifuging are used to remove water or solid contaminants (e.g.,
metallic fragments from engine wear) (Figure 19.4). During settling, used oil is retained in large
tanks for relatively long periods. Heavy contaminants eventually sink and light contaminants rise to
the top. Filtering screens out solids and captures lighter particles that do not settle. During cen-
trifugation at high speeds, the oil is separated from substances with different densities such as water
and solids.
Processing also includes chemical treatment of used oil to remove contaminants and improve
burning characteristics. For example, simple reagents can be added to neutralize acids. During pro-
cessing, used oil can be blended with crude oil to adjust its viscosity or improve its fuel quality.
The major advantage of processing is that it improves the burning quality of used oil.
Processing can also allow off-specification used oil to be upgraded to on-specification used oil so
that it can be burned by a greater number of facilities (see the next section). Processing used oil is
a widespread industry in the United States. Most facilities are small operations that service local
markets (Nolan et al., 1990). Processing is therefore a more readily available recycling option for
used oil generators than is re-refining.

19.8.4 DIRECT BURNING
Direct burning is as the name implies, i.e., burning used oil without prior processing to remove
water, solids, or other contaminants.