146 Refining Used Lubricating Oils
re-rening process, (2) the vacuum distillation/clay process, (3) the vacuum distilla-
tion/hydrotreating processes (hydroprocessing), and (4) the solvent extraction/distil-
lation processes.
The acid/clay process has a long operational history and it is appropriate for a
wide range of circumstances and is thus readily operable in most countries (Table
6.1). However, a number of studies made on ranking of re-rening by-product waste
streams in terms of environmental hazards suggest that the acid/clay process is the
least environmentally sound of the four main re-rening processes. The principal
reason for this is the large quantity of by-product acid tar produced, which presents
difculties in disposal. It is, ...