  The ICARUS T600 detector, the largest liquid Argon Time Projection Chamber
(LAr-TPC) realized after many years of RD activities, was installed and
successfully operated for 3 years at the INFN Gran Sasso underground
Laboratory. One of the most important issues was the need of an extremely low
residual electronegative impurity content in the liquid Argon, in order to
transport the free electrons created by the ionizing particles with a very
small attenuation along the drift path. The solutions adopted for the Argon
re-circulation and purification systems have permitted to reach impressive
results in terms of Argon purity and a free electron lifetime exceeding 15 ms,
corresponding to about 20 parts per trillion of equivalent O2 contamination, a
milestone for any future project involving LAr-TPC's and the development of
higher detector mass scales.
