extends from above through the lid 18 into the interior of agitator vessel 12 for attachment of an agitator and rotation of the agitator in a rotation direction as indicated in fig2 by arrow d the driveshaft 22 has a lower end which carries two struts 24 in opposite disposition for attachment to two sweep blades 26 which form part of the agitator and are thus located substantially in the lower portion 16 of the agitation vessel 12 persons skilled in the art will understand that the arrangement of two sweep blades is done by way of example only of course the agitator may certainly have more or less than two such sweep blades and agitators of such configuration should also be considered within the scope of the present invention as shown in fig1 the sweep blades 26 are arranged in slanting disposition within the agitation vessel 12 such as to extend from a lower inner zone to an outer upper zone relative to the horizontal at an angle u03b1 ranging from about 15 u00b0 to about 60 u00b0 in the following description the term u201c horizontal u201d will denote any direction that is perpendicular to a center axis of the driveshaft 22 while the term.