(57) In a mixer operating under vacuum for the preparation of a mixture for the production
of blocks of stone material, for permitting the addition of additives, in particular
colouring agents, during the mixing cycle without having to restore the atmospheric
pressure in the mixture and then to add the additive and thereafter to provide again
vacuum, there is mounted on the vacuum-tight cover (18) of the mixer at least one
metering device (20), which is preferably also maintained under the same vacuum, comprising
a hopper (30,32) which is closed at the bottom by an element or cylinder (42) which
has a plurality of cavities (50) suitable for being filled with the additive or colouring
agent contained in the hopper (30,32). The plurality of cavities (50) are arranged
alternately with angular offsetting of 180° so that, by rotating the cylinder (42)
through 180°, the cavities (50) filled with the additive are caused to be discharged
into the mixing chamber (10) of the mixer without communication being established
between the mixing chamber (10), which is under vacuum, and the outside, which is
under atmospheric pressure.
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