Process and apparatus for changing the wetting agent of pigments

A process for changing the wetting agent of pigment solids from a first agent that is an aqueous phase to a second agent that is an oily phase, said process comprising the steps of:

• introducing said solids and said first and second agents into an extruder at a predetermined volume rate, said extruder including an extruder barrel having a supply input and a discharge output, at least one screw which has screw threads over its length, and helical spaces between adjacent screw threads, said at least one screw being arranged within said extruder barrel to extend from said supply inlet to said discharge outlet,
• providing at least one lateral opening in said extruder barrel between said supply inlet and said discharge outlet;
• providing a portion of said screw downstream of said at least one lateral opening with at least one counter-thread so as to seal said helical spaces,
• rotating said at least one screw at a predetermined speed;
• selecting said predetermined volume rate and said predetermined speed so as to fill only part of each of said helical spaces over the length of said screw from said supply inlet to said lateral outlet with solids and said first and second agents to segregate said first agent as to allow said first agent to escape through said at least one lateral outlet.