wherein the at least four electrolytic cells are provided in the form of at least two stacks of at least two electrolytic cells, said stacks are connected in parallel and allow the separation of at least one of the at least two stacks at a time for washing/regeneration of a cathode during a preparation of the dye suspension while catholyte solution is circulated through at least one of the at least two stacks,

wherein the electrolytic cells of each stack are connected in parallel and

wherein the electrolytic cells comprise a cathode compartment and an anode compartment separated by a separator, the separator comprising a semipermeable membrane, said cathode compartment comprises a multitude of freely suspended conductive, non-spherical granules in contact with a conductor forming at least the side wall opposite to the separator, said cathode compartment is provided with an upper grid and a lower grid, the width of the mesh and the positioning of said grids are chosen such that the liquid catholyte can pass through the upper grid and the lower grid but the granules are prevented from passing through the upper grid and the lower grid so that they are retained in the cathode compartment,

wherein the anode compartment has a wall opposite to the separator,

wherein said cathode compartment is connected to a main vessel via a catholyte supply pipe and a reduced catholyte return pipe for circulating catholyte through the cathode compartment, and said anode compartment is connected to an anolyte vessel via an anolyte supply pipe and an anolyte return pipe,

wherein said cathode compartment and said anode compartment are also provided with means for supplying acidic cleaning solutions and water and optionally basic solution to the cathode compartment and the anode compartment, said means comprise at least one cleaning medium supply pipe and at least one cleaning medium removal pipe at opposite ends of the cathode compartment and of the anode compartment for supplying and removing acidic cleaning solutions and water and optionally also a basic solution, said cleaning medium supply pipes and said cleaning medium removal pipes being either bypasses of the catholyte supply pipe and/or the anolyte supply pipe and/or the reduced catholyte return pipe and/or the anolyte return pipe, or independent pipes with own inlets and outlets adjacent to the ones of the catholyte supply pipe and the reduced catholyte return pipe, or the anolyte supply pipe and the anolyte return pipe, said at least one cleaning medium supply pipe is connected to at least one vessel for supplying acid and optionally base and is also connected to a water line supplying deionized water, and the at least one cleaning medium removal pipe is connected to a waste water treatment plant (WWTP) or to a waste water vessel.