The disclosure pertains to an electrochemical reactor (1, 13), in particular but not exclusively for vatting sulphur dye or vat dye as well as to methods of using such a reactor and to uses of such a reactor. The electrochemical reactor comprises at least one liquid compartment (3) in which a multitude of freely suspended granules (14) is enclosed, wherein at least one of the side walls (5) of the compartment is an electrode and an opposite side wall is formed by a separator element (6), typically a membrane (6), wherein there is provided a bottom inlet (7, 9) and a top outlet (8, 10) of the compartment for a liquid catholyte (9) or a liquid anolyte (7), wherein the inlet region as well as the outlet region of the compartment (3) are provided with an upper grid (11′) and a lower grid (11″), the width of the mesh and the positioning of which is chosen such as to allow the liquid catholyte (9) or the liquid anolyte (7) to pass through from bottom to top but to prevent the granules (14) to pass through the grids (11′,11″) to leave the compartment (3), and wherein the upstream flow of the liquid catholyte (9) or of the liquid anolyte (7) can be adjusted so that in operation the multitude of granules (14) is dragged against the upper grid (11′) while the lower grid (11″) is substantially not in contact with the granules (14).