A method of treating process water of a flotation plant for the recovery of a valuable material, the flotation plant comprising a mineral flotation line, the mineral flotation line comprising - a grinding mill; - a classification circuit for classifying a feed of ground ore from the grinding mill into classifier overflow and classifier underflow; and - a mineral flotation circuit for treating classifier overflow as infeed of ore particles comprising valuable material suspended in slurry, the flotation circuit comprising a rougher part for the separation of slurry infeed into rougher overflow of recovered valuable material and rougher underflow of reject, and a cleaner part arranged to receive rougher overflow from the rougher part as slurry infeed, for the separation of slurry into cleaner overflow of recovered valuable material and cleaner underflow arranged to flow back into the rougher part as slurry infeed, the flotation plant further comprising a process water circuit for treating underflow and/or overflow of the mineral flotation line, the process water circuit comprising a gravitational solid-liquid separator for dewatering underflow and/or overflow of the mineral flotation line to separate sediment from supernatant comprising at least water and unrecovered fine particles comprising valuable material; and a recover water tank for collecting process water comprising overflow and/or underflow from the mineral flotation line, characterized in that, prior to leading supernatant from the gravitational solid-liquid separator into the recover water tank, supernatant is subjected to cleaning flotation, in which at least 90 % of the flotation gas bubbles have a size from 0,2 to 250 μm, in a cleaning flotation unit for collecting at least unrecovered fine particles comprising valuable material; for separating fine particles comprising valuable material from the supernatant into cleaning flotation overflow as recovered valuable material; and for forming purified process water as cleaning flotation underflow; and in that purified process water is recirculated into the mineral flotation line, or collected into the recover water tank as collected process water. - a grinding mill; - a classification circuit for classifying a feed of ground ore from the grinding mill into classifier overflow and classifier underflow; and - a mineral flotation circuit for treating classifier overflow as infeed of ore particles comprising valuable material suspended in slurry, the flotation circuit comprising a rougher part for the separation of slurry infeed into rougher overflow of recovered valuable material and rougher underflow of reject, and a cleaner part arranged to receive rougher overflow from the rougher part as slurry infeed, for the separation of slurry into cleaner overflow of recovered valuable material and cleaner underflow arranged to flow back into the rougher part as slurry infeed, the flotation plant further comprising a process water circuit for treating underflow and/or overflow of the mineral flotation line, the process water circuit comprising a gravitational solid-liquid separator for dewatering underflow and/or overflow of the mineral flotation line to separate sediment from supernatant comprising at least water and unrecovered fine particles comprising valuable material; and a recover water tank for collecting process water comprising overflow and/or underflow from the mineral flotation line, characterized in that, prior to leading supernatant from the gravitational solid-liquid separator into the recover water tank, supernatant is subjected to cleaning flotation, in which at least 90 % of the flotation gas bubbles have a size from 0,2 to 250 μm, in a cleaning flotation unit for collecting at least unrecovered fine particles comprising valuable material; for separating fine particles comprising valuable material from the supernatant into cleaning flotation overflow as recovered valuable material; and for forming purified process water as cleaning flotation underflow; and in that purified process water is recirculated into the mineral flotation line, or collected into the recover water tank as collected process water. the flotation plant further comprising a process water circuit for treating underflow and/or overflow of the mineral flotation line, the process water circuit comprising a gravitational solid-liquid separator for dewatering underflow and/or overflow of the mineral flotation line to separate sediment from supernatant comprising at least water and unrecovered fine particles comprising valuable material; and a recover water tank for collecting process water comprising overflow and/or underflow from the mineral flotation line, characterized in that, prior to leading supernatant from the gravitational solid-liquid separator into the recover water tank, supernatant is subjected to cleaning flotation, in which at least 90 % of the flotation gas bubbles have a size from 0,2 to 250 μm, in a cleaning flotation unit for collecting at least unrecovered fine particles comprising valuable material; for separating fine particles comprising valuable material from the supernatant into cleaning flotation overflow as recovered valuable material; and for forming purified process water as cleaning flotation underflow; and in that purified process water is recirculated into the mineral flotation line, or collected into the recover water tank as collected process water.