Fume treatment plant comprising: - a main fume treatment line which is provided with at least one filtering device and a stack to discharge the treated fumes into the environment; - a by-pass branch of said main fume treatment line which is positioned downstream of said at least one filtering device and is fluidically connectable to said main fume treatment line by means of an inlet by-pass valve and an outlet by-pass valve; - a system for abating mercury emissions, comprising a fixed bed of chemi-adsorbents fluidically inserted in said by-pass branch; - at least one mercury concentration analyzer (40, 41), which in use is suitable to detect the concentration of mercury in the fumes flowing through said main fume treatment line; - a control system which is connected to said at least one mercury concentration analyzer (40, 41) and to said inlet and outlet by-pass valves and is programmed to: - open said by-pass valves (21, 22) in order to let the fumes flow through said by-pass branch if the analyzer (40, 41) detects a mercury concentration in the fumes higher than a predetermined threshold value, and - close said by-pass valves (21, 22) so as to prevent the flow of fumes through said by-pass branch, if the analyzer (40, 41) detects a mercury concentration in the fumes lower than or equal to said predetermined threshold value, or if, after a peak concentration detection higher than said predetermined threshold value, the average mercury concentrations in the fumes may be estimated to fall within pre-established average emission limits on a predefined time basis, characterized in that it comprises a thermostatting system (26a, 29a; 26b, 29b) of said system for abating mercury emissions, wherein said thermostatting system (26a, 29a; 26b, 29b) is activatable by said control system to maintain the fixed bed of chemi-adsorbents within a predefined operating temperature range between 100°C and 250°C, at least when said by-pass valves (21, 22) are closed and the flow of fumes coming from the main line through said by-pass branch is prevented. - a main fume treatment line which is provided with at least one filtering device and a stack to discharge the treated fumes into the environment; - a by-pass branch of said main fume treatment line which is positioned downstream of said at least one filtering device and is fluidically connectable to said main fume treatment line by means of an inlet by-pass valve and an outlet by-pass valve; - a system for abating mercury emissions, comprising a fixed bed of chemi-adsorbents fluidically inserted in said by-pass branch; - at least one mercury concentration analyzer (40, 41), which in use is suitable to detect the concentration of mercury in the fumes flowing through said main fume treatment line; - a control system which is connected to said at least one mercury concentration analyzer (40, 41) and to said inlet and outlet by-pass valves and is programmed to: - open said by-pass valves (21, 22) in order to let the fumes flow through said by-pass branch if the analyzer (40, 41) detects a mercury concentration in the fumes higher than a predetermined threshold value, and - close said by-pass valves (21, 22) so as to prevent the flow of fumes through said by-pass branch, if the analyzer (40, 41) detects a mercury concentration in the fumes lower than or equal to said predetermined threshold value, or if, after a peak concentration detection higher than said predetermined threshold value, the average mercury concentrations in the fumes may be estimated to fall within pre-established average emission limits on a predefined time basis, - open said by-pass valves (21, 22) in order to let the fumes flow through said by-pass branch if the analyzer (40, 41) detects a mercury concentration in the fumes higher than a predetermined threshold value, and - close said by-pass valves (21, 22) so as to prevent the flow of fumes through said by-pass branch, if the analyzer (40, 41) detects a mercury concentration in the fumes lower than or equal to said predetermined threshold value, or if, after a peak concentration detection higher than said predetermined threshold value, the average mercury concentrations in the fumes may be estimated to fall within pre-established average emission limits on a predefined time basis,