Regenerator for transforming a solidificate containing Ca 2, CaCO 3, Ca 2 + MgCO 3 and a rest into calcium oxide, comprising: a rotary kiln for roasting the solidificate and thereby separating steam and the gaseous contents from the remaining content and obtaining calcium oxide, the rotary kiln having an entry for the solidificate, an exit for the calcium oxide obtained in the rotary kiln, and an exit for hot air containing the steam and the gaseous contents; a motor for the rotary kiln; a burner for providing the rotary kiln with the roasting temperature, the burner comprising an entry for hot air from a cyclone, an entry for fuel, an entry for fresh air, an entry connected to the exit for calcium oxide of the rotary kiln, and an exit for the calcium oxide; a cyclone having an entry, which is in flow connection with the exit for hot air containing the steam and gaseous contents of the rotary kiln, the steam and the gaseous contents going through the cyclone into a filtering unit to be cleansed, an ash collector for collecting the ash left in the cyclone, a first exit for hot air, which is in flow connection with the entry of for hot air of the burner, a second exit for hot air, which is in flow connection with an entry for hot air of a heat exchanger, a heat exchanger which is in flow connection with the second exit for hot air of the cyclone. a rotary kiln for roasting the solidificate and thereby separating steam and the gaseous contents from the remaining content and obtaining calcium oxide, the rotary kiln having an entry for the solidificate, an exit for the calcium oxide obtained in the rotary kiln, and an exit for hot air containing the steam and the gaseous contents; an entry for the solidificate, an exit for the calcium oxide obtained in the rotary kiln, and an exit for hot air containing the steam and the gaseous contents; a motor for the rotary kiln; a burner for providing the rotary kiln with the roasting temperature, the burner comprising an entry for hot air from a cyclone, an entry for fuel, an entry for fresh air, an entry connected to the exit for calcium oxide of the rotary kiln, and an exit for the calcium oxide; an entry for hot air from a cyclone, an entry for fuel, an entry for fresh air, an entry connected to the exit for calcium oxide of the rotary kiln, and an exit for the calcium oxide; a cyclone having an entry, which is in flow connection with the exit for hot air containing the steam and gaseous contents of the rotary kiln, the steam and the gaseous contents going through the cyclone into a filtering unit to be cleansed, an ash collector for collecting the ash left in the cyclone, a first exit for hot air, which is in flow connection with the entry of for hot air of the burner, a second exit for hot air, which is in flow connection with an entry for hot air of a heat exchanger, an entry, which is in flow connection with the exit for hot air containing the steam and gaseous contents of the rotary kiln, the steam and the gaseous contents going through the cyclone into a filtering unit to be cleansed, an ash collector for collecting the ash left in the cyclone, a first exit for hot air, which is in flow connection with the entry of for hot air of the burner, a second exit for hot air, which is in flow connection with an entry for hot air of a heat exchanger, a heat exchanger which is in flow connection with the second exit for hot air of the cyclone.