A method of operating a crystallizing vessel assembly , said vessel assembly comprising: - a crystallizing vessel , said vessel comprising a top wall , a bottom wall and an upright wall extending from the bottom wall to the top wall, said top wall, bottom wall and upright wall defining a lumen , - a superstructure on top of the vessel, said superstructure comprising a bearing , said bearing comprising an upper race member, a lower race member and roller bodies between said upper race member and said lower race member, said lower race member being attached to the superstructure, - a rotor , said rotor comprising: - an upright, rotatable shaft , said shaft comprising i) a top shaft section extending above the top wall and ii) a bottom shaft section (131") in said lumen, said top shaft section being provided with said upper race member, allowing the rotor to rotate along a shaft axis of the upright, rotatable shaft in line with the axis of rotation of the bearing; and - a plurality of rotor arms comprising - arms attached with proximal ends thereof to said bottom shaft section, and - scrapers attached at the distal ends of said arms ; wherein according to the method crystals are grown on the inside of the upright wall and the rotor is rotated to scrape said crystals off said upright wall; characterized in that a plurality of arms of the rotor arms are hollow arms, each arm of the plurality of arms comprising an inlet opening that is relatively close to the shaft and an outlet opening that is relatively far from the shaft. - a crystallizing vessel , said vessel comprising a top wall , a bottom wall and an upright wall extending from the bottom wall to the top wall, said top wall, bottom wall and upright wall defining a lumen , - a superstructure on top of the vessel, said superstructure comprising a bearing , said bearing comprising an upper race member, a lower race member and roller bodies between said upper race member and said lower race member, said lower race member being attached to the superstructure, - a rotor , said rotor comprising: - an upright, rotatable shaft , said shaft comprising i) a top shaft section extending above the top wall and ii) a bottom shaft section (131") in said lumen, said top shaft section being provided with said upper race member, allowing the rotor to rotate along a shaft axis of the upright, rotatable shaft in line with the axis of rotation of the bearing; and - a plurality of rotor arms comprising - arms attached with proximal ends thereof to said bottom shaft section, and - scrapers attached at the distal ends of said arms ; wherein according to the method crystals are grown on the inside of the upright wall and the rotor is rotated to scrape said crystals off said upright wall; characterized in that a plurality of arms of the rotor arms are hollow arms, each arm of the plurality of arms comprising an inlet opening that is relatively close to the shaft and an outlet opening that is relatively far from the shaft. - an upright, rotatable shaft , said shaft comprising i) a top shaft section extending above the top wall and ii) a bottom shaft section (131") in said lumen, said top shaft section being provided with said upper race member, allowing the rotor to rotate along a shaft axis of the upright, rotatable shaft in line with the axis of rotation of the bearing; and - a plurality of rotor arms comprising - arms attached with proximal ends thereof to said bottom shaft section, and - scrapers attached at the distal ends of said arms ; wherein according to the method crystals are grown on the inside of the upright wall and the rotor is rotated to scrape said crystals off said upright wall; characterized in that a plurality of arms of the rotor arms are hollow arms, each arm of the plurality of arms comprising an inlet opening that is relatively close to the shaft and an outlet opening that is relatively far from the shaft. <EOS>