be treated can be maintained as further means for maintaining the temperature for operation when the operation of the apparatus is interrupted there is adopted a method in which the hot secondary dross from which molten zinc has been squeezed out is not discharged but left in the container and it is first discharged when the subsequent dross treatment is initiated according to a preferred example of the manner for attaching scraper plates 9 four arms 8 are fixed to the rotary shaft 7 so that they are spaced from one another by an angle of 90 u00b0 and two each 8 in total of the plates 9 are fitted to respective arms 8 by means of set boards 10 in order to enhance the squeezing effect it is preferred that each scraper plate 9 be inclined with respect to the arm by an angle of 45 u00b0 so that when the rotary shaft 7 is rotated in the clockwise direction in the direction indicated by an arrow in fig2 the scraper plate 9 functions to scrape up the dross along the inclined face of the conical bottom portion of the container and thus squeezes out metallic zinc further in order to collect the squeezed zinc conveniently it is preferred that the lower side of the scraper plate 9 be substantially in parallel to the inclined face of the bottom portion of the container as shown in fig1 the number of arms and the size and number of scraper plates attached to each arm are appropriately determined and.