Multiple hearth furnace improvements

A method for subjecting feed materials to contact with a flowing gas in a multiple hearth furnace, the multiple hearth furnace comprising:

• an external casing having an inner wall surface that is substantially cylindrical;
• a roof structure at an upper end of the external casing;
• a plurality of substantially circular hearths vertically spaced apart from each other within the external casing each hearth comprising an upper surface of a hearth structure and the uppermost hearth being vertically spaced from the roof structure so that directly above each hearth lies a gas space associated with that hearth and each hearth having one of a central drop hole or a plurality of peripherally located drop holes for both downward movement of feed material from that hearth and upward movement of gas into the gas space above that hearth;
• a central shaft assembly extending upwardly through the hearth structures and central drop holes and rotatable about an upright axis;
• and
• within the gas space above each hearth at least one rabble arm secured to the shaft assembly to rotate therewith and having rabbles adapted to move feed material on that hearth structure towards a central or peripheral drop hole in that hearth,
• the method comprising the steps of:
• feeding feed material onto at least one hearth of the multiple hearth furnace;
• passing the feed material over each hearth by means of the rabbles and downwardly from hearth to hearth by gravity via the drop hole or holes in the hearth structure of each hearth;
• simultaneously passing a gas upwardly through the multiple hearth furnace through the drop holes and through the gas spaces above those hearths;
• discharging the gas from the multiple hearth furnace through at least one gas outlet;
• and
• passing the gas entering the gas space directly above at least one specific hearth under an annular baffle before the gas leaves that gas space,
• wherein the annular baffle depends from the hearth structure or roof structure above the specific hearth into the gas space and has a lower edge located to clear the rabble arms associated with the specific hearth,
• and wherein either the annular baffle lies everywhere at a greater radius from the axis than the central drop hole through which the feed material leaves the specific hearth in the case where the specific hearth has a central drop hole or in the case where the specific hearth has peripheral drop holes lies everywhere at a lesser radius than the peripheral drop holes through which the feed material leaves the specific hearth.

A method for reducing mean gas residence times in a gas space directly above a hearth of a multiple hearth furnace, the multiple hearth furnace comprising:

• an external casing having an inner wall surface that is substantially cylindrical;
• a roof structure at an upper end of the external casing;
• a plurality of substantially circular hearths vertically spaced apart from each other within the external casing each hearth comprising an upper surface of a hearth structure and the uppermost hearth being vertically spaced from the roof structure so that directly above each hearth lies a gas space associated with that hearth and each hearth having one of a central drop hole or a plurality of peripherally located drop holes for both downward movement of feed material from that hearth and upward movement of gas into the gas space above that hearth;
• a rotatable central shaft assembly extending upwardly through the hearth structures and central drop holes;
• and
• within the gas space above each hearth at least one rabble arm secured to the shaft assembly to rotate therewith and having rabbles adapted to move feed material on that hearth structure towards a central or peripheral drop hole in that hearth,
• the method comprising the steps of:
• feeding feed material onto at least one hearth of the multiple hearth furnace;
• moving the feed material over each hearth by means of the rabbles and downwardly from hearth to hearth by gravity via the drop holes in the hearth structure of each hearth;
• simultaneously passing a gas upwardly through the multiple hearth furnace through the drop holes and through the gas spaces above those hearths;
• discharging the gas from the multiple hearth furnace through at least one gas outlet;
• and
• passing the gas entering the gas space directly above a specific hearth to flow under an annular baffle in that gas space before the gas leaves the gas space above the specific hearth,
• wherein the annular baffle depends from the hearth structure or roof structure above the specific hearth into the gas space and has a lower edge located to clear the rabble arms associated with the specific hearth,
• and wherein either the annular baffle lies everywhere at a greater radius from the axis than the central drop hole through which the feed material leaves the specific hearth in the case where the specific hearth has a central drop hole or in the case where the specific hearth has peripheral drop holes lies everywhere at a lesser radius than the peripheral drop holes through which the feed material leaves the specific hearth.