Multiple hearth furnace improvements

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

• a plurality of circular hearths vertically spaced apart from each other within a cylindrical external casing and the uppermost hearth being vertically spaced from a roof structure of the external casing so that directly above each hearth lies an associated gas space, each hearth being either an in hearth having for downward movement of feed material and upward movement of gas a central drop hole or an out hearth having for downward movement of feed material and upward movement of gas a plurality of peripherally located drop holes;
• a shaft assembly extending upwardly and centrally through the gas spaces and rotatable about an upright axis;
• and
• within each gas space rabble arms secured to the shaft assembly to rotate therewith and each having rabbles adapted to move feed material inwardly where the hearth associated with the gas space is an in hearth or outwardly where the hearth associated with the gas space is an out hearth,
• the method comprising the steps of:
• providing, in the gas space above a specific circular hearth of the plurality of circular hearths, an annular baffle that comprises a wall that is cylindrical and has an upright axis and that is secured to and extends downwardly from the roof structure or hearth above the specific circular hearth to a lower edge that, in operation of the furnace, is clear of the rabble arms associated with the specific circular hearth so as to divide the gas space above the lower edge into inner and outer annular spaces, the annular baffle surrounding the rotatable central shaft assembly and lying everywhere at a greater radius from the axis than the central drop hole where the specific hearth is an in hearth or everywhere at a lesser radius than the peripheral drop holes where the specific hearth is an out hearth;
• feeding feed material onto the uppermost hearth;
• passing the feed material over each hearth by means of the rabbles and downwardly from the uppermost hearth to successively lower hearths;
• simultaneously passing a gas upwardly through the furnace through the drop holes of each out hearth and through the drop hole of each in hearth and through the gas spaces directly above the hearths.

A method for modifying a multiple hearth furnace to increase gas residence times therein, the multiple hearth furnace comprising:

• a plurality of circular hearths vertically spaced apart from each other within a cylindrical external casing and the uppermost hearth being vertically spaced from a roof structure of the external casing so that directly above each hearth lies an associated gas space, each hearth being either an in hearth having for downward movement of feed material and upward movement of gas a central drop hole or an out hearth having for downward movement of feed material and upward movement of gas a plurality of peripherally located drop holes;
• a shaft assembly extending upwardly and centrally through the gas spaces and rotatable about an upright axis;
• and
• within each gas space rabble arms secured to the shaft assembly to rotate therewith and each having rabbles adapted to move feed material inwardly where the hearth associated with the gas space is an in hearth or outwardly where the hearth associated with the gas space is an out hearth,
• the method comprising the step of:
• providing, in the gas space above a specific circular hearth of the plurality of circular hearths, an annular baffle that comprising a wall that is cylindrical and has an upright axis and that is secured to and extends downwardly from the roof structure or hearth above the specific circular hearth to a lower edge that, in operation of the furnace, is clear of the rabble arms associated with the specific circular hearth so as to divide the gas space above the lower edge into inner and outer annular spaces, the annular baffle surrounding the rotatable central shaft assembly and lying everywhere at a greater radius from the axis than the central drop hole where the specific hearth is an in hearth or everywhere at a lesser radius than the peripheral drop holes where the specific hearth is an out hearth.