Patent Publication Number: US-6700918-B2

Title: Electric furnace for the production of metal oxides

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an electric furnace for the production of metal oxides and, in particular, to the production of lead oxides with high standards of quality of the product and of the process, affording considerable advantages over the prior art. 
     SUMMARY OF THE INVENTION 
     The main objects of the present invention are to achieve a high quality of the final product and great flexibility with regard to the quality of the raw material, combined with easy management of the furnace, with a reliable and precise control system, and with a reduction in problems in the working environment and in the environment outside the plant. Moreover, according to the present invention, it is possible to achieve considerable advantages over currently known furnaces, such as: low investment and installation cost, low energy cost per ton of product produced, low maintenance cost, much quicker start-up, and a reduction in residues produced during start-up. 
     To achieve its objects, the present invention provides for a tubular furnace with a tubular treatment chamber made of stainless refractory steel (AISI-310) and provided with a plurality of heating zones with individual control probes, in particular, three heating zones, each of which comprises six resistors connected so as to achieve a balance in electrical consumption between phases, each heating zone having its optimal adjustment point in dependence on the quality of the raw material, on its physical and chemical characteristics, and on the physical and chemical characteristics of the finished product to be produced. 
     The electrical control panel comprises controllers of consumption per phase and of voltage between phases, and can warn of any electrical problem with the resistors. 
     The heating control system permits adjustment of the electrical consumption of the furnace at any moment in dependence on the state of the furnace, as well as the use of full power during start-up and, during the normal process, consumption controlled by the temperatures of the chamber. 
     The construction of the furnace enables great functional flexibility to be achieved therein in order to adapt to the quality of the raw material used, for which purpose energy consumption is adjusted according to the quality of the raw material. 
     The raw material introduced into the tubular chamber is urged by an agitator shaft towards the end of the furnace body at which the finished product is discharged by a rotary valve. The agitator shaft rotates by virtue of the driving action of a geared motor unit controlled by a frequency meter from the control panel of the furnace. The agitator shaft is of mixed construction, with the use of AISI-310 stainless steel in the portion which is in contact with the product and AISI-304 for the rest. It is constituted by a central tubular element with a square cross-section, constituted by welded plates and a set of vanes welded to the central tube on its respective faces and arranged in a manner such as together to constitute a helical screw. One of the characteristics of the agitator shaft of the furnace of the present invention is that it has, in its outer portions, percussion devices or “hammers” with balls inside them which, by virtue of the rotation of the agitator shaft, cause impacts and vibration inside the shaft, preventing the product from adhering to the shaft. 
     The quality of the final product is adjusted by control of the time spent by the product inside the furnace and also by the output or final production flow, for which purpose the present invention provides for a frequency variator which controls the rate of rotation of the agitator shaft and also the rate of rotation of the helical screw which feeds raw material to the furnace. According to the invention it will also be possible to install two separate speed variators, one for the agitator shaft and the other for the worm screw for feeding the raw material to the furnace itself. The furnace as a whole is thermally insulated by refractory bricks with a temperature classification of up to 1,260° C., the bricks additionally providing the necessary support for the electrical resistors. The insulation is completed by high-density ceramic fibre (128 kg/m 3 ) with a temperature classification of up to 1,260° C. 
     To compensate for the large expansions brought about in the tubular element of the furnace, according to the present invention, movable elements are provided which allow the tube to lengthen freely. In particular, a free extension system is provided, which takes up the expansions of the tubular chamber and of the displacement screw resulting from the temperature difference produced inside the heating chamber. 
     The furnace is controlled by a centralized panel which comprises the elements necessary for the control of temperature in the chamber of the furnace and alarm elements for providing warnings when the values of the control parameters depart from the range of values provided for during the production process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding, some drawings of a preferred embodiment of the furnace of the present invention are appended by way of non-limiting example. 
     FIG. 1 is a partially-sectioned, side elevational view of the furnace of the present invention. 
     FIG. 2 is an elevational view taken from one end of the furnace. 
     FIG. 3 shows a detail in a section taken in the section plane indicated. 
     FIG. 4 show, in cross-section, a detail of the mounting of the heating resistors. 
     FIGS. 5 and 6 show respective details of the main screw of the furnace. 
     FIG. 7 is a schematic side elevational view of the entire main screw. 
     FIG. 8 shows a detail of one end of the main screw. 
     FIGS. 9,  10  and  11  show respective details of the furnace in cross-sections taken in the section planes indicated. 
    
    
     DESCRIPTION OF THE INVENTION 
     According to the embodiment shown in the drawings, the furnace of the present invention comprises a main body  1  of tubular structure, of which the central element is constituted by the tubular chamber  2  of the furnace mounted in the furnace body, which is insulated by means of refractory bricks  3  and insulation layers based on high-density ceramic fibre, indicated in the upper portion by the numeral  4 . 
     The furnace is heated by radiation, by panels of electrical resistors incorporated in the body of the furnace, as can be seen in the detail of FIG. 4 in which the furnace body  1  is mounted on a support frame composed principally of a variable number of upright posts  5  and  6  for fixing the furnace to the floor, in the interior, it is possible to see the mounting of the lateral panels of heating resistors  7  and  8  which are incorporated in the insulating refractory material composed of the refractory bricks  3  having a temperature classification of up to 1,260° C. A plurality of heating zones, preferably three zones, are disposed along the furnace, each zone having six resistors distributed longitudinally. 
     Each of the three heating zones will be provided with two K-type temperature probes for the control of the heating resistors. 
     Each of the heating zones can utilize its optimal adjustment point in dependence on the quality of the raw material, on the physical and chemical characteristics thereof, and on the physical and chemical characteristics of the final product to be produced. 
     The input of raw material will take place through a feed pipe  9 , shown in broken outline, provided with an helical screw for forcing the material to enter the chamber  2  through the duct  10 . There is a simultaneous intake of air  11 . The output of the product, after it has been treated as it travels along the tubular chamber  2 , will take place at the opposite end through a gravity outlet  12 , into a rotary collector  13  driven by an independent geared motor unit  14 . At the actual outlet of the furnace chamber, there will be an air-inlet  15  and optional air extraction  16 , both being controlled by corresponding valves. 
     The raw material is moved along the main chamber  2  of the furnace by an internal screw of special construction, indicated  17  in FIGS. 1 and 5 to  11 . The screw is rotated by a geared motor unit  18  and a transmission system  19  with belts, chains, or the like. 
     The screw  17  is constituted by a square, tubular central element, as can be seen in FIG. 6, in which it is possible to see the screw  17 , which is constituted by a square central tubular element  20  with faces  21 ,  22 ,  23  and  24  on which sets of aligned vanes  25 ,  25 ′, - 25 ″ . . . are fixed, the vanes being arranged at suitable inclinations so as together to form a helical screw for moving the raw material along the tubular chamber of the furnace. 
     Respective percussion devices or “hammers”, indicated  26  and  27  in FIG. 1, are incorporated in the ends of the agitator shaft  17 . Each of these elements is constituted by a central tubular body, FIG. 2, and respective short end extensions  27  and  28  of predetermined inclination, there being disposed inside the elements, some free masses such as spheres  29  and  30 , which can be moved along the tubular elements upon rotation of the screw, giving rise to impacts which prevent adhesion or compaction of the material on the screw. 
     The central tubular element  20  forming the drive screw has ends to which the drive and percussion members can be coupled by keying as can be seen in FIG.  8  and the following drawings. As can be seen, a tubular extension  31  is coupled to the square tubular element  20  by means of an internal coupling sleeve  32  which is coupled with the two tubular elements enabling the end  21  to have openings  33  of suitable shape and arrangement for the coupling of the drive members of the furnace.