Rotary turntable furnace for litharge production

A turntable furnace for heating particulate material and which is particularly suited for the industrial production of litharge. The particulate material is heated inside the furnace on a turntable hearth and distributed radially outwardly from the center of the hearth to a discharge opening by a pair of water cooled screw augers. The rotational speed of the turntable hearth and the screw augers may be independently controlled to vary the amount of mixing and blending of the particulate material and to vary the length of time the particulate material is subjected to heat.

BACKGROUND OF THE INVENTION 
The present invention relates generally to the field of industrial devices 
which are useful for heating particulate material and, more particularly, 
to such devices which are useful in the production of litharge. 
Litharge (lead monoxide) is supplied in industrial quantities for use in 
the pigment industry and in the leaded glass industry. Litharge is 
produced by oxidation of particulate lead into lead monoxide. Presently, 
there are two methods employed in the production of litharge. The first 
method uses a rake furnace to oxidize lead in a batch process. The lead or 
"leady oxide" (PbO+Pb) is heated and stirred by means of a rotating rake. 
The second method is a continuous production process which employs a 
horizontal drying kiln in which the lead is tumbled while being oxidized. 
Both the methods described above suffer certain disadvantages in use in 
that the particulate material does not get evenly heated, resulting in the 
undesired presence of lead and "red lead" in the final product. Moreover, 
in certain applications, such as the leaded glass industry, it is 
important that the litharge meet certain particle size distribution 
specifications. Since the input raw material does not meet such 
specifications, the final litharge product must be milled to meet the 
desired particle size specifications. While the mixing and blending action 
which takes place using the above described methods does serve to somewhat 
reduce the particle size distribution, considerable milling operations are 
still required. 
Each of the following references disclose various types of rotary furnaces 
for heating particulate material: U.S. Pat. No. 3,763,013 to Allred; U.S. 
Pat. No. 4,412,813 to Wulf; U.S. Pat. No. 4,449,924 to Ceretti; U.S. Pat. 
No. 1,208,248 to Wedge; and U.S. Pat. No. 1,064,516 to Miller. None of 
these references, however, disclose the use of rotating helical conveyors 
in a rotary furnace. 
SUMMARY OF THE INVENTION 
One embodiment of the present invention comprises a furnace having top and 
side refractory walls defining a furnace chamber. At least one of the 
refractory walls defines a feed inlet for feeding particulate matter into 
the chamber, a discharge outlet for discharging the particulate material 
outside the chamber, and a burner inlet communicating inside the chamber. 
A heating means is also provided associated with the burner inlet for 
heating the particulate matter within the chamber. There is further 
provided a turntable hearth rotatably disposed in the chamber on a first 
axis of rotation, a first drive means for rotatably driving the hearth, a 
distributing means, including at least one screw auger, for distributing 
the contents of the chamber in a direction radially outwardly from the 
axis of rotation of the hearth by screw movement, and a second drive means 
for driving the distributing means. 
It is an object of the present invention to provide an improved furnace for 
heating particulate material. 
It is a further object of the present invention to provide a furnace of 
improved design which is particularly useful for the industrial production 
of litharge. 
It is a yet further object of the present invention to provide an improved 
furnace for litharge production which allows for higher production volume 
rates, greater uniformity in chemical and physical characteristics of the 
litharge product, and requires less milling of the product to attain the 
desired particle size than previous techniques.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
For the purpose of promoting an understanding of the principles of the 
invention, reference will now be made to the embodiment illustrated in the 
drawings and specific language will be used to describe the same. It will 
nevertheless be understood that no limitation of the scope of the 
invention is thereby intended, such alterations and further modifications 
in the illustrated device, and such further applications of the principles 
of the invention as illustrated therein being contemplated as would 
normally occur to one skilled in the art to which the invention relates. 
Referring now to the drawings in detail, the furnace 10 has a cylindrical 
shape defined by side and top refractory walls 11 and 12, respectively, 
defining a chamber 13 in which particulate material is heated on a 
turntable hearth 18. At the top of furnace 10, a feed inlet 14 
communicates through wall 12 with chamber 13 for feeding particulate 
matter into the chamber. The particulate matter is continuously fed into 
chamber 13 at a desired uniform rate through rotary valve 14a. A burner 16 
communicates heat into the chamber through an inlet in wall 11 and is 
exhausted through exhaust outlet 15. A discharge outlet 17 is provided in 
the bottom of chamber 13 for discharging the particulate material outside 
the chamber 13 after heating. 
Turntable hearth 18 is rotatably disposed at the bottom of chamber 13 and 
rotates on the center axis of a vertically disposed drive shaft 19 which 
is aligned directly below the feed inlet 14. Drive shaft 19 is driven by a 
variable speed motor 20 mounted below hearth 18. Trunnion 21 provides a 
rolling support for the outer portions of hearth 18. 
A pair of screw augers 24 having opposing half pitches are rotatably 
mounted horizontally in chamber 13 above hearth 18. Each screw auger 24 is 
water-cooled through an inner longitudinal cavity extending along the 
respective axis of rotation. The cavities are connected externally of the 
chamber 13 by a conduit 25 to provide a single continuous flow path for 
water circulated through the screw augers 24. The augers 24 are 
independently driven by separate drive motors 26. 
Associated with each screw auger is a scraper 27 for cleaning the screw 
flights of particulate material which tends to gradually build up as the 
augers agitate, mix and convey the material deposited on hearth 18 
radially outwardly from the axis of rotation of hearth 18 to off the 
periphery of the hearth. Each scraper 27 includes a hollow water-cooled 
shaft 28 rotatably mounted at opposite ends of wall 11 and a plurality of 
spikes or teeth 29 corresponding to the number of screw flights in the 
associated auger 24. The spikes 29 are spaced apart along the length of 
and fixedly secured to the shaft 28. Each scraper is drivably connected to 
and rotates in synchrony with the associated screw auger 24 through a 
chain and sprocket linkage 30 driven by a common drive motor 26. 
Drive motor 20 and drive motors 26 include variable speed control means for 
variably controlling the rotational speed of the respective screw augers 
and hearth independently of one another. 
A plurality of scraper blades 31 are fixedly secured at angularly spaced 
apart positions on the underside of hearth 18 at the periphery thereof and 
serve to transport material deposited into annular channel 32 from hearth 
18 to discharge outlet 17 in the bottom of channel 32. A screw conveyor 35 
conveys the particulate material from discharge outlet 17 to a storage 
tank located externally of furnace 10. For the production of litharge, 
screw conveyor 35 is preferably water-cooled to permit the temperature of 
the litharge to be quickly lowered below the temperature range where red 
lead formation occurs. 
The operation of the furnace, as for example in the production of litharge 
in particulate form, may be generally described as follows. Lead or "leady 
oxide" (PbO+Pb) in particulate form is introduced into chamber 13 at a 
desired rate through rotary valve 14a. As the input material is deposited 
on the center of the turntable hearth, it is rotated on the hearth at a 
desired speed. Inside chamber 13, the material is heated to a temperature 
in the range of 650-1200 degrees F. depending upon the degree of product 
oxidation desired. While the material is being rotated on the hearth, 
screw augers 24 mix, tumble and distribute the material radially outwardly 
to the periphery of the hearth. 
Since the screw augers 24 are of opposite pitch, one of the screw augers is 
used to convey the material radially outwardly towards the periphery of 
the hearth while the other of the screw augers, approximately 180 degrees 
angularly spaced apart therefrom, serves to convey the material radially 
inwardly toward the center axis of the hearth. In operation, the screw 
auger conveying the material inwardly rotates at a slower speed than the 
other screw auger so that the net result is radially outward movement of 
the material. By varying the speeds of the respective augers and the 
rotational speed of the turntable hearth, the amount of agitation and 
mixing of the particulate material can be easily variably controlled, as 
well as the length of time the material is subjected to heat inside the 
furnace. 
From the edge of the hearth, the material gravitates into channel 32 where 
scraper blades 31 move the material into discharge outlet 17 for 
transportation by screw conveyor 35 to an externally located storage tank. 
It may be noted that the furnace of the present invention allows for the 
production of particulate material, such as litharge, on a continuous as 
opposed to a batch basis, which eliminates down time and permits 
substantially higher production rates than batch method techniques. 
While the invention has been illustrated and described in detail in the 
drawings and foregoing description, the same is to be considered as 
illustrative and not restrictive in character, it being understood that 
only the preferred embodiment has been shown and described and that all 
changes and modifications that come within the spirit of the invention are 
desired to be protected.