Title:
HEAT PROCESSING OF MINERALS
United States Patent 3671027


Abstract:
Heat processing apparatus of the type including a rotary kiln, drying and preburning sections at the material inlet end to pretreat the material prior to burning in the kiln, and a cooler at the discharge end of the kiln. The drying section is divided into two in-series chambers, with hot gases from the preburning section circulated through the first such chamber and from the cooler through the second chamber.



Inventors:
FRANS ROBERT D
Application Number:
05/075090
Publication Date:
06/20/1972
Filing Date:
09/24/1970
Assignee:
HANNA MINING CO.:THE
Primary Class:
Other Classes:
75/755, 75/762, 432/14, 432/82, 432/117
International Classes:
F27B7/20; (IPC1-7): F27B7/02
Field of Search:
263/32,53
View Patent Images:
US Patent References:



Primary Examiner:
Camby, John J.
Claims:
I, therefore, particularly point out and distinctly claim as my invention

1. In heat processing apparatus of the type employing a rotary kiln, material preburning and drying sections adjacent the inlet end of the kiln, a cooling zone adjacent the outlet end of the kiln, means for advancing the material through the apparatus, and gas flow means to draw exhaust gases from the kiln through the bed of material in the preburning section and for introducing said gases through a portion of the bed in the drying section; the improvement comprising

2. The apparatus of claim 1, wherein the material is advanced through said one chamber of the drying section before passing through the other chamber in which the flow of hot gases from the cooling zone is encountered.

3. The apparatus of claim 1, wherein said hot gases are drawn from the material receiving end portion of the cooling zone.

4. The apparatus of claim 3, wherein the material is advanced through said one chamber of the drying section and then through such other chamber thereof.

5. The apparatus of claim 1, wherein said exhaust gases and said hot gases are caused to flow respectively through the material in the chambers of the drying section in opposite vertical directions.

6. The apparatus of claim 1 wherein the gas conveying means comprises a fan, first conduit means connecting the inlet of said fan to the cooling zone, and second conduit means connecting the outlet of said fan to the said chamber of the drying section.

7. In heat processing apparatus of the type employing a rotary kiln, material preburning and drying sections, said sections being arranged in series to provide a material flow stream from said drying section to the preburning section and then into the rotary kiln, a cooling zone adjacent to the outlet end of the kiln, means for advancing the material through the apparatus, gas flow means comprising first conduit means to draw exhaust gases from the kiln through the bed of material in the preburning section and for introducing said gases through a portion of the bed in the drying section, a bypass conduit connecting a second portion of said preburning section to said first conduit means at a location between the preburning section and the inlet to the preburning section; the improvement comprising

Description:
The present invention relates to improvements in apparatus for heat processing of materials on the order shown in U.S. Pat. No. 3,313,534.

In the apparatus shown in such patent, the material being processed and hot gases from a burner are passed in counter-flow relationship through a rotary kiln. At the material feed or inlet end of the rotary kiln drying and preburning sections, in that order, provide preliminary treatment of the material which proceeds longitudinally therethrough on a traveling grate, and at the discharge end of the rotary kiln there is a cooler to remove heat from the burned or processed material. The heat input in the drying and preburning sections is obtained by flow of hot gases through the bed of material passing through the sections, and exhaust gases from the kiln are utilized for the preburning and from the preburning section for drying.

The problem has been, however, that it frequently is desirable to process the material at a rate beyond the optimum design capacity of such apparatus, and it has been found that when this is done, a disproportionate increase in fuel consumption is incurred. The reason for this is that as the solids feed rate is increased, the gas flow through the beds of material also must be increased and resistance thereto in the apparatus increases approximately with the square of the velocity of the gas flowing in the system, i.e. exponentially with the volume of the gas flowing. Eventually, the resistance increases to a point where further increase in the flow is not possible, and if the production rate exceeds this level, leakage of cold air occurs in the system. The production rate can be increased beyond design rating, but only at the cost of a disproportionate increase in fuel consumption.

As a further problem, it is a requirement of heat processing apparatus of the type to which the present invention is directed that means be provided for proper heat inputs and temperatures in the preburning and drying sections. For instance, if a maximum temperature is exceeded in the drying section, it is possible that water vapor can be produced too rapidly pulverizing the ore being processed with resultant dust loss. In apparatus where total flow of hot gas from the kiln is passed through the bed of material in the preburning section and then through the bed in the drying section, it clearly is possible, for different materials and different feed rates, for the temperatures and heat inputs in these sections to be at other than optimum.

Accordingly, it is a principal object of the present invention to provide such heat processing apparatus which is not subject to the foregoing limitations and in particular can operate at much higher production rates without disproportionate fuel consumption.

It is also an object of the present invention to provide, in such heat processing apparatus, a system for passing hot gases through the beds of material in the drying and preburning sections which is relatively independent of the apparatus production rate.

It is a still further object of the present invention to provide heat processing apparatus including a rotary kiln in which the temperatures and heat inputs in the drying and preburning sections for the kiln can be adjusted accurately and independently of burn temperature and heat input in the kiln itself.

To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principles of the invention may be employed.

In such annexed drawing:

The single FIGURE is a schematic elevation, partly in section, of apparatus in accordance with the present invention.

Referring to the drawing in detail, the heat processing apparatus 10 comprises a rotary kiln 12 having an inlet end 14, relative the direction of flow of granular material 16 in the kiln, and a discharge end 18. The kiln is of conventional design comprising an elongated cylindrical shell 20 lined by refractory 22. The shell is adapted to rotate about its longitudinal axis, and is sloped, so that material introduced at the inlet end 14 travels downwardly to pass from the kiln at the lower discharge end 18.

Adjacent the inlet end, the apparatus includes drying and preburning sections 24 and 26 arranged in that order in end-to-end or series relationship. A firing hood 28 encloses the discharge end 18 of the kiln, burner 30 penetrating the hood and being aimed to direct a flame and hot gases into the kiln. A cooler 32 beneath the hood 28 receives material discharged from the kiln and cools the same.

The drying and preburning sections 24 and 26 at the inlet end of the kiln are arranged in a single housing 34 divided by a baffle 36 positioned about half way between the ends of the housing delineating the sections. A traveling grate 38 extends longitudinally through the sections, granular material being introduced from means not shown onto the grate for movement first through the drying section 24 and then through the preburning section 26. A chute 40 at the inlet end of the rotary kiln carries material from the grate onto the kiln.

The cooler 32 is divided by baffle 42 into preliminary and final cooling chambers 44 and 46, respectively to receive and cool in that order material burned in the rotary kiln 12. The chambers are disposed over a grate 48. Beneath the latter, blowers 50 and 52 force cold air upwardly via windboxes 54 and 56 through the grate 48 and the bed of material on the grate. As shown, part of the air forced upwardly into the preliminary cooling chamber 44 passes into the firing hood 28 to provide air for combustion of the fuel introduced by the burner 30. The cooling air passed through the bed in the final cooling chamber 46 is vented to atmosphere by stack 58.

As indicated earlier, it has been conventional to heat the material in the drying and preburning sections 24 and 26 by the flow of hot gases through the beds of material in the sections, with the exhaust gases from the rotary kiln 12 being used for preburning the material in chamber 26 and those drawn through the bed in the preburning chamber 26 used for drying. In the past, this has been accomplished by means of a vacuum system including windbox 60 positioned beneath the traveling grate in the preburning section, a windbox such as 62 positioned beneath the grate in the drying section, and vacuum equipment broadly identified by the number 64 between the two windboxes. The vacuum equipment normally has included a vacuum fan 66, a dust collector 68 and suitable ductwork 70. Additionally, there has been provided a bypass ductwork 72 leading directly from the preburning section to the inlet side of the dust collector 68. Damper means 74 in the bypass ductwork proportions the flow of hot gases in the ductwork with respect to that flowing through the bed of material in the preburning section.

By means of the bypass ductwork, the heat input into the bed of material in the preburning section has been adjusted independent of the temperature and flow rate of the exhaust gases from the rotary kiln.

It has also been known to employ a cold air inlet 76 in the bypass ductwork, along with damper means 78, to temper the bypassed gases. In this way, the heat input and temperature in the bed of material in the drying section has been controlled to below maximum limits.

The above arrangement has had the advantage that it provides flexibility in adjusting temperatures in the preburning and drying sections to suit the needs of different materials and process operations. More heat input in the preburning section is achieved by closing damper means 74, whereas an increased heat input in the drying section is achieved by closing damper 78.

In accordance with the present invention, the drying section 24 is divided by a baffle 80 into a first drying chamber 82 and a second drying chamber 84. A conduit 86 extends between the preliminary cooling chamber 44 of cooler 32 and the second drying chamber 84. Exhaust fan 88 in the conduit draws heated air from the preliminary cooling chamber into the second drying chamber, via dust collector 90 upstream of the suction fan.

Windbox 62 is made coextensive only with the first drying chamber. An additional windbox 92 is positioned beneath the second drying chamber, coextensive with that chamber, and a suction fan 94 in communication with the windbox via dust collector 96 exhausts gases from the second chamber, through the bed of material in the chamber. The exhaust flow from fan 94 is vented to atmosphere via stack 98.

Suitable damper means 102 in conduit 86 controls the flow of heated air in the conduit.

The first drying chamber is connected to receive flow of hot gases from the preburning section 26, and the temperatures and heat inputs into the preburning section and first drying chamber are adjusted following previous principles. However, the temperature and heat input in the second drying chamber is independently controlled, dependent instead upon the amount and temperature of the heated air transmitted from the preliminary cooling chamber. By suitably selecting the point of connection of the conduit 86 with the cooling chamber, and adjustment of baffle means 102, the exact temperature and heat input required in the second drying chamber can be achieved.

In a particular example in accordance with the invention, for magnetite iron ore, the drying gases are introduced to the bed of such material in the first drying zone at a temperature of 800° F to dry the bed to an average temperature of about 450°. The bed temperature of course varies in the direction of the cross flow of the gases and, at the end of this first zone, the pellets at the bottom will be near 800° F while those at the top will be about 220° F. The preferred temperature of the gases entering the second drying zone from the conduit 86 will be 800° F, with this temperature not exceeding 1,200° F, and the preferred drying temperature can be provided by properly locating connection of the conduit along the length of the cooler, without having to introduce cooling air which would detract from the fuel savings. The conduit at this connection may in fact be provided with a number of longitudinally spaced inlets equipped with dampers to permit selection and use of the one that provides the preferred temperature for the flow.

The temperature of the gases in the preburn section 26 is maintained at 1,800°-2,000° F, with the by-pass system including the conduit 72 diverting a portion of the gases around the bed in this chamber to avoid excessive heating as described in the aforesaid U.S. Pat. No. 3,313,534. The by-passed portion of the gases in this system is cooled by the noted introduction of ambient air to bring the resulting total mixture to a temperature level and weight more suitable for the aforenoted drying, with this system and the flow in the ductwork 70 providing the gases at the desired 800° F temperature.

The final temperature within the kiln is about 2,400° F and the cooling in the first cooler zone will be sufficient to provide the take-off of gases at the 800° F temperature at the conduit 86.

Several advantages of the present invention should be apparent. For one, the gradation of temperature in the cross-flow cooler 32, particularly in the preliminary cooling chamber 44, means that the heated air in the very first part of the cooler is very hot while that at the end of the preliminary cooling chamber is somewhat colder. The invention permits segregation of the colder air from the hotter air making the latter available for the burner and kiln, in the usual way, at the highest temperature possible.

At the same time, delivery of the colder air to the downdraft second drying chamber, supplies to this chamber heated air at the exact temperature required for final drying.

In this respect, it is apparent that this invention achieves an overall increased recovery of waste heat permitting substantial improvement in fuel economy.

As a still further advantage less gas volume is required for preliminary drying. This means that the apparatus can readily accommodate increased capacity, even above design rating, since it involves simply increasing gas flow through the beds in the preburning section and first drying chamber, and in the vacuum system 64 (including dust collector 68 and fan 66) up to design rating for the vacuum system equipment. Increased heat input into the second drying chamber is easily accomplished by adjustment of damper means 102. Alternatively, the invention permits the use of smaller less costly equipment in the choice of components for a new plant.

As a further advantage, temperature level and heat input for final drying can readily be adjusted, as with preliminary drying and preburning, independent of material feed rate and burning conditions simply by operator manipulation of fan and damper controls.

In the description of the drying section 24 of the invention, it was pointed out that the gas flow in the first drying zone was one of updraft through the bed of material, while that in the second drying zone was one of downdraft through the bed. It is apparent that the supplemental gas flow into the second drying zone can be one of updraft, and that in the first drying zone one of downdraft. In addition, the supplemental hot air flow can be drawn from one of several points in the preliminary, or even final cooling chamber. Further, the supplemental air clearly can be delivered to any one of several points upstream of the rotary kiln, with respect to the direction of the material flow.