US3372914A - Shaft kiln and method of operation - Google Patents

Description

March 12, 1968 E. LEATHAM ET AL SHAFT KILN AND METHOD OF OPERATION Filed May 5. 1966 //VV76 A5. EARL L E A 7744 M W/LZ. MM 144 CAM United States Patent Office 3,372,914 Patented Mar. 12, 1968 SHAFT KILN AND METHOD OF OPERATION Earl Leatham, Wexford, Pa, and William W. Campbell and Carl E. Rossi, Ludington, Mich., assignors to Harbison-Walker Refractories Company, Pittsburgh, Pa.,

a corporation of Pennsylvania Filed May 5, 1966, Ser. No. 547,975 6 Claims. (Cl. 263-) ABSTRACT OF THE DISCLOSURE A shaft kiln charging apparatus and gas-collection hood having an integral surge system which accommodates sudden upward blasts of hot, solid material and exhaust gas on the collapse of material hang-ups in the kiln.

A shaft kiln is a refractory and metal structure which defines a long, narrow, hollow column or shaft with openings at the top and bottom through which raw materials may be passed to be burned. A raw material is charged at the top of the shaft kiln, and passes progressively downward through preheating, calcining or burning and cooling zones. Air for combustion and heat transfer is introduced at the bottom of the kiln and passes, countercurrent to the raw material, upwardly through the kiln. Therefore, the preheating zone is located near the top of the kiln, the burning zone is located centrally, and the cooling Zone is located near the bottom of the shaft. Fuel, generally gas, is introduced through various burners located in the calcining or burning zone.

A primary desirable feature of a shaft kiln is its thermal efficiency. This thermal efficiency results because burned materials transfer most of their heat to incoming air before being discharged, and exhaust gases transfer most of their heat to incoming raw material before escaping from the shaft kiln. In theory, if a shaft kiln was long enough, the exhaust gases and discharge materials could leave at the same temperature they entered; but this is not practical. Also, it is not practical to prevent all heat loss through the sides of the shaft kiln.

The rate charge can be moved through the shaft kiln depends, among other things, on the thermal efficiency desired and the rate at which the countercurrent air can be moved through the kiln. It has been found that, for best operation of a shaft kiln, a positive air pressure should be maintained at the lower end and that a negative air pressure be maintained at the upper end. This will insure a suflicient countercurrent air flow. Typical apparatus for maintaining a positive air pressure at the lower end of the shaft kiln is found disclosed in the United States Patent No. 3,165,304, assigned to the same assignee as the present case. It is an object of this invention to provide apparatus which assists in maintaining the negative air pressure at the top of the shaft kiln.

The temperature of the flue gases being removed at the top of the shaft kiln is normally between 600 and 1000 F. These gases cannot be passed into the atmosphere as they contain a substantial amount of fine magnesite dust, which would cause considerable contamination of the atmosphere besides being a waste of a large quantity of material. It is an object of this invention to provide a charging end apparatus which assists in effectively drawing off the flue gases and dust, and which can handle these materials at a temperature between 600 and 1000 F.

Occasionally, during the operation of the shaft kiln, a hang-up will occur. This is the result of bridging of the charge between the shaft kiln walls stopping the passage of the charge through the shaft kiln. In order to break this bridge or hang-up, the positive air pressure maintained at the bottom of the shaft kiln is cut off. The hang-up then violently collapses throwing glowing particles up through the top of the shaft kiln. Some of these particles are as large as a peach stone, and if they become lodged in or on the ductwork at the top of the shaft kiln can melt holes therein. It is an object of this invention to provide a charging end apparatus which will allow for the releasing of said pressure caused by the collapse of hang-ups with little danger to adjacent apparatus.

It is also an object of this invention to provide a method of handling exhaust from a shaft kiln during its normal operation and during collapse of hang-ups. Briefly, according to one aspect of this invention, a charging end apparatus is provided for a shaft kiln having an unflared opening at the top. The charging apparatus comprises a hopper converging to a narrow section which enters the shaft kiln opening and is positioned centrally therein. The feed material or charge is contained within the hopper. Surrounding the hopper and contiguous therewith is a cooling air and surge chamber through which a small amount of cooling air is drawn during normal operation of the furnace and which acts as a pressure release vent when hang-ups are collapsed. Heavier glowing particles thrown up through the surge chamber generally lose their velocity before they reach the top of the duct and thereafter fall back down into the shaft kiln. A collection hood substantially covering the peripheral portions of the shaft kiln opening and extending thereinto is contiguous with the narrow section of the cooling and surge chamber. Exhaust ducts in communication with the hood are spaced thereabout, and are in communication with the exhausting and dust collector systems.

Further features and objects of this invention will be made clear by a study of the following detailed description with reference to the drawings, in which:

FIG. 1 is an elevation view in section through the upper portion of a shaft kiln and a discharge apparatus according to this invention;

FIG. 2 is a section plan view taken along line 2-2 in FIG. 1.

The shaft or hollow column 1 of a shaft kiln is defined by a refractory lining 2 contained within a steel shell 4. The shape of the cross-section of the hollow shaft or kiln is generally ellipsoid. To facilitate the description of this invention, typical dimensions for a particular shaft kiln are given but are not meant to be limiting. Typically, the long axis of the ellipsoid section is about 8 feet and the short axis is about 2 feet. A hopper 5 which is generally rectangular in plan section, is disposed. above the shaft kiln and narrows to a section 6 with substantially parallel sides extending down into the shaft kiln opening approximately 6 inches. The rectangular narrow section is shown centrally disposed within the ellipsoid cross-section of the shaft kiln in FIG. 2. The hopper section need not, however, be rectangular but could, for example, be ellipsoid. Surrounding the hopper is a shell 7 which encloses a cool ing and surge chamber 8 contiguous with the hopper. The surge chamber also extends into the shaft kiln opening about 6 inches. The hopper and cooling chamber shell are fixed together so that they can be moved as a unit. They may be joined by channels (not shown) which also provide reinforcement for the hopper. The hopper is typically about 3 feet high, and its converging sides typically form an angle of about 60 to the horizontal. The distance between the hopper 5 and the shell 7 is typically between 1 and 2 inches. The narrowest dimension of the narrow section of the hopper is typically about 10 inches. It is preferable that the width of the narrow section of the hopper in the direction of the short axis of the ellipsoid section is about one half of the short axis.

An exhaust hood 16 having an inverted U-shaped crosssection surrounds the hopper and cooling chamber and substantially fills the peripheral space between the refractory lining and the surge chamber. The sides 12 of the hood extend into the shaft kiln opening about 6 inches. A cover plate 11 covers the top of the shaft kiln and that portion of the shaft kiln opening not covered by the hood or the hopper. The narrow section of the surge chamber is arranged to fit snugly against the sides of the hood 12.

A plurality of exhaust ducts 14 are spaced around the hood and in communication therewith. These ducts are also in communication with a main duct 15 which leads to the exhausting and dust-collecting systems (not shown). It is preferable that the exhaust ducts approximately form a right angle with the hood.

The charge to a shaft kiln is usually in the form of briquettes which are about the shape of peach stones. These briquettes 16, when piled in the hopper have considerable space therebetween allowing the passage of flue gases therethrough. It is undesirable, of course, if flue gas dust is passed up through the hopper as the fine dust will be spread over the shaft kiln area. Therefore, it is important that substantially all the flue gases be drawn into the hood. This is accomplished partially by putting a negative pressure on the hood. Also, it is necessary to position the hood directly above the shaft kiln so that the flue gases moving upwardly through the kiln do not have to appreciably change direction to enter it. In prior shaft kilns, the upper opening was flared and the hood was placed above the flare. This arrangement was found to be unsatisfactory.

During normal operation of the shaft kiln, cool air is drawn into the surge chamber 8. This air and the exhaust gases enter the hood and are drawn off through the ducts. The cool air drawn through the surge chamber lowers the temperature of the exhaust gases thereby protecting the duct work and facilitating the precipitation of the dust.

When a hang up is collapsed, the upward surge of flue gases, glowing dust, and particles are primarily released through the surge chamber. The heavier particles lose their upward velocity while in the surge chamber and fall back into the shaft kiln. The larger particles, which do not enter the surge chamber but enter the hood, generally do not enter the duct work. This is because of the angle of the join between the ducts and the hood. Without this pressure release feature, the hood and exhaust system would have to be designed to contain much higher pressures than normally encountered. Furthermore, a large quantity of flowing particles would tend to damage the ductwork and exhaust fans.

Having thus described the invention in detail and with suflicient particularity as to enable those skilled in the art to practice it, what is desired to have protected by Letters Patent is set forth in the following claims.

We claim:

1. In a shaft kiln including means defining a vertical, hollow shaft with openings at the top and bottom in which material to be treated passes from top to bottom the improvement comprising said top opening having sides and a charging apparatus comprising a hopper converging to a narrow section which enters the top opening, a surge chamber contiguous with both the hopper and narrow section, said surge chamber opening into the shaft kiln at one end and into the atmosphere at the other a collection hood contiguous with said surge chamber and substantially covering the peripheral portions of the top opening, and exhaust ducts in communication with said hood at spaced positions along said hood.

2. A charging apparatus according to claim 1 in which the narrow section is disposed centrally with the top opening.

3. A charging apparatus according to claim 1 used in a shaft kiln in which the hollow shaft has an ellipsoid section and the width of the narrow section of the hopper along the short axis of the ellipsoid section is about onehalf of the short axis.

4. In a charging apparatus according to claim 1 said hood having an inverted U-shape and said exhaust ducts disposed substantially perpendicular to said hood.

5. In a charge end apparatus according to claim 1 the hopper and surge chamber being in a fixed relationship and forming a pressure fit with said hood.

6. A method of removing exhaust gas from a shaft kiln defined by a vertical hollow shaft with openings at the top and bottom and in which material to be burned is introduced through a hopper at the top and passes from top to bottom of the kiln comprising the steps, (a) during normal operation of drawing exhaust gas from the top of the hollow shaft and simultaneously drawing cooling air from a surge chamber contiguous with the hopper into a hood directly above the hollow shaft, thereafter removing said exhaust gas and cooling air from the hood, (b) the further steps during collapse of hang-ups below said hopper in said hollow shaft of channeling substantially all the sudden upward blast of hot material and gas into the surge chamber where a substantial part of the hot material loses its upward velocity, whereby the hot material that lost its upward velocity falls back into the hollow shaft and the gas passes through the surge chamber to the atmosphere.

References Cited UNITED STATES PATENTS 1,725,763 8/ 1929 Mount et al 26329 2,866,625 12/1958 Sylvest 26330 3,033,545 5/1962 Azbe 26329 FREDERICK L. MATTESON, 111., Primary Examiner. JOHN J. CAMBY, Examiner.

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