BACKGROUND AND SUMMARY OF THE INVENTION This invention is related to a device in kilns comprising a rotatable kiln tube, one end of which forms an inlet for material to be heat-treated and the other end of which forms an outlet for the material, means for providing heat influence on the material in the kiln tube and a dam arranged at the outlet end of the kiln tube, the material having to pass by the dam in order to get out of the kiln tube. The kiln device, in the preferred case, is intended to be used for burning calcium carbonate to calcium oxide.

Various constructions of dams have been proposed until now. The most common construction today is designed as a ring formation within the kiln tube. This ring formation may for instance be erected by brick symmetrically designed all around the kiln tube. Dams also occur which are constructed in the same way as brick dams but are formed by pouring masses or a combination of steel and pouring masses. The dam construction as a symmetrical ring involves, however, a limitation as to the height of the dam. This height is restricted to about 25-28 percent of the internal free diameter of the kiln tube. If this percentage is exceeded, it is no longer possible to obtain

the correct relation between the primary air and the secondary air volumes for the combustion within the kiln. A burner must then be introduced into the kiln at the material outlet end thereof. The burner is normally an oil burner although other fuels may be used. The primary air for burning or combustion is introduced through the burner tube whereas the secondary air is induced to flow into the kiln tube radially outwardly of the burner tube. If the free opening, which at the outlet end of the kiln tube is left open by the ring shaped dam, becomes too small, the problem occurs that not enough secondary air is induced, by the burner, to flow into the kiln tube.

The above mentioned limitation of the height of the dam involves in practice a limitation of the material volume which can be received in the kiln tube. Accordingly, there is a limitation as to capacity. In this connection it should also be pointed out that when the ring shaped dam is given a height close to the limit value, this causes the secondary air passing into the kiln tube (through the opening left free by the dam and the burner) to obtain a relatively high flow velocity. This high flow velocity causes pulverulent proportions of the material under treatment in the kiln tube to tend to move with the secondary air. Such motion of dust or other fine constituents worsens combustion conditions in the burner flame and also worsens heat transfer to the material being treated. Furthermore, the conveyance of dust by means of the secondary air causes a deteriorated treatment capacity since the

secondary air conveys the dust towards the material inlet end of the kiln tube. This internal dust circulation may in unfavorable cases involve a deteriorated treatment volume capacity of up to 15 percent although more normal values are 7 or 8 percent. The dust conveyed by the secondary air may in practice tend to form a veritable screen about the flame generated by the burner. The primary air in the burner in practice corresponds to approximately 20 percent of the required air volume. Thus, the secondary air forms about 80 percent.

In a commercial kiln device of the nature in question it may be necessary to use about 30 tons of brick to build the ring shaped dam (if brick is chosen as the material). This load on the kiln tube necessitates a stable design of the kiln tube and its support bearings. A ring shaped dam of the kind mentioned requires a considerable installation time. Accordingly, a long lasting shut down occurs when the ring dam must be replaced. Finally, it should be mentioned that the prior annular dam makes it very difficult to get out of the kiln tube oversized pieces or balls of lime which are formed within the kiln tube.

The object of the present invention is to provide a kiln device having a dam construction which makes it possible to reduce the deficiencies and disadvantages discussed hereinabove. The object of the invention is achieved in that the kiln tube is rotatably arranged relative to the dam.

Accordingly, the present invention provides how to support the dam by means other than the bearing devices supporting the kiln tube. Since the dam is prevented from participating in the rotation of the kiln tube, it is no longer necessary to design the dam as a circular ring in order to achieve the dam function desired.

In order to obtain sealing between the dam and the outlet end of the kiln tube it is preferable that the dam, possibly with interposition of one or more sealings, bears on the edge portion of the outlet end of the kiln tube. It is then suitable to design the dam as movable in the longitudinal direction of the kiln tube, whereby the dam may be displaced away from the kiln tube, e.g. in order to carry out service thereon. In this connection, means may be arranged to resiliently actuate the dam, in its active position, towards or against the outlet end of the kiln tube. The largest effective threshold height of the dam in the radial direction of the kiln tube is 30 percent and preferably at least 35 percent, of the free internal diameter of the kiln tube, which is possible since the dam does not form any annular configuration.

The dam has an upper inclined edge. It is preferred that this edge at least in part is inclined in the same sense as the angle of repose of the material in the kiln tube on its normal rotation. Thus, the dam will be efficiently adapted not to comprise any larger surface than generally that which is required to fulfill the dam function.

It is preferred that the dam with its lower and rear portion, as viewed in the direction of rotation of the kiln tube, defines a material outlet located under an imagined extensioning of the inclined upper edge of the dam. Accordingly, a local material outlet from the kiln tube in its bottom area will thereby be obtained, which means a local material discharge from the kiln tube with a minimum of dust spreading.

With reference to the enclosed drawings a more specific description of an embodiment of the invention will follow.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

Fig 1 is a diagrammatical, partly cut side view of the kiln device according to the invention;

Fig 2 is a more detailed view illustrating the possible design in the area of the outlet end of the kiln tube;

Fig 3 is a view taken along the line III-III in Fig 2;

Fig 4 is a diagrammatical view illustrating a known dam construction as viewed from the outlet end of the kiln tube;

Fig 5 is a view similar to Fig 4 but illustrating the dam according to the invention; and

Fig 6 is a diagram illustrating the relation between dam height and production capacity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS)

The kiln device accordingly to the present invention will hereinafter be explained for a specific purpose, namely the burning of lime (CaO) . However, it should be carefully noted that the kiln according to the present invention also may be used for other types of heat-treatments of other materials.

In the production of cooking liquor (NaOH) in a cellulose production process, caustic lime (CaO) is mixed with green liquor (Na2CC>3) . The reaction, which is of a reversible character and exotherm, proceeds as follows:

Na2Cθ3 + H ₂ 0 + CaO Ca(OH) ₂ + Na C0 ₃ 2NaOH + CaC03 The by-product of the reaction is so called lime sludge, i.e. calcium carbonate (CaCθ3) .

In order to obtain a reasonable economy in the cellulose production process, recovery of lime sludge (CaCθ3) to new active lime (CaO) is required. Such recovery may for instance occur by means of the rotatable lime sludge kiln illustrated in Fig 1. More specifically, the kiln device comprises a rotatable kiln tube, one end 2 of which forms an inlet for the sludge to be heat-treated and the other end 3 of which forms an outlet for the burnt lime.

The sludge is introduced into the kiln tube 1 by means of a suitable conveyer 4 or the like. Before the sludge is introduced into the kiln tube 1, it is preferably dewatered, e.g. in one preferred embodiment over a vacuum filter to a dry matter content of about ^" 70 percent. The drying zone of the

kiln 1 is indicated at 5. In this drying zone 5, chains 6 may be mounted hanging in the mantle of the kiln 1 according to a suitable pattern. The chain system serves as a heat exchanger between the sludge and flue gas and contributes to a stirring of the sludge. The remaining portion of the water is removed in this drying zone 5.

The kiln 1 in this embodiment has an inclination from the inlet end 2 to outlet end 3 of about 2.2-4.1 percent, which enables the material to be fed through the kiln tube 1 in a direction towards the outlet end 3 at the same time as the kiln mantle rotates. The kiln tube 1 is rotatably supported by means of suitable bearing devices 7 and driven in its rotating movement by means of a driving device of a type known to those of ordinary skill in the art and not illustrated.

At the outlet end of the kiln tube 1 there is arranged a burner 8 adapted to generate a flame 9 adjacent to the outlet end 3. The burner 8 suitably protrudes partly into the interior of the kiln tube 1 and the burner 8 may for instance be an oil burner, the primary air of which is supplied via the burner tube. The primary air flow generates about 20 percent and a secondary air flow forms about 80 percent of the air supply necessary for combustion.

The burner 8 provides a high temperature within the kiln tube 1. In order to withstand this temperature, the outer kiln mantle 10 manufactured from steel or other suitable material is internally lined with lining brick 11 in a layer having a

suitable thickness. The flue gases from the combustion pass through kiln tube 1 in a direction opposite to the material to be burnt and in the area of the end 2 of the kiln tube 1. The inlet for the material the flue gases are discharged into is a collection chamber 12, to which a flue duct 13 is connected. The secondary air for the combustion may for instance be introduced via the air inlet channel indicated at 14 in Fig 1.

After the sludge has passed through the drying zone 5 it arrives to a heating zone 15, in which the sludge is heated, and then successively reaches the burning zone 16 in the area of the burner flame 9. The radiation from the flame generated by the burner 8 mounted in the center of the kiln tube 1 heats the sludge to a temperature of about 1100-1250°C, the burning of sludge to lime occurring according to the reaction:

CaCθ3 + heat CaO + C0 ₂ If required, a flue gas fan may be arranged to assist in making the flue gases to pass through kiln tube 1 to the flue gas discharge device 12, 13 in counter flow relation to the sludge being treated.

At the outlet end 3 of the kiln tube 1 a cooling zone 17 is obtained since the burning zone 16 obtained by the flame 9 is located within the kiln tube 1 and since the secondary air flowing into the kiln tube 1 provides a cooling function. At the outlet end of the sludge kiln there is provided a dam 18 adapted to dam up the lime material in the kiln to increase the retention time of the lime material in this part of the kiln.

This has turned out to provide an improved heating economy and lime quality and also an improved production capacity as compared to the case where no dam 18 is used.

As already described hereinabove, one has, in accordance with prior art, used an annular dam within the kiln tube. Such a dam is diagrammatically illustrated in Fig 4, where the kiln mantle is denoted 10 and the dam R. Such an annular dam R, uniform all along the periphery, involves however, a limitation as to the height h of the dam. This height h is limited to about 25-28 percent of the internal diameter of the kiln mantle 10. This limitation occurs in order to obtain the correct relation between the primary and secondary air velocities for the combustion in the kiln. The cross sectional area of the dam R divided with the area of the central opening 19 left free by the dam is in Fig 4 approximately equal to 3, which is a maximally allowed value for adequate combustion conditions.

The design in the arrangement according to the present invention of the dam denoted 18 is illustrated in Fig 5 as a comparison in a view similar to the one in Fig 4. The dam 18 is stationary in the sense that it does not rotate with the kiln tube 1 and the dam 18 is not supported by the kiln tube 1. Thus, the kiln tube 1 may rotate substantially freely in relation to the dam 18. Due to this stationary arrangement of the dam 18 it is no longer necessary to design the same as an equally thick ring but instead the dam 18 may be designed so that it is really provided only in those areas where it is really needed.

In Fig 5 the effective damming height of the dam 18 is about 37.5 percent of the internal diameter of the kiln mantle 10, i.e. the height h of the dam is substantially larger than in Fig 4 although the dam 18 in Fig 5 leaves a substantially larger area of the cross section of the kiln tube 1 free at the outlet end; thus, the area of the dam 18 as viewed in Fig 5 is smaller than the area 19 of the cross section of the kiln tube 1 left free by the dam 18. This means that the secondary air for the combustion will be able to pass into the kiln tube 1 over a comparatively larger area although the effective height h of the dam 18 has been increased.

In Fig 2 it is illustrated how the dam 18 is conceived to be arranged to bear on the edge portion of the outlet end 3 of the kiln tube 2. At this outlet end 3 a nose ring 10 may be arranged on the edge of the kiln tube 1 and the dam 18 may be arranged to bear on this nose ring 10 in order to achieve a relatively tight contact.

The dam 18 is movably arranged in the longitudinal direction of the kiln tube 1 to move towards and away from the edge portion of the kiln tube 1. Means 21 are provided to resiliently press the dam 18 against or towards the kiln tube 1. These means 21 may for instance have the shape of one or more springs. It is preferred that the dam 18 is arranged on a movable carrier 22, which has the character of a hood enclosing the outlet end of the kiln tube 1 and which comprises an outlet 23 (diagrammatically indicated also in Fig 1) for the caustic

lime material. The carrier 22 comprises an opening 24 for the burner and a further opening for secondary air of the burner. The carrier 22 is movable in the longitudinal direction of the kiln tube 1 together with the dam 18 and may for instance be movably supported on rollers 25. The dam 18 is held in relation to the carrier 22 by means of members 33.

The effective damming height h (Fig 5) of the dam 18 in the radial direction of the kiln tube is generally 30 percent, and preferably at least 35 percent, of the free internal diameter of the kiln tube. As already explained with the aid of Fig 5, the height h in the example illustrated therein is about 37.5 percent. As shown in Fig 3 as well as Fig 5 the dam 18 has at least a partially inclined upper edge 26. More specifically, this edge 26 is inclined in the same sense as the angle of repose of the caustic lime in the kiln tube 1 on normal rotation thereof (arrow 27). In other words, the material under treatment in the kiln tube 1 will, accordingly on rotation thereof, tend to accompany the kiln tube 1 so that the upper surface of the material will be inclined more or less in the same sense as the edge 26 in dependence upon the rotary speed of the kiln tube 1. Accordingly, the inclined edge 26 causes the dam 18 to be able to carry out its damming function with minimum dimensions.

In the known embodiment illustrated in Fig 4, caustic lime, in order to get out of the kiln tube, passes over the height h of the annular dam R. According to a particularly preferred

embodiment of the present invention this is avoided in that the dam 18 with a lower and rear (as viewed in the direction 27 of rotation of the kiln tube) portion 28 defines a material outlet 29 located relatively low in the kiln tube 1. The material outlet 29 should in any case be located at a greater distance from the center axis of the kiln tube 1 than the smallest distance between this center axis and the inclined edge 26 of the dam 18. As is most clearly evident from Figs 3 and 5, the material outlet 29 is located relatively low in the kiln tube 1 and, as viewed in the direction 27 of rotation thereof, somewhat before the lowest point of the outlet end of the kiln tube 1. The material outlet 29 may be described as being located under an imagined rectilinear extension of the inclined upper edge 26 of the dam 18. The material outlet 29 should fulfill the task to discharge the main part of the burnt material from the kiln so that the material generally does not move out of the kiln by passing over the inclined edge 26 of the dam 18. The edge or side surface 30 of the dam 18 located most closely to the material outlet 29 is suitably substantially vertical.

Fig 6 illustrates the relation between the effective height of the dam and the energy consumption for a typical rotatable kiln. More specifically, the abscissa in Fig 6 indicates the effective height h of the dam as a percentage of the free internal diameter of the kiln tube 1 whereas the ordinate is graded in giga joules per ton of lime. The lines 31 represent

the conditions in a kiln with the prior annular dam R according to Fig 4. This dam R has an effective height h of slightly more than 25 percent of the free internal diameter of the kiln tube and this gives rise to an energy consumption of about 6 giga joules per ton of caustic lime. The lines 32 are indicative of the dam 18 according to the present invention in Figs 3 and 5. The effective height h of this dam 18 is about 37.5 percent of the free internal diameter of the kiln tube 1 and this gives rise to an energy consumption of only somewhat over 4 giga joules per ton of caustic lime.

The relation between production capacity (abscissa) and the effective height of the dam expressed in percent (the ordinate) of the free internal diameter of the kiln tube is indicated in Fig 7. As can be seen the production capacity increases with increasing relative height of the dam. The term height refers to the effective height h according to Figs 4 and 5.

To summarize, it can accordingly be established that the stationary (i.e. non-rotating) dam 18 according to the present invention causes a larger effective damming height h without causing problems concerning a free area of the outlet of the kiln tube 1 that is too small for admitting entry of the secondary air required for combustion. More specifically, the dam 18 according to the invention involves the following advantages: a shorter installation period since the dam 18 is to be arranged within the kiln tube 1. The shorter installation time involves a shorter shut down of the kiln. An ordinary kiln has a production capacity of

250 tons of caustic lime per day. A shut down involves a production loss which today corresponds to SEK 250 000 per day. a higher production capacity. The larger the effective height h of the dam 18, higher capacity is obtained in relation to kiln surface and kiln volume. longer life of the brick or lining in the kiln since the lime may be burnt at a lower temperature. The thermal load in the burning zone is decreased due to the lower temperature and due to the fact that a larger surface will be covered by lime. a lower fuel consumption due to the increased retention time in the burning zone. better aerodynamics in the combustion due to the area of the outlet end of the kiln tube 1 which is left free by the dam 18 for entry of secondary air. the retention time for the lime in the kiln can be controlled by controlling the rotary speed of the kiln. This is a consequence of the unsymmetrical design of the dam 18 relative to the center axis of the kiln tube 1. it is easier to get rid of oversized pieces of lime due to the design of the dam 18 described hereinabove defining a particular material outlet. the dam 18 involves no load on the kiln tube 1 since the dam 18 has its own supporting devices. when the kiln is shut down it is possible to reverse the direction of rotation so that all lime material in the kiln can be removed therefrom.