GB2069669A - Method of and apparatus for the heat treatment of pulverous raw materials - Google Patents

Abstract

In a method of calcining raw material, the raw material is fed to a calciner (1) which is also fed with fuel for combustion purposes, and the raw material is entrained in gas flowing up through the calciner (1). The raw material entrained in the gases is passed through an outlet pipe (2) to a fractionating chamber (3) in which the coarse material is separated from the gas flow for return to the calciner (1) The finer fraction of the treated material, forming the finished product, is separated from the gas in a cyclone (5) for subsequent treatment. <IMAGE>

Description

SPECIFICATION Method of and apparatus for the heat treatment of pulverous raw materials The present invention relates to a method of and an apparatus for heat treatment of pulverous raw material, in which the raw material is introduced to and suspended in an oxygen containing gas passing through a heat treatment zone, calcined or burnt in the heat treatment zone by means of fuel fed thereto, and subsequently carried out of the zone by the flow of gas from which is is separated thereafter. Such a method will hereinafter be referred to as "of the kind described".

By the term "heat treatment is particularly understood calcining, i.e. heating a material to a high temperature, but without fusing, in order to bring about physical and chemical changes in the raw material, e.g. in order to drive off carbon dioxide from limestone or chemically combined water from aluminium hydroxide, as well as roasting of mineral ores.

The extent of heat transfer from the gas to the suspended raw material particles in the heat treatment zone depends on the size of the particles. Therefore, heat treatment of pulverous material with a non-uniform particle size may give rise to problems, as the fine particles are often treated to a greater extent than is necessary in view of their small size, whereas the coarse particles may not be treated sufficiently.

To ensure sufficient treatment of the coarse particles it may be necessary to increase the retention time of the material in the heat treatment zone by increasing the volume of the zone and/or use excessive firing, i.e.

increase the temperature of the zone, but both procedures are inexpedient for reasons of heat economy. Also, in a heat treatment zone with increased volume the risk of blocking the zone through the fall of coarse material is great compared with zones having small volumes, while excessive firing may lead to operational difficulties such as the formation of molten ash which can set in an uncontrolled manner and cause agglomeration of the material treated and/or blocking of the heat treatment zone.

The problem of ensuring adequate heat treatment of the coarse particles which, for example as may occur when calcining cement raw meal, but particularly when calcining CaCO3 to CaO, has over the years been sought to be solved in various ways.

U.S. Patent Specification No. 39 04 353 describes a method according to which the material separated from the gas flow is divided into two sub-flows, one of which is returned to the heat treatment zone (here the calcining zone), and the other of which is discharged as the calcined product and passed to a rotary kiln. There is no division of the flows according to particle size and both sub-flows therefore contain a mixture of fine and coarse particles. U.S. Patent Specification No. 37 16 095 describes an analogous method according to which the material, in suspended form, is treated in a hot gas flow, and after separation from the gas flow is recirculated to the heat treatment zone.The disadvantage of the last mentioned method is that the fine material is returned to the heat treatment zone, i.e. material with grain sizes up to 1 mm and which has already been sufficiently calcined, whereas the coarser, insufficiently calcined, material is separated off as the calcined product.

GB Patent Specification No. 14 89 41 6 describes another method of calcining according to which partly calcined material is separated from the gas flow and is re-calcined in another calcining zone. Also, material already sufficiently calcined is further treated in the heat treatment zone.

By the heat treatment method described in G.B. Paten' Specification No. 14 63 124 optimum retention time in the heat treatment zone of the treated material has been sought to be achieved by suspending the material for a suitably long time in an ascending gas flow.

The coarser particles thereby spend a longer time inside the heat treatment zone than the finer ones due to a suitable design of the zone and appropriate choice of gas velocity. This method however presupposes a "suitably designed heat treatment zone", i.e. a zone of comparatively great volume.

Thus, the known technology apparently does not solve the problem of devising a heat treatment method in which the coarse particles are adequately treated without superfluous after-treatment of fine material, without also entailing the above disadvantages.

An objection of the invention is to eliminate these disadvantages.

According to the present invention in a method of the kind described the separation of the material from the gas flow takes place in two steps, in a first of which coarse mate rial is separated from the gas flow, at least part of the coarse material being returned to the heat treatment zone for further treatment, and in a second of which the remaining finer, calcined or burnt material, forming the finished treated product, is separated from the gas flow.

The invention also includes apparatus for carrying out the method the apparatus comprising a heat treatment chamber having an inlet for oxygen containing gas and pulverous raw material, and an outlet tube for passing the material suspended in the gas flow and treated in the heat treatment chamber to a separator, the outlet tube including a fractionating chamber having means for returning fractionated coarse material to the heat treatment chamber, the fractionating chamber be ing positioned between the heat treatment chamber and the separator.

Preferably the heat treatment chamber has separate inlets for the oxygen containing gas, the raw material and the fuel. The pulverous raw material may be mixed with fuel prior to introduction into the gas flow up through the heat treatment chamber. Gaseous or liquid fuel, e.g. natural gas or oil, or pulverous solid fuel e.g. coal may be used.

Means may be provided for controlling the admission of gas and entrained material into the fractionating chamber, so that both the gas velocity and the amount of recirculated material can be controlled.

The heat treatment chamber may be a vertical, tubular chamber or calciner having a centrally located gas inlet at its lower end. At the lowermost end of the calciner will be provided inlets for the raw material and fuel mixture and for fractionated coarsed material from the fractionating chamber.

An example of material which can be treated according to the method is pulverous crushed limestone (CaCO3) with grain sizes ranging between 0 and 5mm, the grains of 1 to 5mm constituting the coarser material fraction, and the grains of upto 1 mm constituting the fine fraction.

Calculations have shown that the introduction of the fractionating chamber with controllable gas/material admission in the riser pipe immediately after the calciner makes it possible to reduce the volume of the latter to 3/4 of that of a calciner without a following fractionating chamber, and to lower the average temperature of the exit gases in the following cyclones by 50" to 100'C, with a consequent fuel saving of approx. 5 per cent.

Two examples of the method and apparatus of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows parts of a calcining plant in which all the material fractionated from the fractionating chamber is returned to the calciner; and, Figure 2 shows parts of a similar plant in which part of the fractionated material is returned to the calciner, and part discharged as finished calcined product.

In Fig. 1, is shown a calciner 1 having a riser pipe 2 which has a fractionating chamber 3 built in. from which chamber fractionated coarse material can be returned to the calciner 1 through a tube -4. The finer, finished calcined product is conveyed by the exit gases to a conventional separating cyclone 5, in which the fine material is separated from the exit gases and thereafter passed through the tube 7 to a cooler 8 from where the cooled product is discharged at 9. The exit gases, cleaned of impurities in the separating cyclone, are passed through the riser pipe 11 further up through parts (not shown) of a preheater.

From the penultimate stage 1 2 of the preheater preheated raw material fed to the plant is passed to the lowermost part of the calciner 1 to mix with a flow of fuel supplied at 6 together with the material recirculated through the tube 4 from the fractionating chamber 3.

In this way material and fuel are intimately mixed immediately before being suspended in the hot gas flow supplied to the bottom of the calciner from the cooler 8 through the tube 10, after which the material is calcined during its passage through the calciner.

The calcined material is discharged from the top of the calciner through the tube 2 and passes, at the inlet of the fractionating chamber 3, a regulating damper 14 having a regulating mechanism 1 5. By means of the regulating damper the amount of material which it is desired to recirculate via the fractionating chamber 3 may be varied. Equally, the direction of the gas material flow may be controlled. This contributes to, and to some extent controls, the fractionating proper of the coarser material, the impact of the material particles against the damper surface and/or opposite surfaces in the tube 2 and the fractionating chamber 3 causing the particles to leave the direction of flow of the gases.

Fig. 2 shows parts of a similar plant, the same references being used as in Fig. 1. The plant shown in Fig. 2 differs from that of Fig.

1 only in that a splitting gate 1 6 is provided in the tube 4 to divide the stream of fractionated material between tubes 4a and 4b, tube 4a leading to the calciner and tube 4b leading to the tube 7 and thence to the cooler 8. Thus a further possibility of controlling the amount of fractionated, coarser material which it is desired to recirculate to the calciner is achieved.

Claims (8)

1. A method of the kind described in which separation of the material from the gas flow takes place in two steps, in a first of which coarse material is separated from the gas flow, at least part of the coarse material being returned to the heat treatment zone for further treatment, and in a second of which the remaining finer, calcined or burnt material, forming the finished treated product, is separated from the gas flow.

2. A method according to claim 1, in which all of the separated raw material is returned to the heat treatment zone.

3. A method according to claim 1, substantially as described with reference to the accompanying drawings.

4. Apparatus for carrying out a method according to claim 1, the apparatus comprising a heat treatment chamber having an inlet for oxygen containing gas and pulverous raw material, and an outlet tube for passing the material suspended in the gas flow and treated in the heat treatment chamber to a separator, the outlet tube including a fractionating chamber having means for returning fractionated coarse material to the heat treatment chamber, the fractionating chamber being positioned between the heat treatment chamber and the separator.

5. Apparatus according to claim 4, which includes means for controlling the admission of gas and material into the fractionating chamber.

6. Apparatus according to claim 4 or claim 5, in which the heat treatment chamber is a vertical, tubular chamber having a centrally located gas inlet at its lower end, and inlets for raw material, fuel and fractionated coarse material from the fractionating chamber also at its lower end.

7. Apparatus according to any of claims 4 to 6, having separate inlets for the oxygen containing gas, the raw material and the fuel.

8. Apparatus according to claim 4, substantially as described with reference to the accompanying drawings.