GB2029557A - Method of and plant for treating wet raw materials in, for example, cement making - Google Patents

Abstract

In a rotary kiln plant for heat treating wet cement raw material or similar raw materials having a large moisture content comprising a dryer (8) for drying and preheating wet raw material prior to the material being passed to the kiln (1), and a precalcining zone (3) for precalcining the raw material leaving the dryer (8), the kiln waste gases leaving the precalcining zone (3) are separated from the precalcined raw material in a separator (5) and thereafter passed directly to the dryer (8) via pipe means (7) for use therein in drying the wet raw material. <IMAGE>

Description

SPECIFICATION Method of and plant for treating wet raw materials in, for example, cement making The invention relates to a method of and a plant for treating, including burning, cement raw materials or similar raw materials having a large moisture content, where the wet raw materials are dried, preheated and pulverized before being fed to other sections of the plant for further preheating, precalcining and burning.

A previous, well-known method for making, for example, cement was to carry out both drying, preheating, and sintering in one long rotary kiln. This method is known as the "wet process" because of the use of raw materials with a relatively large moisture content. The change-over to making use of cement raw materials having a small moisture content initiated a development which has been marked by the use of separate preheating plants, e.g.

cyclone preheaters, precalciners and smaller and smaller rotary kilns. This method known as the "dry process" has contributed significantly to improvements in the economy of cement manufacture.

In order to take advantage of what is known from the dry process when treating raw materials having a large moisture content i.e. wet raw materials, a dryer, for instance a spray dryer, has sometimes, see e.g. British Patent Specification No. 1,375,566, been inserted before the preheater of a dry process plant to dry and pulverize the wet raw materials in order that they could be further treated in the plant as dry raw materials. In such plants, however, it has been necessary to insert an additional heat generator before the dryer in order to obtain sufficient heat in the dryer to dry the wet raw materials. The exhaust gases from the kiln after having passed through the preheater stages have not contained the necessary heat for drying the wet raw materials in the dryer.

Use of such a heat generator coupled to a dryer in turn coupled before a preheater, which by way of example may be of the cyclone type, results in both a complicated and comparatively expensive manufacturing process and an intricate technical plant, and means an increase in the operating costs of the plant.

It is an object of the invention to device a method and a plant for treating, by the "dry" process, cement raw materials or similar raw materials having a large moisture content, e.g. raw slurry for cement or a flotation concentration of cement raw materials, without the aforesaid disadvantages.

According to the present invention, in a method of heat treating cement raw material or similar raw materials having a large moisture content in a rotary kiln plant which includes preheating and precalcining installation having a dryer to which the wet raw material is fed, the dried preheated raw material from the dryer is suspended in the kiln exist gases and subsequently exposed to a heat treatment sufficient for achieving the desired degree of precalcination, the exist gases, after they have been separated from the precalcined raw material which is passed to the rotary kiln for further heat treatment, being passed to the dryer and the temperature of the exit gases after separation from the precalcined raw material being sufficient for drying the wet raw material in the dryer.

The invention also includes a rotary kiln plant for heat treating cement raw material or similar raw materials having a large moisture content, the plant having a preheating and precalcining installation coupled before the kiln and having a dryer to which the wet raw material is fed, for pulverizing, drying and preheating wet raw material prior to the material being passed to the kiln, and a precalcining zone for precalcining the raw material; means for suspending the dried and preheated raw material in the kiln exist gases to be fed to the precalcining zone; a separator connected to the precalcining zone to separate the precalcined raw material from the kiln exit gases; and means for passing the exit gases from the separator directly to the dryer for use therein in drying the wet raw material.

Thus, during the passage of the exit gases through the dryer, the gas heat is utilized for drying and preheating the raw materials. From the dryer the gases are preferably passed to a precipitator via another separator in which raw materials separated from the gases are, together with the raw materials from the dryer, passed to the kiln inlet or to a lowermost riser pipe.

Thus, the special multistage cyclone preheater from kiln plants previously known may be replaced partly by the dryer, in which also part of the necessary preheating takes place, and partly by a riser pipe with a precalciner, in which the remaining part of the preheating and part of the precalcining take place.

Another advantage is that separate heat generators used up to now for achieving the desired temperature in the dryer maybe replaced by the heat from the precalciner and from the kiln exit gases, by which an improved utilization of the latter is achieved. This is made possible either through the use of a rotary kiln with a built-in dispersing device for raw materials at the upper kiln end ailowing for the use of a relatively short kiln and the obtainment of the necessary precalcination heat for the raw materials at the kiln end and in the riser pipe, which system is known for instance from British Patent Specification No. 1,508,840 or by leading spent cooling air from a cooler coupled after the kiln for cooling the product treated in the kiln as combustion air to the precalcination zone through the kiln or bypassing the kiln or by combining these two ways and feeding the raw materials from the dryer to the kiln inlet and/or to the precalcination zone. In both cases the loss of heat of the kiln and precalciner exhaust gases in a multistage preheater is avoided.

Furthermore, the dryer and the calcinerwith appertaining separating cyclones together make up an extremely compact preheating and precalcining arrangement. The desire to keep constructionally reasonable dryer dimensions makes the method and construction particularly advantageous and cheap when the plant is intended for small production rates.

Two examples of rotary kiln plants constructed in accordance with the present invention will now be described with reference to the accompanying drawings, in which Figure I shows, diagrammatically, a plant with a rotary kiln having a built-in dispersing device for the raw materials at the upper or inlet end; and, Figure 2 shows, diagrammatically, a plant having a by-pass for combustion air from the cooler to the lowermost riser pipe and with raw material feed to the upper kiln end and/or the precalcining zone.

In Figure 1 at rotary kiln 1 is shown having a dispersing device in the form of a scoop chamber la mounted at the kiln inlet 2. A drying, preheating and precalcining installation is coupled to the kiln inlet, which installation consists of a spray dryer 8 with a supply pipe 8a for raw material having a large moisture content i.e. wet raw material and a riser pipe 3, acting as precalciner, and equipped with a burner installation of known type having a fuel supply 4a and a supply 4b for atmospheric air for combustion in the precalciner. To the precalciner 3 and to the spray dryer 8 are coupled respective separating cyclones 5 and 12 for separating treated raw materials from the exit gases.Precalcined raw material separated off in cyclone 5 is passed through the pipe 6 to the lower part of the kiln inlet 2 at the other side of the scoop chamber 1 a for further treatment in the kiln 1, whereas raw material separated off in cyclone 12, through the pipe 15 is passed to the scoop chamber latogetherwiththe remaining raw material dried and preheated in the spray dryer 8. An exit gas pipe 7 conducts the total amount of exit gas from the kiln 1 and the precalciner 3 to the spray dryer 8. The exit gas then passes through the spray dryer 8, leaves it through the pipe 1 and is led through the pipe 13 to a precipitator (not shown) via the cyclone 12. Air sluices 10 and 14 are used for preventing false airfrom being drawn up through the pipes 9 and 15 and into the spray dryer 8 and the cyclone 12 respectively.

In operation, wet raw materials are fed to the spray dryer 8 through the pipe 8a and are dried, pulverized and preheated in the spray dryer 8 by means of the hot exit gases from the kiln 1 and the calciner 3. The raw material then passes through the pipe 9, and, together with the raw material separated off in the cyclone 12 and passed through the pipe 15, is led to the scoop chamber 1 a. At this point the raw materials are dispersed into suspension in the kiln exit gases which have been raised nearly to a precalcination temperature, and entrained in the gases are fed through the riser pipe up into the precalciner 3 for further precalcination. In the cyclone 5 the precalcined raw material is separated from the exit gases and passed through the pipe 6 for further treatment in the rotary kiln 1.Arrows indicate the flow of the exit gases through the plant, and it can be seen that in the example shown the total amount of exit gas is passed through the pipe 7 to the spray dryer 8.

Calculations have shown that if the treatment of the kiln exhaust gases in the upper end of the kiln is 1400-1 6000C, the resultant exhaust gas temperature at the inlet of the precalciner is approximately 800"C, at the outlet from the precalciner approximately 900"C, at the inlet in the bottom of the spray dryer approximately 850"C, and at the outlet from the spray dryer approximately 175 C.

In Figure 2 most of the references are the same as the ones used for Figure 1. However, a conventional kiln seal 1b connects the upper kiln in let 2 with the lowermost riser pipe 3a. Combustion air from a cooler (not shown) coupled afterthe kiln is lead through a pipe 17 to the riser pipe 3a, thus bypassing the kiln. A damper 18 is mounted in the pipe 17 to regulate the combustion air feed. By means of a diverter valve 16 raw materials from the dryer 8 and the cyclone 12 are fed either to the kiln inlet 2 or to the precalciner 3. In this embodiment of the invention the heat at the kiln inlet 2 may be lower than in the embodiment shown in Figure 1 and the by-pass of combustion air enables higher heat than would otherwise be possible to be generated in the precalciner and also enables a greater variety of combustion sources in the precalciner, such as low grade fuel, combustible waste etc. This, however, requires the possibility of directing the flow of raw materials in quantities dependant on the generated heat either to the kiln inlet or to the calciner or divided betwen those two, this being done by means of the divertervalve 16.

Claims (13)

1. A method of heat treating cement raw material or similar raw materials having a large moisture content in a rotary kiln plant which includes a preheating and precalcining installation having a dryer to which the wet raw material is fed, in which the dried preheated raw material from the dryer is suspended in the kiln exit gases and subsequently exposed to a heat treatment sufficient for achieving the desired degree of precalcination, the exit gases, after they have been separated from the precalcined raw material which is passed to the rotary kiln for further heat treatment, being passed to the dryer and the temperature of the exit gases after separation from the precalcined raw material being sufficient for drying the wet raw material in the dryer.

2. A method according to claim 1, in which the dry and preheated raw material from the dryer is fed to a dispersing device mounted at the inlet end of the kiln, the dispersing device, in use, dispersing the raw material into the kiln exit gases.

3. A method according to claim 1, wherein the dry and preheated raw material from the dryer is fed to the precalcining installation and entrained in the kiln exit gases therein.

4. A method according to claim 1, substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.

5. A rotary kiln plant for heat treating cement raw material or similar raw materials having a large moisture content, the plant having a preheating and precalcining installation coupled before the kiln and having a dryer to which the wet raw material is fed, for pulverising drying and preheating wet raw material prior to the material being passed to the kiln, and a precalcining zone for precalcining the raw material; means for suspending the dried and preheated raw material in the kiln exit gases to be fed to the precalcining zone; a separator connected to the precalcining zone to separate the precalcined raw material from the kiln exit gases; and means for passing the exit gases from the separator directly to the dryer for use therein in drying the wet raw material.

6. A rotary kiln plant according to claim 5, wherein the dryer comprises a spray dryer.

7. A rotary kiln plant according to claim 5 or claim 6, wherein the raw material from the dryer is fed to the kiln inlet to be entrained in the exit gases.

8. A rotary kiln plant according to claim 7, including a dispersing device integraliy mounted with the kiln at its inlet for dispersing the raw material in the kiln exit gases.

9. A rotary kiln plant according to claim 5 or claim 6, wherein the raw material from the dryer is fed to the precalcining zone to be suspended in the kiln exit gases therein.

10. A rotary kiln plant according to any of claims 5 to 9, wherein the raw material from the dryer can be fed selectively to either the precalcining zone or the kiln inlet for suspension in the kiln exit gases.

11. A rotary kiln plant according to any of claims 5 to 10, wherein combustion air is supplied to the precalcining zone from the burner mechanism in the precalcining zone.

12. A rotary kiln plant according to any of claims 5 to 11, wherein combustion air is fed to the precalcining zone from a cooler coupled after the kiln for cooling the product treated in the kiln, the air being drawn through the kiln to the precalcining zone, or being fed directly from the cooler to the precalcining zone through a pipe by-passing the kiln.

13. A rotary kiln plant according to claim 5, substantially as described with reference to Figure 1 or Figure 2 of the accompanying drawings.