CA1218083A - Method for the production of cement clinkers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A method for producing cement clinkers using a cement raw meal composed of broken or ground raw stone which is calcined in a furnace, including the steps of forming the raw meal into briquettes and subsequently calcining the briquettes in a tunnel kiln, the briquettes being stacked in layers and arranged spaced apart in order to achieve a good flow of hot gases.

Description

The present invention relates gene.ally to the production of cement clinkers and more particularly to a method in which a cement raw meal composed of broken and ground raw stone is formed into bri~uettes which are heated in a furnace until they undergo sintering.

In the production of cement, the step of burning to clinkers at temperatures exceeding 1400C represents the most important step in the process. From raw materials, which comprise lime and clay, there results a synthetic product composed primarily of calcium silicate compounds, as well as calcium aluminate and calcium aluminate ferrite compounds. This product is finely ground and displays characteristics enabling it to bind with water and remain stable under water. Conseauently, the basic aspect of cement manufacturing is the furnace operation.

Cements were first calcined in shaft furnaces, but they were of a type related more to highly hydraulic lime than to cement as contemplated in the present sense. Furnaces similar to those for the calcining of lime were used which were operated in 2Q charges, wherein the fuel and the material to be calcined were introduced in layers. Natural stone was used whose components of lime and clay corresponded to the desired composition. This rock combination had a good gas permeability in the furnace shaft and was thus capable of producing high outputs without inter-fering with furnace operation. However, since such natural stone formations adapted for use as raw materials were of limited supply, it was necessary to utilize manufactured synthetic S cement raw mixtures of finely ground limestone and clayO However, the expenditures required for homogenizing the raw meal and for granulation thereof in preparation for calcining were very sub-stantial and the technology utilized was paxticularly adapted from the field of brick manufacturing.

The granulation of cement raw meal with the aid of water on a granulating plate represented some progress, wherein the moisture content of the granulated material was on the average of 12 to 14~ by weight. Furnaces which were originally inter-mittently operated were replaced with continuously operated shaft lS furnaces which required solid, low-gas fuels for their operation.
Preferably coke, anthracite or also pitch coal were used and the solid fuel was added separately into the granulated material.

When calcining such materials in shaft furnaces, conditions caused by the characteristics of the material occur in that the granulated materials shrink at temperatures above 1280C and sinter together. This results in a solid clinker block which often moves away from the furnace wall and there result passages for the flue gases which cause uneven calcinlng.

The daily output of a modern cement shaft furnace i5 approximately 200 tons with a furnace having a diameter of 3 m and a shaft height of approximately 10 m. Heat consumption in the shaft furnace is considered favorable and is found to be approximately 800 kcal per kg of clinkers.

An advantage of the shaft furnace is that it requires low capital investment and involves simple operation at high availability. However, a disadvantage is that a uniform clinker calcining is not ihsured. For high grade products, for example, prestressed concrete or concrete for manufacturing~ where exact stripping time is important, shaft furnace clinkers cannot be used. In addition, such calcining requires low-gas, solid ~uels.
Firing with li~uid or gaseous fuels has not been successful thus far.

Another calcining unit used for the production of Portland cement involves a circular kiln which was first used for the production of brick but is now also used for the production of Portland cement. This kiln operates continuously with a moving calcining zone and a stationary calcining material charge, wherein cement briquettes are fed in just like brick. In the periphery of the kiln at regular intervals doors are arranged to charge and empty the individual kiln sections which are walled up during calcining. The cement briquettes are heated in the kiln to a temperature such that they can be removed after calcining as burnt ~Z~ 33 brick from the kiln. For compression of the unfinished pieces, impact stamp presses with two dies are preferably used~

In the calcining of cement, circular kilns were eventually replaced with rotary kilns particularly due to the high work expenditure for charging and emptying the circular kiln which had to be performed under un~leasant working conditions and due to the high alternating temperature stresses arising in the wall of the kiln. The rotary kiln became the most important calcining unit utilized in the cement industry and today such units are almost exclusively used~

In such units, first the calcining material is fed in as sludge and granulation of the calcininy material before use in the furnace is no longer necessary. Today, the raw meal, i.e., a dry powder, is used for calcining.

lS As a result of further developments, the rotary kiln has been provided with a cyclone preheater, also called a suspended gas heat exchanger, wherein the raw meal is also fed into the ~urnace system as a powder without moistening. The raw meal is usually heated in four temperature stages of 25 sec. in duration to approximately 750C to 800C and is then deacidified by approximately 20~.

8;3 In the wet process rotary kiln, heat consum~tion is at approximately 15~0 kcal/kg of clinkers, but in the dry process rotary kiln, it is reduced to approximately 1200 kcal/kg clinkers and in kilns with suspended gas heat exchangers, heat consumption is approximately ~70 kcal/kg. In small plants this value is somewhat higher and in large plants it is somewhat lo~er.

Several advantages of the rotary kiln with the heat exchanger are: its low heat consumption; a fast, almost ideal heat transfer from flue gas to calcining material in the temper-ature range up to 800C; the adaptability to use of solid, liquidand gaseous fuel; and its high production output and high avail-ability.

The significant disad~antage of the rotary kiln is that economical production is only possible in large plants.

Other calcining units for cement production, for example the continuously moving hearth and the turbulent layer furnace, were not accepted for use due to connected deficiencies, particularly in the process area.

Due to the high demand for cement in industrial countries where there is a very high density of cement ConsumPtiOn, the dry process rotary kiln is currently almost exclusively used.
Variations in such use are limited only to the type of preheating, _5_ e.g., cyclone preheater with or without additional heating, and to the type of clinker cooling utilized, e.g., planetary cooler, grate cooler, or shaft cooler. There has thus resulted a great decrease in costs by increasing furnace output. Furnace output has been increased from 300 tons per day in 1950 to a daily output oE 5000 to 6noo tons daily in 1980. Although for a daily produc-tion of 500 tons, a specific heat consumption of 870 kcal/kg clink-ers is re~uired, the heat consumption in the unit producing 300 tons daily output is 760 kcal/kg clinkers. A similar decrease also results in labor costs and in total investment costs related to yearly capacity (specific investment cost). These figures show clearly that a large uni-t offers enormous advantages with regard to production costs. However, such advantages can only be main-tained when there is high demand enabling a plant to be uniformly utilized over an entire year with the market having a correspond-ingly high density of cement consumption. Furthermore, short transport distances are significant because price is established by taking into consideration transportation costs.

However, in many countries of the world, the density of cement consumption is significantly lower than in the industrial countries. This means that, assuming otherwise similar conditions, the cement must be transported to these countries over a sig-nificantly larger transport radius. For instance~ the median sales radius for a large plant of 3000 tons in an industrial country such as West Germany i5 approximately 40 km. However, in a clevelopiny country the radius would be approximately 200km.
This c1i~erence shows clearly that larye plants are not sui-t-able for countries wi-th a low density oE cement consumption.
~ccordinyly, the present invention is based upon the task o~ cieveloping a process for the manufacture of Portland cement clinkers in such a manner that small plants can be built which will be competitive and economical.
According to the present invention there is provided a me~lod for making cement wherein cement clinkers are produced from a cement raw meal composed oE broken or ground raw stone which is calcined in a furnace, said method being particularly suitable for enabling the production of cemen-t clinkers econom-ically in smaller sized plants, comprising -the steps of Eorming said cement raw meal into briquettes while in a dry state wi-th-out the addi-tion of a binding agent by compressing said cement raw meal under pressure of at least 200 bar; stacking said formed briquettes in layers in an arrangement with said formed briquettes spaced Erom each other so as -to enhance flow of hot gases through said stacks and around said brique-t-tes; and calcining said briquettes by passing said stacked layers of briquettes sequentially through a tunnel kiln with said briquettes stacked in said spaced arrangement.
Briefly, the present invention may be described as a process for the production of cement clinkers, wherein a cemen-t raw meal composed of broken or ground raw stone is calcined in a furnace, comprising the steps of forming the cemen-t raw meal into briquettes and subsequently calcining said briquetted raw material in a tunnel kiln with said brique-ttes stacked in layers and arranged spaced apart -to achieve a good flow of hot gases.
Thus, the invention overcomes many of the problems of the prior art in that the cement raw meal is briquetted and is subsequently calcined in a tunnel kiln wherein the briquettes which are s-tacked in layers are arranged spaced apar-t in order jJ, ~ 7 to achieve a satisfactory flow of hot gases.
The invention is based on the realization that decreas-incl cost can be achieved witllout necessarily requiring increas-illC3 plant size. This realiza-tion develops from the fact that the superiority oF large plants in industrial production is based primarily on the deliverations of cost per item. However, if the transportation costs represent a significatn-t profit-reducinc3 Eactor, small plan-ts are particuiarly accep-table when, by means of a different technology, the plant size can be adapted to the market potential whereby more favorable manu-facturing costs result also in small plants. This different technology is defined by the process according to the invention which is also suitable Eor the manufacturing o~ special cements, for instance white cemen-t, which are used in significan-tly smaller amoun-ts than ordinary Portland cement.
In one embodiment oE the ~resent invention solid fuels are mi~ed in with said cement raw meal before forma-tion thereof into said brique-ttes. Desirably said cemen-t raw meal is pressed into briquettes on presses which act on two sides tllereof. Preferably said tunnel kiln is operated during heating and sin-tering in counterflow operation, and during cooling, particularly in the -temperature range of 1450 to 1250C, in one of transverse flow or counterflow operation. Suitably the clinker mineral formation of solid reactions is reinforced by usiny briquettes with a raw densi-ty of at least lo 8 kg/dm3.
The present invention will be further illustrated by way of the accompanying drawings illustrated and described a preferred embodiment of the inven-tion and in which:
Figure 1 is a schematic diagram of a tunnel kiln;
Figure 2 is a section along line II-II in Figure l;
Figure 3 is a schema-tic side view of a double-action hydraulic press for producing briquet-tes;
Figure ~ is a diagram showing -the position of the ., ~.
~; -8-~IL~18~33 two press pistons of the press during the pressing of the briquettes; and Figure 5 is a diagram showing the ejection of th~
briquettes.

DETAII.ED DESCRIPTION OF THE PREFERRED EMBODIMENT
A tunnel kiln shown in Figures 1 and 2 has a pre-calcining (preheating) zone V, a calcining zone B, and a cooling zone K. Three charges 2, 3, 4 of briquettes located in the tunnel kiln 1 are arranged in intermediate spaces 6 in order to achieve a good flow of hot gases (Figure 2). The charges 2, 3, 4 are fed into the tunnel kiln 1 from an input side E and they are removed from the kiln on an output side A~ Cold combustion air flowing in the reverse direction enters on the output side A, heats up at the already calcined charge 2, burns added fuel in the calcining zone B and exits as flue gas on the input side E while heating a freshly fed-in charge 4.

The charges 2, 3, 4 are arranged on traveling carriages 7 which form the kiln floor and which closely follow one another.

Fuel is fed through burners 8 into the tunnel kiln, the burners being arranged in the area of the calcining zone B.
Gaseous, liquid or solid fuels can be burned. An access duct 9 ma~es possible inspection of the carriages 7.

Figures 3, 4, and 5 show the production of bri~uettes 5. In Figure 3 there is shown a double-action hydraulic press -8a-having an upper yoke 10 and a lower yoke 11 connected with one another by means of columns 12. A hydraulic cylinder 13 having a press plate 15 ~astened at a piston rod 14 is supported in the yokes 10, 11. A die 16 is supported between the two press plates.

During the pressing of a briquette 5, the raw material or raw meal is fixst inserted in the die 16, wherein the lower piston 15 forms the bottom of the die. After the die 16 is filled, the pressing process begins as seen in Figure 4. The pressplates orpistons 15move againstone anotherand in so doin~, press the briquette 5. After the final pressing (Figure 5), the upper press plate is moved back and the lower press plate ejects the briquette 5 from the die 16 so that it can be removed from the press. The process now repeats itself in the same manner.

The briquettes 5 are then inserted into the tunnel kiln to form the charges 2, 3, 4.

According to more recent developments relating to clinker calcining, the mineral formation in the calcined material proceeds in dependence on the calcining temperature in such a way that at 600C, the calcium carbonate reacts with SiO2 to form Ca2~i (2Ca~.Sio2) and the aluminate component of clay reacts with the calcium carbonate to form calcium aluminate and ferrite compounds while yieldin~ CO2.

-8b-_ ,. ..

In a rotary kiln with a cyclone preheater, this effect cannot be utilized because the reaction components in the gas/
solid suspension have no contact with one another. In rotary kilns with additional heating in the cyclone, the reaction particles are even isolated up to the temperature range of 850 to -8~-: L2~ `83 900C. A reaction conversion onlv takes place after agglomer-ation in rotary kilns and at higher temperatures.

However, during calcining of bri~uettes in a tunnel kiln, the above-described reactlons can in fact -take place. The resulting new formations give the briquette a ceramic binding.
Free CaO develops in the briquette only at a temperature above 900C when the speed of the limestone dissociation is greater than the lime binding reactions. The early C2S formation and the multitude of C2S crystals which result act positively on the sintering process and the quality of -the finished product. How-ever, it is a prerequisite that the raw meal be ground just as finely as for the rotary kiln opera-tion. Calcining in tunnel kilns consequently offers advantages compared to the rotary kiln operation.

15 Starting at 1280C, the first melting phase develops which is commensurate with sharp shrin~age of the briquette. In a conventional Portland cement raw mixture, the portlon of the melting phase is on the average 20%. However, i~ raw material mixtures which are rich in Fe2O3 and A1203, the melting portion may increase to a maximum of 30%. The melt is viscous so that no problems regarding the stability of the briquette in the sintering zone occur. Ca3Si (3Ca.SiO2) forms from 1280 to 1450C
over the melting phase ofCa2Si and CaO. In view of the above~

.

described process, the sintering temperature in the tunnel kiln is therefore lower than in a rotary kiln. The cooling from 1450C to 1250~C can also take place in the tunnel kiln with the desired speed (rapid cooling), i.e, within approximately 30 minutes, in order to produce a cement with high strength values and normal solidification behavior. Also cooling in the tunnel kiln from 1200C to approximately 200C can take place slowly to improve the grindabillty of the c]inker.

Advantageously, the cement raw meal is brlquetted without the addition of a binding agent in the dry state, i.e., it is pressed to form briquettes, and it is then calcined during which solid fuels may also be mixed in. The moisture content during pressing is advantageously 0 to 5%, see Ullmann's Encyclopedia, 4 Ed., 1977, ~ol. 13, p. 719 reyarding the compres-sion of powders.

The briquetting, i.e., the dry compres~ion of cementraw meal, can advantageously take place in two stages.

This may take place either on a single pressing device wherein the cement raw meal is first precompressed, the compression pressure being reduced for venting of a precompressed article. Subsequently, the compressed article is finally compressed, or the cement raw meal is precompressed in the first compression stage on a separate machine, for instance an oval ~Z3~ 3 briquette press, and the oval briquettes are subsequently in a second stage brought into their final square shape with specific dimensions on a press.

For precompression there may also be used machines S which produce so-called shells. This does not relate to uniformly shaped articles but to board-shaped plates up to lOmm thick which are broken and strained to the desired size.

Precompression serves to remove the air from the powder mixture whereby defect-free, i.e., texture-free, pressed articles are obtained.

For the final compression, specific pressures of at least 200 kp/cm are applied.

During calcining the tunnel kiln can advantageously ~e operated during heating and sintering in counterflow. However, during cooling, particularly in the temperature range of 1450C
to 1250C, it can be operated in counterflow or transverse flow, as desired.

A specific example of the practice of the method of the invention is set forth hereinafter.

Example Utilizing a furnace for clinker production of 260 tons per day, a raw meal briquette of a size of 240 x 115 x 115 mm is selected. This results in a volume of 3.17 1 and a weight of 5.7 kg raw meal (density - 1.2 kg/dm3 = 3.75 kg calcined clinker).

The selected tunnel kiln has a width of 4.0 m and a height of 1.31 m. At a space utilization of 70~ this results in 220 stones/m .

The desi~n of the tunnel kiln is based on a temperature gradient of 120 K/h. With the periods for heàting to the decomposition temperature being as follows:

limestone 8 hours deacidifying 8 hours sintering 2 hours cooling 6 hours . _ there results a transit time of 24 hours.

In view of t~e temperature differences to ~e expected in the tunnel kiln, this time is doubled so that the transit time through the tunnel kiln is 48 hours. The length of a tunnel kiln carriage is 2.50 m and the advancing speed is 2.5 m/h. With a kiln cross-section of 5.24 m2, 1153 brique~tes per meter, corresponding to an amount of 4225 kg of clinkers/m, can be fed in.
With an advancing speed of 2.5 m/h, clinker production will be 10.8 tons/h or 260 tons/day. The length of the tunnel kiln is approximately 115 m (120 m). The briquettes are produced, without the addition of a bindin~ agent, on a hydraulic press which acts, for example, on two sides thereof, with pressures of up to 200 bar and more, and with an operating speed of 400 strokes per hour.
For each stroke, six stones of a size of 240 x 115 x 115 mm in two rows are always pressed with a raw density of at least 1.8 kg/dm3 so that the entire output is 4800 stones; that is, 27.7 tons of raw meal per hour or 18 tons of clinkers per hour. At an operating time of 21 hours per day, this results in an output of 378 tons of clinkers. If the stone size were to be increased to a length of 300 mm, this would resul~ in an output of 475 tons of clinkers per 24 hours which corresponds to a dally output of 500 tons cement.

With such a plant~ the somewhat higher specific capital investment costs and personnel costs result in only slightly higher production costs than in a large plant, bu~ this results in a noticeable cost advantage in areas with low density of cement

2 S con s umpt ion.

~l8~383 Thus, with the process in accordance with the present invention, a cement raw meal composed of broken or ground raw stones is formed into briquettes in a press with pressures of 200 bar or more. The briquettes are produced in the dry state and are stacked in layers and spaced apart on a furnace carriage and are heated in a tunnel kiln up to sinteriny. Due to the use of a tunnel furnace, somewhat higher production costs rPsult than in a large plant with a dry process rotary kiln. Since the transportation costs represent a significant profit-reducing factor, the process results in a noticeable cost advantage when there is a lower density of cement consumption than in industrial countries.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the inventive principles, it will be understood -that the invention may be embodied otherwise without departing from such principles.

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for making cement wherein cement clinkers are produced from a cement raw meal composed of broken or ground raw stone which is calcined in a furnace, said method being particularly suitable for enabling the production of cement clinkers economically in smaller sized plants, comprising the steps of forming said cement raw meal into briquettes while in a dry state without the addition of a binding agent by compressing said cement raw meal under pressure of at least 200 bar; stack-ing said formed briquettes in layers in an arrangement with said formed briquettes spaced from each other so as to enhance flow of hot gases through said stacks and around said briquettes; and calcining said briquettes by passing said stacked layers of briquettes sequentially through a tunnel kiln with said briquettes stacked in said spaced arrangement.

2. A method according to claim 1, wherein solid fuels are mixed in with said cement raw meal before formation thereof into said briquettes.

3. A method according to claim 1, wherein said cement raw meal is pressed into briquettes on presses which act on two sides thereof.

4. A method according to claim 1, wherein said tunnel kiln is operated during heating and sintering in counterflow operation, and during cooling, particularly in the temperature range of 1450 to 1250°C, in one of transverse flow or counterflow operation.

5. A method according to claim 1, wherein the clinker mineral formation of solid reactions is reinforced by using briquettes with a raw density of at least 1.8 kg/dm3.