WO1996026165A1

WO1996026165A1 - Method for manufacturing portland cement clinker

Info

Publication number: WO1996026165A1
Application number: PCT/DK1996/000077
Authority: WO
Inventors: Jesper Sand Damtoft; Peter Lund
Original assignee: Aalborg Portland A/S
Priority date: 1995-02-23
Filing date: 1996-02-21
Publication date: 1996-08-29

Abstract

The invention relates to a method for manufacturing Portland cement clinker, subject to addition of a material to a clinker cooler, by which method cement raw meal may be preheated and calcined prior to being burned into clinker, and where the clinker is cooled in a clinker cooler. The calorific value of the material which is added to the cooler is regulated by mixing of at least two different types of material, where at least one of the materials which is added to the clinker cooler has a calorific value which exceeds the desired value and at least one other material has a calorific value which is lower than the desired value, and in that the total mixture has a certain water content.

Description

Method for manufacturing Portland cement clinker

The present invention relates to a method for manufacturing Portland cement clinker, by which method raw meal is preheated and calcined prior to being burned into clinker, and where the clinker, subject to addition of a material, is cooled in a clinker cooler.

Portland cement clinker is manufactured on the basis of a raw mix which is for example composed of limestone, flyash, sand and pyrite ash. After the raw mix has been ground, preheated and calcined, it is burned in a rotary kiln at temperatures reaching a level of 1450-1500^βC in the burning zone of the kiln. During the burning process in the kiln, the raw materi- als will react with one another, whereby the clinker compo¬ nents, the so-called clinker minerals, are formed. As part of the process, the components are consolidated into nodules, the socalled clinker.

An important feature in the manufacture of cement is the cooling of the clinker. In order to avoid a deterioration of the properties of the finished cement, the clinker temperature must be rapidly reduced from a level around 1300-1400°C down to a level below approximately 200^βC. This is done in a clin- ker cooler.

A very common cooler type is the so-called grate cooler in which the hot clinker from the rotary kiln drops down onto a system of cooler grates. The clinker bed is moved by the movement of the grates backwards in the cooler, while air is being injected through the bed so that the clinker is cooled. Part of the heat which is removed from the clinker during the cooling-down phase is re-utilized in the burning process in that the heated air from the foremost part of the cooler is directed through the rotary kiln and possibly through the calciner, if the latter is incorporated in the plant, in which it is utilized as combustion air. However, the cooling air from the rearmost and coldest part of the cooler is directed backwards to a discharge point to the surrounding environment. That part of the air which is directed from the cooler and through the rotary kiln is called secondary air, and that part of the air which is directed through a calciner, if any, is called tertiary air.

Because of the high temperatures which are applied during the manufacture of Portland cement clinker, and due to the size of the plants, the fuel-related costs will be quite substantial. As a result, secondary fuels are increasingly being put to use, e.g. various industrial by-products or types of waste. Furthermore, many of these fuels have a large content of components which may be regarded as raw materials in connec¬ tion with the cement manufacturing process.

The benefits of utilizing these secondary materials are that the costs in regard to primary raw materials and fuels will be lowered, while, at the same time, materials otherwise targeted for dumping or destruction can be utilized.

Many secondary fuels are, however, of such a form that they cannot be introduced into the kiln system in the same way as the comparable primary raw materials and fuels. For example, many secondary fuels cannot be introduced to the kiln through the burners of the kiln. This may, for example, be ascribed to the fact that they cannot be pulverized in a coal mill, that they cannot be transported in conventional pneumatic or mecha¬ nical systems, or that they cannot be sufficiently atomized in the burners of the kiln.

One possible method for utilizing difficult-to-handle secon¬ dary fuels is to add such fuels to the riser duct which con¬ nects the kiln to a preheater, if any, or to the precalciner or the calciner in the types of kiln equipped with such fea¬ tures. This would, however, require that the form of the secondary fuels is such that the fuels can be added by means of, for example, pneumatic or mechanical conveying devices, such as screw conveyors, elevators or belt conveyors.

Another option involves feeding of the difficult-to-handle fuels to the hot end of a clinker cooler, preferably a grate cooler. Hence, the temperature of the secondary and tertiary air will be increased, so that the fuel feed rate through main burners and burners in precalciners or calciners can be redu- ced.

The point of introduction in the clinker cooler must be selec¬ ted so that the combustible material is totally burned out while falling through the cooler or when deposited on the clinker bed itself. The material must, therefore, be fed to the hottest part of the cooler and it may be advantageous to introduce the material to the kiln hood or the connecting piece which constitutes the interface between the kiln and cooler. By feeding the material to the hot part of the cooler it will also be possible to avoid the risk of gases which are not completely burned being directed backwards in the cooler and thus being released to the surrounding environment.

Feeding of a combustible material to the hot part of the clinker cooler is for instance known from DE Al 26 24 971. In this publication waste and industriel by-products are used as secondary fuels for example by production of cement clinker. The primary object of the method according to this publication is to eliminate hazardous waste products, in a way where both ashes and a large part of the harzardous volatile components are bound in the clinker. The secondary object is to utilize the available calorific value of the waste products and to make use of what the waste product might contain of raw ma¬ terials. In the German publication it is recommended to add secondaty fuels with a large content of volatile matter to the hottest part of the clinker cooler (cp. page 9, lin. 29 - page 10, lin. 13) and to add secondaty fuels with a large content of raw materials before the primary raw materials enters the rotary kiln (cp. page 11, par. 2).

There is a specific limit to the amount of fuels which can be added to the clinker cooler, since addition of excessive volumes may cause the cooler to be overheated, hence elevating the discharge temperature of the clinker to an unacceptable level. There is also a maximum limit to the extent to which the secondary air temperature can be increased without causing too high temperatures in the outlet of the rotary kiln and, as a consequence hereof, damage to the outlet sectors, which must protect the edge of the kiln shell, and the lining in the connecting piece between rotary kiln and cooler.

It is, therefore, the objective of the invention to provide a method for manufacturing of Portland cement clinker by means of which a greater amount of material can be fed to the clin¬ ker cooler, while the secondary air temperature is simultane¬ ously controlled, without causing damage to the kiln outlet, and ensuring, at the same time, that the clinker produced is of a good quality.

This objective is achieved according to the invention by adjusting the caloric value of the material which is added to the clinker cooler by mixing of at least two different types of material, where at least one of the materials which is added to the clinker cooler has a calorific value which ex¬ ceeds the desired value and at least one other material has a calorific value which is lower than the desired value, and in that the total mixture has a certain water content.

A simple method by means of which a material with a given calorific value can be manufactured is to mix at least two different materials, of which at least one material has a calorific value which exceeds the desired value and where at least one other material has a calorific value which is lower than the desired value. By regulating the ratio between the added materials with different calorific values it will also be possible to regulate continuously the temperature of the secondary air. At least one of the added materials must have a certain water content since the water has a cooling effect on the surroundings around the point of introduction.

Used Fuller's earth is a waste product which can be used for carrying out the method according to the invention. Thus, used Fuller's earth has a relatively high calorific value and also has a mineral composition which, without problems, can be incorporated into the cement clinker manufacturing process.

Used Fuller's earth emerges as a by-product when manufacturing miscellaneous vegetable, animal and mineral oil products, e.g. rape-seed oil, margarine and diesel oil. The oils contain a number of turbid features which, because of a very small particle size, cannot be removed by filtration in conventional filters. Therefore, the oil is mixed with Fuller's earth which is manufactured on the basis of naturally occurring clay minerals belonging to the smectite group. These clay types are also referred to as bentonite, montmorillonite and fuller's earth. The Fuller's earth absorbs the turbid features where¬ after it is subjected to filtration in traditional filters, such as chamber filter presses. Hence a homogenous by-product, used Fuller's earth, is generated.

If potential applications for used Fuller's earth cannot be found, the material must either be targeted for dumping at public landfill or it must be disposed of in alternative manner. This may give rise to environmental nuisances for the society, and the manufacturer will be confronted with sub¬ stantial costs in connection with the disposal.

Used Fuller's earth typically has a calorific value of about 3.000 kcal/kg and it contains approximately 40% oil and 60% clay, which means that in connection with the manufacture of clinker it may serve both as a fuel and as a raw material which is used in partial substitution for clay, shale, sand and flyash. Experience gained during the manufacturing process indicates that the used Fuller's earth is ignited and com¬ busted almost instantaneously when it is introduced to the hot end of the cooler. As a consequence hereof, the material is completely burned out when that part of the ash which is not entrained in the secondary airstream into the rotary kiln lands on the clinker bed on the cooler grate. It is thus ensured that burning material is not directed backwards in the cooler, which would entail risk of gases not completely burned being released to the surrounding environment. However, experience has shown that it is not expedient to use used Fuller's earth in pure form. The reason for this is twofold:

- The used Fuller's earth is ignited so rapidly that explosive flames will burst through the feed point, causing the feed device to be damaged.

- Used Fuller's earth is very sticky and conveying devices made up of, for example, screw conveyors, belt conveyors and elevator will be clogged, in a matter of minutes or a few hours, due to accumulations of the sticky Fuller's earth.

Another material which may be used for carrying out the method according to the invention is de-inking sludge derived from the paper manufacturing process.

Paper sludge from de-inking process emerges as a by-product in connection with the manufacture of recycled paper and its water content is typically 30-60%. On a dry basis, paper sludge typically contains 50% paper fibres, 25% clay of the kaolin type, and 25% chalk. The calorific value is around 500-1000 kcal/kg. De-inking sludge is produced in vast quanti¬ ties during the manufacture of recycled paper and, therefore, poses a serious environmental problem unless appropriate fields of applications for the sludge are identified.

It is well-known practice to add de-inking sludge to the kiln system when manufacturing Portland cement clinker, where it is used as a secondary fuel and as a raw material. De-inking sludge is non-sticky and can be transported by means of mecha¬ nical conveying devices such as screw conveyors, elevators or belt conveyors.

Quite surprisingly, it has now been discovered that the appli¬ cation of used Fuller's earth as a fuel and raw material through addition to a clinker cooler is substantially improved if, prior to addition, the used Fuller's earth is mixed with de-inking sludge from the paper manufacturing process. By mixing used Fuller's earth and de-inking sludge in a suitable ratio it is obtained that the mixed material will be less sticky and hence much easier to handle, which means that it can be transported in traditional conveying systems, such as screws, belt conveyors and bucket elevators. It is simultane¬ ously obtained that the mixed material, probably because of the water content in the de-inking sludge, is ignited at a sufficiently slow rate, hence preventing damage to the feed device and kiln outlet.

In regard to the improved handling characteristics obtained when mixing used Fuller's earth with de-inking sludge derived from the paper manufacturing process, this property can be utilized also if it is desirable to use spent Fuller's earth at another stage of the process during the clinker manufac¬ turing process, for example through feeding to a calciner or a riser duct.

In principle it is possible to burn any kind of secondary fuel when using the method according to the invention, as long as it is possible to regulate the caloric value of the secondary fuel as added to the cooler, for example by mixing with anot¬ her or several other materials.

A lot of different kinds of fuels are interesting in connec¬ tion with the method according to the invention, for example liquid substances such as oil, solvents, emulsions containing various combustible substances, slurry containing plastic etc., pasty or solid substances such as coal, petcokes, shred- ded automobile tyres, used oil filters, plastic waste, oil sludge, de-inking sludge derived from the paper manufacturing process, used Fuller's earth, dewatered or dried sewage, house or town refuse etc..

It has proven to be particularly advantageous to use the method according to the invention for manufacturing minerali¬ zed clinker. The manufacture of mineralized clinker involves addition of mineralizers to the raw meal in small quantities, hence reducing the burning temperature while promoting the formation of alite, which is the most important strength-en¬ hancing and hydraulic component of the cement.

A combination of fluoride and sulphate, where the clinker contains at least 0.15% F and 1.5% S0₃ of the clinker weight has proven to be particularly suited as a mineralizer.

It is estimated that the use of the described combination of mineralizers will cause the temperature in the burning zone of the rotary kiln to be reduced approximately 100^βC, but, still, the most important effect achieved when using these minerali- sers is not the reduction of the burning temperature, which will only provide moderate savings in terms of fuel usage. The most important effect is that the alite content of the clinker can be increased and that the hydraulic properties of the clinker components are improved. These benefits can be used to manufacture cement with improved properties or as a means of reducing the price of the cement by inclusion of various additives, such as flyash or limestone, without leading to a deterioration of the properties of the cement in comparison with pure Portland cement manufactured without any use of mineralizers.

Application of the described combination of mineralizers involves, however, certain problems during the manufacturing process.

One problem relates to the deterioration of the formation of clinker. The clinker formed becomes more porous than is the case when manufacturing conventional Portland cement clinker, and the dust load in the kiln is substantially increased. This will impede the reactions which lead to the formation of clinker minerals because of insufficient physical contact between the constituent parts of the clinker and due to the fact that the dust will have a negative impact on the trans¬ mission of heat from the burner flame to the clinker.

Since, as mentioned above, the temperature in the burning zone of the kiln is reduced by approximately 100^βC when using the described combination of mineralizers, the temperature of the clinker in the clinker cooler will also be lower than is the case during the manufacture of conventional Portland cement clinker. This is reflected in a lower temperature of the secondary air, leading to a corresponding reduction of the amount of heat which is transmitted from the clinker cooler to the burning zone.

All of these factors will have an unfavourable influence on the heat profile of the kiln, and may result in unsteady kiln operation. The amount of free lime in the clinker rises, which will lead to an increase in the dust load. Normally, attempts are made to compensate for this phenomenon by increasing the fuel consumption, but it will not always be possible to rever- se this unfavourable trend, which may cause the kiln to become too cold, hence making it necessary to interrupt production.

Quite surprisingly, it has now been ascertained that the addition of fuel to the hot end of the clinker cooler will have a favourable influence on the manufacturing process. It has also emerged that when manufacturing Portland cement clinker by means of mineralizers it will be possible to in¬ crease the addition of fuel to the clinker cooler to a higher level than is achievable during the manufacture of conventio- nal Portland cement clinker, without causing the cooler to be overheated or without the discharge temperature of the clinker being elevated to an unacceptable level.

Unsteady kiln operation during the manufacture of mineralized clinker due to a deterioration of the temperature transmission in the kiln and a reduction in the secondary air temperature may thus be prevented by adding combustible material to the hot end of the clinker cooler. The addition of material will increase the temperature of the secondary air, improve the heat distribution in the burning zone of the rotary kiln, reduce the content of free lime in the clinker and limit the dust load. Kiln operation will be stabilized and the fuel consumption of the main burner can be reduced.

A further aspect of the invention emerges if the material which is fed to the clinker cooler has a certain content of clay minerals, which, for example, is the case for Fuller's earth and de-inking sludge. Such a content of clay minerals may be considered to be an extra addition of pozzolanic ma- terial. According to literature (see e.g. Mielenz R.C., Witte, L.P., and Glantz, O.J.: "Effect of calcination on natural pozzolana." Symp. on Use of Pozzolanic Materials in Mortars and Concretes, STP-99, American Society for Testing and Ma¬ terials, Philadelphia, 1950, 43-91) it is a well-known fact that clay minerals will obtain pozzolanic properties, i.e. they contribute to the strength development of the finished cement if heat-treated within a temperature range around 600-900"C in such a way that the crystal water of the minerals is liberated and the crystal structure is destroyed, without any recrystallization of the amorphous minerals.

This condition applies, for example, both for the manufacture of mineralized and for the manufacture of conventional Port¬ land cement clinker. The added clay minerals will thus contri- bute towards developing the strength of the cement, hence leading to a reduction in the price of the cement through substitution of a corresponding quantity of clinker. The influence of the temperature on the clay minerals which are fed to the clinker cooler must be controlled so that the clay minerals are exposed to temperatures of 600-900^βC during an appropriate time period in order to achieve the optimum degree of pozzolanicity. This control action can be performed by varying the calorific value of the feed material, which may, for example, be a mixture of used Fuller's earth and de-inking sludge, where the calorific value is regulated by varying the mixing ratio.

The following two examples illustrate how Fuller's earth and de-inking sludge can be used in connection with the anufac- ture of conventional Portland cement clinker and mineralized Portland cement clinker, respectively:

Example 1. A mixture of used Fuller's earth and paper sludge was added to a semi-wet process during the manufacture of conventional Portland cement clinker. The plant deployed comprised a rotary kiln with a two-stage cyclone preheater and SLC unit (Separate Line Calciner), and a grate cooler.

At a clinker output of 5,400 t/day, the used Fuller's earth and paper sludge were fed to the grate cooler in a ratio of 1:4 at a total rate of 4 t/hour. A limited rise in the secon- dary air temperature was observed. Apart from this, the addi¬ tion did not have any noteworthy effect on the kiln operation.

Example 2.

A mixture of used Fuller's earth and paper sludge was fed to the same plant as described in Example 1 during the manufac¬ ture of mineralized cement clinker.

At a clinker output of 4.600 t/day, used Fuller's earth and paper sludge were fed to the grate cooler in the ratio 2:1 at a total rate of 2.5 t/hour. The secondary air temperature was increased drastically as a result of the addition. This led to a reduction of the dust load in kiln, a reduction of the free lime content in the clinker, and stabilization of kiln opera¬ tion.

Claims

PATENT CLAIMS

1. A method for manufacturing Portland cement clinker, by which method raw meal is preheated and calcined prior to being burned into clinker, and where the clinker, subject to addi¬ tion of a material which has a specifically adapted calorific value depending on the type of clinker produced, is cooled in a clinker cooler C H A R A C T E R I Z E D I N T H A T the caloric value of the material which is added to the clinker cooler is regulated by mixing of at least two different types of material, where at least one of the materials which is added to the clinker cooler has a calorific value which ex¬ ceeds the desired value and at least one other material has a calorific value which is lower than the desired value, and in that the total mixture has a certain water content.

2. A method according to claim 1, C H A R A C T E R I Z E D I N T H A T the material which has a calorific value which exceeds the desired value is used Fuller's earth.

3. A method according to claim 1, C H A R A C T E R I Z E D I N T H A T the material which has a calorific value which is lower than the desired value is de-inking sludge with a water content of 30-60%.

4. A method according to claim 1, C H A R A C T E R I Z E D I N T H A T the manufactured clinker is mineralized clinker and in that the material which is added to the clinker cooler has a calorific value < 1500 kcal/kg material.

5. A method according to claim 4, C H A R A C T E R I Z E D I N T H A T the material has a calorific value of 1800 - 2500 kcal/kg material.

6. A method according to claim 4 or 5, C H A R A C T E ¬ R I Z E D I N T H A T the material is a mixture of Fuller's earth and paper sludge with a water content of 35-55%, so that calorific value of the finished material is maintained within the given range.

7. A method according to claim 1,

C H A R A C T E R I Z E D I N T H A T the material contains a fraction of clay minerals which are given an appropriate time of retention in the cooler within the temperature range i 600-900^βC.