GB2467602A

GB2467602A - A Method of Treating Cement Kiln Bypass Dust

Info

Publication number: GB2467602A
Application number: GB0902169A
Authority: GB
Inventors: Andrew Mark Elliot
Original assignee: CASTLE WASTE SERVICES Ltd
Current assignee: CASTLE WASTE SERVICES Ltd
Priority date: 2009-02-10
Filing date: 2009-02-10
Publication date: 2010-08-11
Also published as: GB0902169D0

Abstract

A method of treating cement kiln bypass dust wherein the dust, comprising chlorides, is mixed with a liquid to form a solution in which the chloride compounds dissolve. The solution is then separated from the residual particulate matter which did not dissolved in the liquid. The liquid is preferably water and the process removes chloride salts such as potassium chloride (KCl) and sodium chloride (NaCl). The separation stage is preferably performed by filtering using a compression filtering device such as a membrane filter press. The flow of liquid for washing the dust is controlled and the chloride content of the separated solution is monitored. The process is stopped when the level of chloride in the solution reaches an acceptable level. The residual particulate matter is dried and introduced into the cement kiln as raw material for the production of cement.

Description

I

A Method Of Treating Cement Kiln Bypass Dust

CROSS REFERENCE TO RELATED APPLICATIONS

This application represents the first application for a patent directed towards the invention and the subject matter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of treating cement kiln bypass dust, and to a method of manufacturing cement.

2. Description of the Related Art

Currently, most of the cement manufactured in the world is produced in a rotary kiln. A mixture of materials is fed to one end of a long rotating cylindrical kiln and a large concentric flame provides heat into the cylindrical kiln at its opposite end. As the kiln is rotated the mixture of materials travels along the cylinder towards its hottest end and undergoes several chemical and *... 15 physical changes resulting in cement clinker emerging from the opposite end S...

of the cylinder.

In recent years, it has been known to supply the rotary kiln with preheated material from a precalciner. The precalciner itself receives fuel that is at least partially burnt within the calciner to heat the mixture of materials and :: 20 start the process, including calcination, before the materials enter the rotary kiln.

The process of producing the cement clinker requires large amounts of energy, and consequently a large proportion of the cost of manufacturing cement comes from the cost of the fuel required. Because of this, it has been known in recent years for secondary fuels to be used in the precalciner that are essentiafly waste products. For examp!e, it has become known to use wood, used tyres, meat and bone-meal from rendered cattle carcasses, solid fuels made from fluffs from paper and plastics, and mixtures of hazardous industrial waste including paints, resins, adhesives, solvents, oils, etc. Ashes from the combustion of such waste fuels typically become combined with the raw materials used to make the cement and therefore become part of the end product.

A problem with using such secondary fuels, is their relatively high chloride content and high content of alkali metals including sodium and potassium. Without controlling the chloride, sodium and potassium content, the cement produced would generally have a chloride, sodium and/or potassium content that is above what is allowed by its specification. In addition, due to cycling of gases within the kiln/precalciner, and dust suspended within the gases, it is known for chloride compounds to condense in cooler parts and consequently have a detrimental effect on the operation of the kiln.

*:::: To control the chloride content of the materials entering the kiln, it is known to remove bypass dust from the kiln. The chloride compounds tend to concentrate in lighter (less dense) particles of dust that are suspended within the gas flows of the kiln and precalciner. Consequently, it is known to have : apparatus, such as an arrangement of cyclones, which sorts the lighter * particles from the heavier particles suspended within the gas flows. The heavier (denser) particles are kept within the system and consequently are fed into the rotary kiln, whereas the lighter particles, containing a higher proportion of chloride, are removed as cement kiln bypass dust.

Although the bypass dust is only a small proportion of the total mass of materials used in the cement production process, it may still amount to several tonnes per day for just one kiln. The bypass dust is typically disposed of by landfill, and due to its high content of chloride, and possibly heavy metals, hazardous waste regulations apply to the landfill. Consequently, there is a cost in the disposal of the bypass dust by landfill.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a method of treating cement kiln bypass dust as claimed in claim 1.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Figure 1 shows a method of manufacturing cement embodying the present invention; Figure 2 shows an overview of the decontamination process 111 of Figure 1; Figure 3 shows an example of apparatus used for the method described with respect to Figure 2; Figure 4 shows the filtering device 308 of Figure 2; * Figure 5A shows a cross-sectional side view of one of the filter components removed from filter device 308; and Figure 5B shows front view of the filter component of Figure 5A. *.**

*. 20 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 A method of manufacturing cement embodying the present invention is illustrated in Figure 1. The process uses a conventional rotary kiln 101 and precalciner 102. As is known, a crusher 103 receives raw material 104 including limestone, clay, and shale. The crusher 103 crushes the raw material into small particles that are supplied to the precalciner.

The mixture received by the precalciner is heated within the precalciner to drive off water and, at least, commence the calcination process.

Consequently, a mixture of dry particulate material is supplied to the rotary kiln 101 at a temperature of typically 900°c. The material within the rotary kiln is further heated by burning a fuel 105 at the opposite end of the rotary kiln.

The rotary kiln comprises a long cylindrical tube having a first end 106, receiving material from the precalciner 102, that is slightly higher than the opposite end 107, so that the rotary kiln has an axis that is a few degrees to the horizontal. The rotary kiln is made to rotate about its axis, and consequently material from the precalciner is slowly moved from the first end 106 to the second end 107. During its travel the material is heated to a temperature of typically 1400°c causing the material to be completely calcined and sintered to produce cement clinker 108. Thus, cement clinker 108 emerges from the second end of the rotary kiln 107. The cement clinker is then cooled and further processed by grinding within a ball mill (not shown) with gypsum to produce the final cement product.

: In order to heat the product in the precalciner, fuel 109 is supplied to, S. **** and burnt within, the precalciner. The fuel 109 may include conventional fuels such as coal but in the present case it also comprises a secondary fuel containing a chloride compound. This secondary fuel would otherwise be *555 regarded as a waste product, but by burning within the precalciner it becomes a useful source of energy. The secondary fuel typically comprises waste 0****S * timber, used tyres, meat and/or bone-meal from cattle rendering, solid fuels made from fluffs from paper manufacture, plastics, paint, resins, adhesives, solvents, oils, other similar liquid hazardous waste products that may be burnt to generate heat, etc. As a consequence of burning the secondary fuel, airborne particles are generated within the precalciner and rotary kiln that have a high chloride content. Typically, much of the chloride is found in the form of sodium chloride and/or potassium chloride.

It has already been found that the chloride compounds tend to concentrate in the lighter (less dense) particles of dust within the precalciner and rotary kiln atmosphere. Consequently, the lighter particles are sorted out from the heavier particles, typically in a cyclone arrangement of the precalciner. The heavier particles containing a relatively lesser amount of chloride are allowed to remain in the cement manufacturing system, whereas the lighter dust particles, having a relatively higher chloride content, are removed from the precalciner as contaminated bypass dust 110.

Conventionally the contaminated bypass dust 110 is disposed of within a landfill site licensed to receive hazardous waste. However, the present method of manufacturing cement includes a decontamination process 111 that receives the contaminated bypass dust 110 and generates decontaminated material 112. The decontamination process removes much of the chloride *: found in the contaminated bypass dust 110 and consequently, the decontaminated material may be used as a raw material in the cement manufacture process. Thus, the decontaminated material is added to the crusher 103 (or a similar crusher of another cement kiln) along with the a...

conventional raw material 104. a a a. * .. a

* Figure 2 An overview of the decontamination process 111 of Figure 1 is shown in the flowchart of Figure 2. At step 201, cement kiln bypass dust is loaded into a mixing tank, and at step 202, water is added to make a slurry. The alkalinity of the slurry is typically between pH 10 and pH 14 depending upon the efficiency of extraction. Consequently, the apparatus used to treat the bypass dust is constructed from materials selected to withstand high salt concentrations at high pH.

At step 203, the slurry is loaded into a filtering device. At step 204 excess water is removed from the slurry by compression as the slurry is pumped into the fiftering device. In an embodiment, the pressure of slurry within the filtering device is measured, and slurry is added up to a predetermined pressure threshold of typically 3 bar (3x105 Nm2).

Steps 201 to 203 are illustrated in Figure 3. An example of a filtering device as used at steps 204 to 209 is shown in Figure 4.

At step 205, the slurry is compressed between filter components. In an embodiment, this compression takes place until a predetermined pressure value is reached. Typically this predetermine pressure value is approximately 4 bar (4x105 Nm2). As a result of the compression, the slurry is dried such that it is then generally regarded as a filter-cake, rather than a slurry.

A flow of water is passed across each filter component at step 206.

During step 206, the filter-cake is kept under pressure at the predetermined *:::: value to ensure that the water passes through it and cannot flow through alternative routes that bypass the filter cake. Although water is used in this embodiment, another liquid capable of dissolving sodium chloride and S...

*. : potassium chloride may be used. A question is asked at step 207 as to * whether the water is clean enough. The water that has passed across each * filter component as described at step 206 and detailed in Figure 5 is tested on exiting the equipment. A predetermined threshold is set such that if the salinity of the water is below the threshold then the question asked at step 207 will be answered in the affirmative. An example threshold is a chloride concentration of less than six thousand parts per million. If the measured chloride level is below the threshold then step 208 is performed. In contrast, if the question asked at step 207 is answered in the negative such that the water does not fall below the threshold required then procedures loop back to step 206.

At step 208, a stream of air is blown across the filter-cake to dry the filtered material and at step 209 a final compression takes place to remove residual water. The air blowing is typically performed while the filter-cake is held under pressure at the predetermined value, and the final compression is then performed at a higher pressure of typically 7 bar to 8 bar(7x105 Nm2 to 8x105 Nm2). Consequently, the slurry is dried such that the water content is less than 40% by mass, and typically 30 to 35% by mass. This level of moisture content is considered to be sufficiently low for the decontaminated material to be reintroduced into the cement kiln along with other raw materials.

In the present embodiment a stream of air is blown across to improve drying of the filtered material. However, in alternative embodiments where moisture content of the end product is less critical, the air-blowing step 208 is eliminated.

At step 210, the decontaminated material is removed from the filtering device for subsequent transportation to a cement kiln for reuse as a raw *:*. 20 material. * * S...

Figure 3 An example of apparatus used for the method described with respect to Figure 2 is shown in Figure 3. A silo 301 stores contaminated cement kiln bypass dust 110 received from one or more cement kilns. At this stage the bypass dust contains a mixture of including: calcium oxide, typically 20% by mass; sodium chloride, typically 15% by mass; and potassium chloride 15% by mass.

Transporting means 302, such as a screw conveyor or conveyor belt is provided to transport the bypass dust from silo 301 to a mixing tank 303, into which the bypass dust is loaded (at step 201). Once in mixing tank 303, the bypass dust has water 304 added to it. A slurry is thus produced (at step 202) in mixing tank 303 and this is mixed by mixing means 306. The sodium chloride and potassium chloride begin to dissolve into the water during mixing, but the calcium oxide and other components of the mixture are generally insoluble.

After mixing, a pumping device 307 (which may for example be a centrifugal pump) is used to pump the slurring from the mixing tank 303 to filtering device 308 (at step 203). Further detail of filtering device 308 is given in Figure 4.

Figure 4 Filtering device 308 is illustrated in Figure 4. In this example the compressible filtering device 308 is a plate and chamber membrane filter press. Filtering device 308 comprises a plurality of filter components such as component 401, component 402 etc. Filter components such as 401 and 402 can be compressed together to expel excess water. A frame 403 is provided to support filter components such as 401, 402 etc. In this example, filter * * * component 402 has a plurality of inlets such as 404, 405 and 406. The use of *S these inlets will be further described with reference to Figure 5.

* In the present embodiment, the filtering device contains between 4 and ****a.

* 10 cubic metres of slurry.

Figure 5 One of the filter components removed from filter device 308 is shown in the cross-sectional side view of Figure 5A and front view of Figure 5B. In Figure 5A, filter component 402 is shown adjacent to a further filter component 401 as can be seen in Figure 4a, components 401 and 402 are of differing construction. Component 402 comprises a rigid portion 501 and a membrane 502. Inlets 503 and 504 are provided to allow fluids to enter and exit the apparatus. A further inlet is provided at 505 to allow the entry and exit of slurry, which is shown at 506.

Component 401 has a rigid portion 507, an intermediary portion 508 and a membrane 509. An inlet is provided at 510. When in use, cavities provided at 511 and 512 can be filled with a fluid in order to compress the slurry 506. When compressed, the components of slurry 506 that are able to pass through membranes 502 and 509 do so and are able to leave the apparatus by any open inlets such as 503 and 504. Intermediate portion 508 is movable such that when cavities 511 and 512 are filled with fluid it adjusts so as to squeeze against membrane 509 thus compressing slurry 506.

In Figure SB an example of the use of this apparatus can be seen. Filter component 402 is shown in a front view. Inlets 404, 405, 406 and 513 are shown. Each of these inlets can be open or closed depending on the S. *.. : application. In the present example, inlet 404 is open and inlet 513 is open. *SS S

Although these are referred to as inlets they also acts as outlets depending upon the configuration. An impurity removing liquid, in this case water, enters the apparatus at 404 and is only allowed to exit at 513. This is achieved by *.S.

. : keeping inlets 405 and 406 closed and pumping the water into 404. Once the * water enters at 404 it passes through 503, through membrane 502, and ***..S * 25 permeates across the slurry 506, at this point referred to as a filter cake as it is compressed within the filter press. Because of the configuration of the inlets the water tends to move in a direction indicated by arrows at 514. Thus the slurry or filter cake 506 is washed. As the water crosses the filter cake 506 impurities are picked up on the way. This occurs while the slurry is compressed between filter elements and their membranes.

The chloride that is held in the filter cake is picked up by the controlled flow of water travelling along path 414 and leaves the apparatus at 413. Other particulate matter, such as lime, that is substantially insoluble in water, remains within the filter press.

It has been found that approximately two cake volumes of fresh water are required to remove the chloride, sodium and/or potassium to an acceptable level as described with reference to steps 206 and 207 in Figure 2.

Once the water leaving apparatus is sufficiently clean, the water supply to the inlet 404 is stopped. Air is then blown across the filter cake to remove excess water at step 208, and the filter cake is further compressed at step 209 to remove further excess water.

Having passed through the filter press the water (containing the chloride salts) is disposed of. However, one embodiment is envisaged in which the sodium chloride and potassium chloride are crystallised out of the solution and stored for use, for example as road salt.

*S* . . . . *.. : Washing the filter cake in situ, within a filter press, uses far less water S...

than alternative methods that do not wash in situ. Thus, this method is environmentally and financially preferable.

Having removed most of the sodium chloride and potassium chloride in the filter press, it is possible to safely dispose of the decontaminated material S by landfill. However, in the present embodiment, after removing the fitter cake *.S.** * 25 from the filter press, the decontaminated material is transported to a cement kiln for reuse as raw material in the manufacture of cement. Thus, the expense of landfill is avoided and the decontaminated material provides a useful raw material. I... * * * ** *

SSI * S *5S* S. S * . S * S. S... * S S... *. . * . * * ..

S S. * .

Claims

Claims 1. A method of treating cement kiln bypass dust comprising: obtaining dust from a cement kiln bypass, said dust comprising a mixture including a chloride compound; mixing said dust with a liquid to form a solution of said chloride compound; and removing said solution from residual particulate matter, not dissolved by said liquid, to leave a mixture having reduced chloride content.
2. A method according to claim 1 in which said liquid comprises water.
3. A method according to claim 1 or claim 2, in which said mixture having reduced chloride content is introduced into a cement kiln as a raw material for the production of cement.
4. A method according to any one of claims 1 to 3 in which said solution is removed from said residual particulate matter by filtering. * * S *S S *5.I
5. A method according to claim 4 in which filtering is performed while compressing said particulate matter in a compressing filtering device. **S. * S ** **

*:*.
6. A method according to claim 5 in which said filtering device is a * membrane filter press.

*SS*SS * S
7. A method according to any one of claims 1 to 6 in which a controlled flow of liquid is passed through said dust to rinse said chloride compound from said dust.
8. A method according to claim 7 in which the concentration of said solution is monitored, and said controlled flow is stopped when the chloride content is acceptable.
9. A method according to claim 7 or claim 8 in which filtering is performed while compressing said particulate matter in a compressing filtering device and said controlled flow of liquid is generated by the selected opening and closing of inlets to said compressible filtering device.
10. A method of manufacturing cement, comprising: burning a secondary fuel that contains a chloride; removing bypass dust comprising a chloride compound from the cement kiln; and treating said removed bypass dust in accordance with any one of claims 1 to 9.*.. :
11. A method of treating cement kiln bypass dust substantially as p...herein described with reference to the accompanying Figures. * * S * *S a... * *.,I * . * * S * p.S p *