Abstract:
A method of cement clinker manufacture comprises feeding a clinker feed material containing a source of calcium carbonate into a feed end of a cement kiln, the feed material is heat processed in the kiln to produce cement clinker with emission of carbon dioxide from thermal decomposition of said source of calcium carbonate and discharge of the carbon dioxide from the kiln, and cement clinker is discharged from a discharge end of the kiln; a coal ash derived from burning pulverized lignite or sub-bituminous coal is included in the feed material fed into the feed end to replace a portion of the source of calcium carbonate, and provides a source of calcium as well as other components notably silicon and aluminum, in the formation of the cement clinker, with a lowering of the emission of carbon dioxide in the kiln, per unit weight of cement clinker produced.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to a process for producing a cement clinker having a content of coal ash as a source of calcium in the feedstock introduced into the cement kiln; more especially the invention is concerned with such a process in which the coal ash is derived predominantly from lignite and sub-bituminous coal sources. The fly ash component of this type of coal ash is referred to as Class C. 
     2. Description of the Prior Art 
     In a cement plant, cement clinker is created at elevated temperatures in a cement kiln from cement clinker raw ingredients which travel through the kiln from a feed end to a discharge end, while passing through different processing zones at elevated temperature. These processing zones include a calcining and a clinkering or burning zone. 
     Cement clinker comprises various cement compounds formed from the raw ingredients, for example dicalcium silicate, tricalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite. Formation of these cement compounds requires sources of calcium, silicon, aluminium hand iron in the raw ingredients fed to the cement kiln. 
     The cement clinker raw ingredients include a source of calcium carbonate, usually limestone, as a source of calcium for the cement compounds of the cement clinker. The calcium carbonate is thermally decomposed to lime and carbon dioxide in the calcining zone. The carbon dioxide emissions represent a pollution problem as they exit from the kiln, the significance of which has heightened with geopolitical concerns surrounding the Kyoto Accord. 
     Coal ash is derived from the burning of lignite, bituminous and sub-bituminous coal in power plants; the fly ash is recovered from the exhaust gases of the coal burning plants, and bottom ash is recovered from the bottom of the boiler as a granular coal ash. 
     Coal ash including class C fly ash has been blended or interground with cement clinker, because of its pozzolanic nature, to produce blended cements or has been used as a pozzolanic admixture in concrete, but has not previously been considered as a source of calcium in the production of cement clinker. Moreover fly ash is formed as very fine particles and is normally utilized in that form, with a minimum of processing. The fine particles can present a handling problem. Bottom ash which can also be used as a source of calcium does not have this problem because of its larger particle sizes. 
     Fly ash is produced in huge tonnages and while uses have been developed for fly ash, the enormous quantities produced still present a disposal problem. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to employ coal ash as a source of calcium in cement clinker production. 
     It is a particular object of this invention to employ a coal ash derived predominantly from lignite and sub-bituminous coal as a source of calcium in cement clinker manufacture. 
     It is a further object of his invention to provide such processes with reduced emissions of carbon dioxide per unit weight of cement clinker produced. 
     In accordance with the invention there is provided in a method of cement clinker manufacture in which a clinker feed material containing a source of calcium carbonate is fed into a feed end of a cement kiln, the feed material is heat processed in the kiln to produce cement clinker with emission of carbon dioxide from thermal decomposition of the source of calcium carbonate and discharge of the carbon dioxide from the kiln, and cement clinker is discharged from a discharge end of the kiln, the improvement wherein a coal ash derived from burning pulverized lignite or sub-bituminous coal is included in the feed material in said kiln to replace a portion of said source of calcium carbonate in the formation of said cement clinker, with a lowering of the emission of carbon dioxide in said kiln, per unit weight of cement clinker produced. 
     In another aspect of the invention there is provided a method of producing cement clinker with reduced emission of carbon dioxide from a change of cement clinker raw ingredients comprising: 
     i)providing a rotary cement kiln having a feed end and a discharge end; 
     ii) establishing predetermined levels of at least calcium, silicon and aluminium for a cement clinker, based on a raw ingredient feed material comprising a source of calcium carbonate, a source of silicon and a source of aluminium, 
     iii) providing a raw ingredient feed material formulation comprising said sources, in which a portion of said source of calcium carbonate is replaced by a coal ash derived from lignite or sub-bituminous coal while maintaining said predetermined level of calcium; 
     iv) feeding said formulation into said kiln, 
     v) exposing said formulation to elevated temperatures in said kiln, while feeding said ingredients from said feed end towards said discharge end, to calcine said source of calcium carbonate with formation of calcium oxide and liberation of carbon dioxide, and chemically combine and integrate said calcium oxide with said sources of silicon and aluminium, and said coal ash, as a cement clinker, and 
     vi) discharging said cement clinker from said discharge end, whereby carbon dioxide emissions are reduced in proportion to the replacement of said source of calcium carbonate by said coal ash. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawing, in which: 
     FIG. 1 is a simplified schematic illustration of a kiln assembly including a kiln and a cooler. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     i) Coal Ash 
     The coal ash as employed in this invention refers to the residue produced in coal burning furnaces from burning pulverized anthracite or lignite, or bituminous or sub-bituminous coal. Such coal ash includes fly ash which is the finely divided coal ash carried from the furnace by exhaust or flue gases; and bottom ash which collects at the base of the furnace as agglomerates. 
     The coal ash employed in the invention is, more especially, one having a significant content of calcium, particularly a Class C fly ash; or bottom ash similarly having a significant calcium content such as results from burning lignite or sub-bituminous coals in coal burning boilers or furnaces. 
     The Class C fly ashes referred to above are defined by CSA Standard A23.5 and ASTM C618, both incorporated herein by reference. 
     The coal ash may typically contain 10 to 40%, more usually 10 to 35%, of calcium determined as calcium oxide. For the purposes of this invention, since the fly ash is being employed as a source of calcium, coal ash of high calcium content especially 20 to 40%, more especially 30 to 40% are preferred, but based on wide availability coal ash having a calcium content of 25 to 35% will more generally be employed. 
     Coal ashes also have significant contents of silicon, aluminium and iron which are all important elements in cement clinker formation. Typical contents determined as their oxides are silicon 30 to 40%, aluminium 15 to 25% and iron 3 to 8%. 
     All percentages herein are by weight unless otherwise indicated. It will be understood and readily recognized by persons in the art that calcium, silicon, aluminium and iron are not present as elements in the fly ash or even, necessarily as oxides of the elements, however the weight % of such elements is conveniently and traditionally expressed as oxides whether the elements are actually present as oxides or otherwise. 
     Typically a majority of the fly ash, at least about 80%, by weight, comprises particles of less than 45 microns. 
     Bottom ash typically is recovered from the base of the furnace as granules of which 70%, by weight, have a size in the range of 100 microns to 8 cm. It has similar chemistry to the fly ash being from the same coal source and occurs in lesser amounts than the fly ash, typically 10-20% of the coal ash produced. 
     ii) Process 
     The process is further described by reference to the embodiment in which the coal ash is a Class C fly ash. 
     The fly ash is suitably introduced to the feed inlet of the cement kiln so that all or a majority of the fly ash travels with the other cement clinker raw ingredients towards the discharge outlet of the kiln, and is not entrained by exhaust and combustion gases travelling within the cement kiln. In order to achieve this the fly ash may be premixed in its fine particle form, with the other raw ingredients, before being introduced into the kiln. The fly ash may also be fed independently to the cement kiln feed inlet, and in such case the fine fly ash particles may conveniently be agglomerated to larger particles, prior to introduction to the kiln. The fly ash may also be introduced independently into the kiln downstream of the feed inlet but upstream of the zone of the kiln in which clinker formation occurs. Thus the fly ash may be introduced to the kiln in the drying zone or the calcining zone. 
     In particular the fly ash is introduced to the kiln at a zone upstream of a zone in which heat processing to form cement clinker occurs, and more especially is a zone in which the calcium oxide chemically combines and integrates with the sources of silicon and aluminium to form cement clinker. 
     One property of Class C fly ash, not present in ashes from bituminous coals, F ashes, is their self cementing properties created by some of the compounds that also occur in cement. 
     Such agglomeration may be carried out in a conventional agglomerator for fine particle materials, but also to take advantage of the self hardening by adding water to layers of the fly ash and allowing the layers to solidify due to hydraulic behaviour to rock-like layers which may then be crushed to a convenient particle size for handling, for example in the size ranges for sand , gravel and cobbles. 
     To this end agglomerated fly ash particles in which at least 80% by weight, have a particle size of 0.1 mm to 6 inches, especially 5 mm to 70 mm are convenient for handling in the process of the invention. 
     Introducing the fly ash in a non-air entrainable state, for example in a water damp or moistened state, is another means of minimizing loss of the fly ash into the emission gases generated in the cement kiln. In this case the damp or moistened fly ash is subjected to drying as it travels with the clinker raw ingredients, and the fly ash is either entrapped in pockets between adjacent clinker ingredient particles or has chemically integrated or combined with the raw ingredient sufficiently downstream in the kiln that it is not entrained by the gases generated in the kiln. 
     Furthermore since the inclusion of the fly ash as a source of calcium lowers the requirement for calcium carbonate with a consequent lowering of the emissions, especially carbon dioxide, the likelihood of entrainment of fly ash particles by such gas emission reduction is also lowered. 
     Bottom ash being coarse, it can be introduced into the raw limestone circuit and through the primary grinding mills or as required, introduced at mid kiln through openings/scoops in the kiln shell 
     In particular the coal ash is introduced to the feed inlet or mid kiln if preferred of the kiln for the clinker raw ingredients, such that the coal ash is conveyed with the other clinker raw ingredients towards the discharge outlet, and the coal ash is exposed to a temperature and residence time effective for melting of the coal ash and chemical integration and combination with the other ingredients to form the cement clinker. Since the main elements of a coal ash are calcium, silicon, aluminium, iron and oxygen, the main components of cement clinker are all found in the coal ash but with the additional benefit that the coal ash is previously formed at high temperature and thus does not have thermal decomposition products. 
     Since the calcium carbonate addition to the kiln, as the prime source of calcium, can be lowered in proportion to the employment of coal ash as a source of the calcium, emissions of carbon dioxide in the kiln can be lowered proportionately. 
     The final proportions require analysis of the balance of each component, calcium, silicon, alumina and iron, all contributed by the coal ash, and all the other raw materials used by the cement plant to meet the intended class of cement clinker, Types 1,2,3,4,5 and special cements listed in ASTM. 
     Various delivery means may be employed for introducing the coal ash to the kiln or to the clinker raw ingredients for premixing, and the design and location of suitable delivery means in the upstream of the feed inlet of the kiln is well within the skill of persons in the art. For example, when fly ash is selected, a damp or moistened fly ash may be introduced by a reclaim screw auger, the fly ash could also be pneumatically delivered to the feed inlet and, for example might be pneumatically injected into the interior of the clinker raw ingredients for enhanced commingling with the clinker raw ingredients. Introduction into the interior of the raw ingredients also minimizes possible loss of the fly ash by entrainment in gases generated in the kiln. 
     Fly ash is commercially available and normally employed in a dry state, in which it is a fine powder. As such it is more difficult to successfully introduce fly ash into the kiln, and retain it with the clinker raw ingredients for combination and integration therewith in the kiln. One would have losses of the fine powder by entrainment in gases flowing in the kiln. Bottom ash would behave differently because of its coarser particles. 
     The coal ash is suitably introduced to the clinker raw ingredients or directly at the feed inlet of the kiln in an amount to provide 5 to 45%, typically 10 to 30%, by weight, of the calcium content of the cement clinker being produced. 
     iii) Chemical Processing 
     In a typical cement clinker production site the raw ingredients for clinker production are assessed prior to introduction into the kiln and proportioned to produce a cement clinker with contents of calcium, silicon, aluminium, iron and other metals in predetermined amounts or ranges, so as to provide a cement clinker for producing a cement of a certain desired class, such as one having early strength during setting, or high ultimate strength. 
     In accordance with preferred embodiments of the invention, the content of calcium carbonate as a source of calcium, in the clinker raw ingredients is lowered or reduced according to the calcium provided by the coal ash. In this way the balance of calcium desired in the cement clinker is maintained. The reduction in the amount of the source of calcium carbonate, for example limestone, results in a lowering or reduction in the amount of carbon dioxide generated in the kiln per unit weight of cement clinker produced. 
     The amount of other ingredients in the cement clinker raw ingredients may also be conveniently adjusted in accordance with the desired amounts or ranges for the other elements in the cement clinker, based on the content of these elements in the coal ash, and the amount of fly ash employed as replacement for the source of calcium carbonate. 
     Brief Description of the Drawing 
     Further features and advantages of the present invention will become apparent form the following detailed description, taken in combination with the appended drawing, in which: 
     FIG. 1 is a simplified schematic illustration of a kiln assembly including a kiln and a cooler. 
     Detailed Description of Preferred Embodiment of the Invention with Reference to the Drawing 
     With further reference to FIG. 1, a kiln assembly  10  includes a feed inlet  12 , a rotary kiln  14  and a cooler  16 . 
     The kiln  14  is mounted for rotation relative to feed inlet  12  and cooler  16 . 
     Rotary kiln  14  has a drying zone  20  for use in a wet process, a calcining zone  22 , a burning zone  24  and an initial cooling zone  26 . 
     Rotary kiln  14  extends between a feed port  18  and a clinker outlet  28 . 
     A burner assembly  30  mounted externally of kiln  14  has a burner nozzle  32  mounted in a firing hood  38  which nozzle  32  extends through outlet  28  into kiln  14 . A flame  36  is developed at nozzle  32 . 
     Cooler  16  has an entry port  42  which communicates with clinker outlet  28  of kiln  14 , and an exit port  44 . A cooler grate  40  is mounted in cooler  16  and air jets  46  disposed below cooler grate  40  feed jets of cooling air upwardly through cooler grate  40  and a bed  52  of clinker supported on cooler grate  40 . 
     Cooler  16  has an air discharge  48 . 
     Cooler grate  40  comprises a plurality of plates in side-by-side relationship. Some of the plates have openings therethrough to allow passage of the cooling air. Some plates are fixed and other are mounted to oscillate, back and forth. The movement of the oscillating plates agitates the clinker. 
     The cooler grate  40  is inclined downwardly from the entry port  42  to exit port  44 . The bed  52  of clinker is advanced towards exit port  44  by the oscillation of some of the plates, in conjunction with the inclination and the build-up of clinker introduced into cooler  16  from kiln  14 . 
     In operation raw cement clinker ingredients  50  which include a coal ash, in particulate form, are fed through inlet  12  and feed port  18  into kiln  14 , where they first enter drying zone  20 . The kiln  14  rotates slowly, and is inclined downwardly from port  18  to outlet  28 . With the rotation of kiln  14 , the ingredients advance slowly and sequentially through drying zone  20 , calcining zone  22  and burning zone  24 , into which a flame extends from burner nozzle  32 .As a variation, the coal ash may be fed through mid kiln access ports or scoops. 
     In drying zone  20  the temperature typically ranges from 300° C. to 800° C. In calcining zone  22  the temperature typically ranges from 825° C. to 1000° C. and in burning zone  24  the temperature is typically 1400° C. to 1425° C. 
     Clinker formation is completed in burning zone  24 . 
     Hot clinker produced in kiln  14  is discharged, through clinker outlet  28  and enters cooler  16  at entry port  42  where it falls onto the cooler grate  40  which advances the hot clinker towards exit port  44 . The hot clinker falling onto cooler grate  40  forms a bed  52  of clinker particles which typically has a thickness or depth of 6 to 24 inches. 
     Air is injected under pressure through air jets  46  located below cooler grate  40 , the air permeates through plates in the cooler grate  40  and the bed  52 , the clinker being progressively cooled by the air from jets  46  as it advances towards exit port  44 . The cooler  16  is typically operated under low pressure or partial vacuum and the air permeating upwardly through bed  52  flows either along the path indicated by the arrows A into kiln  14  or along the path indicated by the arrows B exiting from the downstream end of the cooler. The path of travel of the bed  52  is indicated by the arrow C. 
     EXAMPLE 
     i) Trials 
     Different formulations of clinker feed ingredients are summarized below in the Table, to demonstrate the variation of contribution of Ca, Al, Si and Fe to the resulting clinker, that can be achieved by replacement of different amounts of limestone (calcium carbonate) by different amounts of coal ash, in accordance with the invention. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 SiO 2  % 
                 Al 2 O 3  % 
                 Fe 2 O 3  % 
                 CaO % 
                 % used 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 MIX 1 
                   
                   
                   
                   
                   
               
               
                 Limestone 1 
                 12 
                 2.8 
                 1.5 
                 43 
                 46 
               
               
                 Limestone 2 
                 5 
                 0.9 
                 0.5 
                 52 
                 43 
               
               
                 Ash 
                 42 
                 21 
                 5.6 
                 11 
                 6 
               
               
                 Iron 
                   
                   
                 56 
                   
                 0.7 
               
               
                 sand 
                 73 
                   
                   
                   
                 4 
               
               
                 mix chemistry 
                 14 
                 3.2 
                 1.7 
                 43 
               
               
                 clinker 
                 22 
                 5 
                 2.7 
                 66 
               
               
                 chemistry 
               
               
                 C2S 
                 14.5 
               
               
                 C3S 
                 63 
               
               
                 C3A 
                 8.8 
               
               
                 C4AF 
                 8.22 
               
               
                 MIX 2 
               
               
                 Limestone 1 
                 12 
                 2.8 
                 1.5 
                 43 
                 45 
               
               
                 Limestone 2 
                 5 
                 0.9 
                 0.5 
                 52 
                 42 
               
               
                 Ash 
                 35 
                 21 
                 5.6 
                 17 
                 11 
               
               
                 Iron 
                   
                   
                 56 
                   
                 0.7 
               
               
                 sand 
                 73 
                   
                   
                   
                 4 
               
               
                 mix chemistry 
                 13.5 
                 3.9 
                 1.5 
                 43.1 
               
               
                 clinker 
                 21 
                 6 
                 2.4 
                 66.6 
               
               
                 chemistry 
               
               
                 C2S 
                 12.2 
               
               
                 C3S 
                 63 
               
               
                 C3A 
                 12 
               
               
                 C4AF 
                 7.16 
               
               
                 MIX 3 
               
               
                 Limestone 
                 6.7 
                 0.4 
                 0.7 
                 51 
                 81 
               
               
                 Ash 
                 46 
                 17 
                 8 
                 19 
                 18 
               
               
                 sand 
                 90 
                   
                   
                   
                 1 
               
               
                 mix chemistry 
                 14.7 
                 3.5 
                 2 
                 44 
               
               
                 clinker 
                 22 
                 5.2 
                 3.1 
                 66.2 
               
               
                 chemistry 
               
               
                 C2S 
                 15 
               
               
                 C3S 
                 63 
               
               
                 C3A 
                 8.5 
               
               
                 C4AF 
                 9.3 
               
               
                   
               
             
          
         
       
     
     Minor oxides are not accounted for in the in the final chemistries summarized in the Table.