Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
2001-02-16
2002-05-21
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
73, 73, 73, 73
Reexamination Certificate
active
06391105
ABSTRACT:
BACKGROUND OF THE INVENTION
i) Field of the Invention
This invention relates to a process for enhancing the yield of cement clinker recovered from a cement kiln assembly.
ii) Description of 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.
The resulting hot cement clinker which typically has a temperature of about 1400° C. on leaving the burning zone and at the discharge end of the kiln, is fed into a cooler, a system to cool the clinker and travels as a bed of clinker from the cooler entry port to the cooler exit port, for example, on a cooler grate. Here air is blown through the bed from jets disposed below the grate to cool the hot clinker. Depending on the cooler configuration the clinker at the cooler entry port has a temperature slightly below that of about 1400° C. and the clinker at the cooler exit port has a temperature of about 120° C.
The cooled cement clinker is ground to a desired fineness and is employed as such or in some cases it may be admixed with extenders, especially extenders having pozzolanic properties, to produce a blended cement. The extenders provide a saving in the cement clinker content of the product cement. The extenders are, in particular, by-products from industrial processes which by-products have a high content of silica and contain calcium and/or aluminium in an oxidized form, especially oxides or carbonates. Such extenders include coal ashes, more especially fly ashes and bottom ash; blast furnace slag and silica fume. In addition modifiers such as lime, cement kiln dust and waste cement or cement clinker could also be used to adjust chemistry or as a benefit to handling the extender where for example agglomeration is desired.
Since about 1986, the emphasis on controlling emission gases from power plants has resulted in the installation of low NO
x
burners as a means of reducing oxides of nitrogen in emission gases. The impact of these controls has been an increase in carbon content of Type F and to a lesser extent Type C pozzolanic flyash. Furthermore the addition of low levels of petroleum coke to lignite, bituminous and sub-bituminous coal at some plants has also increased the level of carbon in flyash.
Carbon such as contained in coal ash is a detrimental contaminant in cement, having the effect of absorbing chemicals with resulting deterioration of concrete performance.
Prior attempts to remove the carbon from the flyash include electrostatic separation; mixing the flyash with a fluid such as kerosene and separating the carbon out by foaming; and treating the flyash in a fluid bed combustion chamber for combustion of the carbon.
Bottom ash also contains carbon as a contaminant and it is preferable to remove the carbon if the bottom ash is to be incorporated in cement.
It has previously been proposed to add coal ash to cement clinker in the cooler of a cement kiln assembly in the production of blended cements (U.S. Pat. No. 5,837,052). In this prior proposal the integrity and identity of the coal ash is maintained and the contaminating carbon is oxidized by the heat of the hot clinker as it cools in the cooler to produce a blended mixture of cement clinker and coal ash free of carbon.
It has also been proposed in U.S. Pat. No. 5,976,243 to add blast furnace slag to the cement clinker in the cooler of a cement kiln assembly to drive off water in the slag and produce a blended mixture of cement clinker and blast furnace slag in which the integrity and identity of the blast furnace slag is maintained. In U.S. Pat. No. 5,650,005, it has been proposed to elevate the free lime content of a cement clinker by adding a source of free lime to the cement clinker
SUMMARY OF THE INVENTION
It is an object of this invention to provide a process for increasing or enhancing the yield of cement clinker recovered from a cement kiln assembly.
In accordance with the invention there is provided a process for enhancing the yield of a cement clinker recovered from a cement kiln assembly comprising a cement kiln for production of cement clinker and a cooler for cooling cement clinker from said kiln comprising:
a) producing hot cement clinker from cement clinker raw ingredients in a cement kiln;
b) feeding hot cement clinker from step a) into a cooler;
c) feeding a particulate material comprising silica and an oxide of at least one of calcium and aluminum into contact with said hot cement clinker downstream of the formation of said hot cement clinker and allowing said material to melt to a partially fused material, chemically reacting said partially fused material with the hot clinker to produce a pyroprocessed cement clinker composition of partially fused crystalline hydraulic silicates, cooling said cement clinker composition in said cooler; and
d) recovering a cooled cement clinker composition from said cooler, said composition having a cement clinker content greater than that of the hot cement clinker in step b).
DESCRIPTION OF PREFERRED EMBODIMENTS
The process of the invention increases or enhances the yield of cement clinker recovered from the cooler of a cement kiln, without altering the chemistry, ingredients or process or operating parameters of the cement kiln.
This is a significant advantage since cement kiln operators are conservative by nature and demonstrate great reluctance to modify in any way the parameters of operation of a cement kiln which is operating satisfactorily.
i) The extenders employed in the invention are materials of high silica content and additionally contain calcium, aluminum or both in a form which will react with the silica at elevated temperatures in the cooler of a clinker kiln assembly, to form a composition consisting predominantly of crystalline, hydraulic calcium and aluminum silicates. The calcium and aluminum will typically be present as calcium oxide and aluminum oxide, respectively, but lesser amounts may be present as silicates, for example, calcium silicate and aluminum silicate, and aluminosilicates such as calcium aluminosilicate.
The extender may be amorphous or crystalline and typically will contain silica and calcium and/or aluminum compounds.
Suitable extenders chain silicate molecules which may be calcium or alumino silicates or both, typically containing the same silicates as cement clinker but with significantly less calcium, for example, coal ashes, blast furnace slag and silica fume, which are all by-products of industrial processing or manufacture. Modifiers could also be used to adjust chemistry or as a benefit to handling the extender where for example agglomeration is desired. These could include materials such as lime, cement kiln dust and cement.
At elevated temperatures downstream of the formation of the cement clinker in the kiln, under pyroprocessing conditions the particulate extenders melt to a partially fused state which chemically reacts with the hot cement clinker producing a cement clinker composition consisting predominantly of crystalline hydraulic calcium silicates and which composition is a pyroprocessed composition.
In a first embodiment the pyroprocessing reaction occurs in the upstream end of the cooler adjacent the exit of the cement clinker where the temperature is from 1400° C. to 1000° C.
In a second embodiment the pyroprocessing reaction may also take place in the cement kiln downstream of the zone of the kiln in which the cement clinker is formed and more especially within the kiln at the discharge end of the kiln; the discharge end of the kiln is an especially advantageous site for the addition of extenders in the form of coarse particles. The chemical reactions in the kiln which form the hot cement clinker product of the kiln, take place in the burning zone of the kiln and thus in this second embodiment the addition of the extender takes place downstream of the burning zone.
The particulate extender should have a particle size which permits a satisfactory
Cail Kevin Moire
Cross James Edward
Hopkins Donald Stephen
Lehoux Paul Honore
Oates David Bridson
Lafarge Canada Inc.
Marcantoni Paul
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