There is currently a large amount of attention being paid to the use of additive materials in cement in order to maintain or increase the strength of the cement while reducing the overall energy required to produce the material. In practice, a number of natural and manufactured materials are added to clinker in order to reduce the need for clinker minerals in the cement. These materials include limestone, waste slag from the manufacture of steel and iron, and naturally occurring pozzolan. Disadvantages exist to the use of these materials in practice. Quality concerns limit the introduction of limestone, as limestone naturally provides little to the strength of the finished product. Certain types of slag can be utilized positively for the introduction of strength to cement, but as a waste product of the manufacture of other compounds, the slag often does not have a consistent chemistry. Slags can also contain large amounts of free iron, which can cause premature wear of grinding elements used in the manufacture of cement. Pozzolan provides positive strength development in finished cement, but as a naturally occurring material, is not generally available in locations where the primary raw materials used in the manufacture of cement are mined.
In recent years, a number of processes have gained prominence in the production of artificial pozzolan from the calcining of clay. The manufacture of artificial pozzolan requires lower temperatures and less energy than the production of cement clinker, and is therefore gaining importance among cement manufacturers for its lower cost of production, as well as the positive effects of producing lower emissions (particularly CO2).
In practice, however, while the chemistry may be consistent with a positive effect on strength development, the production of these artificial pozzolans may create materials which are colored differently than the clinker used in the manufacture of cement. This is problematic where the color of the finished product is an important concern, such as when multiple sources of cement may be used for a single project. These issues with the coloration of the final product serve to limit the introduction of these synthetic pozzolans in the production of cement.
Therefore, it is an object of the present invention to provide a method for producing synthetic pozzolan having desired color characteristics, and in particular having a light grey color that many cement producers find desirable.
The above and other objects are achieved by the process of the present invention according to which the coloration of the artificial pozzolan produced may be controlled as desired. Having a synthetic pozzolan product with desirable color characteristics will enable the end user to introduce higher amounts of pozzolan into the finished cement, thus resulting in a higher quality product produced utilizing lower fuel consumption than other cement producing systems.
The invention broadly comprises breaking apart a starting raw material, such as an alumina silicate such as a kaolinic clay, a diatomaceous earth, or a diatomaceous amorphous alumina silicate, to a small feed size, heat treating the raw material to a product pozzolan, and then by affecting the oxidation state of the color-producing components of the artificial pozzolan product, particularly iron and aluminum, through the creation of localized reducing conditions as the pozzolan product cools to a temperature below its color-stabilizing temperature, which color-stabilizing temperature is determined by the amount and identity of color-producing components in the raw materials and therefore in the resulting synthetic pozzolan.
More specifically, wet raw feed materials capable of producing an amorphous alumina silicate when heat treated as described herein, including kaolinic clay, diatomaceous earth and diatomaceous amorphous alumina silicate are fed to a device for sufficient material drying and disagglomeration/crushing of larger material (a “drier crusher”). The product from the drier crusher is collected in a cyclone, and directed to a calciner. Fuel is fed to the calciner to maintain an exit temperature from the calciner that will provide sufficient dehydration and activation of the product. The feed material is heated at least to a temperature (the “activation temperature”) at which the pozzolanic properties, such as the strength of the end material, are optimized and at which, in effect, the raw material is converted to a synthetic pozzolan. This activation temperature will generally range between about 700° C.-900° C., depending upon the properties of the specific raw material being utilized.
The product from the calciner is collected, such as in a collection cyclone, and the material is fed to a cooler where it is cooled from its activation temperature. The gases from the collector may optionally be used for drying and conveying material through the drier crusher. Reducing conditions are maintained in the cooler for at least a portion, and most preferably the initial portion, of the cooling process. When only a portion of the process of cooling the synthetic pozzolan from its activation temperature to its color-stabilizing temperature is performed under reducing conditions, it is preferred that the balance of the cooling process be performed in an oxygen depleted environment.
Pozzolan material fed to the cooler may be treated with a small amount of fuel (preferably oil) to maintain a reducing atmosphere near the material inlet. Further into the cooler, water may be optionally sprayed to assist in cooling of the pozzolan to below its color-stabilizing temperature while maintaining a low oxygen environment. Alternatively, an oxygen depleted gas can be passed through the cooler along with or in place of the water vapor to cool the pozzolan to below its color-stabilizing temperature while maintaining a low oxygen environment. The product from the cooler may then be introduced into one or more optional additional coolers, such as a cyclone cooling system, for further cooling. If the material entering the any additional downstream coolers is at a temperature below its color-stabilizing temperature, a reducing or oxygen-depleted atmosphere will not have to be maintained in such additional cooler. The finally cooled product is thereafter collected. The preheated gases from any additional cooler may be optionally directed to the calciner as hot tertiary air.
The invention is described with reference to the drawings, in which like numerals represent similar elements, and in which:
In all the figures, dashed arrows represent the flow of gas, while solid arrows represent the flow of solid material. With reference to
Most of the dried, crushed material collected in the drier crusher cyclone 4 is directed to the calciner 13 via chutes 10a or 10b. Optionally, a small amount of the dried, crushed material collected in the drier crusher cyclone 4 may be directed to duct 16 for temperature control of the gas in duct 16. The calciner 13 shown in
Optionally, a stoichiometric excess of fuel may be utilizing in calciner 13 to promote heat treatment under reducing conditions.
Fuel can also be fired in a separate air heater (not shown) that receives either ambient air and/or heated air from duct 26; the exhaust gas from this air heater is directed into the calciner 13.
The crushed, dried materials can be directed into the calciner 13 through a single location or multiple locations 10a and 10b. The split of material in chutes 10a and 10b is determined by the de-hydration and activation properties of the raw materials and the split also can be used to help control the combustion of the fuel in the calciner 13. In the calciner the hydrated moisture will be dried off and the material will be heat treated to its activation temperature. The desired activation temperature in the calciner 13 will depend on the chemistry of the feedstock and the associated minerals in the raw feed and will be between 500° C. and 900° C. and most prevalently between about 700° C. and 850° C. Most of the synthetic pozzolan will thereafter become entrained in the gas stream in the calciner 13 and exit via duct 14.
The entrained pozzolan in duct 14 is captured by the calciner cyclone 15 and is directed to cooler 20, which as depicted is a rotary cooler, via chute 17a, with a portion being optionally re-circulated back to the calciner 13 via chute 17b. The operator may desire to utilize the recirculation feature to increase the retention time in the calciner for reasons such as, for example, system height restrictions, for better temperature control and/or improved fuel burnout.
A small amount of fuel, between 10 to 40 kcal fuel per kg of synthetic pozzolan, is added to the synthetic pozzolan via inlet 18 and preferably immediately prior to the pozzolan entering cooler 20. The preferred fuel is fuel oil. The fuel creates local reducing conditions, i.e., an oxygen depleted or low (from about 0% to about 5% by volume) oxygen environment and either CO and/or volatized hydrocarbons, near the synthetic pozzolan during at least the initial part of the cooling process. Downstream from the cooler area in which the small amount of fuel was added, water sprayer 22 is utilized to spray water onto the synthetic pozzolan to contribute to cooling the pozzolan below the color-stabilizing temperature of the color producing metals, particularly iron, which generally between about 150° C. and about 600° C., and more typically between about 180° C. and about 400° C., with the actual color-stabilizing temperature depending on the composition of the pozzolan, and specifically the amount of iron content. Since the synthetic pozzolan is kept well above 100° C. the synthetic pozzolan remains dry. The water vaporizes upon contact with the hot pozzolan. The generated water vapor occupies most of the space inside the cooler 20, this helps to maintain an oxygen depleted atmosphere (i.e. no more than about 10% oxygen) in that portion of the cooler which retards the oxidation of metals. The water vapor exits the cooler 20 via the riser 28. A portion of the fuel oil will volatilize and exit the cooler 20 via the riser 28. In addition some CO produced by burning the fuel and excess water vapor will exit cooler 20 via riser 28. By preventing the oxidation of iron, in particular, and other metals including aluminum, magnesium, manganese and chromium during the cooling process, the pozzolan is prevented from changing to a reddish or other color and may be fixed as white or light grey.
As a supplement or alternative to using water as described above an oxygen depleted gas can be passed through the cooler to cool the pozzolan below the color-stabilizing temperature of the color producing metals. Two possible sources of the oxygen depleted can be the exhaust stream 9 or the gas exiting fan 6; however, any oxygen depleted gas can be used.
In an optional embodiment, the objects of the invention can be achieved if the raw material is heat treated to form synthetic pozzolan under reducing conditions by utilizing a sufficient amount of excess fuel during the heat treating process and thereafter continuing to cool to the “color-stabilizing temperature” under reducing and/or oxygen depleted conditions.
The term “color-stabilizing temperature” as used herein means the temperature at which the pozzolan can continue cooling, such as in ambient air, without significant oxidation of the primary color-producing species in the pozzolan taking place. This temperature will vary according to the relative proportion by weight of color-producing species, which is defined as those compounds which go from a white or light grey shade to a red or other color when oxidized, and which constitute primarily iron, but also to a lesser extent aluminum, chromium, manganese, titanium and magnesium, in the cooling pozzolan material. Typically, this temperature will range from about 180° C. to about 400° C. If oxidation of a substantial (i.e. at least 90 wt percent) amount of the primary color-producing species is inhibited while the material is cooled to its color-stabilizing temperature, the final cooled product will typically have a light grey shade.
The activation and color stabilization temperatures, as defined herein, for a given sample of material can be determined by one skilled in the art by a number of test procedures. For example, the activation temperature for a given raw material may be determined by running a furnace test or a thermogravimetric analysis on the sample and the color stabilization temperature may be determined by running thermal studies on the cooling synthetic pozzolan material made from said raw material.
As used herein, the term “reducing conditions” or “reducing atmosphere” means that the overall conditions in the cooler (or the calciner) favor reduction of the color-changing species in the pozzolan. As used herein, the term “oxygen depleted” or “oxygen deprived” atmosphere or conditions means that while overall conditions do not promote reduction of the color-changing species in the pozzolan, there is also not sufficient oxygen to promote their oxidation.
The synthetic pozzolan exits the cooler 20 via chute 21 and is directed into duct 24 where it is further cooled by air 23. The entrained synthetic pozzolan is captured by cyclone 25 and leaves the system as the synthetic pozzolan product 27. The air preheated by the synthetic pozzolan exits cyclone 25 and is directed to the calciner 13 via duct 26. The temperature of the air in duct 26 will be almost the same as the product 27.
Optional region 100 in
In the embodiment of
Region 200 in
However, as the number of stages is increased, the drying capacity of the drier crusher will be reduced, while the fuel consumption in the calciner will decrease. Therefore, the preferable number of cyclones will depend upon the moisture content of the raw material and the tradeoff between the capital cost of the cyclones versus the operational cost savings.
Per
Most of the dried, crushed material collected in drier crusher cyclone 4 is directed to the duct 61 via chutes 60a, while some the dried, crushed material collected in drier crusher cyclone 4 may be directed to duct 63 via chute 60b for temperature control of the gas in duct 63. The dried, crushed material in duct 61 is transported to cyclone 62 where it is captured and directed to duct 16 via chute 64. The dried, crushed material in duct 16 is transported to cyclone 65 where it is captured and directed to the calciner 13 via chutes 10a and 10b.
According to
Optionally, fuel oil may also be inserted behind flame 84 in rotary kiln 80, via inlet 18a, to begin exposing the synthetic pozzolan to a low oxygen environment in an area of the kiln in which the temperature experienced by the pozzolan begins to decrease from the maximum temperatures experienced within the kiln. The insertion of fuel oil in the rotary kiln will always be done in concert with maintaining at least a portion of cooler 20 under reducing conditions. In addition, cooler 20 may also provide for the removal of water vapor and oxygen depleted gas through a dust collector in the manner depicted in
This application is a continuation-in-part of, and claims priority from, pending non-provisional application Ser. No. 12/966345, filed Dec. 13, 2010.
Number | Date | Country | |
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Parent | 12966345 | Dec 2010 | US |
Child | 13323306 | US |