Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Reexamination Certificate
1999-02-10
2001-12-18
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
Reexamination Certificate
active
06331207
ABSTRACT:
FIELD OF THE INVENTION
This invention deals with the reclamation and treatment of cement kiln dust (CKD) so that it may be used as feed in the clinker-producing process.
BACKGROUND OF THE INVENTION
CKD is the particulate matter carried from the feed end of a cement (clinker-producing) kiln (industrial furnace) by the exhaust gases resulting from combustion and calcination. CKD typically consists of particles of raw materials, partly to wholly calcined raw materials, reaction process intermediates, fine clinker, and inorganic fuel solids; it usually also includes condensates and reaction products from the inorganic raw material and fuel volatiles.
A particular CKD can vary in composition (see Table 1) from virtually unaltered kiln feed (raw meal) to over 90% alkali sulfates and chlorides (Haynes and Kramer, “US Bureau of Mines Circular 8885,” 1982; and personal observation), depending on process type, kiln configuration, raw materials, fuel(s), process characteristics, and point(s) of collection. It can vary in particle size from that of fine sand or silt to that of clay, with particle size distribution ranging from very broad to very narrow depending on material and process parameters.
TABLE 1
ANALYSES SHOWING EXTREMES OF CKD COMPOSITION
(Relative to Kiln Feed)
OXIDE
MINIMUM CHANGE*
MAXIMUM CHANGE
SiO
2
12.11
1.07
Al
2
O
3
4.44
0.28
Fe
2
O
3
1.44
0.24
CaO
47.29 (43.0)
2.19
MgO
1.08
0.10
LOI
35.00
1.50
Na
2
O
0.12
4.36
K
2
O
1.54
43.31
SO
3
1.15
45.96
Total
103.77
99.01
*Analysis #63 in Haynes and Kramer. The total indicates this analysis to be of low quality. CaO analysis is higher by about 5% and SiO
2
lower by about 2% relative to typical expectation for kiln feed.
The quantity of dust generated from a particular kiln depends on the factors noted above as affecting composition and particle size, the internal configuration of the kiln, the quantity of gases passing through the kiln, and other operating conditions. In general, the amount of dust generated from a kiln system ranges from about 5% to over 20% of kiln feed, with an average “emission factor” of around 12% (“Emission Factors for Industrial Processes,” EPA publication AP-42) of clinker produced; EPA indicates relatively little dependence upon process types.
United States clinker capacity of 75 to 80 million tons per year has remained relatively steady for the past 20 years (“US and Canadian Portland Cement Industry: Plant Information Summary, December 1995”; Portland Cement Association, Skokie, Ill., November 1996). Applying the EPA Emission Factors suggests that about 9 to 10 million tons of CKD are generated per year; that about 300 million tons have been generated over the past 30 years; and that over 1 billion tons have been generated since the start of the US cement industry over a century ago.
Kiln dust is a major problem at many cement manufacturing plants. Dust is generated in large quantities and is often not suitable for direct return to the cement-producing process as a feed, except in relatively small amounts, often because of high concentrations of alkalies and sulfates, because of incompatibility of the dust with the process, or because of limitations of the process equipment. It is estimated that less than half of the dust generated each year is returned to the process. (Some plants return all of their dust while others return none of it.) Since large quantities of dust cannot be returned directly to the kiln in all cases, that material must be disposed in some manner. Beneficial uses have been suggested, but the amounts of dust so used are relatively small. For some uses, only a few dusts are acceptable, while for others, such as agricultural liming, use is seasonal, but generation is continuous. Frequently, dust that cannot be directly returned or reused is discarded in waste piles, or it may be placed in land- or quarry-fills. Such disposal methods are inherently unsatisfactory because they involve wasting a material for which significant processing and handling costs and effort have been incurred. In addition, as environmental regulations have matured, the costs and problems of disposal have become more onerous, and continued disposal of kiln dust has become ever less desirable and more expensive.
PRIOR ART
The problems relating to CKD have long been recognized, and various methods have been proposed for their solution. Methods proposed by prior art have had one or more goals such as:
kiln dust recovery for reuse;
kiln dust utilization for other purposes;
byproduct recovery; and
pollution control.
One method suggested by prior art for treating cement kiln dust is to leach the dust with water to remove alkalies. Such procedures have been used in the past and have suffered from several problems:
only part of the alkalies were readily soluble, often half or less;
typical ratios of water to dust were 10:1 to 20:1, or higher;
an effluent high in pH (>10) and dissolved solids was discharged;
dissolved solids tended to precipitate in the receiving waters;
the high pH effluent was detrimental to the biosphere;
the recovered solids were high in water content, often over 70%; and
adjustments to kiln feed chemistry were required when treated dust was returned to the kiln.
These problems were so severe that the leaching methods of the past were largely banned by the US EPA by virtue of effluent limitations that were unachievable [see “Development Document for Effluent Limitations Guidelines and New Source Performance Standards for the Cement Manufacturing Category.” US EPA, January 1974]. Most leaching processes were shut down in the late 1970's and early 1980's, with the last one being discontinued in August of 1998. With that closure there are, to my knowledge, no commercially viable reclaim and recycle facilities for CKD in the United States, suggesting that there may actually be no relevant prior art.
Another method for treating cement kiln dust is described in Patzias (U.S. Pat. No. 2,991,154, July 1961) wherein kiln dust is mixed with water in a ratio of 2.5 to 3 parts water to 1 part dust, with the resulting mixture heated to about 358° F. in a closed vessel at a pressure of 150 psi for about 30 minutes. The slurry is then filtered to separate the solution containing the alkalies from the residual solids; the separated solution is treated by neutralization with sulfuric acid, evaporation, centrifugation, or a combination thereof, to recover alkali sulfates. The residual kiln dust solids are recycled to the cement-making process (with no indication of further treatment after alkali removal). This process is not practical for a number or reasons. Water is used in high ratio to dust, high temperature and high pressure (rather than carbonation) are used to effect the dissolution of alkalies, and significant amounts of calcium will be dissolved. Kiln dust solids would differ significantly in composition from normal kiln feed (requiring kiln feed correction). The residual solids could be returned to some kiln processes, but they would not be suitable for all processes and the amounts returnable would often be limited.
Similarly, McCord (U.S. Pat. No. 4,031,184, June 1977) leaches CKD at high temperature (but not at high pressure), using potassium chloride (KCl) to enhance solubility, and then flocculates the CKD solids using oil and a fatty acid and pelletizes the precipitate for reuse. The KCl is partly precipitated by cooling, making use of the large difference in solubility of KCl in hot and cold water. In all probability, the inventor has misidentified the chemical species that precipitates. The Haynes and Kramer survey of 113 US cement kiln dusts (i.e. most of those produced in the United States) indicates only 11 dusts where the sulfate does not exceed the chloride by at least two orders of magnitude (i.e. by at least a factor of 100). Even in those 11 dusts, sulfate exceeds chloride by a factor of two to five. Since the solubility of potassium chloride is higher than that of potassium sulfate by more than a factor of two in both hot and co
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