The U.S. cement industry has adopted a year 2020 voluntary target of a 60% reduction (from a 1990 baseline) in the amount of CKD landfilled per ton of clinker produced. In general, CKD is a very heterogeneous mix both by chemistry and particulate size, and these characteristics are dependent on the raw materials, fuels, kiln pyroprocessing type, overall equipment layout, and type of cement being manufactured. It is incorrect to label all dusts collected in the PMCD as waste. Many facilities return all or a major portion of the CKD to the kiln as a feedstock while other facilities sell the dust for numerous beneficial uses, such as soil consolidation, waste stabilization, and other uses. Recycling this byproduct reduces the need for limestone and other raw materials and helps conserve energy.
The most common reasons for not returning CKD to the kiln system* are equipment limitations for handling the dust and chemical constituents in the dust that would be detrimental to the final cement product. The fraction of CKD that is not returned to the kiln or otherwise beneficially used is placed in landfills.(1)
The Beneficial Uses of Cement Kiln Removed from the Cement Manufacturing Process are:(2)
The focus is in the first two points as they were combined into one research [Utilization of Cement Kiln By-Pass dust (in construction & agriculture purpose)]
1- CKD Cement Blend/additive:
The use of CKD as an addition to Portland cement has been evaluated by a number of researchers. A summary of some examples can be found in Detweiler et al. (1996). M. S. Y. Bhatty (1983, 1984a-c, and 1986) published a series of reports on the use of CKD blended with Portland cement as well as fly ash and ground granulated blast furnace slag. The studies found that cements containing only CKD had reduced workability, setting times, and strength. The loss of strength was attributed to alkalies in the dust. It is believed that the use of fly ash with CKD diluted the alkalies and thus improved the strength. Addition of slag to a cement-CKD blend generally decreased workability, but produced higher strengths than blends containing no slag. It was found that blended cement with high sulfate produced the greatest strength, and that the impact of the alkalies in the dust could be negleted by the fly ash and/or slag. Overall, the ratios of alkali, chlorides, and sulfates in the dust are important in determining the properties of the blended cement. Ravindrarajah (1982) reported that kiln dust could be used in masonry and concrete blocks without loss of strength or workability. His study showed that up to 15% of the Portland cement could be replaced with CKD. If higher percentages of dust were used, the setting was retarded, workability was reduced, and water demand was increased. Daugherty and Funnell (1983) reported that the use of up to ten percent interground CKD did not have any adverse effects on the setting time, soundness or shrinkage of the final Portland cement concrete. However, the strength results varied, most likely attributed to the changing dust composition.
Abo-El-Enein et al. (1994) studied the mechanical properties of blended cements using by-pass dusts. The initial and final setting times of cement pastes were decreased due to the high free lime content in the CKD. Blended cements with up to 15% kiln dust had increased compressive strengths and accelerated hydration. Compressive strengths decreased when more than 15% of the Portland cement was replaced with CKD.
2- Agricultural soil Amendment:
Because of the high lime and potassium concentrations, CKD is used as a soil amendment or fertilizer in many parts of the world. The acid neutralizing capacity of the lime in CKD counteracts the acidic soils that result from years of farming. Neutral soils are a better growing environment for crops and also enhance herbicide effectiveness. The dust may provide potassium and trace metals that are also depleted from agricultural soils due to plant withdrawal requirements. The following describe some of the work that has been completed on this subject within the U.S.
For agricultural uses of CKD in the U.S. before 1975, Davis and Hook (1975) completed a survey of dust applications in their report for the USEPA. They found that kiln dust was suited to replace liming agents and potassium fertilizers. Baker et al. (1975) developed non-putrescible soil-like products for general agricultural applications by combining CKD with sewage sludge before vacuum filtering. Risser et al. (1981) produced a lime-potash soil additive composed of 35% CaO, 6% MgO, 5% K2O, and 4% SO3 from CKD having a controlled composition. Taylor (1987) also reported the usefulness of CKD as a substitute for lime and fertilizer elements. Since the quality of CKD varies with each cement plant, however, the nutrient value may also change. In Australia, Dan et al. (1989) found that CKD increased the crop yield equally well as crushed limestone. Fraiman et al. (1991) recommended the use of bypass dust containing high levels of K2O, SO3, and Cl, in fertilizer applications. Because potassium is the most valuable fertilizer element contained in the dust and the least desirable element for recycling, CKD with high potassium contents could be adequately utilized as a fertilizer. It should be noted that nutrients such as nitrogen and phosphorus are still required from other sources regardless of whether limestone or CKD is used on agricultural soils.
Because the placement of a fine dust, such as CKD, on agricultural lands is difficult, it has been suggested that granules or agglomerates of dust should be made (Kachinski, 1983; Wommack et al.,2001). The larger particles help to limit fugitive releases of dust while transporting, handling, and placing the CKD. Conversely, care must be taken so that the granules are not so rigid that rain and other natural processes cannot dissolve or break down the particles to release the beneficial constituents of the dust. One of the concerns with using CKD as a fertilizer or soil amendment is the level of trace metals it may contain. The effect of those metals on the food chain through possible extraction by soil and subsequent movement into vegetation should be determined before such applications are to be considered. However, in a specific study on the use of CKD as a fertilizer in Iowa, Preston (1993) demonstrated that trace metals in CKD were well below the permissible levels for land application. The USEPA (1999b) studied metals and other constituents of fertilizers including CKD. Their findings showed that the metals content of various fertilizers ranged over several orders of magnitude but did not provide any specific recommendations on the use of CKD on agricultural lands.
Kanare (1999) completed a study comparing CKD to soils, agricultural limestones (aglime), and sewage sludges. He found that sludges generally have the highest levels of chromium, lead, mercury, nickel, and silver compared to CKD, aglime, and North American soils. The highest levels of thallium and selenium are found in CKD. Other elements of interest are present in these materials at non-detectable or comparable concentrations.
Note:- CKD contains insignificant amounts of trace metals and therefore metal concentrations are not usually a concern for most applications. A comprehensive study evaluated the presence of trace metals in CKD from 79 plants in the United States and 10 plants in Canada using both conventional and waste derived fuels (PCA 1992). Each CKD was tested for the eight RCRA metals: arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver. The samples were also analyzed for antimony, beryllium, thallium and nickel, which are regulated under the Boiler and Industrial Furnace rule for hazardous solid wastes. Results showed that the average level of trace metals found in the CKD were significantly below the regulation limits (PCA, 1992).
*for latest achievement in this regard by M/s A-tech follow this link: http://www.atec-ltd.com/blog/index.php/bypass-dust-treatment/
[(1)REPORT ON SUSTAINABLE MANUFACTURING 2011 PCA]
[(2)BENEFICIAL USES OF CEMENT KILN DUST,By:IEEE-IAS Cement Industry Committee,Wayne S. Adaska, P.E., Director, Public Works, Portland Cement Association,Donald H. Taubert, Director, Promotion & Technical Service, Capitol Cement,Presented at 2008 IEEE/PCA 50th Cement Industry Technical Conf., Miami, FL, May 19-22, 2008]