Final Technical Report
September 1, 1995, through August 31, 1996
Project Title: Production of Inorganic Pellet Binders from Fly-Ash
ICCI Project Number: 95-1/3.1A-1
Principal Investigator: S. K. Kawatra, Department of Metallurgical and Materials Engineering, Michigan Technological University
Other Investigators: T. C. Eisele, Department of Metallurgical and Materials Engineering, Michigan Technological University
Project Manager: D. Banerjee, ICCI
ABSTRACT
Utility companies which burn Illinois coals are being faced with progressively increasing landfilling costs for disposal of their combustion ashes. It is therefore important for them to find ways to market heir fly-ashes rather than landfilling them. However, many of these Illinois utilities produce fly-ashes that contain more than 6% unburned carbon, particularly utilities which use the new Low-NOx combustors. Such high carbon levels make fly-ashes difficult to utilize. In this project, a completely new application for Illinois fly-ashes is being developed which is less carbon-sensitive. The fly-ash is being used to produce binders for inorganic particulates, which are widely used by teh iron-ore producers in the Great Lakes region to produce iron-ore pellets. This project has found that Illinois fly-ashes are the closest sources of suitable fly-ashes for these producers, thus making utilization of Illinois fly-ashes for this purpose very attractive.
In this project, pelletization studies showed that fly-ash in combination with calcium hydroxide could be used to produce pellets with satisfactory strength when hardening accelerators, such as calcium chloride, were added. In studies conducted using high-calcium Illinois fluidized-bed combustor fly-ash, satisfactory pellets could be produced without any added calcium hydroxide, although calcium chloride did markedly increase the strength of dried pellets made using this binder. It was also shown that fly-ash carbon contents as high as 11.11% did not have any harmful effects on pellet quality.
Following the laboratory studies, cooperative test work was carried out in the facilities of the Cleveland Cliffs Iron Co. (CCI) to determine whether fly-ash-bonded pellets would sinter properly i their pilot-scale pellet-sintering apparatus. It was demonstrated that pellets of satisfactory strength could be produced and sintered acceptably at the CCI facilities. It was found that the intensity of the mixing of the fly-ash with the magnetite is critical to the dried pellet strength. Strong pellets could be produced using CCI's high-intensity mixer, but their low-intensity mixer produced much weaker pellets. It was also determined that increasing the moisture content of the magnetite during mixing resulted in stronger pellets.
EXECUTIVE SUMMARY
This project investigated the use of Illinois fly-ashes as binders for the iron-ore pellets produced in the Lake Superior iron district, with potential future applications to binding other inorganic particulates. The U. S. Geological Survey estimated that 62 million metric tons of iron-ore pellets were produced in the US in 1995, mainly in the Lake Superior district, and that approximately 637,000 metric tons of binder were consumed by this market. Since Illinois is the closest source of suitable fly-ash for this application, this represents a significant market for Illinois fly-ashes.
Iron-ore pellet binders are only one potential application for fly-ash-based binders. Metal foundries also require binders for their molding sands, and U. S. foundries consumed 885,000 metric tons of binder in 1995(1). There are 237 foundries in Illinois, including very large installations such as those operated by Caterpillar and John Deere, which would benefit directly from the availability of fly-ash binders as a replacement for the bentonite binders that are currently in use. With an average price of $58/ton for conventional bentonite binders in 1995, the value of fly-ash based binders can be significant.
Fly-ashes have never been used in this type of application before, with most of the fly-ashes which are currently utilized being sold as cement admixtures. Since iron-ore binders have different requirements than do cement admixtures, this new application will provide a means for utilizing those those ashes that are currently unmarketable. In particular, this project has demonstrated that iron-ore binders can tolerate high levels of unburned carbon, and that fluidized-bed combustor fly-ash is a good pellet binder even without any other additives. The ability to tolerate high carbon contents will become progressively more important for users of Illinois coals in the future, because the adoption of Low-NOx combustors is leading to higher levels of unburned carbon in the fly-ash. Fluid-bed combustors are becoming more widely used in order to control sulfur oxide emissions from combustion of Illinois coals, and so successful utilization of their fly-ashes is also of great importance.
Objectives
The goal of this project was to use currently unmarketable Illinois fly-ashes as a binder for inorganic powders. Such a binder would be useful for several high-volume markets, including production of iron ore pellets for blast-furnace feed. Low calcium Class F fly-ashes were used in combination with calcium hydroxide as a binder for iron ore pellets, with calcium Chloride being added as a hardening accelerator. The importance of carbon content and calcium content of the ash to the quality of the final pellets was investigated. The use of fluid-bed combustor fly-ash was also investigated, as such ashes are very high in calcium oxide and therefore have particularly good binder properties.
In the production of iron-ore pellets, the individual pellets must have a dry crushing strength of at least 5 pounds force per 1/2 inch diameter pellet. The pellets must also be sinterable at temperatures up to 1200 degrees C without a significant degree of spalling or cracking, and should have final sintered strengths of at least 400 pounds force per 1/2 inch diameter pellet. Pellets were therefore tested to determine whether they met these requirements.
Fly-Ashes Studied
This project concentrated on Illinois fly-ashes that are currently unmarketable. The following fly-ashes have been collected from Illinois power plants, and were used in the binder studies:
A) Ashes from the E. D. Edwards plant, operated by the Central Illinois Light Co. These ashes are unmarketable due to high carbon content. This project used ash both in its raw state, and after being treated by froth flotation to remove unburned carbon.
B) Fly-ash fro the fluid-bed combustor operated by ADM, Inc. This ash is very high in calcium compounds because of the limestone that is added to the combustor as a sulfur absorbent. It is therefore capable of acting as a binder without supplemental calcium hydroxide additions.
The compositions of the raw ash samples received, as determined in this project, are given in Table 1.
Table 1: Chemical Analyses of the Illinois fly-ashes being used, with the analysis of a typical western bentonite included for comparison purposes. The "Edwards Wet -150 mesh Decarbonized" fly ash was produced by screening ash collected from the plant waste pond, and removing the carbon by froth flotation.
| ADM Fluid-Bed Combustor | Edwards Dry Low-Carbon | Edwards Dry High-Carbon | Edwards West -150 mesh, Decarbonized | Western Bentonite | |
| Silicon Dioxide, SiO2 | 19.62 | 50.07 | 48.00 | 51.69 | 39.62 |
| Aluminum Oxide, Al2O3 | 6.60 | 24.41 | 23.87 | 26.94 | 23.16 |
| Iron Oxide, Fe2O3 | 5.04 | 9.51 | 9.34 | 10.83 | 5.49 |
| SiO2+Al2O3+Fe2O3 | 31.26 | 83.99 | 81.21 | 89.46 | 68.27 |
| Calcium Oxide, CaO | 46.19 | 2.72 | 2.63 | 2.77 | 9.63 |
| Mag. Oxide, MgO | 3.33 | 0.85 | 0.84 | 0.92 | 2.11 |
| Sodium Oxide, Na2O | 0.69 | 0.61 | 0.55 | 0.50 | 1.06 |
| Potassium Oxide, K2O | 0.81 | 2.68 | 2.61 | 2.48 | 0.39 |
| Titanium Dioxide, TiO2 | 0.43 | 1.22 | 1.25 | 1.45 | 1.25 |
| Manganese Dioxide, MnO2 | 0.13 | 0.05 | 0.05 | 0.06 | 0.01 |
| Phosphorus Pentoxide, P2O5 | 0.72 | 0.09 | 0.12 | 0.12 | 2.20 |
| Strontium Oxide, SrO | 0.08 | 0.15 | 0.14 | 0.17 | 0.39 |
| Barium Oxide, BaO | 0.05 | 0.16 | 0.16 | 0.19 | 0.53 |
| Sulfur Trioxide, SO3 | 12.53 | 0.61 | 0.54 | 0.14 | 5.93 |
| Loss on Ignition | 3.80 | 6.90 | 11.11 | 1.73 | 13.24 |
Results from the Project
Previous work by the investigators was carried out to remove carbon from fly-ashes, with the intent of converting the valueless high-carbon ashes into more marketable low-carbon ashes. Froth flotation has proven to be extremely effective for removing the unburned carbon from fly-ash, and can reduce its Loss-on-Ignition to less than 2%, even when the fly-ash contains up to 11% carbon. The tremendous success in decarbonization was largely due tot he use of Dow M210 froth conditioner as an additive to the fuel oil. This reagent increased the effectiveness of the fuel oil by approximately a factor of ten. High carbon content has long been the major stumbling block in fly-ash utilization, and froth flotation provides a means for dealing with the problem. Some of the treated fly-ashes from this previous work were used int he present study.
When the Edwards low-calcium Class F fly-ashes were used, fly-ash/calcium hydroxide binder mixtures have been found to be effective binders for iron-ore pellets, provided that accelerators such as calcium chloride or magnesium sulfate salts are added. High-calcium fly-ash, such as ADM fluid-bed combustor fly-ash, was an effective binder in its raw state, but was even more effective with added calcium chloride as shown in Figure 1 and Figure 2. These figures show that addition of calcium chloride systematically increases the dry strengths of the pellets, while still allowing production of fired pellets with strengths greater than 400 pounds force/pellet. At the highest fly-ash dosages, the pellets made with unslaked ash tended to crack upon firing, leading to low strengths. This problem could be prevented by addition of calcium chloride.
Figure 1 could not be converted to HTML - to view this table, please request a hard copy from ICCI
Figure 2 could not be converted to HTML - to view this table, please request a hard copy from ICCI
Experiments with the Edwards Class F fly-ashes, with carbon contents ranging from 0.94% to 11.11%, demonstrated that as the carbon content of the ash increased, a slight increase in the strength of the pellets dried at 105 degrees C was observed. This is believed to be due to natural accelerator salts contained in the highest-carbon ashes. Upon firing at 1200 degrees C, a tendency for decreased strength at higher carbon contents was observed. However, the magnitude of the decrease was comparable to the random variations in fired pellet strength, and was not as large as was originally feared. This indicates that iron-ore pelletization can utilize fly-ashes which contain much more than 6% carbon.
Pilot-scale mini-pot-grate kiln sintering tests, to determine the quality of the finished pellets after hardening at 1200 degrees C, were carried out by CCI personnel as the second part of CCI's contribution to this project. The pellets sintered without spalling, and the strongest sintered pellets had an average crushing strength of 497 +/- 37 pounds force per 1/2" diameter pellet, which exceeds the minimum requirement of 400 pounds force/pellet.
It was noted in the course of these tests that the type of mixer used for combining the ADM fly-ash wit the magnetite had a major effect on the pellet properties. The differences between mixers were due to differences in the intensity of their mixing actions, with the high-intensity mixers resulting in higher dry pellet strengths. It was also noted that the crushing strength of the dried pellets was dependent on the moisture content of the magnetite during mixing with the binder. As the moisture content increased, the pellet dry strengths also increased, up to the maximum practical limit for the moisture content (12% moisture). Moisture contents higher than 12% are not practical, because the magnetite becomes sticky and impossible to pelletize with standard equipment.
(1) US Geiological Survey (1996), Mineral Commodity Summaries, available by World-Wide-Web, http://minerals.er.usgs.gov:80/minerals/pubs/mcs/clays.txt, http://minerals.er.usgs.gov:80/minerals/pubs/mcs/ironore.txt.