Mercury Removal from Induration Off Gas by Wet Scrubbers

Abstract

During the induration of taconite pellets, green balls are heated to greater than 2200°F. A previous study indicated that greater than 90 percent of the mercury contained in the green balls is volatilized during induration. Some of the volatilized mercury is removed by the gas scrubbers. Studies on coal burning power plants indicate that the mercury in flue gas is present as either elemental mercury or as divalent mercury. In power plant scrubbers, the majority of the divalent mercury is removed, but very little elemental mercury is removed by the scrubbers. The particulate matter in the off gas appears to remove a significant portion of the mercury that is removed. It is thought that the off gas chemistry and the scrubber water chemistry could affect the removal of mercury. To determine if the scrubber water chemistry could affect the removal of mercury from taconite pelletizing off gases, the Minnesota Department of Natural Resources' (MNDNR) environmental cooperative funded a study to sample around the scrubbers from the plants equipped with wet scrubbers to determine if water chemistry affects mercury removal. Another objective of the study was to determine the role of solids entrained in the off gases and removed by the scrubbers. These solids are returned to the process. If they were discarded, then some amount of mercury could be eliminated from the system, but at a cost of iron units.

Samples were obtained from Minntac, EVT AC, Minorca, Hibtac and Northshore. With the exception of the mercury analyses, all chemical analyses were conducted at Coleraine. Mercury analyses were run by Frontier Geosciences of Seattle, Washington.

While the various plants have different scrubber configurations and scrubber water chemistries, these differences appeared to have no significant affect on mercury removal. Accurate mercury balances were not possible because mercury content in the fired pellets from all of the plants was below the detection limit of about 0.6 parts per billion (ppb). Solids entrained in the off gases removed significantly more mercury than the scrubber water. Of the mercury removed in the scrubber systems, the amount contained in the solids ranged from 75 percent at Northshore to greater than 99 percent at EVTAC. The minus 10 micron fraction of the solids in the off gases appears to remove the most mercury. Analysis of the solids that are continually recycled to the Minorca wet scrubbers indicates a high capacity for mercury removal (the solids assayed over 3000 ppb mercury). This result indicates that the mercury should remain with the solids and should not leach if the solids were sent to the tailings basin.