Date of Award

January 2012

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

First Advisor

Steve Benson

Abstract

The taconite industry located in the Mesabi iron range has been identified as one of the major contributors of the atmospheric mercury in the Lake Superior basin by the Lake Superior Lakewide Management Plan (LaMP). Mercury is a leading concern among air toxic metals due to its volatility, persistence and bioaccumulation as methylmercury in the environment, and its neurological health impacts.

Previous research work performed at taconite processing plants by Minnesota Department of Natural Resources (DNR) identified the taconite ore as the main source of mercury during the taconite processing. Magentite iron ore pellets are produced by balling moist concentrates to green balls, which are then dried, oxidized to hematite, sintered, cooled and transported to steelmaking plants. Mercury is released during the heat processing (induration) step of these green balls to a final product called as taconite pellets. In order to address the mercury emission problem, an approach was proposed by the University of North Dakota (UND) team which explores the possibility of oxidizing the mercury and thus, increasing the mercury capture from scrubbers.

The proposed technology employed a low corrosion method where brominated activated carbon (ESORB-HG-11) was added to green balls to promote mercury oxidation. In Phase I, mercury oxidation potential of ESORB-HG-11 was established. In Phase II, green balls produced from the ore concentrate and additives obtained from five different plants were mixed with trace amounts of ESORB-HG-11. The green balls were

then subjected to heating experiments to determine the mercury oxidation potential of the additive.

Heating tests of the green balls from four of the taconite facilities showed the mercury oxidation levels ranging between 43% and 78%, with averages of 52% (±8 %) and 58% (±11%) for 0.1 and 0.5 weigh percent loading respectively. Baseline oxidation levels averaged to 18% (±6%), while oxidation levels due to addition of ESORB-HG-11 averaged 42% (±9%) and 48% (±13%) for the 0.1 and 0.5 weight percent loading respectively. Results were in accordance with Phase I indicating that 0.1 wt% is optimum loading for mercury oxidation. The results from the fifth taconite facility have not been included in the averages since they showed significantly lower mercury oxidation levels when compared to other plants.

Phase analysis experiments and results from Phase I and Phase II suggest that there was little or no gas-phase mercury oxidation occurring during tests performed using the lab scale apparatus. This suggests that the mercury oxidation observed during these tests is a solid phase phenomenon occurring most likely on the carbon surface and within the green ball. Previous work indicated that the gas-phase mercury oxidation does occur in taconite facilities. Consequently, a full-scale demonstration of the technology might result in higher levels of mercury oxidation than observed during the bench scale tests in this project. The impact of the carbon additive on the fired taconite pellet needs to be investigated in future testing to further develop the process.

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