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A research project supported by the Minnesota Department of Natural Resources and the Iron Ore Cooperative Research Committee was undertaken to reduce the dissolution of fluoride into process water during taconite processing. Leach tests were conducted using rod mill feed samples and water containing varying concentrations of calcium and sodium. Laboratory liberation grinds were performed to evaluate fluoride rejection during grinding and magnetic separation. Tube furnace tests were conducted to evaluate fluoride volatilization during pellet induration. The results indicate that fluoride concentration of process water is a function of both calcium and sodium concentration in the process water. It appears that less than 1% of the fluoride present in rod mill feed will dissolve in process water during grinding. More than 90% of the fluoride present in rod mill feed is rejected with tailings during size reduction and magnetic separation. The furnace tests demonstrate that bentonite and fluxstone (calcite and dolomite) addition to taconite concentrate during pellet production increases the amount of fluoride that volatilizes during induration. Lime hydrate addition to pellets decreases the amount of fluoride that volatilizes during pellet induration. During acid pellet production, approximately three times more fluoride enters the wetter during induration-off-gas scrubbing than during grinding. When fluxed pellets are produced, three to eight times more fluoride enters the water during off-gas scrubbing than during taconite grinding.

A research project was undertaken to determine if coal addition to fluxed-magnetite prefired pellets would reduce nitrogen oxide (NOx) emissions during pellet induration. Iron-oxidation-rate tests, coal-oxidation-rate tests and calcinations-rate tests were conducted in a laboratory furnace to determine the amount of coal necessary to provide adequate heat for limestone calcination without producing detrimental effects to magnetite oxidation. Pilot-scale pot-grate tests were then performed both with and without coal in the prefired pellets to verify pellet-quality parameters and nitrogen oxide emissions. The off-gas during each pot-grate test was analyzed for NOx, CO2, CO, SO and O2. There was approximately a 10% reduction in NOxemissions with coal addition during the normal pot-grate firing cycle. There was a 75% reduction in NOxemissions during firing cycles that used 25% to 75% excess air during firing rather than the normal 300% excess air.

This paper represents a synopsis of five referenced research projects conducted by the author. Included are summaries of problems that occur during agglomeration and filtering due to water chemistry, water treatment and magnetite-particle surface area. High concentrations of divalent-cations in process water, particularly calcium and magnesium, have severe detrimental effects on prefired pellet quality when bentonite clay is the pellet-binding agent. The presence of fluoride, sulfate and calcium in process water can negatively affect filtering. Chemical water treatment can affect agglomeration and filtering either negatively or positively, depending on the ions that are being removed from or added to the process water. Water-treatment mechanisms that ensure improved filtering and improved prefired pellet quality are described.

A research project was undertaken to determine if coal addition to fluxed-magnetite prefired pellets would reduce nitrogen oxide (NOx) emissions during pellet induration. Iron-oxidation-rate tests, coal-oxidation-rate tests and calcinations-rate tests were conducted in a laboratory furnace to determine the amount of coal necessary to provide adequate heat for limestone calcination without producing detrimental effects to magnetite oxidation. Pilot-scale pot-grate tests were then performed both with and without coal in the prefired pellets to verify pellet-quality parameters and nitrogen oxide emissions. The off-gas during each pot-grate test was analyzed for NOx, CO2, CO, SO and O2. There was approximately a 10% reduction in NOxemissions with coal addition during the normal pot-grate firing cycle. There was a 75% reduction in NOxemissions during firing cycles that used 25% to 75% excess air during firing rather than the normal 300% excess air.