1 

### Evaluation of Minnesota Taconite Wet Scrubbers at Evaluation of Minnesota Taconite Wet Scrubbers at Minntac, Keewatin Taconite, Hibbing Taconite, and U nited Taconite
by John Engesser, P.E. Minnesota Department of Natural Resources, Division of Lands & Minerals

This paper is a review of water treatment methods t hat can be used to increase sulfur dioxide scrubbing at Minntac, Keewatin Taconite, Hibbing Ta conite, and United Taconite. The paper will look at the impacts that more efficient sulfur dioxide scrubbing will have on process water chemistry, tailing water chemistry, and iron oxide recovery.

Taconite Water Chemistry

Minnesota taconite facilities have been operating f or 40 to 50 years. During this time the concentration of dissolved solids in recycled proce ss water and tailing water has increased. This increase in the ionic strength of taconite-process water has occurred because of the following four chemical reactions: 1) sulfide mineral oxidat ion and the subsequent formation of sulfuric acid, 2) the removal of soluble acid gases (sulfur dioxide, hydrogen chloride, hydrogen fluoride) from induration exhaust by wet scrubbers, 3) neutra lization of acid gases by carbonate minerals, and 4) dissolution of soluble salts.

Sulfide minerals, present in taconite, oxidize and hydrolyze to form sulfuric acid when exposed to air and water during grinding and induration. S ulfuric acid is neutralized by calcium carbonate and magnesium carbonate, which are both p resent in taconite. These reactions cause an increase in the ionic concentrations of calcium, magnesium, and sulfate in process water and tailing water. The following reaction equations re present what occurs during grinding, induration and tailing disposal.

4FeS 2 + 15O 2 + 14H 2 O  4Fe(OH) 3 + 16H + + 8SO 4 = (grinding and tailing disposal)

4FeS 2 + 11O 2  2Fe 2 O 3 + 8SO 2 ↑ (Induration)

2SO 2 + 2H 2 O + O 2  4H + + 2SO 4 = (Scrubber Reaction)

H + + CaCO 3  Ca +2 + HCO 3 - (Neutralization)

H + + MgCO 3  Mg +2 + HCO 3 - (Neutralization) As the concentration of sulfate increases, the conc entration of both magnesium and calcium increase. The following graph illustrates the corr elation between hardness (calcium and magnesium) and sulfate in taconite process water.

Sulfate Concentration Corrleated to Calcium and Man gsium Concentrations in

Taconite Process Water

R 2 = 0.9829

0 1 2 3 4 5 6 7 8 9

1 2 3 4 5 6 7 8 9 10

Ca + Mg (mmoles/liter)Sulfate (mmoles/liter)

Data Points2

Increased sulfate concentration results in increase d process water hardness, which can cause processing problems in flotation and balling. High calcium concentrations in process water results in precipitation of calcium carbonate in pi pes. Calcium sulfate precipitation can also occur in certain areas of the plants when the calci um sulfate solubility limit is reached. The

ultimate result of the increased water hardness is an increase in taconite production costs.

There are very few soluble salts present in taconit e. However, with the advent of fluxed pellets, soluble chlorides (calcium, magnesium, sodium and p otassium) are introduced into the water during fluxstone (calcite and/or dolomite) grinding . Chloride is volatized during pellet induration and is subsequently removed by the wet s crubbers as hydrochloric acid. The following reactions represent the formation of hydr ochloric acid during induration and the neutralization reactions that occur during scrubbin g and after scrubbing.

CaCl 2 + H 2 O  CaO + 2HCl ↑ (Induration)

HCl + CaCO 3  HCO 3 - + Ca +2 + Cl - (Neutralization)

Small amounts of naturally occurring insoluble fluo ride minerals (mostly fluorapitite) are present in Minnesota taconite. Approximately 90 to 95 perc ent of the fluoride is rejected to the tailings during magnetite concentration. The remaining flu oride (0.001% to 0.002% of the concentrate) enters the induration process. Fluoride minerals a re also added to the concentrate with the addition of bentonite clay, limestone and dolomite prior to greenball formation. Approximately forty percent of the fluoride entering the indurati on process is volatilized during induration. The quantity of hydrogen fluoride that forms during ind uration increases as the concentration of sodium in the process water increases and the conce ntration of calcium in the process water decreases. Wet scrubbers remove the hydrogen fluor ide that forms during pellet induration. Approximately 0.01 pounds of hydrogen fluoride diss olves in the process water for each ton of taconite processed. Most of the fluoride that diss olves in the process water precipitates as calcium fluoride. Fluoride concentration in proces s water is limited by calcium concentration because calcium fluoride is fairly insoluble in wat er and readily precipitates. The presence of sodium in process water aids in the dissolution of fluoride salts, which results in increased concentrations of fluoride in process water. The c oncentration of fluoride in taconite process water varies from 2 mg/liter to 25 mg/liter dependi ng on the concentration of calcium and sodium. The following two graphs demonstrate how t he concentration of fluoride in process water is affected by the concentrations of calcium and sodium in process water.

Calcium and Fluoride in Process Water Samples

from Minnesota Taconite Plants

0 5 10 15 20 25

0 20 40 60 80 100

Calcium Conc., ppmFluoride Conc., ppm

Data Points Theoretical CaF2 Solubility3

The following reactions represent the fluoride reac tions that occur during induration and neutralization after scrubbing.

NaF +H 2 O  HF ↑ + NaOH (Induration)

2HF + 2CaCO 3  CaF 2 ↓ + Ca +2 + 2HCO 3 - (Neutralization)

The addition of sodium carbonate and/or sodium hydr oxide to taconite process water results in the precipitation of calcium carbonate, which decre ases the concentration of calcium in the water. The following reactions represent what occu rs when sodium carbonate and sodium hydroxide are added to taconite process water.

Na 2 CO 3 + Ca +2  2Na + + CaCO 3 ↓

NaOH + HCO 3 - + Ca +2  Na + + H 2 O + CaCO 3 ↓

Water Chemistry Conclusions

1) Increased concentrations of sulfate in taconite process water results in increased concentrations of hardness (calcium and magnesium). This increased hardness results in increases in pellet production costs. 2) Sulfate and calcium will precipitate from soluti on once the solubility limit of calcium sulfate is reached. This will occur when the produ cts of the molar concentration of calcium and sulfate reach the solubility limit. So dium sulfate is extremely soluble. 3) Increased scrubber efficiency can result in incr eased sulfate concentration in taconite

process water. 4) The concentration of sodium in taconite process water affects the amount of hydrogen fluoride that forms during pellet induration. Incr eased concentrations of sodium results in increased formation of hydrogen fluoride during pellet induration. 5) The concentration of calcium in taconite process water affects the amount of hydrogen fluoride that forms during pellet induration. Incr eased concentrations of calcium results in decreased formation of hydrogen fluoride during pellet induration. 6) The concentration of calcium in scrubber water a ffects the amount of fluoride that precipitates from solution after hydrogen fluoride scrubbing. Increased concentrations of Sodium and Fluoride in Process Water Samples from

Minnesota Taconite Plants

0 5 10 15 20 25

0 20 40 60 80 100 120 140 160

Sodium Conc., ppmFluoride Conc., ppm

Data Points4

calcium results in increased precipitation of calci um fluoride and decreased concentration of fluoride in the taconite process water. 7) Precipitation of calcium fluoride and calcium su lfate can limit the amount of fluoride and sulfate that remain in solution in taconite process water. 8) The addition of sodium carbonate and sodium hydr oxide to taconite process water

decreases the concentration of calcium in taconite process water.

Past History of Taconite Process Water

There are some observations that can be made by loo king at past practices at Minnesota taconite

facilities. 1) Two taconite facilities, Northshore Mining and L TV Mining used soda ash (sodium carbonate) water treatment to “soften” (precipitate calcium carbonate) process water. The tailing basins at these two facilities contain water that has the highest fluoride

concentrations of all the Minnesota taconite facili ties. Precipitation of calcium carbonate decreased the calcium concentration of the water, w hich decreased calcium fluoride precipitation; thus resulting in high fluoride conc entration in the tailing water. 2) The tailing facility at Minntac contains the hig hest concentration of sulfate because Minntac installed four once through wet scrubbers, which remove sulfur dioxide. The sulfur dioxide is oxidized to sulfate. The tailing basin has no permitted direct discharge other than the existing seeps and so the sulfate co ncentration in the tailing water continues to increase due to the recycling of water from the tailing basin back to the

scrubbers. Minntac has a lower fluoride concentrat ion in tailing water than do most taconite facilities because of high calcium concent ration, which limits the fluoride concentration to the solubility of calcium fluoride

. 3) It appears that the lime re-circulating scrubber at Keewatin Taconite can limit the amount of sulfate and fluoride that enter the tailing wate r. This is accomplished by continually adjusting the scrubber water to the proper pH and a llowing it to circulate until gypsum (calcium sulfate) and calcium fluoride precipitatio n occurs. 4) There is no precipitation reaction that limits t he concentration of chloride in taconite

process water.

Sulfur Dioxide Scrubbing at Taconite Plants

The efficiency of sulfur dioxide scrubbing in tacon ite induration scrubbers depends on the concentration of sulfur in the fuel and pre-fired p ellet, the scrubber water chemistry, the scrubber water flowrate, the composition of the ash from the fuel, the composition of dust formed during pellet induration, and residence time (gas - water contact time and reaction rates).

1) Sulfur dioxide scrubbing efficiency is affected by the concentration of sulfur in the fuel and the pre-fired pellet. During pellet induratio n, the sulfur that is present in the fuel and pre-fired pellet oxidizes to sulfur dioxide. An inc rease in the amount of sulfur entering

the furnace results in increased concentration of s ulfur dioxide in the waste gas. Sulfur dioxide begins to react with the alkalinity in the scrubber water as the sulfur dioxide dissolves in the water. The alkalinity and the pH of the scrubber water begin to decrease as the sulfur dioxide reacts. As the pH of the scr ubber water decreases, the rate of sulfur5

dioxide scrubbing decreases. Once the pH of the sc rubber water decreases to 4.5, the

reaction of sulfur dioxide with bicarbonate is esse ntially complete. Sulfite (SO 3 = ) and bisulfite (HSO 3 - ) are much more stable in water than is sulfurous a cid (H 2 SO 3 ). Sulfurous acid begins to form at about pH 4. Sulfu r dioxide scrubbing that occurs below pH 4.5 is due to the solubility of sulfur dioxide i n water, which is controlled by the partial pressure (concentration) of sulfur dioxide in the w aste gas.

2) Sulfur dioxide scrubbing efficiency is affected by the alkalinity and pH of the scrubber water. The pH of taconite process water is usually very close to pH 8, which is the pH at

which the maximum concentration of bicarbonate alka linity occurs. As bicarbonate reacts with sulfur dioxide, the scrubber water pH d ecreases and the bicarbonate alkalinity decreases. Water that contains high bicarbonate al kalinity concentration is capable of scrubbing more sulfur dioxide than water that conta ins low bicarbonate alkalinity

concentration. Concentration of Sulfite Species

0.0000 0.0005 0.0010 0.0015 0.0020 0.0025

0 2 4 6 8 10 12 14

pHConcentration, moles/liter H2SO3 HSO3- SO3=

Carbonate Species Concentration at Various pH

0 20 40 60 80 100 120 140 160 180

2 4 6 8 10 12 14

pHConcentration, mg/liter H2CO3

HCO3

- CO3=6

3) The scrubber make-up water flowrate affects the amount of bicarbonate that enters the scrubber, which affects the amount of sulfur dioxid e removed from the waste gas. Higher scrubber make-up water flowrates result in more bic arbonate alkalinity available to react with sulfur dioxide in the waste gas. The table be low indicates that sulfur dioxide scrubber efficiency is dependent on the amount of b icarbonate alkalinity (HCO 3 - ) that

enters the scrubber. Keewatin Taconite data is not included in the table below because most of the alkalinity in the scrubber water at Kee tac comes from lime addition rather than the make-up water.

4) Sulfur dioxide scrubbing efficiency is affected by the neutralizing capacity of ash from

the fuel. Coal ash contains little or no acid neut ralizing capacity; however, wood ash contains large amounts of CaO and MgO, which will n eutralize acid components in the waste gas. The ash content of wood is about 2% by weight. Following is a table that lists the ash analyses for northern Minnesota birch and a spen. Using the MgO and CaO analyses, the neutralizing capacity of the wood ash is about 0.44 moles per short ton of

wood burned.

5) Sulfur dioxide scrubbing efficiency is affected by the addition of limestone to the prefired pellets. Taconite plants add from 1% to 10% limestone to pre-fired pellets depending on the type of pellet that is being produ ced. Limestone addition rates of 1% can be used for standard (acid) pellet production; whereas, limestone addition rates of 8% to 10% are used for fluxed pellet production. The taconite pelletizing process produces from 2 to 8 pounds of dust per gross ton of pellets produced. Wet scrubbers remove this dust from the waste gas. The acid neutralizing cap acity of this “scrubber dust” during the production of a 1% limestone pellet is about 0.2 to 0.8 millimoles for each ton of standard pellets produced. The acid neutralizing capacity o f “scrubber dust” during the production of 10% limestone pellets is about 2 to 8 millimole s for each ton of pellets produced.

The following table indicates some calculated sul fur dioxide scrubber efficiency rates. Two efficiency rates are listed in the table. The first scrubber efficiency rate was calculated using the increase of sulfate in scrubbe r water. The second scrubber efficiency rate was calculated using the concentration of sulf ur dioxide in the waste gas entering the scrubber (calculated from mass balance) and the con centration of sulfur dioxide exiting Wood %Fe 2 O 3 %SiO 2 %Na 2 O %K 2 O %CaO %MgO %SO 3 %LOI

Birch 1.30 0.85 8.70 15.1 45.8 11.6 3.1 13.5 Aspen 0.59 5.80 0.17 12.2 52.7 5.44 3.1 18.5 Calc. Water Calc. Air Taconite Scrubber Make-up Make-up HCO3- Reacted HCO3- Re maining HCO3- SO2Scrubber SO2 Scrubber Plant Make-up, gpm HCO3-, lb/min lb mole/min lb mole/min lb mole/min Efficiency Efficien cy
Minntac L6&7 2800 4.10 0.0672 0.0596 0.0077 60% 71%

Minntac L4&5 2800 4.10 0.0672 0.0288 0.0384 53%

Utac L2 870 1.57 0.0257 0.0257 0.0000 26% 27%

Hibtac 3900 7.67 0.1258 0.0187 0.1070 75%7

the stack. The sulfur dioxide removed by the dust was calculated based on a calculated removal rate for bicarbonate alkalinity by the scru bber water that occurred from pH 8 (pH of taconite process water) to the measured pH of th e scrubber exit water (listed in the table). The difference between the total sulfur di oxide removal (efficiency) and the sulfur

dioxide that reacted with the bicarbonate alkalinit y in the water is the amount of sulfur

dioxide that reacted with the dust. Notice that fo r Minntac Lines 4 & 5 the amount of sulfur dioxide removed by the “scrubber dust” was m ore than any other scrubber listed in

the table. This is because wood was used as a fue l and wood ash has a very fine grain size and is therefore very reactive. This data dem onstrates that sulfur dioxide scrubber efficiency is dependent on neutralization of scrubb er water by the dust and fuel ash.

Note: Waste gas flowrates are calculated using, the oxygen conten t of the waste gas, fuel type, fuel composition, fuel rate, pre-fire pellet moisture, and pre-fired pellet com bined water and carbon dioxide content with the assumption that all combustion products are water and carbon dioxide.

6) Liquid gas reactions are dependent on the gas co ncentration that is in contact with the liquid. As the gas concentration decreases, the li quid gas reaction rate decreases. In the case of sulfur dioxide scrubbing the gas is sulfur dioxide and the liquid is water. The following table indicates the concentrations of sul fur dioxide in the taconite waste gas leaving the scrubber (gas after scrubbing). Notice the high pH in the Hibtac scrubber water and the amount of alkalinity remaining in the water that is still available to react with sulfur dioxide. However, the sulfur dioxide c oncentration in the waste gas is very low (5.5 ppm) and, therefore, the reaction is very slow. The graph below indicates that

there is a sulfur dioxide concentration limit that is attainable in a wet venturi scrubber due to such things as residence time and scrubber water flowrate.

Scrubber Scrubber Taconite Water Exit SO2 Remaining HCO3- Plant pH ppm lb mole/min

Keetac 6.4 31.1 NA

Minntac L6&7 5.3 30.0 0.0077

Minntac L4&5 6.2 24.3 0.0384

Utac L2 3.6 102.0 0.0000

Hibtac 7.0 5.5 0.1070 Calc. Water Calc. Air Scrubber SO2 Taconite Waste gas Pellet SO2 Nat Gas Coal Wood Pellet lb SO2 SO2Scrubber SO2 Scrubber Water Scrubbing Plant acfm, 110F Prod lt/hr ppm scfh lb/hour lb/hr Type min ute Efficiency Efficiency pH by dust
Keetac 613,789 693 62.9 76,200 20,175 0 Acid 5.94 50% 6.4 NA
Minntac L6&7 407,951 415 88.0 83,900 16,500 0 Flux 5.53 60% 7 1% 5.3 20%
Minntac L4&5 404,883 415 51.6 136,950 0 11,450 Flux 3.21 53% 6.2 35%
Utac L2 583,428 452 139.7 0 15,325 0 Acid 12.54 26% 27% 3.6 15%
Hibtac 468,871 384 21.9 96,260 0 0 Acid 1.58 75% 7.0 39%8 Lime Scrubber Reactions (Re Lime Scrubber Reactions (Re-

- circulating Lime Scrubber) circulating Lime Scrubber)

Scrubber Reactions with hydrated lime (calcium hydr oxide) Ca(OH) 2 + H 2 O  Ca +2 +2OH - +H 2 O (lime dissolution)

H + + OH -  H 2 O (acid neutralization)

Ca +2 + SO 3 =  CaSO 3 ↓ (calcium sulfite precipitation)

2CaSO 3 + O 2  2CaSO 4 (calcium sulfite oxidation)

2Ca +2 + 2SO 3 = + O 2 +4H 2 O  2(CaSO 4 .2H 2 O) ↓ (gypsum precipitation)

Ca +2 + 2OH - + 2HF  CaF 2 ↓ + 2H 2 O (calcium fluoride precipitation)

Ca +2 + 2OH - + CO 2  CaCO 3 ↓ + H 2 O (high pH calcium carbonate precipitation)

The addition of lime to scrubbers increases acid ga s scrubbing efficiency. As the pH of the scrubber water increases, the amount of acid gas re moved from the waste gas increases. However, the pH of the scrubber water cannot be inc reased to the point where bicarbonate and/or

carbon dioxide are converted to carbonate because c alcium carbonate will precipitate from solution and scaling within the scrubber will occur . As pH increases, the hydroxide (from lime addition) that is present in the scrubber water rem oves carbon dioxide from the waste gas and converts it to carbonate, which then precipitates a s calcium carbonate. Calcium carbonate precipitation will plug scrubbers, which in turn de creases scrubber efficiency. The following two graphs depict the concentration of carbonate at various pH. These graphs are based on water that has a concentration of 120 mg CO 2 /liter. Depending on pH the CO 2 can be carbonic acid (H 2 CO 3 ), bicarbonate (HCO 3 - ), and/or carbonate (CO 3 = ).

Sulfur Dioxide in Scrubber Exit Waste Gas vs Scrubb er Water pH

0 20 40 60 80 100 120

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5

Scrubber Water pHExit Waste Gas SO2, ppm

Minntac, Hibtac & UTac Poly. (Minntac, Hibtac & UTac)9

The above two graphs indicate that as the pH of the water in a re-circulating lime scrubber increases, the concentration of the carbonate ion i ncreases. The carbonate concentration must be maintained at a low enough concentration that will not result in large amounts of calcium carbonate precipitation. The solubility product fo r calcium carbonate is 4.9x10 -9 . A recirculating scrubber that operates at 100F and main tains the sulfate concentration in the water at 1400 mg SO 4 /liter will have water that has a calcium concentra tion of 650mg Ca/liter. This means to maintain a calcium carbonate precipitation rate of less than 1 mg/liter, the

concentration of carbonate has to be less than 10 -5 moles/liter. Examining the log graph above, the pH of the re-circulating scrubber water should be maintained at a pH less than 7.5 to ensure

the carbonate concentration is less than 10 -5

. Log of Concentration of Carbonate Species

-6 -5.5 -5 -4.5 -4 -3.5 -3 -2.5

2 4 6 8 10 12 14

pHLog of Concentration, log(moles/liter) CO3= HCO3

- H2CO3

Carbonate Species Concentration at Various pH

0 20 40 60 80 100 120 140 160 180

2 4 6 8 10 12 14

pHConcentration, mg/liter H2CO3

HCO3

- CO3=10

The solubility of calcium sulfite is less than the solubility of calcium sulfate. The above graph indicates that the concentration of bisulfite (HSO 3 - ) is greater than the concentration of sulfite when the pH of the scrubber water is less than 7. If the sulfite oxidation rate to sulfate is not rapid enough, then calcium sulfite can precipitate from solution, especially when the pH of the scrubber water is above 7. High concentrations of sulfur dioxide in the waste gas can result in high enough concentrations of sulfite in the scrubb er water that precipitation of calcium sulfite can even occur as low as pH 6. Calcium sulfite pre cipitation in the scrubber can cause scaling and plugging problems. For this reason it is diffi cult to operate re-circulating-lime scrubbers at a pH that is above 7 without decreasing scrubber effi ciency by forming precipitates.

Application to Minnesota Taconite Induration Scrubb ers

Keewatin Taconite Scrubber Keewatin Taconite Scrubber – Keewatin Taconite has a re-circulating-lime, venturi-rod scrubber.

In order to maintain maximum sulfur dioxide scrubbe r efficiency, the scrubber should be operated between pH 7 and pH 7.5. If it is operate d at a pH greater than 7.5, calcium carbonate and calcium sulfite scaling will occur in the scrub ber, which will result in decreased scrubber efficiency and could even result in shutting down t he process to clean the scale formation from the scrubber. It would appear that Keewatin Taconi te is currently operating at near optimum sulfur dioxide scrubbing conditions.

Minntac Line 3 Scrubber Minntac Line 3 Scrubber – Minntac installed a re-ci rculating-lime venturi-rod scrubber on the Line 3 pellet induration furnace during 2005 - 2006 . Minntac is currently not operating Line 3; however, when Line 3 was operating, scaling in the scrubber was a problem. The scrubber system will have to optimized to ensure good sulfur dioxide scrubbing without scaling. When this optimization is accomplished, Line 3 should ha ve sulfur dioxide scrubber efficiency similar to the Keewatin Taconite re-circulating lime scrubb er. Concentration of Sulfite Species

0.0000 0.0005 0.0010 0.0015 0.0020 0.0025

0 2 4 6 8 10 12 14

pHConcentration, moles/liter H2SO3 HSO3- SO3=11

Minntac Lines 4, 5, 6 and 7 Scrubbers Minntac Lines 4, 5, 6 and 7 Scrubbers – Minntac has “once through” venturi-rod scrubbers on lines 4, 5, 6, and 7 pellet induration furnaces. It is currently estimated that approximately 70%

of the sulfate in the Minntac basin water is due to sulfur dioxide scrubbing. The wet scrubbers are currently removing more than 50% of the waste-g as sulfur dioxide when Minntac is producing fluxed pellets and/or using wood as a fue l. Minntac produces fluxed pellets on lines 6 and 7 using coal and natural gas as fuel. This com bination (fluxed pellets, coal and natural gas) results in a sulfur dioxide scrubbing efficiency th at is greater than 50%. Minntac produces standard (acid) pellets and fluxed pellets on lines 4 and 5 using wood and natural gas as fuel, which results in a sulfur dioxide scrubbing efficie ncy greater than 50%. These scrubber efficiencies will not be improved by adding chemica ls to increase scrubber water alkalinity or by installing a re-circulating lime scrubber similar t o the Keewatin Taconite Scrubber.

Hibbing Taconite Company Hibbing Taconite Company – Hibbing Taconite Company (Hibtac) has three furnace lines with “once through” venturi rod scrubbers. Hibtac does not use coal as a pellet induration fuel. This means that the only sulfur entering the waste gas i s from the taconite greenballs (pre-fired pellets). The calculated concentration of sulfur d ioxide in the waste gas entering the scrubber is about 25 ppm. The concentration of the waste gas l eaving the scrubber is about 6 ppm. The scrubber water leaving the scrubber still has a pH of 7 and contains 200 ppm bicarbonate alkalinity. This means that the sulfur dioxide scr ubbing reaction is gas concentration limited, which also means that sulfur dioxide scrubbing effi ciency will not be improved by adding chemicals to increase scrubber water alkalinity. T he data also show that the installation of a recirculating lime scrubber similar to the Keewatin T aconite Scrubber will not improve scrubbing.

United Taconite United Taconite – Line 2 at United Taconite (Utac) has two re-circulating venturi rod scrubbers.

The scrubber water from each scrubber is sent to tw o small thickeners. The underflow from each thickener is pumped back to the process at a rate o f 435 gpm or a total of 870 gpm for the two scrubber systems. The overflow from the scrubber t hickeners is sent back to the scrubbers with fresh make-up water. The make-up water is equal to the blowdown plus evaporative losses. The bicarbonate alkalinity entering the scrubber water is equal to the amount of bicarbonate alkalinity present in the make-up water. This limits the amou nt of sulfur dioxide that is removed from the waste gas to the amount of sulfur dioxide that will react with the bicarbonate alkalinity in the make-up water plus the amount that will react with the dust plus a small amount of soluble sulfur dioxide. This operating practice limits the amount of sulfate that enters the process water and the tailing water. The concentration of sulfate in the scrubber water currently increases from about 250 mg/liter to 800 mg/liter. This means that 4.0 pounds of sulfate per minute enters the process water (tailing water) with a scrubber blowdown rate of 870 gpm. The acid neutralization for the blowdown water comes from calcium and magnesium car bonates present in the ore, which limits the solubility of fluoride through calcium fluoride precipitation. Increasing the alkalinity of the

water that is re-circulated from the scrubber thick ener to the scrubber would increase sulfur dioxide scrubbing efficiency.

Following are some scenarios that could be consider ed to increase the sulfur dioxide scrubbing efficiency for Line 2 at United Taconite: 1) Increasing the alkalinity of the scrubber water by adding sodium hydroxide or sodium carbonate to the current system would increase sulf ur dioxide removal. Addition of

either of these chemicals would increase sulfur dio xide removal, but it also would result12

in increased sulfate and fluoride concentrations in the tailing water. Doubling sulfur dioxide scrubbing efficiency by using either sodium hydroxide or sodium carbonate will result in an addition of 8.0 pounds of sulfate per minute to the tailing water (double the current rate). The current process water at United Taconite is 225 to 250 mg/liter. Doubling the sulfate input would result in sulfate concentrations of 450 to 500 mg/liter in the process water. Addition of sodium salts would also result in fluoride concentrations in the process water (tailing water) that could rea ch 20 mg/liter. Use of these salts will

result in increased concentrations of both fluoride and sulfate in tailing water, which would create environmental problems with the tailin g water.

2) Lime addition to the scrubber water will increas e sulfur dioxide scrubbing efficiency and

will also limit the amount of sulfate and fluoride in the tailing water by precipitation of calcium sulfate and calcium fluoride. However, lim e addition could not be used with the current scrubber design. Adding lime to the curren t system would result in scaling problems, which would result in decreased sulfur di oxide and particulate scrubbing efficiency. The current scrubbers and water handli ng system would have to be redesigned to allow for lime addition to the scrubb er water.

3) The installation of a new re-circulating lime sc rubber similar to the Keewatin Taconite scrubber system would result in increased sulfur di oxide scrubbing efficiency. This type of scrubber would nearly double the current sulfur dioxide scrubbing efficiency. The result would be the removal of and additional 2.8 p ounds of sulfur dioxide per minute (700 tons per year at 95% operating time). A scrub ber blowdown rate of 350 gallons per minute would be required in order to maintain the c urrent rate at which sulfate enters the tailing water (4.0 pounds per minute). One of the variables that must be considered when designing a re-circulating scrubber is the solid co ncentration in the water that is

circulated back to the scrubber. If the solids con centration is to high, the efficiency of the

scrubber can be affected and the amount of blowdown has to be increased to keep the solids at the proper concentration. A blowdown rat e of 350 gpm should work if a dust collector, such as a multiclone system, is installe d prior to the scrubber; however, if dust collection cannot be installed prior to the scrubbe r, the blowdown rate would probably have to be about 700 gpm. A scrubber blowdown rate of 700 gpm would not result in the precipitation of calcium sulfate and, therefore, th e amount of sulfate entering the tailing water would be about 8 pounds per minute. This wou ld result in doubling the amount of sulfate that is currently entering the water (4 lb/ min to 8 lb/min).

The installation of a re-circulating lime scrubber without dust collection prior to the scrubber would result in a system that would requir e a blowdown of about 700 gallons per minute. This would result in doubling the curr ent concentration of sulfate and calcium in the process water. The calcium concentr ation in the process water will increase because lime will neutralize the acid gase s present in the scrubber water. The

sulfate concentration and the calcium concentration of the process water would increase to about 500 mg/liter and 160 mg/liter respectively . This increase in hardness would result in operating problems that would be difficul t to predict. It will be difficult to determine the costs associated with the resulting i ncrease in hardness and sulfate concentrations. Some of the problems that would oc cur are:13 a) Increased scaling in plant process pipes due to increased concentrations of calcium. The process water will essentially be sat urated in calcium carbonate. Calcium carbonate precipitation will occur in pipes . It will also occur on mineral surfaces during grinding when water temperatures in crease. b) Calcium carbonate precipitation on mineral surfa ces can have a negative impact on magnetite recovery. The resulting increase in t he loss of magnetite ore to the tailings will increase the cost of production. c) Currently United Taconite does not have flotatio n, but calcium carbonate precipitation on mineral surfaces has a negative im pact on the flotation process, which results in additional flotation reagent consu mption. d) Increased calcium concentration in process waterresults in a deterioration of prefired pellet quality that is corrected by increasin g pre-fired pellet moisture and increasing binder use. United Taconite uses organi c binders. Past research with bentonite clay indicates that an increase of 10 mg/ liter in calcium and/or magnesium (hardness) in the process water results i n a 3% increase in bentonite clay consumption in order to maintain pre-fired pel let quality. The current Ca+Mg concentration in United Taconite process wate r is 80 mg/liter. An increase to 160 mg/liter (double the current concen tration) would result in at least a 24% increase in binder use. Organic binders are usually affected more by water chemistry than is bentonite clay, which means the u se of organic binder would increase by at least 24% and/or the current organic binder might no longer work. e) The scrubber thickener underflow solids currentl y are sent back to the process. This results in approximately 80 to 100 pounds of t aconite concentrate per minute, which is equivalent to approximately 25,000 long tons of pellets per year.

The result would be that if multiclone dust collect ors cannot be installed prior to the wet scrubber. A production loss of approximate ly 25,000 long tons of pellets per year would occur.

Dust collectors prior to wet scrubbers would be nec essary to maintain the current process water quality. The following items would have to b e considered when evaluating an installation of a re-circulating lime scrubber syst em with dust collection prior to the wet

scrubbers at United Taconite: a) Installation of multiclones to collect about 20, 000 long tons of concentrate per year will be needed. A system to recycle the multi clone dust back into the pellet process will also need to be evaluated. It does no t appear that there is enough room in the pellet plant to install multiclones pri or to the current wet scrubbers.

An evaluation will have to be made to determine if the installation of multiclones is possible. b) A re-circulating lime scrubber retrofit installa tion would probably require a new building because all of the necessary equipment wou ld not fit into the confines of the current pellet plant. The new building would ha ve to contain a lime mixing facility, a scrubber recirculation tank, a lime add ition system with pH controls, a

scrubber water solids thickener for the blowdown wa ter treatment, and a filtering system to filter the solids. An alternative to a f iltering system would be a lined settling pond.14 c) The filtering system would have to be sized for 20 short tons per day if

multiclones are installed. d) The current venturi rod scrubbing system would h ave to be evaluated to determine if a new rod design is needed to handle the increas ed solid load that would occur with the re-circulating lime scrubber. e) An evaluation of increased operating costs assoc iated with a re-circulating lime

scrubber would also be needed. These should includ e i. Ore losses associated with the dust loss that wi ll occur due to the loss of reclaimed ore dust. This would be about 5,000 long tons per year if prescrubber dust collectors (multiclones) are installe d. ii. Approximately 2500 short tons of lime per year will be needed to neutralize the scrubber water to pH 7. iii. There will be approximately 8500 short tons of solids per year that will

have to be filtered and hauled to a lined landfill or deposited in a lined settling pond. This will include 5600 short tons o f dust, 2500 short tons of gypsum and 400 short tons of calcium fluoride. Thi s would require 2 trucks per day to haul the material to a landfill.

References:
1. Engesser, J .“Effect of water chemistry, water treatment and Blaine on magnetite filtering and magnetite agglomeration with bentonite clay.” Minerals &Metallurgical Proces sing. Vol 20, No. 3, August 2003.

2. Engesser, J. “Reduction of fluoride dissolution duri ng induration off-gas scrubbing.” Minerals &Metallurgical Processing. Vol 22, No. 1, February 2005.

3. Engesser, J. “Effect of Plant Process Water Chemistry on Silica Flotation and Balling”, University of Minnesota Coleraine Minerals Research Laboratory, Iron Ore Cooperative R esearch Program -Minnesota Department of

Natural Resources , 1994.

4. Engesser, J. “The Removal of Sulfate and Fluoride from Process Water – The Effect of Water Chemistry on Filtering and Flotation” University of Minnesota Coleraine Minerals Research Laboratory, Iron Ore Cooperative Research Program -Minnesota Department of Natural Resources , 1998.

5. Engesser, J. “Taconite Process Water and Tailing Water Qu ality – Cationic Exchange Effect on Balling and Flotation”, University of Minnesota Coleraine Minerals Research Laboratory, Iron Ore Cooperative Research Program -Minnesota Department of Natural Resources , 1999.

6. Engesser, J. “Bentonite Quality – The Effect of Water Ch emistry and Antifreeze Agents on Bentonite Binding Characteristics” University of Minnesota Coleraine Minerals Resear ch Laboratory, Minnesota Technologies Inc

, 2000.

7. Engesser, J. “The Reduction of Fluoride Dissolution during Induration Off-gas Scrubbing” University of Minnesota Coleraine Minerals Research Laboratory, Iron Ore Coo perative Research Program -Minnesota Department of Natural Resources , 2001.

8. Goetzman, H., Bleifuss, R., Engesser, J. “An Evaluation o f Organic Binders as Substitutes for Bentonite Pelletizing”, Proceedings of the 61 st Annual Meeting of the Minnesota Section AIME and 49th Ann ual Mining Symposium , January 1988.15

8.. Stone, R.L. “Relationship Between Zeta Potential of Bento nite and the Strength of Unfired Pellets”, America Institute of Mining Metallurgical and Petroleum Engineers So ciety of Mining Engineers of AIME Transactions

Volume 238 , 1967.

9. Engesser, J. Minntac Agglomerator Sulfur Balance, 1994.

10. Engesser, J. Eveleth Taconite Agglomerator Sulfur Balance, 1998.

11. Engesser, J., Niles, H., “Mercury Emissions from T aconite Pellet Production”, University of Minnesota, Duluth Natural Resources Research Institute, 1997.

12. Berndt, M., Engesser, J., “Mercury Transport in Tac onite Processing Facilities: (I) Release and Capture During Induration” Iron Ore Cooperative Research Program – Minnesot a Department of Natural Resources , 2005.

13. Benner, B., “Mercury Removal from Induration Off-Gas Wet Scrubbers”, Environmental Cooperative Research Program – Minnesota Department of Natural Resources, 2001.