1

Evaluation of Minnesota Taconite Wet Scrubbers at Evaluation of Minnesota Taconite Wet Scrubbers at 

Minntac, Keewatin Taconite, Hibbing Taconite, and United Taconite 

by John Engesser, P.E. 

Minnesota Department of Natural Resources, Division of Lands & Minerals 

This paper is a review of water treatment methods that can be used to increase sulfur dioxide 

scrubbing at Minntac, Keewatin Taconite, Hibbing Taconite, 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 for 40 to 50 years. During this time the 

concentration of dissolved solids in recycled process 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 oxidation 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) neutralization 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. Sulfuric acid is neutralized by calcium 

carbonate and magnesium carbonate, which are both present 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 represent what occurs during grinding, 

induration and tailing disposal. 

4FeS2 + 15O2 + 14H2O  4Fe(OH)3 + 16H+

 + 8SO4

=

 (grinding and tailing disposal) 

4FeS2 + 11O2  2Fe2O3 + 8SO2↑ (Induration) 

2SO2 + 2H2O + O2  4H+ + 2SO4

=

 (Scrubber Reaction)

H

+

 + CaCO3  Ca+2 + HCO3

-

 (Neutralization) 

H

+

 + MgCO3  Mg+2 + HCO3

-

 (Neutralization) 

As the concentration of sulfate increases, the concentration of both magnesium and calcium 

increase. The following graph illustrates the correlation between hardness (calcium and 

magnesium) and sulfate in taconite process water. 

Sulfate Concentration Corrleated to Calcium and Mangsium 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 Points
2

Increased sulfate concentration results in increased 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 pipes. Calcium sulfate precipitation can also 

occur in certain areas of the plants when the calcium 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 taconite. However, with the advent of fluxed pellets, 

soluble chlorides (calcium, magnesium, sodium and potassium) 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 scrubbers as hydrochloric acid. The 

following reactions represent the formation of hydrochloric acid during induration and the 

neutralization reactions that occur during scrubbing and after scrubbing. 

CaCl2 + H2O  CaO + 2HCl↑ (Induration) 

HCl + CaCO3  HCO3

-

 + Ca+2 + Cl- (Neutralization) 

Small amounts of naturally occurring insoluble fluoride minerals (mostly fluorapitite) are present 

in Minnesota taconite. Approximately 90 to 95 percent of the fluoride is rejected to the tailings 

during magnetite concentration. The remaining fluoride (0.001% to 0.002% of the concentrate) 

enters the induration process. Fluoride minerals are 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 induration process is volatilized during induration. The 

quantity of hydrogen fluoride that forms during induration increases as the concentration of 

sodium in the process water increases and the concentration of calcium in the process water 

decreases. Wet scrubbers remove the hydrogen fluoride that forms during pellet induration. 

Approximately 0.01 pounds of hydrogen fluoride dissolves in the process water for each ton of 

taconite processed. Most of the fluoride that dissolves in the process water precipitates as 

calcium fluoride. Fluoride concentration in process water is limited by calcium concentration 

because calcium fluoride is fairly insoluble in water 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 concentration of fluoride in taconite process 

water varies from 2 mg/liter to 25 mg/liter depending on the concentration of calcium and 

sodium. The following two graphs demonstrate how the 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., ppm

Fluoride Conc., ppm

Data Points Theoretical CaF2 Solubility
3

The following reactions represent the fluoride reactions that occur during induration and 

neutralization after scrubbing. 

NaF +H2O  HF↑ + NaOH (Induration) 

2HF + 2CaCO3  CaF2↓ + Ca+2 + 2HCO3

-

 (Neutralization) 

The addition of sodium carbonate and/or sodium hydroxide to taconite process water results in 

the precipitation of calcium carbonate, which decreases the concentration of calcium in the 

water. The following reactions represent what occurs when sodium carbonate and sodium 

hydroxide are added to taconite process water. 

Na2CO3 + Ca+2  2Na+ + CaCO3↓

NaOH + HCO3

-

 + Ca+2  Na+ + H2O + CaCO3↓

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 solution once the solubility limit of calcium 

sulfate is reached. This will occur when the products of the molar concentration of 

calcium and sulfate reach the solubility limit. Sodium sulfate is extremely soluble. 

3) Increased scrubber efficiency can result in increased 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. Increased 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. Increased concentrations of calcium results 

in decreased formation of hydrogen fluoride during pellet induration. 

6) The concentration of calcium in scrubber water affects 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., ppm

Fluoride Conc., ppm

Data Points
4

calcium results in increased precipitation of calcium fluoride and decreased concentration 

of fluoride in the taconite process water. 

7) Precipitation of calcium fluoride and calcium sulfate can limit the amount of fluoride and 

sulfate that remain in solution in taconite process water. 

8) The addition of sodium carbonate and sodium hydroxide 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 looking at past practices at Minnesota taconite 

facilities. 

1) Two taconite facilities, Northshore Mining and LTV 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 facilities. Precipitation of calcium carbonate 

decreased the calcium concentration of the water, which decreased calcium fluoride 

precipitation; thus resulting in high fluoride concentration in the tailing water. 

2) The tailing facility at Minntac contains the highest 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 concentration 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 concentration in tailing water than do most 

taconite facilities because of high calcium concentration, 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 water. This is accomplished by continually 

adjusting the scrubber water to the proper pH and allowing it to circulate until gypsum 

(calcium sulfate) and calcium fluoride precipitation occurs. 

4) There is no precipitation reaction that limits the concentration of chloride in taconite 

process water. 

 

Sulfur Dioxide Scrubbing at Taconite Plants

The efficiency of sulfur dioxide scrubbing in taconite induration scrubbers depends on the 

concentration of sulfur in the fuel and pre-fired pellet, 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 induration, the sulfur that is present in the fuel and 

pre-fired pellet oxidizes to sulfur dioxide. An increase in the amount of sulfur entering 

the furnace results in increased concentration of sulfur 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 scrubber water decreases, the rate of sulfur 
5

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

reaction of sulfur dioxide with bicarbonate is essentially complete. Sulfite (SO3

=

) and 

bisulfite (HSO3

-

) are much more stable in water than is sulfurous acid (H2SO3). 

Sulfurous acid begins to form at about pH 4. Sulfur dioxide scrubbing that occurs below 

pH 4.5 is due to the solubility of sulfur dioxide in water, which is controlled by the partial 

pressure (concentration) of sulfur dioxide in the waste 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 alkalinity occurs. As bicarbonate 

reacts with sulfur dioxide, the scrubber water pH decreases and the bicarbonate alkalinity 

decreases. Water that contains high bicarbonate alkalinity concentration is capable of 

scrubbing more sulfur dioxide than water that contains 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

pH

Concentration, 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

pH

Concentration, 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 dioxide removed from the waste gas. Higher 

scrubber make-up water flowrates result in more bicarbonate alkalinity available to react 

with sulfur dioxide in the waste gas. The table below indicates that sulfur dioxide 

scrubber efficiency is dependent on the amount of bicarbonate alkalinity (HCO3

-

) that 

enters the scrubber. Keewatin Taconite data is not included in the table below because 

most of the alkalinity in the scrubber water at Keetac 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 neutralizing capacity; however, wood ash 

contains large amounts of CaO and MgO, which will neutralize 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 aspen. 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 produced. 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 capacity 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 of “scrubber dust” during the production 

of 10% limestone pellets is about 2 to 8 millimoles for each ton of pellets produced. 

 The following table indicates some calculated sulfur 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 scrubber water. The second scrubber efficiency 

rate was calculated using the concentration of sulfur dioxide in the waste gas entering the 

scrubber (calculated from mass balance) and the concentration of sulfur dioxide exiting 

Wood %Fe2O3 %SiO2 %Na2O %K2O %CaO %MgO %SO3 %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- Remaining HCO3- SO2Scrubber SO2 Scrubber

Plant Make-up, gpm HCO3-, lb/min lb mole/min lb mole/min lb mole/min Efficiency Efficiency

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 scrubber water that occurred from pH 8 (pH 

of taconite process water) to the measured pH of the scrubber exit water (listed in the 

table). The difference between the total sulfur dioxide removal (efficiency) and the sulfur 

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

dioxide that reacted with the dust. Notice that for Minntac Lines 4 & 5 the amount of 

sulfur dioxide removed by the “scrubber dust” was more than any other scrubber listed in 

the table. This is because wood was used as a fuel and wood ash has a very fine grain 

size and is therefore very reactive. This data demonstrates that sulfur dioxide scrubber 

efficiency is dependent on neutralization of scrubber water by the dust and fuel ash. 

Note: Waste gas flowrates are calculated using, the oxygen content of the waste gas, fuel type, fuel composition, 

fuel rate, pre-fire pellet moisture, and pre-fired pellet combined 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 concentration that is in contact with the 

liquid. As the gas concentration decreases, the liquid 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 sulfur 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 concentration 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 minute 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% 71% 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 hydroxide) 

Ca(OH)2 + H2O  Ca+2 +2OH- +H2O (lime dissolution) 

H

+

 + OH-  H2O (acid neutralization) 

Ca+2 + SO3

=  CaSO3↓ (calcium sulfite precipitation) 

2CaSO3 + O2  2CaSO4 (calcium sulfite oxidation) 

2Ca+2 + 2SO3

=

 + O2 +4H2O  2(CaSO4.2H2O)↓ (gypsum precipitation) 

Ca+2 + 2OH- + 2HF  CaF2↓ + 2H2O (calcium fluoride precipitation) 

Ca+2 + 2OH-

+ CO2  CaCO3↓ + H2O (high pH calcium carbonate precipitation) 

The addition of lime to scrubbers increases acid gas scrubbing efficiency. As the pH of the 

scrubber water increases, the amount of acid gas removed from the waste gas increases. 

However, the pH of the scrubber water cannot be increased to the point where bicarbonate and/or 

carbon dioxide are converted to carbonate because calcium 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 removes carbon dioxide from the waste gas and 

converts it to carbonate, which then precipitates as calcium carbonate. Calcium carbonate 

precipitation will plug scrubbers, which in turn decreases 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 CO2/liter. Depending on pH the CO2 can be carbonic acid 

(H2CO3), bicarbonate (HCO3

-

), and/or carbonate (CO3

=

). 

Sulfur Dioxide in Scrubber Exit Waste Gas vs Scrubber 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 pH

Exit 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 increases. 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 for calcium carbonate is 4.9x10-9. A recirculating scrubber that operates at 100F and maintains the sulfate concentration in the water at 

1400 mg SO4/liter will have water that has a calcium concentration 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

pH

Log of Concentration, log(moles/liter)

H2CO3 HCO3- CO3=

Carbonate Species Concentration at Various pH

0

20

40

60

80

100

120

140

160

180

2 4 6 8 10 12 14

pH

Concentration, 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 (HSO3

-

) 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 scrubber water that precipitation of calcium sulfite 

can even occur as low as pH 6. Calcium sulfite precipitation in the scrubber can cause scaling 

and plugging problems. For this reason it is difficult to operate re-circulating-lime scrubbers at a 

pH that is above 7 without decreasing scrubber efficiency by forming precipitates. 

Application to Minnesota Taconite Induration Scrubbers 

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

In order to maintain maximum sulfur dioxide scrubber efficiency, the scrubber should be 

operated between pH 7 and pH 7.5. If it is operated at a pH greater than 7.5, calcium carbonate 

and calcium sulfite scaling will occur in the scrubber, which will result in decreased scrubber 

efficiency and could even result in shutting down the process to clean the scale formation from 

the scrubber. It would appear that Keewatin Taconite is currently operating at near optimum 

sulfur dioxide scrubbing conditions. 

Minntac Line 3 Scrubber – Minntac installed a re-ci Minntac Line 3 Scrubber 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 have sulfur dioxide scrubber efficiency similar 

to the Keewatin Taconite re-circulating lime scrubber. 

Concentration of Sulfite Species

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0 2 4 6 8 10 12 14

pH

Concentration, moles/liter

H2SO3 HSO3- SO3=
11

Minntac Lines 4, 5, 6 and 7 Scrubbers – Minntac has Minntac Lines 4, 5, 6 and 7 Scrubbers “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-gas sulfur dioxide when Minntac is 

producing fluxed pellets and/or using wood as a fuel. Minntac produces fluxed pellets on lines 6 

and 7 using coal and natural gas as fuel. This combination (fluxed pellets, coal and natural gas) 

results in a sulfur dioxide scrubbing efficiency that 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 efficiency greater than 50%. These scrubber 

efficiencies will not be improved by adding chemicals to increase scrubber water alkalinity or by 

installing a re-circulating lime scrubber similar to 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 is from the taconite greenballs (pre-fired 

pellets). The calculated concentration of sulfur dioxide in the waste gas entering the scrubber is 

about 25 ppm. The concentration of the waste gas leaving 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 scrubbing reaction is gas concentration limited, 

which also means that sulfur dioxide scrubbing efficiency will not be improved by adding 

chemicals to increase scrubber water alkalinity. The data also show that the installation of a recirculating lime scrubber similar to the Keewatin Taconite Scrubber will not improve scrubbing. 

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

The scrubber water from each scrubber is sent to two small thickeners. The underflow from each 

thickener is pumped back to the process at a rate of 435 gpm or a total of 870 gpm for the two 

scrubber systems. The overflow from the scrubber thickeners 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 amount 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 carbonates 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 thickener to the scrubber would increase sulfur 

dioxide scrubbing efficiency. 

Following are some scenarios that could be considered 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 sulfur dioxide removal. Addition of 

either of these chemicals would increase sulfur dioxide removal, but it also would result 
12

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 reach 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 tailing water. 

2) Lime addition to the scrubber water will increase 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, lime addition could not be used with the 

current scrubber design. Adding lime to the current system would result in scaling 

problems, which would result in decreased sulfur dioxide and particulate scrubbing 

efficiency. The current scrubbers and water handling system would have to be 

redesigned to allow for lime addition to the scrubber water. 

3) The installation of a new re-circulating lime scrubber similar to the Keewatin Taconite 

scrubber system would result in increased sulfur dioxide 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 pounds of sulfur dioxide per minute 

(700 tons per year at 95% operating time). A scrubber blowdown rate of 350 gallons per 

minute would be required in order to maintain the current 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 concentration in the water that is 

circulated back to the scrubber. If the solids concentration 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 rate of 350 gpm should work if a dust 

collector, such as a multiclone system, is installed prior to the scrubber; however, if dust 

collection cannot be installed prior to the scrubber, 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, the amount of sulfate entering the tailing 

water would be about 8 pounds per minute. This would 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 require a blowdown of about 700 gallons 

per minute. This would result in doubling the current concentration of sulfate and 

calcium in the process water. The calcium concentration in the process water will 

increase because lime will neutralize the acid gases 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 difficult to predict. It will be difficult to 

determine the costs associated with the resulting increase in hardness and sulfate 

concentrations. Some of the problems that would occur are: 
13

a) Increased scaling in plant process pipes due to increased concentrations of 

calcium. The process water will essentially be saturated in calcium carbonate. 

Calcium carbonate precipitation will occur in pipes. It will also occur on mineral 

surfaces during grinding when water temperatures increase. 

b) Calcium carbonate precipitation on mineral surfaces can have a negative impact 

on magnetite recovery. The resulting increase in the loss of magnetite ore to the 

tailings will increase the cost of production. 

c) Currently United Taconite does not have flotation, but calcium carbonate 

precipitation on mineral surfaces has a negative impact on the flotation process, 

which results in additional flotation reagent consumption. 

d) Increased calcium concentration in process water results in a deterioration of prefired pellet quality that is corrected by increasing pre-fired pellet moisture and 

increasing binder use. United Taconite uses organic 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 in a 3% increase in bentonite 

clay consumption in order to maintain pre-fired pellet quality. The current 

Ca+Mg concentration in United Taconite process water is 80 mg/liter. An 

increase to 160 mg/liter (double the current concentration) 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 use 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 currently are sent back to the process. 

This results in approximately 80 to 100 pounds of taconite concentrate per 

minute, which is equivalent to approximately 25,000 long tons of pellets per year. 

The result would be that if multiclone dust collectors cannot be installed prior to 

the wet scrubber. A production loss of approximately 25,000 long tons of pellets 

per year would occur. 

 

Dust collectors prior to wet scrubbers would be necessary to maintain the current process 

water quality. The following items would have to be considered when evaluating an 

installation of a re-circulating lime scrubber system 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 multiclone dust back into the pellet 

process will also need to be evaluated. It does not appear that there is enough 

room in the pellet plant to install multiclones prior 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 installation would probably require a new 

building because all of the necessary equipment would not fit into the confines of 

the current pellet plant. The new building would have to contain a lime mixing 

facility, a scrubber recirculation tank, a lime addition system with pH controls, a 

scrubber water solids thickener for the blowdown water treatment, and a filtering 

system to filter the solids. An alternative to a filtering 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 have to be evaluated to determine 

if a new rod design is needed to handle the increased solid load that would occur 

with the re-circulating lime scrubber. 

e) An evaluation of increased operating costs associated with a re-circulating lime 

scrubber would also be needed. These should include 

i. Ore losses associated with the dust loss that will 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 installed. 

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 of dust, 2500 short tons of 

gypsum and 400 short tons of calcium fluoride. This 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 Processing. Vol 20, No. 3, August 2003. 

2. Engesser, J. “Reduction of fluoride dissolution during 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 Research 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 Quality – 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 Chemistry and Antifreeze Agents on Bentonite Binding 

Characteristics” University of Minnesota Coleraine Minerals Research 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 Cooperative Research Program -Minnesota 

Department of Natural Resources, 2001. 

8. Goetzman, H., Bleifuss, R., Engesser, J. “An Evaluation of Organic Binders as Substitutes for Bentonite 

Pelletizing”, Proceedings of the 61st Annual Meeting of the Minnesota Section AIME and 49th Annual Mining 

Symposium , January 1988. 
15

8.. Stone, R.L. “Relationship Between Zeta Potential of Bentonite and the Strength of Unfired Pellets”, America 

Institute of Mining Metallurgical and Petroleum Engineers Society 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 Taconite Pellet Production”, University of Minnesota, Duluth 

Natural Resources Research Institute, 1997. 

12. Berndt, M., Engesser, J., “Mercury Transport in Taconite Processing Facilities: (I) Release and Capture During 

Induration” Iron Ore Cooperative Research Program – Minnesota 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.