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 ofthe 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 + 1502 + 14H20 7 4Fe(0H)3 + 16II' + 8S04- (grinding and tailing disposal) 

4FeS2 + 1102 7 2Fe203 + 8S02t (Induration) 

2S02 + 2H20 + 02 7 4II' + 2S04 - (Scrubber Reaction) 

II' + CaC03 7 Ca +2 + HC03- (Neutralization) II' + MgC03 7 Mg+2 + HC03· (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. 

9 

-':-8 

" 7 "" II) 6 

0 " 5 E 

.§. 4 

-" 3 2 

" en 1 

0 

Sulfate Concentration Corrleated to Calcium and Mangsium Concentrations in 

Taconite Process Water 

R'= 0.9829 1"..00'" 

1 

-

2 

..:l 

.,.. ... .... 

.... ... 

". -

3 4 5 6 7 

Ca + Mg (mmolesfliter) 

• Data Points 

8 9 10 

1 
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. 

CaCh + H20 -7 CaO + 2HClj (Induration) 

HCl + CaC03 -7 HC03- + Ca+2 + cr (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% ofthe 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 saits, 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. 

25 20 

E 

Co 

"- 15 

,f 

c 

0 10 u 

.. 

." 5 

iL "0 

Calcium and Fluoride in Process Water Samples 

from Minnesota Taconite Plants 

..... 

'" • • • ..... 

o 20 

• Data Points 

i .... 

40 60 80 

Calcium Cone., ppm 

-Theoretical CaF2 Solubility 

T 

100 

2 
e

0 

'tl 

5 

:::I 

iL 0 

o 

"'I --.... 

• 

20 40 60 80 100 120 140 

Sodium Cone., ppm 

• Data Points 

160 

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

neutralization after scrubbing. 

NaF +H20 HFj + NaOH (Induration) 

2HF + 2CaC03 CaF2t + Ca+2 + 2HC03- (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. 

Na2C03 + Ca+2 2Na+ + CaC03t NaOH + HC03- + Ca+2 Na+ + H20 + CaC03! 

Water Chemistry Conclusions 

I) 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 

3 
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 

4 
":a 

• 

0.0025 0.0020 0.0015 

C 

o 

= 

0.0010 

!l 

c 

o 

U 

0.0005 0.0000 

H2S03 

'\ / 

, I 

/ \ 

LJ " o 2 

Concentration of Sulfite Species 

4 

1\ / 

V 

/\ V \ 

6 

pH 

8 

S03= 

10 12 14 

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 concentrntion 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 concentrntion is capable of 

scrubbing more sulfur dioxide than water that contains low bicarbonate alkalinity 

concentrntion. 

180 

160 140 120 

E 

f! 100 o 

80 

!l 

c 60 

o 

u 

40 

20 

o 

2 4 

Carbonate Species Concentration at Various pH 

, 

/ 

./ 

/' HC03-" I 

\ I 

J 

6 

\ 

\ 

\ 

"-

8 

pH 

'\ , 

I 

/ 

/ 

10 

/' 

I 

I 

\ 

\ \ 

'-.. 

12 

COl= 

14 

5 
tban the make-up water. 

Calc. Water Calc. Air 

Taconite Scrubber Make-up Make-up HC03- Reacted RC03- Remaining HC03 S02Scrubber S02 Scrubber 

Plant Make-up, gpm Ib/min Ib mole/min Ib mole/min Ib molelmin Efficiency Efficiency 

Minntac L6&7 2800 4.10 0.0672 0.0596 0.0077 60% 

Minntac L4&5 2800 4.10 0.0672 0.0288 0.0384 53% 

VtacL2 870 1.57 0.0257 0.0257 0.0000 26% 

Hibtac 3900 7.67 0.1258 0.0187 0.1070 

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

tbe 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 tbe 

waste gas. The ash content of wood is about 2% by weight. Following is a table tbat lists 

tbe ash analyses for nortbern Minnesota birch and aspen. Using the MgO and CaO 

analyses, the neutralizing capacity of tbe wood ash is about 0.44 moles per short ton of 

wood burned. 

Wood %FIl10 3 %Si02 %Na2O %K2O %CaO %MI(O %S03 %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 

5) Sulfur dioxide scrubbing efficiency is affected by tbe addition of limestone to tbe prefired pellets. Taconite plants add from 1% to 10% limestone to pre-fired pellets 

depending on tbe type of pellet tbat 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 tbe waste gas. The acid neutralizing capacity of this "scrubber dust" during tbe 

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 tbe 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 tbe table. The first scrubber efficiency rate was 

calculated using tbe increase of sulfate in scrubber water. The second scrubber efficiency 

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

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

6 

71% 

27% 

75% 
Taconite 

Plant 

Keetac 

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. 

Calc. Water Calc. Air Scrubber S02 

Waste gas Pellet S02 Nat Gas Coal Wood Pellet IbS02 S02Scrubber S02 Scrubber Water Scrubbing 

acfm, 11 OF Prod ItIhr ppm scth Iblbour Iblhr Tvoe minute Efficiency Efficiency pH by dust 

613789 693 62.9 76,200 20175 0 Acid 5.94 50% 6.4 NA 

Minntac L6&7 407951 415 88.0 83,900 16500 0 Flux 5.53 60% 71% 5.3 20% 

Minntac lA&5 404 883 415 51.6 136950 0 11450 Flux 3.21 53% 6.2 

UtacL2 

Hibtac 

583428 452 139.7 0 0 Acid 12.54 26% 27% 3.6 

468871 384 21.9 96260 0 0 Acid 1.58 75% 7.0 

.. Note: Waste gas flowrates are calculated usmg, the oxygen content of the waste gas, fuel type, fuel compOSItIon, 

fuel rate, pre-frre 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 S02 Remaining HC03 

Plant pH ppm lbmole/min 

Keetac 6.4 31.1 NA 

Minntac L6&7 5.3 30.0 0.0077 

Minntac lA&5 6.2 24.3 0.0384 

UtacL2 3.6 102.0 0.0000 

Hibtac 7.0 5.5 0.1070 

7 

35% 

15% 

39% 
N 80 

0 

III 

" = 60 " .!

= 

 

40 

20 ............... • 

0 

-

3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 

Scrubber Water pH 

• Minntac, Hibtac & UTac -Poly. (Minntac, Hibtac & UTac) 

Lime Scrubber Reactions eRe-circulating Lime Scrubber) 

Scrubber Reactions with hydrated lime (calcium hydroxide) 

Ca(0H)2 + H20 7 Ca+2 +20K +H20 (lime dissolution) 

W + OK 7 H20 (acid neutralization) 

Ca +2 + S0 3 - 7 CaS03! (calcium sulfite precipitation) 

2CaS03 + O2 7 2CaS04 (calcium sulfite oxidation) 

2Ca +2 + 2S03- + 02 +4H20 7 2(CaS04.2H20)! (gypsum precipitation) 

Ca+2 + 20K + 2HF 7 CaF2! + 2H20 (calcium fluoride precipitation) 

Ca +2 + 20K + C02 7 CaC03! + H20 (high pH calcium carbonate precipitation) 

7.5 

The addition oflime 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 COz/liter. Depending on pH the CO2 can be carbonic acid 

(H2C03), bicarbonate (HC03), and/or carbonate (COn. 

8 
g> 

c o 

1l 

b 

c 

g u 

'0 

g> ·5.5 

..J 

" 2 

180 160 140 

S 

120 

E 

c· 100 

o 

80 

g 60 o 

u 

40 

20 

o 

4 

2 4 

6 8 

pH 

10 12 

Carbonate Species Concentration at Various pH 

'\. \ 

/ 

./ 

/' HC03.'\. 

I 

J \ 

\ \ 

6 "' 8 

pH 

, 

) 

/ 

./ 

10 

/' 

I 

I 

I 

\ 

\ \ 

"' 12 

14 

C03-

14 

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 4.9xlO·9. A recirculating scrubber that operates at 100F and maintains the sulfate concentration in the water at 

1400 mg SOJliter will have water that has a calcium concentration of 650mg Calliter. 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. 

9 
0.0015 

C o 

:; 

0.0010 

o 

o 

o 

0.0005 

0.0000 

\ I 

\ I( 

I 

/ , 

o 2 

r\ / V 

/\ V \ '-

4 6 8 10 12 14 

pH 

The solubility of calcium sulfite is less than the solubility of calcium sulfate. The above graph 

indicates that the concentration of bisulfite (HS03") 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 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. Ifit 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-circulating-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. 

10 
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 (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) 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 of870 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 

11 
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 of350 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 blow down has to be increased to keep the 

solids at the proper concentration. A blowdown rate of350 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 of700 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/miu). 

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: 

12 
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 \0 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 muiticlone 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 multi clones 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. 

13 
1. 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. 

11. Approximately 2500 short tons oflime 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: 

I. 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, Augnst 2003. 

2. Engesser, J. "Reduction of fluoride dissolution during induration off-gas scrubbing." Minerals &Metallurgical 

Processing. Vol 22, No. I, February 2005. 

3. Engesser, J. "Effect of Plant Process Water Chemistry on Silica Flotation and Balling", University of Minnesota 

Coleraine Minerals Research Laboratory, Iron are Cooperative Research Program -Minnesota Department of 

Natural Resources. 1994. 

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