Ore Preheating in Rotary Kiln

P.C.Sudhakar

Why to Preheat Manganese Ore ? To drive away moisture To dissociate MnO2 to Mn2O3

To add useful heat to furnace To use waste heat from low temperature gases To avoid eruptions To explore the possibility of construction of

closed furnace To continue utilization of 100% coal as

reductant To get energy savings To be environment friendly

Preamble:

Around 0.5 MW to 1 MW of low temperature waste heat is available for extraction from each of 16.5 MVA furnaces when operated with Coal as reductant. There are several ways to extract this heat and the critical issues related to this heat recovery are:

Hot air and Hot Water making may be suitable for make up to boiler and air preheater for boiler.

Non coking Coal used as reductant in furnace but as a combustion agent in boiler. The regeneration of power from furnace by closing like boiler is another point of view.

Continued: The huge hot air quantity can be used for

air conditioning /ore preheating. Low pressure steam can be generated for

dust suppression in open low hood furnace. When uncontrolled firing of non-coking coal

happens in furnace the heat is wasted, instead make coal char and ore preheating in kiln retrofitting and use closed furnace to control smelt reduction.

The temperature is to be controlled and dust free environment make the fans consume less power.

Advantages of using low temperature heat for Ore preheating:

Hot water is in too large a quantity for Boilers make up quantity, hence not suitable.

Closing the furnace is further step of Ore pre-heating, hence can be taken up later.

Low pressure steam generation is not possible as per experts in the area.

Hence Ore pre-heating is viable option especially in the case of Manganese Ores.

Using coal as reductant in Submerged Arc furnace mean the following : 1) The flue gas temperatures are around 300 degree

centigrade and our Gas Cleaning Plant bag house can with stand only 200 degree centigrade

2) Hence these are low hood open type furnaces drawing 300000 cubic meter/hour dilution air hence lot of sensible heat is lost as waste.

3) Other plant operators are using Coke in addition coal as reductant and hence the problem is not rise.

4) The huge quantity of gases needs to be cooled before entering bag house to save bag life.

5) Therefore we would like to extract heat with ore pre-heating option.

6) Our auxiliary consumption is high compared to other plants.

7) They are using Forced Draft Coolers before the fan to reduce fan power consumption.

Moisture contents in Ores:

Origin of Ore H2O % ( Approximate)

Australia Ore 3.8%

South Africa Ore 1.2%

MOIL MSL 498 2.0%

MOIL BGF 452 8.4%

Sandur 26-28 0.4%

Radhika 4.2%

Calculation of Moisture Losses Average % of Moisture in Manganese Ore 5% Quantity of Moisture in 2 tons of Ore = 100 kg / ton

of alloy Daily production in 16.5 MVA furnace = 70 tpd Daily moisture removal = 7 tons per day Required heat = 540 kCal/kg *7000 kg ~ 4,000,000

kCal Available heat = 0.2 kCal/kg*300000 cum/hr*24 hrs*0.8 kg/cum ~ 11,000,000 kCal Efficiency required of rotary kiln = (4/11)*100 = 36% Efficiency = {(473-373)/473} *100 = 21%

Dryness of ore need not be 100% Theoretical Dryness = (21/36) *100 = 58% Time of residence of ore = 46 tons /shift ~ 6

tons/hour Most valuable heat is consumed in drying ore

rest of the heat is sufficient for bag house. Dryness of around 50% is sufficient for

avoiding most of the troubles. Rotary drier kiln capacity ~140 ton/day. The kiln size is 46 tons * 0.5 cum/ton / ( 21%)

= 110cum The diameter of kiln = 3.1meter The length of kiln = 14 meter.

Why the kiln problem needs to be solved?

The problem has three dimensions:

Calculus of rotation of charge for exposure to flue heat

Thermal heat transfer for removal of moisture

CFD to solve flow of flue gases 

Why should we keep Pre-collector before kiln? To settle the coarse dust. The diameter of kiln 3.1 meter or 10 feet for

easy implementation of sonic soot blowers. Bucket elevator is easy to put the ore into kiln. Transfer chute for hot discharged ore can be

used Second feeding trolley for hot ores can be

used. The blow/eruption gas adjustment takes in

pre-collector before opening of chimney. The 14 meter length can be adjusted outside

the shed and normal diameter of 2 meter transfer kiln can be made inside the shed.

Kiln literature 1) Moisture in Manganese Ores paper 2) Kiln Calculus thesis 3) CFD of drying thesis 4) Kiln thermal heat exchanger paper 5) Pre-reduction iron ore simulation paper 6) Coal reactivity standard 7) Production of Manganese alloys why

Carbon consumption is more when using highly reactive coal

8) Kiln for Ferro Manganese production in Japan paper

Why close furnace ?Advantages of closing furnace: To avoid fugitive emissions. To use furnace gas for pre-heating ore etc., To avoid unwanted air pollution.Disadvantages of closing furnace: The blows/eruptions may cause dangers. The dried ore and dry coal need to be used. The stainless steel of 316 L shall be used for

protection of furnace inner parts. The hydro cyclone and Venturi scrubber and gas

collection tank shall be used instead of present arrangement.

Why understanding kiln a precursor to closing furnace? The coal will be understood in all aspects for

preheating ore. The huge volume of dilution air shall be

reduced in phase wise increasing the temperature for preheating.

The temperature may be gradually increased for testing as far as possible.

Some aspects of partially closing the furnace with coal as reductant shall be clearly understood.

Once all is clear we can replace pre-collector with Venturi Scrubber and kiln can be heated with reaction gas of furnace.

How to design a rotary kiln ? Assumed Values -- 1 Angle of repose of Manganese ore plays important

role in design. Theoretically measured value of angle of repose is ~ (10 rpm *360 degree)/(60 seconds) = 60 degree /second Bulk density ~ 2 tons/cubic meter The Ore piece average particle size ~ 50 mm ~

0.050 m Volume of 6 tons ~ 6/2 = 3 cubic meter Volume of particle ~ (4/3)*pi()*(5/2)^3 = 6.94 cubic

cm Void fraction =0.5

How to design a rotary kiln ? Assumed Values -- 2 Number of particles = (3*0.5)/(6.94*(10^-6)) ~220000

nos. The necessary revolutions per minute that is required to

flatten the Ore dump cone ~ 10 rpm The residence time in kiln required ~ 8 hours = 480

minutes The production time ~ 220000 nos. /360 seconds = 612

nos. / second Due to irregular shape of Ore Pieces it is better give a

booster angle to Kiln inclination. ~ ( 600 nos. / second) / (60 degree /second ) ~ 10 nos./second

Based on the last booster angle can be determined for irregular shape. For 5 nos./second the angle of repose may be 30 degree/second for production time 150 nos./ second for particles of irregular shape which are highly flat.

Kiln Design Angle of inclination of kiln = (60 degree/second ) /(10rpm/60 seconds) = 360 degree per 8 hours travel time = (360/(8 hours*3600 seconds/hour)) = (0.1/8) = 0.0125 degrees /second per ten

rpm = 0.0125* 10*60 = 7.5 degrees. Booster angle For 30 degree/second = another 3.25 degrees. Total inclination of kiln = 10.75 degrees.

Kiln design references Modelling and optimization of a rotary kiln

direct reduction process H.P. Kritzinger and T.C. Kingsley, Hatch Goba

Numerical Modelling of Granular Beds in Rotary Kilns

M. A. Romero Valle

If Kiln RPM is 1 !!! The angle of inclination is 0.75 degrees The booster angle of inclination is 0.325 degrees The total angle of inclination is 1.075 degrees !!!! The kiln Froude number = ((angular velocity)2 * R) / g Where angular velocity = (2 * π / 60 ) = 0.105 s-1

R =1.55 m g =9.81 m / s2

Froude number = (((0.105)^2)*(1.55))/(9.81) Froude number = 0.0171/9.81 = 0.00174 Bed cross sectional area = (110 cubic meter/14 meter) = 7.86 square meter h = 0.333 m

h bed thickness calculation

D 3.1mR 1.55mh 0.333m

A total 7.9m2

A solids 1.6m2

A gases 6.272131m2

solid angle 76.5288degrees

A solids 1.61607m2

sector A 3.20897m2

Triangle A 1.592897m2

Modes of Operation

Reference for Modes of Operation:

How to arrive at mechanism of Ore movement ? For 21% fill , 1 rpm and 0.333m bed depth

the Froude number is 0.00174 and the mechanism may be slumping.

The other mechanisms for characteristic curve Cascading, Rolling , Slipping, Cataracting and

Centrifuging. For Froude number > 1 obvious is

centrifuging. The exactly opposite for Froude number <

0.001 , Bed depth < 0.1m and rpm < 50 the mechanism is Slipping.

The middle zone is rolling and most suitable for heat transfer and hence rpm should be increased as far as possible.

How to Build Characteristic curves ?

Table: VARIATIONS OF SPECIFIC POWER IN PRODUCITON OF Manganese FERRO ALLOYS.

1) Temperature smelting zone 0.82) Size smelting zone 0.853) Slag Volume ore nature carbonate/oxide/silicate 0.94) Mn Input low grade ores 1.15) Si% lower limit customer requirement 0.956) MnO% in slag TFC/ton of alloy 0.97) Slag Basicity Unwanted gangue content in ores 1.18) Arc voltage Capacity of Manganese furnace and PCD 1.59) Fume quantity Temperature 1.05

10) Coal/coke quality Rank, type, grade etc.

Singareni, South Africa and Australia 1.2

11) Total fixed Carbon/ton Coke /coal bed thickness and conductivity 0.912) Conductivity of coals and

oresLoad pick up and power consumption 0.9

13) Use of carbonate fraction blows, surface raking required 1.214) Ferro Manganese slag

fractionLow melting point slag 1.1

15) Sinter fraction use of fines reduces surface temperatures 1.116) Coal Size coal bed formation 0.917) Coal/Ore Reactivity alloy yield 0.918) Thermal and electrical

lossesdesign of furnace for manganese alloys 1.05

19) Mn/Fe ratio for maintaining Mn% on lower limit border 0.920) Mn/P ratio For maintaining  P% on upper limit 0.921) PYROMET AUTOMATION to

controlbased on resistance and interdependent on

other0.8

22) Electrode Lengths based on slipping index for silico manganese

0.9

23) Calorific values coals use of less electrical energy  for smelt reduction 0.924) Moisture primary specific power increase up to 200

kWh/T1.2

25) Ore consumption per ton High Specific power high consumption of ores 1.1

Gas CalculationTotal number of Moles N 1+z+x+3.0p+y 1294261.648

Mn 54.94 kilo moles ZONE1 kWh kilo moles nco x 544.9732446 15.2647 KgFe 55.85 Mn2O3 6.6203652 ΔH298 -199.8 kJ/mole -367.43 0.5449732 CO nco2 1-x 21571.70128 949.3706 Kg

Si 28.09 H2O 5% 50 ΔH298 44 kJ/mole 611.1111 21.571701 CO2 nn2 z 900000 25200 Kg

C 12.01 H2O 0% 7.2 ΔH298 -41.1 kJ/mole -82.2 50 H2O nh2 p 7200 14.4 Kg

Ca 40.08 7.2 H2 nh2o p 57200 1029.6 kgMg 24.31 Temp ~ no2 x+p 7744.973245 247.8391 kg

Al 26.98 4700C nexcess o2 y 300000 9600 kg

O 16 kilo moles ZONE2 kWh kilo moles mole fractions partial pressures Total 37056.47 kg 46321 Nm3/ton

H 1 CaO 16% 2.853067 ΔH298 178.3 kJ/mole 141.3061 14.365338 CO nco/N pco x/(1+z+x+3.0p+y) 0.000421069 202653 Nm3/hr

MgO 7% 1.7365418 ΔH298 101.1 kJ/mole 48.76788 7.7513361 CO2 nco2/N pco2 (1-x)/(1+z+x+3.0p+y) 0.016667187

MnO 70.94 Mn3O4 4.4135768 ΔH298 -187.8 kJ/mole -115.121 nn2/N pn2 z/(1+z+x+3.0p+y) 0.695377169

Mn3O4 228.82 Fe3O4 0.9549388 ΔH298 -12.9 kJ/mole -3.42186 nh2/N ph2 p/(1+z+x+3.0p+y) 0.005563017

Mn2O3 157.88 nh2o/N ph2o p/(1+z+x+3.0p+y) 0.044195082

MnO2 86.94 no2/N po2 (x+p+y)/(1+z+x+3.0p+y) 0.237776476

SiO2 60.09 kilo moles ZONE3 kWh kilo moles N P 1

CaO 56.08 MnO 10% 1.409642 ΔH298 -16.9 kJ/mole -6.61749 17.527066 CO temp degree K (kelvin) delta z

MgO 40.31 Fe 16% 2.8648165 ΔH298 -12.9 kJ/mole -10.2656 Temp ~ 2300 2co+o2 = 2co2 -162499 J

Al2O3 101.96 C 2.1372292 ΔH298 172.4 kJ/mole 102.3495 20000C 750 h2o+co=h2+co2 -11485 J

H2O 18 k1= (pco2)2/((pco)2 *(po2)) 6589.45577 -168145.832342.055 kWh kilo moles ZONE 4 kWh kilo moles k2= ((ph2)*(pco2))/((ph20)*(pco)) 4.982481377 -10013.76426416.6667 kWh Mn 65% 11.831088 ΔH298 252.3 kJ/mole 829.1621 17.527066 CO Gas Constant 8.314 J/mol.degK

555.5556 kWh Si 16% 5.6959772 ΔH298 754.9 kJ/mole 1194.4153314.278 kWh/ton Direct efficiency 80.8 %

Slag melting

Alloy meltingReaction energy

Total Specific power

MgCO3 = MgO + CO2

3Mn2O3 + CO = 2Mn3O4 + CO2Manganese Monoxide

Carbon

Gas Reduction

Magnesium

Aluminium

Oxygen

molecular weight

Calcium

MnO(l) + C = Mn (l) +CO

SiO2(l)+C = Si (l) +CO

Smelt Reduction

Manganese Dioxide

Silicon Dioxide

Calcium Oxide

Magnesium Oxide

Indirect Reduction

1/3Mn3O4+1/3 CO = MnO + 1/3CO2

1/3Fe3O4 + 4/3CO = Fe + 4/3CO2

C+ CO2 = 2CO

Fe2O3 + CO = Fe3O4 + CO2

Hydrogen Dioxide

Aluminium Oxide

CaCO3 = CaO + CO2

Drying and Calcinationatomic weight

ManganeseIron

Silicon

2MnO2+ CO = Mn2O3+ CO2

Hydrogen

H2O (l) = H2O (g)

H2O (g)+CO = H2 (g) + CO2 (g)

Moisture effect on Gas Calculation

In 27.6 MVA large furnace, 5% of moisture , Actual Specific Power 4100

kWh/ton 10% of moisture , Actual Specific Power 4300

kWh/ton This is happening due large latent heat of

evaporation and high moisture content in flue gas.

In 16.5 MVA small furnace 5% of moisture , Actual Specific Power 3900

kWh/ton 10% of Moisture , Actual Specific Power 4000

kWh/ton

Heat Balance for Ore Preheating

Heat for raising temperature of Solids(140tpd) = +18% ????

Heat for evaporation of Moisture(2.5%) from solids = - 18%

Heat for dropping the temperature of flue gas = +21%

Heat gain required for kiln = -15% !!!!(additional burner may be required for temperature Increase)Derived not from pure heat balance!

Conclusions-The metallurgist point of view is expressed, not heat transfer problem in general.-The problem is maze for hard heat transfer specialist.-Possible heat transfer is considered, but efficiencies aren’t matching -For which moisture % like around <5 % (i.e, 2.5%) the Optimum matching may occur.-The fill in the kiln is low (1/3) causing good heat transfer.

Final

just THANK YOU.