Project Report

Micropelletization of Deep Beneficiated Low Grade Iron Ore for Its Rational Utilization

Project team

S. P. Mehrotra, Principal Investigator

Anubhav Pandey, Senior Project Associate

Sudhir Kumar Singh, M.Tech. Student

Department of Materials Science and Engineering

Indian Institute of Technology, Kanpur

Kanpur, 208016, U.P. (India)

January, 2012

Summary

The Iron and Steel industry in our country so far has been essentially using good grade

iron ores (iron content more than 60%) - iron ore with less than 60% Fe is hardly used.

It is estimated that with the proposed expansion of iron and steel industry in India in

next 10 years or so, the good grade iron ore would be exhausted within the next 25-40

years. For the survival of the industry it is therefore necessary that newer technologies

are developed which allow utilization of lower grade iron ores (between 50-55% Fe

content) in Indian blast furnaces in a competitive manner. One possible option includes

deep beneficiation of low grade iron ore to raise the iron content level, say from 50-

55% to about 65%. This will require fine grinding of the ore to give good degree of

liberation, a prerequisite for deep beneficiation. This fine product, however, will not be

directly usable in the blast furnace. It is therefore necessary to convert it to a suitable

form of agglomerate. Pelletization is one option but setting up new pelletization plants

will require heavy investment which the existing plants may resist.

In this project an alternate strategy is explored. The fines of beneficiated iron ores are

converted into micropellets, spherical pellets in the size range of 4 – 6 mm, which then

can be used as the feed material in the existing Sinter plants. The primary focus in this

project is to come up with the optimum operating conditions which give the optimum

yield of pellets in the desired size range. Not only the size, but the product also needs to

have good enough cold strength to be able to transport the pellets from the

Pelletization plant to the Sinter plant. The operational parameters that are being

studied in this work include: the type of binder and its amount, moisture content, speed

of rotation of the Disc pelletizer, angle of Disc inclination, time of pelletization, etc. The

quality of the product is finally determined in terms of the yield of the pellets in the

acceptable size range, their cold strength and the aging effect, if any.

1. Introduction

In many industrial processes agglomeration of fines is essential for conservation of resources and abatement of pollution. The iron and steel industry in India is vitally concerned with agglomeration of iron ore fines. During the mining of iron ore, around 30-40% fines (particles below 8 mm size) are generated. The ore washing plants also generate huge amount of slimes per year. Besides, there are huge deposits of naturally occurring iron ore fines (blue dust) with iron content of 60-65%. These fines and superfines are not usually suitable for agglomeration through sintering. The pelletization is one of the agglomeration processes developed to treat fine concentrates of iron ore. It is defined as the process of forming larger spherical bodies by rolling moist fine particles on a surface without application of external pressures. The pelletization of iron ore has been found to markedly increase the efficiency of the blast furnace operation. This is due to the spherical shape and close size range of pellets which results in even charge distribution across the blast furnace stack. This reduces channeling and produces good solid-gas contact improving heat and mass transfer which, in turn, reduce the coke consumption in the blast furnace. Also pellets can be made of desired chemical composition depending on the requirement. The reduction rate of lime-fluxed pellets is about 8% higher than that of pure iron ore pellets. The melting point of fluxed pellets is lower than that of pure iron ore concentrate pellets. This again reduces the coke consumption in the blast furnace. Furthermore, the distribution of limestone within the pellets enhances the slagging reactions of the impurities at a lesser energy consumption.

Heat indurated (essential to get desired crushing strength) pelletization on the other hand requires 80-85% fines below 75µm size and temperature of 1200 ~1350 ° C. The heat requirement for this process is considerably high. In the context of scarcity of fuel and its rising cost, efforts are being made worldwide to develop alternate energy saving processes for agglomeration.

Several energy saving agglomeration processes have been developed. RRL-Jorhat process (Iyengar et al., 1968 ), Grang cold process (Svensson, 1969 ), and some other processes (Lotosh, 1973; Lotosh and Efimov, 1973)) are based on the use of ordinary portland cement (OPC) as the binder. Other processes (Goksel, 1977; Hassler and Kihlstadt, 1977) are based on in situ formation of binders like calcium-silicate-hydrate by the reaction of lime and silica or other siliceous materials under hydrothermal conditions. Formation of calcium carbonate by the reaction between lime and carbondioxide is also utilized for agglomeration of ore-fines (Imperato, 1968 ).

Pellets prepared by Grang cold process are found suitable as burden material for blast furnace (Svensson, 1969). Research and Development Centre for Iron and Steel, Steel Authority of India Limited, established the cement (OPC) bonded iron ore pellets as suitable burden material for low and moderate shaft furnaces (Minerals and Metals Review, 1981). The pellets were prepared by using around 9-10% cement followed by curing under normal humid conditions for about 28 days to attain crushing strength of around 130-160 kg/pellet (for 15-20 mm pellets). This much

amount of binder requirement however introduces considerable amount of gangue material in the pellets and therefore lowers the merit of the process. Pellets for use in conventional blast furnace may require strength of 200 kg/pellet or more. Use of comparatively cheap materials like slag-cement in place of OPC may bring economical advantage in pellet making. India produces huge quantity of blast furnace slag but only a fraction of the same is utilized for making slag- cement. The slags are poorer in quality; low in lime (26-38%) and rich in alumina (20-30%). Although poorer in quality, the slags could be activated to make various cementitious binders (Dutta and Borthakur, 1990). Preliminary investigation carried out earlier (Durra et al., 1986) indicate that around 50% of the OPC requirement for pellet making could be substituted by slag to obtain pellets of adequate crushing strength.

In this project the primary objective is not to make pellets of 15-25 mm size to be used as blast furnace feed, but to make spherical micropellets (4-6 mm) which may be used as sinter feed. It is hoped that by using micropellets as sinter feed, the sinter bed porosity will be higher and uniform, and therefore with the same suction pressure a sinter bed of much greater heights (80-100 cm instead of normal 50 cm bed) can be used. This should increase the productivity of the sinter plant by about 80-100 percent. It is also emphasized in this project that the micropellets should have enough cold strength so that these can be easily transported and charged into the sintering machine. Thus, studies have been made to develop adequate cold strength of micropellets without any subsequent firing, as is often the case during conventional pelletizing of fines.

2. Materials and Experimental Procedure

2.1 Materials: Deep beneficiated low grade iron ore (Fe content originally less than 55% enhanced to more than 65% by deep beneficiation process), received from NML Jamshedpur, having the size distribution of 95% passing 140 mesh, 85% passing 200 mesh, 65% passing 270 mesh and 50% passing 400 mesh. Several binding materials used so far include Ordinary Portland cement (OPC), Molasses and laboratory grade bentonite and laboratory grade quicklime (CaO) and slaked lime (Ca(OH)2) have been used for carbonate-bonded pellets.

Tap water is used throughout these experiments. The amount of binders, water and slaked lime added to the feed charge are expressed as weight percentage of the originally dry feed charge.

2.2 Process variables: Following are the process variables and their ranges which have been considered and optimized in this work.

1. Fineness of feed � As received beneficiated ore: No grinding � Beneficiated ore with 3 hrs of grinding � High grade-Pure ore with 3 hrs of grinding

2. Binder and amount used

Binder Amount used, wt% of dry ore Bentonite 1, 2.5, 5, 7.5, 10

Ordinary Portland cement (OPC) 5, 7.5, 10 Slaked lime (Ca(OH)2) 10, 15, 20

Burnt lime (CaO) 10 Molasses 2 Moisture 13.5-18

3. Other operating parameters

Parameters Value used RPM of disk 24, 27, 31, 35

Disk inclination angle 40, 45, 50, 55 Location of moisture addition 450, 600, 750

2.3 Production of pellets:

The pellets have been produced in a disk pelletizer (Fig.1, diameter 66 cm, collar height 16 cm, angle of inclination 400-550 and rpm 20-35) using 500g iron ore fines mixed with binders like burnt lime (CaO), slaked lime (Ca(OH)2), Ordinary Portland cement (OPC), molasses, bentonite as feed material. Binders are expressed in weight % of iron ore. Ore mix (iron ore along with binder) in dry form is added in the pelletizer and then moisture is added to the ore mix in the pelletizer running at prespecified RPM and angle of inclination. Surface tension of water & gravitational force create pressure on particles, so they coalesce together and form nuclei which grow in size to form green balls. The green balls in the size range of 4-6 mm are screened out and used in the present study. Since micropelletization is a relatively new concept and there is no standard method available to check physical properties of these micropellets. So some bigger size pellets in size range of 10-15 mm are made to validate our results with the previously published work.

Fig. 1: Schematic of disk pelletizer

2.4 Carbonate Bond Process Description (CO2 Curing):

When calcium hydroxide and carbon dioxide come in contact with each other, under controlled moisture conditions, calcium hydroxide reacts to form a basic carbonate having considerable mechanical strength. Also, significant increase in crushing strength of pellets could be justified as conversion of Ca(OH)2 to CaCO3 leads to volume expansion and more void filling of pellets.

Ca(OH)2+ nH2O + CO2 = CaCO3 + (n+1)H2O

The molar volumes of

Ca(OH)2 = 33.59 cm3/mol

CaCO3 = 34.10 cm3/mol

Pellets made with Ca(OH)2 as binder first dried in open atmosphere for 1-2 days to bring down retained moisture to 3-4% then curing is done in CO2

environment for 5-15-30 minutes. Cured pellets then kept at room temperature for 2 days. Setup for CO2 curing is shown in Fig. 2.

2.5 Strength determination

Two types of strength tests are generally used for pellets. These are:

1. Drop Test

2. Crushing strength

These tests are briefly described below.

2.5.1 Drop test

For drop test of pellets, standards method describes the maximum number of drop sustained by pellets having size of 15-20mm from a height of 46cm on a steel pellets. If we do the same, carbonate bonded pellets (15% Ca(OH)2-15min of CO2 curing) and pellets made from cement (7.5%) as binder having the size of 4-6mm would easily sustain 20-40 drops without breaking

Fig. 2: CO2 Curing setup

and this is quite acceptable. The curing of cement bonded pellets is carried out in humid condition for 2-3 days followed by room temperature curing for another 30 days. It has been found that 90% of total strength achieved in first 15 days of curing.

2.5.2 Crushing strength measurement

For measuring cold rushing strength (CCS) of cured pellets, a load cell having maximum capacity of 35 kg with accuracy of 2g, shown in Fig. 3, was installed. Load is applied manually and slowly to avoid any jerk.

Cold crushing strength of pellets is expressed in kg/pellet by taking average of strength of 50 pellets of the same size. Pellets made with 20% Ca(OH)2 – cured in CO2 for 30 minutes have shown the maximum crushing strength of 10.5 kg/pellet (average value) for 6±0.5 mm pellets.

Fig. 3: Setup of measuring crushing

strength of pellets

3. Result and Discussion To get the feel of pelletization behavior of newly installed pelletizer, experiments were carried out with high grade iron ore (Fe content>62%) which was available in plenty in our lab at IIT Kanpur. Experiments were then carried out with beneficiated iron ore grounded for 3 hrs. Particle size distribution is given in Table 2. With ore mix of 500g pure iron ore and 2.5% (12.5g) bentonite, several experiments were carried out by changing the disk inclination 400, 450 and 500, RPM of the disk 24, 27, and 31, moisture addition angle 450, 600 and 750. Results of these experiments in terms of weighted average size of pellets and their yield in the range of 4-6mm pellets are presented in table 1. Few experiments were also carried out for disk inclination of 400 and 550. At 550 inclination there was excessive sliding at lower RPM and at higher RPM, pellets started falling down from top instead of rolling down, making them difficult to pelletize. And at 400 inclination, rolling of pellets over pelletizer disk was not optimum due to low rpm (critical speed limitation at this inclination).

Table 1: Typical results of pelletization experiments with natural high grade iron ore (500g ore, 2.5% bentonite, 13.5% moisture)

Angle of disc w.r.t. Horizontal β=45°

RPM Exp No. Place of moisture addition(α) Weighted Avg. (mm) Yield, %

24

1 α1=45° 8.02 8.43

2 α2=60° 6.19 44.93

3 α3=75° 4.88 82.32

27

4 α1=45° 14.74 0

5 α2=60° 10.12 0.94

6 α3=75° 6.40 33.3

31

7 α1=45° 10.26 10.84

8 α2=60° 9.30 3.86

9 α3=75° 9.20 16.54

Angle of disc w.r.t. Horizontal β=50°

RPM Exp. No. Moisture addition point(α) Weighted Avg. (mm) Yield, %

24

10 α1=45° 13.00 0

11 α2=60° 4.97 52.06

12 α3=75° 3.29 24.07

27

13 α1=45° 11.50 0

14 α2=60° 6.15 48.56

15 α3=75° 4.97 83.27

31

16 α1=45° 9.05 0.61

17 α2=60° 7.70 9.03

18 α3=75° 5.86 71.22

Few pictures of pellets made with beneficiated ore by taking best (having the maximum yield) combination of process parameters from table 1

Fig. 4: Pellets made with operating parameters of

Exp. No. 3 in table 1 using binder as 15% Ca(OH)2

and moisture=16%

Yield=83%, avg. size of pellets=4.8 mm

Fig. 6: Pellets made with operating parameters of

Exp. No. 18 in table 1 using binder as

2.5% bentonite and moisture=13.5%

Yield=71%, avg. size of pellets=5.8 mm

Fig. 5: Pellets made with operating parameters

of Exp. No. 15 in table 1 using binder as 20%

Ca(OH)2 and moisture=17.5% Yield=85%, avg. size of pellets=5.3 mm

3.1 Effect of disc inclination and RPM on yield of pellets:

The critical speed at which the pellets or balls no longer roll down but stick to the collar wall under the centrifugal force is given by the relation: ηc

* = 42.3{sin (βd /Dd)}

½

Where

ηc is the critical speed (rpm), βd is the disk inclination angle and Dd is the disk diameter in meter

Disk is usually operated at speeds of 0.6-0.7 times the critical speed. For disk inclination 500, the maximum operating RPM is around 30.

Based on the data, following can be stated:

Yield (4-6 mm) = f (rpm, angle of disc inclination) at fixed Moisture addition point (α) = 75°

Fig. 7: Relation b/w Avg. Size of pellets & place of moisture addition

Since in disk pelletizer, growth of pellets mainly takes place by layering mechanism, so for the pelletization with fixed feed as in our case, final pellet diameter does not depend on the number of disk revolution, Instead of that, it depends on the nucleation rate and the number of nuclei initially formed. As soon as the free feed material is consumed in the pelletizer, pellets growth almost stops. Further insignificant growth takes place by abrasion transfer only at higher RPM (>25RPM). Hence, all pelletization experiments were carried out for 10 minutes. Operating parameters of Exp. No. 15, which has the maximum yield, is selected for further pellets production with beneficiated ore for drop number test and cold crushing determination.

* Advances in chemical engineering, Volume 10, by Thomas B Drew p-59.

3.2 Effect of fineness of feed:

The physical and mineralogical properties of the ore greatly influence its balling behavior. To study the effect of fineness of feed on balling behavior, three different iron ore mix, given in Table 2, were prepared. Mixture 1, which is comparatively coarser, was taken from high grade iron ore which was initially in 1-3 mm size range but then grounded for 3 hrs.

Table 2: Particle size distribution of ore mix Mesh size Mixture -1 (high grade iron

ore-3 hrs grinding), Wt.% Mixture -2 (beneficiated ore-

no grinding), Wt. % Mixture -3 (beneficiate

ore- 3hrs grinding), Wt. %

-100 +140 15 4 0

-140 +200 17 10 8

-200 +270 12 20 12

-270 +400 11 16 20

-400 45 50 60

Mixture 2 is as received deep beneficiated low grade iron ore from NML. Mixture 2 was grounded for 3 hrs to get the mixture 3 which was finest in all three iron ore mixes. Balling behavior of mixture 1(high grade ore) was best even if it was relatively coarser than mixture 2 and 3. Similar type of balling behavior was noticed in the case of ore mixture-3. However, mixture 2 did not follow the normal pelletization pattern. It was seen that very loosely bonded nuclei were formed which were unable to grow. This resulted in an uneven pelletization behavior. The plausible reasons for the above findings are: (1) Most of the clay content of the beneficiated iron ore was removed during beneficiation, and (2) The beneficiation process based on the froth flotation would have resulted in formation of a thin coating of surface active agents over iron ore fines as it can be seen in Fig. 9. Pellets made with mixture 1 and 3 with 15% Ca(OH)2 and moisture addition of 16% are given in Fig. 8 and 10 respectively.

Fig. 9: Pellets made from mixture 2

Fig. 8: Pellets made from mixture 1

Fig. 10: Pellets made from mixture 3

Fig. 11: As received beneficiated ore is black in color while after grinding of 3 hrs, original color of

hematite has been surfaced. This is indicating some type of coating over beneficiate ore.

To avoid the excessive grinding, of as received beneficiated ore from NML, a few pelletization experiment were also carried by blending as received beneficiated iron ore with natural high grade in 80:20 ratio and it was seen that the pelletization behavior significantly improved- it was more or less same as mixture 1 or 3.

3.3 Effect of different binders on strength

The properties of green pellets (diameter 4-6 mm) prepared using beneficiated iron ore (mixture 3) are presented in Table 3. The moisture content of the pellets ranges between 13-18% depending on the binders and its amount.

Table 3: Typical results of Drop No. and CCS of pellets made by beneficiated ore grounded for 3 hrs (Mixture 3)

S. No. Binder Binder Content, wt%

Moisture content, wt%

Drop No.

Cold Crushing Strength of 6±0.5 mm pellets (Kg/pellet) Avg. Min. Max.

1 Bentonite 2.5 13.5 2 1.2 0.5 2 5 15 2.5 1.7 0.8 2.5

2 Cement* 5 14 16 5.8 3.5 7 7.5 15 20 6.5 4 8 10 16 25 8 4.5 10

3 Ca(OH)2** 10 14 23 7.5 5 10

15 16 29 9 6.5 12 20 18 35 10.5 7 13

4 Ca(OH)2** +

Molasses 10%+2% 15 28 8.7 5.5 12

* 3 days of moist environment curing and 30 days of room temperature curing.

** CO2 passed for 15 minutes at retained moisture of 3-4% in the pellets. Comparing strength of 20% Ca(OH)2 pellets with fired pellets (unfired strength=10.5kg/pellet)

As no strength standards are available for pellets in the size range of 4-6 mm, it was decided to

compare the cold strength of our pellets with that of fired pellets of the same size. For this

purpose, pellets of 4-6 mm size were fired at 12000C for 1 hour, and their compressive strength

was determined. It is believed that the pellets having the cold compressive strength 40-50% of

the CCS of fired pellets would be able to withstand the load of transportation as well as that of

the sinter bed.

Table 4: Compressive strength of fired pellets

Firing temperature (holding time= 1 hour)

Compressive strength of 6±0.5 mm pellets (kg/pellet)

Avg. Min. Max.

12000C 19.5 13.5 27

Fig. 12: Effect of Slaked lime (Ca(OH)2) content on CCS of pellets

Fig. 13: Variation of Cold Crushing Strength with amount of cement

3.4 Comparative study of CO2 curing time

CO2 curing is done for different time periods to assess the kinetics of the reaction. Pellets with 20wt% Ca(OH)2 is taken for study. Variation of strength with curing time has shown in Fig. 14.

Fig. 14: Effect of CO2 curing with time on cold crushing strength of pellets

3.5 CaO as a binder

10% CaO is used as binder to see the usefulness of CaO. 14% moisture is added to the feed during pelletization. It is found that just after pelletization; pellets start swelling and cause breakage of pellets. This might be resulted because of sudden volume expansion due to formation of Ca(OH)2 from CaO and also moisture present in pellet is used in the reaction.

CaO (s)+H2O (l)=Ca(OH)2

The molar volumes* of CaO = 16.71 cm3/mol, CaCO3 = 34.10 cm3/mol

4. Conclusion

It is possible to successfully pelletize the deep beneficiated ore only after either grinding to

remove the scaling over ore fines might have been formed by beneficiated process or blending it

with natural high grade ore in at least 80:20 ratio. Pellets of required size range of 4-6 mm can

be made by operating condition selecting any of the combination given in table 1. Higher disk

inclination and higher rpm are preferred as these give more rolling of pellets over the pelletizer

disk causing more compaction. As a result of this study, following conclusions are derived

1. Fineness of the feed is not the only criterion for pelletization. Surface morphology of

fines plays an important role as it has been seen that natural high grade ore which is

coarser than beneficiated ore easily pelletizes while for pelletization of beneficiated ore,

either we have to grind it to alter its surface morphology or blend it with natural high

grade ore.

2. Pellets made with Ca(OH)2 requires higher moisture content than pellets made with

cement or bentonite. Also the moisture content for it is very critical. Even a difference of

1% may very adversely affect the pelletization process. 1% higher moisture content than

the optimum leads to the flooding of pellets, and 1% lower moisture leads to significant

decrease in strength.

3. CO2 cured pellets as well as cement bonded pellets have shown quite good crushing

strength (for 4-6 mm pellets). Significant increase in strength is noticed with increase in

lime or cement content.

4. CO2 curing time of 20 minute is just enough as no increase in strength is seen beyond this

time period.

5. Future Goal

Micropellets made by this method would be finally going to serve as feed material for sinter

plant. So our next target would be to get the optimum physical and chemical properties of these

micropellets. We have done some work on cold crushing strength. Further work would be to

check the abrasion and shatter index, thermal deterioration of pellets etc. Some of these best

experiments will be repeated with beneficiated BHQ (Banded Hematite Quartzite) iron ore

received from IMMT Bhubaneswar.