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Research Article | DOI: https://doi.org/10.31579/ijmse.2019/005
*Corresponding Author: Dare Victor Abere, National Metallurgical Development Centre, Jos, Nigeria.
Citation: Olumide M Ogunronbi., Dare V Abere, Grace M Oyatogun., Sammy A Ojo, Temitope E Alonge,. et al (2019) Effects of Particles Sizes, Binders and Fluxes on the Physical Properties of Iron Ore Pellets. J. Effects of Particles Sizes, Binders and Fluxes on the Physical Properties of Iron Ore Pellets, 1(1); Doi: 10.31579/ijmse.2019/005
Copyright: © 2019 Dare Victor Abere. This is an open-access article distributed under the terms of The Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Received: 22 November 2019 | Accepted: 17 December 2019 | Published: 20 December 2019
Keywords: iron; feed; binders; flux; crushing strength; moisture content; furnaces
This work examined the suitability of Itakpe iron ore, Itakpe, Kogi State, Nigeria to meet the feed requirements of today's iron production methods using different particle sizes, binders and fluxes. Test such as crushing strength, green and dry compressive strength, metallization crushing strength, moisture content were carried out. Also, the tumbler, abrasion, and Shatter Indices of the ore were determined Addition to these, porosity, drop number, Drop Resistance tests were performed. From the result obtained, the Itakpe iron ore was found to have good mechanical properties exemplified with tumble and shatter index data >89.0 wt% and <2.5 wt%, respectively. Furthermore, its reducibility at 0.87%/min is within the acceptable range as a natural material feed for blast furnace and direct reduction furnaces. Also, the energy requirement for heating the ore to 1100°C was found to be higher in the samples containing a wider size range of irregular grains and the largest contaminations. In summary, it was concluded that the Itakpe iron ore has good physical and metallurgical properties to serve as a natural material for the blast furnace and direct reduction furnaces.
Nigeria is blessed with large quantity of iron ore deposit of about three billion (3 x 109) metric tonnes. Even with this large reserve of proven and unproven iron ore deposits, only one of the deposits in the proven reserves is currently being exploited and processed that is the Itakpe iron ore deposit. This deposit has an estimated reserve of about 200 million (200 x 106) metric tonnes and has been earmarked to supply Ajaokuta and Delta Steel Plants [1]. Pelletizing of iron ore was started in the 1950s to facilitate the utilization of finely ground iron ore concentrates in steel production. Two main types of processes have been developed, the straight Grate and the Grate kiln processes. In the straight Grate process, a stationary bed of pellets is transported on an endless traveling grate through the drying, oxidation, sintering and cooling zones. In the grate kiln process, drying and most of the oxidation is accomplished in a stationary pellet bed. Thereafter, pellets are loaded in a rotary kiln for sintering. This way, more homogenous induration in pellets is achieved [2]. Pelletizing requires ultra-fines of over 75% of -325 mesh and porosity of pellets is about 20 – 30%. The shape of pellets is near spherical and hence built permeability of the burden is much better than that of sinter [1]. Use of pellet burden reduces hanging, which is often observed if a high proportion of sinter is used; the used of pellets saves coke by 25kg/t and the productivity by 15% [1].
Iron is a chemical element with symbol Fe. It is a metal in the first transition series. It is by mass the most common element on Earth, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust. Its large quantity in rocky planets like Earth is due to its copious production by fusion in high-mass stars, where it is the last element to be produced with release of energy before the violent collapse of a supernova, which scatters the iron into space. Iron, with its general products, is currently the most widely used metal in the various sectors of the world’s economy. This is due to the good mechanical properties it possesses to the low cost associated with its production. Iron is mainly produced through two methods; the blast furnace, BF, route (pig iron), and the direct reduction, DR, route (sponge iron). According to the World Steel Association, 2011, crude steel production was standing at 1.4 billion tons by the end of 2010. Of these, 70% was produced via the basic oxygen furnace (BOF), which uses pig iron from the blast furnace, and 28% via the electric arc furnace (EAF), which uses sponge iron and scrap [3].
Iron ore can be used directly in its natural form as a raw material for processing iron and it can be upgraded through beneficiation before being used. The feedstock is evaluated for physical and metallurgical properties [4]. Physical properties give an indication of the material’s behaviour during handling and descent in the furnace. Metallurgical properties on the other hand indicate the materials’ behaviour during the reduction process. In selecting iron ore for iron and steel industries, some of the properties which need to be considered include tumbler, abrasion and shatter indices, porosity, chemical composition, loss on ignition, reduction behavior, and thermal degradation [5].
Binders are used in order to produce high-quality “green” pellets that can be handled at ambient temperatures, and that can also tolerate subsequent high-temperature processing at up to 15000C without allowing the pellets to disintegrate, with accompanying loss of product as environmentally objectionable dust, the binders include starch, bentonite which is a clay mineral predominantly consisting of the mineral Montmorillonite mixed with some related minerals like Nontronite and Beidellite [6]. Fluxes are the materials added to the contents of a smelting furnace for example, iron pellets for the purpose of purging the metal of impurities and of rendering the slag more liquid. The flux most commonly used in iron and steel furnaces is limestone which is charged in the proper proportions with the iron [6].
However, the process of pelletization enables converting iron ore fines into uniformed sized iron ore pellets that can be changed into the blast furnaces or for production of Direct Reduced Iron (DRI). Pellets are uniform in size, with purity level of 63% - 65% contributing to faster reduction and high metallization rates. Pellets with their high uniform mechanical strength and high abrasive strength increase production of sponge iron by 25% to 30% with same amount of fuel. Pellets are spherical balls formed by the agglomeration of natural or ground iron ore fines in the presence of moisture and binder. Again, on subsequent induration at around 1300 0C, they become suitable feed for downstream iron making processes like the Blast furnace and direct reduction furnace. Pelletizing process consists of three main stages namely; Raw material preparation, Green pellet formation and pellet hardening (induration) [2].
The functionality of a blast furnace depends mostly on the physical and chemical characteristics of the materials. The load materials, which are charged through the throat, are coke, lump ores, and agglomerated ores in the form of sinter or pellets. Lump ores are significantly cheaper than pellets and sinters. However, they are inferior, particularly with respect to softening-melting and they affect the smooth running of the blast furnace and increase the coke consumption [7]. Swelling and disintegration of iron ore have been two major draw backs in their acceptance as feed for blast furnaces and direct reduction furnaces [8]. Therefore, iron ore as mined from the earth has been almost completely replaced as a feed for iron blast furnaces by sinters and pellets.
In the study of Itakpe ores, it was found that the chemical composition and microstructure of the ore corresponds to the demands on high grade iron ore. Precisely, the Fe, silica, and alumina contents indicate that they can profitably be used for iron production [1]. This particular study examines the physical and metallurgical properties of Itakpe iron ore, Itakpe, Kogi State. It evaluates these with respect to the requirements for the different iron production processes, in order to establish the ore’s suitability in meeting the necessary demands for iron production.
Materials and Method
12kg of Itakpe Iron ore concentrate was pulverized in Ball mill for 3 hours and further pulverized in Ring Pulverizing Machine for 5 minutes after which the Iron ore was sieved with different Mesh sizes to give particle sizes A (+0.125, -0.09), B(+0.09, -0.063) and C(-0.063). 500g of pulverized and sieved Itakpe Iron Ore was collected from each particle size as stated above into three different places, (A, B, C), (A1, B1, C1), (A2, B2, C2) and (A3, B3, C3), 15g of bentonite as binder, 10g of limestone (CaCO3) as flux were added and mixed thoroughly with 60ml of water in palletizing disc of 40cm diameter and 10cm depth, Tilt angle = 380, rotating speed of 15rpm. The above steps were repeated varying the particle sizes, binders and fluxes.
Determination of Physical Properties
The physical properties of the ore were investigated by determining their crushing strength, compressive strength (green and dry), indurating compressive strength or metallization crushing strength, moisture content, tumble, abrasion, and shatter indices, micro porosity, drop number, drop resistance, as well as their bulk density.
Drop Number
Five samples from the ones formed earlier were dried in the oven at 2000C – still in their green state. They were subjected to drop from a marked distance of 60cm height from the ruler to a hard but fixed base before chattering was noted and recorded. The procedure was repeated for the five pellets in which the calculated average gave the actual drop number of the pellets.
Drop Resistance
It is a test done to determine the crushing strength of green balls after three drops from different heights. These values are indications of the admissible height differences at various transfer points during green ball transportation. About 7N are required as minimum strength for three drops from a height of 46cm. In this study, five selected samples were also chosen in green state and then dropped at varying distance of 48cm, 60cm and 72cm height from the ruler to a hard but fixed base. Numbers of drop to shattered point were recorded and the average calculated.
Green Compressive Strength Test
This test was performed when the pellets were not yet totally dried but only heated to some degrees. Five pellets were heated to 2000C and then subjected to chatter test using Hydraulic press crushing machine. Readings were taken and recorded.
Dry Compressive Strength Test
The test was carried out to determine the crushing compressive strength of different samples of the pellets made and to investigate how strong the pellet would be after they had been totally dried up. Five green pellets were heated in the furnace to about 9000C, after which they were subjected to compressive strength test. Also, another five green pellets were heated in the Electric Carbonate furnace to about 9000C and soak for 1 hour (h) after which they were subjected to the same compressive strength test. The results were taken and recorded.
Crushing Strength Test
Certain minimum crushing strength is quite necessary in order for the pellets to withstand the compression load in the pellet bed on a belt conveyor, drying grate, indurating grate, or in a shaft furnace [9] and therefore, the test was performed in three different ways, first, hand press which is operated by hand and the power being hydraulically transmitted, secondly hydraulic press with motorized drive, lastly electrically operated press with a weight place on a movable level area.
Indurating Compressive Strength Test or Metallization Crushing Strength Test
The test was carried out to investigate the physical properties of iron pellet at the temperature it started changing to metal; this was necessary for their behaviour during transportation and primarily during metallurgical treatment either in the blast furnace or in direct reduction plants [10]. The pellets quality was evaluated by adopting appropriate testing methods, which were developed from experience gained mainly in industrial plants. Five green pellets were kept in Electric Carbolite Furnace heated to an indurating temperature of 12000C at which metal phase started to form.
Moisture Content
This was carried out to determine the percentage moisture content of the samples of iron ore pellets made. Weighed quantity of iron ore sample from each mesh sizes was charged into a known weight crucible and transferred into an oven at 1500C. The crucible with its content was then heated for 2 hours after which it was brought out of furnace re-weighed and weight recorded until two same weights were recorded in two consecutive heating. Then the initial weight of sample “X” subtracted from the final heating of total sample “N” gave the volume of water expelled (V) from the iron sample, i.e. “X” – “N” = Total. Therefore, the moisture content per pellet produced can be determined from:
Where V = Volume of H2O in pellet produce
V1= Volume of H2O moisture evolved
V2= Volume of water used
N = No of pellet produced
Tumbler/Abrasive Resistance Test:
A tumble strength test measures two mechanisms of feedstock degradation, that is, the Tumble Index (TI) and the Abrasion Index (AI). It was carried out following the International Standard ISO 3271:1995(E) for determination of Tumble Strength for iron ore [11]. Precisely, 10 pellets weighing 250g, dried at 2000C in an oven were introduced into a drum having diameter of 0.5m and length of 0.25m with two lifters each 0.5cm high located inside the drum which was rotated for 30 minutes in high speed, after which the pellets were screened and fraction +6.3mm and -0.5mm were collected differently. The percentage of separated fractions in proportion to the feed weight is the volume of Tumbled index (TI)(+6.3mm) and abrasion index (AI) (-0.5mm) can be obtained below
Where:
T I = Tumbled Index
Wt = Weight
W= Total weight of pellet
Micro porosity:
The test was done in order to evaluate the reduction velocity in iron pellets. Weight of a fired pellet was measured; when the pellet was dipped into a beaker containing benzene, bubbles were then released until stoppage in bubbles. Then the sample brought out and weighed again to see the difference in weight. Therefore:
Where
P= Porosity
D = Weight of pellet in Benzene
d = Weight of sample
This was repeated for the samples from each group.
Results and Discussion
Physical properties of iron ores are determined by using crushing strength, compressive strength (green and dry), indurating compressive strength or metallization crushing strength, moisture content, tumble, abrasion, and shatter indices, micro porosity, drop number, drop resistance, as well as their bulk density. Tests such as tumbler and shatter tests give an indication of the material behaviour during ore mining, loading, transportation, handling, and screening. They also give an insight into the material’s behaviour, during an initial period of the reduction process in its descent in the furnace. Results obtained during the course of this work are presented below:
Key
Where
X1 = Average Drop Number in 48cm
X2 = Average Drop Number in 60cm
X3 = Average Drop Number in 7cm
Moisture Content for Itakpe Iron Ore:
= 50.08 – 49.84
= 0.96ml
The test was carried out on the Green Pellets fired at 2000C for 1 h and it was observed in the table 1 that even though the binders and the fluxes used might have played one role or the other on the test, it could be observed that particle sizes play some reasonable effects on the test. In the computation of the averages, there was an increase towards the particle sizes. Drop Resistance Test (48, 60, 72cm) was carried out on the Green pellets fired at 2000C for 1 h. The result in table 2 shows that the effect of particle sizes on the drop resistance test cannot be overlooked. The drop number indicates how often green balls can be dropped from a height of 60cm before they show perceptible cracks or crumble. According to experience, the minimum value is four, which means a pellet should be able to withstand without any damage, four drops from a height of 60cm [12]. It was shown clearly that because of the smaller in sizes, they were able to be adhered together strongly by the binders used. Their resistances were increased as we were getting to finer particle sizes. From the table 3, it can be observed that the compressive strength of the ore increases with decrease in particle sizes and this could be attributed to the fact that there was little chemical reaction taken place in the pellets since they were just fired at 2000C for 1 h and hence yielded low result were improved as the particle sizes were becoming finer. High records in the Dry Compressive Strength test was discovered after firing the pellets to 9000C as in table 4. Most significantly, record of high dry compressive strength was noted in the pellets that had Bentonite as binder and Calcium hydroxide as flux. This was because Calcium hydroxide can also serve as binder. To this effect, as Calcium hydroxide was acting as a flux, in some other way, it acted as a binder too therefore increasing the compressive strength of the pellet [13]. It was discovered from table 5 that the dry compressive strength test at 9000C and soaking for 1 hr was negatively affected by CaCO3 being that it only served single purpose and positively affected by Ca(OH)2 because it served duo purpose; binder and flux consecutively. More still the finer the particle sizes, the brighter the result. Also, chemical reaction had already taken place at this point and thus increasing the bond strength of the pellets [14]. At this indurating temperature, also known as metallization point at which the pellets were already forming metal, there was increase in the compressive strength of the pellets but in the case of Bentonite and Ca(OH)2, greater compressive strength was observed in table 6, this is due to the fact that as Calcium Hydroxide was acting as flux in the composition, it also acted more or less as a binder [15]. As indicated in table 8, it can be deduced that the effect of Bentonite as binder and Ca(OH)2 as flux recorded high abrasive index. It shows a good binder has much effect on the strength of the iron pellet [16]. Table 9 shows that the major parameter that played significant role on the micro porosity is particle sizes because finer sizes have more compatibility more than those of coarser ones [17].
Conclusion
Based on the analysis of the effects of Particle sizes, Binders and Fluxes on the Physical and properties of Iron Ore pellets with Itakpe Ore as case study, it could be concluded and suggested that the effect of sizes as shown in the test showed that the finest particle size of (-0.063) has the best quality in time of drop number test, drop resistance and compressive strength test. Also, the effect of binders as shown showed that the samples containing bentonite as binder produced the best physical properties of the pellets. The effect of fluxes as shown in the test, samples containing Calcium hydroxide showed a great improvement in its physical properties; tumbler index, abrasive index, compressive strength at 2000C, 9000C holding, 12000C. Using the composition of binder and flux as 3% and 2% respectively with the Ore weight, not neglecting the par ticle sizes which also played a greater role as that of 0.063mm. Using the composition of 3% binder 2% respectively for bentonite as binder and Ca(OH)2 as the flux being shown in the results, bentonite has a selling and gelling properties and it is sodium ion that helps during bonding – for combination when necessary. It recommended that bentonite should be used with Calcium hydroxide as flux at a composition of 3% and 2% of ore weight when using Itakpe ore for Iron production.
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