Testing of raw materials for blast furnace

Testing of raw materials for blast furnace

The efficiency of a blast furnace is directly dependant on the permeability of the charge in the furnace. The quality of coke should be such that it gives minimum operational difficulties and maximum production rate .The properties of B/F fuel are

1. Chemical composition like fixed carbon, sulphur, phosphorus and other deleterious impurities.

2. Reactivity

3. Cellular and porosity

4. Size range

5. Thermal stability at high temperature

6. Strength an abrasion resistance

1. Chemical composition: 

The chemical composition is important in determining the quality of coke. Chemically the useful component of coke is its fixed carbon and the balance is made up of ash volatile matter and other impurities. Ash usually contains refractory oxides like SiO2, Al2O3, CaO, etc. and needs basic flux to maintain slag basicity. An increase in ash content of coke by 1% decreases the production by 3-6% and increase coke consumption by 4-5%.Coke is the prime source of sulphur so, the total amount of sulphur and phosphorus in coke is usually small. Proximate and ultimate analyses provide different data of coke chemical composition. 

2. Reactivity: 

It is defined as the rate at which the carbon atoms of coke react with oxidizing gasses like O2, CO2 or H2O. A highly reactive coke may react more readily with the CO2 in the ascending gasses in the BF and a low reactivity may delay its burning. Reactivity is measured under standard conditions of experimentation. Knowing the physical properties of Coke is important as it predicts how coke will behave in a Blast furnace. The reactivity test for coke includes Coke Reactivity Index (CRI) and Coke Strength after Reaction (CSR)according to standard ASTM D 5341-93a.

Technique for determining lump coke reactivity in carbon dioxide gas at elevated temperatures and its strength after reaction in carbon dioxide gas by tumbling in a cylindrical chamber referred to as the „I – tester‟. When coke lumps descend in the blast furnace, they are subjected to reaction with countercurrent CO2 and to abrasion as they rub together and against the walls of the furnace. These concurrent processes physically weaken and chemically react with the coke lumps, producing an excess of fines that can decrease burden permeability and result in increased coke rates and lost hot metal production. This test method is designed to indirectly measure this behaviour of coke in the blast furnace. Most blast furnaces will require a coke with a CSR greater than 60 and CRI less than 25. A coke sample of 200 grams with a particle size between 19 – 22.5 mm is placed in a reactor and heated to 1100°C in inert atmosphere. Subsequently, the coke is degassed, i.e. reacting according to the Boudouard or the solution loss reaction, isothermally for two hours in 100% CO2 gas atmosphere, and then cooled with nitrogen gas. After cooling, the coke is weighed and tumbled for 600 revolutions in an I-drum followed by sieving through screens with mesh sizes of +10 and – 0.5 mm. The weight loss of coke represents CRI, and the remaining coke on the +10 mm sieve represents the CSR. 

3. Porosity: 

Apparent and true specific gravity, as determined by this test method, are influenced by the type of coals carbonized and the operating and preparational conditions of that carbonization, that is, charge bulk density, heating rate, and pulverization level. In turn, these properties directly influence the performance in processes using coke. This test method covers the determination of apparent specific gravity and true specific gravity of lump coke larger than 25-mm (1-in.) size and calculating porosity from the specific gravity data. 

4. Size range: 

The characteristics of a good metallurgical coke are its strength and size range. The size range of coke is generally chosen to match the size of other raw materials to ensure maximum bed permeability for smooth furnace operation. The investigations have shown that the coke size has to be sufficient lumpy. It was found that a mean size of burden of 13 mm the mean size of coke should not be less than 53 mm .The optimum size of coke should be 3-5 times than that of iron bearing material. 

5. Thermal stability: 

During its descent in the furnace coke gets progressively heated to 15-1600° C before it burns in front of the tuyeres . The temperature gradient in an individual piece of coke sets in differential contraction and expansion, particularly where large mineral matter is present, resulting in high local stresses and its consequent tendency to degradation increases .The thermal stresses experienced by coke in the blast furnace are greater the larger is the lump size of the feed coke. 

The following factors are therefore believed to favor high thermal stability: 

A. Absence of large lumps in feed coke 

B. Uniformity of coke texture 

C. Minimum inert inclusion of large sizes 

D. High carbonization temperature and heat soak 

E. Prior mechanical conditioning 

F. low chemical reactivity

6. Strength and abrasion resistance: 

The blast furnace coke has to stand the rigorous of handling and which includes drops and flows at several places before landing in the furnace. In addition to this, coke has to stand high temperature and the nearly 20-25 m tall burden lying over it when it reaches the tuyere level. Height of modern furnace is controlled mainly by the strength of the available coke. It is an established practice to estimate the suitability of coke for blast furnace, in terms of its strength and abrasion resistance by measuring its shatter and abrasion indices. The shatter test gives resistance of coke to impact where as the tumbler test is designed to measure the resistance of coke to degradation by a combination of impact and abrasive forces. Shatter tests consists of dropping the obtained sample of certain fixed weight of coke from a standard height on a standard quality floor. The shatter index is expressed as percentage retained in various sieves of certain fixed sizes. The abrasion index is measured by putting a standard weight of sample in a drum and rotating it for a fixed number of revolution at a fixed speed.

Micum test: 

The most popular test is the Micum test as per the German standard DIN 51712 of 1950. In this test 50 kg. of air dried coke sample of +50 mm size is charged in a drum, one meter long and one meter in diameter and rotated for 100 revolutions at the rate of 25 r.p.m. The product is screened on 50, 40, 10 mm screens and results are expressed as M40(% of total charge retained on 40 mm screen) and M10(% of charge passing through 10 mm screen) values. M40 index gives the indication of hardness of coke and M10 its resistance to abrasion. The drum employed here in as longitudinal angle iron baffles from inside.

Testing of Iron Ore:

a) Reducibility: 

The reducibility determines the ease with which oxygen can be removed from the iron oxide in the ore by reducing gases. This influences the productivity and quality of the product. The reducibility is inversely proportional to the time required reach some arbitrarily chosen degree of reduction.

The standard norm is represented by ,

dR/dT = 0.5% per minute ,minimum, which can be verified experimentally. Reducibility of the iore depends on size, shape and distribution of the ore. Hematite ore is chosen for DRI production, because it has better reducibility than magnetite ore. 

b) 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. Precisely, a 15 kg test block sample was tumbled in a circular drum rotating at 25 rpm for 200 revolutions. Subsequently, the ore was screened and fractions +6.3 mm and −0.5 mm were obtained. The percentage of the fractions in proportion to the feed weight is the value of the TI (+6.3 mm) and AI (−0.5 mm). The test was repeated four times and the average values for these tests represent the final TI and AI data. 

c) For estimation of a Shatter Index,a dried lump iron ore sample (10 kg) of size −40 + 10 mm was dropped 4 times from a height of 2 m onto a cast iron floor (0.5 × 0.5 × 0.03 m). Thereafter, the iron ore was screened and the shatter index expressed as the wt% passing through a 5mm sized screen (i.e., −5mm fraction). This procedure followed a test procedure, which were carried out on other ores. 

d) Decrepitation: 

When iron bearing materials are suddenly exposed to the exhaust gas temperature at the stock level on charging, breakdown may occur due to thermal shock. This is known as decrepitation. Experimentally it is measured by dropping a known weight of material in a furnace previously heated to a temperature level of 400600°C, under normal atmosphere, inert atmosphere or under mildly reducing conditions. After the charge attains the temperature it is removed, cooled and sieved to measure the breakdown.

In a typical test 500 g of 20-40 mm size undried ore is dropped in a furnace previously heated to a temperature level of 400°C and retained there for 30 min under a flow rate of 5000 litres of nitrogen per hour. The sample is then removed, cooled and the percentage of 0·5 mm and -5·6 + 0·5 mm material in the product is determined by sieving. It is believed that ores with more than 10% porosity will not decrepitate

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