Sintering of iron ore

Transcription

1 GOVERNMENT POLYTECHNIC DHANBAD Sintering of iron ore [project & seminar] By abhishek malakar 2015 Please myabhishekmalakar.wordpress.com use purchased version to remove this message.

2 Sintering of Iron Ore (project & seminar) Page 2 of 32

3 Contents: 1. Introduction 2. Definition of sintering 3. The need for sinter 4. Literature survey 5. Raw material 6. Principle & mechanism of sinter 7. Different process of sintering 8. Dwight-Lloyed sintering process 9. Flow sheet of sintering 10.Sinter machine specification 11. Types of sinter 12 Effect of fine iron ore 13. Quality aspect of sinter w.r.t. BF performance 14. Factor affecting sinter quality 15. Parameter controlling sintering process 16. Sintering technology improvement & Economics 17. Advantages of sinter 18. Disadvantage of sinter 19. Future of sintering 20. Conclusion Page 3 of 32

4 Q. sintering of iron ore 1. INTRODUCTION:- 40 to 50% iron ore becomes under size which may create pollution in the plant of the mine site. For the utilization of the variable ore, it is necessary to agglomerate the fines into the required size for charging in the blast furnace. Agglomeration is the size incensement process of the fines ore. Agglomeration is defined as the process to prepare a suitable Blast furnace feed for smooth, proper and efficient running of the Blast furnace operation. The process of agglomeration can be classified as follows: i) Briquetting. ii) Nodulising iii) Vacuum Extrusion process. iv) Sintering. v) Pelletizing. Page 4 of 32

5 There are two process of agglomeration for iron ore: 1. Sintering 2. Pelletisation Less than 100 mesh fine iron ore is used for sintering whereas more than 100 mesh is used for the pelletisation. Therefore, they are not competitor. They are complimentor of each-other. 2. DEFINITION OF SINTERING: Sintering is a process of agglomeration of fine mineral particles into a porous and lumpy mass by incipient fusion caused by heat produced by combustion of solid fuel within the mass itself. Sinter: It is the product of sintering. It is a porous mass. 3. THE NEED FOR SINTER (i) To utilize the fines generated during the mining operation. Page 5 of 32

6 (ii) To utilize different additives like mill scale, flue dust, hearth slag etc. in an integrated steel plant. (iii) The need for charging prepared burden in Blast Furnaces to increase productivity and lower fuel rate 4. LITERATURE SURVEY Humans have used the process of sintering for thousands of years. Some of the first sintered products were bricks heated in open-pit fires to add strength. After that the waste fines material began to sinter. It has been seen that the fines of ore can be used effectively to agglomerate it. So, the theory of sintering came into practice with the aim of utilizing the waste material. This technology was developed for the treatment of the waste fines in the early 20th century. Since then sinter has become the widely accepted and preferred Blast Furnace burden material. Presently more than 70% of hot metal in the world is produced through the sinter. In India, approximately 50% of hot metal is produced using sinter feed in Blast Furnaces. Large sinter strands 6 m wide and with a sintering area of Page 6 of 32

7 400 m2, are capable of producing t/m2/day. 5. RAW MATERIALS USED FOR SINTERING 1. Iron bearing material: i. Fine iron ore=> -10mm to 100 mesh ii. Mill scale 2. Coke breeze=> less than 6mm The amount of coke breeze requires for sintering is 6-8% for C temperature. 3. Flux: i. Fine limestone=> -10mm ii. LD converter slag iii. Sand dune- it is used to control the alumina content. Advantages of adding flux to sinter It generates slag with the impurities present in the iron ores and solid fuels producing a suitable matrix for cohesion of the particles It improves the physical and metallurgical properties of sinter It reduces the melting temperature of the iron ore blend Page 7 of 32

8 It promotes the calcination reaction of the limestone (CaCO3 =CaO + CO2) outside of the blast furnace hence saving heat consumption in the blast furnace. 4. Moisture: Moisture is mixed for the heat carrier and for giving the sticking property to the fines % water is necessary in the sintering process. 6. PRINCIPLE AND MECHANISM OF SINTERING The principle of sintering involves the heating of iron ore fines along with flux and coke fines or coal to produce a semi-molten mass that solidifies into porous pieces of sinter with the size and strength characteristics necessary for feeding into the blast furnace. It is basically an agglomeration process achieved through combustion. The iron ore sintering is carried out by putting mixture of iron bearing fines mixed with coke breeze on as permeable grate. The top layer of this sinter bed is Page 8 of 32

9 heated to the sintering temperature by a gas or oil burner and air is drawn downward. The top layer of this sinter bed is heated to the sintering temp. (1200C-1300C) inside a Ignition Hood furnace. In the ignition hood the air is drawn downwards, through the grate with the help of exhaust blowers (Waste Gas Fan) connected by means of Waste gas main. The narrow combustion zone developed initially at the top layer by layer to the sintering level. The cold blast drawn through the bed cools the already sintered layer the thereby gets itself heated. The heat contained in the blast is utilized in drying and preheating the lower layers in the bed. In advance of combustion therefore each

10 layer gets dried and preheated by the heat transferred from the upper combustion zones. The lower portion of the bed absorbs much of the heat in the gases. In the combustion zone, bonding takes place between the grains and a strong and porous aggregate is formed. The process is over when the combustion zone has reached the lowest layer of the bed. The sinter cake is thus tipped from the grate in hot condition. It is then broken, cooled in sinter cooler cold sized and sent to the Blast furnace. The heat wave travels down in the bed with approximately a constant velocity given by an expression: Where, H= heat capacity of gas per unit volume h= the heat capacity of solid per unit volume W= normal volume of fluid per unit cross-section of the bed per minute

11 F= the void fraction per unit volume of bed. Bonding action During the sintering process, binding takes place due to: 1. Mechanical bonding 2. Chemical bonding Mechanical bonding The process of the bonding of the fine particles at high temperature takes place due to recrystallisation, grain growth and diffusion process. Recrystallisation: it is the rearrangement of the crystal structure above the recrystallisation temperature. = (0.3 to 0.5) Where,T r = recrystallisation temperature T m = melting temperature in Kelvin the coefficient is 0.5 for the iron. Grain growth: the rate of change of diameter of the grain is inversally proportional to the grain at constant temperature. Page 11 of 32

12 = / A grain boundary(gb) is the transition area or interface between adjacent crystallites (or grains) of the same chemical and lattice composition, not to be confused with a phase boundary. The adjacent grains do not have the same orientation of the lattice thus giving the atoms in GB shifted positions relative to the lattice in the crystals. Due to the shifted positioning of the atoms in the GB they have a higher energy state when compared with the atoms in the crystal lattice of the grains. It is this imperfection that makes it possible to selectively etch the GBs when one wants the microstructure visible. Striving to minimize its energy leads to the coarsening of the microstructure to reach a metastable state within the specimen. This involves minimizing its GB area and changing its topological structure to minimize its energy. This grain growth can either be normal or abnormal, a normal grain growth is characterized by the uniform growth and size of all the grains in the specimen. Abnormal growth is Page 12 of 32

13 when a few grains grow much larger than the remaining majority. Diffusion: During the heating of ores or grains atoms or molecules migrate from one grain to another grain and cause to bind the grains. Chemical bonding = It is the process of bonding due to incipient fusion iron silicate called as fayalite. Its chemical formula is. and its melting point is about Iron ore contains silica and mill scale is added. During the heating of coke, iron ore also reduced as: + + For the bonding action, about 3% FeO is required. 7. DIFFERENT PROCESSES OF SINTERING:- Huntington and Heberlein Pot Process-fpr non-ferrous metal Industry. Batch Sintering-Greenwalt Single Pan Process Page 13 of 32

14 Allmanns Ingenoirs Bryans Multi Pan Process Dwight-Lloyd Continuous Sintering Process Dwight-Lloyd sintering process is used iron ore sintering. 8. Dwight-Lloyed sintering process:- The machine is made of pallets, mount or two rotating drums having grate. Accessories of the sinter machine: 1. Raw material storage bins

15 2. Mixer: Generally most of the sinter plants are provided with separate mixing and balling drums. But the latest generation of sinter plants are provided with a combined mixing and balling drums. The main purpose of mixing drum is to homogenize the sinter mix. The diameter of the drum, the RPM and the space factor play a major role in achieving higher degree of mixing. 3. Hopper 4. Labler 5. Ignition hood 6. Drums- for the movement of endless chain:- The balling drum (Nodulising drum) ensures that fines are coated on the nuclei particles, thus produce higher size balls. This facilitates in improving the mean size of sinter mix and hence the permeability of mix. Here again the diameter, RPM and space factor play a major role in achieving higher degree of balling. Very little water is added in mixing drum and major quantity of water is added in the balling drum Page 15 of 32

16 The amount of water added and the method of water addition in the balling drum also control the degree of balling and hence the permeability of sinter mix. 7. Endless chain- made of pallets 8. Suction pipe & Suction box: The rate of flow of the air through the bed is controlled by the vacuum under the bed and the permeability of the bed. The optimization of the gas dynamics parameters of the sinter machines enables one to achieve higher under grate suction and thus substantial improvements in the techno-economic parameters of the sinter production. 9. Sinter box. 10.Sinter crusher Process A layer of controlled size sinter (bedding) is fed to the bottom of the sinter machine grates for the protection of the grates. After this the moistened micro pellets of the raw materials mix is fed and leveled. After the material is leveled on the sinter machine, the surface of the charged material on the sinter machine is Page 16 of 32

17 ignited using gas or oil burners. Air is drawn through the moving bed causing the fuel to burn. Sinter machine velocity and gas flow are controlled to ensure that burn through (i.e. the point at which the burning fuel layer reaches the base of the strand) occurs just prior to the sinter being discharged. During the machine movement the sintering of the material bed on the grate proceeds downward. Waste gas circuit is to be fully leak proof, not allowing air from atmosphere to be sucked by the system. This results into saving of power in the waste gas circuit. At the end of the machine the sintered material in the form of cake is discharged into the hot sinter crusher. Here the hot sinter cake is crushed to a pre-determined maximum particle size. From here the sinter is discharged onto sinter cooler which can be either straight line or circular cooler. After cooler the sinter is transferred to the screening section. In the screening section the product sinter, bedding and return fines are separated. Return fines, not suitable for Page 17 of 32

18 downstream processing, are conveyed to a bin for recycling in the sintering process. Waste gases are treated for dust removal in a cyclone, electrostatic precipitator, wet scrubber or fabric filter or to the chimney. Fixing of feed rate :- For sending raw mix to sinter machine for sintering, fixation of feed rate of materials is done considering capacity of the sinter machine and quality requirement of blast furnace. Feed rate fixed is : Iron ore fines 250 T/hr. Flux 75 T/hr. ( Feeding of Flux depends on available lime in sinter required in blast furnace.available lime means CaO SiO 2 in sinter.) Coke 20 T/hr. Waste materials 20 T/hr. Sinter return 60 T/hr. Lime dust 02 T/hr. Page 18 of 32

19 Table: Indian sintering plant and their performance Steel plant Rated capacity Mt/year Sintering area m 2 x no. of strands Suction w.g. mm Bed height mm Sinter production t/m 2 /hr 1. bokaro /312 x bhilai x Rourkela x Durgapur x2 and x 1 5. Tata x 2 and 1000 and steel 192 x VSP % sinter in B.F., Burden 9. Flow chart of sintering:-

20 Sinter machines Sinter machines are of two types i) circular and ii) Straight line. Straight line Machines are also being known as Dwight and Lloyd machines. Dwight and Lloyd constructed the first continuous sinter plant in 1906.Circular sinter machines are normally suitable for blast furnaces having useful volumes of 650 Cu m and less.. Subject unit 12 Sqm 17 Sqm 25 Sqm 33 Sqm machine machine machine machine Annual Production 1000 tpa Total Power requirement Land needed for plant Kw Sqm Land needed for Sqm building Table: main parameters of circular sinter machine Various features of the circular machines are as below. When compared with the straight line machines the capital investment costs are low and the construction periods are short. Page 20 of 32

21 Sealing is better and air leakage is less in these machines since the wind boxes move synchronously with grates and since water sealing is adopted. Discharging system makes the size of cold sinter such that there is no need of an additional crusher. The circular machines are having high operational flexibility. Straight line machine: Straight line machines are normally used for large sinter plants. Present straight line machines are installed having widths from 2 meters to 5 meters and with effective sintering areas from 200 to 600 Sqm. The productivity of such machines typically ranges from 30 to 46t/Sqm/day. Capacities of such machines range from 190,000 tons per annum to 6.5 Mtpa. 10.Sinter Machine Specification for each machine Length - 78 M No. of pallets Sintering area 252M2 Bed height - 480mm Exhauster - 02 Nos. Page 21 of 32

22 Aspirator - 02 Nos. Cooler Blower- 06 Nos. Balling Drum - 02 Nos. Drum Cooler - 01 No. Straight line Cooler - 01 No. 11. TYPES OF SINTER Depending upon weather bases have been incorporated in the Sinter mix, sinters are divided into three broad classes: - (i) Non Fluxed OR ACID SINTERS: - Those where no flux is present or is added in the ore. (ii) BASIC SINTER OR Self Fluxing SINTER: - Those where sufficient flux has been added in the sinter mix to provide a basicity that is desired in the final slag, taking into consideration only the burden acids. An extra flux is added to the BF burden, to take care of coke ash acids. (iii) SUPER BASIC OR SUPER FLUXED SINTER: - In these type of sinters an additional flux is added to the mix to provide for Page 22 of 32

23 the desired final slag basicity, taking into account the acids content of both ore as well as the coke ash. 12. EFFECT OF IRON ORE FINES SIZE ON SINTERING: Sl.No Size Lime %yield VSS Prod. T.I (mm) (Kg/t) (+5mm) mm/min t/m 2 /h % QUALITY ASPECT OF SINTER-WITH RESPECT TO BLAST FURNACES PERFORMANCE CHEMICAL 1. Fe% in Sinter 2. CaO % in Sinter 3. SiO2 % in Sinter 4. MgO% in Sinter Page 23 of 32

24 5. Al2O3 % in Sinter 6. FeO % in Sinter 7. K2O % in Sinter PHYSICAL 1. SINTER SIZE ANALYSIS in terms of Cum+10mm and - 5mm 2. TUMBLER INDEX 3. SHATTER INDEX 4. RDI (Reducibility Degradation Index) 5. RI (Reducibility Index) 14. FACTORS AFFECTING SINTER QUALITY (1) Size of The Charge Mix: The strength of sinter is directly related to the size distribution of the charge mix. If size is large, the contact area will be less and the strength of the sinter will be low and conversely if size is too small the contact area of particles will be large and the strength will be high. Page 24 of 32

25 Ideal size of ore Fines -10 mm to mesh Coke breeze -3.2 mm 85% Flux mm 85% (2) Fuel content: - Variation in Fuel content in Charge Mix affect the peak Temperature attained during sintering, the combustion zone will not be uniform leading to poor bed permeability, This increases return fines generation (3) Moisture: - The presence of moisture in the Charge mix has several advantages. It maintains proper permeability in the bed during sintering. This is beneficial from the point of view of heat transfer during sintering. (4) Re-circulating load or Return fines addition: - For higher output of the sinter strand the circulating load should be low. A low circulating load however, reduces the permeability of the bed. An optimum-circulating load is established for maximum output of the acceptable sinter to the Blast Furnaces. 15. Parameters Controlling Sintering Process Page 25 of 32

26 Fuel content for heat input Ignition intensity- Temperature of Ignition Hood Furnace Moisture content of mix to control its permeability. Machine speed control to obtain complete Burn through Return Fines Addition Waste Gas Temperature Sintering Temperature or Burn through Temperature Pressure drop across the Sinter Bed- Main Suction Bed Height Calcined Lime addition- to improve bed Permeability. 16. SINTERING TECHNOLOGY IMPROVEMENTS High intensity mixing and nodulizing Ignition furnace for optimum maintenance and operation Travelling Grate with longer lasting pallets Minimized off-gas volumes with effective sealing. Reduction in off-gas cleaning capacity through Selective Waste Gas Page 26 of 32

27 Recirculation Discharge station for long service life Direct charging to cooler with maintenance-free cascade classifier for natural segregation and improved cooling efficiency and reduced emissions Efficient sinter cooling and installation of heat recovery system Energy savings together with reduced emissions Sinter plant control systems Economics Typical figures indicating capital cost of setting up of a sinter plant are shown below : Civil work % of total cost Foundation 10 Buildings 23 electricals 15 sinter machine 16 sinter cooler 7 Blower, apron etc. 7 RMP Equipment 10 Gas main, blunker etc. 7 Page 27 of 32

28 Miscellaneous 5 Total 100 The operating cost-breakdown is typically as follows: Wages 30% Repairs maintainance, Supplies, utilities etc. 50% Transportations & general Survices 5% Fixed expense 15% Total ADVANTAGE OF SINTER i) Agglomeration of fines into hard, strong and irregular porous lumps which gives better bed permeability. ii) Elimination of % of sulphur and Arsenic (if present) during sintering. iii) Elimination of moisture, hydrated water and other volatiles on the sinter strand with a cheaper fuel. iv) Increased the softening temperature and narrowing down of the softening range. Page 28 of 32

29 v) As the calculation of flux takes place in sinter strand, super-fluxing saves much more coke in the furnace. vi) It increases the Blast Furnace productivity. vii) Lime rich bosh slag hinders reduction of silica, absorbs vaporized silicon and sulphur to produce low- Si, low-s iron. viii) Increase of sinter percentage in Blast Furnace burden, increases the permeability, hence reduction and heating rate or burden increases, so the productivity also. ix) Utilization of solid wastes generate within steel works. 18. DISADVANTAGE OF SINTER i)100% sintered (iron ore) can not be charged in the blast furnace. By sintering one can not create uniform sizes. 19. FUTURE OF SINTERING The future of sintering and pelletising is directly related to the future of the iron making processes that use sinter and pellets, mainly the Blast furnace and DR process. Page 29 of 32

30 The future of these processes in turn depends upon steel production and consumption, which have been on an upward trend. Environmental interest Commitments to minimise CO2 emission mainly by striving to minimise overall energy consumption. 20. CONCLUSIONS: Indian economy is growing at 8-9% currently and likely to grow at rates >10%. Steel demand and supply will grow in the same way riding piggyback on the growing infrastructure. India is likely to develop a steel making capacity of MTPA by the year 2020 and around 400 MTPA by the year Page 30 of 32

31 Since good iron ore deposits are depleting fast beneficiation technologies will have to be adopted to meet iron ore demand. Agglomeration technologies such as pelletisation and sintering will have to be added to Indian steel plants so that concentrates can be used and the agglomerated products used in iron making to produce iron and steel economically and in eco-friendly way. Existing sinter plants need to be upgraded to use concentrate. The Agglomeration technologies are constantly being upgraded to meet stringent environment standards. The same need to be incorporated in existing units to make these more eco-friendly. The end. Page 31 of 32

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