Grate-Kiln Process - Iron Ore Pelletising The production of Iron ore pellets is a complex business measured in millions of tons of production. Pellet plants are built to meet any or a combination of criteria like Captive / Merchant , Magnetite / Hematite or/and Blast furnace type / DRI type. Like all businesses, Iron ore Pellet producers have to deliver higher quality at a lower cost. Iron ore palletizing has been continuously improving where every penny saved is multiplied across millions of tons per year. There are two major processes for the production of Iron ore pellets: Straight Grate and Grate-Kiln. This paper focuses on Grate-Kiln process and the changes that have contributed for increased availability, lower operating cost and higher quality in the Grate-Kiln pelletizing process besides briefly covering topics on current and future trends. Original Design Iron ore pelletizing was originally developed by Allis Chalmers in the USA (a predecessor company to Metso) utilizing Straight Grate technology (SG). Allis Chalmers subsequently developed Grate-Kiln technology from the Lepol Grate-Kiln system that was originally designed for cement production. The main aim was to use Coal as principal fuel and higher quality of pellets. The SG technology was licensed to A.G.McKee (now Aker Kvaerner) where as Allis Chalmers retained and improved Grate-Kiln technology. The first Grate-Kiln Iron ore pelletizing plant was installed for Cleveland Cliffs Company at Humboldt, Michigan mines, USA in 1960. Since then 51 other Grate-Kiln plants have been built all over the world and the 52nd is under construction. The total installed capacity of Grate-Kiln plants exceed 122 million tons per year (year 2004) compared to two other SG plant suppliers, each with an installed capacity of about 82 million tons a year (Year 2004 figures). Advantages of Pellets as Feed Stock The process objective is to transform Iron ore concentrate into pellets suitable to feed Blast Furnace or Direct Reduction plant or Corex. With high-grade Iron ore rapidly depleting, the necessity of Iron ore beneficiation is becoming a reality. Beneficiation of Iron ore is much more economical than getting rid of gangue at higher end of value chain like in BF or BOF / Electric furnace. We believe that it will cost 5 to 10 times more to remove gangue in BF / BOF or Electric arc furnace than in Beneficiation plant. In order to beneficiate the Iron ore, we need to crush and grind the Iron ore to the liberation size to maximize the benefits from the process. Typically Iron ore is liberated when the ore is ground to sub 45 microns size. Metso Minerals has extensive knowledge and expertise in Beneficiation of Indian Iron ore and has supplied India’s largest Hematite ore beneficiation plant. Pelletization was invented to make use of Blue dust and ultra fine concentrate generated in the Iron ore beneficiation plants. Pellets have the benefit of lower gangue on account of beneficiated ore. They also produce fewer fines during transportation, in the reduction furnaces like BF/Corex and DRI besides their high reducibility. There are many Blast furnaces and DRI plants, running with 100% pellets prove that benefit from pellets offsets the costs. Process Brief Description Concentrate / ground Iron ore of typical size 80% sub 45 microns are required to be at 9% moisture. Suitable binder (Bentonite) or organic binders are added to the concentrate and are thoroughly mixed in High intensity mixer. Sometimes Coal or Coke breeze is added to the concentrate prior to mixing to bring down the fuel consumption.
Hardened pellets result from heating green balls to a temperature between 1290C to 1340C. With sufficient retention time at this temperature, the green ball moisture bonds are replaced with mineral bridges and slag bonds that enhance the pellet’s physical properties such that stockpiling, shipping and subsequent processing can be achieved with minimum fines generation. In some cases this treatment (Induration) causes certain chemical reactions to occur that change pellet’s specific metallurgical properties. These reactions may include the oxidation of magnetite and dehydration of earthy hematites; in many cases “fluxed pellets” are produced by additions of limestone, dolomite, silica, etc. to the balling feed. These additions react with the gangue in the iron ore to enhance the performance of the pellets in certain downstream processing steps. The GK System consists of three machines in series; a traveling grate, a rotary kiln and an annular cooler. Overall, the GK System is a countercurrent gas/solids heat exchanger. Each machine in a GK system can be independently controlled. For example, the speed of the grate can be slowed to allow more drying time of the green balls. The kiln speed can then be modified to keep a constant residence time for proper induration. In a SG, a change to increase the drying changes the induration time because the SG is one connected conveyor. This ability to independently control the three processes (drying/preheat, induration, cooling) is what gives the GK system it’s well-known fast start-up times. It is also why a plant can switch from processing magnetite to hematite in only a few hours, never stopping production. For an operator looking to process a variety of ores during the operating life of the plant, the GK flexibility is vital. Travelling Grate The traveling grate is used primarily to dry and preheat green balls for feeding into the rotary kiln, where they are indurated. The traveling grate provides the means for efficient heat transfer with high, medium and low temperature gases. These gases transfer heat by convection so intimate gas solids contact is required for effective heat transfer. Dried green balls do not have the physical properties necessary to survive direct feeding to the rotary kiln and must be semi-indurated. For acceptable semi-induration, it is necessary for green balls to be substantially heated and fluxstone carbonates (if used) to be substantially calcined. Good gas-solids contact is required to cause bonding to occur and to remove CO2 so that calcination of the fluxstone can proceed at reasonable rates. This semi-induration or preheating is accomplished by flowing the gases exiting the annular cooler and rotary kiln through the bed of dried balls in the tempered preheat and preheat zones respectively of the traveling grate. Rotary Kiln The rotary kiln is a downwards-sloping cylinder from the traveling grate to the annular cooler. The speed of rotation controls the rate of solids flow through the rotary kiln and imparts a mixing action to the pellet burden. The mixing action is important because it creates a homogeneous pellet product. All of the
pellets are exposed to the burner flame for an equal amount of time. This residence time is sufficient to uniformly indurate all pellets, thereby minimizing the ball-to-ball quality differences inherent in pellets discharging from the static bed processing typical of a SG. At the discharge end of the rotary kiln is a single burner, which utilizes the hot recuperated (secondary) air from the annular cooler for combustion. The burner can be designed to fire a single fuel or a combination of fuels. Oil, gas (LBG/Coke oven gas/ BF gas/ Corex gas) or coal can be used singly or in varying mixtures. Because of the single burner, controlling the induration process is a simple temperature loop. Grate-Kiln plant processing Hematite ore may have few additional burners in pre heating zone. This is very different from a SG system that may require up to 50 burners. Annular Cooler The annular cooler is functionally the same as the traveling grate except for its annular configuration. Hot pellets discharging from the rotary kiln are distributed in the annular cooler as a level bed. Ambient air is forced upwards through the conveying elements (pallets) and the bed. Thus machine parts are not exposed to high temperatures. The pellets are leveled in the annular cooler to a bed depth of 660 mm and conveyed over upto 4 cooling zones. In each cooling zone, sufficient cooling air is provided to produce the mass of air at a temperature required by the rotary kiln and traveling grate. The various cooling zones are designed in such a way that they will recover the maximum heat from the hot pellets and various ducts carry this hot air to rotary kiln/ traveling grate zones. Cooled pellets discharge through the cooler’s discharge hopper at a controlled rate to a product load-out system. Advantages of Grate-Kiln System Pellets Quality: Straight Grate has static bed and firing is done only at the top of the bed, leaving over fired pellets at the top of the bed and under fired pellets at the bottom of the bed. This can be easily verified by CCS range of any SG plants. Grate-Kiln has kiln which tumbles the pre heated pellets very slowly and exposes the whole lot of pellets to uniform heat, which results in excellent product quality. Tumbling won’t generate any significant fines as pellets coming out of pre heating zone will have sufficient strength and kiln rotates at the speed of 1.5 to 2 rpm which is too slow for any significant dust generation. The pellets produced by Grate-Kiln have nearly nil percentage of pellets under 100 kg/P CCS compared to 5 to 8% pellets from Straight Grate. These weak pellets generate fines in the BF/ DRI and bring down the gas permeability resulting in lower productivity. To minimise these weak pellets, SG operators over fire the pellets at the cost of higher fuel consumption. Good reducibility and sufficient mechanical strength during reduction are two most important properties demanded from pellets by the BF/DRI/Corex operators. Good porosity and low swelling index, close size range of fired pellets are the derivatives of the above two requirements.
Power consumption: Due to shallow bed on the Grate and with no gas flow obstruction in the kiln, Grate-Kiln power consumption is almost 40% less in Induration section of the pellet plant compared to similar sized SG plant. SG plants generally have a fan power consumption of 30 kWh/ton against GrateKiln power consumption levels of 18 kWh/ton. By virtue of their moist characteristics permeability of the Green balls is pretty low. In Grate-Kiln system, Grate-Kiln considers 150 to 175mm of Green balls bed depth where as SG design considers a Green balls bed depth of 400 to 450mm. The Process fans need more power to suck or push the air through higher bed depth resulting in much higher fan power requirement. The fan power requirement is proportional to the increase in bed height. Fuel: Grate-Kiln plants generally consume about 5 to10% less fuel due to non usage of hearth layer, no excess false air leakages owing to high pressure drops across the bed, no inefficiencies out of multiple smaller burners. But in reality, fuel cost could be to the tune of 40% of SG plant due to usage of Coal even if imported. SG plants can’t fire coal due to smaller burners where as Grate-Kiln can fire coal in the kiln burner. Fuel accounts for apprx. 50% of production cost in Indian Pellet plants and every penny saved counts. Maintenance cost: Grate-Kiln has lower maintenance cost compared to SG plant as SG plants use expensive grate bars and pellet cars to withstand high Induration temperatures. The alloy used in Pallets and Grate bars of Straight Grate plants are of premium grade to withstand higher thermal shock and load. Their replacement cost is much higher than the components used in the Grate chain of Grate-Kiln plant exposed to less severe conditions. SG plant's grate bar typical consumption is 20gm/ton against 8gm/ton of Grate-Kiln. It has been observed that SG plant has about 1/2 USD per tone more maintenance related cost compared to Grate-Kiln plant due to higher wear parts consumption. Test Center: Metso Minerals Pyro Division operates the Process Research & Test Center (PRTC) in Danville, Pennsylvania, USA and is perhaps the only Pellet plant supplier having inhouse test facility. The PRTC is a fully equipped facility with the capabilities to perform complex material and process testing and evaluations, as well as simulating a complete pilot flow sheet. For iron ore pelletizing this includes mixing and balling, small and large Pot-Grates, and rotary batch kiln to develop the process parameters for the Grate-Kiln System for specific ores, concentrates and product specifications. The Metso PRTC can also assist the Buyer later with new process conditions should there be any major change in the feed materials or end product specifications. Current Trends IN Grate-Kiln Plants Increased plant capacity: It is a universally accepted logic that higher the plant capacity lower is the operating cost. With consolidation of players in minerals globally, the plant size has gone up and Metso is offering 6 Million-Ton Per Year in Hematite and 7 Million-Ton Per Year in Magnetite.
CFD designs: Traveling grate and grate manifold design have benefited from computational fluid dynamics analysis (CFD). This analysis has helped to reduce pressure drop in the system reducing power requirement. Balling drum size: Size has been increased to 5m diameter giving an output of over 200 TPH of net green balls per drum. This has resulted in lesser balling drums for given plant capacity. Higher availability: Current plant designs are for 330 days of operations in a year. Many Grate-Kiln plants operate more than 8000 hours a year. Use of specialty alloys: The quality of alloys used for Grate parts, annular cooler parts and their machining tolerances have gone up leading to higher reliability and reduced maintenance cost. Fuel reduction: Metso’s patented computerised H&M balance has been developed in close interactions with existing high efficiency Grate-Kiln plant operators. Decades of Grate-Kiln plant running experience are coupled with theoretical design knowledge to produce Industry’s best thermal modeling. VFD drives for the fans: The improvement in the capacity and the reliability of Variable Frequency Drives makes them suitable for pellet plant operations. They are expected to save 20% power and help to achieve exact control of process. Future Developments Predictive Control Systems: OCS software developed by Metso Cisa, combines expert system, fuzzy logic, adaptive model structure, neural networks, statistic functions, etc. If incorporated OCS software can predict the pellet qualities in advance or can produce desired pellets meeting desired norms like fuel consumption, pollution norms etc. OCS software optimises the plant peformance on a continuous basis, correlates changes continuously and makes corrections online. VisioPelletTM : Metso has a presence in Iron ore value chain from crushing to pelletization and understands the importance of every activity on subsequent stages. In pelletization Green pellet size distribution and sphericity are essential for final pellet quality and grate productivity. VisioPelletTM measures green pellet size distribution and •"sphericity" and automatically adjusts the Balling drum/ disc speed or feed rate to achieve the desired size distribution and sphericity, resulting in consistent green balls to Induration. CONCLUSION The Grate-Kiln system provides the lowest cost method of making high quality pellets. High availability, low fuel including that of Coal burning capability, low power use and low maintenance costs make it the system of choice for Iron ore pellet producers and users be it Hematite or Magnetite ore.
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GK Sistem terdiri dari tiga mesin di seri; perapian bepergian, rotary kiln dan pendingin annular. Secara keseluruhan, Sistem GK adalah gas / padat penukar panas lawan.
Setiap mesin dalam sistem GK dapat dikontrol secara independen. Misalnya, kecepatan perapian dapat diperlambat untuk memungkinkan waktu pengeringan lebih bola hijau. Kecepatan kiln kemudian dapat dimodifikasi untuk menjaga waktu tinggal konstan untuk indurasi yang tepat. Dalam sebuah SG, perubahan untuk meningkatkan pengeringan perubahan waktu indurasi karena SG adalah salah satu conveyor terhubung. Kemampuan untuk mandiri mengontrol tiga proses (pengeringan / pemanasan awal, indurasi, pendinginan) adalah apa yang memberi sistem GK itu kali start-up cepat terkenal. Hal ini juga mengapa tanaman dapat beralih dari pengolahan magnetit ke hematit hanya dalam beberapa jam, tidak pernah berhenti produksi. Untuk operator yang ingin mengolah berbagai bijih selama umur operasi pabrik, fleksibilitas GK sangat penting.
Keuntungan dari Sistem Grate-Kiln
Pelet Kualitas: Parut Lurus memiliki tempat tidur statis dan pembakaran dilakukan hanya di atas tempat tidur, meninggalkan lebih dari menembakkan pelet di atas tempat tidur dan di bawah pelet dipecat di bagian bawah tempat tidur. Hal ini dapat dengan mudah diverifikasi oleh CCS berbagai setiap tanaman SG. Parut-Kiln memiliki kiln yang Tumbang pelet dipanaskan pra sangat lambat dan memperlihatkan seluruh banyak pelet panas seragam, yang akan menghasilkan kualitas produk yang sangat baik. Tumbling tidak akan menghasilkan apapun denda signifikan pelet keluar dari zona pemanasan pra akan memiliki kekuatan yang cukup dan kiln berputar pada kecepatan 1,5 sampai 2 rpm yang terlalu lambat untuk setiap generasi debu yang signifikan. Pelet yang dihasilkan oleh ParutKiln memiliki persentase hampir nihil pelet di bawah 100 kg / P CCS dibandingkan dengan 5 sampai 8% pelet dari Grate Lurus. Maskapai pelet lemah menghasilkan denda di BF / DRI dan menurunkan permeabilitas gas sehingga menurunkan produktivitas. Untuk meminimalkan ini pelet yang lemah, operator SG atas api pelet pada biaya konsumsi bahan bakar yang lebih tinggi.
Reducibility yang baik dan kekuatan mekanik yang cukup selama pengurangan adalah dua sifat yang paling penting yang dituntut dari pelet oleh BF / operator DRI / Corex. Porositas yang baik dan indeks pembengkakan rendah, jarak dekat ukuran pelet dipecat adalah turunan dari atas dua persyaratan.
KESIMPULAN Sistem Parut-Kiln menyediakan metode biaya terendah pembuatan pelet berkualitas tinggi. Ketersediaan tinggi, bahan bakar rendah termasuk yang terbakar kemampuan Coal, penggunaan daya rendah dan biaya perawatan yang rendah membuat sistem pilihan bagi produsen bijih besi pelet dan pengguna baik itu hematit atau magnetit bijih.
The Grate Kiln Process dikembangkan oleh Allis Chalmer dan pabrik pertama pada teknologi ini dibangun pada tahun 1960 Dalam proses Parut-Kiln (Gambar 3) grate bepergian digunakan untuk mengeringkan dan memanaskan pelet. Bergerak Material lurus perapian perjalanan sampai ia mencapai suhu 800 ° C sampai 1000 ° C. Setelah itu dipindahkan ke refraktori berjajar rotary kiln untuk indurasi mana suhu terus ditingkatkan up-to 1250 - 1300 ° C. Pada 800 ° C, Magnetite besi Bijih akan dikonversi menjadi Fe2O3 dalam reaksi eksotermis. Panas yang dibebaskan mengeras bola hijau yang bermanfaat untuk menahan dampak jatuh di rotary kiln. Sebuah pendingin melingkar digunakan untuk pendinginan pelet dipecat.
Perbandingan dua proses diberikan dalam Tabel 1 Tabel 1-comarison antara Lurus Traveling Grate (STG) dan Grate Kiln (GK) Proses
Sl Tidak Lurus Traveling Grate (STG) Proses Grate Kiln (GK) Proses
1 Pengeringan, pemanasan awal, Indurasi dan siklus pendinginan dilakukan dalam satu unit tunggal Pengeringan, pemanasan, dan siklus pendinginan Indurasi dilakukan di unit yang berbeda 2 Green pelet tetap tidak terganggu selama proses Seluruh proses berlangsung dalam tiga peralatan yaitu bepergian grate, rotary kiln dan lebih dingin melingkar maka perpindahan pelet berlangsung. 3 mobil Parut bergerak pada kecepatan yang sama di zona pengeringan, indurasi dan pendinginan. Setiap gangguan dalam satu zona mempengaruhi zona lain kontrol Independen tiga zona maka memiliki fleksibilitas operasional yang lebih baik 4 Denda generasi diabaikan karena tidak ada pengalihan bahan Sejak transfer material terjadi di beberapa tempat generasi maka lebih tinggi dari denda 5 Tidak ada persyaratan kekuatan produk setengah Sebelum transfer ke Kiln pelet hijau harus cukup mengeras 6 Ketersediaan Proses lebih tinggi ketersediaan proses Bawah 7 konsumsi energi yang lebih tinggi spesifik rendah konsumsi energi spesifik
8 Lesser Pemeliharaan Higher Pemeliharaan Pembentukan debu 9 generasi debu Rendah Lebih Tinggi 10 biaya investasi yang lebih tinggi biaya rendah investasi 11 Cocok baik untuk hematit dan magnetit bijih Proses lebih cocok untuk bijih magnetit ..
