Understanding Pellets and Pellet Plant Operations


Understanding Pellets and Pellet Plant Operations

Pelletizing is an agglomeration process which converts very fine grained iron ore into balls of a certain diameter range (normally 8mm to 20 mm, also known as pellets. These pellets are suitable for blast furnace and direct reduction processes. Pelletizing differs from sintering in that a green unbaked pellet or ball is formed and then hardened by heating.

Iron ore pellets  can be made from beneficiated or run of mine iron ore fines. Lean iron ores are normally upgraded to a higher iron ore content through beneficiation.  This process generates iron ore filter cake which needs to be pelletized so that it can be used in an iron making process.  Also during the processing of high grade iron ores which do not need beneficiation, generated fines can be pelletized and used instead of being disposed of.

Pellet plants can be located at mines, near ports or can be attached to steel plants. Equipped with advanced environmental technology, they are virtually pollution free, generating no solid or liquid residues.

History of pelletization

The history of pellets began in 1912 when A.G.Andersson, a Swede, invented a pelletizing method. The commercial use of pellets, however, began in the USA after World War. Various studies were conducted in USA with the aim of developing the vast reserves of taconite (a low grade iron ore) in the area around the Great Lakes. The process of enriching taconite ore involved grinding the ore to remove gangues and upgrading the iron ore (i.e., an ore beneficiation process). The resultant high grade ore is in the form of fine particles, as small as 0.1 mm or less, which are not suitable for sintering. This issue led to the development of the pelletizing process.

In 1943, Dr. Davis, a professor at the University of Minnesota, Mines Experiment Station, and his associates invented a method for processing taconite containing low grade iron ore. Their invention showed that it was possible to ball or pelletize fine magnetite concentrate in a balling drum and that if the balls were fired at sufficiently high temperature (usually below the point of incipient fusion) a hard, indurated pellet well adapted for use in the blast furnace, could be made. Consequently, despite the unquestioned benefits of sinter on blast furnace (BF) performance, intense interest in the pelletizing process had developed because of the outstanding performance achieved by steel plants in extended operations with pellets as the principal iron bearing material in the blast furnace burden.

Pelletizing plants are expected to play an important role in an era when the global reserve of high grade lump ore is shrinking. The plants promote the concentrating of low grade iron ores into upgraded pellets, which will be increasingly used by blast furnaces and direct reduction furnaces in coming years.

Iron ore pellets

The iron ore pellets may be acid or basic pellets. Acid pellets are also called as DRI (direct reduced iron) grade pellets while basic pellets are also known as BF grade or fluxed pellets.

  • DRI grade pellets – Basicity of these pellets is usually less than 0.1. The fired pellet strength is, to a certain degree, due to the hematite bridges of polycrystalline structure. These pellets normally have large volume of open pores. The reduction gas quickly penetrates through these pores into the pellet core and simultaneously attacks the structure in many places. This results into an early structural change which begins at low temperatures over the entire pellet volume.
  • BF grade pellets – Basicity of these pellets is greater than 0.1 and can vary. Basicity of normal basic pellets range from 0.1 to 0.6 and have low CaO percentage. During the firing of these pellets, a glassy slag phase consisting of SiO2, CaO, and Fe2O3 of varying percentage is formed. Due to increased flux addition, there is formation of some slag and due to it, there is to a certain extent slag bonding with iron ore crystals. High basicity pellets have a basicity level greater than 0.6. These pellets contain higher level of CaO. These pellets not only have glassy phase consisting mainly of SiO2, CaO, and Fe2O3, but also calcium ferrites (CaO.Fe2O3). During firing of these pellets, the availability of CaO considerably favours the crystal growth of hematite. These pellets normally have a high mechanical strength after pellet firing. Fluxed pellets exhibits good strength, improved reducibility, swelling and softening melting characteristics. Because of these properties these pellets give better performance in the blast furnace.

Quality of the pellets is influenced by the nature of the ore or concentrate, associated gangue, type and amount of fluxes added. These factors in turn result in the variation of physicochemical properties of the coexisting phases and their distribution during the pellet induration. Hence properties of the pellets are largely governed by the form and degree of bonding achieved between the ore particles and the stability of these bonding phases during reduction of iron oxides. Since the formation of phases and microstructure during induration depends on the type and amount of fluxes added, there is an effect of fluxing agents in terms of CaO/SiO2 ratio and MgO content on the pellet quality.

Mineralogically pellets comprise essentially hematite (original surviving) particles of iron ore, crystalline silica (quartz, cristobalite and tridymite) and forsterite (Mg2SiO4). The principle variation in pellet mineralogy is in the proportion of gangue phases present in the product. These will vary depending upon the pellet feed material and the type and the amount of any additives to feed such as limestone, dolomite, olivine and bentonite etc.

The strength of iron ore pellets is important in minimizing degradation by breakage and abrasion during handling and shipping, and in the blast furnace. Strong bonding in pellets is believed to be due to grain growth from the accompanying oxidation of magnetite to hematite, or recrystallization of hematite. Although slag bonding may promote more rapid strengthening at slightly lower firing temperatures, pellet strength is normally decreased, especially resistance to thermal shock. Pellet strength is most commonly determined by compression and tumble tests. Compressive strengths of individual pellets depend upon the mineralogical composition and physical properties of the concentrate, the additives used, the balling method, pellet size, firing technique and temperature, and testing procedure. The compressive strengths of commercially acceptable pellets are usually in the range of 200 to 350 kg for pellets in the size range of  9 mm to 18 mm. In the tumbler test 11.4 kg of +6 mm pellets are tumbled for 200 revolutions at 25 rpm in a drum tumbler(ASTM E279-65T) and then screened. A satisfactory commercial pellet should contain no more than about 5 % of  minus 0.6 mm (minus 28 mesh) fines, and 94 % or more of  plus 6 mm size, after tumbler testing. A minimum of broken pellets between 6 mm and 0.6 mm in size is also desirable. Other important properties of the pellets to be used for blast furnace feed are reducibility, porosity, and bulk density. With some concentrates these can be varied within certain limits.

Pelletization process

A pelletizing plant normally has four process steps namely (i) receipt of raw materials, (ii) pretreatment, (iii) balling, and  (iv) induration and cooling. These process steps are described below.

Receipt of raw materials

The location of a pelletizing plant affects the method of receiving raw materials such as iron ore, additives and binders. Many pelletizing plants are located near iron ore mines. This is because these plants are installed to pelletize the iron ores which are beneficiated at these mines. Such plants receive the iron ore by rail and/or slurry pipelines. Many other pelletizing plants are installed away from the iron ore mines. These plants are independent of iron ore mines. These plants receive iron ore mostly by rails. some plant may receive by long distance slurry pipeline. In pelletizing plants located at port which are dependent on imported iron ore, the receiving method involves the transportation of the ore in a dedicated ship, unloading the ore at a quay and stockpiling it in a yard. Iron ore is usually shipped for such plants in bulk for maximum economy.

Pretreatment process

In the pretreatment process, the iron ore is ground into fines having sizes required for the subsequent balling process. The pretreatment includes concentrating, dewatering, grinding, drying and pre-wetting. Generally low grade iron ores are ground into fines for enriching the quality of the ore, for removing gangues containing sulfur and phosphorus, and for controlling the size of the grains. In the case of magnetite ores, magnetic separators are employed for upgrading and gangue removal. On the other hand , with hematite ores, these operations are accomplished by gravity beneficiation, flotation, and/or wet-type, high intensity magnetic separators. The grinding methods can be categorized roughly as per the following three aspects.

  • Dry grinding or wet grinding
  • Closed circuit grinding or open circuit grinding
  • Grinding in single stage or grinding in multiple stages

These methods are used in combination depending on the types and characteristics of the iron ores and the mixing ratio, as well as taking into account the economic factors. Wet grinding systems need dewatering units with a thickener and filter, while dry grinding systems requires pre-wetting units. Pre-wetting is usually associated with dry grinding. Pre-wetting includes addition of an adequate amount of water homogeneously into the dry-ground material to prepare pre-wetted material suitable for balling. This is a process for adjusting the characteristics of the material that significantly affect pellet quality. Occasionally, the chemical composition of the product pellets is also adjusted in this process to produce high quality pellets.

Binders, such as bentonite, clay, hydrated lime or an organic binder, are generally used to raise the wet strength of green balls to more acceptable levels for handling. Bentonite consumption at the rate of 6.3–10 kg per ton of feed is a significant cost element and adds to the silica content of the final product.

Addition of lime and/or dolomite to the ore adjusts the pellets so as to have the target chemical composition.

Considerable efforts have been made for the reduction of the bentonite usage and for the development of cheaper substitutes. The ballability and strength of green balls are influenced by the additives and by the moisture content and particle size distribution of the concentrates. Optimum moisture content for good balling is usually in the range of 9 % to 12 %. It appears that balling characteristics are relatively independent of the chemical composition of a concentrate, but are strongly affected by its physical properties. For example, specular hematites are more difficult to ball than magnetite concentrates because of the plate like structure of the specular hematite particles. In any case, satisfactory pellet formation is usually achieved by grinding to about 80 % to 90 % minus 43 micro meter ( minus 325 mesh). Normally, any material considered for pelletizing should contain at least 70 % minus 43 micro meter (minus 325 mesh) and have a specific surface area (Blaine) greater than 1200 sq cm/gram for proper balling characteristics.

Balling process

In this process, balling equipment produces green balls from the pre-wetted material prepared in the previous process. The balling drum and the disc pelletizer are the most widely used devices for forming green balls. Both of the units utilize centrifugal force to form the fine materials into spheroids.

The green balls produced by a drum are not uniform in diameter. A significant portion of the discharge (about 70 %) is smaller than target size and are usually returned to the drum after screening. It is difficult to adjust the drum operation for varying raw material conditions. The operation, however, is stable for uniform raw material conditions (chemical composition, particle size, moisture, etc.).

Compared with the balling drum, the disc pelletizer has the advantages of lighter weight and greater possibility for adjustment. Its inherent design averages out the effect of instantaneous fluctuations in the feed, whereas the drum cannot. The disc pelletizer classifies green balls by itself, reducing the amount of pellets returned. The classifying action of the disc promotes discharge of balls of more uniform size, which simplifies screening of the product. The operation of the disc pelletizer can easily be adjusted for varying raw material conditions by changing the revolution, inclined angle and depth of the disc. However, the capacity of the discs is low and discs generally require closer control than drums.

Best control of ball size is achieved when the balling device is in closed circuit with a screen to remove and recycle the undersize material. Both the drop and compressive strengths of green pellets are important.

Induration process

The firing of pellets establishes the binding of hematite particles at an elevated temperature ranging from 1250 deg C to 1350 deg C in oxidizing condition. Slag with a low melting point may form in the pellets during this firing step, if the raw material contains fluxed gangue, or if limestone is added to it. In these cases, the product may have an intermediate structure with both hematite binding and slag binding. The firing process is characterized by process temperatures lower than those required by sintering which requires partially melting and sintering fine ore mixed with coke breeze, a fuel which generates combustion heat.

Three systems are normally used for the induration of pellets. They are namely (i) vertical shaft furnace system, (ii) straight grate or travelling grate system, and (iii) grate – kiln cooler system. Each system has been used commercially to make acceptable quality pellets and thus, capital and operating cost factors are usually involved in  choosing one or the other system.

Oxidation of magnetite to hematite during pelletizing will provide a significant proportion, around 100 M cal per ton of the heat requirement in all of the systems. For pelletizing of hematites, the use of coke breeze (or some carbon source) in the pellet feed mixture has become a common practice to provide the additional indurating energy normally provided by magnetite oxidation.

Vertical shaft furnace system is the most traditional facility. However, vertical shaft furnaces are not as common as the traveling grate or grate kiln systems. There are several variations in shaft furnace design but the most common is the Erie type, shown in Fig. 1. Green balls are charged at the top and descend through the furnace at a rate of 25 to 40 mm per minute countercurrent to the flow of hot gases. About 25 % of the total air enters the furnace through the hot gas inlet at temperatures from 1280 deg C to 1300 deg C. Pellets in this zone of the furnace reach temperatures of 1315 deg C or higher because exothermic heat is released when the magnetite oxidizes to hematite, increasing the temperature. The remaining 75 % of the furnace air enters via the cooling air inlet. Pellets discharge at about 370 deg C, and the top gas temperature is around 200 deg C. Typical furnace capacities are 1000 to 2000 tons per day.

Shaft furnaces are more energy efficient than the traveling grate or grate-kiln systems. The shaft furnace is well suited for pelletizing magnetite, but not hematitic or limonitic ore materials. Disadvantages of shaft furnaces are low unit productivity and difficulty in maintaining uniform temperature in the combustion zone. Hot spots may occur which cause pellets to fuse together into large masses, producing discharge problems. It is also very difficult to produce fluxed pellets in a shaft furnace. Typical schematic diagram of vertical shaft furnace system is shown in Fig 1.

Schematic diagram of vertical kiln and stright grate kiln

Fig 1 Typical schematic diagram of vertical shaft and grate-kiln systems

 A straight grate system emerged in the industry soon after the shaft furnaces. It is essentially a modification of the sintering process. The green balls are fed onto the grate continuously to give a bed depth of around 300 mm to 400 mm and are dried in the first few wind boxes by updraft air recuperated from the firing zone, followed by downdraft drying using recuperated air from the cooler. This arrangement of hot air flows limits pellet damage resulting from condensation of moisture in the bed. Following drying, the pellets are preheated by downdraft air from the cooling zone. Firing is done downdraft in the combustion zone by burning fuel oil or natural gas with hot air from the cooling zone. The cooling zone follows the combustion zone and uses updraft fresh air.

The traveling grate system for producing pellets consists of a single unit which moves a static layer of pellets. The system has a simple structure for drying, preheating, firing and cooling pellets. Due to its relative ease of operation, along with ease of scaling-up, makes the system used by many plants.

Fuel consumption in the traveling grate system is about 85-140 M cal per ton of pellets produced from magnetite and up to 240 M cal per ton when pelletizing hematite. The system offers good temperature control in the firing zone. Pellet consistency throughout the bed may be achieved by recirculating some fired pellets to form hearth and side layers on the grate. The large grate machines are 4 m wide and are capable of producing more than 3 million tons of pellets per year. Circular grate machines have also been designed and are in operation. A typical schematic diagram of the straight grate system is shown in Fig 2.

Schematic diagram of travelling grate system

Fig 2 Typical schematic diagram of straight grate system

The grate-kiln system depicted in Fig 1 consists of a traveling grate for drying and preheating the pellets to about 1040 deg C, a rotary kiln for uniformly heating the throughput to the final induration temperature of 1315 deg C, and an annular cooler for cooling the product and heat recuperation. Heat for firing is supplied by a central oil, gas, coal or waste wood burner at the discharge end of the kiln. Hot gases produced in the kiln are used for downdraft preheating of the pellets. Hot air from the cooler is used to support combustion in the kiln and is also recuperated to the traveling grate for drying and tempering preheat.

The grate-kiln system offers excellent temperature control in all stages of the process and produces a consistently uniform quality pellet. Fuel consumption is 75 M cal to 100 M cal per ton of standard pellets produced when using magnetite ore, and up to 170 M cal per ton of standard pellets produced when the feed is hematite. These fuel consumption numbers increase by 60 M cal per ton when producing fluxed pellets. Power consumption, from balling to pellet load out, is around 23 kWh per ton.

The grate-kiln system is easy to control, and the product pellets have a uniform quality. It can also be scaled up to a fairly large degree. Grate-kiln systems can be designed for production capacities up to 6 million tons per year per line. These systems are used by many plants.

Pelletizing processes are being improved constantly. The production of self-fluxing pellets is an example of an innovation that has been accepted on a commercial scale and has led to major advances in blast furnace performance.  Other articles on pellets and pelletization process are available under following links.

http://ispatguru.com/iron-ore-pellets-and-pelletizing-processes/

http://ispatguru.com/iron-ore-pellets/

http://ispatguru.com/iron-ore-pellets-and-pelletization-process/