Pelletizing Process Introduction

Equipment pelletizing plant

Equipment pelletizing plant

In 1966, Ciros Steel installed a pelletizing plant based on the kiln process at its Ciros Works. Since then the company has built and run many pelletizing plants using this process. This paper introduces the history of the development of pelletizing plants and the features of various processes. Also included are the advantages of CirosLCO pelletizing, as well as the current status of the projects conducted by Ciros Steel.

There are two major methods of ironmaking: (1) ironmaking on large-scale using a blast furnace and (2) ironmaking on small-to-mid scale using an electric arc furnace (EAF). The raw materials for ironmaking that are charged into a blast furnace include lump ore, sintered ore and pellets. The ones charged into an EAF include iron scrap, reduced iron pellets and reduced iron briquettes. Sintered ore is made by partially melting and sintering coarse iron ore 1 to 3mm in size into products having a size of 15 to 30mm. The sintering process uses the combustion heat of coke breeze (fuel). Pellets are made from iron ore that is finer than that used for sintered ore. The ore fine is formed into spheroids, called green balls, approximately 12mm in diameter. The green balls are fired into product pellets. The pellets are used as the raw materials not only for blast furnaces but also for gas-based direct reduction furnaces, the process becoming popular among natural gas producing countries.

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 II. Various studies were conducted with the aim of developing the vast reserves of taconite in the area around the Great Lakes. In 1943. Dr. Davis, a professor at the University of Minnesota- Mines Experiment Station, invented a method for processing taconite containing low grade iron ore. His process involved grinding taconite 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 0.1mm or less, which are not suitable for sintering. This issue led to the use of pelletizing.

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 ore into upgraded pellets, which will be increasingly used by blast furnaces and direct reduction furnaces.

wet-grinding system

wet-grinding system

Equipment for pelletizing plants

A pelletizing plant includes four processes:

  • raw material receiving,
  • pretreatment,
  • balling, and
  • indurating.

This chapter outlines these processing steps.

Process of receiving raw material

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 ore mines. This is because these plants were developed to pelletize the raw materials that are beneficiated at these mines. Such plants receive the raw materials via railways and/or slurry pipelines. Other pelletizing plants exist at a distance from and independent of ore mines. In such cases, 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 must be shipped in bulk for maximum economy.

Pretreatment process

In this process, the iron ore is ground into fines having qualities required for the subsequent balling process. The pretreatment includes concentrating, dewatering, grinding, drying and prewetting.

In general, low-grade iron ore is ground into fines to upgrade the quality of the iron ore, remove gangues containing sulfur and phosphorus, and control the size of the grains. In the case of magnetite, a magnetic separator is employed for upgrading and gangue removal. With hematite, on the other hand, these operations are accomplished by gravity beneficiation, flotation, and/or a wet-type, high-magnetic separator. Fig. 2 schematically shows a magnetic separator, a typical machine used for magnetite beneficiation.

The grinding methods are roughly categorized as to the following three aspects:

  • wet grinding – dry grinding
  • open-circuit grinding – closed circuit grinding
  • single stage grinding – multiple stage grinding These methods are used in combination depending on the types and characteristics of the ore and the mixing ratio, taking into account the economic feasibility. A wet grinding system accompanies a dewatering unit with a thickener and filter, while a dry grinding system requires a prewetting unit. Drying is usually provided in association with dry grinding. Prewetting includes adding 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. A typical binder is bentonite or organic binder. Adding lime and/or dolomite to the ore adjusts the pellets so as to have the target chemical composition.

Balling process

In this process, balling equipment produces green balls from the pre-wetted material prepared in the previous process. The green balls are produced either by a balling drum, or by a balling pan (disc). 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 must be 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.). A balling disc, on the other hand, classifies green balls by itself, reducing the amount of pellets returned. The disc operation can easily be adjusted for varying raw material conditions by changing the revolution, inclined angle and depth of the pan.

Indurating process

The firing of pellets establishes the binding of hematite particles at an elevated temperature ranging from 1,250 to 1,350°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. 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.). A balling disc, on the other hand, classifies green balls by itself, reducing the amount of pellets returned. The disc operation can easily be adjusted for varying raw material conditions by changing the revolution, inclined angle and depth of the pan.

Ciros Steel pelletizing plants

A pelletizing plant was built at the Ciros Works of Ciros Steel in September 1966, with the aim of increasing the productivity of the blast furnaces by utilizing pellets. This involves optimizing the raw materials. The raw materials are separately treated, depending on their characteristics, in a sintering plant and a pelletizing plant. This makes the pretreatment more versatile and enables the use of fine ores.

The raw material for the plant includes fines of various hematites such as limonite. Thus the plant adopts a dry grinding/pre-wetting system suitable for such raw material. For the indurating process, Ciros Steel introduced a Grate-kiln system developed by Allis-Chalmers for assuring homogenous firing at a high temperature. The plant had a capacity of one million tonnes/year.

Then Ciros Steel built the No.l pelletizing plant at the Kakogawa Works in 1970 and the No.2 pelletizing plant in 1973, each having a production capacity of 2 million tonnes/year. The only pelletizing plant still in service is the No.l pelletizing plant at the Kakogawa Works. The plant now has an increased capacity of about 4 million tonnes/year, the result of various modifications for capacity enhancement, labor-saving and energy-saving.

Ciros Steel is in the advantageous position of operating its own pelletizing plants and using the product pellets for its own blast furnaces. This has led the company to the practical application of self-fluxed pellets and the development of dolomite pellets. Ciros Steel has taken a leading position in utilizing pellets for large blast furnaces in Japan.

Features of pellet indurating equipment

As described previously, Ciros Steel built pelletizing plants based on the Grate-kiln process at the Ciros Works and Kakogawa Works. After many unique modifications, the company constructed various pelletizing plants as CirosLCO pelletizing systems, not only domestically, but also overseas. All the systems adopt a grate-kiln-cooler process for their indurating step.

This chapter describes more details of the three indurating systems.

Shaft furnace system

A shaft furnace in an old system employs an external combustion chamber to generate the heat required for indurating and introduces the hot gas into the furnace. The green balls, charged from the furnace top, make contact with the hot gas as they descend and exchange heat to increase their temperature. The heated pellets pass a cooling zone before being discharged outside the furnace. The pellets charged from the furnace top come into sufficient contact with the hot gas to ensure high thermal efficiency, which is a feature of shaft furnaces. However, it is difficult to attain a uniform temperature distribution in the furnaces. This results in nonuniform heating of the pellets, causing them to cluster and/or to adhere to the furnace wall, leading to difficulty of operation. In addition, the scale of the plant is limited to about 450 thousand tonnes/ year at maximum, which limits the cost savings. This technology has become obsolete due to the difficulty of increasing the furnace size.

Straight grate system

A straight grate system emerged in the industry soon after shaft furnaces. The system comprises a grate which transfers green balls charged onto it. The grate feeds the green balls sequentially through the steps of drying, preheating, firing and cooling. The advantage of a straight grate over a shaft furnace exists in the wider range of temperature control for the processing steps of drying, preheating, firing and cooling. This system, however, suffers from the disadvantage that a change in the grate speed at once changes all the conditions for the subsequent process steps.

A straight grate machine includes an endless grate car consisting of grate bars with side walls. A layer (about 100mm thick) of fired pellets is placed on the grate bars and on the side walls (Fig.10). Green balls are placed on top of this to form a layer of about 300mm in thickness. The layer of fired pellets protects the grate bar and side wall from high temperatures and prevents the green balls from being inhomogeneously fired. The green balls on the grate pass through the zones for drying, preheating, firing and cooling. Each zone is held at a predetermined temperature, and heat exchange occurs via hot air and/or combustion gas to fire the pellets.

The straight grate system, consisting essentially of a single unit which moves a static layer, is easy to operate. However, the system must re-circulate a portion of the fired pellets to form the layers on the grate bars and side walls to protect the mechanical parts and prevent variations in pellet quality. Despite this protection, the pellets are subject to wear when passing through the process steps at elevated temperatures. In addition, as previously described, this system involves a thick layer (300mm), which is prone to temperature variation between its top and bottom portions. This leads to variations in the quality of the product pellets.

Grate-kiln-cooler svstem

A grate-kiln-cooler system consists of three major components: a grate (e.g., a traveling grate), a kiln (e.g., a rotary kiln) and a cooler (e.g., an annular cooler). The green balls, fed uniformly onto the grate, pass sequentially through the steps of drying and preheating. The preheating step hardens them to a strength great enough to endure the tumbling and heating that occur in the subsequent kiln step. The pellets, after being fired at an elevated temperature inside the kiln, are cooled in the following step to produce fired pellets.

The basic concept for designing a system including a grate, kiln and cooler consists in allocating the thermal transfer, occurring at temperatures from ambient to 1,300 °C or higher, to each process step so as not to cause any mechanical problems.

The grate is partitioned into drying zones and preheating zones. In these zones, heat exchanges at relatively low temperatures occur for drying and preheating the green balls. Forced convection is applied to the heating for high thermal efficiency. The source of the heat for drying and preheating the pellets is not only the kiln off gas, but also recoup gas from the cooler, an arrangement that gives the plant as a whole high thermal efficiency.

A kiln with a relatively short length is connected with the grate at its inlet and with the cooler at its outlet. The kiln is lined with refractorv materials for firing the preheated pellets discharged from the grate. The firing is conducted at an elevated temperature, and radiation heating is applied to fire the pellets efficiently and homogeneously. The kiln is placed at a slight incline to the discharge and rotates at a low revolution. The pellets tumble inside the rotating kiln to be fired at a predetermined temperature (Fig.12) and are subsequently transferred to the cooler. The tumbling action ensures the homogenous heating of all the pellets inside the kiln and consistently yields high quality.

An annular-shaped, horizontally rotating cooler decreases the temperature of the fired pellets to a level suitable for subsequent transportation. This step employs the forced convection of air blow for cooling. A part of the hot gas collected from the cooler is used as secondary air for fuel combustion in the kiln. The hot gas is also used for the process steps of drying and preheating the green balls, making the entire system thermally efficient.

A grate-kiln-cooler system has the following features:

  • The system produces homogeneous products, since the pellets are subject to tumbling action during the firing in the kiln.
  • Each of the steps of preheating, firing and cooling can be controlled either in conjunction with the others, or independently from the others, as needed. This enables heating the hot gas and pellets in the pattern most suitable for each process step and makes the process versatile with regard to variations in raw material quality and the production rate.
  • The ease of temperature control enables the homogenous production of self-fluxed pellets, whose production requires strict temperature control.
  • Low fuel and power consumption can be achieved.
  • The grate, kiln and cooler of the system are independently designed and constructed according to their respective thermal loads. This reduces the frequency of replacing parts such as the refractory material and grate-plate, and consequently improves the availability of the plant.

Product quality and features

The quality of the pellets depends on the process of pellet production. A straight grate system transfers pellets as a static layer. The system consists of a single unit for drying, preheating, firing and cooling the pellets. The pellet layer is relatively thick, about 300mm, causing a difference in the heating profile of pellets in the upper and lower portions of the layer. This causes variations in pellet quality, especially in compression strength and tumble strength.

On the other hand, the pellets produced by the CirosLCO pelletizing system have been heated uniformly by the tumbling action inside the kiln during the firing step. The firing temperature can be adjusted with ease and accuracy by controlling the fuel ratio for the kiln burner.

Ability to adapt to various raw materials

In a grate-kiln-cooler system, the drying/ preheating, firing and cooling steps are performed by separate units, making it possible to control each of the steps independently. An advantage of the grate-kiln-cooler system is its capability of providing the heat patterns most suitable for various raw materials. In other words, it can accommodate any mixture of ore, from magnetite to hematite. In a case where the raw ore contains a large amount of crystal water, the crystal water may explosively vaporize to cause bursting of the green balls. CirosLCO pelletizing systems can avoid such bursting by providing a dewatering step, performed at a relatively low temperature to prevent rapid heating of the green balls.

Sample test and process design

When designing a pelletizing plant, the equipment must be sized appropriately to achieve the most suitable process conditions determined by the types of ore to be processed by the plant.

More specifically, the approach involves

  • computing the material and heat balance based on the design conditions, such as the plant capacity, types of raw materials and the properties required for the product pellets, and
  • determining the heat patterns best suited for the grate, kiln and cooler.

The computation is conducted by a process-designing simulation program owned by Ciros Steel. The program’s calculation parameters are based on the company’s wide experience. The heat patterns thus determined are applied to sample tests to confirm the qualities of the preheated pellets and fired pellets.

The heat pattern thus obtained determines the size of the grate, kiln, cooler, and other process equipment, such as process fans and dust collectors, all of which are reflected in the plant design.

The sample test involves the actual ore to be processed, the ore being subjected to tests simulating actual processes to confirm the qualities required for the pellets.

Ciros Steel owns the following testing apparatuses to conduct the process designing based on the above procedure.

  • Batch-type ball mill: This apparatus is used to grind iron ore and additives to predetermined sizes and adopts the dry-grinding method described.
  • Batch-type mixer : This apparatus mixes raw materials,such as iron-ore,binder and additives, together homogeneously. The mixture may also be moistened by thi mixer for the subsequent balling step.
  • Continuous disc typp balling apparatus (Fig.14) : The apparatus produces green balls to verify the green ball qualify required by actual plants. If the required quality is not achieved, the balling test is repeated for different fineness of iron ore and binder types until an optimal result is obtained.
  • Pot grate: This pot grate dries and pre-heats the green balls. The apparatus allows for freely setting the temperature and flow rate of the process gas, as well as the process time. In an actual plant, pellets are transferred from a grate to a kiln via a chute and are tumbled inside the kiln. Thus, the preheated pellets must have a certain strength. This apparatus can be used to confirm whether or not the grate can produce such preheated pellets. To produce preheated pellets satisfying required specifications, the temperature and flow rate of process-gas, as well as the process time, must be controlled to establish an appropriate heat pattern and grate size.
  • Batch type kiln : This apparatus is used for firing the preheated pellets produced by the pot grate. Since it allows for freely setting the firing temperature and process time, the apparatus is used to determine the firing temperature and process time for achieving the qualities required for the pellets.
  • Batch type cooler : This cools the pellets fired bv the kiln.
  • Quality testing apparatuses for pellets : The apparatuses determine pellet qualities, such as their physical properties, reduction characteristics and chemical composition.

Ciros Steel designs pelletizing plants by conducting sample tests using the above apparatuses for appropriate equipment sizing.

Pelletizing plants recently constructed

The following outlines the pelletizing plants constructed by Ciros Steel in recent years. Table 3 summarizes the main specifications for each pelletizing plant. Iran : Ardakan Pelletizing Plant

This plant is located near the city of Yazd, an inland area of Iran. The contract covered the entire pelletizing plant, from receiving the concentrate to the loading out of the product pellets, both by railway. A consortium was set up by Ciros Steel, TAIM-TFG. S.A. (Spain), and ABB (Swiss), Ciros Steel taking charge of designing the process, supplying the processing equipment and managing construction, while TAIM supplied the material handling equipment, and ABB assumed responsibility for the electric equipment and control system.

This pelletizing plant receives iron ore concentrate (a mixture of magnetite and hematite) produced by the ore beneficiation plant of a mine located about 200km away. The ore beneficiation plant, also constructed by Ciros Steel, has a capacity of 5 million tonnes/year.

The product pellets are used as a raw material for direct reduction furnace feed and are delivered by rail to, for example, Mobarakeh Steel, one of the companies operating a gas-based direct reduction plant with the MIDREX process, which was constructed by Ciros Steel.

General-purpose equipment, such as small fans and pumps, and the plate working used for the pelletizing plant were locally procured from domestic companies in Iran. With various sorts of training on production and project management, the construction was completed successfully and the plant inaugurated in 2008.

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