Pellet ore roasting

Calcination is a key process in the production of pellets. The technical and economic indicators such as production, quality and energy consumption of the products are mainly determined by the roasting process.
At present, there are three types of equipment for roasting pellets: belt roasting machine, chain grate-rotary kiln system, and shaft furnace.
1. Belt- type roasting machine The production of pellets by belt roasting machine began in the 1950s. At present, the world's most widely used band roaster, which accounts for nearly 60% of the total output, is produced by it.
The belt roaster is similar in form to the belt sintering machine, but the difference is substantial. The production of sinter is basically based on the internal heating of solid fuel, while the roasting pellets rely on external heating. According to the length of the machine, it is generally divided into five belts: drying, preheating, roasting, soaking and cooling. The length of each belt and the thermal system vary from material to material, and the thickness of the ball layer on the machine is about 500.

The earliest belt roaster was built in the United States and is called the Mckee belt roaster. Its smoke cycle is shown in Figure 1. At that time, the drying belt was divided into two parts: blast and suction. In order to improve the heat efficiency, the hot air obtained by cooling the pellets was used for combustion and drying.

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In the past 30 years, the scale, equipment and technology of belt roasters have developed greatly. The types of belt roasters vary from country to country, and Figure 2 is one of them. The commonalities of various belt roasting machines are as follows:
(1) Use primer and bedding to improve the quality of roasting and extend the life of the trolley.
(2) The combination of blast drying and air drying is used to improve the drying process and improve the quality of the pellets.
(3) The blast air cools the pellets and directly recycles the hot air to assist the combustion zone.
(4) Only exhaust gas with a low temperature and high moisture content, or contaminated flue gas, is discharged into the chimney.
Regardless of the type of belt roaster, it is an important goal to make full use of energy. Since the seventies, a belt roaster to large-scale development direction, machine-wide increased from 1.83 m to 4.0 m fifties, the effective working area is increased from 94 m ² to 704 m ^2, a single output of 800,000 tons per year Increase to 5 million tons. In addition, due to the improvement of refractory and thermal insulation materials and the increasingly rational design of the process, the energy consumption of the calcined pellets is greatly reduced. Over the past firing one ton of iron ore pellets magnetic energy to be about 19 million kcal, is now down to 8.82 ~ 101,000 kcal. The amount of hematite in the ingredients is increased, and the energy consumption is correspondingly increased. All of the hematite is used, and the energy consumption will be doubled.
In 1973, China introduced a 162 m ² belt roasting machine from Japan to a factory in North China. Due to raw materials, it has not been produced normally. A company in the Northeast has introduced a set of pellets with an annual output of 2 million tons from Australia and is currently under construction.
Second, the chain machine - rotary kiln system

The grate-rotary kiln system includes a chain grate-rotary kiln and a cooling machine. The flow of a typical grate-rotary kiln is shown in Figure 3. The grate is similar to the belt roaster, but it is only used for dry ball drying and preheating. The thickness of the sphere is only 150 to 200 mm, and no primer is required. The hot exhaust gas discharged from the tail of the rotary kiln is 1000 ° ~ 1100 ° C, first introduced into the preheating section, the temperature is reduced to 300 ° ~ 400 ° C and then led to the drying section, and the last exhaust gas is discharged into the chimney. In order to adapt to different raw materials and improve the drying process, the grate machine is divided into two sections, one section of preheating; one section of drying, two chambers and three sections, two sections of drying, one section of preheating; and three chambers and four sections. [next]
The preheated pellets already have a certain strength and go directly into the rotary kiln.
The rotary kiln is a cylinder lined with refractory bricks with a slope of 3 to 6% around the centerline. In the kiln with a burner, heavy oil or coal gas as the fuel. The calcination temperature in the kiln is 1300 ° ~ 1350 ° C. The pellets are added from the tail, and the kiln is filled with about 7%. The pellets are reversed from the hot gas stream and flipped while roasting. When it moves to the kiln head, the roasting process ends, then falls onto the cooler and cools down.
The cooling pellets are mostly ring-cooled, and the thickness of the ore layer is 710-760 cm, and the blast is cooled. The heat exchange is sent to the kiln at 800 °C, which is used as secondary air to assist combustion.
The grate-rotary kiln system began to develop in the late 1960s, and currently around 40% of the world's pellets are produced. The rotary kiln continues to develop in the direction of large-scale. Its diameter has increased from 3.05 meters in 1960 to 7.62 meters today, and the stand-alone production capacity has increased from 330,000 tons to 4.5 million tons per year.
There is no large-scale chain kiln-rotary kiln system in China, and several sets of small equipment have been built in the northeast, north and south-central regions. The diameter of the rotary kiln is only 2 meters, which is an industrial test equipment.
3. The shaft furnace is the earliest pelletizing and roasting equipment in the world. The United States Yili Company built the first experimental shaft furnace in 1947, and officially established the factory in 1950. After that, 31 factories in various countries built 102 shaft furnaces, with an annual output of 29 million tons of pellets. By the early 1960s, half of the world's pellets were produced by shaft furnaces.

The earliest shaft furnace shape in the United States is shown in Figure 4. The raw balls are uniformly packed into the furnace from the upper furnace mouth, and high-temperature gas generated in the baking chamber is injected from both sides, and heat exchange is performed in the reverse flow. The green balls are dried, preheated, calcined and consolidated, and then cooled in the process of falling down with the cold air blasted from the bottom. However, in fact, the hot gas flow is disturbed by the bottom-up cooling airflow, the airflow distribution along the cross-section of the shaft furnace is uncontrollable, and the temperature and atmosphere are extremely uneven, resulting in poor product quality; in addition, the productivity of the shaft furnace is low, a furnace The shaft furnace with an area of ​​8 m ^{2 has} an annual output of 300,000 tons of pellets. The 15 m ² large shaft furnace has an annual capacity of 500,000 tons. Moreover, the shaft furnace can only roast the ball with magnetite as raw material. Mission mine. Therefore, since the 1970s, with the development of belt roasters and grate-rotary kiln, the shaft furnace has stopped developing. At present, only a few manufacturers in foreign countries use shaft furnaces, the largest of which is the founder of shaft furnaces - Erie [next]
China began to build a shaft furnace in 1996. In 1968, the first shaft furnace was put into operation in Jinan, Shandong. Later, several others were built in Zhejiang and other places. China's raw material conditions are poor, the fine powder has a coarser particle size, higher SiO ₂ content, and lower gas calorific value. At the beginning of the shaft furnace opening, many difficulties were encountered, and even the pellets were formed into large pieces in the furnace, so that production could not continue.
In 1969, the engineers and workers of the Jinan Iron and Steel Plant in Shandong Province carried out technical transformation of the shaft furnace. Invented the drying bed and the wind guiding wall for the problem that the temperature inside the shaft furnace was too high and the two air streams interfered with each other. The development of shaft furnaces has created a new path. The shaft furnace that has been technically modified is shown in Figure 5.

The role of the air guiding wall is to make the cooling air pass through the wall, avoiding the interference of the hot air flow from the combustion chamber, making the temperature of the baking belt uniform, and reducing the resistance of the cooling air through the column, increasing the cooling air volume and improving the cooling air volume. Cooling effect.
A drying bed is added to the furnace mouth to make a real drying belt appear in the shaft furnace. The raw ball is placed on the drying bed, the thickness of the bed is 150-200 cm, and the slope of the drying bed is larger than the natural pile angle of the pellet. As the pellets in the furnace move downward, the green balls slide down the slope of the drying bed, from the top to the mouth for about 6 minutes, during which time the hot air passing through the drying bed is dried. The field test results show that under normal conditions, the water content of the raw ball can be reduced from 8 to 9% to 1.5%.
The pelletized ore that has been dried slides toward the center at the mouth of the natural pile. The kinetic energy of the big ball is larger, tends to the center, and the small ball and a small amount of powder and fragments remain on the wall. Therefore, it helps to control the edge effect of the furnace wall, and the hot gas flow from the combustion chamber is developed toward the center, which is favorable for uniform temperature distribution.
The wind guide wall and drying bed technology changed the appearance of the shaft furnace, the productivity increased, the energy consumption decreased, and the quality of the pellets improved. This technology was quickly promoted in China.
The field test proves that after the wind tunnel adopts the air guiding wall and the drying bed, there are five bands of drying, preheating, roasting, soaking and cooling in the furnace. Compared with the belt roasting, the drying belt of the shaft furnace is too Short, the remaining four belts are longer than the belt roasting machine. It seems that drying is still a limiting part of the shaft furnace roasting pellets.
The innovation of China's shaft furnace has attracted the interest of American Yili Company. Now the wind guide wall and drying bed technology have been used in the shaft furnace of Yili Company for trial use.
Comparing the above three kinds of roasting pellets equipment, it can be known that the equipment of the belt roasting machine is concentrated, easy to handle and maintain, the baking cycle is short, the roasting system is convenient to adjust, and the adaptability to raw materials is strong. It there is a problem, non-uniform height along the seam pellet quality, the carriage and the grate shall be made of heat resistant alloy steel; grate - kiln advantage is that uniform roasting, better product quality, can adapt Raw materials do not require heat resistant alloy steel. The problem is that the equipment is huge, difficult to repair, and the kiln is easy to form a ring, which affects the operation rate; the structure of the shaft furnace is simple, and there is no special requirement for the material. The disadvantage is that the productivity of the single furnace is low, and the adaptability to the raw materials is poor. Only the roasting magnetite pellets have a better effect, the roasting is uneven, and the quality of the product is poor. However, after the shaft furnace adopts the drying bed and the guiding phoenix wall, the output, quality and energy consumption are improved. Its equipment is simple, although the single furnace output is not large, it may be more suitable for small and medium-sized factories.

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