JPS63130730A - Production of reduced chromium ore pellet - Google Patents

Abstract

PURPOSE:To improve the yield of the carbonaceous material to be internally charged to the surface layer part of preheated pellets and to improve reducibility by successively providing a preheating chamber for the pellets and rotary kiln and controlling the temp. of the waste combustion gas to be admitted from the rotary kiln to the preheating chamber. CONSTITUTION:The green pellets are produced by mixing the granular chromium ore, reducing agent and binder at a prescribed ratio and humidifying and pelletizing the mixture by a disk pelletizer 1. The green pellets are conveyed by a grate conveyor 3 into a preheater 2 and are successively passed through a 3rd chamber, 2nd chamber and 1st chamber by which the pellets are gradually heated to higher temps. and are preheated. The waste combustion gas 8 is directly fed from the rotary kiln 4 into the 1st chamber and, therefore, said gas contains an oxidative gas and has the higher temp. A spray nozzle 7 is, therefore, disposed in the 1st chamber and the gas temp. is lowered by direct spraying of water by which the decarburization reaction is suppressed and the yield of the carbonaceous material in the surface layer part of the pellets is improved to a remarkably higher level.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a process for producing reduced chromium ore pellets in a reduction furnace (great kiln) that is connected to a preheater and a rotary kiln. By lowering the temperature of the combustion exhaust gas,
Internal carbon material on the pellet surface layer and CO in the exhaust gas in the preheating machine
2. This invention relates to a technique for suppressing the decarburization reaction with H2O, improving the yield of carbonaceous material contributing to reduction, and thereby improving the reduction rate of reduced chromium ore pellets.

[Conventional technology]

In the production process of reduced chromium ore pellets using a grate kiln, there has conventionally been no means for controlling the temperature of the combustion exhaust gas of the rotary kiln to be lowered. In fact, in the process of manufacturing iron ore pellets, burners are even installed to further heat the combustion exhaust gas.

In the conventional process, the decarburization reaction C+CO2→2CO C+H20→CO+H2 between the internal carbon material on the surface layer of the pellet that comes into contact with the gas in the preheater and the CO2 and H20 in the combustion exhaust gas from the rotary kiln is unavoidable, and the amount of decarburization is This amounted to approximately 20% of the total amount of interior carbon material. As a result, the maximum reduction rate of reduced chromium ore pellets using a great kiln was limited to about 80%.

[Problem that the invention seeks to solve]

The present invention provides a method for minimizing the amount of decarburization in a preheater, and aims to dramatically improve the reduction rate of reduced chromium ore pellets.

[Means for solving problems]

The present invention controls the temperature of the combustion exhaust gas flowing from the rotary kiln into the preheater in accordance with the reduction rate of the pellets when producing reduced chromium ore pellets in a reduction furnace (great kiln) in which a preheater and a rotary kiln are connected. This is a method for producing reduced chromium ore pellets.

A preferred embodiment of the present invention is a method for producing reduced chromium ore pellets, characterized in that the temperature of the combustion exhaust gas flowing from the rotary kiln into the preheater is controlled by water cooling.

[Effect]

A desired number of spray water nozzles are arranged near the joint between the rotary kiln and the preheater or inside the preheater, and the combustion exhaust gas flowing from the rotary kiln into the preheater is directly sprayed and cooled, thereby lowering the combustion exhaust gas temperature and achieving the above-mentioned results. This aims to improve the reduction rate by suppressing the decarburization reaction and improving the internal carbon material yield in the surface layer of the preheated pellets.

An example of a process flow sheet for producing reduced chromium ore pellets using a grate kiln is shown in FIG.

First, a mixture of powdered chromium ore, fine coke as a reducing agent, and a small amount of caking agent such as bentonite in a predetermined ratio is mixed with a disk pelletizer l for example
Humidify and granulate to 20 mm to obtain green pellets. Chromium ore contains Fe and Cr oxide (Fe
OlCr203), and these are used as interior carbon material (C:8
0 to 90 wt. % coke) at high temperatures (1300-
Reduction is carried out in a rotary kiln 4 at a temperature of 1400° C. according to the following formula.

7Cr20B+27C +2Cr7 C3+21CO 7FeO+I OC-+Fe7 C3+7CO It is common to add the necessary amount of internal carbon material to reduce these Cr and Fe oxides by 100%, and the carbon content of the granulated green pellets is the same as that of the ore. Although it varies depending on the type, it is usually about 15 to 17% by weight.

The granulated green pellets are dried and heated in a preheater 2 while being conveyed by a great conveyor 3. The preheater 2 is usually divided into three chambers, and the gas temperature is, for example, 220°C in the third chamber (!, 550°C in the second chamber, and too much in the first chamber).
℃, and in particular, the first chamber is the chamber into which the combustion exhaust gas from the rotary kiln 4 directly enters, so the gas temperature is the highest. The pellets preheated by the preheater 2 are heated and reduced by a burner 6 placed in a high-temperature rotary kiln 4, and then cooled by a cooler 5 to become reduced chromium ore pellets. The combustion exhaust gas composition of the rotary kiln 4 contains about 30% of oxidizing gas (H20 + CO2), for example, H2O: 15.6% CO: 4.1%, CO2: 13.5% N2: 66.8%. In addition, since the gas temperature is as high as about 1000° C., the carbon material present in the surface layer of the pellet in the first chamber of the preheater 2 is decarburized by the reaction C+H20→CO+H2 C+CO2→2CO. Since this amount generally reaches about 20% of the total internal carbonaceous material, achieving a reduction rate of 80% of the reduced pellets was the limit in conventional processes.

Table 1 shows the relationship between the amount of decarburization and temperature when green pellets produced using an actual machine were left for 60 minutes in an atmosphere of 30% CO2 and 0% N2.

From Table 1, as the temperature decreases from 1000°C to 800°C, the amount of carbon material removed from the green pellets by the reaction of CO2+C=2CO is approximately 1/
3, which shows that the reduction in combustion exhaust gas temperature has a great effect on improving the yield of interior carbon material in the first chamber of the preheater 2.

Therefore, the spray nozzle 7 is arranged in the first chamber of the preheater 2 into which the high-temperature exhaust gas from the rotary kiln directly flows, or near the connection between the preheater 2 and the rotary kiln 4, and sprays water directly onto the combustion exhaust gas 8. By lowering the gas temperature, the decarburization reaction can be suppressed and the yield of carbonaceous material in the surface layer of the pellet can be dramatically improved. Note that it is desirable to use a mist spray to speed up the evaporation of the sprinkled water droplets and increase the cooling efficiency of the gas.

Based on the above experimental results, an experiment was conducted to control the temperature of kiln flue gas using spray water using an actual machine, and the effect of kiln flue gas temperature on the reduction rate of reduced chromium ore pellets was investigated. The results are shown in Figure 2. Kiln exhaust gas temperature is 7
The reduction rate reaches its maximum value at around 80°C. The reason for this is as shown in Figure 3 (a), as the temperature of the kiln exhaust gas decreases, the amount of decarburization from the pellets in the preheater decreases, but if the temperature is lowered too much, there will be a lack of heat in the kiln, and the pellets will be decarburized. This is because the internal carbon material is not effectively used for reduction because the temperature of the pellet does not rise sufficiently, and as shown in FIG. 3(b), the amount of carbon remaining as free carbon in the finished pellet increases.

The kiln gas temperature at which the reduction rate is maximum varies slightly depending on operating conditions such as production volume, fuel consumption rate, and air ratio. Therefore, in order to always operate at the point where the reduction rate is maximum, it is necessary to periodically change the kiln exhaust gas temperature within a range of about ±50°C and always know the kiln exhaust gas temperature at which the reduction rate is maximum. is desirable. Based on past operating experience, the kiln exhaust gas temperature at which the reduction rate is maximum is 750.
It can be said that it exists in the range of ~850°C.

By the way, the required amount of water is g of kiln exhaust gas at 1000℃.
Approximately 180 fL/ is required to lower the temperature to o o 'c, and the gas amount after watering is approximately 7% higher than before watering.

Note that in the first chamber of the preheater 2, a method of lowering the kiln exhaust gas temperature by using a heat exchanger or cold air suction can be considered, but using a heat exchanger has problems such as dust abrasion and rising equipment costs. This is considered to be one of the most promising methods. On the contrary, the method using cold air suction increases the 02 partial pressure of the kiln exhaust gas and promotes decarburization, which is contrary to the purpose.

[Example] Seven mist spray nozzles (nozzle specification 50 k
g/cmX IO41/win, mist droplets with a diameter of 10,100 pL were arranged at intervals of 50 cm. The maximum amount of watering was 7017 minutes (equivalent to a production rate of 15 t/hour and a water amount of 2801/l), and the amount of watering was controlled by pressure. The pump used was a 60 kg/c rn' x 22 kw plunger pump.

The effects of applying the present invention were investigated using this equipment. Table 2 shows a comparison of operating specifications for 20 days before and after application of the present invention.

By applying the present invention, the reduction rate of reduced chromium ore pellets has been dramatically improved from 74% to 85%. Furthermore, despite the increase in reduction energy, the unit consumption of coke oven gas, which is fuel gas, has hardly changed. This is because it has become possible to suppress the decarburization of green pellets in the preheater, which has made it possible to increase the air ratio and actively burn the CO generated through reduction, making it possible to operate with a higher exhaust gas oxygen potential. . By the way, by applying the present invention, the CO concentration in the exhaust gas, which was conventionally about 5%, has become possible to operate at about 2%.

In addition, from cross-sectional observation of the finished pellet, conventional pellets had a two-layer structure with completely different structures on the surface and inside, but by applying the present invention, it was possible to achieve a nearly uniform structure on both the surface and inside. found. From this, it is considered that by applying the present invention, the reduction of the surface layer of the pellet proceeded almost in the same way as the inside, and as a result, the structure of the pellet became almost uniform within the grain.

〔Effect of the invention〕

Up to now, the reduction rate of reduced chromium ore pellets has been at most 8.
0% was thought to be the limit, but with this invention, 90%
We have reached the point where it is almost possible to achieve this goal. As a result, the usefulness of the present invention is extremely large, such as reducing various basic units in the stainless steel blowing process of a converter and improving productivity by shortening blowing time.

[Brief explanation of the drawing]

Figure 1 is a production flow sheet for reduced chromium ore pellets using a grate kiln, Figure 2 is a graph showing the relationship between kiln exhaust gas temperature after water sprinkling and reduction rate, and Figure 3 (a) is kiln exhaust gas temperature after water sprinkling. Figure 3(b) is a graph showing the relationship between the temperature of the kiln exhaust gas after water sprinkling and the amount of free carbon/total carbon input. 1...Disc pelletizer 2...Preheater 3...Grate conveyor 4...Rotary kiln 5...Cooler 6...Burner 7...Spray nozzle

Claims (1)

[Claims] 1. When producing reduced chromium ore pellets in a reduction furnace in which a preheater and a rotary kiln are connected, the temperature of the combustion exhaust gas flowing from the rotary kiln to the preheater is controlled according to the reduction rate of the pellets. A method for producing reduced chromium ore pellets characterized by: 2. The method for producing reduced chromium ore pellets according to claim 1, wherein the temperature of the combustion exhaust gas flowing from the rotary kiln into the preheater is controlled by water cooling.