JPS5847632B2 - Method for preventing deposits in rotary kiln - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing the formation of deposits on the inner walls of a rotary kiln.

The method of reducing metal oxides in a rotary kiln allows the use of inexpensive solid carbon as a reducing agent and has low construction costs, so it has been used for a long time as a method for direct steelmaking, pre-reduction treatment without ferroir production, or steelworks. It has been used for dust treatment, etc., but the deposits formed on the inner wall of the rotary kiln can deteriorate the properties of certain products and cause operational failures such as a decrease in yield, and its scope of application is limited to very small-scale production facilities. has been limited.

In other words, if deposits form in the rotary kiln, this will worsen the transfer of raw materials within the rotary kiln, causing variations in the temperature distribution of the raw materials, and at the same time, if the deposits are separated from the furnace wall, they will become lumps, which will cause problems during transfer. This crushes the reduced product and causes powdering, which leads to a decrease in the yield of the reduced product with a predetermined particle size, and furthermore, it is impossible to produce a product with a constant reduction rate. In addition, if the growth of the kiln inner wall-shaped deposits continues, the rotation of the input raw materials in the kiln will be obstructed, making it impossible to collect finished products from the rotary kiln, and eventually the operation will be stopped. bring about a situation where it is necessary.

The inventors have a length of 71 m, an outer diameter of 4.57 m, and an inner diameter of 4.0 m.
The following investigation was conducted using a 4m actual rotary kiln.

That is, in order to obtain a reduced product having a particle size of 7.5 to 18.9 mm, the raw material pellets shown in Table 1 are loaded with coke powder shown in Table 2 as a reducing agent under the mixing conditions shown in Table 3. The temperature of the inner wall of the kiln and the softening start temperature of the raw materials were investigated during operation at a rotational speed of 0.6 to 0.7 rllm.
After 098 hours of operation, the kiln was shut down, and after cooling, the formation of deposits inside the kiln was investigated. The results are shown in FIG.

Similarly, the raw material softening start temperature was determined by measuring the raw material sampled with a trowel at each longitudinal direction inside the kiln during operation using a load softening temperature measuring device after cooling with N2 gas.

Figure 1 shows the length from the raw material charging end of the rotary kiln, and
FIG. 2 is a relational diagram showing the temperature distribution of the inner wall of the kiln, the temperature distribution at which raw material softening starts, and the shape or condition of deposits.

In FIG. 1, 1 indicates a rotary kiln, and 2 indicates deposits inside the kiln.

The deposit 2 is formed in the range of 32 to 62 doors from the raw material charging end of the kiln. On the other hand, looking at the temperature distribution during rotary kiln operation, the inner wall temperature (A Curve) shows a value lower than the raw material softening start temperature (Curve B), but within the formed range, the inner wall temperature (Curve A) is higher than the raw material softening start temperature (Curve B).

That is, it has been found that the in-kiln deposits 2 are always formed when the temperature of the kiln inner wall (curve A) is higher than the softening start temperature of the raw material in contact with the inner wall (curve B).

Next, heat transfer inside the kiln will be explained.

Heat transfer in the radial direction of the kiln is divided into heat passing through the inner wall and shell of the kiln and heat radiating from the surface of the shell, and the amount of heat Q1 passing through per unit length and unit time is expressed by equation (1).

to: kiln internal gas temperature, t1: kiln internal wall temperature, t2
: shell outer surface temperature, ro: kiln inner wall inner diameter, r1: steel shell inner surface radius, r2: steel shell outer surface radius, α: gas film heat transfer coefficient, λ1: kiln inner wall brick thermal conductivity, λ2: iron Skin thermal conductivity.

Further, the amount of heat dissipated Q2 is expressed by equation (2).

t2: Temperature and cold of the outer surface of the iron skin, t3: Air temperature, α: Boltzmann constant, ε: Blackness, β: Convective heat transfer coefficient, F outer surface area of the iron skin per unit length.

Since thermal equilibrium is established during rotary kiln operation, Q1 = Q. 2 ・・・・・・・・・
...... (3), and from equations (1) and (3), Q' ...... (4) t1=
to-2x.

. is obtained.

From equation (4), the following method can be considered as a method for reducing the kiln inner wall temperature t1 in order to prevent deposits inside the kiln.

That is, ■ gas temperature t in the rotary kiln.

How to lower.

However, method (2) causes a decrease in the temperature of the raw material, making it impossible to obtain a product with a predetermined reduction rate.

■Kiln inner wall radius r.

How to reduce. However, in method (2), the gas flow rate increases, which increases the heat transfer coefficient, so it is not as effective as reducing the kiln inner wall temperature.

Therefore, the methods of securing the gas temperature to maintain the raw material temperature necessary for reduction and lowering only the inner wall temperature are: (1) increasing the amount of heat dissipated from the outer surface of the kiln shell Q2;

is optimal.

■Method C: Possible methods to increase the amount of heat dissipated include forcibly cooling the outer surface of the steel shell with water spray, etc., and increasing the thermal conductivity or reducing the thickness of the kiln inner wall bricks. However, in the latter method, water sprinkling, etc., is required to prevent the temperature of the steel shell from rising (because the temperature of the steel shell increases).
This requires forced cooling.

The purpose of the present invention is to prevent the deposits that conventionally form on the inner wall of a rotary kiln, and the outer skin of the rotary kiln is forcedly cooled by water spraying, etc., so that the temperature of the inner wall of the entire length of the kiln changes to the point at which the raw material that comes in contact with it starts to soften. The kiln is characterized in that by controlling the kiln to maintain the temperature at an appropriate temperature lower than the kiln temperature, the generation of deposits can be prevented without lowering the internal temperature of the kiln.

Hereinafter, the present invention will be explained in detail based on the drawings. FIG. 2 is an explanatory diagram showing a method of controlling the inner wall temperature of a rotary kiln.

Reference numeral 3 denotes a thermocouple for measuring the temperature of the raw material, which is inserted so that it penetrates the steel shell and the inner wall at several places in the axial and circumferential directions within a predetermined range from the charging end and comes into contact with the raw material 15 in the kiln. .

The temperature detected by the thermocouple 3 is sent via a slip ring carbon brush 4 to a raw material thermometer 5l.

The raw material temperature and raw material reduction rate are set in advance in the calculation setting device 60, and the raw material temperature measured by the raw material thermometer 5 is sent to the calculation setting device 60, which estimates the raw material reduction rate. From the relationship between the reduction rate and the preset raw material softening temperature (1), the raw material softening start temperature T can be calculated using the following formula T = f (t)...
... Determine by (4), and then multiply T in (4) by the safety factor α to determine the raw material softening start temperature αT.

A thermocouple 7 measures the temperature of the inner wall of the kiln, and is inserted into the inner wall 16 of the rotary kiln by penetrating the steel shell at several locations in the axial and circumferential directions.

The current temperature detected by the thermocouple 76 is sent to the steel skin temperature control device 8 via the slip ring carbon brush 4'.

The adjusting device 8 compares the inner wall temperature detected by the inner wall temperature measuring thermocouple 7 with the raw material softening start set temperature determined by the calculation setting device 6, and converts the temperature difference into an adjusting signal ( .

The cooling water fed from the water tank 10 to the main water supply pipe 12 by the pump 11 is automatically regulated by the opening/closing operation of the flow rate regulating valve 9 operated by the regulating signal G obtained by the regulating device 8. In the form of deposits on the inner wall of the rotary kiln, a certain area around the outer periphery of the outer shell 17 is cooled by water spraying from the water spray nozzle 14 of the provided water sprinkling branch pipe 13, and the amount of heat dissipated from the outer surface of the outer shell 17 is controlled to increase. 6.Thus, the inner wall temperature is controlled to be lower than the raw material softening start temperature.

The present invention will be explained by examples.

Example 7 In the actual rotary kiln shown in FIG.
．． In order to obtain a reduced product having a particle size of 5 to 189 peaks, the raw material pellets shown in Table 1 are mixed with the coke powder shown in Table 2 as a reducing agent in the mixing ratio as shown in Table 3 (inside the trowel kiln). O
Figure 3 shows the results of investigating the shape of deposits when the kiln was charged with water, operated at a rotational speed of 0.6 to 0.7 rpm, and cooled with water spray over a range of 40 to 60 m from the raw material charging end of the kiln. Shown below.

In Figure 3, the vertical axis represents the formation state of inner wall deposits as the ratio of lumps of 100 mm or more in the discharged product, and the horizontal axis represents the temperature between (raw material softening start temperature) and (inner wall temperature) in the kiln. Show the difference.

From Figure 3, as the temperature difference between (raw material softening start temperature) and (inner wall temperature) increases due to water spray cooling of the kiln shell, the ratio of lumps in the shell decreases, and the deposits formed on the inner wall decrease. It is clear that there has been a decrease.

By controlling the inner wall temperature to be 50'C or more lower than the raw material softening start temperature using the steel shell forced cooling G of the present invention, the rate of lumps in the product due to the formation and detachment of inner wall deposits can be reduced to almost 0%. ? I was able to rub it.

As described in detail above, according to the method of the present invention, it is possible to maintain the kiln inner wall temperature always lower than the melting start temperature of the raw materials in contact without lowering the raw material temperature in the kiln.
As a result, it is possible to prevent the formation of deposits on the inner wall of the kiln, which was the cause of poor product quality and yield decline in conventional rotary kilns. It is possible to produce large amounts of reduced iron by taking advantage of the advantages.
There is something called a dog that contributes to improving productivity.

It goes without saying that the present invention can be widely applied to kilns such as ore reduction kilns and cement kilns, where the problem of welding of raw materials to the inner wall of the kiln occurs.

[Brief explanation of the drawing]

FIG. 1 is a relationship diagram showing the length from the raw material charging end of the rotary kiln, the porosity distribution in the kiln, and the shape of deposits. FIG. 2 is an explanatory diagram schematically showing a piece of equipment 11 for carrying out the method of the present invention. FIG. 3 is a graph showing the effect of the method of the present invention. 1: Rotary kiln, 2: Deposits, 3: Thermocouple for measuring raw material temperature, 4: Slip ring carbon brush, 5: Raw material thermometer, 6: Arithmetic setting device, 7: Thermocouple for measuring inner wall temperature, 8: Iron skin temperature control device, 9: flow rate adjustment valve, 10: water supply tank, 11: water supply pump, 12: water supply main pipe, 13: watering branch pipe, 14: watering spray nozzle, 15: raw material, 16
:Inner wall, 17: Iron skin.

Claims (1)

[Claims] 1. A method for preventing the generation of deposits in a rotary kiln, which comprises cooling the inner wall of the kiln during operation of the rotary kiln to control the temperature of the inner wall over the entire length of the kiln to be lower than the softening start temperature of the raw material in contact with the inner wall of the kiln.