JPS635174B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a method for regenerating foundry sand, and in particular to a method for efficiently roasting foundry sand in an almost natural state by utilizing the thermal energy of combustible materials contained in the binder of old sand. This relates to a method for recycling recycled foundry sand.

Conventionally, many mechanical methods have been proposed and widely used for regenerating this type of foundry sand, such as organic self-hardening sand. However, in the case of green sand, it is difficult to regenerate using mechanical methods alone, so the old sand is first roasted in a fluidized roasting furnace and then recycled using mechanical methods. Although it is possible to obtain recycled sand of relatively high quality, it is generally unprofitable due to the low thermal efficiency of fluidized torrefaction furnaces, and there are few examples of this method outside of large-scale factories, and there is no more economical method. The reality is that in most factories, old sand is simply disposed of.

Furthermore, for the purpose of reducing the cost of recycling the old sand, various proposals have been made for improving the thermal efficiency of fluidized torrefaction furnaces, and some examples will now be described.

First, Figure 1 shows the general configuration of a basic fluidized roasting furnace. That is, in this FIG. 1, the roasting furnace 1
The foundry sand a introduced into the furnace from the hopper 2 forms a fluidized bed 5 near the bottom of the furnace by fluidized air sent by the blower 3 and blown out from the air nozzle 4. The foundry sand a' forming the fluidized bed 5 is heated into the furnace by a burner 6 provided on the furnace wall, roasted, and then discharged to the outside from the outlet 7.

However, in the case of a fluidized roasting furnace with a lever configuration, uniform roasting is possible due to the characteristics of the fluidized bed.
Moreover, since the residence time of the foundry sand in the fluidized bed can be extended, recycled sand of relatively high quality can be obtained.

However, in such a fluidized torrefaction furnace, the combustion conditions for the foundry sand in the fluidized bed are insufficient, so more thermal energy is consumed than necessary, and the amount of heat emitted as combustion exhaust gas is reduced. The heat efficiency was extremely low. That is, the foundry sand is heated in the upper part of the fluidized bed 5 by the flame and combustion gas of the burner 6, and although the temperature is increased, the foundry sand is particularly heated due to the lack of oxygen.
The combustion rate of the carbon-based combustibles contained in a' is slow, and when this foundry sand a' descends in the fluidized bed 5 as it flows, it is heated up by the fluidized air blown out from the air nozzle 4. Although it becomes an oxygen atmosphere, on the other hand,
The flowing air that is blown out lowers the temperature and suppresses combustion. In other words, the foundry sand a' forming the fluidized bed 5 undergoes a heating stage under oxygen-deficient conditions and a cooling stage under oxygen-enriched conditions repeatedly, so that the carbon-based combustibles contained therein are sufficiently removed. In addition to requiring a long time to burn and remove, which is extremely inefficient, there is also the disadvantage that a large amount of thermal energy is consumed as described above.

Therefore, in order to improve this point, conventional efforts have been made to improve thermal efficiency by recovering waste heat. Examples of this include, for example, a method of installing a multi-stage grate above the fluidized bed (as described in Publication of Utility Model Publication No. 54-28411), and a method of installing a rigid heat exchanger at the exhaust port (publication of Utility Model Publication No. 54-28411).
28412 Publication), a means for introducing the roasted foundry sand into a cooling chamber and exchanging its sensible heat (Utility Model Publication No. 54-
3126 Publication), means for forming a preliminary combustion zone by alternately arranging conical and inverted conical guide plates in several stages above the fluidized bed (described in Japanese Patent Publication No. 57-59015), and a fluidized bed. There is a method using a layered heat exchanger (published in Japanese Utility Model Application Publication No. 58-4249).

Although the waste heat of the fluidized roasting furnace is recovered relatively efficiently by each of these means, the temperature of the fluidized air preheated by the waste heat does not rise despite the large amount of waste heat recovered. Since the heat energy density of the heat recovered from exhaust heat is relatively small, it is not possible to improve the incomplete combustion conditions in the fluidized bed, which is the essential drawback of the fluidized torrefaction furnaces described above. Each example of energy-saving measures naturally had its limits.

In view of these conventional drawbacks, the inventors thoroughly investigated the combustion action of fluidized roasting furnaces and discovered the following new technology. In other words, the disadvantage of the fluidized torrefaction furnace is that the synergistic effect of the cooling effect of the relatively low-temperature fluidized air in the fluidized bed and the oxygen-deficient state caused by the burner exhaust gas reduces the combustion of carbon-based combustibles contained in the foundry sand. is suppressed. On the other hand, when the flowing air temperature reaches, for example, 600℃ or higher, the foundry sand naturally generates thermal energy without the need for heating with a burner, and the amount of heat generated at this time is equal to or even less than the total amount of waste heat. If it is larger than , the fluidized bed temperature can be maintained at the desired high temperature,
Roasting action can be achieved with almost no auxiliary fuel.

This invention focuses on the above-mentioned point, and aims to cause carbon-based combustibles contained in recycled foundry sand to self-combust by effectively recovering waste heat and increasing the temperature of the flowing air as much as possible. It is characterized by what it did.

In order to recover waste heat effectively here, it is best to recover heat directly from the highest temperature zone of the fluidized torrefaction furnace, and foundry sand immediately after torrefaction is optimal for this highest temperature zone. In this case, the means for moving the foundry sand from the fluidized bed to the heat exchange section and the passage of fluidized air from the heat exchange section to the air nozzle must be kept to a minimum in order to avoid a drop in temperature. On the other hand, although the exhaust gas directly above the fluidized bed has a high temperature comparable to that of the highest temperature zone, the heat transfer coefficient is about 1/5 to 1/10 lower than that of foundry sand.
It is inappropriate to recover heat from here; on the other hand, it is more effective to use the heat of the exhaust gas from here for drying and preheating the molding sand that is introduced. The minimum necessary temperature of the fluidizing air is determined by the calorific value and processing amount of the foundry sand, the furnace capacity, etc.

Next, one embodiment of the method of this invention will be described in detail with reference to FIGS. 2 to 4.

FIG. 2 shows a schematic configuration of a fluidized roasting furnace to which this embodiment method is applied. In this Figure 2,
In the roasting furnace 11, a preheating section 12, a roasting section 13, and a fluidized air heating section 14 are sequentially formed from above.
5. Burner 1 is installed on the furnace wall of the roasting section 13 toward the inside of the furnace.
6 and the lower furnace bottom of the fluidized air heating section 14 are respectively provided with an outlet 17, and the space between the roasting section 13 and the fluidized air heating section 14 below is partially closed by a fluidized air nozzle 18. There is no chute or other additional means between the two parts 13 and 14, and this flowing air nozzle 18
A pipe connecting between the blower 19 and the blower 19 outside the lower part of the furnace body is located within the fluidized air heating section 14 and forms a heat exchanger 20.

Therefore, in the case of this configuration, first, the molding sand a is charged from the hopper 15 to a predetermined level and then stopped, and the air blowing from the blower 19 is started to pour the molding sand a.
After forming a fluidized bed 21 by a' at the lower part of the roasting section 13 above the air nozzle 18, the burner 16 is ignited to heat the foundry sand a' forming the fluidized bed 21.
After roasting and waiting for the fluidized bed 21 to rise to the roasting temperature, the outlet 17 is opened and at the same time, the injection of the foundry sand a is restarted.

Here, the foundry sand a is charged into the preheating section 12, dried and preheated by exhaust gas, and then transferred to the roasting section 1.
3 and reaches the fluidized bed 21, and this fluidized bed 2
The foundry sand a' that has reached the temperature of 1 is roasted by the burner 16 and heated to the maximum temperature in the furnace. Next, the roasted foundry sand a'' flows down through the fluidized air heating section 14, which is continuously formed directly below the fluidized bed 21, with almost no temperature drop, and passes through the heat exchanger 20.
The sensible heat is exchanged with the flowing air passing through it, and is eventually discharged to the outside from the outlet 17.

As this process is repeated, the average temperature of the fluidized air heating section 14 increases, and at the same time, the temperature of the fluidized air with which heat is exchanged also gradually increases. In addition, the temperature of the fluidized bed 21 blowing out this fluidized air also shows a tendency to rise, but in order to keep the roasting temperature constant, the burner 16 is gradually throttled down, and eventually the burner 16 is completely stopped. The roasting temperature will be maintained and the system will automatically shift to natural roasting operation. If this natural roasting is continued, the roasting temperature will gradually change, but when the temperature tends to rise, the amount of casting sand a is increased, and on the other hand, when the temperature tends to fall, the burner 16 is turned on as an auxiliary The roasting temperature can be controlled by, for example, igniting a flame.

By the way, an example of a trial operation with this device configuration is shown in Figure 3, and in this case, after the start of operation, approximately 3
Within a short period of time, we transitioned to full natural roasting operation.

Note that A in FIG. 3 shows the case where the amount of casting sand a is increased, and B shows the case when the air blowing by the blower 19 is stopped. Furthermore, the dotted line in c shows the temperature change near the outlet where the fluidized air blows into the fluidized bed 21.According to this, at the same time as the injection of molding sand starts, the foundry sand that has been roasted to a high temperature flows into the fluidized air heating section. 14, the temperature of the fluidized air rose rapidly, and it is understood that when the temperature exceeded about 600°C, the foundry sand completely shifted to natural roasting operation. However, it goes without saying that the lower limit temperature of the fluidized air that is possible for self-combusting roasting of foundry sand varies depending on the capacity of the roasting furnace, the heat retention state, the amount of molding sand input, and the like.

Further, the recycled sand obtained in this example can be used as is for self-hardening molds without any particular problem, but when used as shell sand, it requires further recycling treatment as in the conventional method. The recycled foundry sand obtained in this example was used, for example, in
After being recycled using the mechanical recycling device described in Publication No. 42411, the bending strength of shell sand was measured using JISK6910, as shown in Figure 4.
As shown in code a, this recycled sand showed bending strength greater than that of new sand after being processed five times, and it was obtained as shown in code b, which was almost the same as the conventional recycled sand that was not subjected to natural roasting. Ta. Incidentally, the symbol c is an example of recycled sand obtained only by a mechanical regeneration method.

As detailed above, according to the method of the present invention, in a method for recycling foundry sand in which foundry sand containing a combustible binder is roasted in a fluidized roasting furnace, The foundry sand is caused to flow down into a fluidized air heating section that is continuously formed directly below the fluidized bed, and is allowed to stay in the heating section for a predetermined period of time, and a heat exchanger is disposed within this staying foundry sand. The fluidized air flowing through the heat exchanger is heated to around the roasting temperature by the heat of the retained foundry sand, and then the heated fluidized air is passed through a nozzle provided at the end of the heat exchanger to the fluidized air. Since the molding sand containing the combustible binder is naturally roasted by blowing air into the bed, the molding sand that has just been roasted in the fluidized bed does not enter the nozzle from which the air blows out, but instead flows through the nozzle. The heat of the foundry sand that has just been roasted in the fluidized bed can be effectively recovered by the fluidized air in the heat exchanger.

Moreover, in the case of the present invention, since the fluidized air just before blowing out from the nozzle or the fluidized air a little earlier can be exchanged with the heat of the foundry sand that has just been roasted in the fluidized bed, there is no place where the temperature will drop; The more the fluidized air moves towards the nozzle, the faster it exchanges heat with the foundry sand that has just been roasted in the fluidized bed, and the temperature of the fluidized air tends to rise more and more significantly.
Therefore, heat exchange between the molding sand roasted in the fluidized bed and the fluidized air can be performed in the most efficient manner.

Therefore, for example, even if a large amount of molding sand with a low content of combustible binder is charged into the furnace, natural roasting can be performed more economically without requiring a large amount of burners or combustible materials. This makes it possible to regenerate foundry sand at a low cost not only in large-scale foundries, but also in small-scale foundries, where conventional technology was unprofitable, saving resources, energy, and preventing pollution. It has features that can greatly contribute to

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view showing the general configuration of a basic fluidized roasting furnace, Figure 2 is a cross-sectional view showing the general configuration of a fluidized roasting furnace to which an embodiment of the method of the present invention is applied, and Figure 3 is the same as above. FIG. 4, which is an explanatory diagram showing an example of operation, is a graph showing test results of recycled sand obtained by the method of the present invention. 11... Roasting furnace, 12... Preheating section, 13... Roasting section, 14... Fluid air heating section, 16... Burner, 18... Fluid air nozzle, 19... Blower,
20... Heat exchanger, 21... Fluidized bed.

Claims (1)

[Claims] 1 In a method for regenerating foundry sand in which foundry sand containing a combustible binder is roasted in a fluidized roasting furnace, foundry sand roasted in a fluidized bed is continuously formed directly below the fluidized bed. A heat exchanger is disposed in the retained foundry sand, and the fluidized air flowing through the heat exchanger is allowed to flow down into the heated part and stay in the heated part for a predetermined period of time. After heating the molding sand to around the roasting temperature,
Blowing out the heated fluidized air toward the fluidized bed using a nozzle provided at the end of the heat exchanger,
A method for recycling foundry sand, characterized in that foundry sand containing a combustible binder is naturally roasted.