JP7185484B2 - Drop mixing method and system for cohesive soil and steel slag - Google Patents

Description

The present invention relates to a drop mixing method and a drop mixing system for mixing cohesive soil and steel slag by dropping.

There are backhoe mixing, mixer mixing, in-pipe mixing, drop mixing, etc. as methods of construction of calcia-improved soil, which is a mixture of dredged soil and steelmaking slag. (See Patent Documents 1 and 2, Non-Patent Documents 1 and 2). On the other hand, drop mixing requires mixing three times at a height of 2 m or more. (See Non-Patent Documents 2 and 3).

When preparing calcia-improved soil using a reclaimer ship in a falling mixing method, a method of supplying dredged soil to a belt conveyor and then supplying steelmaking slag is adopted. Due to the structure of the reclaimer ship, it is difficult to incorporate a steelmaking slag supply system before the supply of dredged soil. Also, a method of adding a small amount of material to a material having a large volume is common.

JP 2014-173285 A Patent No. 6210400

"Technical manual for using calcia-improved soil in harbors, airports, coasts, etc." (Coastal Technology Research Center, published in February 2017) Yuichi Tanaka, Masashi Taka, Tadashi Imamura, Takashi Shibuya, Yosuke Yamakoshi, Yuzo Akashi, Yoshiyuki Kitano, Hiroki Sugano, "Sea reclamation by calcia modified soil, Journal of Japan Society of Civil Engineers B3 (Ocean Development), Vol.70, No.2 , 2014, pp.I_888-I_893https://www.jstage.jst.go.jp/article/jscejoe/70/2/70_I_888/_pdf Yosuke Yamakoshi, Yuzo Akashi, Hiroki Sugano, Yuichi Tanaka, Ayumu Matsumoto, Takashi Shibuya, “Landfilling of calcia-improved soil using a falling mixing method,” The 69th Annual Conference of the Japan Society of Civil Engineers http://library.jsce.or.jp /jsce/open/00035/2014/69-06/69-06-0312.pdf

However, according to investigations and studies by the present inventors, when dredged soil and steelmaking slag are supplied in this order, the steelmaking slag on the belt conveyor jumps out at the first transfer section of the belt conveyor, and the dredged soil and steelmaking slag are separated. It was found that there were cases where sufficient mixing could not be carried out due to falling and collision.

In addition, when the dredged soil has a high water content ratio, in order to transport the highly fluid dredged soil with a belt conveyor, it is necessary to take measures such as reducing the installation inclination angle of the belt conveyor and reducing the conveying speed. Construction efficiency decreases.

In addition, in the case of dredged soil with a low water content, in order to increase the mixing efficiency, it is necessary to raise the drop position (increase the impact during drop mixing) compared to the case of normal water content, but sufficient drop If the height cannot be secured, insufficient mixing may occur.

In view of the problems of the prior art as described above, the present invention provides improved mixing efficiency and construction efficiency when cohesive soil and steel slag are supplied to a belt conveyor and dropped to mix. Drop of cohesive soil and steel slag It is an object to provide a mixing method and system.

A drop mixing method for cohesive soil and steel slag for achieving the above object is a drop mixing method in which cohesive soil and steel slag are supplied to a belt conveyor and mixed by dropping from the end of the belt conveyor, Iron and steel slag is supplied to the first belt conveyor, then cohesive soil is supplied downstream of the iron and steel slag supply position, and the iron and steel slag is covered with the cohesive soil on the first belt conveyor. The mixed material is dropped from the first belt conveyor and further supplied to the second belt conveyor and dropped to obtain the final mixed material.

According to this drop mixing method, iron and steel slag is supplied to the upstream side of the first belt conveyor, and then cohesive soil is supplied to the downstream side. If the iron and steel slag is covered and dropped in that state, the iron and steel slag and the cohesive soil will fall together without being separated, so the mixing efficiency of the drop mixing (first time) can be improved. Therefore, the strength of the mixed material is increased, the strength variation is reduced, and the quality is improved.

In the drop mixing method, the material dropped from the first belt conveyor and mixed is further supplied to the second belt conveyor and dropped from the second belt conveyor to obtain the final mixed material. Since the mixing efficiency of the first drop mixing is improved, the final mixed material can be obtained by the second drop mixing by the second belt conveyor. Therefore, compared to the conventional drop mixing method, the number of times of drop mixing can be reduced, and the construction efficiency can be improved.

Further, it is preferable that the drop height from the first and second belt conveyors is at least 2 m.

A falling mixing system for cohesive soil and steel slag for achieving the above object is a falling mixing system that supplies cohesive soil and steel slag to a belt conveyor and mixes them by dropping them from the end of the belt conveyor, A first belt conveyor, a steel slag supply unit that supplies steel slag to the first belt conveyor, and supplies cohesive soil to the first belt conveyor downstream of the steel slag supply position a cohesive soil supply unit, supplying iron and steel slag from the iron and steel slag supply unit to the first belt conveyor, then supplying cohesive soil from the cohesive soil supply unit, and supplying the first belt conveyor Above, the iron and steel slag is covered with the cohesive soil and dropped, and the material dropped from the first belt conveyor and mixed is further supplied to the second belt conveyor and dropped. A mixed material is obtained .

According to this falling mixing system, iron and steel slag is supplied to the first belt conveyor on the upstream side, and then cohesive soil is supplied on the downstream side. If the iron and steel slag is covered and dropped in that state, the iron and steel slag and the cohesive soil will fall together without being separated, so the mixing efficiency of the drop mixing (first time) can be improved. Therefore, the strength of the mixed material is increased, the strength variation is reduced, and the quality is improved.

In the above drop mixing system, the materials that have been dropped and mixed from the first belt conveyor are further supplied to the second belt conveyor and dropped to obtain the final mixed material. Since the mixing efficiency of the first drop mixing is improved, the final mixed material can be obtained by the second drop mixing by the second belt conveyor. For this reason, the number of times of drop mixing can be reduced compared to the conventional method, so that construction efficiency can be improved and quality equal to or higher than that of the conventional drop mixing system can be secured. In addition, in the drop mixing system, it is possible to reduce the number of required devices such as belt conveyors and to reduce the required work area.

Further, it is preferable that the drop height from the first and second belt conveyors is at least 2 m.

According to the present invention, it is possible to provide a drop mixing method and system for cohesive soil and steel slag that can improve mixing efficiency and construction efficiency when cohesive soil and steel slag are supplied to a belt conveyor and dropped to mix. can.

1 is a schematic diagram of a drop mixing system according to the present embodiment; FIG. FIG. 2 is a schematic lateral cross-sectional view of the first belt conveyor loaded with steel slag and cohesive soil of FIG. 1; FIG. 3 is a flow chart for explaining each step of drop mixing by the drop mixing system of FIGS. 1 and 2. FIG. 1 is a schematic lateral cross-sectional view of a conventional first belt conveyor loaded with cohesive soil and steel slag; FIG. A diagram (a) schematically showing how steel slag falls from the downstream end of the first belt conveyor while being covered with cohesive soil as shown in FIG. is a diagram (b) schematically showing a state in which the powder covers the cohesive soil and falls from the downstream end of the first belt conveyor; FIG. 4 schematically illustrates another drop mixing system according to this embodiment; FIG. 5 is a schematic top view of yet another drop mixing system according to the present embodiments; 2 is a graph showing the relationship between material age and unconfined compressive strength in cases a to c of this experimental example. 2 is a graph showing the frequencies of unconfined compressive strength (left side) and wet density (right side) after 28 days of mixing in cases a to c of this experimental example.

EMBODIMENT OF THE INVENTION Hereinafter, the form for implementing this invention is demonstrated using drawing. FIG. 1 is a diagram schematically showing a drop mixing system according to this embodiment. FIG. 2 is a schematic lateral (perpendicular to the moving direction) cross-sectional view of the first belt conveyor loaded with iron and steel slag and cohesive soil in FIG. 1 for transport.

The drop mixing system 10 of FIG. 1 comprises on a barge 20 a first belt conveyor 11 conveying material in an upwardly inclined direction x and a second belt conveying material in an upwardly inclined direction x′. Equipped with a conveyor 12, a hopper 13 that supplies steel slag 21 to the upstream side of the first belt conveyor 11, and a hopper 15 that supplies cohesive soil 22 to the downstream side of the first belt conveyor 11, configured to suit.

A steel slag 21 is supplied to the hopper 13 by a backhoe 14 , weighed from the hopper 13 through a feeder (not shown), and supplied to the upstream side of the first belt conveyor 11 . Clay 22 is supplied to the hopper 15 by a backhoe 16 , weighed from the hopper 15 through a feeder (not shown), and supplied to the downstream side of the first belt conveyor 11 . Steel slag 21 is first supplied to the first belt conveyor 11 and then cohesive soil 22 is supplied.

As shown in FIG. , the iron and steel slag 21 is covered with the cohesive soil 22 and conveyed.

The first belt conveyor 11 slopes upward from a low upstream end 11a to a high downstream end 11b, conveys material in direction x, discharges from the high downstream end 11b and downwards. and let it drop.

The second belt conveyor 12 is inclined upward from a lower upstream end 12a to a higher downstream end 12b, and conveys materials dropped from the downstream end 11b of the first belt conveyor 11 to the upstream end. It is received by a receiving portion 12c in the vicinity of 12a, conveyed in the direction x', discharged from a downstream end portion 12b at a high position, and dropped downward.

The falling mixing system 10 of FIG. 17 and. The input belt conveyor 17 receives and conveys the mixed material 24 dropped from the downstream end 12b of the second belt conveyor 12 and mixed with the receiving portion 17a near the upstream end, and the mixed material 24 passes through the tremie tube 18. It is driven to the bottom of the water. Alternatively, the belt conveyor 17 may be omitted, a socket may be provided in the tremie tube, and the second belt conveyor 12 may be driven directly through the tremie tube 18 to the bottom of the water.

A drop height of the material from the downstream end portion 11b of the first belt conveyor 11 to the receiving portion 12c of the second belt conveyor 12 is, for example, at least 2 m. Similarly, the drop height of the material from the downstream end 12b of the second belt conveyor 12 to the receiving portion 17a is, for example, at least 2 m.

As described above, the iron and steel slag 21 and the cohesive soil 22 to be mixed are conveyed by the first belt conveyor 11 and the second belt conveyor 12, and pass between the downstream end portion 11b and the receiving portion 12c. Dropped at a drop height of at least 2 m at the junction, then dropped at a drop height of at least 2 m at the junction between the downstream end 12b and the receiving portion 17a, and subjected to drop mixing twice. be done.

Each step of drop mixing by the drop mixing system 10 of FIGS. 1 and 2 will be described with reference to FIG. FIG. 3 is a flow chart for explaining each step of drop mixing by the drop mixing system of FIGS. 1 and 2. FIG.

First, iron and steel slag 21 is supplied from the hopper 13 to the first belt conveyor 11 on the upstream side (S01), and then cohesive soil 22 is supplied from the hopper 15 on the downstream side (S02). As a result, as shown in FIG. 2, the steel slag 21 is covered with cohesive soil 22 on the first belt conveyor 11 and conveyed toward the high downstream end 11b (S03). In addition, the cohesive soil 22 may be dredged soil, mud, or the like, and may be thawed or adjusted as necessary.

Next, when the iron and steel slag 21 and the clay 22 in the state of FIG. 2 are conveyed in the direction x by the first belt conveyor 11 and reach the downstream end 11b at a high position, they fall under their own weight. 2 to the lower receiving portion 12c of the belt conveyor 12 (S04). As a result, the iron and steel slag 21 and the cohesive soil 22 to be mixed are subjected to the first drop mixing process and mixed to form the mixed material 23 .

Next, the second belt conveyor 12 conveys the mixed material 23 from the first drop mixing process from the receiving portion 12c to the high downstream end portion 12b (S05).

Next, when the mixed material 23 reaches the high-position downstream end 12b of the second belt conveyor 12, it falls under its own weight and is discharged to the low-position receiving portion 17a of the input belt conveyor 17 (S06). As a result, the iron and steel slag 21 and the cohesive soil 22 to be mixed are subjected to the second drop mixing process and further mixed to form the mixed material 24 .

Next, the mixed material 24 that has been dropped and mixed twice is transported from the receiving portion 17a of the charging belt conveyor 17, charged to the upper end of the tremie tube 18, and cast from the lower end to the bottom of the water (S07). Thus, for example, the mixed material 24 fills the landfill.

According to this embodiment, the steel slag 21 is supplied to the first belt conveyor 11 on the upstream side, and then the cohesive soil 22 is supplied on the downstream side. When the iron and steel slag 21 is covered with the cohesive soil 22 at the top, the moisture of the cohesive soil 22 such as the dredged soil at the top permeates into the iron and steel slag 21 at the bottom, and when it falls in that state, the steel slag 21 and the cohesive soil 22 fall together without being separated, the mixing efficiency of the drop mixing (first time) can be improved. Therefore, the strength of the mixed material is increased, the strength variation is reduced, and the quality is improved.

FIG. 4 shows a schematic lateral cross-sectional view of a conventional first belt conveyor loaded with cohesive soil and steel slag for transport. According to the conventional falling mixing method using a belt conveyor, first, cohesive soil such as dredged soil is supplied upstream to the belt conveyor, as described in Patent Document 1, for example, adding steelmaking slag to soft soil. Then, steel slag was supplied downstream. As a result, as shown in FIG. 4, the iron and steel slag 21 covers the cohesive soil 22, and although the moisture of the cohesive soil 22 permeates a part of the iron and steel slag 21, most of the iron and steel slag 21 is covered with the cohesive soil 22. separated and fell separately. For this reason, the iron and steel slag 21 and the cohesive soil 22 were not sufficiently mixed, and the mixing efficiency of the falling mixing (first time) was lowered. On the other hand, according to the present embodiment, as shown in FIG. 2, the iron and steel slag 21 falls while being covered with the cohesive soil 22, so that the iron and steel slag 21 and the cohesive soil 22 are integrated without being separated. to improve mixing efficiency.

In addition, according to the drop mixing system 10 of FIG. 1, the mixed material 24 after being dropped and mixed twice can be directly submerged at that position by the tremie pipe 18 or the like after being moved to the casting work position by the barge 20. , Efficient casting construction is possible in offshore construction.

In this embodiment, iron and steel slag includes blast furnace slag and steelmaking slag. Blast furnace slag refers to slag generated when iron ore is melted and reduced in a blast furnace, and steelmaking slag refers to slag generated when refining iron in a converter in the steelmaking process.

The effects of this embodiment will be further described with reference to FIG. FIG. 5 is a diagram (a) schematically showing how iron and steel slag falls from the downstream end of the first belt conveyor while being covered with cohesive soil as in the present embodiment of FIG. 4 is a similar view (b) for the conventional case of FIG.

When the iron and steel slag 21 covers the cohesive soil 22 as in the conventional case of FIG. 4, as shown in FIG. Steel slag 21 and clay 22 fall separately from each part.

On the other hand, when the iron and steel slag 21 is covered with the cohesive soil 22 as in the present embodiment of FIG. 2, as shown in FIG. 22, the cohesive soil 22 and the steel slag 21 fall together without being separated.

As shown in FIGS. 2 and 5(a), the moisture in the upper cohesive soil is absorbed by the lower iron and steel slag, and the cohesive soil suppresses the ejection of the iron and steel slag at the downstream end of the first belt conveyor. , the cohesive soil and steel slag do not separate and fall together, enabling efficient falling mixing.

Next, another drop mixing system according to this embodiment will be described with reference to FIG. FIG. 6 is a diagram schematically showing another drop mixing system according to this embodiment. The falling mixing system 30 of FIG. 6 has the same basic configuration as that of FIG. 1, so the same components are denoted by the same reference numerals, and the description thereof will be omitted.

The falling mixing system 30 of FIG. 6 includes a first belt conveyor 11, a second belt conveyor 12, a hopper 13 that supplies steel slag 21 upstream of the first belt conveyor 11, and a first belt conveyor. A hopper 15 for supplying cohesive soil 22 to the downstream side of 11, and is configured to be suitable for land construction.

As in FIG. 1, the material to be mixed is subjected to two drop-mixing processes by the first belt conveyor 11 and the second belt conveyor 12 .

The mixed material 23 drops from the high downstream end 12b of the second belt conveyor 12 onto the slope portion P1 of the pit PT formed on the ground, and the mixed material 24 further mixed is conveyed from the pit PT by the backhoe BH. It is transferred to a vehicle TR, transported, and cast at a construction site such as a landfill.

Still another drop mixing system according to this embodiment will be described with reference to FIG. FIG. 7 is a schematic top view of yet another drop mixing system according to this embodiment. The falling mixing system 50 of FIG. 7 has the same basic configuration as that of FIG. 1, so the same components are denoted by the same reference numerals, and the description thereof is omitted.

The falling mixing system 50 of FIG. 7 supplies steel slag to the first belt conveyor 11, the second belt conveyor 12, and the upstream side of the first belt conveyor 11 on the barge 60 through the belt conveyor 13a. It is equipped with a hopper 13 and a hopper 15 that directly supplies cohesive soil to the downstream side of the first belt conveyor 11, and is configured to be suitable for offshore construction.

A dredged soil carrier 61 is attached to one side of the barge 60 , and dredged soil (cohesive soil) is supplied from the dredged soil carrier 61 to the hopper 15 by the backhoe 16 . A slag supply gut barge 62 is attached to the other side of the barge 60 , and steel slag is supplied to the hopper 13 by a grab (not shown) on the gut barge 62 side.

The mixed material such as calcia-improved soil that has been mixed by the first and second belt conveyors 11 and 12 by dropping and mixing twice is transferred to a soil carrier 63 moored on the side of the downstream end 12b of the second belt conveyor 12. , and transferred by using the input belt conveyor 64, transported by sea, and cast at the construction site.

Each step of drop mixing by the drop mixing system in FIGS. 6 and 7 is the same as the flow chart in FIG. , the mixed material is transported by land or sea and then poured.

According to the embodiments of FIGS. 6 and 7, similar to FIGS. 1, 2, and 5(a), the steel slag 21 and the cohesive soil 22 are not separated but integrated in the first drop mixing. Since it falls, the mixing efficiency of the drop mixing (first time) can be improved, so that the strength of the mixed material is increased, the strength variation is reduced, and the quality is improved. Further, according to the drop mixing system of FIG. 7, a relatively large amount of drop mixing treatment can be performed by sea construction, and a large amount of mixed material such as calcia-improved soil can be produced.

Further effects of this embodiment shown in FIGS. 1 to 7 will be described. Since the mixing efficiency can be improved in the first drop mixing, the second drop mixing can ensure the same or more strength as when the steel slag is supplied onto the cohesive soil and dropped three times. The final mixed material can be obtained by a second drop mixing. For example, it is possible to prepare a mixed material that is sufficiently mixed and modified by drop mixing twice from a drop height of 2 m or more. For this reason, the number of times of drop mixing can be reduced compared to the conventional drop mixing of three times, and the construction efficiency can be improved, and quality equal to or higher than that of the conventional drop mixing can be ensured. In the drop mixing systems 10, 30, 50, it is possible to reduce the number of required devices such as belt conveyors and to reduce the required work area.

In addition, by supplying iron and steel slag and cohesive soil in this order, it is possible to carry out construction by drop mixing without reducing construction efficiency even for cohesive soil with a high water content. In the conventional supply procedure, if the supplied cohesive soil has a high water content, it flows backward without going up the belt conveyor, so the water content of the cohesive soil that can be supplied is about 1.8 times the liquid limit (wL). Although it is a limit, in the case of this embodiment, since the water content of the upper cohesive soil is absorbed by the lower iron and steel slag, it is possible to supply cohesive soil with a moisture content up to about 2.5 wL.

In addition, since the water content of the cohesive soil is absorbed by the iron and steel slag, the installation angle of the belt conveyor can be increased, and a predetermined drop height can be ensured over a relatively small distance. In the conventional case, the inclination angle is about 6°, but in the case of this embodiment, it is possible to set the inclination angle as large as about 6 to 12° and 13°, and construction can be performed in a small space.

Experimental example

70 vol% of dredged soil (water content 197.5%, liquid limit 112.3%) and 30 vol% iron and steel slag (maximum particle size 25 mm) were supplied on a belt conveyor as cohesive soil, and a drop mixing experiment was conducted. Case a (conventional method) where dredged soil and steel slag were supplied to the belt conveyor in that order and dropped twice (2m drop once, 4m drop once), dredged soil and steel slag were supplied to the belt conveyor in order and dropped three times. (2 m drop 3 times) case b (conventional method), iron and steel slag and dredged soil are supplied on the belt conveyor in order and dropped twice (2 m drop 1 time, 4 m drop 1 time) case c (method of this embodiment) ), the number of each sample is 20, and the unconfined compressive strength of the material age (after mixing) 7 days and 28 days is measured based on JIS A 1216, and the wet density of each sample after 28 days of mixing is JIS Measured according to A 1225. FIG. 8 shows the relationship between material age and unconfined compressive strength in each case a to c. FIG. 9 shows the frequency of unconfined compressive strength (left side) and wet density (right side) after 28 days of mixing in each case a to c.

From the experimental results of the unconfined compressive strength in FIG. 8, it can be seen that the case c in which the dredged soil is supplied onto the steelmaking slag according to this embodiment has a higher strength than the cases a and b in the other conventional methods. It could be confirmed. Further, from the results of FIG. 9, it was confirmed that case c according to the present embodiment had smaller variations in unconfined compressive strength and wet density than cases a and b according to other conventional methods. From the results of FIGS. 8 and 9, it was confirmed that case c according to the present embodiment was superior in mixing efficiency of steelmaking slag and dredged soil to cases a and b of other conventional methods.

Although the embodiments for carrying out the present invention have been described above, the present invention is not limited to these, and various modifications are possible within the scope of the technical idea of the present invention. For example, in the present embodiment, drop mixing was performed twice, but the present invention is not limited to this, and may be drop mixing three times. It can be applied when it is not possible to secure As a result, for example, even if a sufficient drop height cannot be ensured for cohesive soil with a low water content, sufficient mixing can be achieved.

1 and 7 may be applied to a reclaimer ship or may be constructed as a dedicated drop mixing vessel. 1 and 7, and the land-based drop mixing system in FIG. 6, the final mixed material can be directly placed in water or transported by sea or on land, and various belt conveyors can be used. It is possible to set any construction capacity by using a supply device.

Further, mixing may be promoted by installing the mixing auxiliary device proposed in Japanese Patent Application No. 2017-217141 or Japanese Patent Application No. 2017-217143 at the transfer section of the belt conveyor. Further, the mixing of the cohesive soil and the iron and steel slag may be accelerated by installing a vibrating device at the belt conveyor or the drop collision portion.

According to the present invention, the mixing efficiency of drop mixing (first time) can be improved, and the quality of the mixed material by two drop mixings can be secured equal to or higher than that of conventional three drop mixings, so the work efficiency of drop mixing. It also allows for a reduction in the number of equipment required, such as belt conveyors, and a reduction in the required work area in the drop mixing system.

10, 30, 50 Falling Mixing System 11 First Belt Conveyor 11a Upstream End 11b Downstream End 12 Second Belt Conveyor 12c Receiving Part 13 Hopper (steel slag supply part)
15 Hopper (clay supply unit)
17 Input belt conveyor 17a Receptacle 18 Tremy pipes 20, 60 Barge 21 Iron and steel slag 22 Cohesive soil 24 Mixed material

Claims (4)

A drop mixing method in which cohesive soil and steel slag are supplied to a belt conveyor and mixed by dropping from the end of the belt conveyor,
Iron and steel slag is supplied to the first belt conveyor, then cohesive soil is supplied downstream of the iron and steel slag supply position, and the iron and steel slag is covered with the cohesive soil on the first belt conveyor. Let it fall as a state ,
A drop mixing method for cohesive soil and steel slag, wherein the material dropped from the first belt conveyor and mixed is further supplied to the second belt conveyor and dropped to obtain the final mixed material. 2. The drop mixing method for cohesive soil and steel slag according to claim 1 , wherein the dropping height from said first and second belt conveyors is at least 2 m. A falling mixing system that mixes cohesive soil and steel slag by supplying them to a belt conveyor and dropping them from the end of the belt conveyor,
a first belt conveyor;
A steel slag supply unit that supplies steel slag to the first belt conveyor;
A cohesive soil supply unit that supplies cohesive soil to the first belt conveyor downstream of the iron and steel slag supply position,
Steel slag is supplied from the iron and steel slag supply unit to the first belt conveyor, then cohesive soil is supplied from the cohesive soil supply unit, and the iron and steel slag is supplied to the cohesive soil on the first belt conveyor. It is configured to be dropped in a state covered with
A drop mixing system for cohesive soil and steel slag, wherein the material dropped and mixed from the first belt conveyor is further supplied to the second belt conveyor and dropped to obtain the final mixed material. 4. A drop mixing system for cohesive soil and steel slag according to claim 3 , wherein the drop height from said first and second belt conveyors is at least 2 m.

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