TOTAL RECYCLING SYSTEM

Transcription

1 TOTAL RECYCLING SYSTEM Ko Hasegawa, Toho Gas Co., Ltd. 1. TOTAL RECYCLING SYSTEM Excavated soil, asphalt concrete lumps, cement concrete lumps and used gas pipes produced by gas piping work are sorted and carried into our recycling yard. These s for recycling are made into products through respective recycling processes, which can be used as s for backfilling excavated sites after gas piping work, restoring pavement, and all. The conceptual diagram of this total recycling flow is shown in Fig. 1 below. [Legend] Produced (used) Recycling process Recycled product Surface layer Roadbed Subgrade Gas pipe Asphalt cement lumps Recycled concrete crushed stones Improved roadbed Excavated soil Recycled asphalt composite, PE added asphalt composite Improved soil Gas pipe location sign sheet PE pipe Cast-iron/steel pipe Materials for recycling (s for iron and steel making) Gas pipe location sign pole Fig. 1 Total recycling flow The recycling yard has an area of about 33000m 3, large enough to stock excavated soil, asphalt concrete lumps, cement concrete lumps and used pipes produced by gas piping work. It has an improved soil and improved roadbed manufacturing plant, an asphalt concrete lump and cement concrete lump crushing plant and an asphalt composite manufacturing plant (Fig. 2) to recycle excavated soil and asphalt cement lumps and cement concrete lumps collectively and efficiently. 1

2 Asphalt composite manufacturing plant Site area: 2,500m 2 Processing capacity: Max kg/h Asphalt concrete lump and cement concrete lump crushing plant Site area: 2,500m 2 Processing capacity: Max kg/h Improved soil and improved roadbed manufacturing plant Site area: 20000m 2 Processing capacity: Max kg/h Used gas pipe stockyard Raw stockyard Asphalt cement lump and concrete cement lump crushing plant Asphalt composite manufacturing plant Improved soil and improved roadbed manufacturing plant Product stockyard Raw stockyard Raw stockyard Fig. 2 Layout of the recycling yard The improved soil and improved roadbed manufacturing plant improves soil by adding unslaked lime to excavated soil and manufactures subgrade (improved soil) or roadbed (improved roadbed ). The asphalt concrete lump and cement concrete lump crushing plant crushes asphalt concrete lumps and cement concrete lumps produced by gas piping work and manufactures recycled aggregate and recycled concrete lump crushed stones. The asphalt composite manufacturing plant heats and mixes recycled aggregate and virgin aggregate and manufactures recycled asphalt composite by mixing the raw s, such as asphalt and stone dust (Fig. 3). Improved soil and improved roadbed manufacturing plant Excavated soil Screening and grain size adjustment Unslaked lime Mixing Screening and grain size adjustment Cement concrete lump crushed product Mixing Screening and grain size adjustment Asphalt concrete lump and cement concrete lump crushing plant Asphalt concrete lumps, cement concrete lumps Crushing Asphalt composite manufacturing plant Asphalt concrete lump recycled aggregate Asphalt and stone power recycled additive Virgin aggregate Heating and mixing Mixing Improved soil Improved roadbed Recycled concrete crushed stones Recycled asphalt composite Fig. 3 Recycling flow of excavated soil 2

3 Used gas pipes are stocked at the collection yard. PE straight pipes are carried into a recycling manufacturer and flaked and pelletized for manufacture into various recycled products (Fig. 4). Used PE pipes (pipe shreds, removed pipes) PE straight pipe part Cutting and separating PE joint part PE pipe recycled raw manufacturer Iron and steel manufacturer, and blast furnace manufacturers Removing foreign objects Cleaning and cutting Crushing (flaking) Crushing (flaking) Reducer for blast furnaces (coke furnace) Pelletizing Manufacturing recycled products Gas pipe location sign sheets Gas pipe location sign poles Novelty goods PE added asphalt composite (additive) Fig. 4 Recycling flow of PE pipes Copper wire contained PE pipe joints are used as a reducer for raw s for blast furnaces (chemical recycling). Steel pipes and cast-iron pipes are sold to the outside as raw s for iron and steel, and reused as raw s for recycled iron and steel by iron and steel manufacturers. For example, gray-iron pipes are recycled as ductile-cast-iron pipes by adding magnesium or the like. 2. HISTORY OF RECYCLED PRODUCT DEVELOPMENT 2.1 Recycled products of excavated soil, asphalt concrete lumps and cement concrete lumps Excavated soil, asphalt concrete lumps and cement concrete lumps produced by gas piping work have been disposed (e.g., landfilling) since they cannot be compacted sufficiently as they are when they are subjected to rolling compaction. For backfilling excavated roads, natural s (e.g., pit sand, crushed stones) have been used instead. For asphalt composite, virgin using 100% virgin aggregate as raw has been used. These have caused problems as follows: (1) It is difficult to acquire dumping sites for excavated soil, asphalt concrete lumps and cement concrete lumps (since dumping sites have been exhausted, and even if some are available, they are in the distance and disposition claims high cost). (2) It is difficult to obtain quality pit sand and crushed stones (since sand pits and stone pits are available only in the distance and procurement claims high cost). (3) Transport of excavated soil, asphalt concrete lumps, cement concrete lumps, pit sand and crushed stones causes traffic pollution, and the transport itself is not economical. (4) Collecting pit sand and crushed stones destroys the environment. In view of these problems and in order to utilize natural resources effectively, Toho Gas has been working on the technical development for recycling excavated soil, asphalt concrete lumps and cement concrete lumps and the construction of a recycling plant since As a result, we have succeeded in recycling all backfilling s and pavers produced by road excavation work into 3

4 recycled products as follows: [Introduction time of recycled products] Recycled asphalt composite (paver for road surface layer) : June 1984 Improved soil (backfilling for subgrade) : April 1986 Recycled concrete lump crushed stones (backfilling for roadbed) : October 1998 Improved roadbed (backfilling for roadbed) : January Recycled products of used gas pipes For gas pipes to be laid underground, ferrous pipes, such as steel pipes and cast-iron pipes, had been used previously, but in recent years, polyethylene (PE) pipes having high resistance to corrosion and quakes have been used instead. Toho Gas introduced PE pipes in 1982, and has tried to expand their application since then. Today, we use PE pipes for over 90% of all pipes to be laid in gas piping work. Judging from the rate of expansion in PE pipe application, increase in the volume of used PE pipes (pipe shreds and removed pipes produced by dimensional adjustments) from gas piping work was anticipated as seen today. On the other hand, various problems with waste disposition, such as the exhaustion of waste dumping sites, increase in waste disposition cost and the enforcement of the Recycling Law, have become controversial more and more. In an attempt to solve these problems, we started the study and technical development for recycling PE pipes in 1992, and have introduced the products until today as listed below. On the other hand, recycling method for used gas pipes made of ferrous s, such as steel pipes and cast-iron pipe) into s for iron and steel manufacturing has been established for a long time. Accordingly, used ferrous pipes are reused as recycled steel pipes and cast-iron pipes. [Introduction time for main recycled products] Gas pipe location sign poles : March 1997 Gas pipe location sign sheets : September 1997 PE added asphalt composite : April 1999 Others: Clear files, ball-point pens, plant pots, leisure sheets, garbage bags, etc. 2.3 Recycle of PE joints When PE straight pipes are laid underground in roads, the PE straight pipes are joined each other. To supply gas to a consumer, service lines should be branched from a PE straight pipe. One of the pipeline elements used for such joining or branching is called PE joint. Since PE joints contain copper wires, it is difficult to pelletize or flake PE joints unlike PE straight pipes. Therefore, since 2006, PE joints have been subjected to chemical recycling into reducer for blast furnaces. By steaming crushed PE joints together with iron ore and coke in a blast furnace, oxygen in iron ore is reduced and thereby pig iron can be produced. Here, according to the Basic Law for Establishing the Recycling-Based Society enacted in 2000, more advanced usage of wastes is preferable and recycling (for use as PE ) is preferable to chemical recycling. Along such suggestions, Toho Gas plans to study the possibility of recycling for PE joints by separating copper wires from them. 3. MANUFACTURING METHODS FOR RECYCLED PRODUCTS 3.1 Recycled products of excavated soil Manufacturing methods for improved soil and improved roadbed In general, excavated soil contains gravel, sand and clay heterogeneously. This makes excavated soil so soft that they cannot be compacted firmly and therefore are not suitable for use as road backfilling. For this reason, in the improved soil and improved roadbed manufacturing plant, excavated soil is subjected to soil improvement process where large lumps like gavel are crushed on a crusher into fine grains, screened for grain size adjustment, and unslaked lime is added, to improve excavated soil into improved soil suitable as road backfilling. Furthermore, in the adjacent improved roadbed plant, concrete lump crushed product made by crushing cement concrete lumps and adjusting their grain sizes is supplied and mixed during the improved soil manufacturing process (after the soil improving process) into recycled improved roadbed suitable as roadbed backfilling. The improved soil and improved roadbed manufacturing plant was manufacturing only 4

5 improved soil when it started operation. When it was decided to manufacture improved roadbed, a new plant was not constructed for this purpose but a plant for adding concrete lump crushed product was added to the improved soil manufacturing plant. As a result, the existing plant was turned to be capable of mass-producing quality improved roadbed with economical investment (Fig. 5). Plant for manufacturing improved roadbed Cement concrete lump crushed product Supply hopper Unslaked lime Unslaked lime Mixing Weightometer, moisture meter Screen No. 3 (No. 1 screen) Crusher (Hydraulic jaw crusher) Mixer (Ribbon mixer) Supply Screen hopper No. 2 (Trommel) Screen No. 1 (Vibration grizzly) Screen No. 4 (No. 2 screen Excavated soil Crusher (Impact crusher) Crushing Crusher (Roll crusher) Improved soil and improved roadbed Fig. 5 Manufacturing flow for improved soil and improved roadbed Soil improving method utilizing unslaked lime (Points of improved soil development) By adding unslaked lime to excavated soil, the following four soil improvement effects were obtained with increased soil strength: Hydration: Unslaked lime reacts on water to lower moisture content. Pozzolanic reaction: Carbonation: Unslaked lime reacts chemically on mineral in soil to aggregate clay. Unslaked lime reacts on carbon dioxide in soil to generate calcium carbonate to raise soil strength. Ion-exchange reaction: Ultra-fine soil particles are aggregated by ion-exchange reaction to make soil easy for compaction. However, adding too much unslaked lime to soil makes the soil so strong that re-excavating the same place in the future would be difficult, and adding too little unslaked lime to soil would prevent unslaked lime from exerting its effect completely so as to make the soil insufficient in strength. To find the optimum amount of unslaked lime to add, laboratory tests were repeated. As a result, the optimum addition ratio of unslaked lime to the total weight of the soil was found to be approx. 1 to 1.5wt%. As apparent from the comparison of the test results as shown in Table 1, the excavated soil was improved to be superior to pit sand and showed the quality characteristics as follows: The CBR* value is stable, i.e., the ground strength is stable after compaction. Little deformation is caused by imbibition. Owing to imperviousness to water, there is no over-compaction phenomenon in construction in rainy weather. The settlement amount of improved soil is far smaller than that of gravel soil and pit sand. Owing to small grain size of maximum 15mm and uniform grain-size distribution, time required for covering, leveling and rolling compaction can be shortened. * CBR = California bearing ratio: An index indicating the strength of the soil as subgrade or roadbed. 5

6 Sample type Improved soil Pit sand Excavated soil Soil particle density g/cm Natural moisture content % Gravel part % Particle size Sand part % Silt and clay part % Classification Sand mixed with fine particle part Sand mixed with silt Clay sand CBR Testing method Compacted soil Compacted soil Compacted soil Mean CBR % Table 1 Soil test results Points of improved roadbed development The quality of improved roadbed depends on the compounding ratio of excavated soil, cement concrete lump crushed product and unslaked lime. Considering this, the optimum compounding method that could satisfy three requirements, i.e., having sufficient strength, having gain-size distribution conformable to the grain-size distribution standard of crushed stones and having sufficient storability, was determined. This optimum compounding ratio is shown in Table 2. Raw Compounding ration (wt%) Excavated soil Cement concrete lump crushed product Unslaked lime 1.50 Table 2 Optimum compounding ratio The improved roadbed manufactured with the optimum compounding ratio was subjected to various quality tests specified by the City of Nagoya where Toho Gas is located and its conformance to the standard of the city on all items was proved (Table 3). Item Maximum grain size Immediate CRB (mm) (%) Modified CBR (%) Evaluation Developed product Good Standard value 40 or less 80 or more 80 or more Table 3 Quality confirmation values The on-site construction efficiency for and roadbed strength immediately after construction for improved roadbed were also confirmed. As a result of comparative evaluation with conventional roadbed (crusher-run) as shown in Tables 4 and 5, the construction efficiency was equivalent to that for the conventional roadbed and the roadbed strength immediately after construction was higher than that of the conventional roadbed. Thus, the development of roadbed with very high quality was completed. Item Results Workability Equivalent to conventional roadbed Finished condition Equivalent to conventional roadbed Table 4 Confirmation results for on-site construction efficiency Site density Plate bearing Improved roadbed (KC-40) N/ Conventional roadbed (crusher-run C-40) N/ Standard value 93 or more 0.177N/ or more Table 5 Comparative confirmation of roadbed strength 6

7 3.2 Recycled products of asphalt concrete lumps and cement concrete lumps Manufacturing methods for recycled asphalt composite and recycled concrete lump crushed stones Since the quality of asphalt in asphalt concrete lumps is deteriorated due to aging and the grain of aggregate is varisized, technology for preventing the quality deterioration is required to recycle asphalt concrete lumps into asphalt composite. As this technology, recycled aggregate made of asphalt concrete lumps can be recycled into stable asphalt composite of high quality when it is added with virgin aggregate for over 50% and then with additive. Here, asphalt concrete lumps left unused for turning into recycled asphalt composite are used as recycled concrete lump crushed stones. Used cement concrete lumps are so large in grain size and so ill-balanced in grain-size distribution that they cannot be used as they are as crushed stones for roads (roadbed ) suitable for backfilling. For this reason, used cement concrete lumps are screened for grain size adjustment at the crushing plant into recycled concrete lump crushed stones (RC-40) as stable roadbed of high quality. These crushed stones are also used as raw for improved roadbed. Fig. 6 shows the manufacturing flow of recycled product of asphalt concrete lumps and cement concrete lumps. Asphalt tank Virgin aggregate hopper Asphalt concrete lumps and cement concrete lumps Recycling additive Meter Stone powder tank Meter Virgin aggregate heater dryer 180 Meter Hot bin Meter 160 Mixer Recycled aggregate hopper Recycled aggregate heater dryer 140 Recycled concrete lump crushed stones Crushing equipment Improved roadbed Recycled asphalt composite Fig. 6 Manufacturing flow of recycled product of asphalt concrete lumps and cement concrete lumps Points of recycled asphalt composite development As described above, recycled asphalt composite is manufactured by heating and mixing recycled aggregate, virgin aggregate, stone dust, asphalt, etc. However, the recycled asphalt composite thus manufactured is susceptible to the uneven quality of recycled aggregate, and, therefore, should be placed under strict quality control. Particularly, since the volume of used asphalt concrete lumps contained in the recycled asphalt composite has a major effect on the temperature for the optimum compaction and the durability of pavement, close attention should be paid to the compounding design of recycled asphalt composite. In view of the above, the optimum compounding design of recycled asphalt composite is determined through quality test assuming every possible environment (outdoor air temperature) and on-site condition. Table 6 shows the quality comparison between recycled asphalt composite with recycled aggregate and virgin aggregate compounded at 1:1 (addition of recycled aggregate: 50%) and virgin. As evident from this table, recycled asphalt composite nearly equivalent in quality to virgin was obtained. 7

8 Composite type Fine grained asphalt concrete 13 mm Quality item Added with recycled aggregate for 50% Virgin Penetration (25, 1/10mm) Percentage of void (%) Degree of saturation (%) Flow value (1/10mm) Degree of stability (4.9kN or more) Grain size distribution Within standard range Within standard range Fluidity resistance (DS value: times/mm Table 6 Quality of recycled asphalt composite 3.3 PE pipe recycled products Manufacturing process for PE pipe recycled products Used PE pipes (pipe shreds, removed pipes) produced at gas piping work sites are piled up at the recycling yard. Foreign objects are removed from the used PE pipes and mud, dirt, etc. are washed off the used PE piles by a recycled raw manufacturer. The used PE pipes are cut to around 30 cm for easy supply into the crusher. The cut pipes are supplied into the crusher to be crushed into grains (flakes) of 5 to 10mm. Then, the grains are heated, molten and made into finer grains (pellets) of 2 to 3mm. These flakes and pellets are used for manufacturing various recycled products. The PE flakes are processed into gas pipe sign location poles or used as additive to PE added asphalt composite. The PE pellets are processed into PE gas pipe location sheets for use in gas piping work, and clear files, ball-point pens, leisure sheets, plant pots, garbage bags, etc. Here, the PE added asphalt composite is manufactured by adding PE flakes for 1 to 2% to heating asphalt composite for use in improving road pavement. The relevant technology was developed by referring to the information reported by Professor Hajime Honda of Osaka City University and others (Professor Yamada of Osaka City University, and Obayashi Road Ltd.) Points of PE pipe recycled product development To study the recyclability of used PE pipes, i.e., the possibility of processing used PE pipes into various products, the properties of the pellets and flakes were investigated. PE can be divided roughly by density into low-density PE, medium-density PE and high-density PE. About 70% of PE products available in the market use low-density PE, while PE gas pipes use medium-density PE. The main physical properties of the raw manufactured by recycling such PE pipes are shown in Table 7. Physical property item Used PE pipes PE pipes available in the (medium-density PE) market (mainly low-density PE) Melt flow rate (MFR)* g/10min 1.0g or more/10min Density g/cm g/cm 3 Tensile yield strength MPa Tensile breaking strength % Melting point *: Melt flow rate (MFR): A value indicating fluidity. The larger the MFR is, the higher fluidity and workability are in the molten state. Table 7 Main physical properties of used PE pipes and PE pipes available in the market From this table, it is clear that the PE pipe is raw characterized by its low fluidity (hard to flow), and suitable for such molding methods as extrusion (PE pipe molding method), press forming, blow molding, inflation molding and compression molding, but not suitable for precision molding methods, such as injection molding. 8

9 4. ACHIEVEMENTS AND EFFECTS 4.1 Production volume of recycled products Tables 8 and 9 show the achievements of the collection of excavated soil, asphalt concrete lumps, cement concrete lumps and used gas pipes and the manufacture of their recycled products. Unit: 10 6 kg Fiscal year Item Produced volume Excavated soil Recycled volume Recycling ratio 40% 48% 52% Asphalt concrete lumps Produced volume and cement concrete Recycled volume lumps Recycling ratio 94% 97% 97% Table 8 Recycling of excavated soil, asphalt concrete lumps and cement concrete lumps Unit: 10 3 kg Fiscal year Item Produced volume 5,040 4,806 4,736 Steel and cast-iron pipes Recycled volume 5,040 4,806 4,736 Recycling ratio 100% 100% 100% Produced volume PE pipes (including joints) Recycled volume Recycling ratio 100% 100% 100% Table 9 Collection and recycling volume of used gas pipes Excavated soil, asphalt concrete lumps and cement concrete lumps are collected from cities, towns and villages located within about 20 km radius from the recycling yard, turned out into recycled products at the plants within the recycling yard, and used as backfilling s and pavers in gas piping work. All used gas pipes are collected to the recycling yard, the used steel pipes and cast-iron pipes are sold to the outside as s for iron and steel manufacturing, and the used PE pipes are processed into recycled products by a PE pipe recycling raw manufacturer or used as reducer for blast furnaces. In recycling excavated soil, asphalt concrete lumps and cement concrete lump, about 50% of the excavated soil (produced volume: about kg) is used for improved soil and improved roadbed, while nearly 100% of asphalt concrete lumps and cement concrete lumps (produced volume: about kg) are recycled and nearly 100% of used gas pipes are recycled. 4.2 Effect of waste volume reduction As shown in the following table, there is a certain waste volume reduction effect by way of landfill disposition, for example: Waste item Waste volume reduction effect Excavated soil About 300,000,000kg/year Asphalt concrete lumps and cement About 80,000,000kg/year concrete lumps Use PE pipes About 100,000kg/year Used steel pipes and cast-iron pipes* About 5,000,000kg/year *: Used steel pipes and cast-iron pipes have been recycled outside into s for iron and steel manufacturing. 4.3 Effect of resource saving and energy saving (1) Owing to the effect of saving natural resources as shown in the following table, recycling is promoted for protecting the environment from destruction and using natural resources effectively: 9

10 Item Pit sand, crushed stones Plastic s (PE) Iron and steel s Volume reduction effect for waste, etc. About 380,000,000kg/year About 100,000kg/year About 5,000,000kg/year (2) Recycling improved soil, asphalt concrete lumps and cement concrete lumps brings about reduction in transport energy (energy saving effect) as shown in the following table: Diesel oil: Saving by 1,140m 3 /year Reduction in transport energy (1) Transport energy (diesel oil consumption) before recycling (Transport volume) / (Load capacity per truck: 10 tons) (One-way transport distance) (Pit sand, etc. + excavated soil, etc. transport frequency: 2 times) (Round transport: 2 times) / (Truck fuel consumption: 2 kg/liter) (380,000,000 kg) / (10,000 kg) (20 km) (2 times) (2 times) / (2km/10-3 m 3 ) = 1,520 m 3 (2) Transport energy (diesel oil consumption) after recycling (Transport volume / Load capacity per truck: 10 tons) (One-way transport distance) (Improved sand, etc. + excavated soil, etc. transport frequency: 1 time) (Round transport: 2 times) / (Truck fuel consumption: 2kg/liter) (380,000,000 kg) / (10,000 kg) (10 km) (1 time) (2 times) / (2 km/10-3 m 3 ) = 380 m 3 (3) Reduction in transport energy = (1) (2) Diesel oil: 1140m 3 /year 5. POSTFACE By establishing a total recycling system for excavated soil, asphalt concrete lumps, cement concrete lumps and used PE pipes produced by gas piping work, all processes of gas piping work from road backfilling to road pavement restoration are now able to be carried out with recycled products alone. Furthermore, all recycled products are of high quality that conforms to the relevant standards and spread widely in and around Nagoya City where Toho Gas is located. Furthermore, Toho Gas is promoting the production control of excavated soil, asphalt concrete lumps and cement concrete lumps by applying the non-open digging piping work method, the rehabilitation-based repair method and the same-day restoration method to minimize the waste production earnestly while promoting the total recycling system. Toho Gas will be making efforts to develop and spread the total recycling system and thereby contributing to the development of recycling-based society. 10