More than 250 years have passed since the Industrial Revolution (late 18th century), and currently, human-made mass, especially from manufacturing units and construction, exceeds all living biomass [1
]. There are growing concerns about global warming associated with an increase in the consumption of natural resources, owing to the mass consumption of mineral resources and an increase in greenhouse gas (GHG) emissions. Under these circumstances, the importance of a circular economy (CE) has been globally recognized, for developing a recycling-oriented society and achieving carbon neutrality, which is one of the prime Sustainable Development Goals (SDGs) adopted by the United Nations (UN) in 2015 [2
]. Industries, governments, and businesses have contributed to achieve a recycling-oriented society. Remanufacturing, the goods from which have been categorized as “reused or recycled goods,” is effective in saving resources through the reuse of subparts and is more economically efficient than other means of recycling resources. Therefore, remanufacturing is used as a major means of achieving economic and environmental benefits.
The term “remanufacturing” refers to restoring a machine in operation on site, with its quality condition being nearly the same as that of a new machine and supplying it to the market. Notably, remanufacturing consists of collecting used components, disassembling and assembling, testing, and painting [3
As shown in Figure 1
, a wide variety of products are being remanufactured, and the remanufacturing market is growing exponentially, especially in recent times. Market research suggests that the remanufacturing market has a significant impact on various industries, with a market size of USD 43 billion in the United States of America (USA) (2011), USD 33 billion in Europe (2015), and USD 3.5 billion in China (2020). Notably, in the future, the remanufacturing business is expected to continue to grow globally [4
]. An increasing number of studies on remanufacturing from the USA, Europe, and China reflect the growth of the remanufacturing market [7
]. In the aerospace industry, because the quality of maintenance is directly related to service life and cost, which accounts for ~35% of the total market, the maintenance of each aircraft is strictly managed by not only manufacturers, but also airline companies (end-users) [8
]. In the auto parts industry, which accounts for ~19% of the automotive market, there is a large market for remanufactured parts, owing to a high demand for auto parts, although the unit cost of parts is low [9
Commercial vehicles, such as buses and trucks, which are at the top of the remanufacturing market, are included in heavy-duty off-road (HDOR) equipment. In the construction and mining machinery industries, in which machinery is operated for ~24 h a day and 365 days a year, remanufactured parts are supplied from a logistic center near the construction site; this enables the quick replacement of parts on the site and reduces machine idle time. Therefore, there is a high demand for remanufactured parts. In particular, mining companies often use remanufactured parts throughout the process after cars are delivered from the first overhaul to scrapping. Remanufactured parts for mining machinery (mining shovels and trucks) accounted for a large share of the market, with an estimated market size of USD ~2 billion in 2015 [10
There are many reports on the study and development of remanufacturing technology, with a focus on the process, demand forecasting, and machine and repair technology [3
]. In the field of auto parts, environmental assessment studies have focused on the life cycle assessment (LCA) and economic assessment of remanufactured alternators and liquefied natural gas (LNG) and diesel engines [13
]. Similarly, in the field of photocopiers and multifunction copiers, the LCA’s of remanufactured copiers have been conducted [15
]. Owing to the difficulty in obtaining data, there are only a few studies on the environmental assessment of remanufacturing, with a focus on construction and mining machinery. Xiao et al. [16
] assessed the future economic performance and an LCA for three cases: a machine mounted with remanufactured parts, a machine with parts remanufactured at the manufacturer’s factory, and a machine with parts remanufactured by a local dealer. They developed a hypothetical operational model in their assessment, and their LCA was only an estimate of the environmental impact of the manufacturing process only till the level of the production of materials, such as steel products, and was not an assessment of the detailed manufacturing process [16
]. Jun et al. [17
] conducted an LCA and estimated the environmental impact of the manufacturing process by multiplying the weight and material of each part of a hydraulic cylinder and reducer (=device) used in a construction machine and the replacement rate of the part by the characterization factor for the material. Additionally, previous studies have estimated the environmental impact of a construction machine by multiplying its estimated weight with the characterization factor of the material [17
]. A study was conducted on the LCA of new construction and mining machines. Kwak et al. [19
] calculated the global warming potential (GWP) of a wheel loader mounted with a Tier 4-certified diesel engine throughout its lifecycle, from material mining to end-of-life disposal. In addition to calculating the GWP during the service life of the machine, they performed a sensitivity analysis, while considering the differences in the load factor and fuel consumption of the engine, according to clients. The GWP during the service life of the machine accounted for ~80% of the GWP throughout the lifecycle, whereas the GWP in the maintenance process (remanufactured parts are not used) was estimated to be ~6% [19
]. Using an LCA approach, Ebrahimi et al. [20
] quantified the environmental impact of the engine and other components of a construction machine, which was compliant with European emission standards Stage V, at different stages of the lifecycle, from manufacturing to end-of-life disposal. They deduced that a cleaner alternative method was effective for treating end-of-life tires, owing to the tires’ very high freshwater aquatic ecotoxicity [20
]. The abovementioned studies used the values estimated from the ecoinvent database, because foreground data, such as the electric power used in manufacturing, were not available.
The contribution and novelty of this study comprise a detailed environmental impact assessment of new and remanufactured components used in construction or mining machinery during the heat treatment and machining stages of the manufacturing process. As such, in this study, we assessed the environmental impact per machine of new and remanufactured components used in a mining machine by systematically examining the data obtained during the entire manufacturing process, from material refining to assembly, and in the remanufacturing process, during regular maintenance. In particular, the aim of our study was to portray the environmental impact of remanufacturing through quantitative assessments, with a focus on the hydraulic cylinder and reducer, which are heavy machines consisting of many constituting parts. Particularly, we evaluated the GWP of new and remanufactured components, based on data from an inventory analysis conducted during the new manufacturing and remanufacturing stages.
The organization of the rest of this article is as follows. Section 2
presents the research method, which includes descriptions of the target parts and function unit, system boundary, inventory analysis, and impact evaluation. Section 3
presents the results of the assessment, including the calculated environmental impact of the remanufacturing of each mining machine and a relative evaluation for the case of using a new one. Section 4
discusses the results and evaluates their potential environmental impact on a global scale. Finally, Section 5
gives a comprehensive conclusion and discusses potential next steps of the study.
3.1. Environmental Impact of the Manufacturing Process for Subparts
The manufacturing process of the subparts consists mainly of smelting, machining, and heat treatment. As shown in Figure 6
a–d, we selected heavy parts of the machine as typical parts: cylinder rod and tube, reducer gear, and gear carrier. First, the GWP was generally the highest during the smelting process. Next, the GWP in the surface treatment processes, such as chrome plating and heat treatment, was 1674 kg-CO2
eq./pcs for the boom cylinder rod and 537 kg-CO2
eq./pcs for the travel reducer gear, owing to heat treatment (carburization, high-frequency hardening, and nitriding), portraying that these processes have a relatively high environmental load. A possible reason for the high GWP is that the equipment having large power consumption for plating, carburization, and nitriding is operated for a long period of time, and it consumes a large amount of chrome plating solutions, such as LPG and LNG. Notably, for the boom cylinder tube and travel reducer carrier, which do not require surface treatment, the GWP of the machining process occupies the next share of smelting.
The hydraulic pump was not a heavy part, but we assessed it for comparison purposes. Figure 7
a–c portrays the estimates calculated by aggregating the GWPs of new and remanufactured subparts (including heavy parts, other than those described above) in the manufacturing process, from material smelting to welding and heat treatment. The GWPs in the material smelting process of the parts (the range indicated by the white arrow) tended to be the highest: (a) the GWP for the boom cylinder in the process, from smelting to welding, was ~9820 kg-CO2
eq./pcs for the new subparts, and 6029 kg-CO2
eq./pcs for the remanufactured ones; (b) the total GWP for the travel reducer was 9598 kg-CO2
eq./pcs for new subparts in the process, from smelting to machining and heat treatment, and 4151 kg-CO2
eq./pcs for remanufactured ones. Characteristically, these parts also have a high GWP; and (c) the total GWP for the hydraulic pump in the process, from smelting to machining and to heat treatment, was relatively low, with 1155 kg-CO2
eq./pcs for the new subparts, and 435 kg-CO2
eq./pcs for the remanufactured ones.
These results suggest that in the manufacturing process of the subparts, product weight has a significant impact on the environment. A possible reason for this is that the weight increased in proportion to the processing time in each process.
All parts portrayed a high GWP in the smelting process because they were made mainly of cast iron or special steel and were thus, heavy, although they had a lower characterization factor than those made of mechanical structural carbon steel.
3.2. Environmental Impact of the Manufacturing Process for Assy Parts
The estimation of the GWP of subparts in the assembly stage of the manufacturing process of an assy part is shown in the range indicated by the black arrow in Figure 7
a–c: (a) for the boom cylinder, which is a heavy part, cleaning and assembling accounted for a small share of the GWP. Notably, testing and painting accounted for 55% and 40%, respectively. Particularly, in the painting process, many large, long parts with a large surface area were painted, and thus, paint was used in large quantities, ~3 kg/pcs; (b) for the travel reducer, the GWP was generally low, because each process did not require equipment with high power consumption; and (c) for the hydraulic pump, the GWP in the testing process tended to be high, with 167 kg-CO2
eq./pcs for the new parts and 232 kg-CO2
eq./pcs for the remanufactured parts, despite them being relatively light weight. The main reason was that the testing process consumed a large amount of power. In the testing process, the hydraulic pump was tested by simulating the behavior of the engine when the machine was in operation. Therefore, the test required a high-power motor, control panel, and electric heater to heat the oil to a high temperature. The secondary reason for the high-power consumption was that the pump had more parameters that were tested, compared to the other parts, including low-load operation, resulting in a longer testing time. For the boom cylinder, the power output of the testing equipment was relatively low, and the test was completed in a short time. Therefore, the cylinder had a low GWP in the same testing process, with 48 kg-CO2
eq./pcs for the new parts and 9 kg-CO2
eq./pcs for remanufactured parts.
Each stage of the cleaning process was described as a distinctive feature. The remanufactured parts collected from the market had different appearances: some with dirt, sand, and paint removed, and some with a partially rusted surface. Therefore, it was important to clean the surface of the parts with a high-pressure water cleaner first and then, clean the inside of the parts in the assy state, after disassembly and before reassembly. Some parts could not be reused. In this case, the anti-rust oil was removed from the new parts (they must be cleaned). Because the cleaning process involved several stages, the number of subparts was directly linked to the level of GWP in the cleaning process.
For example, the GWP for the travel reducer in the cleaning process accounted for 76% of the GWP for all the processes. In this case, the number of subparts was 223 pcs/assy. In addition, because this was the same for the hydraulic pump in the cleaning process and many subparts were used in each assy part (346 pcs/assy), cleaning was the second highest in GWP, after testing. Because the number of subparts for the boom cylinder was 72 pcs/assy, which was relatively small compared to the other parts, the GWP was lower for the boom cylinder.
3.3. Environmental Impact of the Logistic Process
portrays shows the changes in GWP for different components in the logistic process. The GWP in the logistic process was estimated on a weight basis, suggesting that the GWP changed according to the weight of the component. In particular, a component with a high GWP was the hydraulic cylinder. Of all the components, the boom cylinder had the largest environmental load, and the GWP was estimated to be ~1275 kg-CO2
eq./pcs. Notably, because the GWP in the logistics process of remanufactured parts depends on their reuse rate, the GWP values are reflected in the reuse rate.
3.4. Environmental Impact of the Remanufacturing Process
illustrates the GWP in the remanufacturing process (cleaning–disassembly–assembly–testing–painting) of individual components that are assembled into a machine in multiple quantities. Generally, as in the manufacturing process of new ones, the hydraulic pump and motor portrayed a high GWP, despite their low weight. Among the components assessed in this study, the hydraulic pump had the highest GWP of 726 kg-CO2
eq./machine. As described in the previous section, this was largely due to the testing process, which accounted for ~90% of the GWP for all remanufacturing processes. Hydraulic travel and swing motors exhibited the same trend. This was probably because only one electric motor was used in the pump and motor testing, and a high-power motor was selected.
Among the other components, the boom cylinder had a GWP of 126 kg-CO2eq./machine during the remanufacturing stage and a GWP of 176 kg CO2eq./machine in the manufacturing stage. The difference in GWP between the new and remanufactured products was relatively small at ~50 kg-CO2eq./machine.
4.1. Total Evaluation
illustrates the GWP per mining machine throughout its life cycle, which was comprehensively assessed, based on the results described above.
It was assumed that the components (hydraulic cylinder and hydraulic pump) with a replacement cycle of 10,000 h would be replaced six times until disposal, and those (reducer and hydraulic motor) with a replacement cycle of 20,000 h would be replaced three times. However, because the components of a machine with an operating time of 0 h are in a new condition, the GWP was assumed to be the same for Scenario 1, where new components were replaced on a regular basis. For Scenario 2, wherein remanufactured components were replaced on a regular basis, they were excluded from the calculation. As a result, the total GWP of all the components was ~415 ton-CO2eq./LC in Scenario 1 and 221 ton-CO2eq./LC in Scenario 2. Thus, our results revealed that a total of 194 ton-CO2eq./LC can be reduced. The difference in GWPs between Scenarios 1 and 2 was 23 ton-CO2eq./cycle for a replacement cycle of 10,000 h and 42 ton-CO2eq./cycle for a replacement cycle of 20,000 h. This is because the material smelting process, which has a large environmental load, can be omitted; thus, significantly reducing the GWP of the remanufactured components. Notably, setting standards, along with the use of equipment and technology to ensure quality, are important for achieving this goal.
Based on the above results, we estimated the GWP for the mining machinery on a global scale, using the number of mining machines in operation [26
]. In this report, the number of mining machines sold per year was estimated at 571 in 2020. From this number, with respect to Scenarios 1 and 2, the GWP was 240,000 ton-CO2
eq./year in Scenario 1, wherein new components were used as replacements during the maintenance of a machine, and 130,000 ton-CO2
eq./year in Scenario 2, wherein remanufactured components were used as replacements. Our calculations portray that the GWP can be reduced by ~110,000 ton-CO2
eq./year in Scenario 2.
The results suggest that the reduction in environmental load of remanufacturing as described above is large. On the other hand, as a prospect of this research, the methodological approach could be made more reliable by considering the environmental impact assessment of other parts such as the engine, or for electrification, as de-carbonization becomes more common in both construction and mining machinery.
4.2. Effects of Green Manufacturing
Green manufacturing refers to energy and resource savings, as well as the control of harmful substances in the manufacturing process [27
]. Therefore, regarding the manufacturing process of new and remanufacturing parts in this research, the process with the highest environmental load is material. That process is prominent. On the other hand, surface treatments such as heat treatment and chrome plating also show a high proportion (e.g., 20% for boom cylinder, and 30% for travel device (see in Figure 7
a,b)). Regarding the assy process, there are concerns about increased power consumption in the pump test process and emission volatile organic compounds (VOCs), Propan-2-ol and Xylene using organic solvent paints [28
] in the paint process. Therefore, there will be challenges to achieve power saving and facilitate the replacement of water-based paints [28
] with lower VOC amounts in these processes, which are important for the overall assy process.
Particularly for the remanufacturing process, it is important to increase the reuse rate of components by developing machining, repair, and nondestructive testing technologies. Notably, increasing the reuse rate can significantly reduce the environmental load. The use of plating solutions and gases for surface treatment in the manufacturing of new subparts portrayed a higher impact on the environmental load than the consumption of power. Therefore, it is important for achieving a carbon-neutral society to more efficiently use plating solutions or switch to a new surface treatment having a lower environmental load. The environmental load of the cleaning process has been further reduced by developing and using a cleaning method [29
]. In addition, if remanufacturing or direct reuse is not possible, a new value can be created by reusing the components among different industries, such as automotive lithium-ion batteries, catalysts, and smartphones, which can make a significant contribution to the circularity of resources [30
4.3. Barrier Review
As described in the previous section, remanufacturing operations are known to have economic and environmental advantages but are not widely used in the industry. This is attributable to three factors: (1) issues related to collection and system, (2) technological issues, and (3) market issues [3
In particular, concerning the collection and system-related issues, some countries have import restrictions on used items. Brazil, Argentina, and the Middle Eastern countries have provisions to ban the imports of retreads and used tires. Indonesia has strict import restrictions on core and remanufactured parts from other countries, including used parts. In particular, companies that develop an import business globally are required to have a license called an Angka Pengenal Importir Umum (API-U), in accordance with the Regulation of the Minister of Trade. However, the license cannot be used to import goods other than those listed in the Harmonized System Code (HS Code), for which a prior application has been made [35
As described above, in some countries, import restrictions on scrapped and used parts apply to core and remanufactured parts, either because remanufactured parts are not well-recognized or to domestic industries. Therefore, there are unintended import restrictions on remanufactured parts. In these regions, one possible option is to operate a remanufacturing business on a local production and consumption basis, instead of catering to global business operations.
Notably, remanufacturers are classified into three types: original equipment manufacturers (OEM), contracted remanufacturers (CR), and independent remanufacturers (IR) [38
]. Owing to local laws, many repair and rebuilding services provided by local CRs and IRs are available in these regions and compete against OEM.
In this study, we assessed the environmental impact of the new and remanufactured components commonly used in mining machines. We obtained highly reliable data by assessing the individual processes in detail. The major findings of this study are as follows:
The GWP of a machine can be reduced by 194 ton-CO2eq./LC throughout its lifecycle per machine, using remanufactured components, instead of new ones. This is because the share of the manufacturing process of subparts decreased as the number of reused sub-parts increases, making a significant contribution to reducing the environmental loads. In addition, considering the number of mining machines in operation worldwide, the GWP can be reduced by ~110,000 ton-CO2eq./year.
Of all the processes, the manufacturing process of the subparts portrayed the highest environmental impact. In particular, material smelting and surface treatment accounted for a large share of the GWP. For example, the GWP of a new boom cylinder rod was 3233 kg-CO2 eq./pcs during smelting. The environmental load of the surface treatment process was significantly affected by the consumption of chrome plating solutions. Hydraulic pumps and motors, which consume a large amount of power in the testing process, can be effective in reducing the GWP and can thus, be powered by renewable energy and energy-saving motors, so as to develop a quality assurance system for low-power motors.
The cleaning process of the remanufactured components portrayed the largest environmental impact. This is because the outside and inside of the components must be cleaned before and after they are disassembled and reassembled. Thus, improving the efficiency of the cleaning process will further contribute to reducing the environmental load of remanufactured components.
Previous studies have not quantified the environmental impact of remanufactured components per mining machine. In our study, we conducted a highly reliable LCA of mining machine components by estimating the GWP of the components, while considering the data measured in the new manufacturing and remanufacturing stages, which have a large environmental load, with regional differences. As a result, this study clarified the lower environmental load which can be achieved by using remanufactured components.
The aim of this study was to assess new and remanufactured components commonly used in mining machinery manufactured by the OEM. By including rebuilt and reused components in the assessment, we expect a further comprehensive discussion of the environmental impact of remanufacturing. However, it is important to assume the replacement cycle and reuse rate of different components, owing to the differences in their quality.