Water Footprint Inventory Construction of Cathode Copper Products in a Chinese Eco-Industry

Abstract

Copper is an important strategic resource for the national economy and social security of China. Water use is a significant component of copper production. However, water shortages and water pollution are two global crises in water resource management. In this study, a copper production industry in China was studied from the perspective of water footprint (WF) and ecological industry (eco-industry). A WF inventory was built by accounting for the entire production and supply chain process, including mining, ore dressing, transportation, smelting, and electrolysis. An index system comprising target, criterion, and variable layers was established to evaluate the sustainable utilization of water resources. It was observed that the studied industry showed a good sustainability for water resource utilization. Only 65.67 tons of freshwater per ton of product was inputted in the entire process due to virtual water (VW) and the use of reclaimed water. However, the WF of each ton of cathode copper product was 162.58 t, and the imported VW of the eco-industry accounted for 92.45%. Increasing the VW import and reducing the VW export can alleviate water shortages. A detailed WF analysis showed that the effects of evaporation and different types of losses on the blue WF (BWF) were significant and should be considered. Upstream water consumption of electricity and other energy sources were also observed to be an important part of the BWF. Regardless of whether freshwater or recycled water was used, the WF can be reduced only by effectively reducing water consumption.

1. Introduction

Copper is one of the three important strategic metals among copper, iron, and aluminum [1], which is important for the economic and social security of China. More than 90% of the modern industries require copper products [2]. China is the world’s largest manufacturer of refined copper, accounting for about 37% of the output. However, there are still many problems, including the expansion of copper processing industries, and conflicts between resources and the environment [3]. According to a study by the U.S. Department of the Interior and the U.S. Geological Survey on water consumption in copper ore flotation, it takes 1.5–3.5 tons of new water to deal with 1 ton of copper sulfide ore through a traditional comminution-lapping–flotation concentrate process [4]. Arsenic-containing wastewater produced by copper smelting is a hazardous waste with extremely high concentrations of arsenic and sulfuric acid, posing a huge challenge to human health and the ecological environment [5]. Water is one of the most important resources for human survival and development. Globally, only 3% of freshwater is available for direct use by humans [6]. Urban, domestic, and industrial water use together account for 31% of global freshwater withdrawals [7]. Water shortage and water pollution are the two major global crises in water resource management [8]. China is rich in water resources, but the available water per capita is only one quarter of the world’s average [9]. Mining and extraction of non-ferrous metals are among the top 10 water-consuming industries in China.

China has included VW in its national water strategy to ease water pressure. Once physical water (PW) is used in production, it becomes VW that is embedded in the product, and can flow through the supply chain between economic sectors and regions [10]. International trade can transfer water resources between regions. China was the largest importer of VW in 2001, and the VW imports became more than double to 71 km³ in 2007 [11]. A large amount of VW has been imported to support the growing consumer demand.

The concept of VW was first proposed by Allan (1996) to denote the water embodied in imported food, and was later defined as the water needed to produce goods and services in the industrial chain [12]. Water footprint (WF) refers to the amount of water consumed by goods and services in a defined geographical area (or an industrial sector or population) [13,14]. It is a multidimensional indicator of water usage and pollution, which shows the “lost” water from the system and indicates the allocation of water resources at a particular time and place [15]. WF evaluation has become vital to sustainable water development [16], and provides decision support for water resource management [14]. WF accounting mainly includes bottom-up and top-down methods [17]. Bottom-up refers to process analysis, which uses a detailed description of a single production process. The top-down approach resembles Input-Output-Analysis which is adopted in economic and environmental domains. The WF Network (WFN) [18] and International Organization for Standardization (ISO 14046) [17] have simultaneously developed the international WF standards.

ISO 14046 believes that environmental impacts are the key to understanding the WF, and the formulated international WF standards consider environmental impacts as the core principle [19]. The WF assessment of products, processes, or organizations in Environmental Management—Water Footprint—Principles, Requirements, and Guidelines, formulated by ISO 14046 in 2014 is based on the principles of their life cycle assessment (LCA). However, LCA studies have been designed to assess the overall environmental impact of products. The impacts on water resource utilization and water quality are only two among a series of environmental impacts of products [20]. The LCA research group focuses on the impact of water use on the local environment while ignoring the larger global water shortage problem [21].

Currently, WF research is mainly focused on the assessment of water consumption in agricultural product systems, while limited information is available on the WF of primary metal production [22]. WF during 2006–2015 attached great importance to VW trade and flow, agricultural biodiesel production, sustainable consumption and water pollution, water–energy relationship, and resource consumption [23]. Studies have also been conducted on the WF of different diets and food loss/waste, crop yields, and application of water efficiency in irrigation. [23]. However, few studies exist on the WF of copper products. In the process of copper production, the most amount of water is consumed in the concentrator [22]. Studies on the WF of the copper industry mainly focus on the ore dressing process [24], overlooking the WF of copper smelting and cathode copper production processes. Several studies have been conducted focusing on the final product of accounting of the copper industry, while the detailed WF inventory of each production unit of cathode copper is missing. Since the copper production industry consumes large amounts of water, they are the focus of local water resource managers. Therefore, the inventory construction of the WF and water use sustainability evaluation of the local, large water-consuming industry can help to explore the problems in water resource utilization by industry, and provide a theoretical basis for local managers to make decisions.

In this study, the cathode copper production industry in China was taken as an example, and the production and supply of cathode copper was considered as the research boundary. The enterprise is a national-level eco-industry, which refers to the industrial production organization that transfers the surplus energy and materials in the production process to other production processes so as to improve the resource and energy utilization efficiency of the whole production process, and reduce the amount of waste and pollutants. Its production water comes from the reclaimed water treated by the local sewage treatment plant. Freshwater is used only for daily living. Analysis was made from the perspective of the WF and eco-industry. The bottom-up approach was used to calculate the WF of the cathode copper products according to the WFN method, while the WF inventory of the cathode copper production and supply chain was established to aid in the sustainable development of water use. An index system was established to evaluate the sustainable utilization of water resources. Relevant countermeasures and suggestions for water resource protection in the copper smelting industry are proposed based on the results of this study.

2. Materials and Methods

2.1. Study Area and Data Source

Extraction of pure copper (99.99%) from sulfide ores with <1% copper content is a multi-step process. Chalcopyrite (CuFeS₂) is the dominant copper sulfide ore [25]. Copper concentrate is produced from natural ore through mining and beneficiation (including crushing, flotation, separation, concentration, dehydration, etc.). There is a pyro-metallurgic route and a hydro-metallurgic route for copper production. Pyro-metallurgic route was adopted in the research enterprise. It refers to the process of producing anode copper from copper concentrate through pre-drying, flash melting, basic oxygen furnace blowing, anode furnace refining, and anode casting. Finally, cathode copper and anode mud were produced through electrolysis, and the anode mud was processed to produce gold, silver, and other by-products [26]. This study analyzed the production and supply of cathode copper, including mining, ore dressing, transportation, smelting, and electrolysis, calculated the WF of each unit and link of the process, and established the WF inventory of cathode copper.

The studied enterprise is located in northern China, and its main products are cathode copper, sulfuric acid, and rare metals such as gold and silver. They adopt “steam drying—flash smelting—flash blowing—rotary anode furnace refining—permanent stainless-steel cathode electrolytic” method to produce cathode copper. A Karl furnace recovers the precious metals associated with copper concentrate in the anode slime. Sulfur was recovered from the copper concentrate by dynamic dilute acid washing and purification, and a two-turn acid absorption process. According to statistical data [27], the Falkenmark Water Stress Indicator of the city where the ore refinery is located is 96.75 m³, indicating extreme water shortage.

Due to the shortage of copper resources in China and the low grade of the mineral, the company mainly imports raw copper concentrate from Chile. In northern Chile, copper ore is mined from an open-pit mine and transported to a concentrator for subsequent crushing, grinding, and screening, followed by flotation enrichment to obtain ~30–40% copper concentrate [22]. The boundaries of this study are shown in Figure 1.

Smelting and electrolysis data were obtained from field investigation. Shipping data were obtained through telephonic conversation with the shipping company. The production and transportation data of mining and ore dressing in Chile were obtained from relevant literature [22]. Pollutant discharge data were calculated according to the Manual of Production and Emission Accounting Method and Coefficient of Emission Source Statistical Survey [28] and Technical Specifications for Mining and Mineral Processing Wastewater Treatment of Copper, Nickel, and Cobalt (HJ2056-2018) [29]. Indirect water consumption information was obtained by consulting the Ecoinvent 3.1 (Competence Centre of the Swiss Federal Institute of Technology, Zürich, Switzerland. www.ecoinvent.org, accessed on 2 November 2021), and the Chinese Reference Life Cycle Database (CLCD) (www.ike-global.com/#/products-2/chinese-lca-database-clcd, accessed on 2 November 2021), jointly developed by the Sichuan University and Ike Environment in China.

2.2. WF Accounting Method

The WF includes green, blue, and grey waters. Green WF refers to rainwater and snowmelt stored in soil roots that evaporates back into the atmosphere, and is mainly related to agricultural and forestry products [18,30,31]. Blue WF (BWF) refers to surface or groundwater that evaporates or is added to products, and is generally consumed by irrigation, industries, and households [18,30,31]. Grey WF (GWF) refers to the water required to absorb pollutants associated with a particular activity to meet the local water quality standards [30,32]. Thus, the WF measures the occupancy of freshwater as a natural resource (through the green and blue freshwater resources index) and the use of freshwater as a medium for waste assimilation (through the grey freshwater resource index) to unite water quantity and quality in one indicator [33]. The bottom-up approach departs from the smallest unit feasible in assessing VW and WF, and then aggregates each unit to desired scale and period [31]. It provides a more complete picture of water use. The purpose of this study was to establish a list of water footprints detailed to each production unit of the process, and a bottom-up approach is more appropriate for this. In this study, the bottom-up method was adopted based on the WF accounting standard proposed by the WFN. The WF of the final “product” was the sum of the WF of each production step [18].

Cathode copper production does not involve green water consumption. The blue BWF included evaporated water, water added to the product, and water that does not backflow or simultaneously return to the same catchment [18]. The calculation is as follows:

$${\mathrm{WF}}_{\mathrm{blue}}=\mathrm{BlueWaterEvaporation}+\mathrm{BlueWaterIncorporation}+\mathrm{LostReturnflow},$$

(1)

The GWF is the pollutant load (L; expressed by mass) divided by the environmental water quality standard of the pollutant (maximum acceptable concentration, C_max; expressed by mass/volume) and its natural concentration in the receiving water (C_nat; expressed by mass/volume) [18]:

$${\mathrm{WF}}_{\mathrm{grey}}=\frac{\mathrm{L}}{{\mathrm{C}}_{\max }-{\mathrm{C}}_{\mathrm{nat}}} ,$$

(2)

GWF is determined by the most polluting pollutant [34]. The final copper WF of the cathode is calculated as follows:

$$\mathrm{WF}={\mathrm{WF}}_{\mathrm{blue}}+{\mathrm{WF}}_{\mathrm{grey}} ,$$

(3)

2.3. Sustainable Evaluation Method

Eco-industry is a new form of industrial organization designed on the theory of circular economy. It refers to the comprehensive use of technical, economic, and management measures to transfer the surplus energy and materials generated to other production processes, so as to improve the resource and energy utilization efficiency of the whole production process and reduce the amount of waste and pollutants. Eco-industries follow the 3R principle of reduction, reuse, and recycling of the circular economy, and their goal is to minimize regional waste and improve the resource utilization rate [35]. To a certain extent, the sustainable utilization of water resources is consistent with the development goals of eco-industries. In order to promote the construction of ecological civilization in the industrial field, and regulate the construction and operation of national eco-industrial demonstration parks, China formulated the Standard for National Demonstration Eco-industrial Parks (HJ274-2015) [36]. The index system construction of this study referred to this standard. The standard stipulates the evaluation methods, evaluation indexes, data collection, and calculation methods of national eco-industrial demonstration parks. The evaluation indexes in the standard include five categories: economic development, industrial symbiosis, resource conservation, environmental protection, and information disclosure. For an enterprise, there was no industrial symbiosis. Information disclosure mainly affected the social level. The purpose of this study was to evaluate the sustainability of enterprise water resources utilization, and the indicators included environmental, economic, social, and resources. Specific variable indexes were determined according to the situation of cathode copper production enterprises, reference standards, and expert opinions.

We adopted the evaluation method of the hierarchical index system to establish the index system with the target, criterion, and variable layers. The index weight was determined by the expert ranking method, which was proposed by Professor Cheng Shuxiao in China [37]. It consisted of three steps: First, a questionnaire was issued to rank the importance of the indicators, with the first importance recorded as 1 and the second importance recorded as 2; if the latter two indicators were equally important, then they were recorded as the average of the latter two rankings, e.g., 3.5, and the rest recorded as 5, 6, etc. Second, the questionnaire was retrieved for data processing that included reliability test and weight calculation. Kendall’s harmony coefficient method [38] was used for the reliability test:

$$\mathsf{\omega }=\frac{\mathrm{S}}{{\mathrm{m}}^2({\mathrm{n}}^3-\mathrm{n})/12},$$

(4)

$$\mathrm{S}={\displaystyle \sum }{\left[{\mathrm{R}}_{\mathrm{i}}-\frac{{\displaystyle \sum }{\mathrm{R}}_{\mathrm{i}}}{\mathrm{n}}\right]}^2,$$

(5)

where ω is the reliability, n is the number of indicators, m is the number of experts, and Ri is the sum of each indicator level. When the calculated reliability is ≥0.70, the evaluation result was considered acceptable. The weight calculation formula is:

$${\mathrm{w}}_{\mathrm{i}}=\frac{2\left[\mathrm{m}\left(1+ \mathrm{n}\right)-{\mathrm{R}}_{\mathrm{i}}\right]}{\mathrm{mn}\left(1+ \mathrm{n}\right)}$$

(6)

Each index value was calculated from the industry survey and statistical data, and the research industry was scored by comparing it with the average level of the industry. The full score of each variable layer index was 100 points. Finally, the index scores were weighted, and the sustainability degree of the industry’s water resource utilization was obtained by referring to the evaluation standard. Sustainability evaluation criteria were obtained from the suggestions of the experts (

Table 1. Evaluation standard of hierarchy index system method.

Range	Sustainability
85–100	Good
70–85	Sustainable
60–70	Poor
0–60	Unsustainable

).

3. Results

3.1. WF Accounting

3.1.1. Mining and Ore Dressing in Chile

Chile has the richest copper resources in the world, accounting for approximately 1/4 of the world’s total reserves. It is the main producer and exporter of copper concentrate and electrolytic copper. China’s copper resources are low-grade taste, and difficult to mine and concentrate; therefore, it imports a large amount of copper concentrate [39] from Chile. Copper production is divided into pyrometallurgy (mining, dressing, smelting, and refining) and hydrometallurgy (mining, heap leaching, and solvent extraction and electrodeposition). Generally, pyrometallurgy is used for copper sulfide, and hydrometallurgy is used for sulfur oxides [40]. The research industry of this study used copper sulfide.

After extraction, the ore was transported to the concentrator for subsequent crushing, grinding, and screening. Pena and Huijbregts (2014) chose Codelco Norte Production as a representative case in Antofagasta, Northern Chile [22]. The BWF of sulfide and oxidized ores in beneficiation, smelting, and refining processes that included material consumption of more than >1 wt. %, and consumption of all electricity and fuel were calculated. The PW consumption of mineral processing was the largest with ~60.00 t water per ton of cathode copper, while the VW consumption was 8.18 t per ton of cathode copper. The PW and VW consumption in the mining process was ~5.4 t and 0.8 t [22] (

Table 2. BWF of the mining and ore dressing process in Chile.

Process	PW (t)	VW (t)	WFblue (t)
Mining	5.40	0.80	6.20
Ore Dressing	60.00	8.18	68.18
Total	65.40	8.98	74.38

). Therefore, the BWF of cathode copper in Chile was 74.38 t per ton.

The grey water (GW) produced in mining and ore dressing came from the discharge of pollutants from their wastewaters. Mining wastewater included the gushing water of the mine pit (well), and the leaching water of the dump and waste rock field that contains heavy metals, and is supplied by rainfall; ore dressing wastewater contained heavy metals and agents produced by processing, overflow water of the tailing reservoir, seepage water under the tailings dam, rainwater in the early stage, and ground flushing water of the workshop. Presently, most of the wastewater is treated and reused, and the removal rate of pollutants can reach 95–99%. Due to the lack of literature on the analysis of copper mining wastewater in Chile, the pollutant discharge concentration and discharge amount of mining and beneficiation wastewater were obtained from the Manual of Production and Emission Accounting Method and Coefficient of Emission Source Statistical Survey [28], and Technical Specifications for Mining and Mineral Processing Wastewater Treatment of Copper, Nickel and Cobalt (HJ2056-2018) [29]. The GWF per ton of copper cathode was 6.19 t and 51.53 t, respectively.

3.1.2. Transport

Copper concentrate is transported from Chile to China via sea routes. The WF mainly includes indirect water consumption of fuel oil, blue water, and grey water consumption of personnel. The fuel consumption, transportation time, and personnel quota of each road section were obtained from relevant literature and consultation with freight companies. The WF of fuel oil (light and heavy) were obtained from the Ecoinvent 3.1 Database and CLCD. A person’s WF can be obtained from www.waterfootprint.org (accessed on 15 October 2021), a website developed by the University of Twente in the Netherlands. The WF of 1 t cathode copper during sea transportation was 0.54 t.

3.1.3. Production Process in China

Along with the main process of “steam drying—flash melting—flash blowing—rotary anode furnace refining—permanent stainless steel cathode electrolysis” copper production of the research industry, it also includes flue gas and other waste treatments, as well as processing of sulfuric acid, rare metals, and other by-products. The water used in these processes is mainly the treated water from the local sewage treatment plant, with water links including pure water stations, power supplies, raw materials’ supply, melting–blowing–refining–electrolytic cathode copper production process, such as sulfuric acid production, reclaimed material processing, mineral processing, oxygen, rare metal production, and railway maintenance. The investigated factory had two sewage treatment stations, where one treated the concentrated water produced by the pure water station, which was processed and then transferred to the local sewage treatment plant, and the other was used for the acid-bearing wastewater produced by sulfuric acid production, which was used for the slow cooling process of the slag. Domestic water came from fresh groundwater, and domestic sewage was discharged into the local sewage treatment plant through pipes. The sewage treatment plant accepted the wastewater of other industries and residents in the area as well, which was treated and transported to the research industry through pipelines. The water flow situation of the industry was estimated through field investigation (Figure 2). The blue arrow in Figure 2 represents the PW consumption of blue water (BW), and the grey arrow represents the GW.

The PW per ton of cathode copper products was directly obtained by industry field surveys conducted. The VW was calculated by consulting the Ecoinvent and CLCD Databases through investigation and statistics of all upstream processes, such as raw and auxiliary materials and energy use, including electricity and natural gas. The BWF per ton of cathode copper product in China was 29.54 t, of which the PW consumption was 10.61 t and the VW was 18.93 t.

GW consumption came from the discharge of domestic sewage, circulating cooling water, and sewage from the sewage treatment station. These three types of sewage were processed by the local sewage treatment plant, and then discharged into the same water body, with the same pollutant discharge standard and concentration. The GWF per ton of cathode copper product in China was 0.40 t, including 0.15 t for employees, 0.19 t for pure water station, 0.06 t for power supply, and cathode copper and sulfuric acid production, while no or negligible GW was produced in other processes.

3.2. Cathode Copper WF Inventory

From the analysis of the production and the WF calculation of the entire life cycle of the cathode copper products, the WF inventory of the cathode copper production was constructed within the boundaries of the study (

Table 3. WF inventory of the cathode copper product.

Process	WFblue (t)	WFgrey (t)	WF (t)
Mining	6.20	6.19	12.39
Ore Dressing	68.18	51.53	119.71
Transport	0.54	0.00	0.54
Domestic	0.16	0.15	0.31
Pure Water Station	0.05	0.19	0.24
Power Supply	1.50	0.02	1.52
Material Supply	3.69	0.00	3.69
Cathode Copper Production	13.14	0.02	13.16
Laboratory	0.02	0.00	0.02
Sulfuric Acid Production	4.27	0.02	4.29
Reclaimed Material Treatment	0.21	0.00	0.21
Beneficiation	0.80	0.00	0.80
Oxygen Production	2.55	0.00	2.55
Rare Metal Production	0.33	0.00	0.33
Railway Maintenance	0.02	0.00	0.02
Wastewater Treatment Station	0.64	0.00	0.64
Slag Cooling	2.17	0.00	2.17
Total	104.46	58.12	162.58

). The WF of cathode copper product in the whole process is 162.58 t, in which BW was 104.46 t and GW was 58.12 t. The WF of mining and ore dressing in Chile accounted for 81.25%, while that of smelting in China was only 18.42%. A detailed analysis of smelting showed that BW consumption accounted for 98.66%, among which the WF of the main production accounted for the highest proportion (43.95%), while the other processes (sulfuric acid production, raw material supply, oxygen production, slag slow cooling, and power supply processes) were >5%. Figure 3 shows the proportion of WF for each process. The grey WF was only 0.40 t because the wastewater of the industry was recycled and fully utilized in the sewage treatment plant.

Through the whole production chain, the freshwater input per ton of cathode copper product was calculated, as shown in

Table 4. Freshwater input per ton of cathode copper product.

Process	Freshwater Input (t)
Mining	5.40
Ore Dressing	60.00
Smelting and electrolysis	0.27
Total	65.67

. Due to VW and the use of reclaimed water, freshwater input was only 65.67t, far less than the WF.

3.3. Sustainability of Water Use

Sustainability includes environmental, economic, social, and resource sustainability. The target layer was the sustainable use of water resource, while the criterion layer included the environment, economy, social, and resources. Referring to the indicators of economic development, industrial symbiosis, resource conservation, and environmental protection in national standard [36], indicators of the variable layer were obtained through expert consultation, which were screened based on their connotation, correlation, independence, necessity, operability, comparability, and pertinence to obtain the evaluation indicators [41]. The variable level index was determined, including the environmental aspect of water emissions of the unit industrial output value and its corresponding economic aspect of freshwater use, compliance rate of water emissions and running condition of water pollution facilities, proportion of high and new technology, the social aspect of urban water shortage index and the share of water resources, the resource aspects of water recycling efficiency, clean energy utilization, and resource use efficiency (

Table 5. Hierarchical evaluation index system.

Target Layer	Weight	Criterion Layer	Weight	Variable Layer	Weight
Sustainability of water resources utilization	1	Environment	0.25	Water emissions of unit industrial output value	0.50
				Water emissions compliance	0.28
				Running condition of pollution treatment facility	0.22
		Economy	0.25	Freshwater use of unit industrial output value	0.63
				High and new technology proportion	0.37
		Society	0.25	Urban water shortage index	0.63
				Share of water resources	0.37
		Resource	0.25	Water recycling efficiency	0.43
				Clean energy utilization	0.40
				Resource use efficiency	0.17

). The indicator explanation is given in Appendix A.

The weight of the target layer was 1, and the sum of the weight of the criterion layer was 1. The sum of variable layer index weights of each criterion layer was 1. As for the impact of the sustainable utilization of water resources in the target layer, this study proposed that the weight of the environmental, economic, social, and resource factors in the four-layer criteria are equal, i.e., 0.25. The weight of the variable layer was calculated by the expert ranking method. The reliability results of the expert ranking questionnaire were calculated to be 0.79, 0.82, 0.82, and 0.76, based on the Kendall harmony coefficient (Equations (4) and (5)), which were acceptable. The weight of each variable index was calculated by using Equation (6) (Table 5).

Finally, the score of the research industry was calculated to be 85.99 (

Table 6. The evaluation results.

Variable Layer	Value	Score	Total
Water emissions of unit industrial output value	0.18 t	99.60	85.99
Water emissions compliance	100.00%	100.00
Running condition of pollution treatment facility	99.12%	99.20
Freshwater consumption of unit industrial output value	0.05 t	100.00
High and new technology proportion	93.03%	94.00
Urban water shortage index	96.73 m3	40.00
Share of water resources	90.16%	77.20
Water recycling efficiency	97.26%	92.00
Clean energy utilization	98.22%	91.20
Resource use efficiency	75.94%	98.60

). Compared with the standard (Table 1), the sustainable utilization of the water resources of the research industry was found to be good. The most significant factor affecting the sustainability of the industry was extreme water shortage of the city where it is located. If the industry were to be located in another water-rich city, the degree of sustainability would have increased remarkably.

4. Discussion

4.1. Water Resources Management Strategy

As an eco-industry, all the water used in production was reclaimed water discharged from sewage treatment plants. From the mining process, the freshwater input of each ton of cathode copper product was only 65.57 tons, and the water resource utilization was sustainable. However, the total WF was 162.58 tons. Detailed analysis was made from the perspective of the WF.

Table 7. BWF consumption of cathode copper product in China.

Process	Water Consumptive Use	PW (t)	VW (t)
Power Supply	Water evaporation; Different type of water losses; Electricity	0.81	0.69
Material Supply	Water evaporation; Different type of water losses; Materials	0.06	3.63
Cathode Copper Production	By-product production; Water evaporation, Different type of water losses, Materials; Electricity	2.9	10.24
Sulfuric Acid Production	Product production; Water evaporation, Different type of water losses; Materials; Electricity	2.92	1.35
Oxygen Production	Water evaporation; Different type of water losses; Electricity	0.47	2.08
Slag Cooling	Water evaporation; Different type of water losses	2.17	0.00

shows the BW consumption of processes with WF ratios >5.00%, in which the VW accounts for 65.84%, mainly from raw materials, auxiliary materials, electricity, and natural gas. Figure 4a shows the proportion of each source, in which power water consumption accounts for 38.67%. The WF of smelting also reached 23.51%. Therefore, saving energy during smelting, especially power consumption, is of great significance in reducing the WF of cathode copper products. Furthermore, improving the utilization rate of raw materials and other resources can also help reduce the BWF. The PW of blue water was mainly in production, evaporation, and loss in different ways, among which copper smelting, sulfuric acid production, and slow cooling of slag contributed significantly. Figure 4b shows the proportion of water consumption in each method, in which the proportion of evaporation and loss is 72.56%. Industries need to strengthen the overhaul and maintenance of circulation cooling systems and other airtight water facilities, and reduce water evaporation and loss during pipeline transportation. Notably, the WF of the slow cooling process of slag was also due to evaporation. In this process, industries used the production wastewater that had been treated by the wastewater treatment station of the production factory, as well as some new industrial water (water from the sewage treatment plant). According to traditional management philosophy, this process did not use freshwater, and at the same time, reduced sewage discharge, which helps protect the environment. However, the WF of this process accounted for 23.26% of the PW of BW in the whole plant, which was not sustainable with respect to WF. Therefore, it was necessary to strengthen the water recycling in this department and manage facility airtightness.

Throughout the production process, China imported 132.1 t of VW per ton of cathode copper, accounting for 81.25% of the WF, which was beneficial for saving water resources. The VW imported by the eco-industry reached 92.45% for the city where the industry is located, which helped to alleviate the extreme water shortage in the city. Increasing VW imports can reduce the use of locally produced PW. The more VW is imported, the less local water is used. Conversely, reducing VW exports can also save local water resources. From a water conservation perspective, for a region with severe water shortages, the existence of raw material primary processing enterprises with large water consumption should be reduced as far as possible to reduce the export of VW.

4.2. Limitations

Although this study refined the entire production and supply chain of cathode copper to unit WF, there are still limitations that require further study. First, the BW data of Chile were obtained in 2009. As global problems, such as environmental pollution and resource shortages have become increasingly prominent, and environmental protection, resource conservation laws, and regulations in various countries and regions have become increasingly efficient, the WF of mining and mineral processing in Chile in recent years may be smaller than that used in this study.

Second, there may be some errors in the statistical process of the production data in China, which was mainly due to the difficulty in estimating the evaporation and losses. The water evaporation data of the circulating cooling water in copper smelting and refining were mainly obtained by the difference between the freshwater recharge and circulating wastewater discharge. The electrolytic cell in the workshop was covered with a steel plate, and evaporation was estimated by recharge and discharge. The sewage treatment system mainly depended on the inflow and outflow measurements of water. The GWF changed with the change in the pollutant discharge concentration. Water consumption varied with the level of production operations and management of employees. We investigated the industry data of 2015–2019, and calculated the average over the years to reduce the uncertainty of data as much as possible.

Third, the VW data of the supply chain were from Ecoinvent 3.1 and the CLCD Database. Most of the data in the databases comes from China and the European Union, and their results may vary with the origin of different materials. In this study, the source of the materials was determined according to the location of the production process. Pollutant discharge data in Chile were calculated from the coefficients obtained from the Chinese Pollution Discharge Technical Manual. Data uncertainty exists due to different production processes and technical levels. The GWF of the supply chain process was not considered in this study, and we hope to improve it in future research.

5. Conclusions

In this study, the cathode copper products’ inventory of the WF was established, with respect to its entire life cycle, starting from mining, ore dressing, and cross-border transportation in Chile to the copper smelting in China and the auxiliary technology. An index system with target, criterion, and variable layers was established to evaluate the sustainable utilization of the water resources of the industry, and it was observed that the industry had good sustainability. The freshwater input of each ton of cathode copper product was only 65.57 tons throughout the whole production process due to VW and the use of reclaimed water, while the total WF was 162.58 tons. A detailed WF analysis of the production processes in China was carried out. VW was produced mainly through trade. In the process of raw material procurement, 92.45% of VW was imported, and the local water intake in the entire production was only 10.61 t. A large amount of imported VW helped alleviate extreme water shortages in the city. Simultaneously, further improvements can be made in the industry’s water resource management and environmental protection. First, the industry should reduce process-based energy consumption, especially power consumption, recycle heat as much as possible, and improve the resource utilization rate. Second, the industry should strengthen the overhaul, and maintain the circulating cooling system and airtightness of other water facilities, as well as reduce the evaporation and loss of water during pipeline transportation. Third, attention should be paid to recycling of the water used in the slow cooling process of slag, and the airtight management of facilities.

The BWF can be reduced only by effectively reducing water consumption, while strengthening water recycling and reuse can further reduce GWF. The evaporation and loss of water from unknown paths have a great influence on the BWF, and should be paid more attention. The upstream water consumption of electricity and other energy is also an important part of the BWF, and saving resources and energy consumption, as well as improving the utilization rate can also help reduce the WF. Additionally, reducing VW export and increasing VW imports are important for alleviating water shortages.