1. Introduction
Copper anode slime is a by-product of crude copper electrorefining, and a substance that deposits at the bottom of the electrolytic cell during the process of copper electrorefining [
1]. The mass is generally about 0.2–1.0% of the anode plate. Anode slime is one of the main raw materials for extraction of rare and precious metals, because it contains a large amount of gold, silver, copper, selenium, tellurium, and platinum group metals [
2,
3,
4,
5]. The annual output of copper anode slime in China is about 71,100 tons [
6], which is an important source of metallic silver production. With the rapid increase in China’s copper and other non-ferrous metals production [
7], the extraction of silver and other precious metals from anode slimes will have increasingly important economic and environmental impacts. The improvement of related extraction technology and its effect on the environment will also attract more and more attention.
The main method to recover silver from anode slime is pyrometallurgy [
8,
9]. This method usually involves a series of pyrometallurgical processes and consumes a lot of energy [
10], and has disadvantages such as high energy consumption, poor silver recovery, and severe air pollution (e.g., SO
2, lead-containing fumes, arsenic, and antimony) [
11]. The wet method and semi-wet method are adopted by most companies in China. The ammonia method is mostly used in the silver separation process—i.e., ammonia and hydrazine hydrate are used as reducing agents to reduce silver ions into elementary substances. However, due to the higher cost and toxicity of hydrazine hydrate, as well as the high volatility of ammonia, the environmental impact is obvious [
12].
Semi-hydrometallurgical technology has been developed in order to resolve these disadvantages. The technology includes the following steps: sulfate roasting, sulfuric acid leaching of copper, and sodium sulfite leaching of silver [
13,
14]. However, this method also has disadvantages, including high consumption of leaching agent and release of harmful gases such as SO
2. During the pretreatment process, the oxidation of sulfide in the anode slime produces SO
2 during the sulfation step of baking, thus possibly inducing pollution due to careless operations. During the leaching process, due to its poor stability, the measured Ag (SO
3)
32− is easily oxidized by the oxygen dissolved in the aqueous solution [
15]. Thus, it is inevitable to use high-concentration sodium sulfite. As a result, the leaching agent is consumed excessively.
The Institute of Process Engineering of the Chinese Academy of Sciences has proposed an efficient hydrometallurgical technology for recovering silver from anode slime, with sodium thiosulfate as the sliver leaching agent and sodium hydrosulfite for silver reduction. This method has a high recovery rate of silver of up to 95.4% under the optimal conditions. The technology was tested on site by metallurgical enterprise in Jiangxi, with the ammonia method for silver separation, commonly used in China, transformed into a thiosulfate method. With unchanged existing equipment and fine-tuning of the technology process flow, the direct material cost per ton of gold slag dropped from 1654.04 yuan to 802.90 yuan—a decrease of over 50%—thereby achieving good economic benefits. However, the changes in the environmental impact have not yet been analyzed.
There are relatively few studies on the environmental impact of the extraction of precious metals in anode slime. Tang et al. [
16] calculated the waste discharge and emission reduction of copper anode slime utilization through joint process analysis. Chai [
17] determined the migration process of major pollutants of the waste copper smelting production line via material flow analysis and identified the emission items of the accumulation of pollutants such as smelting slag, smoke, and dust. Nuss [
18] analyzed the flow of tellurium in copper anode mud by means of material flow analysis and put forward suggestions on the environmental impact management of tellurium. Iannicelli-Zubiani et al. [
19] used the LCA method to analyze the environmental impact of recycling important metals such as copper and gold from electronic waste and identified industrial processes with greater environmental impact. These studies are more static studies on the environmental effects of the production line, lacking comparison of the environmental effects of different processes, and it is therefore difficult to point out the direction of process improvement. From a methodological point of view, direct process analysis and material flow analysis of a production line are limited to the evaluation of the direct environmental impact of the production line. It is difficult to measure the pollution caused by the production of raw materials and waste treatment in the upstream and downstream industries. Relatively, life cycle assessment traces the source of all input and output items, and evaluates its environmental impact more comprehensively.
Life cycle assessment (LCA) is a process of assessing a product, technology, or activity—i.e., an environmental load-related process throughout the entire life cycle, including raw material collection, production, transportation, sales, utilization, recycle, maintenance, and final disposal. Firstly, it identifies and quantifies the consumption and environmental release of energy and materials throughout the life cycle, then it evaluates the impact of consumption and release on the environment, and finally identifies and evaluates the opportunities to reduce the impact [
20]. Usually, LCA is divided into four steps: definition of goals and scope determination, inventory analysis, impact evaluation, and improvement evaluation. This method quantifies and evaluates the resource consumption, ecological pressure, and human health impact of specific substances throughout their entire life cycle. Moreover, it further analyzes the impact of differences between the raw materials or products on the environment [
21]. Compared with other environmental impact evaluation methods that directly evaluate production, it can more completely assess the entire environmental impact of a specific product or process.
In this paper, life cycle assessment (LCA) method is used to analyze the final resource consumption and pollutant emissions per unit output of the main product—metallic silver—in two different silver separation processes, the ammonia method and sodium thiosulfate method. Then, the impact on different aspects of the ecological environment is analyzed to compare the impact intensity and characteristics between the two processes. On the basis of the overall impact analysis, we analyzed the sensitivity of the environmental impact of all input–output items to select the input–output items that have the greatest ecological impact, and then the direction of technology improvement is proposed. The novelty of this paper is the first life cycle environmental impact assessment of silver extraction from copper anode slime. This assessment will help to identify the links and input–output items with greater environmental impact, and provide direction for future process improvement. The structure of this paper is as follows:
Section 2 describes the methodology and data sources used in this paper. Part 3 explains and discusses the analytical results. Part 4 presents final conclusions and suggestions according to the result analyzed, explores the disadvantages, and touches upon a further research direction.
4. Conclusions and Recommendation
The life cycle environmental impact of the sodium thiosulfate-based silver separation technology is much lower than that of the original process. The weighted person equivalent is approximately 21.94% of the original process. The ecological impact, human health impact, and resource availability are equal to 13.72%, 45.78%, and 23.73% of the original process respectively. The new process has good environmental and economic benefits and broad application prospects. The new process brings in an additional economic income of 8.12 yuan per kilogram of silver. China has an annual output of 71,100 tons of copper anode slime [
6], from which about 4000 tons of metallic silver can be extracted. If the sodium thiosulfate method is adopted for silver separation, an additional economic income of more than 30 million yuan can be generated annually. Meanwhile, greenhouse gas emissions (1.3 × 10
4 tons of CO
2 equivalent), fossil energy consumption (1.6 × 10
4 tons of oil equivalent), acidification (2.5 × 10
3 tons of sulfur dioxide equivalent), and particulate matter emissions (3.1 × 10
2 tons of PM2.5 equivalent) can be reduced.
The new process has the greatest impact on human health, accounting for 47.06% of the total weighted human equivalent. Subdivided further, the impact is mainly divided into climate change, photochemical smog, and ionizing radiation, which account for 37.23%, 17.87%, and 13.38% of the total equivalents respectively. The sum of the three items exceeded two-thirds of the impact on total resources and environment. In terms of input and output, the main impact of the new process on the resources and environment is concentrated on the use of sodium hydroxide, accounting for 33.98% of the total equivalent, followed by sodium thiosulfate and sodium carbonate, accounting for 26.10% and 18.30% of the total equivalent respectively. These input and output items are the key areas that need attention in future technology improvement.
In this paper, the overall environmental impacts of two copper anode slime silver separation processes were analyzed using the life-cycle assessment method, and the ultimate resource consumption and pollutant emissions arising from the two silver separation processes were evaluated. Additionally, two types of environmental impact in different areas were analyzed through subdivision analysis, revealing the raw materials or emission items that should be improved first. This conclusion has greater guiding significance for the improvement of the production technology.
This paper has some shortcomings caused by the test progress. First, limited by the shortcoming of technical analysis, this paper fails to further analyze the technology mechanism of the main raw material consumption and waste discharge; second, this article only evaluates the environmental impact of existing production methods, and proposes improvement directions on this basis, but failed to propose a clear technological path for improvement. These issues will be important directions for our further research.