A new method of full resource utilization of copper slag

Highlights

• Copper slag has achieved full resource utilization.

• The silicon and iron in the copper slag is well-utilized.

• The “circulation” leads to an increase of the recovery rate of metal ions.

Abstract

Copper slags can hardly be utilized because the fayalite in it produces silica gel during the dissolution process. This work proposes a new process on sulfuric acid treatment of copper slag (Jinchuan Group, Gansu Province, China.). The results show that over 98% of recovery efficiency of Cu²⁺, Co²⁺, Ni²⁺ and Zn²⁺ was achieved through the whole process cycle. Ammonia was used to control the pH of the solution in order to completely precipitate Fe²⁺, and then produce hematite powder. More than 85% of the non-ferrous metal ions were left in the solution, and the utilization method of stepwise extraction was evaluated. The generation of silica gel during the acid digestion of copper slag was not restricted, and the silica gel in the acid-leach residue was subsequently dissolved with NaOH solution to obtain sodium metasilicate solution. Acidification and other steps were carried out to produce high-purity silica with a purity of 99.9%. Magnetite and pyroxene were obtained by using a simple magnetic separation process to segregate the remaining slag phase. The whole process achieves the full resource utilization of copper slag, and exhibits great potential for the future industrial utilization of copper slag in an economically and environmentally friendly way.

Introduction

With the rapid development of construction, electronics, machinery, metallurgy and other fields, the production and consumption of copper have skyrocketed, with its production reaching 28.5 million tons in 2019 alone (Tian et al., 2021). About 80% of the annual copper production in the world is generated by pyrometallurgy process (Han et al., 2018). Processing a ton of copper will produce approximately 3 tons of copper slag (Khalid et al., 2019). The accumulation of large amounts of copper slag will occupy massive land. In the meantime, some external factors, such as wind erosion and rainfall, will bring the heavy metal ions and sulfides into the soil and groundwater. The level of heavy metals tested in surrounding animals and plants always exceeds the standard, which increases the risk of human disease (Xue et al., 2009; Tang et al., 2018; Cheng et al., 2016a). The main mineral phases in copper slag are fayalite, pyroxene, magnetite and metal sulfide (Li et al., 2019). In addition to rich silicon and iron elements, there are non-ferrous metals such as copper, zinc, cobalt and nickel (Perederiy and Papangelakis, 2017). Most of the non-ferrous metals exist in the form of sulfide, and a small part of them replaced ferrous ion in magnetite (Gabasiane et al., 2021; Zhou et al., 2021). The use of copper slag to extract silica and metal elements not only reduces solid waste, but also creates considerable economic benefits. Therefore, the effective treatment of copper slag has become a topical issue in the field of solid waste utilization.

At present, there are three different treatment methods for copper slag, namely beneficiation, smelting and chemical extraction. We have briefly collated some previous work on metallurgical slag treatment in Table 1, listing the treatment method, the type of reagents added, and the recovery efficiency of valuable elements. Besides, some researchers focus on improving the extraction efficiency of copper, by adding CaO and other reagents during the smelting process to lower the melting temperature and viscosity, while an appropriate amount of inert gas was blown into the molten copper slag to accelerate the growth of copper droplets (Roy and Rehani, 2015; Heo et al., 2013). Although the amount of copper in the final slag can be reduced, a significant amount of iron and silicon, as well as abundant nonferrous metals, still remain in it (Coursol et al., 2012).

Flotation is the dominant beneficiation method at present, which mainly aims at utilizing the residual copper in the slag (Zhou et al., 2021; Piatak and Parsons, 2015). However, in order to float out copper sulfide encapsulated in fayalite, a large amount of energy is required to fully grind the copper slag, while other valuable elements are not utilized (Guo et al., 2018a). Smelting method involves the reduction of slag by high temperature roasting, where the amount of additives, reduction temperature, reduction time and the reduction behavior of metal phase all influence the final utilization (Guo et al., 2018b). Sarfo et al. (2017) carried out carbothermal reduction process of copper slag (from the United States) by mixing 12.5% carbon with copper slag and reacting at 1420 °C for 0.75 h. The initial copper slag is reduced to low-grade pig iron with the recovery efficiency of up to 90% (Sarfo et al., 2017). However, non-ferrous metal elements can also enter pig iron during the smelting process, which is difficult to separate. At the same time, the amount of copper slag is so large that direct high temperature reduction consumes large amounts of energy and emits large amount of CO₂.

Chemical extraction is mainly divided into acid leaching, alkali leaching and biological leaching. Bioleaching depends on the metabolic process of microorganisms, and the leaching mechanism depends on the type of microorganisms (Kaksonen et al., 2017). For example, acidophilic bacteria can convert sulfide in minerals into sulfate and oxidize Fe²⁺ into Fe³⁺ (Sethurajan et al., 2013). Kaksonen et al. (2016) dissolved about 80% of copper and 25% of zinc in copper slag (from Finland) by providing H₂SO₄ continuously to acidophilic bacteria at a temperature of 27 °C and pH of 2.5 (Kaksonen et al., 2016). Bioleaching has the advantage of being environmentally friendly and low cost, but the conditions during its production process need to be strictly controlled and the reaction time is extremely long, which limits its large-scale application. Ammonia leaching is the most popular alkaline leaching process (Ku et al., 2016). Its principle is that non-ferrous metal elements such as copper, cobalt and nickel in slag can form water-soluble complexes with ammonia (Bidari and Aghazadeh, 2015). In the leaching process, pH is related to the stability of metal ammonia complex, which directly affects the recovery efficiency (Park et al., 2006). Roy et al. (2016) leached copper slag (from India) with ammonia at a concentration of 1 mol/L, and about 75% of copper was successfully recovered. They also conducted a sulfuric acid leaching experiment for comparison, and 89% recovery efficiency of copper could be achieved. It can be seen that single ammonia leaching was less effective than single sulfuric acid leaching (Roy et al., 2016). In addition, iron ions cannot form complexes with ammonia, but remain as insoluble precipitates in copper slag, which cannot be well utilized.

In the acid leaching method, sulfuric acid is the best leaching agent (Mussapyrova et al., 2021). When the concentration of sulfuric acid increases, the leaching efficiency of metal can be improved, but the silicon dissolved from fayalite will form silica gel, which will affect the subsequent separating operation. At the same time, the presence of iron ions in the leachate also increases the difficulty of subsequent separation and metal recovery (Banza et al., 2002; Zhang et al., 2021). Many researchers have tried to increase the leaching efficiency while limiting the formation of silica gel and preventing iron ions from entering the liquid (Liao et al., 2021; Seyrankaya and Canbazoglu, 2021). Zhang et al. (2010) added NaClO₃ and neutralized it with calcium oxide, which greatly reduced the content of silicon and iron in the leaching solution, and the leaching efficiency was 98%, 97% and 89% for cobalt, zinc and copper, respectively (Zhang et al., 2010). Shi et al. (2020) performed pressure leaching of copper slag from a copper smelter in Yunnan province China. The leaching efficiency of copper reached 97% at 200 °C and 600 kpa. The leaching efficiency of ferrous ions was controlled to less than 1%. Iron remained in the leach residue in the form of hematite (Shi et al., 2020). Table 1 reveals that although the leaching efficiency of nonferrous metals is guaranteed, the recovery of Si is rarely reported, and the subsequent separation of Si and Fe is still a challenge.

In this work, a new process of copper slag treatment was designed to make full use of the valuable resources. First, we studied the process of sulfuric acid digestion of copper slag and investigated the process of silica gel formation. Then we studied the process of separation of iron ions to prepare hematite powder that meets Chinese national standard (GB/T 1863–2008). For the remaining nonferrous metals, a stepwise extraction method was also proposed for the subsequent separation. Most notably, we have successfully solved the problem of utilizing silica gel produced during the acid digestion of copper slag and have successfully prepared a high purity silica product (99.9%). Eventually, a considerable amount of magnetite powder was separated from the remaining leach residue by a simple beneficiation method. The whole process achieves full resource utilization and conforms to the concept of harmless treatment of solid waste.