Production of cobalt from copper-cobalt ores on the African Copperbelt – An overview
Introduction
The African Copperbelt, or colloquially “the Copperbelt”, is the largest sediment-host copper province in the world. It is estimated that the Copperbelt contains 5 billion tonnes of ore containing 3.3% copper, yielding 185 million tonnes of copper. Industrial-scale mining on the Copperbelt has taken place for over a century. Cobalt is produced as a by-product of this copper mining. However, the increasing demand for energy storage and electric vehicles has increased the demand for cobalt (Azevedo et al., 2018), moving the status of cobalt from speciality metal to essential metal as the world’s energy sources change.
The global production of cobalt has increased over the period 2011–2018, as shown in Fig. 2. The data given in Fig. 3 indicates that the increase in global production has been sourced almost entirely from the DRC, which in 2018 produced about 60% of the world’s cobalt (see Fig. 3). This concentration of primary cobalt production in the DRC is mirrored by the concentration of refinery production in China, where about 60% of refinery production occurs (Azevedo et al., 2018, Shedd, 2019).
The price history of cobalt is shown in Fig. 4. Because cobalt is produced as a by-product of either nickel or copper production (Crundwell et al. 2011), the price of cobalt exhibits the volatility associated with by-product commodities. In 2018, the price spiked above 90,000 USD/t but has since dropped. Such spikes in price are not an uncommon experience, with price peaks in 1980, 1995, 2008 and 2018.
The dominance of the DRC in terms of total production and new production is clear from the data shown in Fig. 2, Fig. 3. This dominance is reflected in the biggest cobalt-producing mines and metallurgical operations, as shown in Table 1. All the current top producers are from the DRC. Some of these producers, like Mutanda Mining and Chambishi Metals, are temporarily closed, either for development or for business reasons.
The purpose of this paper is to provide an overview of the metallurgical methods used for separating cobalt and copper as currently practised on the African Copperbelt (see Fig. 1), the largest source of cobalt. Many of these mines have been in existence for decades. However, the processes used and the metallurgy employed have evolved with changes in the technology, market demand and societal pressures. Several other reviews have been published but they are not focused on the current industrial practice of the extractive metallurgy of cobalt. Instead, these reviews are focused on possible options for future development (Sole et al., 2019), on only the mineral processing (Shengo et al., 2019), or on design considerations (Miller, 2009, Fisher and Treadgold, 2009, Fisher, 2011). Other reviews, such as Rumbu, 2016, Prasad, 1989, Crundwell et al., 2011 are either too broad or require updating. In this work, the focus is on the techniques of extractive metallurgy that are currently used on the African Copperbelt to produce cobalt products.
There are two factors that have a bearing on the choice of products from the mine and hence the extractive metallurgy employed. The first influence is the requirements of manufacturers who use cobalt. In the next section, the supply chain to various cobalt products is discussed. The second influence is the mineralogy of the ore mined. This aspect is discussed after the supply chain. Following these preliminary topics, the production of cobalt from the region is discussed, after which an overview of metallurgical processes is given. Following this overview, each of the relevant process areas, or unit operations, is discussed in further detail.
Section snippets
The supply chain from mine to consumer products
Cobalt is used in superalloys, hard metals, pigments, catalysts, magnets, tyre adhesives, wear-resistant coatings, and lithium-ion batteries. The supply chain for cobalt from mining to final product is shown in Fig. 5. In our assessment of cobalt metallurgical facilities, the processing of copper and cobalt has been divided into the hydrometallurgy refinery, which produces copper cathode and partially-purified cobalt hydroxide, and the metal refinery, which re-dissolves the cobalt hydroxide,
Mineralogy of cobalt in the copperbelt
The value-bearing minerals are divided into two categories commonly used in the copper industry: “sulphide” and “oxide”. On the Copperbelt, the term “oxide” must be interpreted as meaning “non-sulphide”, since the minerals referred to are seldom true oxides, but are mainly carbonates and hydroxides.
The principal sulphide ore minerals chalcopyrite (CuFeS2), bornite (Cu5FeS4) and carrollite (CuCo2S4) are mainly found in dolomites and shales in Congo, and shales and sandstones in Zambia.
Fig. 5
An overview of the extractive metallurgy employed
The flowsheets currently producing cobalt fall into one of three generic classes, which are illustrated in Fig. 7. These classes are (i) the hydrometallurgical treatment of mined ore (referred to as ‘whole ore’) to produce cobalt hydroxide; (ii) the roasting of flotation concentrate followed hydrometallurgical refining to cobalt metal; and (iii) the smelting of flotation concentrate to produce a cobalt alloy, referred to as a white alloy or alliage blanc. In addition to these three products
Unit operations for cobalt extraction
The production of high-grade products from the Copperbelt requires focus from a combination of metallurgical disciplines. As illustrated in Fig. 7, there are five general unit operations that are used. In this section, the contributions of concentration, roasting, smelting, hydrometallurgical refining, and cobalt metal refining are discussed in further detail.
Changing market requirements
There is a tidal change in the product pattern of cobalt. In the past, cobalt from the region was produced as cathode of relatively high grade. Currently, cobalt is produced as an impure hydroxide, resulting in a discounted price and increased transportation charges. This change is facilitated partly by the market, where it is argued that consumers are willing to accept impure cobalt hydroxide. However, mining companies traditionally do not easily accept a discount to the metal price, and it is
Conclusions
This paper has described the ways in which cobalt is separated from copper into various forms of saleable products. The great variety of different circuit configurations and operating conditions is testament to the ingenuity of the engineers and the companies of the region. The biggest change of the latest 15 years has been the introduction of copper solvent extraction into the hydrometallurgical flowsheets. This has enabled the production of high-quality copper cathode but has at the same time
CRediT authorship contribution statement
F.K. Crundwell: Writing - original draft. N.B. Preez: Validation. B.D.H. Knights: Validation.
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