**2. History of Technology Development**

From November 1991 to June 1992, a plant trial was conducted at Shuikoushan (SKS) Smelter for a period of 217 days to smelt copper concentrate in a bottom-blowing furnace [15,16]. The name of SKS Smelting Technology was therefore used initially to represent the bottom-blowing copper smelting technology. Figure 1 below shows the flowsheet of the SKS trial [16]. It can be seen from the figure that carbon fuel was used during the trial, and the feeds were palletised during the trial. The smelting slag was

**Citation:** Zhao, B.; Liao, J. Development of Bottom-Blowing Copper Smelting Technology: A Review. *Metals* **2022**, *12*, 190. https:// doi.org/10.3390/met12020190

Academic Editor: Mark E. Schlesinger

Received: 30 December 2021 Accepted: 19 January 2022 Published: 20 January 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

treated by flotation process to recover copper and the concentrate from the flotation was sent back to the bottom-blowing furnace (BBF). Only the coarse flue dust from the BBF and PS converter was sent back to the BBF. The detailed operating parameters are given in Table 1. It can be seen from Table 1 that the BBF treated 50 t feeds per day during the trial and Cu in the concentrate was approximately 20%. Matte with 50% Cu was produced and Cu in the slag was in the range of 1–3%. Oxygen utilization at 100% was claimed and a lance life of 5000 h was estimated.

**Figure 1.** Flowsheet of SKS copper smelting technology, adapted from [16].


**Table 1.** Detailed operating parameters during SKS trial. Data derived from [16].

In 2005 the first industrial scope BBF was built in Sin Quyen smelter in Vietnam. The size of the furnace was Ø3.8 m × 11.5 m and the capacity was designed to be 10,000 t Cu per year [6,17]. However, the BBF in Sin Quyen smelter seemed not operating properly as no operating details were reported. In 2008 the first real commercialised BBF started in Dongying Fangyuan Nonferrous Metals (Fangyuan) [7]. The main equipment was a horizontal cylindrical furnace shown in Figure 2. The size of the furnace was Φ 4.4 m × 16.5 m and it was lined with 380 mm thick chrome-magnesia bricks. The BBF had nine gas lances arranged in two rows on the bottom, five lances at 7 degrees and four lances at 22 degrees towards the outside. Each lance consisted of an inside tube and an external shroud. The inside tube injected pure oxygen and the shroud injected airflow as a coolant to protect the lance. The furnace operated with a rotation mechanism so the lances could be rolled above the molten bath during maintenance, repair, power failure or another emergency. Mixed feed materials with 7–10 wt% moisture were continuously transported by a belt conveyor into the high-temperature melt in the furnace through a feed mouth located above the reaction zone. High-pressure oxygen and air were blown constantly by the lances into the copper matte, in which iron and sulphur were rapidly oxidised. The by-product SO<sup>2</sup> was sent to the acid plant and the slag was tapped regularly followed by the flotation process to recover copper. Matte was tapped regularly and sent to the PS converters by a ladle.

Table 2 shows key process parameters in the Fangyuan smelting plant in January 2012 compared with the initial design. The effort made by the management and engineers enabled unexpected performance to be achieved in the first commercialised BBF. Differently from the trials in Shuikoushan, the following advantages were demonstrated in the Fangyuan BBF:



**Table 2.** Typical process parameters for Fangyuan smelting plant in January 2012. Data derived from [7].

Compared with other bath smelting technologies, BBF shows several advantages:

## (1). Less preparation of feeds

Top blowing technology (Ausmelt and Isasmelt) requires pelletised feeds and side blowing technology (Teniente Converter) needs to inject dry and fine particle feeds. BBF can treat a wide range of size feeds up to 100 mm diameter with moisture up to 10%.

## (2). High gas pressure and long lance life

Much higher gas pressure in BBF has many advantages confirmed by Kapusta et al. [18–23]. As one of the significant developments at Fangyuan, a series of trials were undertaken to identify the optimum pressures of the oxygen and air in the lances. It was found that optimum gas pressures were related to the feeding rate, the viscosity of the slag, thicknesses of the slag and matte layers. The following requirements need to be met: (1) no backflow of matte into the lances; (2) enough stirring for rapid reactions and generation of the surface wave; (3) no strong splashing of the slag and matte to block the feed mouth and (4) a protection accretion is formed at the tip of the lance. Figure 3 shows the "mushroom" formed at the tip of the lances which enabled the lance life to be over 10 months [7]. These mushroom tips not only protect the lances but also distribute the injected gases to small bubbles.

## (3). Autogenous Smelting

Autogenous operation was achieved in the BBF at Fangyuan for smelting of normal copper concentrates. This operation reduced energy consumption and CO<sup>2</sup> emission. The autogenous smelting was obtained by (1) low-temperature operation; (2) low slag rate resulting in a high Fe/SiO<sup>2</sup> ratio; (3) low off-gas volume resulting in high oxygen concentration; (4) high thermal efficiency as most of the oxidisation reactions occur in the lower part of the bath, the heat generated from these reactions can be efficiently absorbed by the molten matte and slag.

#### (4). Low-Temperature Smelting

The slag temperatures measured at the taphole of the Fangyuan BBF were usually in the range of 1150 to 1170 ◦C which was much lower than the liquidus temperature of the slag [7,24]. Approximately 20% solid spinel was present in the slag which increased the slag viscosity significantly. High-pressure gases injected from the bottom of the molten bath can generate surface waves that push the viscous slag out of the furnace through the taphole. The advantages of low-temperature smelting are (1) no extra fuels are required to maintain smelting temperature (although oil jets were available on both ends of the BBF, they were rarely used); (2) low consumption of the refractory. There was no significant erosion of the refractory on most parts of the furnace after one and a half years operation since spinel accretion was formed to protect the refractory [24–27]; (3) relatively higher viscosities of the matte and slag allowed higher pressures of the injected gases without

strong splashing; (4) a higher-grade matte can be produced at a lower temperature at a given oxygen partial pressure.

**Figure 3.** "Mushroom" tips formed on the lances of the Fangyuan BBF. Reprinted with permission from ref. [7]. Copyright 2021 John Wiley and Sons.

#### **3. Fundamental Studies to Support Development of BBS Technology**

#### *3.1. Slag Chemistry*

In the conventional theory of pyrometallurgy, the slag temperature must be higher than its liquidus temperature to avoid the formation of the solid phase. In Fangyuan BBF operation, high matte grade and high Fe/SiO<sup>2</sup> in the slag resulted in a high liquidus temperature of the smelting slag. In contrast, the BBF slag temperature was lower than other copper smelting technologies [7,28–30]. It was doubted widely that either slag composition or temperature was not reliable. Researchers from the University of Queensland measured the slag tapping temperatures carefully using a K-type thermocouple and collected quenched slag samples. In combination with electron probe X-ray microanalysis (EPMA) and thermodynamic analyses [31], it was confirmed that it is possible to operate BBF with a viscous slag containing solid phase.

Figure 4 shows a typical microstructure of the quenched BBF slag compared with the FSF slag [7]. It can be seen from Figure 4a that liquid, spinel and matte phases were present in the slag. The shape and size of the spinel phase indicate that the spinel was the primary phase presented at high temperatures. The slag tapping temperature of the BBF was therefore lower than its liquidus temperature. The matte droplets in different sizes were also present in the slag. In contrast, it can be seen from Figure 4b that no solid phase was found in the quenched flash smelting slag. The compositions of the phases present in a quenched BBF slag were measured by EPMA and are shown in Table 3 together with the bulk composition measured by XRF. Both EPMA and XRF can only measure elemental compositions. The compositions shown in the table were calculated by assuming that the elements were present in certain forms of oxides or elements in the slag. It is estimated from the compositions that the proportion of the spinel is approximately 16.8 wt%.

Reheating experiments were carried out using the slag shown in Table 3 under ultrahigh purity Ar flow. The experiments were undertaken at 1150, 1200, 1250 and 1300 ◦C respectively using Pt foil. In these experiments, the oxygen partial pressure was not controlled, it was assumed that there was no oxygen exchange between the slag and Ar gas. FactSage was used to simulate the reheating experiments. The experimentally determined proportion of liquid phase from mass balance was compared with FactSage predictions. It can be seen from Figure 5 that both experimental data and FactSage predictions showed the proportion of liquid phase decreased slowly with decreasing temperature between

1150 and 1300 ◦C [7]. This indicates that equilibrium was obtained during the reheating experiments and the FactSage predictions were relatively reliable under the condition used. Reheating experiments were not undertaken below 1150 ◦C. It can be seen from the figure that, according to the FactSage predictions, the proportion of liquid phase decreases sharply if the temperature is lower than 1150 ◦C due to the formation of other solid phases such as olivine and silica.

**Figure 4.** Typical microstructures of quenched smelting slag from Fangyuan BBF (**a**) (Reprinted with permission from ref. [7]. Copyright 2021 John Wiley and Sons) and a flash furnace (**b**).

**Table 3.** Compositions of the phases present in a quenched BBF slag measured by EPMA and bulk composition measured by XRF (wt%). Data derived from [7].

