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Dec 11, 2019

Hydrometallurgical smelter dust treatment - the case for dealing with impurities

Impurity metals are a constant challenge in metals production and warrant special attention as they can have a significant impact on both operations and final products. They can be removed in the smelter and copper refinery from different sources. The most problematic impurities in smelting processes are arsenic, antimony, bismuth, lead, and zinc. Concentrate pre-treatment methods like blending and partial roasting of concentrates are widely used to control impurity metal levels in both the smelter and refinery. Levels can also be controlled using slagging and slag cleaning at the smelter and electrolyte purification in the refinery. These methods are well known and widely used.
flash smelting

This case gives an overview of hydrometallurgical processes that remove impurity metals and recover copper from flue dust. With impurity metal levels in concentrates increasing, one option to enhance impurity removal from the smelter refinery circuit is hydrometallurgical treatment of smelter flue dust.

The conventional method for controlling flue dust has been recirculation back to the smelter furnace. When impurities start to accumulate in the circuit and there are unacceptable impurity levels in anodes, different solutions need to be used. Even though impurity levels have increased over time, the quality requirements for anodes remain restrictive as higher impurity levels would mean a greater need for solution purification at the electrolytic refinery and more bleed treatment. But as only some of the impurities can be removed with bleed and solution purification, more bleed treatment would mean increased costs for refineries, which cannot be tolerated.

Based on raw material variations and impurity behavior, smelters have become more interested in alternative purification methods. The hydrometallurgical treatment of flue dust is a potential way to control impurities as treating flue dust components externally means impurities do not accumulate, leading to improved impurity control.

Impurity levels can be balanced between flue dust and different slag qualities. Depending on the impurities present, the slag can also be the exit point from a flowsheet with the process tailored for improved recoveries. Based on plant economics, a common feature is that copper needs to be recovered. This can be done in various ways such as directly in copper cathodes, via precipitation, or through crystallization as copper sulfate, which is a powerful coolant when returned to smelter furnaces.

Arsenic, antimony, and bismuth levels in electrolyte also influence electrorefinery efficiency and must be taken into account. The primary reason for hydrometallurgical processing is that there isn’t any other reasonable pyrometallurgical processing method for high-impurity flue dust. A secondary reason is that in some cases the dust contains valuable elements. Especially when the lead and zinc content in flue dust is extremely high and accumulation of these elements in the smelter mass balance needs to be avoided, it is essential to recover these elements using hydrometallurgical processing.

Impurity metal removal and copper recovery can be done, for example, by leaching the flash smelting furnace boiler, converting furnace boiler, and ESP flue dust. Impure wash acid and acidic wastewater from the acid plant can be recycled to the hydrometallurgical plant where it is used for leaching smelter dust (Figure 1).

block diagram of hydrometallurgical plant
Figure 1. Schematic block diagram of hydrometallurgical plant showing how to remove impurity metals and recover copper from smelter dusts

Copper smelting dust behaves differently depending on its source, and the dust mineralogy and chemical composition is highly dependent on several factors such as feed material quality, the technology used, and the furnace operating conditions. The solubility of the elements in the dust also varies significantly depending on the source.

For example, metals in flash smelting furnace (FSF) dusts are mostly in sulfidic forms (i.e. copper sulfide and arsenopyrite), whereas in Peirce-Smith converter (PSC) dust they are mostly in oxidic forms (i.e. lead oxide, zincite, lead arsenate, and arsenic oxide) with some components in sulfidic or sulfate forms (i.e. lead oxide sulfate and anglesite).

Chemical composition of flue dusts
Table 1. Chemical composition of flue dusts

Process description

The hydrometallurgical smelter dust treatment process normally requires leaching, arsenic precipitation, and copper recovery. Valuable metals recovery takes place using mechanisms such as precipitation, electrowinning, or crystallizing, with impurity metals precipitated and directed to the tailings pond or waste area. The needs of the smelting process can also influence the recovery method. If coolant is needed for a smelting furnace, copper is recovered as copper sulfate crystals instead of copper cathodes or sulfide precipitate. Zinc can also be extracted and recovered as cathodes if there are no zinc refineries nearby; if there are, it can be recovered as zinc carbonate (Zn(CO3)2). The recovery method can depend on the market structure, the availability of chemicals, or even the price of energy (e.g. electricity or steam).

The advantage of the hydrometallurgical dust treatment process is that the smelter and acid plant are integrated. Hot flue dust energy can be used for leaching reactor heating and impure wash acid or wastewater from the acid plant can be utilized in leaching. Sometimes the only target is to separate the waste components and return the copper back to the smelter or copper refinery to minimize copper loss. The leaching of arsenic, antimony, and bismuth depends on leaching conditions like acid concentration, temperature, and oxidation potential. Precipitation is then carried out with pH control and/or reagents like SO2 as shown in Figure 2.

FSF ESP dust would be leached in two stages and PSC dust in just one stage for a greater impact on impurity levels in the smelting circuit. Water leaching at an ambient temperature achieves good separation of copper from harmful impurities and therefore the filtrate can be precipitated completely by neutralization and recycled back to the copper smelter. The remaining residue from FSF water leaching and the PSC ESP dust can be leached in the same reactor using chloride leaching under oxidation at an elevated temperature (90–100°C). Complete leaching in the second stage is desirable for all components. Leaching is then followed by sulfide precipitation, where the copper-rich precipitate is recycled back to the FSF furnace. The remaining filtrate contains most of the unwanted impurities and they can be precipitated using neutralization. The neutralization step after the sulfide precipitation requires iron in order to make sure that the Fe/As (molar ratio) is suitable for ferric arsenic or scorodite precipitation.

Block diagram of a hydrometallurgical plant treating FSF and PSC dusts
Figure 2. Block diagram of a hydrometallurgical plant treating FSF and PSC dusts.

The first method removes copper first from the solution using selective precipitation with sulfides as a CuS precipitate. The second method removes arsenic first by partial neutralization with precise pH control. The neutralization step is followed by copper precipitation after the arsenic precipitate has been filtered. A third option, often chosen when there is an electrowinning plant nearby the smelter, is to use an SX/EW processes for impurity management.

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