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Typical e-waste recycling processes by pyrometallurgy

In this article we explore the recovery of metals from electronic waste by pyrometallurgical processing.
image of a molten metal
© Raghild Aune NTNU

Pyrometallurgical and hydrometallurgical processes are commonly used to extract base metals and precious metals. The use of microorganisms and their metabolites through bio-hydrometallurgical processing, or bioleaching, has also been investigated to extract metals from e-waste. All these technologies have advantages and disadvantages that affect their suitability for practical implementation. Pyrometallurgy is, however, today the traditional and most common approach for base metal and precious metal recovery from e-waste.

Pyrometallurgy uses high-temperature processes in oxidative or reductive conditions to secure physical and chemical transformations in the raw material allowing the recovery of the metals of interest.

Pyrometallurgical treatment of e-waste commonly involves smelting in furnaces at high temperatures, incineration, combustion, and pyrolysis, and in these unit processes the metals are separated based on their chemical and metallurgical properties.

The Aurubis smelter in Germany, the Noranda smelter in Canada, the Boliden Rönnskar smelter in Sweden, the Umicore smelter in Belgium, and the DOWA smelter in Japan are some of the thermal plants available globally today for formal processing of e-waste. Some details for these smelters are presented in Table 1.

Table 1. A summary of typical pyrometallurgical methods that use e-waste as a feedstock for recovery of metals.

Examples of pyrometallurgical smelters

The largest pyrometallurgical facility that processes e-waste today is the Umicore smelter with a capacity of smelting 350,000 tons of e-waste/year (see Figure 1), and due to the increase in e-waste generation there are plans to expand the plant capacity to 500,000 ton/year.

The material sent to the Umicore smelter has typically been dismantled or pre-processed to remove large plastic parts, Fe, and Al. The e-waste is smelted in an ISA SMELTTM furnace where the plastics and other organic substances acts as reducing agents and thereby the energy source instead of coke. In the smelting step the precious metals and copper are separated from the other base metals into copper bullion that undergoes further processing using electrowinning and precious metal recovery.

Most of the other metals are separated into a lead slag phase that also undergoes further treatment using base metal operations, i.e., processed using a lead blast furnace, lead refinery, and special metals plants. In addition to pyrometallurgical techniques, the plant also utilises hydrometallurgical and electrochemical processes.

Schematic diagram of the full process of smelting at Umicore- Hoboken

Figure 1. Schematic diagram for the Umicore’s integrated metals smelter and refinery smelting processing

In the Noranda smelter (see Figure 2) the feed stock that enters the reactor are immersed in a molten metal bath and mixed with supercharged air (up to 39% oxygen). Even in this case the energy cost is reduced by combustion of plastics and other flammable materials in the feedstock. Impurities such as Fe, Pb and Zn are converted into oxides which become fixed in a silica-based slag. The slag is cooled and milled to recover more metals before disposal. The copper matte containing precious metals is removed and transferred to the converters. After an upgrading in the converter, liquid blister copper is refined in an anode furnace and cast into anodes with a purity of 99.1%. The remaining 0.9% contains the precious metals, including Au, Ag, Pt and Pd together with other recoverable metals such as Se, Te, and Ni which are recovered by electrorefining of the anodes.

Schematic diagram for the Noranda smelter

Figure 2. Schematic diagram for the Noranda smelter and refinery smelting processing

Another pyrometallurgical process is practiced at the Boliden Rönnskar smelter (see Figure 3) where scraps can be fed into the process in different steps depending on their purities. High copper containing scrap is fed into the converting process directly, and low-grade e-waste is fed into the Kaldo furnace where an oxygen lance supplies the needed O2 for combustion. Off-gases are subjected to additional combustion in post-combustion step. The Kaldo furnace produces a mixed copper alloy that is sent to copper converting for recovery of metals such as Cu, Ag, Au, Pd, Ni, Se, and Zn, and the dusts (containing elements such as Pb, Sb, In and Cd) are sent to other operations for metal recovery.

Schematic diagram for the Boliden smelter

Figure 3. Schematic diagram for the Boliden Rönnskar smelter and refinery smelting processing.


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© Raghild Aune NTNU
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