Pyrometallurgy, nonferrous

Pyrometallurgy, nonferrous

The branch of extractive metallurgy in which processes employing chemical reactions at elevated temperatures are used to extract and refine nonferrous metals from ores, concentrates, and recycled materials. The entire process from feed to finished metal may be pyrometallurgical, or a pyrometallurgical step may be used in conjunction with other technologies. Increasingly, a mix of processes maximizes the efficiency and advantages of an overall operation.

The processes in pyrometallurgy in general can be classified as preparatory, reduction, oxidation, and metal refining. Treatment of a given raw material or metal may involve all these steps, or some of the steps may form a part of the total processing system, which may include nonpyrometallurgical operations.

In preparatory processes, the concentrate or upgraded ore or other feed is converted by chemical reaction to a form suitable for further processing. The most common subprocesses are drying and calcination, pyrolysis and hydrolysis, roasting, sintering, and chlorination. Even though a chemical reaction does not actually take place during drying, this subprocess is included since it is often part of a subsequent high-temperature operation such as smelting. In some cases, the preparatory process is carried out to provide a material that is amenable to treatment by hydrometallurgical processing, such as the roasting of zinc concentrates to produce a zinc calcine (essentially a zinc oxide intermediate product) which is leached with sulfuric acid solution for zinc production, or it is a step such as calcination in the preparation of alumina for aluminum smelting.

Reduction processes effect the high-temperature reaction of a metal compound to the metal and its separation from the residue, as represented by the reaction below,

where MX is the metal compound, R the reacting or reducing agent, and M the metal. The reducing agent and reaction conditions (for example, temperature and pressure) and the concentration of reactants and products are selected to achieve a rapid or spontaneous reaction. These reactions usually require energy input.

The amount of reducing agent used should be low and inexpensive, relative to the value of the metal produced, while the product RX should be readily separable from the metal. Reducing agents commonly used in nonferrous pyrometallurgy include carbon (usually as coke), carbon monoxide gas (from coke), natural gas, iron and ferrosilicon (for Mg production), aluminum (for Ca production), and magnesium (for Ti, Zr, and Hf production).

For thousands of years, pyrometallurgical smelting of sulfide materials has been the key production method for nonferrous metals, in particular for copper, nickel, tin, lead, and zinc. This still remains the case on account of lower costs associated with new intensive technology and lower overall energy consumption. Formerly, it was common to roast such feed materials prior to the actual smelting operation. Roasting is still a major processing step in zinc and tin production. However, the roasting step for copper and nickel production was gradually eliminated, and during the latter part of the twentieth century continuous smelting processes were developed to directly treat sulfide concentrates, producing (by oxidation of the sulfide material) a high-grade copper matte product (∼70% Cu) in a single step. In the case of lead, the metal itself can be readily produced directly. Oxidation processes are normally exothermic, a characteristic that has generally led to the development of autogenous processes, requiring virtually no fossil fuel.

Two basic types of smelting processes are used for copper or nickel production: flash smelting and bath smelting. In flash smelting, a fine concentrate feed is introduced into the furnace chamber, along with oxygen-enriched air, and the reaction principally occurs in a gas-phase system between the oxygen-bearing gas and solid particles. In bath smelting, a concentrate feed is introduced into the furnace melt, which is blown and kept highly agitated by submerged tuyeres (injecting the oxygen-enriched air), such that the feed is enveloped and reacts within the turbulent bath.

The new continuous lead smelting process can produce lead directly, while on account of the thermodynamics of the copper smelting system with the immiscible Cu-Cu2S phases being present, copper production is normally carried out in two stages: (1) copper concentrate smelting to produce a high-grade matte (typically 60–75% Cu, 4–12% Fe, ∼21% S), a slag (approximately 27–30% FeO, 15–20% Fe3O4, 25–30% SiO2, 1–5% Cu), and a sulfur dioxide-rich gas (9–15% SO2 at acid plant); and (2) copper matte converting, wherein the matte is oxidized or converted to metallic copper, producing a small amount of slag and sulfur dioxide gas.

A significant amount of copper is produced from recycled materials (such as from used automobiles, motors, old electrical appliances), and the pyrometallurgical processes are well able to handle this feed load on account of the flexibility as to feed type.

In metal refining processes, the starting material is generally an impure metal, usually produced in a primary production process. Impurities are removed to yield a final metal product, meeting a product specification. The processes are classified as (1) volatilization (separation of metal or metal compound as a gas from a liquid or solid); (2) drossing and precipitation (separation of the metal or impurities as a solid from the liquid melt); and (3) slag refining (separation of metal or impurities by their extraction from one liquid into a second immiscible liquid phase).

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