Precipitation from Solution

Production of metal powders by hydrometallurgical processing is based on leaching an ore or ore concentrate, followed by precipitating the metal from the leach solution. Although basic precipitation reactions have been known for more than 100 years, commercial use of this process did not flourish until the 1950s, as a result of increasing interest and work on lower-grade ores. Metal precipitation from solution can be accomplished directly by electrolysis, cementation, or chemical reduction. Indirect precipitation can be achieved by first precipitating a compound of the metal (hydroxide, such as carbonate, or oxalate, for example), followed by heating, decomposition, and reduction.

The most widely used commercial processes based on hydrometallurgy are copper cementation and the separation and precipitation of copper, nickel, and cobalt from salt solutions by reduction with hydrogen (Sheritt Gordon process). In the 1960s and 1970s, several pilot plants using hydrometallurgical processing (solvent extraction and reduction with sulfur dioxide and hydrogen) were operating temporarily in the United States for the production of high-purity copper powder.

In its simplest form, copper cementation recovers copper from acidic dump leach solutions as an impure powder precipitate. Due to the presence of significant amounts of iron and silicates, low apparent density, and high green strength, such copper powders find use in P/M friction composite components. They are not used in conventional structural parts.

Nickel powders are now produced in large quantities directly from their ores by precipitation. The powder has a purity of at least 99.8%, with the major impurities being cobalt, iron, and sulfur, due to their presence in the original nickel ore. Powder can be produced in a variety of size distributions and is quite uniform. Similar techniques have been developed for cobalt.

Processing Conditions. For divalent ions, these processes consist of precipitation from an aqueous source using hydrogen. The basic concept is that a metallic ion such as nickel, copper, or cobalt in the solution reacts with gas (hydrogen) by the following reaction:

or if the solution is ammoniacal:

Generally, processing begins with leaching of ores and includes purification and separation stages prior to reduction. Reduction potential may be estimated by comparing the electrochemical potential of the metal ion and that of the hydrogen ions as a function of the partial pressure of hydrogen and pH of the solution. For reduction to occur, the hydrogen potential must be greater than the metal potential. Metal concentration has a minimal effect on electrochemical potential. These relationships are discussed in detail in Ref 2 and are shown in Fig. 2, which indicates that copper can be reduced in very acidic solutions. Higher pH values are required for nickel and cobalt. To obtain practical reaction rates, the actual process is carried out at elevated temperatures and pressures. For complete reduction, pH is increased by adding ammonia.

Fig. 2 Potential of 1 and 10-3 molar metal solutions and hydrogen potential at varying pH at 25 °C (75 °F). Source: Ref 2

Separation of metals is based on differences in the stability of complexed metal ions (ammines) subjected to hydrolyzing and oxidizing reactions. Potential-pH diagrams with the thermodynamics of oxygen reduction and hydrogen oxidation provide the necessary information to predict separation potential within a given system. Figure 3, a potential-pH diagram for the copper-ammonia-water system, is discussed in detail in Ref 3.

Fig. 3 Potential-pH diagram for the Cu-NH3-H2O system. Total NH3 equals 1 mol/L. Source: Ref 3

The use of additives and control of nucleation, particle growth, and particle agglomeration allow the production of powders with a wide range of particle sizes, particle density, and particle shape with specific surface areas from less than 1 m2/g to about 8 m2/g.

Coprecipitation or successive precipitation of different metals from solution allows the production of alloyed and composite powders. Spray drying extends this capability to innumerable combinations. For more information, see the article "Spray Drying and Granulation" in this Volume.

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