Refining of Metals

the removal of impurities from crude metals. After extraction from raw materials, crude metals are between 96 and 99 percent principal metal, the rest being impurities. Crude metals cannot be used by industry at this stage because of inferior physical, chemical, and mechanical properties. The impurities found in crude metals may have value in themselves; the gold and silver recoverable from copper, for example, pay for the entire cost of the refining process.

The three basic refining methods are pyrometallurgical, electrolytic, and chemical. All these methods are based on distinctive properties of the individual elements, such as melting temperature, density, and electronegativity. Pure metals are frequently obtained by employing several refining methods in succession.

Pyrometallurgical refining. Pyrometallurgical refining, which is carried out at a high temperature in a melt, has a number of variations. Oxidizing refining is based on the tendency of some impurities to form compounds with O, S, Cl, and F that are more stable than compounds of the principal metal and these elements. This technique is used to refine Cu, Pb, Zn, and Sn. For example, when a stream of air is forced through molten copper, oxides are formed by admixtures of Fe, Ni, Zn, Pb, Sb, As, and Sn, since the admixtures have a greater tendency to react with oxygen than copper; the oxides rise to the surface of the tank and are removed.

Liquation separation is based on differences in the melting temperatures and densities of alloy constituents and on the low level of mutual solubility of the constituents. For example, when molten crude lead is cooled, copper crystals (dross) separate out at established temperatures and, because of their low density, float to the surface and can be removed. This method is used to remove Cu, Ag, Au, and Bi from crude lead, to remove Fe, Cu, and Pb from crude zinc, and to refine tin and other metals.

Fractional recrystallization utilizes the difference in the solubilities of a metallic admixture in solid and liquid phases and the slow diffusion of impurities in the solid phase. This method is used in the production of semiconductor materials and in the preparation of high-purity metals; it is employed in zone melting, plasma metallurgy, the removal of single crystals from a melt, and directed crystallization.

Rectification, or distillation, is based on the difference between the boiling point of the principal metal and that of the impurities. Refining is carried out as a continuous refluxing process in which volatilization and condensation of the fraction being separated are repeated many times. Rectification may be accelerated considerably if it is performed in a vacuum. This method has applications in the removal of Cd from Zn or Zn from Pb, in the separation of Al and Mg, and in Ti metallurgy. Vacuum filtration of a liquid metal through ceramic filters removes suspended solid impurities; the process is used in Sn metallurgy.

When steel is refined in a casting ladle with liquid synthetic slag, the contact surface of the metal and the slag is appreciably greater (because of the mixing) than when the refining processes are carried out in a melting aggregate. This greatly improves the efficiency of desulfurization, dephosphorization, and deoxidation of metals in removing nonmetallic impurities. Inert gases are blown through molten steel in a refining procedure that removes suspended slag particles or solic oxides from the metal. The impurities adhere to gas bubbles and float to the surface of the melt.

Electrolytic refining. Electrolytic refining, the electrolysis of aqueous solutions or salt melts, yields metals of high purity. It is used for thorough purification of most nonferrous metals.

Electrolytic refining with soluble anodes involves the anode dissolution of the metals to be purified and the deposition of the pure metals on the cathode; in this process, electrons from the external circuit are captured by ions of the principal metal. Electrolysis can be used to separate metals because the principal metal and the admixtures have different electrochemical potentials. For example, the standard electrode potential of Cu relative to a standard hydrogen electrode—taken as zero—is + 0.346; the values for Au and Ag are greater, and those for Ni, Fe, Zn, Mn, Pb, Sn, and Co are negative. During electrolysis, copper is deposited on the cathode, the noble metals do not dissolve but instead settle to the bottom of the electrolytic cell as a slurry, and metals with negative electrode potentials accumulate in the electrolyte, which is periodically cleaned. Occasionally—in the hydrometallurgy of zinc, for example —electrolytic refining is carried out using insoluble anodes. Here, the principal metal is in a solution from which impurities are carefully removed in advance, and during electrolysis it is deposited on the cathode as a compact mass.

Chemical refining. Chemical refining is based on the different solubilities of a metal and its impurities in acid or alkaline solutions. Impurities gradually accumulate in a solution and are removed by chemical means, such as hydrolysis, cementation, the formation of sparingly soluble compounds, and purification by extraction or ion exchange. The affinage of noble metals is an example of chemical refining. Au is refined in boiling sulfuric or nitric acid; Cu, Ag, and other metallic impurities are dissolved, whereas the purified gold remains as an insoluble residue.