3. Important Chemicals in Hydrometallurgy

Hydrometallurgy is about producing metals. However, many of the chemical reagents we use are not based on metals. The hydrometallurgist needs to be familiar with these, and with their typical uses. In this section some of the common, relevant industrial chemicals are listed. In addition, the common cations and anions are also listed. The chemical formulas and names need to be learned, particularly for those especially common in hydrometallurgy.

Acids

For the purposes of hydrometallurgy acids are molecules or ions that ionize to form protons, H+, in aqueous solution. A fuller treatment of acids and bases is given in Part II of this review. Bases are species that accept protons, i.e. a base has some affinity to chemically combine with H+.

Bases

Oxidants

Redox reactions (electron transfer) are a very important class of chemical reactions. Note: An oxidizing agent is a species which accepts electrons. A reducing agent is one which donates electrons. In the process of electron transfer the oxidant (a reactant) gets reduced, or accepts electrons. The reductant (a reactant) gets oxidized, or donates electrons. It is very important to remember these distinctions.

Reductants

The world is an oxidizing place, at least on the surface. Hence reductants are not commonly available, except where generated in: 1. anoxic (O₂-free) environments (coal, natural gas, sour gas (H₂S) and elemental sulfur), and 2. through photosynthesis, e.g. cellulose, starch). Consequently, reductants tend to be more expensive.

Complexing Agents

Complexing agents are molecules or ions that can bond with metals (atoms or ions). One ore more of the complexants may be bound to the metal. The chemical species formed is thought of as a complex of discrete ligands (the complexing agents) bound to a central metal. More on this later in this review.

Note: cations in salts are typically not involved in the complexes, other than as counterions.

Precipitating Agents

Many anions form quite insoluble compounds with many cations. Precipitation can be used to recover a metal ion (wanted or unwanted) from aqueous solutions.

Other Organic Chemicals

Numerous organic chemicals and mixtures are used as process additives. Organic reagents will be described and discussed as needed.

Common Simple Anions

The common simple anions are listed below. Note: Some O^2- does not exist in aqueous solution! It occurs in many solid metal oxides (e.g. CaO), but O2- itself is such a strong base that it reacts completely with water:

\[\ce{O_2^- + H_2O -> 2OH^-}\tag {117}\]
The prefix “bi” indicates that a proton is added to an anion, e.g. S^2- is sulfide; HS^– is bisulfide.

Common Simple Cations

The common simple cations are listed below. In water they form aquo complexes (water as the ligand). It is important to be able to write the formulas and recognize the names. More on coordination chemistry later in this review.

• H+ (or H3O+; the proton is strongly hydrated in water)
• NH4+ ammonium
• Li+
• Na+
• K+
• Rb+ (low natural abundance)
• Cs+ (low natural abundance)
• Mg+2
• Ca+2
• Sr+2 (low natural abundance)
• Ba+2 (low natural abundance)
• Cr+3
• Mn+2
• Fe+2 ferrous
• Fe+3 ferric
• Co+2
• Ni+2
• Cu+ cuprous (unstable in water)
• Cu+2 cupric
• Ag+
• Zn+2
• Cd+2
• Hg+2 mercuric
• Hg2+2 mercurous
• Al+3
• Tl+ thallous
• Tl+3 thallic
• Pb+2
• VO+2 vanadyl
• VO2+
• UO2+2 unranyl
• BiO+
• La+3 (also many other lanthanide +3 cations)

Cations and anions of course occur in combination with each other as electrically neutral salts, with varying degrees of solubility in water and other solvents.