Glossary

An electrode where oxidation occurs.

An autoclave is a machine that uses steam under pressure to kill harmful bacteria, viruses, fungi, and spores on items that are placed inside a pressure vessel.

Non-ferrous, metals excluding precious metals. The term "base" indicates "low-born," i.e. of less worth. May also refer to metals that are the more easily corroded.

A metal oxide product produced from roasting or heating. For example a zinc calcine is an impure ZnO product obtained from roasting of a ZnS concentrate,

\[
\ce{ZnS_s + \frac{3}{2}O2_g = ZnO_s + SO2_g}
\]

Calcium oxide, CaO, is obtained by calcination of CaCO3:

\[
\ce{CaCO3_s = CaO_s + CO2_g}
\]

An electrode where reduction occurs.

Any ion or neutral molecule that involves one or more metal ions and one or more coordinated ions or molecules, e.g. [Zn(H₂O)₆]²⁺, [AuCl₄]³⁻, [Cu(CN)₃]²⁻, Ni(CO)₄, etc. (See Chemistry Review Part II).

The mineral of interest in a more concentrated form than was present in the ore. A variety of largely physical process (as opposed to processes that chemically alter the mineral) may be used to separate the desired mineral(s) from the surrounding rock. The result is a concentrate in which the desired minerals are present, but without much of the unwanted material. Generally the minerals remain in the original form. A concentrate may go on to leaching or pyrometallurgical extraction. Note that a concentrate refers to something distinctly different (mineral processing) from purification/concentration.

Unit of electric charge.

The reduced and oxidized species of a single half reaction. For example, the H⁺/H₂ couple refers to the half reaction 2H⁺ + 2e⁻ = H₂. Sometimes only the principal species actually undergoing electron transfer are specified. For example, the Mn(VII)/Mn(II) couple in acid solution refers to the half reaction MnO₄⁻ + 5e⁻ + 8H⁺ = Mn²⁺ + 4H₂O.

Equal to the non-standard reduction potential, E, referenced to the standard H⁺/H₂ half cell.

The sum total of positive charge is precisely matched by the sum total of negative charge. Matter is naturally electrically neutral. Solutions containing ions maintain this requirement.

An electrically conducting material used as a surface where electron transfer occurs.
2. Any half reaction, including the electrode at which it occurs. The electrode itself may, or may not, be involved as a reactant in the half reaction. An example is 2H⁺ + 2e⁻ = H₂ on Pt.

See Potential difference.

Electrolysis practiced specifically for plating metal. Also abbreviated as EW.

See Potential difference.

See electrowinning.

A diagram depicting the sequence of steps and associated information for an chemical process.

The unwanted minerals that constitute the unwanted minerals of an ore. These are usually substantial fractions of the total mass.

Grade, or tenor, refers to the concentration of the minerals or elements of interest in the ore or deposit. Units may be % by weight, g/t, kg/t, oz/t (oz = ounces; the troy ounce is the common unit for precious metals. It equals 31.1034768 g.)

This refers to the individual mineral crystals in a deposit or sample.

1. One compartment of an electrochemical cell, where either reduction or oxidation occurs. 2. A half reaction.

An electrode surface for electron transfer that is not involved in the chemical reaction and does not undergo chemical transformation. An example is Pt used with the H⁺/H₂ half cell. The half reaction only involves H⁺ and H₂. Pt is there only to provide a surface at which electron transfer may occur.

Many minerals exhibit this property, which is variable composition within the same structure. This may occur when metal ions of similar size substitute for some of the principal metal ions. Some examples are pentlandite (Ni,Fe)₉S₈, in which variable amounts of nickel substitute for iron, and powellite Ca(Mo,W)O₄.

The ions or molecules that are coordinated to metal ions in complexes.

The earth's crustal rocks. The oceanic and non-oceanic lithospheres differ. In practice extractive metallurgy has used the land-based lithosphere. Oil and gas industries have made extensive use of the oceanic lithosphere.

The metal-enriched product from roasting in pyrometallurgical treatment of sulfide concentrates. The composition of matte from chalcopyrite (CuFeS₂) roasting is roughly Cu₂S·FeS.

Strictly speaking, a mineral is a naturally occurring, homogeneous inorganic substance of definite composition and crystal structure. Native elements (e.g. sulfur, gold) and compounds are included. A great many solid materials in nature fit this description. However, some solid materials have variable composition, e.g. pyrrhotite, FeS_1−x, and pentlandite, (Fe,Ni)₉S₈, where, respectively, varying fractions of iron and sulfur and nickel and iron occur. These are still considered minerals. Finally, coal, which is not exactly an inorganic substance nor of definite composition, may also be considered a mineral, broadly speaking.

A mineral deposit is a body of natural material that contains specific minerals.

The study of minerals: crystallography, chemistry, physical properties, classification and ways of distinguishing minerals. In the context of hydrometallurgy and mineral processing mineralogy refers to the type, concentration and distribution of minerals in a deposit. How the minerals are dispersed is very important. Gold may be present in the ore at a certain concentration, but is it localized between grains of silica or is it finely dispersed within a pyrite matrix? Issues like that make all the difference to how an ore is processed.

These are minerals which are a single chemical element. Examples are sulfur, mercury, antimony, copper, silver, gold, platinum, etc. Often these materials are not actually pure. They may be alloyed or combined with other elements to a minor extent. For example gold may contain some copper or silver.

The average concentration (in % by weight) of an element on the earth. (Strictly speaking it refers to the average concentration of any specified isotope. In hydrometallurgy the usual concern is with the average concentration including all naturally occurring, stable isotopes of an element.)

Metal elements that are not easily corroded; their simple cations have relatively high standard reduction potentials.

An ore is a mixture of minerals, one or more of which can be economically exploited as a source for one or more materials. These materials might ultimately be produced as metals, alloys, compounds or concentrates. Note that the key is economically viable production. This requires a certain grade of the mineral(s) to be present. Not all mineral deposits are ore bodies, but all ore bodies are mineral deposits. With advances in technology, or increases in price what was only a mineral deposit at one time, might become an ore later.

A reactant that accepts or takes electrons from another reactant. The oxidant gets reduced.

Electrons being taken away from a reactant. The term may be used specifically for a half reaction or for a balanced reaction where the main half reaction of interest is the oxidation half reaction.

A concentration unit, usually mg (micrograms) of species of interest per kg of total mass of the sample. Since the density of water is nearly 1000 g/L (998 g/L at 21°C), and the density of a dilute solution is close to that of water, ppb is also often used synonymously with mg/L, though this is not strictly correct.

A concentration unit, usually mg (milligrams) of species of interest per kg of total mass of the sample. Since the density of water is nearly 1000 g/L (998 g/L at 21°C), and the density of a dilute solution is close to that of water, ppm is also often used synonymously with mg/L, though this is not strictly correct.

These include: Ru, Os, Rh, Ir, Pd, Pt. They are generally rare, noble metals and precious. PGM's and some of their compounds are noted for their catalytic properties.

In the context of electrochemistry, the driving force for electrons to transfer from one species to another under specified conditions of temperature, pressure and composition (concentrations, or, better, activities). The units are volts. When a charge of 1 C passes through a potential difference of 1 V energy equal to 1 J is released or consumed (depending on whether the charge flows with or against the electric field). By definition, 1 J º 1 VC. The standard cell voltage (see "standard conditions") is designated △E°. For non-standard conditions, it is designated △E.

The rare and highly valuable metals. Historically, Au, Ag, and PGM's. These also tend to be the metals that are less prone to corrosion. These in turn are known as the noble metals.

Various processes are used to form a solid from solution. This may be a compound, such as Al(OH)₃ after bauxite leaching and solution purification, or the metal itself, e.g. Cu metal produced from electrolysis of a CuSO₄ solution.

The solution product of leaching, most commonly bearing the desired metals. Usually the metals are dissolved as cations (always with suitable counterions, like sulfate, SO₄²⁻), e.g. Cu²⁺, Zn²⁺, etc., or as complex ions, such as CuCl₂⁻, Au(CN)₂⁻, Zn(NH₃)₄²⁺, etc.

A reactant that donates or loses electrons to another reactant. The reductant gets oxidized.

Electrons being accepted by a reactant. The term may be used specifically for a half reaction or for a balanced reaction where the main half reaction of interest is the reduction half reaction.

Process of heating and decomposing a mineral at high temperatures (pyrometallurgy). In sulfide roasting, the sulfide is at least partially burned off, mostly as SO₂ gas. Metal oxides, sulfates, or sulfides with high (enriched) metal content may be produced.

One of several types of apparatus for joining half cells that allows for electrical connection between them. A salt bridge is usually comprised of a U-shaped tube filled with an aqueous gel, such as agar, that contains a high concentration of a simple salt, like KCl or KNO₃. The salt is chosen so as to be compatible with both half cells; the salt must not interfere with the half reactions. Electrons flowing through the external circuit of a cell establish a current. For the circuit to be complete there must be provision for the current to pass through the solution. Electrons can't do that; they do not survive in solution. Rather the current is carried by migration of ions in solution. Anions flow out of the reduction half cell and into the anode half cell. Equivalently, cations can move out of the anode half cell and into the cathode half cell. This maintains electrical neutrality. Without this the current flow would cease virtually immediately. (Bulk charge separation is enormously energetic, as a lightning storm amply demonstrates.) Other methods can also be used to facilitate ionic migration, such as ion exchange membranes.

A mixture of solution and solids. This is the state of an ore plus solution during and at the end of leaching. Complete dissolution does not occur.

For electrochemical systems standard conditions are a combination of specified pressure, temperature and composition. Traditionally, 1 atm pressure for reactants and products and unit activities (molal scale) of the ions/compounds involved. Pure solids and liquids have unit activity by definition. Usually standard reduction potentials are tabulated for 25°C. Conversions to other temperature can be made. Some time ago, the pressure standard was changed to 1 bar = 100 kPa = 100,000 N/m2 = 0.98692 atm. Some care is needed to determine what conditions were employed when using a table of standard potentials.

The solid residue left after leaching. If this has further value it may be processed in subsequent steps. Otherwise it is waste. Tailings solids are often associated with wash water. The tailings mixture is stored or impounded in a tailings pond.

A common unit of mass when speaking of precious metals. 1 Troy ounce = 31.1034768 grams.

These are individual steps in an overall process. They are commonly understood in the industry, including the typical basic principles and procedures. A list of the general unit operations follows, along with associated terms. These overlap with elementary process units mentioned above. Unit operations are more concrete, well defined and specific.

This is the net amount of water that must be discharged from, or taken into a process. It is a critical parameter with respect to discharge of effluents and availability of process water in dry locations.