Since Pourbaix presented Eh versus pH diagrams in his “Atlas of Electrochemical Equilibria in Aqueous Solution”, diagrams have become extremely popular and are now used in almost every scientific area related to aqueous chemistry. Due to advances in personal computers, such diagrams can now show effects not only of Eh and pH, but also of variables, including ligand(s), temperature and pressure. Examples from various fields are illustrated in this paper. Examples include geochemical formation, corrosion and passivation, precipitation and adsorption for water treatment and leaching and metal recovery for hydrometallurgy. Two basic methods were developed to construct an Eh-pH diagram concerning the ligand component(s). The first method calculates and draws a line between two adjacent species based on their given activities. The second method performs equilibrium calculations over an array of points (500 × 800 or higher are preferred), each representing one Eh and one pH value for the whole system, then combines areas of each dominant species for the diagram. These two methods may produce different diagrams. The fundamental theories, illustrated results, comparison and required conditions behind these two methods are presented and discussed in this paper. The Gibbs phase rule equation for an Eh-pH diagram was derived and verified from actual plots. Besides indicating the stability area of water, an Eh-pH diagram normally shows only half of an overall reaction. However, merging two or more related diagrams together reveals more clearly the possibility of the reactions involved. For instance, leaching of Au with cyanide followed by cementing Au with Zn (Merrill-Crowe process) can be illustrated by combining Au-CN and Zn-CN diagrams together. A second example of the galvanic conversion of chalcopyrite can be explained by merging S, Fe–S and Cu–Fe–S diagrams. The calculation of an Eh-pH diagram can be extended easily into another dimension, such as the concentration of a given ligand, temperature or showing the solubility of stable solids. A personal computer is capable of drawing the diagram by utilizing a 3D program, such as ParaView, or VisIt, or MATLAB. Two 3D wireframe volume plots of a Uranium-carbonate system from Garrels and Christ were used to verify the Eh-pH calculation and the presentation from ParaView. Although a two-dimensional drawing is still much clearer to read, a 3D graph can allow one to visualize an entire system by executing rotation, clipping, slicing and making a movie.

This paper deals with the recovery of gold from waste printed circuit boards (WPCBs) ash by high-pressure oxidative leaching (HPOL) pre-treatment and iodide leaching followed by reduction precipitation. Base metals present in WPCB ash were removed via HPOL using a diluted sulfuric acid solution at elevated temperatures. Effects of potassium iodide concentration, hydrogen peroxide concentration, sulfuric acid concentration, leaching temperature, and leaching time on gold extraction from pure gold chips with KI–H₂O₂–H₂SO₄ were investigated. The applicability of the optimized iodide leaching process for the extraction of gold from the leach residue obtained after HPOL were examined at different pulp densities ranging from 50 g/t to 200 g/t. Results show that the removal efficiency was 99% for Cu, 95.7% for Zn, 91% for Ni, 87.3% for Al, 82% for Co, and 70% for Fe under defined conditions. Under the optimal conditions, the percentage of gold extraction from the gold chips and the residue of WPCBs was 99% and 95%, respectively. About 99% of the gold was selectively precipitated from the pregnant leach solution by sequential precipitation with sodium hydroxide and L-ascorbic acid. Finally, more than 93% of gold recovery was achieved from WPCB ash by overall combined processes.

Study of the acid bake-leach process has shown potential advantages for the treatment of enargite (Cu3AsS4) concentrates. Among the most important advantages of the process is the transformation of enargite to water-soluble copper sulfate and highly soluble arsenic trioxide (arsenolite). Because arsenic is retained in the condensed phase during the baking, the vapor pressure of arsenic trioxide should be estimated at typical baking temperatures (e.g. 473 K). To that end, the vapor pressure of As4O6 (g) was estimated under the baking conditions based on published thermodynamic values. The vapor pressure of arsenolite at 473 K was found to be approximately 9.03 × 10-4 atm. Based on the linear regression analysis of the published vapor pressure values for arsenolite in the temperature range 366−579 K, the equation for the best fit line was found to be as follows, with a correlation coefficient of 0.9973:logPArsenolite (atm) = (-5780.7)/(T (K))+9.16. Available information on arsenic trioxide does not allow a definite conclusion regarding the arsenolite/claudetite transformation temperature and their exact melting points. However, the transition temperature has been reported to be in the wide range of 240−506 K in different references. Furthermore, the thermodynamic information concerning arsenolite/claudetite is sparse and at times not consistent. An effort has been made in this paper to compile the most reliable thermodynamic information for arsenic trioxide (arsenolite and claudetite).

Copper sulfide from ore deposits and recycled copper from collected wastes are the two main resources for the production of copper metal. Thermodynamic calculations can be used to serve as a guide for characterization of ore deposits and their mineralogy as well as for process development by mineral processing and by waste separation. The concentrated ore and separated waste can then be processed by pyro- and/or by hydro-metallurgical operations. Such operations may be controlled by oxidation-reduction potential, temperature, complexing reagents (gas, solid or dissolved species), oxidant- reductant, and mass of components. Commonly used stab ility diagrams for aqueous system include speciation (counting alpha) and EH-pH (Pourbaix) diagrams and, for non-aqueous systems, are partial pressure (Kellogg), binary and ternary phase, and Ellingham diagrams.For multicomponent systems, the appropriate approach for the construction of the stability diagram is to use a complete mass balance approach rather than draw the equilibrium line between two adjacent species. Not limited to two variables, a three- dimensional diagram can also be constructed.This paper illustrates the use of stability diagrams for investigating and interpreting how operation variables are important with a focus on extractive metallurgical operations to produce copper. Industria l operation conditions are marked on the diagrams.