Thermodynamics of Aqueous Systems with Industrial Applications

32 Application of Thermodynamics in Hydrometallurgy A State-of-the-Art Review HERBERT E . BARNER Kennecott Copper Corp., Lexington, M A 02173

Downloaded by EMORY UNIV on May 29, 2014 | http://pubs.acs.org Publication Date: October 29, 1980 | doi: 10.1021/bk-1980-0133.ch032

ROGER N. KUST Exxon Research and Engineering Co., Florham Park, NJ 07932

In the past several years interest in hydrometallurgical processing of ores has grown significantly. This growth can be attributed to several factors, among which are the necessity for environmental protection and pollution control, the increase in cost and decrease in availability of oil and natural gas, the increased complexity of ores processed, and the increased exploitation of non-sulf ide ores. Whether or not hydrometallurgical processing will provide satisfactory answers to many of the problems facing the minerals industry today is yet to be demonstrated. However, there have been several instances in recent years where hydrometallurgical processing, totally or in part, has proven to be advantageous. Hydrometallurgical processes involve the treating of a raw ore or concentrate with an aqueous solution of a chemical reagent in areaotor. The desired metal values are leached from the ore or concentrate, and the residue, after washing, is rejected as tailings. The leach liquor, containing one or more metal values in solution is then processed to the metals. The leaching reactor can be a conventional stirred tank or autoclave, or, as in the case of dump leaching, can be a large pile of rejected ore. In in-situ type leaching, the reactor is the ore-body itself, into which a suitable lixiviant is pumped. Hydro processes operate at lower temperatures than pyro processes, usually 50-250 C, and as a result the rates at which reactions occur are frequently several orders of magnitude slower. Consequently, in the development of such processes, kinetic studies become important. However, application of thermodynamics can still give valuable insight into the nature of various processes and should be used to determine process limitations. One important way in which thermodynamics is utilized in hydrometallurgical process development is as a guide to experimental planning and process selection and evaluation. A high degree of accuracy is not needed at this stage of process development. Estimates which are within 15 to 20% of the true value are frequently adequate for making feasibility calculations.

0-8412-0569-8/80/47-133-625$05.00/0 © 1980 American Chemical Society

In Thermodynamics of Aqueous Systems with Industrial Applications; Newman, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

626

THERMODYNAMICS OF

AQUEOUS SYSTEMS W I T H INDUSTRIAL

APPLICATIONS

There are numerous areas which can be treated by thermodynamic analysis. Some of them are: 1. Calculation of the solubility of simple and complex salts and gases, including estimation of "metal loading" in leach liquors 2. Estimation of vapor pressures of volatile components 3. Determination of the extent of reaction or position of equilibrium under various conditions of temperature, pressure and concentrations. 4. Estimation of extent of complex ion and ion pair formation. 5. Calculation of distribution coefficients for ion exchange processes.


Earlier Reviews A number of earlier reviews on this subject have been published. MacDonald (1) has addressed the electrochemistry of metals in aqueous systems at high temperature, including the extrapolation of low temperature data and the activity coefficients of ions, water and dissolved gases. Two contributions from the Warren Spring Laboratory in England (2, 3) have reviewed pressure hydrolysis at high temperature and the extrapolation of potential-pH (Pourbaix) diagrams to high temperatures. Kwok and Robbins (4) have also considered the thermodynamics of high-temperature solutions with emphasis on separation of metal values from leach solutions; the CuS0 - H S 0 - H „ 0 was treated in some detail. Peters (5) has reviewed the leaching of copper, nickel, zinc, lead and molybdenum concentrates in terms of the thermodynamic stability of the sulfide minerals of these metals. Process developments associated with the most favorable decomposition paths are considered. From a geochemistry point of view, Helgeson 7, 8) has presented the properties of water at high temperature and pressure, Debye-Huckel parameters as a function of temperature and pressure, and the partial molal properties of electrolytes. Helgeson (9) has in addition published a comprehensive monograph on the thermodynamic aspects of hydrothermal ore-forming solutions. Barnes (10) has also presented a review of hydrothermal geochemistry with emphasis on thermodynamic interpretation and experimental measurements at high temperature and pressure. 4

2

4

Species in Solution Before any thermodynamic estimates can be made, there must be an assessment of the major solution, solid phase, and gaseous phase species present in a given system. In some instances physico-chemical measurements must be made to identify the species to be considered, but frequently chemical intuition and judgement flavored with experience are sufficient to provide a basis for initial estimating. Generally, for metal species in solution one needs to know how metal ions are hydrated, the number of ligands associated with the metal, the charges on the complex ions present, and whether or not the metal species are primarily mononuclear. For many metals such as copper, zinc, nickel, silver, etc. there is reasonable agreement as to what type of complexes are formed with the relatively simple ligands of interest to hydrometallurgy, e.g., hydroxide,


32.

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Thermodynamics in Hydrometallurgy

627

chloride, sulfate, nitrate, cyanide, thiocyanate. For a few metals, such as molybdenum and tungsten, the species formed are extremely complex in nature and there is little agreement on their constitution, particularly in weak acid or neutral solutions. A recent report on the recovery of metal values from geothermal brine indicated the presence of lead, silver, copper, and iron in the cat ionic form. Because the brine contained 155 g/1 of chloride the metals must be present as chloroanions. The presence of chloroanions rather than aquated cations suggests a different type of processing and would certainly alter the results of thermodynamic estimates.


Equilibrium Constants and Free Energy of Formation It is necessary to consider a number of equilibrium reactions in an analysis of a hydrometallurgical process. These include complexing reactions that occur in solution as well as solubility reactions that define equilibria for the dissolution and precipitation of solid phases. As an example, in analyzing the precipitation of iron compounds from sulfuric acid leach solutions, McAndrew, et al. (11) consider up to 32 hydroxyl and sulfate complexing reactions and 13 precipitation reactions. Within a restricted pH range only a few of these equilibria are relevant and need to be considered. Nevertheless, equilibrium constants for the relevant reactions must be known. Furthermore, since most processes operate at elevated temperatures, it is essential that these parameters be known over a range of temperatures. The availability of this information is discussed below. Data at 25°(^. Free energy of formation, equilibrium constants and related data at 25 C exist for a wide range of minerals, other solids, gases and aqueous species, including ions and complexes (see later discussion on data sources). Availability of data at this reference temperature is usually not a major stumbling block. On the other hand, new solution species are being identified. For example, some polynuclear species and some ion pair complexes are now recognized as being more significant in aqueous solutions than previously thought. There is therefore a need to develop, extend and up-date the data on new species which come to be recognized as significant. Temperature Effects. The most reliable source of information on the temperature dependence of a specific equilibrium constant is experimental measurement. Occasionally, sufficient experimental data are available. In the more usual situation, however, only sporadic high-temperature data, if any at all, are available. It is then necessary to use some form of extrapolation procedure to extend the 25 C data to higher temperatures. A number of approaches are available to extrapolate low-temperature equilibrium constants. Various aspects of this problem have been discussed and some comparisons to experimental data have been made. See, for example, Criss and Cobble (12), Helgeson (13), MacDonald (1), and Manning and Melling (3). We are not, however, aware of any recent comprehensive evaluation, error analysis or overall assessment. We summarize below what we believe to be the present situation.


THERMODYNAMICS

628

OF AQUEOUS

SYSTEMS WITH

INDUSTRIAL APPLICATIONS

The equation that relates the Gibbs free energy of a chemical reaction at temperature Τ to that at the reference temperature 298°C is given by + Δ 0

Τ. =

Δ 0

Δ8

298 " 298

Δ

Τ

Τ Δ

dT

Τ

^

Thermodynamics of Aqueous Systems with Industrial Applications

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