--The chemistry of gold extraction

The chemistry of gold extraction by John Marsden and lainHouse. Ellis Horwood Series in Metals and AssociatedMaterials. Chichester, Ellis Horwood. £80.

The chronology has been well researched, and the bookdefmes all the milestones. It is a pity that there is no map ofthe early gold finds in South Africa, which would haveoffered a comparison with those shown of Australia and theUSA. Nevertheless, the 'Pre-cyanidation, Cyanidation andTechnical Development' eras fall logically into place, andreflect on the advances in social history and civilization, aswell as in technology.

ORE DEPOSITS AND PROCESS MINERALOGY

This chapter is of immense value in providing all thebasic information about the chemical and physicalproperties of native gold and gold-bearing minerals, andhow these affect the performance of all the chemicalprocesses involved in gold extraction. The importantcharacteristics of placer ores versus free-milling ores areexplained, as are the reasons for specific problems relatingto gold extraction from oxidized ores, sulphides,carbonaceous ores, etc. Particularly useful are the sectionsdealing with the occurrence of gold in process products suchas flotation and gravity concentrates, tailings, andrefmery materials.

State-of-the-art methods for the quantitativecharacterization of gold ore that can lead to increasedrevenues or reduced costs are described.

FACTORS AFFECTING PROCESS SELECTION

The all-important factors affecting the extraction processare neatly divided into six categories, which are given in-depth treatment. Specifically, the metallurgical response ofan ore (or concentrate) to a process can be thoroughlydetermined through the application of a detailedprogramme of testing, which is shown diagrammatically inflowsheet form. The unit process options used in extractionare neatly displayed in a similar manner, supported bydetailed text.

Flowsheet options are described for the different types ofore (showing that a 'horses for courses' approach isnecessary), and provide a very useful source of reference.Clearly, circuit complexity, and hence cost, vary with thedegree of refractoriness of the gold-containing material. Inthis regard, a general guide is given for relative capital andoperating cost factors for selected flow sheets, for whichcertain fIXedparameters are assumed.

GOLD HYDROMETALLURGY

In this and succeeding chapters, the book's title topic isaddressed in detail. Physical-chemistry principles ofreaction equilibria and reaction kinetics are generallydescribed, but are also specifically applied to goldchemistry where appropriate. The vital role played by

Reviewer:R. Graham

particle characteristics in leaching reactions is stressed,both physical and mineralogical. The experimentaltechniques that can be employed to provide information onreaction mechanisms are briefly reviewed.

Oxidatlve Pretreatment

The reasons for the necessity of pretreating 'refractory'ores are given, and the various oxidative processes aredetailed, i.e. hydrometallurgical sulphide, low-pressureoxygen, high-pressure oxygen (acidic and non-acidic),nitric acid, chlorine, biological, and pyrometallurgicaloxidation. In all cases, the reaction chemistry and kineticsare reviewed, and attention is paid to effluent streams andgaseous emissions. The exciting concept of biologicaloxidation is given sound theoretical and practical treatment,identifying it as a cost-effective and environmentallyacceptable process for gold-bearing sulphide ores andpotentially for carbonaceous ores. The description ofpyrometallurgical techniques benefits greatly from theliberal use of diagrams showing changes in structure,phase-stability diagrams, tables, and graphs.

Leaching

Eh;>H diagrams are used to full effect in this chapter.The chemistry of cyanidation as the classic gold-leachingtechnique is developed methodically, and the effects ofinterfering elements are discussed. A description is given ofpractical applications using agitated tanks, vats, heaps, etc.,and alternative lixiviants and associated reaction chemistryare reviewed. However, the point is made that developmentand cost constraints impede the establishment of large-scalecommercial processes using these reagents.

Solution Purification and ConcentrationThis chapter describes the vitally important step of

concentrating and purifying gold-bearing solutions. Theadsorption of the gold complex by activated carbon is dealtwith in much detail, together with elution and the effects onthe process parameters and the presence of other ionicspecies. The processes considered include carbonpreparation, carbon-in-pulp, carbon-in-leach, carbon-in-column, elution, and reactivation. Ion-exchange resins aresimilarly dealt with, and resin-in-pulp and resin-in-solutionprocesses are considered. Solvent extraction is reviewedbriefly, its general principles being explained and specificgold-extraction systems being highlighted. The point ismade that solvent extraction is not applied commercially topurify and concentrate gold-bearing solution.

RecoveryDetails are given of the electrochemistry of the

cementation (precipitation) of gold by zinc metal, effective

page290;

282 OCTOBER 1992 JoumaI of tie South Amcan Institute of Mining and MelBHutgy

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