Different methods used in pre-treating preg-robbing graphite ores in Zimbabwe prior to leaching.￼
In the past, gold processing tended to concentrate on simple, free-milling oxide ores. As these have become scarcer, attention has turned to refractory deposits of ore, which have previously been untreated. Generally, these have been left due to technical difficulties arising during cyanidation, because of the minerals that make up the ore. This “refractory behaviour” has many definitions and causes. The term refractory is in common use in gold processing and is applied to two concepts about the extraction of gold from an ore. Firstly, a general definition for refractoriness is that the ore will not give an acceptable recovery of gold by cyanidation. The second concept refers to the gold that is not extracted from an ore following cyanidation. This may be the refractory component of the ore. (La Brooy, et al., 1994) suggested a classification for ore refractoriness, based on the total extraction possible by cyanidation.
Figure 1: Types of gold ores according to (Marsden & House, 2006)
From a metallurgical perspective, gold ores can be broadly subdivided into:
Free milling Ores: These consist of the most profitable gold ores. Over 90% of the gold can be recovered by conventional cyanidation. Ores of this nature are easily crushed, washed, and milled to recover the gold contained within it. Examples are quartz rose ores and sugar quartz (Salter & Chryssouils, 1987).
Refractory Ores: Most metallurgists regard refractory ores as those where the gold is in some way locked in the sulphide fraction. Two other terms are sometimes used to describe gold ores that are difficult to treat.
Many of the methods now used for gold extraction are based on techniques that have been known or established for centuries. Gravity concentration, amalgamation, cyanide leaching, chlorination, zinc precipitation and carbon/charcoal adsorption are all processes that have been used for at least a hundred years and a combination of these remain as the basis for most gold recovery flowsheets (Marsden & House, 2006). Leaching gold with a cyanide solution remains the most widely used hydrometallurgical process for the extraction of gold from ore and concentrate.
In the context of gold extraction, leaching is the dissolution of a metal or mineral in a liquid (usually, but not always a solvent). The chemical reaction for the dissolution of gold “the Eisner’s equations, follows:
Eisner’s equation of dissolution
8CN–(aq) + 4Au (s) + O2 (g) + H2O (l) → 4Au (CN–)2 (aq) + 4OH–(aq)
Gold + Free cyanide ion + oxygen + water → gold cyanide complex ion + hydroxyl ion
The ore must be ground to a specific size before it can be reacted with cyanide in the presence of oxygen, the gold in the ore will be dissolved to form gold cyanide complex ion. For non-refractory gold ores, carbon is then added to adsorb all the dissolved gold in the solution in a process called adsorption. Despite the success of cyanidation, its applicability is limited by some gold mineralization. These types of gold mineralisation are referred to as “refractory gold ores” (Turney & Smith, 1989). Refractory ores have two main elements that make it difficult to leach the gold from the ore and these are sulphides and carbonaceous matter.
Sulphides prevent the lixiviant from contacting the metal of interest during leaching and thus, reduce recovery. Carbonaceous matter, on the other hand, adsorbs the metal of interest from the pregnant solution and thus reduces recovery in a phenomenon referred to as preg-robbing (Zaitstseva, et al., 1973). Preg-robbing occurs when the ore contains active carbon that adsorbs gold from pregnant solution thus acting like activated carbon (Hausen & Buchnam, 1984).
In gold processing, various carbonaceous materials of different types and characteristics may be encountered, and these would affect gold recovery at different degrees. Several researchers have reported on carbonaceous matter in gold ores to include anthracite, graphite, coal, shale, decaying wood, wood ashes and other vegetable matter (Adams, et al., 1996).
Early characterization studies by Radtke and Scheiner into the physical and chemical nature of carbonaceous gold ores identified two distinct organic components in addition to an activated carbon type material. High molecular weight hydrocarbons were thought to coat the surface of the activated carbon type component, while the other organic component was considered to comprise an organic acid with functional group capable of reacting with gold complexes. After the findings in this investigation, numerous attempts have been made to characterize carbonaceous gold ores in terms of hydrocarbons, organic acid and activated carbon type content and assess the influence these components have on preg-robbing (Radtke & Scheiner, 1970).
All mines have unique ore mineralogy characteristics, which determines the ore processing and metallurgical gold recovery method (Anderson & Harrison, 1990). Owing to the depletion of readily amenable gold ores and the discovery of complex deposits, this has inspired the need for knowledge improvements and search for commercially effective techniques in extracting gold from refractory gold ores.
Assessment of options available for economic processing of preg-robbing gold ores.
One of the challenges facing the gold industry in the twenty-first century is the continuing need to identify new reserves of economically treatable ores. Discoveries of large, metallurgically simple ore bodies are becoming increasingly rarer, forcing companies to investigate options for the processing of ores which may present recovery, economic and/or environmental issues, requiring innovative approaches to their treatment.
Oxidative processes have been used as a pre-treatment for sulphide, carbonaceous, and telluride ores and concentrates to increase the extraction of gold by standard hydrometallurgical processing techniques, usually cyanidation. These methods are applied when direct treatment by cyanidation gives unacceptably low gold recovery or is uneconomic, for one of the following reasons:
- Gold is locked in reactive gangue minerals, often sulphides, and cannot be adequately liberated, even by fine grinding.
- Gold occurs with minerals that consume unacceptable quantities of reagents, for example, pyrrhotite, marcasite, and arsenopyrite.
- Gold occurs with carbonaceous materials that adsorb gold during leaching.
The simplest method of treating a carbonaceous ore is the method described by R.W. Nice (1971) at the Mclntyre Porcupine Mine where the carbon, which contains very little gold, is floated off and discarded. The remaining ore is then responsive to cyanidation. This does not work at Carlin because the carbonaceous compounds have high gold content. A considerable amount of flotation work was done on Carlin ores, however, to produce a high-grade concentrate for possible shipment to a smelter and a tailing which could either be discarded or directly cyanided. All the concentrates contained both carbonaceous material and pyrite and showed low recoveries of gold and poor selectivity.
The effectiveness of the use of blinding agents, ionic exchanger resins and the combination of them in improving gold extraction from simulated preg-robbing ore was discussed Mohammad Zaki and Prasetyo Sandha at the Institute of Technology of Bandung in Indonesia. The blinding agents used were Kerosene, diesel oil and pine oil, while the ion exchange resins used were Lewatit Monoplus MP 800. Preg-robbing conditions were simulated by blending fine activated carbon with ore sample. The investigation results showed that the presence of artificial carbonaceous material at 2% (w/w) in the gold ore significantly reduces gold recovery. Pre-treatment of artificial preg-robbing ore prior to cyanidation by mixing with kerosene, diesel oil and pine oil for 0.5h improved gold recovery up to 25.4%.
Over the last 40 years, numerous investigations have focussed on the preg-robbing capacity of carbonaceous material during gold ore processing as summarized above. Various techniques have been applied to characterize the physical and chemical properties of the carbonaceous material in efforts to predict its preg-robbing capacity. The present study, which forms part of an ongoing investigation, is specifically designed to increase the understanding of preg-robbing in ores containing carbonaceous material especially those found in the Bulawayo greenstone belt in Zimbabwe.