The industrial processing of ores containing precious metals such as gold is generally very mining site specific. The variety of various mineral processing techniques employed is extensive and includes size reduction, size classification, gravity separation, roasting, cyanidation, chemical and/or bacterial leaching, ion exchange and/or solvent extraction, and froth flotation. Gold occurs in the earth’s crust on the average in very low concentration estimated by various sources to be between two and five parts per billion.
For a specific ore deposit to be considered as an economic source of gold, generally a concentration of near one part per million (one gram/Mton) or more is required. From a mineralogical viewpoint, gold often occurs in its native or free form. However, it can also occur as gold compounds of telluride, antimony, and selenium contained within either sulfide minerals such as pyrite, arsenopyrite, and pyrrhotite or, to a less extent, within silicate, oxide, and carbonate minerals. In many locations, alloying of gold with silver is found.
The optimal processing method for recovering gold is determined by many factors such as the mineralogy of the gold bearing minerals as well as the associated mineralogy of the gangue minerals, the type of liberation profile of the gold bearing minerals upon size reduction, and the particle size of the gold itself. Free or native gold particles can vary in size from the extreme of large nuggets to very finely disseminated gold associated in a complex sulfide mineral matrix.
Typically free gold particles of several hundred microns or larger are efficiently recovered by one or more of a number of gravity based techniques due to the large specific gravity differential of gold (19.3 to 15) versus typical gangue minerals (5 to 2.7). When gold is associated with a sulfide (usually pyrite) matrix, historically most industrial plants involving size reduction and subsequent liberation have operated in a size range of 50 to 80 percent less than 75 microns. Grinding finer than this has been normally not economical unless the ore has a high gold content in the feed ore.
Except for the notable exception of the finely disseminated nature of the gold ores of the Carlin Trend in Nevada, typically the processing scheme includes a flotation concentration step before or after a cyanidation step. The use of cyanide as a gold leaching process is generally inhibited by the presence of species such as pyrrhotite, arsenopyrite, stibnite. marcasite, active carbonaceous material, and some copper minerals such as chalcocite and malachite. The cyanide leaching process is generally inhibited by the above materials either due to the consumption of oxygen which slows down the rate of gold cyanidation and/or the consumption of large quantities of cyanide resulting in poor process economics. In the case of active carbonaceous materials being present, such materials can behave like activated carbon and adsorb gold from cyanide solutions. For ores that are refractory in nature, roasting and bacterial/pressure leaching is used to liberate gold prior to cyanidation.
Froth flotation in gold processing has been used for many years on a broad scale in many areas of the world, e. g. Klimpel and Isherwood (1993) and O’Connor and Dunne (1994), with South Africa being a very large scale user of gold flotation technology for many years. The purpose of this article is to summarize this author’s experiences over the last 20 years in the selection and/or optimization of reagent schemes with 24 different gold ores. This article will not discuss reagent chemistry mechanisms but rather concentrate on application methodologies. There are a number of useful review articles available on general sulfide collector flotation fundamentals which document some of the theoretical background behind reagent mechanisms on mineral surfaces, e. g. Fuerstenau et. al. (1985), Aplan and Chander (1988), and Fuerstenau (1989).
In addition, some useful plant oriented references on general sulfide applications and trends include Lynch et. al. (1981) and Klimpel (1989, 1995), Crozier (1992) and Malhotra (1993). Some of the material presented in this article is based on the background information contained in the above references. No attempt will be made to reference all of the literature articles available on gold flotation but rather the emphasis will be on presenting an integrated picture of some overall plant reagent use trends. The incentives for using flotation as a processing step include it’s high throughput at relatively low capital and operating costs, it’s high mineral specificity, and it’s high operating flexibility, e. g. Klimpel (1995). The ever increasing availability of effective collectors and frothers for gold flotation along with improved application methodology are also helping to increase the use of froth flotation as an integral step in the overall gold processing arena.
From the viewpoint of gold mineralogy interacting with reagent chemistry, it is useful to characterize the various possible reagent philosophies into two general categories: the flotation of free or native gold species and the recovery of gold associated with a variety of sulfide minerals. These two categories will be discussed in detail with illustrating examples presenting important factors that appear to be somewhat general in application. All of the experimental examples presented in this paper are previously unpublished and, in most cases, represent a substantial experimental effort to arrive at the data.
Flotation Collector Type and Dosage
The second major factor in the flotation of native or free gold is the choice of collector type and dosage. In this regard, it has consistently been this author’s experience that with inherently naturally floating minerals, the most effective collectors are those collectors that are uncharged and water insoluble. These hydrophobic collectors operate best when used at or near the natural pH of the mineral slurry, e. g. Klimpel (1995, 1997b). Typical uncharged and water insoluble collectors useful in such flotations include: thionocarbamates, RHN-(C=S)-OR xanthogen formates, RO-(C=S)-S-(C=S)-OR” mercaptans, RSH, and dialkyl sulfides, R’-S-R”.
In contrast to this insoluble class are charged water soluble collectors such as xanthates, RO-(C=S)-S- M+ and dithiophosphates, (RO)- (P=S)-S-M+. The reason for the natural pH is that Ca++ is generally a native or free gold depressant and as will be shown in a later section of this article, the presence of lime slows down the flotation kinetics of gold species. Thus if alkaline pH adjustment is necessary, it should be achieved with caustic, NaOH, soda ash, KOH, etc. and generally for acidic pH adjustment, sulfuric acid, H2SO4, is used although the use of hydrochloric acid, HCl, can be beneficial.
There are a number of reasons why insoluble collectors are preferable for the recovery of inherently naturally (collectorless) floating species. The first is that the use of water soluble (hence hydrophilic) collectors such as xanthates and dithiophosphates definitely slows down the rate of flotation of naturally floating minerals including native gold. As will be shown in many different ways in the article, gold flotation, under even the best of conditions, is inherently a slow rate process compared to other naturally floating minerals such as chalcopyrite or to other minerals requiring strong collector action such as chalcocite, sphalerite, etc. Thus any condition that slows down the gold recovery process needs to be avoided if at all possible.
The second reason is that much higher dosages of water soluble collectors are required for equivalent optimal equilibrium recoveries than are required for water insoluble collectors. Even with the water soluble xanthate species, for example, only the larger carbon content R groups such as amyl and butyl appear to be effective as gold collectors at any dosage. Ethyl and propyl R groups in xanthates are very seldom considered for gold flotation. The third reason is that naturally flotable minerals have a tendency to be naturally selective over many gangue minerals including pyrite. The use of a uncharged water insoluble collector helps to maintain this inherent selectivity whereas using a charged water soluble collector rather quickly destroys this selectivity. Once this natural selectivity is destroyed, the plant is often then forced into using depressants such as cyanide, pyrite depressants of various types, pH regulators for mineral surface modification, etc.
Figure 3 shows the trends of R and K parameters as measured on another native gold ore from the Pacific Rim. Six different collector conditions including collectorless flotation were measured at a natural ore pH of 7.3. This ore has primarily free gold species, a very small amount of gold associated with pyrite, and 5.6% by weight of the ore is pyrite. The trends shown in this figure are very typical of a large number of plant ores that this author has worked with. Also, all of the reasons given above for the desirability of using uncharged water insoluble collectors just given for the optimal recovery of inherently naturally flotable ores are verified in graphical form in Figure 3.
To begin with, it can be seen that both the collectorless flotation equilibrium recovery and the associated flotation rate are higher than with small dosages (12.5 g/Mton) of xanthate and dithiophosphate. The equivalent small dosage of 12.5 g/Mton of the thionocarbamate (ethyl isopropyl) and the dialkyl sulfide clearly gives superior recoveries and rates. It is only at high dosages of amyl xanthate, 100 g/Mton or more, that the equilibrium recovery R for the xanthate equals that of the R for the insoluble collectors at 12.5 g/Mton. The collector RKS-100 also shown in Figure 3 is an experimental
uncharged collector being developed by RK Associates specifically to enhance the recovery of naturally floating minerals.
The di-isobutyl dithiophosphate results are similar to those of the xanthate except that a low equilibrium gold recovery is achieved regardless of the dosage used. Whatever gold recovery the insoluble collectors are going to achieve is basically achieved by 25 to 50 g/Mton and any higher dosages of these specific collectors really accomplishes little. The higher dosage of charged collectors also has another characteristic of poor pyrite selectivity which will be shown shortly.
- The selection of an optimal flotation reagent scheme for gold flotation is very dependent on the specific mineralogy of the gold ore to be processed.
- Free or natural gold is optimally recovered using a reagent approach consistent with that successfully utilized in the recovery of copper sulfide minerals that have inherent natural or collectorless floatability. This implies natural or near natural pH and the use of small amounts (< 10 g/Mton) of an uncharged water insoluble collector.
- Inherently naturally floating minerals float fast kinetically, are relatively selective against other sulfides including pyrite, are relatively insensitive to water chemistry variations allowing for even seawater to be used effectively, and are effectively recovered at natural ore pH’s.
- Gold associated with iron sulfides can be recovered optimally in either acidic or alkaline conditions usually with larger amounts (> 50 g/Mton) of charged water soluble collectors. Effective collectors are available for either pH regime. Xanthates are the primary major collector component on the alkaline side and mercaptobenzothiazole is the major collector component on the acidic side. Blends of different collectors are particularly effective with gold associated with iron sulfides or with base metal sulfide ores. The dithiophosphates
are, in any given application, an effective additional component to a collector blend to enhance gold recovery.
- Gold flotation is inherently a slow kinetic process. Thus any operating and/or reagent condition that slows down the rate should be avoided. Typical adverse conditions are excessive dosages of collectors, high slime presence, low pulp densities, and cold process water conditions. There is nothing more golden in gold flotation than having lots of retention time.
- The choice of a particle size balanced frother having high flotation rate is important in gold flotation with generally glycol or polypropylene glycol methyl ether frothers being preferred over alcohol frothers.
- Larger free gold particles float slower than finer particles and also require more collector to recover by flotation. Thus care has to be taken to avoid over-reagentizing the fines and thus losing fine particle recovery in an attempt to increase the coarse particle recovery, i. e. to avoid the R/K trade-off that can occur with all minerals in flotation.
- Whenever possible, to reduce reagent and capital costs and to get the highest possible gold recoveries with mixed mineralogy gold ores having some minerals that float naturally, a two stage rougher flotation scheme is recommended. In the first stage, the inherently naturally floating minerals are removed first at natural pH with small (< 10 g/Mton) dosages of an uncharged water insoluble collector and then the remaining gold species are recovered in a second stage using higher collector dosages and also collector blends again at or near natural pH. The two stage rougher concentrates can be cleaned together sometimes requiring the use of a small amount of lime.