Quantitative analysis of sub‐microscopic and surface preg‐robbed gold in gold deportment studies Stamen S. Dimov* and Brian Hart Surface Science Western, Western University, 999 Collip Circle, London, On, N6A 5B7, Canada, Senior Research Scientist, Phone: +519 661 2173, e‐mail: [email protected] ABSTRACT In refractory ores, a substantial part of the gold content may be present as a sub‐microscopic component within the mineral matrix of the various carriers. This type of gold may be present as finely disseminated colloidal size gold particles (<0.5μm) or as a solid solution within the sulphide mineral matrix. Such ores are not amenable to gold recovery by direct cyanidation. In order to liberate the sub‐microscopic gold, the ore has to be oxidized before being subjected to gold cyanidation and extraction. Autoclave pressure oxidation (AC POX) of the sulphide ore and subsequent cyanidation is a common technology used to liberate the gold in such refractory ores. The additional presence of active carbonaceous matter in such ores (double refractory ores) represents another major obstacle for efficient gold recovery during the process of pressure oxidation and cyanidation. This carbonaceous matter (c‐matter) can adversely affect the gold recovery due to its ability to adsorb, or pre‐grob gold from the cyanide solution. Advanced micro‐beam analytical techniques such as Dynamic Secondary Ion Mass Spectrometry (D‐SIMS) and Time‐of‐Flight Secondary Ion Mass Spectrometry (TOF‐SIMS) have become powerful tools for characterization of different forms and carriers of gold for the needs of the mineral processing industry. Major advantages of these techniques are related to their ability to analyze individual mineral particles and provide quantitative analysis with detection limits in the low ppm/ppb concentration range. This paper describes various micro‐beam techniques and procedures implemented at Surface Science Western (SSW) for quantification and characterization of the sub‐microscopic gold and the preg‐robbed surface gold as part of full gold deportment studies in feeds and process stream products. INTRODUCTION The scope of a full deportment study is to establish all forms and carriers of gold present in the sample. Accurate, detailed information about the forms and carriers of gold in a feed or process stream sample is of crucial importance for optimization of the gold recovery process and identifying potential losses. The last several decades have seen the evolution of micro‐beam technology towards routine analysis for both problem solving and development capacities in the mineral processing industry. The implementation of advanced, ion and laser based micro‐beam analytical techniques for the needs of the mineral processing industry has led to new applications and establishing new protocols to accurately determine carriers and forms of gold in ore characterization and gold deportment studies. Based on their unique capabilities to provide quantitative analysis in the low ppm/ppb range, combined with imaging capabilities and the ability to directly analyze individual mineral particles, micro‐beam techniques such as Dynamic Secondary Ion Mass Spectrometry (D‐SIMS) (Chryssoulis, Cabri & Lennard, 1989; Chryssoulis & Cabri, 1990; Dimov & Hart, 2011) and Time‐‐of‐Flight Secondary Ion Mass Spectrometry (TOF‐
SIMS) (Dimov, Chryssoulis & Sodhi, 2003; Dimov, Hart & Chattopadhyay, 2009; Hart & Dimov, 2010) have become benchmark techniques for trace element and surface analysis for the needs of the mining industry. METHODOLOGY The Dynamic SIMS technique is a benchmark technique for quantitative analysis of sub‐
microscopic (invisible) gold and other precious metals in minerals. The sub‐microscopic gold detected and quantified by the Dynamic SIMS is refractory gold, i.e. it is locked within the crystalline structure of the mineral phase (most often in sulphide minerals) and it can not be directly released by the cyanide leach process. This type of gold may be present as finely disseminated colloidal size gold particles (<0.5μm) or as a solid solution within the mineral matrix. The Dynamic SIMS technique utilizes an energetic primary ion beam which sputters consecutive layers of material from the surface of polished mineral grains. Some of the sputtered particles are ejected as positive or negative ions which carry information about the composition of the sample and are further analyzed in a magnetic sector mass spectrometer. By rastering the primary ion beam across the surface of the polished mineral grain it is possible to map the distribution of the sub‐microscopic gold present in the mineral grain. The detection limits for the technique are in the range of 0.2ppm with imaging capabilities at a spatial resolution of 1 μm. The quantification of the D‐SIMS data is based on mineral specific ion implanted standards which results in elimination of any mineral matrix effects. The use of such matrix matched standards results in calibration curves covering large dynamic range from 0.2ppm to several percent (Chryssoulis, Cabri & Lennard,1989, Dimov & Hart, 2011). Since the D‐SIMS analysis is performed on polished mineral grains embedded in polished section mounts, it provides information on the sub‐microscopic gold content within the volume of the mineral grain. In addition, the D‐SIMS data provide valuable information on the type of the sub‐microscopic gold in each carrier; solid solution gold or finely disseminated colloidal size (<0.5 μm) gold particles. By analyzing a sufficient number of individual grains from each mineral phase of interest, it is possible to achieve a good statistical representation and an accurate quantitative estimate of the sub‐microscopic gold content in the various mineral carriers present in the sample. 2
Recognizing the importance and the effect of gold preg‐robbing on carbonaceous matter during the gold recovery process, an analytical protocol has been developed which addresses two different aspects of this problem: i) characterization of the c‐matter in an ore sample which helps to predict the impact of the c‐matter on the gold recovery (Helm, Vaughan & Staunton, 2011; Hart, Dimov & Mermillod‐Blondin, 2011) and ii) accurate evaluation of the surface gold preg‐
robbed on c‐matter from process stream products (Dimov, Hart & Chattopadhyay,2009). The procedure for characterization of the preg‐robbing properties of c‐matter in a feed sample involves a set of complementary micro‐beam analytical techniques (SEM/EDX, Laser Raman Spectroscopy) along with surface area measurements (BET) and standardized doping tests. It provides information on all the variables affecting the preg‐robbing properties of the carbonaceous matter, namely its composition, maturity and surface area and it can be used as a predictive tool. Our data library on preg‐robbing capacities of c‐matter from a large number of commercial mines shows a very large dynamic range with differences up to 200 times (Hart, Dimov & Mermillod‐Blondin, 2011). A comparative analysis of the data indicates that (along the TOC content and exposed surface area, BET) the crucial parameter defining the preg‐robbing capacity is the maturity or the degree of disorder of the naturally occurring ore carbon. Carbon spectra generated by Laser Raman spectroscopy provides valuable information on the nature/maturity of the c‐matter and can be used as a simple predictive tool on the expected preg‐
robbing capacity. The characterization of surface gold preg‐robbed on c‐matter is important for gold deportment studies in process stream products such as direct cyanidation tails or AC POX CIL tails. It is accomplished by the TOF‐SIMS technique. This advanced surface analytical technique provides non‐ destructive organic and inorganic surface analysis with detection sensitivity in the low ppm/subppm range. It is semi‐quantitative and it can provide information on the ”speciation of surface gold” (metallic or compound gold such as Au(CN)2, AuCl2, Au(SO3)2, etc.) preg‐robbed on c‐matter. The quantification of these forms of surface gold is provided by specific external standards. This information not only helps to balance the gold in these products but also provides valuable insight into the chemistry and origins of gold losses during these processes. The applications of the above discussed analytical micro‐beam techniques in full gold deportment studies will be demonstrated in two separate case studies on a feed and AC POX CIL residue samples. EXPERIMENTAL I. Full gold deportment study in a feed composite sample Background information A feed composite sample (P80=60mkm) has a total assayed gold content of 5.25g/t. The mineralogical analysis shows the presence of various sulphide minerals with pyrite being the major sulphide mineral phase. Pre‐concentration by gravity separation and subsequent optical microscopy have shown the presence of visible gold. The sample had insignificant content of carbonaceous matter (TOC). Objectives 1.
To characterize the visible gold in terms of composition, liberation and size distribution. 3
2.
3.
To identify all mineral carriers of sub‐microscopic gold and quantify the sub‐microscopic gold content in each carrier. To determine the fraction of the gold associated with silicates. Methodology The visible gold was characterized using high resolution visible gold scans produced by a field emission SEM‐EDX instrument (model Hitachi 6600 VP‐FEG‐SEM). By using both backscatter imaging and EDX compositional analysis , the SEM‐EDX scans provide detailed information on the composition of the gold particles and their host minerals and as well as on their size distribution and liberation. The magnification used allowed to detect and characterize visible gold particles with dimension down to 1‐2 microns. The sub‐microscopic gold was quantified by D‐SIMS instrument, model Cameca IMS 3/4f. The as received sample was assayed in duplicate for Au, S= , total organic carbon, TOC and other elements. After sizing by wet screening and gravity separation, a sulphide tip, middlings and gravity tail were produced. The gravity tails were further processed by heavy liquid separation in order to obtain clean silicates/binaries fractions. The fractions were assayed for Au, S= and other elements. Polished section mounts were prepared for both the visible gold scan and the D‐SIMS studies. Mineral grains from various mineral phases present in the sample were marked under optical microscope and identification maps were prepared. These selected mineral grains were analyzed for sub‐microscopic gold content by D‐SIMS. Quantification of the D‐SIMS data was carried out by using mineral specific implants standards. Results of the study The established gold deportment diagram for the feed sample is shown on Figure 1. The gold is present in two forms: as visible and sub‐microscopic gold. The independently determined by D‐
SIMS sub‐microscopic gold content in the sulphide minerals represents 83.8% of the total assayed gold in the sample. The pyrite mineral phase is the major carrier of sub‐microscopic gold among the sulphide minerals: it contains 59.6% of the total gold content in the sample. The arsenopyrite is the second major carrier with 21.1% of the total assayed gold. The copper sulphide minerals and the iron oxides in the sample are minor carriers of sub‐microscopic gold; 2.5% and 0.6% of the assayed gold is contained in the chalcopyrite and iron oxides fractions. The gold assayed in the rock minerals accounted for 3.8% of the total assayed gold in the sample. 4
Total assayed Au=5.25 g/t
6
5
Visible gold Carriers of sub‐
microscopic gold 12.4%
29.6
Au, g/t 4
59.6%
3
Sub‐
microscopic gold Pyrite 83.8%
2
21.1%
Arsenopyrite 1
2.5%
Au in rock: 3.8% Chalcopyrite Iron oxides: 0.6% Figure 1. Gold deportment diagram representing all major forms and carriers of gold in the composite feed sample. The relative distribution of gold per carrier is given in % of the total assayed value for gold in the sample. Four different morphological types of pyrite were identified: coarse, porous, microcrystalline and disseminated pyrite, Figure 2. The D‐SIMS data showed large dynamic range of sub‐microscopic gold concentrations within the various morphological types of pyrite, with highest estimated average values of sub‐microscopic gold concentrations in the microcrystalline and the disseminated types of pyrite. The relative abundances of various morphological types were determined by grain counting using optical microscopy and these values were used to weigh the contribution of different morphological types of pyrite into the gold balance. The data for arsenopyrite which is present as coarse and porous type were handled in a similar way. Pyrite coarse
Pyrite porous
Pyrite microcrystalline
Figure 2. Morphological types of pyrite analyzed by the Dynamic SIMS 5
Pyrite disseminated
The Dynamic SIMS technique can provide high resolution maps with distribution of trace elements with concentrations down in the low ppm range. An example of such a map with distribution of sub‐microscopic gold and arsenic in a pyrite mineral grain is shown on Figure 3. 50 µm
Figure 3. Images of distribution of sub‐microscopic gold and arsenic in a pyrite mineral grain produced by Dynamic SIMS. Also shown is a map for the matrix element S and an optical photograph of the grain The visible gold content was calculated using the assayed gold values and the independently determined by D‐SIMS sub‐microscopic gold content in the feed sample. The composition, size distribution and the liberation of the visible gold were determined from the high resolution visible gold scans provided by the field emission SEM‐EDX instrument, Table 1 The visible gold was present in the forms of native gold and electrum. Examples of a high resolution backscattered electron and a secondary electron images along with EDX spectrum for selected areas are shown in Figure 4. Table 1. Composition, size, liberation and host for identified Au and Au+Ag grains in a feed sample. Grain # 80 Gold 46 Gold 94 Gold 93 Gold 95 Gold 56 Gold 92 Gold + Silver 89 Gold + Silver 45 Gold + Silver 88 Gold + Silver Au 99.2 99.8 99.8 99.8 100.0 99.6 46.8 63.6 83.3 70.5 Ag 0.2 0.2 0.4 52.7 35.6 16.2 28.7 Te 0.8 0.2 0.5 0.8 0.5 0.8 Aspect Ratio 1.4 Breadth (um) 4.4 Length (um) 5.9 1.8 1.5 1.3 1.3 1.3 2 1.5 1.3 1.3 2.6 2.1 1.6 1.8 1.6 1.8 2.1 1.6 1.5 4.6 3 2.2 2.2 2.1 3.5 3 2.2 1.8 6
Perimeter Liberation (um) 15.5 Liberated/Attached 11.2 Liberated 7.5 Liberated/Attached 5.7 Liberated/Attached 5.7 Liberated/Attached 5.2 Liberated/Attached 8.2 Locked 7.3 Locked 5.4 Locked 4.6 Locked Mineral Association Pyrite Pyrite Pyrite Pyrite Pyrite Pyrite Pyrite Py/Cpy Pyrite Pyrite Figure 4. BSE and SEI images along with EDX spectra of Au‐Ag (electrum) grain and host phase (pyrite) in the feed sample. II. Full gold deportment study in an AC POX CIL tail sample Background information A sulphide concentrate (POX feed) is processed in an autoclave pressure oxidation plant with the subsequent POX discharge subjected to a cyanide leaching. In this example the POX feed sample contains about 38 % pyrite, 3% arsenopyrite and trace amounts of chalcopyrite. The sub‐
microscopic gold contained in the sulphides is the dominant form of gold in the sample (95% of the total assayed gold content, Au=18.5g/t) with the rest being free gold. The POX feed sample also contained 2.75% total organic carbon (TOC). The gold assayed in the POC CIL tail sample was 3.4g/t. A full gold deportment study on the POC CIL tail sample was carried out. Objectives: 1.
2.
3.
To identify all relevant gold carriers in this sample. To identify and independently quantify all major forms of gold. Base on the established gold deportment to identify relevant cause(s) for losses in the process of the gold recovery. Methodology The standard procedure for gold deportment in process stream product involves assaying, sample sizing, gravity separation and heavy liquid separation. Polished section mounts are prepared for the mineralogical analysis and the quantitative D‐SIMS study for sub‐microscopic 7
gold. The TOF‐SIMS technique (ION‐TOF IV TOF‐SIMS instrument) was used for characterization and quantification of surface gold preg‐robbed on carbonaceous matter. For that purpose, as received, individual carbonaceous particles from the POX CIL tail sample were picked under the optical stereoscope and their surfaces analyzed. SEM‐EDX and Raman Spectroscopy were used to characterize the composition/distribution and the nature of the c‐
matter. D‐SIMS was used to quantify the sub‐microscopic gold in the sample. Results of the study: The mineralogical analysis established the presence of an unoxidized pyrite fraction and a secondary hematite fraction. Visible gold or arsenopyrite were not identified in the POX‐CIL tail sample indicating a complete oxidation of the arsenopyrite and complete cyanidation of the free gold. The determined gold balance in the sample is shown on the diagram in Figure 5. This diagram represents the established forms of gold in the sample along with the corresponding carriers, and the techniques used to independently quantify all these forms/carriers of gold. The established gold deportment accounts for 94% of the total assayed gold in the sample. 4.0
Technique
Forms/Carriers of gold
Total assayed
Au=3.4 g/t
Assayed 1.1%
Soluble gold salts
D‐SIMS 24.7%
Sub‐microscopic gold in hematite D‐SIMS
17.4%
Sub‐microscopic gold in unoxidized pyrite TOF‐SIMS 46.5%
Surface gold preg‐robbed on c‐matter Assayed 4.4%
Gold enclosed in rock
Au, g/t 3.0
2.0
1.0
Figure 5. Gold deportment diagram representing all major forms and carriers of gold established in the POX CIL tail sample and the corresponding techniques utilized for independent quantifications of these forms of gold. The relative distribution of gold per carrier is given in % of the total assayed value for gold in the sample. 8
The composition and the nature of the carbonaceous matter in the POX CIL tail were analyzed by SEM‐EDX and Raman spectroscopy. This carbonaceous matter was present mainly as finely disseminated carbon in rock particles, Figure 6. The Raman Spectroscopy analysis of the c‐matter showed that the nature of this carbon is close to that of activated carbon (Figure 7) which is a marker for highly preg‐robbing properties. Figure 6. Backscattered electron image of a disseminated carbonaceous matter (DCM) particle from the POX CIL tail sample along with Raman spectra from two different surface areas. Figure 7. Raman spectra from three different DCM particles from the POX CIL tail overlapped with reference Raman spectra from activated and graphitic carbon. The Raman spectra from the DCM particles are similar to that of activated carbon Three major causes for gold losses during the process of gold recovery were identified in this 9
study: i)
ii)
iii)
Incomplete oxidation of the pyrite mineral phase which accounts for 17.4% of the gold losses. Incomplete leaching of the sub‐microscopic gold contained in the porous secondary hematite phase which account for 24.7% of the gold losses. Preg‐robbing of surface gold on the carbonaceous matter is the major cause for gold losses. It amounts to 46.5% of the total losses. The speciation of the surface gold from the TOF‐SIMS analysis shows that most of the surface gold is in the form of Au(CN)2 (67%) with the rest being metallic gold. CONCLUSIONS The implementation of advanced micro‐beam analytical techniques in gold (and other precious metals) deportment studies has greatly improved and expanded our capabilities to accurately characterize and quantify various forms of gold in individual mineral carriers. Their advantages are rooted in their ability to analyze individual mineral particles and to provide quantitative data with detection limits down in the low ppm/ppb concentration range making them benchmark techniques for these types of analysis. REFERENCES 1. 2. 3. 4. 5. 6 7. 8. Chryssoulis,S.L ,Cabri, L.J. and Lennard,W.(1989) “Calibration of the Ion Microprobe for quantitative trace precious metal analysis of ore minerals”, Economic geology, Vol.84, pp. 1694‐1689. Chryssoulis,S.L. and Cabri, L.J. (1990) “Significance of gold mineralogical balances in mineral processing”, Trans. Instn. Min.Metall, (Sect. C:Mineral process Extr. Metall), Vol. 99, C1‐C10 Dimov,S.S. and Hart,B. (2011) “Applications of micro‐beam techniques in gold deportment studies”, World Gold 2011, Proceedings of the 50th conference of metallurgists, Montreal, Canada Ed. By Deschenes G., Dimitrakopoulos R. and Bouchard J, pp.17‐26 Dimov,S.S., Chryssoulis,S.L. and Sodhi,R.H. (2003) “Speciation of Surface Gold in Pressure Oxidized Carbonaceous Gold Ores by TOF‐SIMS and TOF‐LIMS”, Applied Surface Science, Vol. 203‐204 , pp. 644‐647. Dimov,S.S.,Hart,B. and Chattopadhyay, A,.(2009) “Speciation and quantification of surface gold in carbonaceous matter from AC POX stream products by TOF‐SIMS”, COM 2009, In proceedings of the 48th annual conference of metallurgists, Sudbury, Canada, Ed by: Hamilton, C., Hart, R. and Whittaker P.J., p.85‐91. Hart,B. and Dimov,S.S. (2010) “TOF‐SIMS surface analysis as a diagnostic and predictive tool for mineral processing: new developments and application” CMP 2010, In proceedings of 42nd annual meeting of the Canadian Mineral Processors, Ottawa, Canada, pp.121‐142. Hart,B., Dimov,S.S. and Mermillod‐Blondin,R.(2011) “Procedure for characterization of carbonaceous matter in an ore sample with estimation towards its preg‐robbing capacity”, World Gold 2011, Proceedings of the 50th conference of metallurgists, Montreal, Canada,. Ed. By Deschenes G., Dimitrakopoulos R. and Bouchard J, pp.35‐50. Helm, M. M., Vaughan J.P. and Staunton, W.P. (2011)” Evaluation of Preg‐Robbing in Goldstrike Carbonaceous Ore Using Raman Spectroscopy”. World Gold 2011, Proceedings of the 50th conference of metallurgists, Montreal, Canada,. Ed. By Deschenes G., Dimitrakopoulos R. and Bouchard J, pp.595‐606. 10