Practice Brief GOLD PROCESSING. Revision Date: October 23, 2015

Transcription

1 Practice Brief GOLD PROCESSING Revision Date: October 23, 2015 Prepared by 330 Alison Blvd. Fredericton, New Brunswick Canada, E3C 0A9 Telephone: (506)

2 Gold Processing Practice Brief October 2015 Page 2 of 9 Introduction Over the past five years, gold prices have declined to less than US$ 1,200 per ounce as of 3 rd Q 2015 (see Figure 1). This steady downward trend has sparked an interest in the development of lower cost production methods, especially owing to the fact that in many cases gold prices may be not be strong enough to support production from difficult to process or low grade ores despite rising capital, reagent and mining costs. has also developed highly sophisticated, dynamic economic models that provide our clients with a means of assessing the economic viability of the project throughout every stage of commercial development. For a more complete description of Thibault & Associates Inc. s process development testing and economic modelling capabilities, please refer to the respective Practice Briefs for these services. Gold Mineralogy and Types of Gold Ores The optimum recovery methods for a particular gold ore are largely dictated by the gold mineralogy, i.e. the forms, carriers, association and grain size of gold in the ore. The form of gold refers to its chemical state, while a carrier of gold is the particular mineral particle that contains gold in one or more of these forms. Native gold and electrum (a gold silver alloy) are the most common forms of gold. Table 1 lists some of the typical forms and carriers of gold, while Figures 2 and 3 depict images of gold mineralization in a sulphide gold ore. Many gold ores will contain more than one form and carrier of gold over a wide range of particle sizes, all of which need to be considered when selecting the appropriate processing technology. Figure 1: Five Year Price Trend for Gold To achieve highly competitive production costs, exceptional process engineering support is critical for development of technically and economically viable recovery methods. has successfully completed numerous projects for upgrading and recovery of precious metals from a wide variety of refractory and non refractory ores through application of a broad range of process technologies, including: flotation, gravity concentration, bio oxidation leaching, pressure oxidation leaching, cyanide leaching, carbon in leach, carbon in pulp and resin in pulp roasting, and coal gold agglomeration. offers complete client care through our unique process engineering service concept, which combines in house bench and pilot scale test facilities with a full range of process technology development and plant design services. This assures efficient technology transfer from test programs to definitive design as well as seamless progression of design from initial concept definition to aftercare programs. Table 1: Typical Forms and Carriers of Gold Type Microscopic (Visible) Gold Submicroscopic (Invisible) Gold Forms Native Gold (>80% Au) Electrum (<80% Au, 20 50% Ag) Kustelite (<50% Au, >50% Ag) Aurostibite (AuSb 2 ) Maldonite (Au 2 Bi) Auricupride (AuCu 3 ) Gold Tellurides (AuTe 2, (Au,Ag)Te 2, AuAgTe 4, Ag 3 AuTe 2 ) Solid solution gold or fine grained gold inclusions (< 0.1 micron) in carrier minerals Carriers Free/liberated gold mineral grains. Attachments to other minerals or gangue in the ore. Inclusions locked inside other minerals or gangue in the ore. Pyrite (FeS 2 ) Arsenopyrite (FeAsS) Tetrahedrite (Cu 12 As 4 S 13 ) Chalcopyrite (CuFeS 2 ) Pyrrhotite (Fe 1 x S) Gold ores can be classified as free milling (non refractory), refractory, or complex, depending on the relative ease of extracting gold from the ore, as described in Table 2. Often times there may be both a free milling and a refractory component to a gold ore. For example, a sulphide ore may contain native gold or electrum that is easily liberated in addition to some submicroscopic gold locked in pyrite. The bulk of gold ores currently being processed are free milling, however, complex and refractory gold processing is becoming more commercially important as free milling ores are depleted.

3 Gold Processing Practice Brief October 2015 Page 3 of 9 Free Milling (Non Refractory) Refractory Complex Table 2: Classification of Gold Ore Types Characteristics Gold minerals can be sufficiently liberated (not necessarily free ) for direct attack by cyanide. >90% gold recovery is typically achievable in a conventional cyanide leach. Gold mineralization is locked within other minerals and not amendable to cyanidization during conventional leaching. <90% gold recovery is typically achievable in a conventional cyanide leach. Gold locking may be due to visible gold particles encapsulated by other minerals, submicroscopic gold, or presence of mineral forms not soluble in cyanide (e.g. tellurides, AuSb 2, Au 2 Bi). Can be considered semirefractory. Gold minerals sufficiently liberated for cyanide leach. >90% gold recovery in a conventional cyanide leach, but at higher reagent consumptions. Reactive minerals in the ore may consume oxygen or cyanide, and re adsorb gold after it is dissolved. Typical Ores Placers, quartz vein, oxide ores, some sulphide ores. Most often sulphide ores such as pyrite, arsenopyrite, chalcopyrite. Sulphide ores containing elevated copper content or pyrrhotite, carbonaceous ores, ores with high clay content. employed to treat the grinding circuit cyclone underflow and produce a high grade free gold concentrate that can be sold directly to refiners. Figure 2: Carriers of gold particles present in sulphide ore: 1 electrum locked within a liberated pyrite particle, 2 aurostibite attached to a galena particle, 3 native gold locked within arsenopyrite, 4 composite particle containing electrum, aurostibite, quartz and sulphide minerals. Gold Ore Processing Technologies The type of gold ore, along with grade and liberation particle size all have an influence in selecting the technology bestsuited for upgrading and recovering gold from a particular deposit. Figure 4 provides an overview of some of the more common gold processing options. The simplest ores would use none of the optional processing steps, while the more complex/refractory ores would require some or all of the optional recovery processes as illustrated. Gravity Recovery of Free Gold The grain size of free gold influences its recoverability in a gold processing plant. If there is a significant quantity of free, coarse gold that is liberated in the grinding circuit, it can tend to accumulate in the grinding mills, eventually becoming flattened or over ground to a fine size. If flotation preconcentration is used, coarse and flat gold particles will not float well, and particles that are too fine will also not be recovered, leading to a loss of gold. In order to enhance overall gold recovery in the plant, gravity recovery is often used in the grinding circuit when there is coarse free gold present. Centrifugal gravity concentrators are commonly Figure 3: Forms of free or liberated gold particles present in sulphide ore: 1 native gold, 2 electrum, 3 aurostibite surrounded by oxidation rim of aurostibate, 4 fine electrum <5µm in size. Gold Pre Concentration Considerable capital and operating cost savings can often be achieved if the run of mine ore can be upgraded to a higher grade gold concentrate prior to cyanide leaching. Waste rock is rejected directly to tailings and the downstream processing equipment can be significantly downsized; reducing the capital investment. Froth flotation is the most common method for preconcentration and is effective for floating valuable gold bearing minerals and free gold, leaving gangue material behind. Upon the addition of specific collectors and frothing agents, air is bubbled through the slurry causing the desirable minerals to attach to air bubbles and float to the surface. The resulting froth is collected as an enriched gold concentrate for further processing.

4 Gold Processing Practice Brief October 2015 Page 4 of 9 Figure 4: Block Diagram of Typical Processing Options for Gold Ores Gravity separation is a lower cost method for upgrading gold ores, which eliminates the need for reagent addition in the pre concentration step. Using this method, a higher grade feedstock for downstream processing is obtained through physical separation of gold bearing minerals and gangue material based on the different specific gravities of the minerals. Treatment of Free Milling Gold Ores Most free milling gold ores today are processed using the approach of crushing, grinding, agitated cyanide leaching, carbon in pulp, and gold electrowinning as shown in Figure 4. The crushing and grinding steps sufficiently liberate gold minerals so that they can be directly leached in a cyanide

5 Gold Processing Practice Brief October 2015 Page 5 of 9 solution. High grade ores are usually directly leached after grinding, while low grade ores suitable for pre concentration are typically upgraded into a concentrate prior to leaching. Cyanide is then used to selectively dissolve gold from the host minerals. A special case (and the most simple) of the free milling gold ores is Placer deposits, where gold is present in the form of free (liberated) gold that is coarse enough for direct recovery by gravity processing. The heavier gold particles are recovered to a concentrate that is saleable to a smelter and cyanide leaching is not required. Some low grade, high tonnage gold ores are processed using a simplified cyanide leach process with direct cyanide leaching of the crushed ore in heaps or vats. In order to avoid the normal grinding step, these ores need to have high natural permeability to allow direct cyanide leaching of the crushed ore. Leaching is slow and recoveries are typically lower compared to agitated tank leaching. Gold is recovered from solution in the conventional manner using carbon adsorption in columns followed by electrowinning of gold from the concentrated, purified solution. Treatment of Refractory Gold Ores Figure 5: Commercial Flotation Circuit Designed by for Pre Concentration Activated carbon is used to recover dissolved gold from the leach solution using either Carbon In Leach (CIL simultaneous leaching / adsorption) or Carbon In Pulp (CIP leaching followed by adsorption as two separate unit operations). The Merrill Crowe zinc cementation process is now only recommended over CIP/CIL in limited applications such as for ores having high silver content. The choice of CIP or CIL depends on the ore characteristics, the relative gold leaching kinetics and the possible presence of carbonaceous preg robbing matter in the ore. After being adsorbed onto the activated carbon, the gold is stripped into a high concentration, high purity gold solution that is fed to an electrowinning cell for recovery of metallic gold. The plated gold is further refined using selective fluxing reagents to separate impurities and cast into gold dore bars using a gold furnace (see Figure 6). Refractory gold ores are processed in the same manner as the free milling ores with the addition of pre treatment steps designed to break down the host minerals and allow cyanide leaching of the gold. Pre concentration of the run of mine ore may or may not be required, depending on the ore grade and mineralogy. Some refractory gold ores contain visible gold particles that are still encapsulated in other minerals at a conventional grind size (e.g. 80% passing 75 microns). Sometimes, this type of refractory ore (or pre concentrate) can be treated by ultrafine grinding (e.g. 80% passing 10 microns) to sufficiently liberate the gold for cyanide leaching. Other refractory gold ores contain encapsulated visible gold particles or sub microscopic gold that cannot be liberated by ultrafine grinding. This physically locked gold is often associated with sulphide mineralization. In addition, gold may be chemically locked within gold minerals that are not readily soluble in cyanide (e.g. tellurides, Au 2 Bi, AuSb 2, etc.). The normal processing route for physically and chemically locked gold is to use a pre oxidation process to destroy the locking minerals, leaving the gold behind so it is accessible for cyanide leaching. Pre oxidation of host mineralization can be performed using various technologies, such as oxidative leaching, roasting, or bioleaching. Treatment of Complex Gold Ores Figure 6: Gold Furnace/Gold Dore Pour Like refractory gold ores, complex gold ores are treated in a similar manner to free milling gold ores with some modifications to the process. If a complex gold ore contains a reactive sulphide mineral like pyrrhotite that consumes oxygen, a pre aeration step prior to the cyanide leach may be

6 Gold Processing Practice Brief October 2015 Page 6 of 9 used. Some cyanide consuming metal sulphides can be selectively removed prior to the cyanide leach using preconcentration technologies such as flotation. Since gold is readily adsorbed onto carbon, gold ores that naturally contain carbonaceous minerals may demonstrate lower recoveries resulting from re adsorption of dissolved gold onto the residue during leaching. These ores are known as preg robbing and their treatment depends on the level of severity of the preg robbing. For minor preg robbing, using CIL instead of CIP can assure gold is adsorbed onto activated carbon instead of the carbonaceous minerals in the ore. Additives can also be used in the leach to de activate the carbonaceous components in the ore. In more severe cases of preg robbing, pre oxidation treatment similar to that used for refractory gold ores can be employed as a pre treatment step. Effluent Cyanide Destruction/Heavy Metal Removal Using cyanide solutions for extracting gold requires special attention to the treatment of waste streams. Depending on the cyanide addition rates, ore characteristics, and cyanide complexes formed by side reactions, a dedicated treatment system for cyanide destruction and heavy metals removal is required to ensure environmental compliance. Process Flowsheet Development Test Programs New Operations Each gold deposit has a distinct set of chemical and physical characteristics that influence the selection of the optimum unit operations to maximize gold recovery. Our firm is dedicated to developing a unique and commercially viable flowsheet tailored to each specific deposit to generate high quality products in the most cost effective and efficient manner. In developing a test program for a new gold ore, our staged approach includes: initial characterization of the ore (mineralogy, free milling vs. refractory, potential for pre concentration or gravity gold recovery, need for pre oxidation); identify processing alternatives and complete preliminary bench scale testing; assess technical and economic viability of alternative process routes, and; phased development of the process flowsheet based on the most viable process options. All test programs are tailored to the client s needs and the status of the project development. As the project advances, more comprehensive test programs are required at each stage to define the flowsheet and design parameters in more detail. In essence, the test program supports the economic assessment and process design, initially providing a preliminary flowsheet and grade/recovery data for preliminary economic assessment, with more advanced testing ultimately leading to definitive plant design and process equipment performance specifications. provides test programs to assess all aspects of gold processing, including: liberation characteristics; crushing and grinding assessment; gravity gold recovery (free gold); pre concentration by gravity or flotation; pre aeration and pre oxidation leaching; cyanide leaching; carbon in pulp and carbon in leach gold adsorption and stripping; solid/liquid separation data, and; cyanide destruction and heavy metal fixation. All key variables are assessed such as optimum grind size, reagent schemes and dosages, grades, recoveries, reaction kinetics, and operating parameters (temperatures, ph, etc.). We perform initial tests using batch bench scale equipment followed by locked cycle testing or mini pilot continuous testing where required for scale up and design of commercial facilities. is proficient in assessing, designing and implementing a variety of pre oxidation technologies for freeing refractory gold from microscopic and submicroscopic inclusion in sulphide matrices and has successfully designed commercially viable flowsheets for many of our clients. Existing Operations or Toll Processing Facilities also conducts in plant test programs (field trials) to assist with operational issues (e.g. to improve on recovery, debottleneck unit operations, or assist with environmental compliance). As well, we regularly evaluate different ores for toll processing opportunities for existing operators. In the case of toll processing, the suitability of a candidate ore is assessed in the lab relative to the existing plant design operating parameters to determine recoveries and operating costs before the client commits to processing the ore. Minor plant equipment or operating practice modifications can be recommended to best suit the ore to be toll processed.

7 Gold Processing Practice Brief October 2015 Page 7 of 9 Typical Test Program Results The most common types of scoping tests or initial ore characterization tests completed at Thibault & Associates Inc. s in house laboratory includes bench scale flotation testing, Mozley mineral separator gravity release testing, and batch cyanide and pre oxidation leaching. The objective of this initial series of bench scale tests, typically completed during the discovery phase of the project development, is to assess the relative response of the ore to various methods of upgrading and select the process unit operations as defined by a block diagram. Some typical results of flowsheet development test programs completed by are shown in Figures 7 through 9. Figure 7 depicts a kinetic profile for flotation of gold to a bulk sulphide concentrate. From the graph, the impact of residence time in the flotation cell on recovery of gold is clearly demonstrated and is used for scale up plant design (technology transfer); which is essential to assure commercial viability of the full scale operation. Cumulative Metal Recovery (wt%) Time (min) Au (%) As (%) Fe (%) Figure 7: Flotation Pre concentration Kinetic Test for a Sulphide Gold Ore In Figure 8, a Mozley Laboratory Mineral Separator was used to assess pre concentration of a sulphide ore by gravity. This release analysis is used to define the optimum liberation size of the particles for gravity concentration along with producing grade recovery curves for evaluation of commercial viability. The results of the release characteristics of the ore are used to model a pre concentration flowsheet using gravity separation unit operations. Cumulative Gold Recovery (%) Figure 8: Gravity Pre concentration Release Analysis Test for a Sulphide Gold Ore Figure 9 shows the results of a typical batch cyanide leach completed in an agitated vessel with continuous air injection and varying initial cyanide concentrations. This type of preliminary bench scale testing gives an indication of leach kinetics, reagent consumption and susceptibility of the ore or concentrate to cyanide leaching within the confines of a relatively simple and cost effective test program. For projects in the more advanced stages of development, also offers more detailed bench scale test programs, as well as continuous pilot programs, to define both operating and process equipment design/scale up parameters to support detailed construction cost assessments and plant design. Gold Extraction (wt%) 100% 80% 60% 40% 20% 0% 100% 80% 60% 40% 20% 0% 500 to 1000 μm 250 to 500 μm 150 to 250 μm 106 to 150 μm 53 to 106 μm 20 to 53 μm 20 to 1000μm Cumulative Gold Grade (g/tonne) 0.5 g/l NaCN 1.0 g/l NaCN 1.5 g/l NaCN 2.0 g/l NaCN Leach Time (h) Figure 9: Gold Cyanide Leach Kinetics in Mechanically Agitated Batch Leach Tests

8 Gold Processing Practice Brief October 2015 Page 8 of 9 Gold Processing Plant Design offers a full range of engineering design services from conceptual studies to final detailed design, construction technical assistance and plant commissioning/start up. We also provide services for plant retrofits and process intensification studies that may be triggered by requirements for plant expansion or a change in ore type. The typical stages of design include: discovery; scoping / preliminary economic assessment; pre feasibility; feasibility, and; definitive design and budget control estimates. In addition to our expertise in metallurgical and hydrometallurgical process development and design, Thibault & Associates Inc. maintains in house capabilities for project management/coordination of multi discipline engineering design of processing facilities including civil/structural, electrical, process control, mechanical, and environmental permitting. Every project is based on a fully integrated approach to execution and delivery. Our firm has developed in house flowsheet design and scaleup models to relate the bench scale data to full scale commercial equipment. For example, cyanide leach circuit scale up is based on both gold leaching kinetic data and use of retention time distribution models to size leach tanks in series under continuous operation. Carbon in pulp circuits are designed based on reaction kinetics and equilibrium models developed from the lab data that defines the required number of stages and tank sizing to extract gold to its full potential. Figure 10 shows a typical CIP circuit equilibrium curve used to predict the number of stages required for a particular gold ore. Prior to definitive plant design, project execution is based on an assessment of the ore metallurgical characteristics and variability of the ore within different zones of the mine plan. Our approach is to assess the complexity of process chemistry and identify measures to overcome technical risks. Selection of the process chemistry for plant design is also based on the project earning power as defined by our economic modelling of the production strategy. Benchmarking and characterization of the gold ore for either greenfield plant design, modernization of a brownfield production facility or process intensification of an existing processing plant, is based on a fully integrated assessment of the technical and economic viability of the processing strategy and includes but is not limited to: identification of refractory ore degree of oxidation required/grind size specifications; quantification of heavy metals in pregnant leach solution (PLS) fixation of metals to comply with environmental guidelines; characterization of preg robbing ores high affinity of carbonaceous ore for adsorption of auro cyanide complex; analysis of silver in PLS and the relative impact on CIP/CIL loading based on gold displacement and selectivity; analysis of poisons in PLS and the relative impact on loading and regeneration of carbon or resin; characterization of resistance of carbon or resin to i) scale from water hardness or calcium and magnesium in the ore, ii) fouling, iii) clays, and iv) harsh environments, and; selection of a flux for selective upgrading of gold dore. also has experience and is qualified in the preparation of Canadian (NI ) and international securities commission compliant economic assessment studies including preliminary economic assessment (scoping study), pre feasibility and feasibility studies for advancement of capital projects throughout all stages of development. Gold Loading on Carbon (g/t) Stage 3 Stage 2 Stage 1 Gold Processing Economics To ensure process development remains focussed on the most economically viable options, we provide dynamic economic modelling of the process technology throughout every stage of project development. The use of economic modelling allows for early rejection of non viable process options and facilitates the execution of trade off studies to assess alternative processes and added value production opportunities. CIP Tails CIP Feed Gold in Solution (mg/l) Figure 10: Equilibrium Curve for Staged Operation of a CIP Circuit has performed numerous studies comparing and assessing the relative capital and operating costs of alternative processing routes for various types of gold

9 Gold Processing Practice Brief October 2015 Page 9 of 9 ores, and the economic models generated for these clients have been proven to be invaluable in assuring that the project advances down a technically and economically viable path from concept to commercial reality. The inherent flexibility of the economic models developed by provides a means for straightforward assessment of economy of scale for new projects or to determine the optimum production rate based on the project net present value (see Figure 11) and cost of production (see Figure 12) for different plant sizes. Pre-Tax Net Present Value (Millions of $CDN) Gold Production Cost (CDN$/oz) Optimum Range of Plant Capacity Plant Capacity (tpd ROM Ore) Pre-Tax NPV Pre-Tax NPV at x2 2x CAPEX Figure 11: Example of Procedure for Assessment of Economy of Scale Relative to Net Present Value Optimum Range of Plant Capacity Plant Capacity (tpd ROM Ore) Figure 12: Example of Procedure for Optimization of Plant Capacity Relative to Production Costs range of 2,000 to 8,000 tonnes per day) impact on the production costs per troy ounce of gold produced as illustrated by Figure 13. Both our flowsheet development programs and plant design standards focus on measures to minimize production cost. Annual Gold Production (Troy oz/yr) 800, , , , , , , ,000 Figure 13: Example of Typical Economic Assessment Procedure for Gold Projects From initial development of production concepts to bankable feasibility, detailed design and after care, Thibault & Associates Inc. fully defines the technical aspects and economic constraints involved in the development of gold projects. Our team approach to achieving your vision with our innovation assures optimum utilization of capital project development budgets, providing both exceptional technical service and value to our clients. Our Firm Total Gold Production OPEX Legend (US$/oz ±20%) > $500 $400 to $ g/t Au 2.5 g/t Au 5.0 g/t Au 10.0 g/t Au 0 0 2,000 4,000 6,000 8,000 10,000 Plant Capacity (tpd ROM ore) < $ provides process engineering services, specializing in tailored design of metallurgical and hydrometallurgical flowsheets for the production of concentrates and refined metals, with an experience base that includes recovery of base, transition, rare and precious metals since Our firm assures a fully integrated approach to project execution by offering in house bench and pilot scale test facilities, process and economic modelling capabilities, process intensification studies, coordination of multi discipline engineering design and after care services. In addition to trade off and risk assessment studies for alternative production strategies, our dynamic economic modelling capabilities provide an effective tool to assess runof mine cut off grades and target reserves relative to the earning power of the project. Both run of mine grade of gold (in the range of 1.0 to 10.0 g/t) and production rates (in the Contact: Stephanie M. Goodine, P. Eng. Lead Plant Design / Process Chemical Engineer s.goodine@thibault process engineering.ca 330 Alison Blvd. Fredericton, NB E3C 0A9 Telephone: (506) process engineering.ca