Refining of Gold and Silver-Bearing Doré PDF

Chapter 34 Refining of Gold- and Silver-Bearing Doré M.B. Mooiman1 and L. Simpson2 1 Franklin Pierce University, Manchester, NH, USA; 2Elemetal Refining, Jackson, OH, USA 1. INTRODUCTION This chapter describes the treatment of gold- and silver-bearing doré materials to produce refined bullion-grad...

Chapter 34 Refining of Gold- and Silver-Bearing Doré M.B. Mooiman1 and L. Simpson2 1 Franklin Pierce University, Manchester, NH, USA; 2Elemetal Refining, Jackson, OH, USA 1. INTRODUCTION This chapter describes the treatment of gold- and silver-bearing doré materials to produce refined bullion-grade gold and silver. The business of gold and silver refining, as it relates to doré treatment, is described and the processing routes for high- and low-gold doré are provided. The processing methods for gold and silver refining comprise two main flowsheet configurations: prerefining followed by electrorefining and dissolution followed by precipitation. The prerefining processes can be pyrometallurgical or hydrometallurgical in nature. In contrast to the refining of industrial precious metals scrap and old jewelry, the refining of doré has unique challenges associated with feedstock variability and the presence of deleterious elements. Although the basic refining flowsheets have not changed much in the past 100 years, process development and incremental improvements are still taking place and some recent developments are described. 2. INDUSTRY STRUCTURE AND THE GOLD AND SILVER REFINING BUSINESS 2.1 Comparison of Gold and Silver Refining with Platinum-Group Metals Refining As with many other metals-related industries, the key aspects of the precious metals business are mining, extraction, refining, product fabrication, and recycling (Figure 34.1). The business structure of the gold and silver industry is, however, different than that of the platinum-group metals (PGMs, comprising Pt, Pd, Rh, Ir, Ru, and Os). PGM refineries are more complex and expensive and often use proprietary technology; as a consequence, they are usually intimately associated with mining companies and extractive operations (Mooiman et al., 2016). The few stand alone PGM refineries are usually associated with integrated smelting operations, such as Umicore or large, integrated PGM companies, such as those of BASF, Heraeus, or Johnson Matthey. These companies are prominent on the product side of the business, focusing on the fabrication of PGM chemicals, catalysts, alloys, and other products. Although their refineries exist mainly to treat their own scrap generated during their product-fabrication operations, some do undertake refining for other customers. In contrast, gold- and silver-refining operations are seldom associated with mining and extractive activities: very few miners operate their own refineries. The main reason is that there are readily available, large-volume sources of old jewelry and industrial scrap material available. Gold and silver refining is also technically much simpler than PGM refining. These operations are consequently easier to establish, and there are many independent gold and silver refiners that bid aggres- sively for the refining of mining doré. Competition is high, refining fees are low, and advance payments are a routine component of the competitive nature of this business (clients typically receive 90% of their value within 1 or 2 days of receipt by the refiner and the difference within about 1 week). Compared with the ease and low cost of sending doré to an independent refinery, it seldom makes sense for a mining company to operate its own refinery, with the attendant capital and operating costs and the cash-flow implications inherent in inventory holdup. A few gold operations have, at times, operated their own refineries, but these inevitably close down when the operating challenges, better pricing from refiners, Gold Ore Processing. Mike D. Adams (Editor), http://dx.doi.org/10.1016/B978-0-444-63658-4.00034-7 595 Copyright © 2016 Elsevier B.V. All rights reserved. 596 PART | II Unit Operations FIGURE 34.1 Structure of the precious metals industry. (a) Gold and silver refining, fabrication, and recycling occur largely separately from their mining and extraction activities, while (b) PGM refineries are typically more integrated with the upstream operations and can include recycling operations. After Mooiman et al. (2015a). and inventory holdup issues become apparent. Harmony Gold Mine, a South African primary producer, is a case in point. Harmony set up and operated an on-site gold-refining operation capable of producing 700,000 oz/year of >99.99% purity as 400 oz good delivery bars (Sole et al., 1998). However, security and logistical difficulties, compounded by the large in- process gold inventory and dearth of technical expertise, saw the mine return to sending doré to the Rand Refinery just a few years later (Mooiman et al., 2016). Silver-refining operations, usually associated with gold-refining businesses, operate on a similar principle, but some larger silver operations are associated with base-metals extraction companies and operate their own refineries. Examples include the lead and zinc operations of Teck (Canada), Kazzinc (Kazakhstan), and Met-Mex Peñoles (Mexico). 2.2 Industry Components and Participants Gold and silver refiners receive impure forms of precious metals from mining companies as doré and turnings, filings, or blanks from fabricators, such as jewelry manufacturers. Another source of precious metals comes from collectors. These are businesses that collect precious metals from various spent resources, such as electronic scrap, industrial waste, abandoned jewelry from pawnshop operations, and the “cash for gold” operations that are now found in many communities. Because refiners process materials from many different sources, they play a central role in the precious metals industry, as shown in Figure 34.2. Refiners are the conduit through which impure precious metals flow and are converted into readily tradable and useable forms. Refiners purify the metals so that they can be sold to banks, traders, and fabricators. Fabricators of precious metals products, such as jewelry companies, can purchase metals directly from refiners, but precious metals are more typically purchased from banking and trading partners. When a precious metals product, such as a jewelry item, reaches the end of its useful life, it can be purchased, as noted earlier, by precious metals collectors. Collectors have the ability to pay for very small lots, which they consolidate and then ship to larger refining operations that separate and refine the precious metals.  Refining of Gold- and Silver-Bearing Doré Chapter | 34 597 FIGURE 34.2 Participants in the precious metalserefining industry. 2.3 Operations of a Precious Metals Refiner There are three key undertakings in any precious metals refining operation, viz., evaluation, separation, and conversion (see Figure 34.3). An integrated precious metals refinery carries out all three tasks. The first task, and that of most importance to both the refiner and their customer, is evaluation, which is the accurate determination of the precious metals content in the customer’s material. Evaluation consists of two separate operations: sampling and assaying. Sampling involves the homogenization of the precious metals containing feedstock, normally by melting of high-grade material, then taking a sample of material that is representative of the entire melt. This sample is analyzed for precious metals content using a variety of instrumental and fire assay techniques. Although the industry places much emphasis on accurate analytical techniques, it must be appreciated that effective sampling of the feedstock is just as important: if poor sampling techniques are used, even the most sophisticated analytical techniques cannot compensate for a material that is not representative of the received material. Poor sampling can lead to low returns and unhappy customers or excessive returns and disgruntled refiners. Both circumstances can be avoided by the application of correct sampling practice and analytical procedures. To ensure accurate and transparent practices, customers will often visit the refinery to FIGURE 34.3 The three main activities of a precious metals refinery. 598 PART | II Unit Operations witness the weighing and sampling of their material and will leave with a sample of their material that they can analyze in- house and compare with the assay reported by the refiner. The comparison of analytical results takes place through a formalized process called assay exchange; the resolution of any discrepancies, within agreed-on limits, between the customer and the refinery’s results are resolved by the established splitting of differences. If the discrepancies exceed the agreed variation, the sample is sent to a mutually agreed assay laboratory for independent determination of the precious metals content. After evaluation, the customer is paid and the refinery is now responsible for the chemical separation of the gold, silver, and PGMs present in the feedstock. These separations are complex and involve an in-depth knowledge of chemistry, hydrometallurgy, pyrometallurgy, and physical metallurgy, as well as the safe use of hazardous chemicals. Many smaller refineries, sometimes called secondary refiners or “melt-and-sample houses,” elect to avoid this step by sending evaluated material to another refinerda so-called integrated or primary refinerydthat will reevaluate the material, carry out the chemical separations, and convert the precious metals to a saleable form. Primary gold and silver refiners include the larger precious metals companies, such as Asahi Refining, Elemetal Refining, Metalor Technologies, Rand Refinery, and The Royal Canadian Mint. Some smaller refineries also carry out their own chemical refining operations, albeit on a more limited scale, but these companies tend not to be active in the doré business. The final step in an integrated precious metals refining operationdand the main value-adding stepdis to convert the refined precious metals into a form that can be sold. Products usually take the form of precious metals bars, weighing 1 kg, 100 troy oz, or 400 troy oz for gold and 100 troy oz or 1000 troy oz for silver. Some refiners convert refined precious metals into smaller-denomination investment bars and coins (typically 1 troy oz), precious metals chemicals, alloys, grain, or other fabricated parts. Much of the refiners’ profits are earned from these added-value products. Precious metals refining is often a low-margin business: the key to a successful refining operation is the ability to move metal quickly through the refining process and to minimize in-process inventories. These two parameters frequently determine the choices that refineries make regarding their refining process routes. 2.4 Business Transactions of a Precious Metals Refiner At first glance, a precious metals refining enterprise appears to be a straightforward business but it is, in fact, very complicated. It is, in essence, a combination of an analytical laboratory, a chemical manufacturing operation, a bank, and a hedging and trading operation all wrapped up in a highly secure environment. Refiners participate in two main types of business transactions. In the first type of transaction, a refinery charges the owner of a precious metals feedstock a refining fee (a toll) to refine the precious metals and to convert the precious metals to a form, such 400 oz bullion bars, that can be delivered to a bank or trading partner. Under these circumstances, the refining company does not take ownership of the material: it remains the property of the customer (such as a mining company) and it is simply processed, purified, converted to a saleable form, and sent to the customer’s bank or trading partner. This is known as toll refining. In the second type of transaction, the owner of the precious metals sells the metals directly to the refinery. The refiner pays the customer for the contained precious metals less the refining fee. Under these circumstances, the refinery then has the choice to convert the precious metals to a tradable ingot, which can be sold to a bank, or it can be converted into other products, such as coins, powders, or chemicals, that can earn some added value above the metal price. Fees for both toll refining and outright purchases typically comprise two components that are explicitly stated in the refining contract. The first is a refining fee, which is based on the weight and grade of the received material; the second is a small prenegotiated precious metals retention that is assessed by returning slightly less than 100% of the precious metals in a lot. Retentions are highly variable and can range from 0.05% to 5%, depending on volumes, grades, precious metals prices, and negotiated agreements between the refinery and its customer. Not all refined precious metals are considered equal. Accredited refiners are those whose refined and branded material has received approval and certification from key precious metals exchanges, the most important of which are the London Bullion Market Association (LBMA) and the New York based Commodity Exchange (also known as COMEX). Metals from accredited refiners can be freely traded on these markets. Refiners certified by the LBMA are accorded good delivery status, and their precious metals ingots are known as good delivery bars (LBMA, 2014). Such certified bars are usually traded with only limited restrictions on the world precious metals markets. Smaller refining operations often do not have good delivery status and can experience difficulties in selling their refined material. Their products are usually sold to a party not concerned about buying and using gold from nonaccredited sellers or to buyers who will purchase the uncertified precious metals at a discount to the spot price.  Refining of Gold- and Silver-Bearing Doré Chapter | 34 599 3. GOLD AND SILVER DORÉ Refiners typically receive three types of material: jewelry-based materials from end-of-life items, scrap from precious metals fabrication operations, or doré from mining companies. Jewelry and fabrication scrap tend to be straightforward materials, with grades ranging between 40% and 75% gold and copper, silver, zinc, and nickel as common contaminants. In contrast, mining doré is a far more complex material: it is highly variable in gold concentration and can contain large amounts of silver and base metals, such as zinc, iron, copper, as well as a host of deleterious minor elements (such as mercury, lead, cadmium, selenium, tellurium, and arsenic), that can significantly complicate evaluation and processing. Doré materials are either high-gold or high-silver, referred to as gold doré and silver doré, respectively. High-gold materials contain from 30% to 98% gold, with the difference made up by silver and varying quantities and types of base metals. High-silver doré typically contains >50% silver, with smaller quantities of gold (1e30%) and various base metals, most commonly copper and zinc. However, some silver doré material can contain large quantities of deleterious elements, such as lead, selenium, and cadmium. Doré materials are highly variable in composition and are very dependent on the location and facilities of the origi- nating mining company, the characteristics of the ore body currently being processed, the nature of the extraction oper- ations, and the level of processing in the smelting operations on the mine site. Doré bars delivered to a refinery vary significantly in weight, size, and appearance. 4. REFINING OF HIGH-GOLD DORÉ MATERIALS After evaluation and settlement with the customer (Step 1 in Figure 34.3), the doré is ready for refining. The two basic gold refining schemes are shown in Figure 34.4: the prerefining/electrorefining flowsheet is shown schematically on the left, and the dissolution/precipitation flowsheet is shown on the right. The most commonly used prerefining process involves melting the impure material and injecting chlorine gas into the melt to convert the base metal contaminants and silver to their respective chloride salts. This so-called Miller process upgrades the gold to approximately 95% purity. The partially purified gold is then passed to an electrorefining operation, known as the Wohlwill process, in which the impure gold is dissolved into concentrated hydrochloric acid at the anode and is plated out as high-purity gold (usually 99.9% purity or higher) at the cathode. The byproduct base metals and silver chloride salts, produced in the prerefining step, are treated to recover and recycle the entrained precious metals. Byproduct- recovery processes are themselves complicated and extensive operations. In the dissolution/precipitation refining route, the impure material is completely dissolved in concentrated hydrochloric acid with the aid of an oxidizing agent, often nitric acid. The 3:1 (vol:vol) mixture of concentrated hydrochloric and nitric acids used in the dissolution process is known as aqua regia; hence, the term aqua regia refining is often used for this refining approach. Once all the gold is dissolved and the insoluble residues are removed via filtration, the gold is selectively precipitated from the solution, using a reducing agent such as sulfur dioxide gas, to produce a high-purity gold sponge or powder. This process also produces precious metals containing byproducts that need to be treated to recover and recycle FIGURE 34.4 The two basic refining schemes for gold and silver. 600 PART | II Unit Operations the entrained precious metals. Generally speaking, however, the quantity of byproducts arising from the dissolution/ precipitation refining scheme is far lower than that of the MillereWohlwill approach. The main disadvantage of the dissolution/precipitation scheme is that the process is not as robust as the Millere Wohlwill scheme and is limited in the range of feed material compositions that it can process. The difficulty is that gold dissolution is compromised by high silver concentrations. During leaching, silver forms a passivating layer of silver chloride on the surface of the material, which prevents complete dissolution of the underlying material. General industry experience is that the upper limit for silver in material that can be treated in the dissolution process is about 15%. Many doré materials have higher silver concentrations, which limits the applicability of the dissolution/precipitation process. Refiners have wrestled with the pros and cons of these two distinct refining approaches for many decades. The MillereWohlwill approach has the advantage of robustness, and it can be used to refine almost any grade of material, including doré containing high levels of silver. However, it also produces substantial volumes of gold-bearing byproducts, including chloride salts, slags, refractories, and scrubber solutions, that need to be treated to recover and recycle the gold and silver. The electrorefining operation, in particular, requires high levels of in-process gold inventories. On a comparative basis, inventory levels in the MillereWohlwill process are significant: residence times in these circuits range from 3 to 7 days, depending on equipment design and operating conditions. On the other hand, the dissolution/precipitation process, although limited to treating low-silver materials, has relatively low gold inventory holdups. Some operations are designed to complete an entire dissolution and precipitation cycle within a 24-hour period, and residence times in these circuits are typically of the order of 1 to 3 days. For refiners dealing mainly with high-carat gold alloys (
75% Au), the aqua regia process is probably the simplest and most robust of the chemical refining methods available. 4.1 Prerefining of Gold Prerefining of gold is carried out either pyrometallurgically or hydrometallurgically. Both routes focus on reducing the concentrations of base metals and deleterious elements in the doré to enable the effective and efficient electrorefining of the gold. Hydrometallurgical routes are seldom used today. 4.1.1 Hydrometallurgical Prerefining of Gold e The Parting Process Parting refers to the separation of silver from gold via nitric acid dissolution of the silver component. In the analytical fire-assay procedure, the removal of silver by nitric acid parting is normally the final chemical separation step that allows the gravimetric determination of gold in a sample. A successful parting process requires the presence of sufficient silver in the sampledat least 66%dso that passivation of the unleached silver by a high concentration of gold does not occur. Fire- assay sample preparation therefore involves the addition of supplementary silver at the start of the process to elevate the silver content to the correct level. This same approach can be adopted on a large scale: silver can be added to a difficult-to-treat doré to increase the silver content, followed by nitric acid leaching to dissolve the base metals and silver, leaving behind the insoluble high-grade gold product ready for the next stage of refining. Silver is also highly soluble in sulfuric aciddsolubilities in excess of 100 g/L at elevated temperatures have been noted (Lietzke and Stoughton, 1956)dso boiling sulfuric acid can be used in place of nitric acid. In fact, prior to the development of the chlorination and electrorefining processes in the 19th century, sulfuric acid parting was the process used to refine gold alloys (Marsden and House, 2006). 4.1.2 High-Temperature Chlorination e The Miller Process The separation of silver and other metals from gold by treatment with chlorine gas was known in the early 1800s. It was discovered in 1838 by Louis Thompson (Rose, 1905) but was first patented and put into practice in 1865 by Francis Bowyer Miller at the Sydney Mint in Australia (Miller, 1868) and has since been known as the Miller process. Figure 34.6 shows the processing scheme of a modern Miller operation and byproduct treatment. The feedstock is melted in an induction furnace, and chlorine gas is then injected into the melt. The key reactions are the formation of metallic chlorides, which either volatilize into the gas stream (e.g., FeCl3, ZnCl2) and are captured in a scrubber or float on top of the melt as molten chlorides (e.g., CuCl, AgCl) and are periodically skimmed off. Table 34.1 shows some of the main reactions that occur, the melting and boiling points of the metallic chlorides, and their free energies of formation at reaction temperatures. Figure 34.5 presents some experimental data from the Rand Refinery (South Africa), which show how the order of chlorination follows, in general, the free energies of chloride formation. However, in an actual chlori- nation reactor, the concentrations and thermodynamic activities of the various metallic components are constantly changing, which has a significant effect on the order and extent of chlorination (Nadkarni et al., 1991).  Refining of Gold- and Silver-Bearing Doré Chapter | 34 601 TABLE 34.1 Melting Point, Boiling Point, and Gibbs Free Energy of Formation of Selected Metal Chlorides Reaction Chloride Melting Point (8C)a Chloride Boiling Point (8C)a DGrxn (kcal/mol @11508C)b Fe þ 1.5Cl2 / FeCl3 304 332 47.7 Zn þ Cl2 / ZnCl2 318 732 55.4 Pb þ Cl2 / PbCl2 501 953 47.7 Cu þ 0.5Cl2 / CuCl 430 1212 49.2 Ag þ 0.5Cl2 / AgCl 455 1564 33.8 Au þ 1.5Cl2 / AuCl3 180 (subl.) 229 17.7 a Data from Dean (1985). b Data from Nadkarni et al. (1991). FIGURE 34.5 Chlorination of gold-bearing material in the Miller process at the South African Rand Refinery. After Fisher (1987). The Miller process can chlorinate almost all of the base metals and silver in the melt and can directly produce a good delivery gold product of 99.5% purity, as is done at the Rand Refinery. The challenge with chlorinating doré to this extent is that, at low base-metal and silver concentrations, gold itself begins to chlorinate and is driven into the gas phase, leading to a great deal of gold ending up in the ductwork and scrubber. Most refineries monitor the extent of reaction and elect to halt chlorination at between 93% and 98% gold content to avoid volatilizing the gold. The refining process is then completed in the electrorefining step. Owing to the corrosive nature of chlorine gas and metallic chlorides, equipment for the Miller operation requires continual upkeep and maintenance. Furnace hoods and ductwork are often fabricated from inert materials, such as titanium and high-chromium alloys (such as HastelloyÔ ) or, where appropriate, fiberglass or polyvinylchloride (PVC). Conden- sation of Miller fume causes accumulation of considerable amounts of metal chlorides in the ductwork, and regular cleanout is required. Scrubbing of Miller off-gas streams is complicated by their corrosive nature and the requirements to scrub chlorine, volatile metal chlorides, and fine particulates from the gas stream. Combinations of a high-pressure drop venturi, packed-bed scrubber, and/or wet electrostatic precipitator are typically employed. The Miller process produces scrubber sludge and chloride salts containing precious metals that need to be reclaimed. Miller salts, which are recovered from the top of the melt during chlorination, contain predominantly AgCl, CuCl, base- metal chlorides, and entrained gold particles. The gold content of the salts ranges from 0.5% to 3% but depends very much on the process operating conditions. The molten Miller salts are typically granulated into water and then subjected to an oxidative leach to convert the insoluble CuCl to soluble CuCl2 while leaving gold and silver chloride as insolubles. Oxidizing agents used in this operation include hydrogen peroxide (H2O2), chlorine (Cl2), sodium hypochlorite (NaClO), and often sodium chlorate (NaClO3) (see Reaction (34.1)): 6CuClðinsol.Þ þ 6HCl þ NaClO3 /6CuCl2ðsol.Þ þ NaCl þ 3H2 O (34.1) 602 PART | II Unit Operations The key to this oxidative leach is control of the oxidationreduction potential (ORP) to ensure that the oxidizing conditions are sufficient to oxidize Cuþ to Cu2þ but not too oxidizing to dissolve gold. After copper dissolution, the insoluble material consists mainly of silver chloride and gold entrained in the salts. The silver chloride is then hydrometallurgically reduced to metallic silver in a slurry using any one of several reducing agents, including copper, iron, zinc, hydrazine (H4N2), or dextrose in highly alkaline conditions. The reduced silver is dried, melted, and passed through the nitrate-based silver electrorefining process, where the silver dissolves at the anode and the gold reports the insoluble anode sludge, which is then directed back into the Miller process e see Figure 34.6. FIGURE 34.6 Flowsheet for the MillereWohlwill gold refining process.  Refining of Gold- and Silver-Bearing Doré Chapter | 34 603 The challenge associated with this process is that multiple treatment steps are required to recover the gold and so the resultant residence time of the gold in the byproduct streams is lengthy. In an effort to reduce residence times, some refiners, including the Rand Refinery, add soda ash (Na2CO3) directly to the molten chloride salt (Fisher, 1987). Soda ash reduces some of the silver chloride in the salt, and the metallic silver collects the entrained gold particles into a metallic button that separates from the rest of the salts on cooling. This step, referred to as de-golding, has the advantage of collecting the bulk of the gold from the molten salt and quickly returning it to the main refining process. 4.2 Electrorefining of Gold The gold anodes produced by the Miller process are electrorefined in concentrated hydrochloric acid electrolytes. The impure anodes are electrolytically dissolved, and pure gold is electrodeposited at the cathode. The basic electrorefining configuration, shown in Figure 34.7 and known as Wohlwill refining, has changed little since its invention in 1878 by Ernest Wohlwill at the Norddeutsche Affinerie in Hamburg, Germany (Pletcher and Walsh, 1990). The anodic dissolution and cathodic reduction processes are straightforward (Reactions (34.2) and (34.3)), but the process is complicated by the formation of the Auþ species in solution, AuCl2  , which disproportionates into metallic gold and Au3þ (Reaction (34.4)), depositing considerable amounts of gold mud on the bottom of the electrowinning cell. Periodic harvesting of the mud is required to mobilize the stagnant gold inventory at the bottom of the cell. Anodic dissolution Au þ 4Cl /AuCl4  þ 3e (34.2) Cathodic reduction AuCl4  þ 3e/Au0 þ 4Cl (34.3) Disproportionation 3AuCl2  /2Au0 Y þ AuCl4  þ 2Cl (34.4) In most Wohlwill operations, gold is deposited onto titanium cathodes and is mechanically harvested using a variety of approaches, most of which use manual labor. Some refineries avoid manual separation of deposited gold from the cathode blanks by plating directly onto starter sheets of pure gold. Although this does cut down on the manual labor required, the fabrication and inherent inventory of gold in the starter sheets introduces other operating costs. In another variation, some operations confine the anodes in bags to separate the anode mud from the bulk of the electrolyte and to capture the insoluble materials, particularly AgCl, produced at the anode. One of the biggest challenges associated with electrolytic refining is maintenance of the electrolyte. The electrolyte requires a high level of gold, >100 g/L, and the concentrations of base metals and PGMs, which continually accumulate in solution, need to be monitored and reduced, as necessary, to avoid exceeding the control limits. The gold concentration in solution continually depletes during electrolysis due the imbalance between the selective gold deposition at the cathode and the nonselective dissolution of gold, silver, and base metals at the anode. The disproportionation reaction also depletes soluble gold in solution. Electrolyte adjustment is carried out daily by removal of spent electrolyte and the addition of fresh electrolyte containing a higher gold concentration. FIGURE 34.7 Configuration of a Wohlwill gold electrorefining cell. 604 PART | II Unit Operations To limit the continual depletion of gold from the electrolyte and the extent of the disproportionation reaction, some refiners add nitric acid to the electrolyte. The nitric acid has two functions: the first is to oxidize Auþ to Au3þ and thereby reducing the amount of mud produced by gold disproportionation; the second is to dissolve gold from the mud and from the anode via the aqua regia dissolution reaction (Section 4.3.1). Some refineries add substantial quantities of nitric acid to their electrolyte, so much so that they characterize their process as aqua regiaebased electrolysis (Mostert and Radcliffe, 2005), while others continually meter in small quantities throughout the electrolysis cycle. Gold is readily recovered from the spent electrolyte by any of several reducing agents, including sulfur dioxide gas (SO2), sodium sulfite (Na2SO3), sodium hydrogen sulfite (NaHSO3), ferrous sulfate (FeSO4), or Zn power. The precipi- tated gold is normally redissolved and used as make-up solution for the electrolytic cell. PGMs, particularly platinum and palladium, are recovered by zinc cementation or by precipitation as their respective hexachloride salts by the addition of sodium chlorate and NH4Cl or KCl, in the so-called red salt process. The reduction of gold from spent electrolytes and the redissolution of the precipitated gold sponge, along with the redissolution of gold mud from the cell, comprise an inherent circulating loop, which increases process inventories. 4.3 Chemical Dissolution The chemical nobility of gold makes it difficult to dissolve using common acids on their own: a strong oxidizer, together with a strong complexing agent, is required. The most common dissolution systems used today are either aqua regia (a mixture of nitric and hydrochloric acids) or a combination of hydrochloric acid with chlorine gas. In both of these processes, gold is oxidized and then retained in solution as the stable chloroaurate complex, AuCl4  , by the presence of excess chloride provided by the free hydrochloric acid. A generic flowsheet for the chemical dissolution/reduction process, depicting the various stages in gold refining, can be seen in Figure 34.8. 4.3.1 Aqua Regia Dissolution of Gold Dissolution in aqua regia proceeds by either of the following two reactions: Au þ 3HNO3 þ 4HCl/HAuCl4 þ 3NO2 þ 3H2 O (34.5) Au þ HNO3 þ 4HCl/HAuCl4 þ NO þ 2H2 O (34.6) The off-gases have a distinct brown hue and contain a mixture of NO and NO2, often referred to as NOx. The relative proportions of NO and NO2 are highly variable and depend on the acid concentrations, temperatures of dissolution, air flow rates, and a host of other variables. Gold and most other metals contained in the feed dissolve in aqua regia; however, silver forms an insoluble silver chloride precipitate under typical conditions used. It is therefore necessary to restrict the amount of silver in the feed to a maximum of 15%; a greater quantity than this will create the risk of incomplete dissolution of the gold by forming a passivating layer of silver chloride on the surface of the metal. Refiners overcome this limitation by blending doré materials with other feedstocks containing low silver concentrations or even base metals, such as copper, to reduce the silver concentration below 15%. Another approach is to pretreat the metal with nitric acid to remove silver as silver nitrate before dissolution in aqua regia (see Section 4.1.1). A more-tedious approach is to carry out the dissolution until the point at which passivation initiates. The undissolved material is then filtered off and contacted with zinc powder to convert the passivating silver chloride to metallic silver. The material is then treated with nitric acid to dissolve the available metallic silver, and the now unpassivated material is returned to the aqua regia dissolution. This approach, which is sometimes repeated two or three times, can be effective on some intractable materials but is a laborious and lengthy process. To aid dissolution in aqua regia, it is usual to first melt and granulate the material (essential when lots are blended) to provide a large-enough surface area to ensure acceptable reaction kinetics. The solution is also usually heated during dissolution to ensure rapid reaction. Once the dissolution is complete, the excess nitric acid must be removed because the oxidizing power of nitrates can significantly inhibit the subsequent reduction of gold from solution. Failure to remove all the nitric acid can also affect the downstream recovery of PGMs. Nitrate removal is typically done by boiling down the solution and making up the volume with concentrated hydrochloric acid. This boil-down process is repeated until all the nitric acid is destroyed. The reaction for the destruction of the nitric acid is as follows: HNO3 þ 3HCl/NOCl þ 2H2 O þ Cl2 (34.7)  Refining of Gold- and Silver-Bearing Doré Chapter | 34 605 FIGURE 34.8 Flowsheet for gold refining by the dissolution/precipitation route. Once the nitric acid is destroyed, the leach slurry is cooled to below 35  C to minimize silver solubility and prevent contamination of the gold by silver chloride in the gold reduction step. Silver chloride, AgCl, is typically insoluble in chloride solutions under typical conditions used. At high chloride concentrations, however, such as those found in aqua regia leach solutions, complex species, such as AgCl2  and AgCl3 2 , form, which increase silver solubility in these solutions: soluble silver concentrations of 1e2 g/L are typical. The solubility of silver is highly dependent on temperature 606 PART | II Unit Operations and chloride concentration: any lowering of the temperature by cooling or the chloride concentration by dilution will cause some silver to precipitate from solution as AgCl. Silver is the most common source of contamination in refined gold, so dilution and cooling are employed to minimize AgCl solubility before filtration. Additional care must be taken to maintain filter integrity during filtration of pregnant gold solution to avoid silver chloride contamination. The dissolution of gold-bearing material into aqua regia is simple, fast, and effective, and high gold concentration solutions are produced. However, the process produces large volumes of NOx-bearing off gases that are problematic to scrub. There are several methods for controlling NOx emissions. Gas scrubbing by sodium hydroxide (NaOH) can be used. In this system, NO2 is scrubbed from solution but it does release NO in the process (Reaction (34.8)). NO is a troublesome component to deal with because its absorption into caustic solutions requires its oxidation to NO2: for this oxidation to occur via atmospheric oxygen (Reaction (34.9)), very large scrubbers with long residence times are required. 3NO2 þ H2 O/2HNO3 þ NO (34.8) 2NO þ O2 /2NO2 (34.9) Alternative approaches to NOx control involve the promoted oxidation of NO by the addition of oxidants, such as hydrogen peroxide (H2O2), sodium chlorate (NaClO3), sodium hypochlorite (NaClO), potassium permanganate (KMnO4), and ozone (O3), to the scrubbing solutions (Yang and Chen, 2000). Hydrogen peroxide oxidation systems have been applied in several precious metals operations. The series of oxidation reactions is as follows: 3NO2 þ H2 O/2HNO3 þ NO (34.10) 2NO þ HNO3 þ H2 O/3HNO2 (34.11) HNO2 þ H2 O2 /HNO3 þ H2 O (34.12) Another commonly used method for NOx removal takes the opposite approach and reduces the nitrogen oxides to nitrogen gas via the use of sodium sulfide (Na2S) or sodium hydrosulfide (NaHS). This makes use of the very rapid re- action between NO2 and Na2S and ensures that all of the noxious brown fumes, due to the presence of NO2, are also removed. The main reaction between nitrogen dioxide and sodium sulfide in the scrubber is: 2NO2 þ 2Na2 S þ H2 O/N2 þ Na2 S2 O3 þ 2NaOH (34.13) A major drawback of this scrubbing system is the hydrogen sulfide odor given off by sodium sulfide solutions: Na2 S þ 2H2 O/2NaOH þ H2 S (34.14) To counteract this, the final scrubbing stage in this type of operation is often operated at elevated pH values to minimize the release of H2S. In all NOx control processes, pH control is paramount and instrumentation redundancy is often prudent. 4.3.2 Other Dissolution Systems Other oxidizing agents that have been used to dissolve gold into hydrochloric acid include hydrogen peroxide and sodium chlorate. The most viable alternative to aqua regia, however, is the use of chlorine gas in combination with hydrochloric acid. In this process, chlorine gas is bubbled through a vigorously agitated suspension of atomized gold powder and hydrochloric acid. Gold dissolution occurs by the following reaction: 2Au þ 2HCl þ 3Cl2 /2HAuCl4 (34.15) As in the aqua regia process, silver levels in the feedstock should be keep below 15% to avoid passivation of the solids by silver chloride. The dissolution rate of gold in aqua regia is reported to be up to 10 times faster than that using chlorine and HCl (Geoffroy and Cardarelli, 2005). To achieve comparable dissolution kinetics, the use of chlorine normally requires the gold feed to be atomized into a fine powder prior to dissolution. Aqua regia, by virtue of its reaction mechanism that involves NOx evolution at the dissolution interface, can tolerate far larger particle sizes. Further, since the rate of gold dissolution is dependent on the solubility of chlorine gas in the solution, improved dissolution kinetics are achieved by conducting the dissolution in a pressurized vessel. The partial pressure created by unabsorbed chlorine in the space above the solution in a pressurized vessel is a useful reaction control parameter. As with aqua regia dissolution, excess oxidizing agent must be purged from solution to minimize the detrimental effects of chlorine on the subsequent reduction process. Residual dissolved chlorine is easily removed by displacement  Refining of Gold- and Silver-Bearing Doré Chapter | 34 607 with air while the solution is hot. This operation requires a lot less time and energy than the repeated and prolonged addition of HCl and boiling down of solutions required to remove nitrates from an aqua regiaebased leach solution. Another advantage of the chlorine-based process is that unused chlorine in the gas phase is far easier to scrub than the NOx gases produced by aqua regia dissolution. These advantages should, however, be balanced against the need for atomizing the dissolution feedstocks for the chlorine process and the requirement for more sophisticated pressurized equipment and automated controls. Moreover, the storage, handling and regulatory controls associated with chlorine gas are far more significant than those for nitric acid. 4.4 Reduction of Gold from Solution Once separated from the insoluble silver chloride, the gold-bearing solution is treated with a reducing agent to selectively precipitate pure gold. Of the available reducing agents, sulfur dioxide (SO2), sodium metabisulfite (Na2S2O5), ferrous sulfate (FeSO4), and oxalic acid (HCOOH) are most widely used. The respective reduction reactions are shown in Reactions (34.16) to (34.19): 2HAuCl4 þ 3SO2 þ 6H2 O/2Au þ 8HCl þ 3H2 SO4 (34.16) 4HAuCl4 þ 3Na2 S2 O5 þ 9H2 O/4Au þ 10HCl þ 6NaCl þ 6H2 SO4 (34.17) HAuCl4 þ 3FeSO4 /Au þ HCl þ FeCl3 þ Fe2 ðSO4 Þ3 (34.18) 2HAuCl4 þ 3HCOOH/2Au þ 8HCl þ 3CO2 (34.19) The most common and effective method of recovering dissolved gold is to sparge the pregnant solution with SO2 gas in the presence of approximately 6 g/L sodium ions (Stanley et al., 1987), added to the solution as sodium chloride. Sodium ions promote the formation of an easily filterable and dried gold sponge. Although the reduction can be done in a single step, care must be taken to avoid overdosing with SO2, especially if the concentrations of base metals and PGMs are high. Once all the gold is completely reduced from solution, any subsequent reduction of these metals will contaminate the gold sponge product. A Noranda patent (Stanley et al., 1987) advocates the use of a two-stage approach, in which up to 95% of the gold is precipitated in the first stage by controlling the ORP to about 650 mV (vs. the saturated calomel electrode): once 650 mV is reached, the precipitated gold is filtered from solution and washed with dilute hydrochloric acid and hot water. This produces a >99.99% pure gold sponge, which is then dried and melted in preparation for the manufacture of bullion or added-value products. The depleted solution and wash water are reheated and sparged with SO2 until the ORP drops below 350 mV to precipitate the remaining gold from solution. The gold produced in the second stage is typically more contaminated than the first-stage product. This impure gold is filtered, washed, and recycled to the dissolution step. The resulting gold-free solution is suitable for recovery of platinum, palladium, and other PGMs in a downstream process. The biggest disadvantage of a two-stage reduction is the built-in recirculating gold inventory: depending on the size of the plant, the holding costs can be significant. 4.5 Use of Solvent Extraction Variants of the dissolution/precipitation process use solvent-extraction technology. In these operations, gold-bearing chloride leach solutions are subjected to a solvent-extraction step to selectively remove gold from a highly contami- nated aqueous phase. The Vale Acton refinery in the United Kingdom uses dibutyl carbitol to selectively extract gold from a PGM-rich chloride leach solution, following which the gold is precipitated from the organic phase using oxalic acid (Barnes and Edwards, 1982). Johnson Matthey (United Kingdom) and Anglo American Platinum’s Precious Metals Refinery (South Africa) similarly use methylisobutylketone to extract gold from PGM-bearing liquors, followed by direct reduction of gold from the organic phase with oxalic acid (Crundwell et al., 2011). The MinataurÔ process, developed at Mintek in South Africa and operated by Harmony Gold for several years (Sole et al., 1998; Bower and Bazhko, 2015), was designed to directly leach gold sludge produced in the electrowinning step of the cyanide extraction circuit at the mine site. The sludges were dissolved using HCl and chlorine. The key feature of this process was the introduction of a solvent-extraction step between the dissolution and precipitation steps. The solvent-extraction process selectively extracted gold from a solution that contained silver, aluminum, zinc, copper, magnesium, iron, nickel, PGM, and other minor contaminants. The HCl-rich raffinate from the extraction circuit was recycled to the leach, with a small bleed to control the buildup of impurities. Small quantities of coextracted impurities were scrubbed from the loaded organic before it was stripped to produce a purified, concentrated gold 608 PART | II Unit Operations solution. Gold was recovered as a metal powder by direct reduction from this loaded strip liquor. A further degree of selectivity can be introduced in this step by the choice of the reducing agent: precipitation by oxalic acid produced gold of 99.999% purity, while sulfur dioxide produced gold of 99.99% purity. The MinataurÔ process allowed Harmony Gold Mining Company to circumvent the need for preparing doré bars and sending the material to a refinery (see Section 2). The process is noted to be particularly useful for materials containing significant amounts of base metals (Feather et al., 1997). 5. REFINING OF HIGH-SILVER DORÉ MATERIALS The refining schemes used for high-silver doré materials are similar to those shown for gold in Figure 34.4: the prerefining/ electrorefining and dissolution/precipitation routes. The former process is most commonly used but many refiners, espe- cially those dealing with relatively high-grade silver feedstocks (>80% Ag), manage to electrorefine silver without incorporating a prerefining step. 5.1 Prerefining of Silver Hydrometallurgical and pyrometallurgical processes are used for prerefining of silver. Both target reducing the concen- trations of base metals and deleterious elements to enable the effective and efficient electrorefining of silver. 5.1.1 Hydrometallurgical Prerefining of Silver by Sulfuric Acid Leaching Silver feedstocks containing high amounts of base metals are leached in the presence of an oxidizing agent, typically oxygen or hydrogen peroxide, to selectively dissolve the base metals in sulfuric acid. However, because silver has an appreciable solubility in sulfuric acid (see Section 4.1.1), it is essential to ensure leaching conditions that promote the selective dissolution of base metals over silver followed by treatment of the leach solution to remove any soluble silver. Silver recovery is carried out by cementation with a base metal, such as copper, zinc, or iron, or even the copper-bearing silver feedstock itself. Dissolved silver can also be recovered by precipitation as silver chloride. The success of this prerefining approach depends on the surface area of the feedstock (granulation or even atomization may be required), the leaching conditions (temperature, acid concentration, oxidizing agent, and agitation), and the effective recovery of any solubilized silver. The appeal of this route is that the base metals, and in particular copper, ultimately report to a sulfate solution which can then be directed to a copper electrowinning operation for recovery of copper metal. In contrast, the pyrometallurgical route (Section 5.1.2) requires subsequent treatment of the slags and dross by sulfuric acid dissolution to release the base-metal units. 5.1.2 Pyrometallurgical Prerefining of Silver Pyrometallurgical prerefining is most commonly used, normally conducted via the high-temperature oxidation of molten silver feedstock in a stationary mode, such as a reverberatory furnace, or in a rotating vessel, such as a top-blown rotary converter or Kaldo furnace. These operations involve the diffusion or direct injection of oxygen or air into the molten metal: the base metals are converted to oxides, which then report to the slag phase or form a removable dross that floats on the molten silver. Pyrometallurgical processes can be very quick and effective but, on a small scale, they are very energy and maintenance intensive and produce several difficult-to-handle byproducts, such as slags, refractories, and baghouse dust, that need to be treated for precious and base metals recovery. The pyrometallurgical oxidation of silver can be carried out in several different furnace configurations. 5.1.2.1 Reverberatory Furnace Reverberatory (“reverb”) furnaces have been used by many primary silver producers over the years (Barker, 1967), but their best-known refining application was at the Handy and Harman Refining Group (HHRG) for over 20 years. A typical reverb furnace configuration is shown in Figure 34.9. A direct-fired burner is wall-mounted above the melt. There is no direct contact of the flame with the melterather, the flame and heat strike the roof and opposite walls and are reflected back and around (i.e., they reverberate around the furnace) to heat up the melt. Heat transfer occurs mainly by radiation and some convection. Reverb units are generally known for high operating costs, low capital costs, and moderate oxidation rates. Their footprint requirements tend to be large because the molten silver bath is fairly shallow to aid oxidation rates and to ensure that the entire metal load remains molten.  Refining of Gold- and Silver-Bearing Doré Chapter | 34 609 FIGURE 34.9 Reverberatory furnace configuration used for the pyrometallurgical prerefining of silver. After NPTEL (2006). There is no slag cover in these units: the molten silver feedstock is directly exposed to the atmosphere. The metal contaminants, mainly copper, are oxidized and form a dross on top of the melt that is periodically raked off the surface. The copper-oxide dross, which contains a fair amount of entrained silver, is treated to recover silver by leaching in sulfuric acid. The copper oxide is dissolved and the resulting copper sulfate solution is treated with NaCl to precipitate any dissolved silver and then copper is precipitated as copper hydroxide in the wastewater treatment system. 5.1.2.2 Slagging Furnace This is a gas-fired horizontal rotating furnace to which metal and fluxes are added. Once heated, the furnace is rotated and the metal is exposed to the atmosphere by the rotating action of the furnace. The oxidized base metals report to the slag. These furnaces are useful for the removal of volatile, oxidizable species, such as tin and zinc, but they are not effective for oxidizing large volumes of copper from the melt. 5.1.2.3 Top-Blown Rotary Converter In a top-blown rotary converter (TBRC), the metal is melted by a direct gas-fired flame and flux is added to ensure that the metal is covered by a layer of fluid slag. The furnace is rotated and oxygen is directly injected into the melt via a water- cooled submerged steel lance. The base metals are oxidized and solubilized in the slag by the flux components. The slags are usually borosilicate or sodium silicate based. These units have also been used for base and volatile metal removal from anode slimes produced by copper or nickel electrorefining operations (see Chapter 46). The basic configuration of a TBRC furnace is shown in Figure 34.10. Compared with reverberatory furnaces, TBRCs offer many advantages, including a smaller footprint and rapid oxidation, but they are subject to more operating challenges, caused by greater refractory wear and large amounts of base metals and silver reporting to the off gases. 5.2 Electrorefining of Silver Silver electrorefining is straightforward, involving the anodic dissolution of silver and base metals from the anode and the simultaneous cathodic reduction of silver at the cathode (Reactions (34.20) and (34.21), respectively): Anodic dissolution : Ago /Agþ þ e (34.20) Cathodic reduction: Agþ þ e/Ago (34.21) Silver is produced as fine needle-like crystals at the cathode, which are continually scraped off to avoid short-circuiting between the anode and cathode. The silver crystals fall to the bottom of the electrolytic cell and are harvested, manually or automatically, on a continuous or batch basis. Additives, such as tartaric acid, can be added to control the morphology of 610 PART | II Unit Operations FIGURE 34.10 TBRC furnace configuration used for the pyrometallurgical prerefining of silver. silver crystal product. Under ideal conditions, the cathodic process selectively reduces silver, but in practice the extent of selectivity is dependent on the solution composition, applied voltage, and current density. Anodic dissolution of the impure silver is not selective, and any readily oxidizable metallic impurity in the anode will dissolve. The main contaminant in the anode is copper, with minor amounts of lead, cadmium, and zinc. Metals such as gold and platinum, which are insoluble in nitric acid, do not dissolve and report to the anode mud. The production in the anode compartment of a gold-bearing slime, which can cause passivation of the silver anodes, must be periodically removed, creating materials-handling challenges. The anode compartment must be isolated from the cathode compartment by a membrane or porous bag to avoid contaminating the silver crystals with anode slime. Because the anodes are impure and the cathodic reduction is selective for silver, dissolved copper and other base metals continually build up in solution as the silver content of the electrolyte is depleted. It is necessary to periodically remove some of the impure electrolyte and to top it up with clean electrolyte containing high concentrations of dissolved silver. The silver in the spent electrolyte is recovered and returned to the refining circuit, which, together with incompletely dissolved anodes, leads to a sometimes significant recycling loop within the electrorefining process. When there is an elevated concentration of insoluble elements in the anode material, the inner core of silver anode material becomes encapsulated by these materials and is not available for anodic dissolution via Reaction (34.20). This causes passivation of the anode, leading to the production of free acid and oxygen by the anodic oxidation of water (Reaction (34.22)): Anodic evolution of oxygen 2H2 O/O2 þ 4Hþ þ 4e (34.22) The presence of excess free acid in the electrolyte is problematic because it redissolves the electrorefined silver, thereby reducing overall current efficiency and decreasing the silver yield. This process also releases NOx into the atmosphere, which creates an exposure hazard for the operators. High free acid also promotes the formation of hydrogen at the cathode (Reaction (34.23)), which further reduces the efficiency of silver electrorefining: Cathodic evolution of hydrogen 2Hþ þ 2e/H2ðgÞ (34.23) Some operators allow excess nitric acid to build up (or even add fresh nitric acid) to promote the redissolution of silver crystal in situ. This reduces the amount of silver that has to be dissolved offline to make up the silver concentration in the cells but it does reduce the current efficiency. A process flow diagram for a typical silver electrorefining operation is shown in Figure 34.11. Two important unit operations are the production of freshly made electrolyte and the treatment and recycling of silver in the spent electrolyte. Fresh electrolyte, containing up to 400 g/L silver, is typically produced by the dissolution of refined but off-specification  Refining of Gold- and Silver-Bearing Doré Chapter | 34 611 FIGURE 34.11 Basic silver electrorefining process showing silver dissolution operations, electrorefining, and spent electrolyte treatment. silver crystal into nitric acid. Silver is recovered from spent electrolyte by the precipitation of silver chloride, which is filtered off, reduced to metal, and recast into silver anodes. Two basic cell designs are used in silver electrorefining: the BalbachThum (horizontal) cell and the Moebius (vertical) cell (Figure 34.12). Common features of both cells include the anodes, cathodes, electrolyte, and separation of the anode in a bag to capture any insoluble material (usually gold and PGMs) released at the anode. Both configurations have advantages and disadvantages, but most refiners operate Moebius-type cells due to their small footprint, the large number of anodecathode pairs that can be accommodated in a single cell, and the lower labor requirements for solution adjustment and harvesting of the silver crystal. Compared with gold electrorefining, silver electrorefining operations are larger and more labor intensive and many refineries have introduced varying degrees of automation. Automated operations can include the simple scraping of silver crystal from the anodes, harvesting of silver from the bottom of cells using screw conveyors or bottom-discharge ports, and the use of particulate anodes with the continual removal of anode mud (Mostert and Radcliffe, 2005). 5.3 Dissolution and Precipitation An alternative approach to silver refining involves the complete leaching of the silver-bearing feedstock into nitric acid followed by silver precipitation and reduction. The silver-containing feedstock is chemically dissolved and the solution is 612 PART | II Unit Operations FIGURE 34.12 Configurations of (a) BalbachThum and (b) Moebius silver electrorefining cells. filtered to remove any insoluble material, such as gold or PGMs. An attractive feature of this flowsheet is that it releases the gold content earlier in the process, leading to lower gold inventories than a traditional electrorefining process in which the gold is tied up in anodes and anode slimes. Several methods are employed for silver recovery. In the traditional process, silver is precipitated as silver chloride followed by its reduction to metallic silver. Reducing agents include dextrose in a caustic solution, zinc, or iron powders. Production of bullion-grade silver is, however, seldom achieved. As in the production of silver powder for electronic applications, metallic silver can also be reduced directly from solution using a reducing agent such as hydrazine, but in the presence of large amounts of base metals, the selectivity of this process is poor. A more promising approach is direct electrowinning of silver from the leach solution. Instead of a soluble anode, a dimensionally stable anode, usually made of titanium or niobium, is used. The anode reaction is the evolution of oxygen and generation of acid (Reaction (34.21)). Interest in silver electrowinning has recently experienced a resurgence owing to the development of a cylindrical electrowinning cell by Electrometals Corporation, the so-called EMEW cell. Reports indicate that 70e80% of the silver can be recovered directly as 99.99% pure Ag in a single pass (Treasure, 2001). Attractive features of the EMEW process and cell design include high selectivity, automated harvesting of electrodeposited silver, and a modular design that permits expansion as needed. 6. DELETERIOUS ELEMENTS IN REFINING OF GOLD AND SILVER DORÉ The extraction of precious metals from ores and their recycling from industrial and end-of-life scrap is frequently asso- ciated with the presence of deleterious elements, including mercury, lead, cadmium, selenium, tellurium, bismuth, beryllium, arsenic, and others (Mooiman, 2012). Frequently, insufficient attention is paid to the nature and fate of  Refining of Gold- and Silver-Bearing Doré Chapter | 34 613 deleterious elements during the recovery and refining processes. This can lead to the inadvertent exposure of refinery workers to toxic materials, contamination of operations, and out-of-compliance air and wastewater discharges. Deleterious elements cause three types of problems: they (1) interfere with the sampling and assaying process, making evaluation of the precious metals content difficult; (2) complicate the extraction and subsequent processing of the precious metals by reducing yields, introducing contaminants and the need for complex processing chemistries, and requiring expensive pollution control measures; and (3) create health and safety concerns for workers and the surrounding com- munities and environmental contamination. As an example, mercury is a byproduct of gold mining in Nevada, USA. The sulfide ores contain 0.1e100 ppm of mercury, which can make its way into doré (Miller, 2007). Many artisanal and small-scale mining operations also still use mercury amalgamation technology to recover gold. Mercury is also often present in recycled precious metals, such as dental amalgams and switches and sensors in electronic scrap. Mercury presents a notable processing and occupational health challenge for many precious metals recyclers and refiners, because they are often unaware of its presence in their feedstocks. Its presence only manifests during processing, which can lead to inadvertent releases, exposure of personnel, and high costs to purge process equipment and operations of contamination (Mooiman and Apprahamian, 2012). Selenium is another element often found in doré that is notorious for creating problems in gold and silver refining circuits. It complicates separations and poses challenging wastewater treatment choices (Mooiman and Cettou, 2013). Cadmium is a common contaminant in many doré materials, particularly high-silver doré in which levels as high as 4% have been observed. Cadmium is a listed carcinogen and fumes off during the evaluation and prerefining processes. Any cadmium not removed during these upfront operations concentrates in solutions and electrolytes and can compromise the production of high-purity precious metals (Mooiman et al., 2015). 7. FUTURE DEVELOPMENTS IN DORÉ REFINING The refining processes for doré have, in the case of the MillereWohlwill process, been used for over 100 years; the dissolution/precipitation processes go back even further to the times of the alchemists. Although the basic chemistry and operations have not changed, incremental improvements in equipment design, materials of construction, process controls, chemical analyses, and automation have transformed these operations. Glass reactors are still extensively used but modern refineries use materials such as silicon nitride, titanium, HastelloyÔ , and polyvinylidenedifluoride. Solution and gas flows are monitored and controlled by flow sensors, load sensors, and automatic valves. Regular and rapid analysis of refining products are carried out by X-ray fluorescence and inductively coupled plasma opticaleemission spectroscopy analysis, and refined gold and silver are analyzed by advanced techniques such as glow-discharge mass spectroscopy. To reduce manual labor requirements and to promote process safety and reliability, some refiners have introduced significant automation of the production and handling of anodes, the granulation and atomization of feed material for dissolution, the harvesting of electrorefined gold and silver, and the mixing and filtering of solutions. One of the more interesting recent process developments is the “acid-less” process developed in Russia (Martini, 2015). This involves the melting of the gold alloy or doré and then applying a vacuum to distill off the volatile base metals, such as zinc, silver, and lead. The volatilized metals are captured on a chilled condensation plate placed ahead of the vacuum pump. By appropriate selection of temperatures and vacuum conditions, the various metals can be separately distilled from the melt. The product is an ingot that contains largely gold and copper and that can then be further refined by electrolysis. This is essentially a prerefining process and can be viewed as a potential replacement for the Miller process. Although the process does not eliminate the use of acid, its application results in the use of less acid during refining. The process has appeal because it eliminates the use of chlorine and the generation of large quantities of byproducts containing small amounts of precious metals. Further developments are eagerly awaited. For the moment, applications of the two core refining schemes (prerefining/electrorefining and dissolution/ precipitation) are well established and will endure. Incremental improvements in doré refining will continue, particularly in the implementation of process controls and automation to reduce labor requirements and process variability. When dealing with a material as variable in composition as mining doré, a certain degree of process robustness and flexibility, along with a good deal of process knowledge and judgment, will continue to serve operators of gold and silver refineries well. ACKNOWLEDGMENT The authors would like to thank their good friend, Dr. Kathy Sole, for her input and editing of the final document. 614 PART | II Unit Operations REFERENCES Barnes, E.J., Edwards, J.D., 1982. Solvent extraction at Inco’s Acton precious metal refinery. Chem. Ind. (London) 5, 151e164. Barker, I.L., 1967. Slimes treatment and silver refining operations by the Cerro De Pasco Corporation at La Oroya, Peru. In: Butts, A., Coxe, C.D. (Eds.), Silver: Economics, Metallurgy and Use. Van Nostrand Company, Inc., Princeton, NJ, pp. 95e103. Bower, W., Bazhko, O., 2015. A reflection on the MinataurÔ gold refining technology e past, present and future. In: World Gold 2015. South African Institute of Mining and Metallurgy, Johannesburg, pp. 77e88. Crundwell, F.K., Moats, M.S., Ramachandran, V., Robinson, T.G., Davenport, W.G., 2011. Extractive Metallurgy of Nickel, Cobalt and the Platinum- Group Metals. Elsevier. Dean, J. (Ed.), 1985. Lange’s Handbook of Chemistry, thirteenth ed. McGraw Hill. Feather, A., Sole, K.C., Bryson, L.J., 1997. Gold refining by solvent extraction e the Minataur process. J. South. Afr. Inst. Min. Metall. 97, 169e173. Fisher, K.G., 1987. Refining of gold at the Rand Refinery. In: Stanley, G.G. (Ed.), The Extractive Metallurgy of Gold in South Africa, vol. 2. 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Yang, C.L., Chen, L., 2000. Oxidation of nitric acid in a two-stage chemical scrubber using DC corona discharge. J. Hazard. Mater. B80, 135e146. Michael B. Mooiman, MBA, PhD, has master’s degrees in chemistry (University of the Witwatersrand, South Africa) and business (Northeastern University, USA) and a PhD in metallurgical engineering (University of Utah). He has spent most of his career in the precious metals business, starting as a gold miner and doing research at Mintek in South Africa. He has served as vice president of production and general manager for Metalor Technologies USA, one of the largest precious metals com- panies and refiners in the United States, and has published over 25 articles in the field  Refining of Gold- and Silver-Bearing Doré Chapter | 34 615 of precious metals. He is also the holder of two U.S. patents for precious metals recovery. Mike is the president of Argo Advisors International, a consultancy and engineering company that specializes in the precious metals industry, and has worked and consulted for major precious metals operations in the United States, Europe, Can- ada, and Africa. He is an associate professor in the MBA program at Franklin Pierce University, New Hampshire, where he coordinates the Energy and Sustainability Pro- gram. During the summer of 2009, he was a Visiting Scholar at the Federal Reserve Bank in Boston, and in 2015e2016 he was a Fulbright Scholar at the University of Botswana. Leo Simpson has a master’s diploma in chemical engineering. He has spent most of his professional career in the precious metals refining and chemical industries, starting as a research technologist with Mintek and DeBeers Diamond Research Laboratories in South Africa. After moving to the United States in 1996, he spent 12 years at Metalor Technologies USA, initially as a process engineer and then as the engineering manager. He then spent a 7-year period as the director of manufacturing at Technic Inc., a precious metals chemical manufacturing company located in Rhode Island, USA. In 2013, he joined Elemetal Refining, a large gold and silver refinery in Ohio, USA, where he currently serves as the director of engineering.