ARSENIC HYDROMETALLURGY; FUNDAMENTALS, TECHNOLOGY AND APPLICATIONS

Transcription

1 ARSENIC HYDROMETALLURGY; FUNDAMENTALS, TECHNOLOGY AND APPLICATIONS * C.G. Anderson 1, L.G. Twidwell 2, R.G. Robins 3, and K.D. Mills 1 1 Kroll Institute for Extractive Metallurgy Colorado School of Mines George S. Ansell Department of Metallurgical and Materials Engineering Golden, Colorado USA (*Corresponding author: cganders@mines.edu) 2 Montana Tech of the University of Montana Butte, Montana USA University of New South Wales Sydney, New South Wales, Australia ABSTRACT Arsenic is a metalloid which has attracted great environmental scrutiny in recent decades. Moreover, its control and deportment is becoming increasingly important in the metallurgical processing of modern complex ores and concentrates. As such the authors of this publication have a combined professional experience of well over 100 years in the study and hydrometallurgical treatment of arsenic. Hence, this paper will cover arsenic fundamentals along with a review of technology for removal, fixation and stabilization. Finally, an application of industrial Alkaline Sulfide Leaching (ASL) of a copper enargite concentrate will be elucidated. KEYWORDS Arsenic, hydrometallurgy, scorodite, ferrihydrite, alkaline sulfide leaching.

2 INTRODUCTION The name arsenic comes from the Latin arsenicum, Greek arsenikon, and yellow orpiment identified with arsenikos, meaning male, from the belief that metals were different sexes. Arabic Az-zernikh was the orpiment from Persian zerni-zar for gold. It is abbreviated as As and it is believed that Albert Magnus obtained arsenic as an element in 1250 A.D. In 1649 Shroeder published two methods of preparing the element (Haynes and Lide 2011). Sources of Arsenic Elemental arsenic occurs in two solid forms: yellow and gray or metallic. Several other allotropic forms of arsenic are reported in the literature. Arsenic is found in its native form, in the sulfides realgar, enargite, tennantite and orpiment, as arsenides and sulfarsenides of heavy metals, as the oxide, and as arsenates. Mispickel, arsenopyrite, (FeSAs) is the most common mineral, from which on heating the arsenic sublimes leaving ferrous sulfide. (Haynes and Lide 2011). Properties of Arsenic Arsenic has an atomic number of 33 on the periodic table with an atomic weight of grams/mole. It can have a valence of -3, 0, +3, or +5. Yellow arsenic has a specific gravity of 1.97 while gray, or metallic, is Gray arsenic is the ordinary stable form. It has a triple point of 817 C, sublimes at 616 C and has a critical temperature of 1400 C. The element is a steel gray, very brittle, crystalline, semimetallic solid; it tarnishes in air, and when heated is rapidly oxidized to arsenous oxide (As 2 O 3 ) with the odor of garlic. Arsenic and its compounds are poisonous. Exposure to arsenic and its compounds should not exceed 0.01 mg/m 3 as elemental arsenic during an eight hour work day. Natural arsenic is made of one isotope 75 As. Thirty other radioactive isotopes and isomers are known (Haynes and Lide 2011). Applications of Arsenic Arsenic trioxide and arsenic metal have not been produced as primary mineral commodity forms in the United States since However, arsenic metal has been recycled from gallium-arsenide semiconductors. Owing to environmental concerns and a voluntary ban on the use of arsenic trioxide for the production of chromate copper arsenate wood preservatives at year end 2003, imports of arsenic trioxide averaged 6,100 tons annually during compared with imports of arsenic trioxide that averaged more than 20,000 tons annually during Ammunition used by the United States military was hardened by the addition of less than 1% arsenic metal, and the grids in lead-acid storage batteries were strengthened by the addition of arsenic metal. Arsenic metal was also used as an antifriction additive for bearings, to harden lead shot, and in clip-on wheel weights. Arsenic compounds were used in fertilizers, fireworks, herbicides, and insecticides. High-purity arsenic ( %) was used by the electronics industry for gallium-arsenide semiconductors that are used for solar cells, space research, and telecommunication. Arsenic was also used for germanium-arsenide-selenide specialty optical materials. Indiumgallium-arsenide was used for short-wave infrared technology. The value of arsenic compounds and metal consumed domestically in 2011 was estimated to be about $3 million (Brooks 2012). Arsenic is used in bronzing, pyrotechny, and for hardening and improving the sphericity of shot. The most important compounds are white arsenic (As 2 O 3 ), the sulfide, Paris green 3Cu(AsO 2 ) 2 Cu(C 2 H 3 O 2 ) 2, calcium arsenate, and lead arsenate. The last three have been used as agricultural insecticides and poisons. Marsh s test makes use of the formation and ready decomposition of arsine (AsH 3 ), which is used to detect low levels of arsenic, especially in cases of poisoning. Arsenic is available in high-purity form. It is finding increasing uses as a doping agent in solid-state devices such as transistors. Gallium arsenide is used as a laser material to convert electricity

3 directly into coherent light. Arsenic (99%) costs about $75 for 50 grams. Purified arsenic ( %) costs about $50 per gram (Haynes and Lide 2011). US EPA Position on Arsenic Arsenic occurs naturally throughout the environment but most exposures of arsenic to people are through food. Acute (short-term) high-level inhalation exposure to arsenic dust or fumes has resulted in gastrointestinal effects (nausea, diarrhea, abdominal pain); central and peripheral nervous system disorders have occurred in workers acutely exposed to inorganic arsenic. Chronic (long-term) inhalation exposure to inorganic arsenic in humans is associated with irritation of the skin and mucous membranes. Chronic oral exposure has resulted in gastrointestinal effects, anemia, peripheral neuropathy, skin lesions, hyperpigmentation, and liver or kidney damage in humans. Inorganic arsenic exposure in humans, by the inhalation route, has been shown to be strongly associated with lung cancer, while ingestion of inorganic arsenic in humans has been linked to a form of skin cancer and also to bladder, liver, and lung cancer. The US EPA has classified inorganic arsenic as a Group A, human carcinogen. Arsine, AsH 3, is a gas consisting of arsenic and hydrogen. It is extremely toxic to humans, with headaches, vomiting, and abdominal pains occurring within a few hours of exposure. The EPA has not classified arsine for carcinogenicity. As an example, the following figure shows regulatory values for inhalation exposure to arsenic (USEPA, 2012) Figure 1. Health Data from Inhalation Exposure (Inorganic Arsenic). ACGIH TLV American Conference of Governmental and Industrial Hygienists' threshold limit value expressed as a time-weighted average; the concentration of a substance to which most workers can be exposed without adverse effects. NIOSH IDLH-- National Institute of Occupational Safety and Health's immediately dangerous to life or health concentration; NIOSH recommended exposure limit to ensure that a worker can escape from an exposure condition that is likely to cause death or immediate or delayed permanent adverse health effects or prevent escape from the environment. NIOSH REL ceiling value--niosh's recommended exposure limit ceiling; the concentration that should not be exceeded at any time. OSHA PEL--Occupational Safety and Health Administration's permissible exposure limit expressed as a time-weighted average; the concentration of a substance to which most workers can be exposed without adverse effect averaged over a normal 8-h workday or a 40-h workweek

4 AQUEOUS ARSENIC REMOVAL PROCESSES The removal of arsenic from process solutions and effluents has been practiced by the mineral industries for many years. Removal by existing hydrometallurgical techniques is adequate for present day product specifications but the stability of waste materials for long term disposal may not meet the regulatory requirements of the future. The aqueous inorganic chemistry of arsenic as it relates to the hydrometallurgica methods that have been applied commercially for arsenic removal, recovery, and disposal, as well as those techniques which have been used in the laboratory or otherwise suggested as a means of eliminating or recovering arsenic from solution. The various separation methods which are then referenced include: oxidation-reduction, adsorption, electrolysis, solvent extraction, ion exchange, membrane separation, precipitate flotation, ion flotation, and biological processes. The removal and disposal of arsenic from metallurgical process streamss will become a greater problem as minerals with much higher arsenic content are being processed in the future. It is mostly the arsenic sulfide minerals which cause impurity levels in hydrometallurgical processes. The main sulfide mineral to cause arsenic impurity problems in arsenopyrite, FeAsS, but in certain locations enargite, Cu 3 AsS 4, tennantite, Cu12As 1 4 S 13, cobaltite, CoAsS, rammelsbergite, NiAs 2, skutterudite, (Co, Ni, Fe)As 3, safflorite, (Co, Fe)As 2, ararammelsbergite, NiAs 2 2, and seligmannite, PbCuAsS 3, are the major source. After smelting of sulfides or in wholly hydrometallurgica treatment, arsenic appears in solution as either arsenic (iii) or arsenic (v) but occasionally as arsenic (-iii). Arsenic speciation in an uncomplexed solution is described most conveniently by means of the potential- ph diagram shown below. Figure 2. Eh-pH equilibrium diagram for the As-H 2 O system at 25 C and unit activity of all species (Robins 1988).

5 Oxidation-reduction reactions between arsenic (v) and arsenic (iii) is possible using sulfur dioxide or sulfite. On an industrial scale this process is used to precipitate arsenic trioxide from arsenic acid solutions as a commercial commodity. There appears to be little likelihood of applying more powerful reductants in hydrometallurgical processing due to the concern of producing arsine, AsH 3. Arsine gas is produced commercially, however, as an intermediate to pure arsenic metal for semiconductor use. Arsenate complexes are very similar to those of phosphate, and there is a fairly extensive literature on the metal phosphate complexes which has been reviewed by Robins, Twidwell and Dahnke. A model for ferric arsenate complexing has been proposed by Khoe and Robins which has significant effect on free energies of formation which have been used previously to describe the solubility of ferric arsenate (FeAsO 4 2H 2 O) a compound of low solubility which is used extensively for removing arsenate from hydrometallurgical process solutions (Robins 1988). Arsenic can be leached specifically from enargite using various methods such as alkaline sulfide leaching, acidic sulfate and chloride media, acidified ferric sulfate, and others. The alkaline sulfide process will be discussed later in this paper. Arsenic Fixation Processes Because arsenic is most hazardous when mobile, it must be fixed as a solid precipitate to get it in a stable form for long-term storage. Two preferred stable forms include ferrihydrite and scorodite which are discussed in the sections to follow. Ferrihydrite Ferrihydrite is a ferric oxyhydroxide precipitate that forms very small particles with a large surface area. In treating hydrometallurgical solutions and waste streams for the removal of arsenic, the use of coprecipitation with Fe (III) has been specified by the US EPA as the Best Demonstrated Available Technology (BDAT). This technology has been widely adopted over the last century, and developments have been well reviewed (L. G. Twidwell, Robins, and Hohn 2005). This technology has also been selected as one of the Best Available Technologies (BAT) for removing arsenic from drinking waters (L. Twidwell and McCloskey 2011). R.G. Robins was the first investigator to recognize and to alert the gold industry that arsenic storage as calcium arsenate was inappropriate. Twidwell & McCloskey have continued work until the present and a number of research summaries are available from the EPA Mine Waste Technology Program (MWTP), e.g. arsenic, arsenic & selenium cementation using elemental iron and catalyzed elemental iron, formation and stability of arsenatephosphate apatites, ferric and ferrous treatment of mine waters (Berkeley Pitlake and Acid Drainage mine water), ferrihydrite/arsenic co-precipitation and aluminummodified-ferrihydrite (AMF)/arsenic treatment of waste water and long-term storage, influence of anion species on ferrihydrite/arsenic co-precipitation and long-term storage, and ferrihydrite/amf/metals co-precipitation and longterm storage. Twidwell quoted two other authors; one says arsenical ferrihydrite can be considered stable provided that: the Fe/As molar ratio is greater than 3, the ph is slightly acidic, and it does not come into contact with reducing substances such as reactive sulfides or reducing conditions such as deep water, bacteria or algae. Another author says that there is no clear experimental evidence that either process is better for safe disposal of arsenic. Local storage conditions will greatly affect stability of arsenic product. Some factors influencing arsenic removal include initial arsenic concentration, valence state, Fe/As mole ratio, presence of associated solution ions, structural modifications to ferrihydrite, mode of precipitation (co-precipitation, post-precipitation, adsorption), ph, temperature and time. To form ferrihydrite different reagents can be used; usually ferric nitrate, ferric chloride, and ferric sulfate. The adsorption capacity is related to the method of preparation (L. G. Twidwell, Robins, and Hohn 2005). Important reviews detailing conditions for formation and the stability of ferrihydrite are presented by Schwertmann and Cornell, who have published a recipe book that presents details of how to prepare iron oxides in the laboratory, including ferrihydrite, hematite and goethite. Many of the experimental studies reported in the literature reference this publication (L. Twidwell and McCloskey 2011).

6 Two ferric precipitation arsenic removal technologies are presently practiced by industry: ambient temperature ferrihydrite/arsenic co-precipitation and elevated temperature precipitation of ferric arsenate. The ambient temperature technology is relatively simple and the presence of commonly associated metals such as copper, lead and zinc and gypsum have a stabilizing effect on the long term stability of the product. The disadvantages of the adsorption technology are the formation of voluminous waste material that is difficult to filter, the requirement that the arsenic be present in the fully oxidized state as arsenate, and the question as to long term stability of the product in the presence of reducing substances. The disadvantages of the ferric arsenate precipitation are that the treatment process is more capital intensive, the compound may dissolve incongruently if the ph is >4, and it may not be stable under reducing or anaerobic bacterial conditions (L. G. Twidwell, Robins, and Hohn 2005). Ferrihydrite is characterized by x-ray diffraction as having a two-line or six-line structure, which relates to the number of broad peaks present. Two-line ferrihydrite is formed by rapid hydrolysis to ph 7 ambient temperature. Six-line ferrihydrite is formed by rapid hydrolysis at elevated temperature and is generally more crystalline than two-line ferrihydrite (L. Twidwell and McCloskey 2011). However, Schwertmann and Cornell have demonstrated that either can be formed at ambient temperature by controlling the rate of hydrolysis (i.e., less crystalline two-line forms at rapid hydrolysis rates whereas, six-line forms if the precipitation is conducted at lower rates, and lepidocrocite forms if the rate of addition of sodium hydroxide is slow enough) (Schwertmann and Cornell 2012). The rate of transformation of ferrihydrite to hematite or goethite has been discussed in great detail by Cornell and Schwertmann in their book. The rate of transformation is a function of time, temperature and ph (e.g., conversion of two-line ferrihydrite to hematite at 25 C is half complete in 280 days at ph 4 but is completely converted at 100 C in four hours) (Cornell and Schwertmann 2003). It has been pointed out by many investigators that ferrihydrite converts rapidly and that the conversion results in a significant decrease in surface area. However, the ferrihydrite conversion rate may be mitigated (changed from days to perhaps years) by the presence of other species and solution conditions during precipitation and subsequent storage (L. Twidwell and McCloskey 2011). General factors that have been shown to decrease the rate of conversion of two-line ferrihydrite to more crystalline forms include: lower ph, lower temperatures, presence of silicate, aluminum, arsenic, manganese, metals, sulfate, and organics (L. Twidwell and McCloskey 2011; Cornell and Schwertmann 2003). Scorodite Scorodite, FeAsO 4 2H 2 O, is a naturally occurring mineral formed in oxidized zones of arsenic-bearing ore deposits. Its wide occurrence in comparison to other secondary arsenate minerals has led many to advocate it as an acceptable carrier for the immobilization of arsenic released during pyrometallurgical or hydrometallurgical processing of arsenic-containing ores and those of gold, copper, and uranium. The production of scorodite, especially from arsenic-rich and iron-deficient sulfate solutions offers a number of operational advantages such as high arsenic content, stoichiometric iron demand, and excellent dewatering characteristics. There are two process options of industrial relevance; the hydrothermal option that involves autoclave processing at elevated temperature ( 150 C) and pressure and the atmospheric process based on supersaturationcontrolled precipitation of scorodite at C. In addition to hydrothermal production of scorodite the work done by Demopoulos has determined that it is feasible to produce scorodite by step-wise lime neutralization at 90 C. The atmospheric scorodite possesses the same structural and solubility characteristics with the hydrothermally produced scorodite. Thermodynamic calculations determined that scorodite is stable in the presence of ferrihydrite under oxic conditions up to ph 6.75 at 22 C or higher ph at lower temperature and gypsum-saturated solutions (Demopoulos 2005). Crystalline scorodite has been prepared many ways. Dove and Rimstidt prepared scorodite by mixing ferric chloride and sodium arsenate solutions and equilibrating the resultant slurry for two weeks at ~100 C (Dove and Rimstidt 1985).

7 Stability of Arsenic-Bearing Residues A review of methods for the environmentally acceptable disposal of arsenic-bearing residues, such as those produced from hydrometallurgical operations, indicated that chemical precipitation as a metal arsenate offered the best solution, not only of precipitating arsenic from process liquors, but also of producing a residue sufficiently stable (giving <5 mg As/L in solution) for disposal. Since published thermodynamic data suggested that metal arsenates were not as stable as had previously been thought, the Noranda Research Centre undertook a comprehensive laboratory study of the stability of metal arsenates, such as might be precipitated from typical hydrometallurgical process solutions, as a function of time and ph. The results indicate that (i) the presence of excess ferric iron (Fe/As molar ratio >3) co-precipitated with ferric arsenate confers a high degree of stability to arsenical residue at ph 7, (ii) the presence of small quantities of base metals (Zn, Cu, Cd) in solution, in addition to excess ferric iron, at the time of precipitation confers stability on the residue in the ph range 4-10, and (iii) naturally-occurring crystalline ferric arsenate (scorodite) has a solubility some two orders of magnitude lower than the chemically-precipitated amorphous form (Harris and Monette 1988) FUNDAMENTAL INDUSTRIAL ALKALINE SULFIDE HYDROMETALLURGY Hydrometallurgical methods can be employed for treatment of gold, arsenic, antimony, tin and mercury containing materials, concentrates and ores as well as complex ones containing any number of metals. The alkaline sulfide leaching (i.e. ASL) system is one of these and is essentially a mixture of sodium sulfide and sodium hydroxide with other meta-stable alkaline species as required. This is a unique hydrometallurgical system as it is a very selective lixiviant for the distinct leaching of tin, gold, antimony, arsenic, tellurium, silver and mercury (Glazkov & Tseft, 1961; Gnatyshenko & Polyvyanni, 1961; Polyvyanni, 1963; Nadkarni, Kusik & Heissner, 1975; Nadkarni & Kusik, 1988; Anderson, 2000, 2001a, 2001b, 2002, 2003, 2007, 2012, 2014; Anderson, Dahlgren, Jeffrey & Stacey, 2004; Anderson, Dahlgren, Huang, Miranda, Stacey, Jeffrey & Chandra, 2005; Anderson & Twidwell, 2008a, 2008b, 2010). Worldwide, ASL technology has been employed industrially in the former CIS, China and the United States for the production of antimony (Anderson, Nordwick, & Krys,1992; Anderson & Krys, 1993; Nordwick & Anderson, 1993; Anderson, Nordwick & Krys, 1994;. Holmes, 1944; Holmes, 1944; van Stein, 1971; Kaloc, 1967). As an example, when the alkaline sulfide hydrometallurgical system is applied to an arsenic containing material like orpiment, As 2 S 3, a sulfide complexed species of sodium thioarsenite in solution is formed. This is illustrated as: Na 2 S + As 2 S 2NaAsS 3 (1) 2 NaAsS 2 + Na 2 S Na 3 AsS 3 (2) When applied to arsenic trioxide, As 2 O 3, sodium arsenite is also formed but the oxides generate hydroxide. The reaction is as follows: 1.5 H 2 O + 2 Na 2 S + ½ As 2 O NaAsS NaOH (3) NaAsS 2 + Na 2 S Na 3 AsS 3 (4) Dissolution of elemental sulfur in sodium hydroxide is also used as a lixiviant for alkaline sulfide leaching of arsenic. The combination of sodium hydroxide and elemental sulfur results in the formation of species other than just sulfide (S -2 ). Both sodium polysulfide (Na 2 S X ) and sodium thiosulfate (Na 2 S 2 O 3 ) are created along with sulfide. Figure 3 illustrates the equilibrium diagram for sulfur while Figure 4 illustrates the meta-stable sulfur diagram more commonly encountered and utilized in ASL industry applications.

8 Figure 3 - Equilibrium potential/ph (Eh-pH) diagram for sulfur. (Pourbaix, 1966) Figure 4 - Meta-stable potential/ph (Eh-pH) diagram for sulfur. (Huang, 2007) The generation scenario. of these predominant meta-stable species is illustrated simplistically in the following 4S o + 6NaOH 2Na 2 S + Na 2 S 2 O 3 + 3H 2 2 O (X-1)S o + Na S Na 2 2 S X (where X= 2 to 5) (5) (6) Due to the oxidizing power of polysulfide on sodium thioarsenite, the major species in solution is normally sodium thioarsenate (Na AsS 3 ). This can be viewed as follows: 4 Na 2 S + (X-1)Na X 3 AsS 3 (X-1)Na 3 AsS + 4 Na 2 S (7) Fundamental information and data in this technology is very limited or it is restricted. Due to space constraintss in this paper and the relative image clarity available for inclusion in this publication, the few other available illustrative Pourbaix Eh ph diagrams in the alkaline sulfide system may be found in the referenced

9 literature as noted. (Tian cong, 1988; Anderson, 2003; Anderson et al, 2005; Young & Robins, 2000; and Robins, 2000) The new proposed proprietary flowsheet, based on proven ASL industrial unit operations and practice is shown in Figure 5. Figure 5 - Proposed modern ASL flowsheet for selective removal of arsenic with direct production of high purity antimony oxide and gold from enargite. In practice, the antimony, arsenic and some gold and silver are leached in the ASL process. Then, liquid solid separations are carried out using both thickeners and media filters. The gold in solution is recovered to a final activated carbon product amenable to a common carbon ADR circuit. The antimony is then processed to an oxide of superior quality. This was done using industrial hydrometallurgical oxidation followed by purification. Finally, the arsenic in solution is precipitated with iron salts to produce a stabilized solid. All of these unit operations for Au, Sb and As separation and treatment have been carried out successfully in industrial operations. As shown in chemical reaction 8 below, the arsenic from enargite selectively dissolves arsenic into solution as a soluble salt. Table 1. lists the composition of a commercially available enargite concentrate while Figure 6 is an automated mineralogy image of that material. This was concentrate was tested with industrial ASL technology and the laboratory results were optimized using Stat Ease software.

10 Table 1: Industrial Enargite Concentrate Elemental Composition and Mineralogy. Elemental Composition ICP & FA QEMSCAN Mineralogy Cu (%) Pyrite (%) 54.2 Fe (%) 28.3 Enargite (%) As (%) 5.89 Cu/As/Sb Group 4.78 Ag (g/tonne) 51.6 Covellite 4.18 Au (g/tonne) 1.93 Cu Clays 1.96 Sb (ppm) 678 Bournite 1.45 Figure 6. MLA analysis of major phases present in a particle of concentrate. 2Cu 3 AsS 4 (s) + 3Na 2 S (aq) 3Cu 2 S (s) + 2Na 3 AsS 4 (aq) (8) Based on the lab results, the Stat Ease optimized computer models produced the following equations of fit: Arsenic [As]= (time) (temperature) (solids) [OH - ] [S -2 ] (9) Antimony log([sb]) = (time) (temperature) (solids) [OH - ] [S -2 ] (10)

11 Gold [Au] = (time) (temperature) (solids) [OH - ] [S -2 ] (11) Silver [Ag] = (time) (temperature) (solids) [OH - ] [S -2 ] (12) Figure 7. is an example Stat Ease optimized response surface image generated in the study. Figure 7 Enargite concentrate arsenic content leaching response at optimal conditions with changes to the main contributing factors, sulfide concentration and temperature. The minimum arsenic content occurs at high temperature and high sulfide concentration. The optimized open cycle leach conditions were found to be a 2.0 hour leach time at 90 C with 57 g/l solids, 24.7 g/l OH - and 68.8 g/l S 2- concentrations. This resulted in about a 16% concentrate weight reduction with very selective and high removal of As and Sb along with some Ag and Au lixiviation. No copper or iron were leached. For reference, Table 2 lists the qualitative and quantitative analysis of the optimized ASL treated enargite concentratee with the appearance of a chalcocite phase and the disappearance of the enargite phase noted.

12 Table 2: Optimized ASL Leached Enargite Concentrate Elemental Composition and Mineralogy. Elemental Composition ICP & FA QEMSCAN Mineralogy Cu (%) 24.5 Pyrite (%) Fe (%) 33.7 Chalcocite (%) As (%) 0.23 Cu/As/Sb Group 4.98 Ag (g/tonne) 56.0 Covellite 4.02 Au (g/tonne) 1.58 Cu Clays 1.76 Sb (ppm) 23.3 Bournite 1.53 From the noted optimization responses, a preliminary process was designed. Economic analysis was performed based on plant design and operating conditions formulated in the design criteria. Capital expenses (CAPEX) and operating expenses (OPEX) were calculated based on industrial experience with the process. Further, the design was evaluated for a 15 year life on a before tax basis. The CAPEX calculation came to just under $49 million USD (±35%). At 400 dry tonnes per day of enargite concentrate processed, the operating expense comes out to approximately $32 million per year. The highest cost of operation is reagents which could be lowered with further optimization, recycle and the inclusion of reagent regeneration. For reference, a typical industrial copper smelter penalty listing is shown in Table 3 which was used in the economic evaluation. The final estimated economic value comes out to a savings of $97 million per year or $665 per tonne of concentrate processed. This takes into account the reduction in arsenic content, the reduction in enargite concentrate weight, the sale of an antimony by product, and the direct sale of the leached gold and silver loaded onto carbon. The concentrate weight reduction contribution of about 16 % after arsenic removal reduced costs in two ways; it saved in shipping costs to the smelter and increased the copper and other concentrations in the final solids (which gives more favorable prices from the smelter). These values result in a total savings of $1.23 per pound of copper. This is a significant savings for a product that currently sells now for about $3 per pound. Table 3. Typical Copper Smelter Penalty Element Costs. Penalty Element Limit/ DMT 1 Penalty Cost Arsenic (As) 0.10% $ 5.00/0.1 % ( up to 0.5 % As) $ 11.00/ 0.1 % ( >0.5 % As) Bismuth (Bi) 200 ppm $ 4.00/ 100 ppm (up to 1200 ppm Bi) $ 6.00/ 100 ppm ( >1200 ppm Bi) Selenium (Se) 0.05% $ 5.00/0.01 % Se Antimony (Sb) 0.10% $ 4.00/0.1 % Sb Cadmium (Cd) 200 ppm $ 4.00/100 ppm Cd Lead (Pb) 1% $2.75/0.5 % Pb 1/ Dry Metric Tons The yearly expenses and the yearly savings were used to determine various economic evaluations. Over an assumed 15 year project life, the project is expected to have a rate of return of 132%, an NPV 8 of $505 million, and a present value ratio (PVR) of Payback for this investment is just over 9 months. A sensitivity analysis was

13 performed for changes in NPV and IRR. Figures 8 and 9 show thatt the project is robust with up to 20% changes in the discount rate, OPEX and CAPEX. Figure 8. ASL Enargite Process NPV Sensitivity Analysis. Figure 9. ASL Enargite Process IRR Sensitivity Analysis. SUMMARY This paper has outlined the properties of arsenic. As well, the fundamentals behind arsenic hydrometallurgy and its aqueous precipitation and stabilization have been elucidated. Finally, an examplee of the successful

14 application of industrial alkaline sulfide leaching (ASL) technology to a commercially available copper enargite concentrate was provided. REFERENCES Anderson, C.G., Nordwick, S.M. & Krys, L.E., (1992). Processing of Antimony at the Sunshine Mine, Residues and Effluents - Processing and Environmental Considerations, ed. R.G. Reddy, W. P. Imrie, P.B. Queneau (San Diego, CA: AIME-TMS, Anderson, C.G. & Krys, L.E., (1993). Leaching of Antimony From a Refractory Precious Metals Concentrate, Proceedings Of The Fourth International Symposium On Hydrometallurgy, Utah Anderson, C.G., Nordwick, S.M. & Krys, L.E., (1994). Antimony Separation Process, U.S. Patent No. 5,290,338, March 1 Anderson, C.G. (2000). A Survey of Primary Antimony Production, Minor Elements 2000 Processing and Environmental Aspects of As, Sb, Se, Te and Bi, Courtney Young Ed., SME, Colorado, Anderson, C.G. (2001a). Industrial Nitrogen Species Catalyzed Pressure Leaching and Alkaline Sulfide Gold Recovery from Refractory Gold Concentrates, Precious Metals 2001, 25th Annual IPMI Meeting, Tucson, Arizona, June. Anderson, C.G. (2001b). Hydrometallurgical Treatment of Antimony-bearing Industrial Wastes, Journal of Metals, TMS, Volume 53, Number 1, Anderson, C.G. (2002). The Chemical Analysis of Industrial Alkaline Sulfide Hydrometallurgical Processes, Proceedings of The Society of Mineral Analysts and the Canadian Mineral Analysts Annual Meeting, Spokane, Washington, April. Anderson. C.G., (2003). The Industrial Alkaline Sulfide Hydrometallurgical Treatment of Mercury Bearing Antimony Ores and Concentrates, EPD Congress 2003, TMS Annual Meeting, San Diego, CA., February. Anderson, C.G, Dahlgren, E., Jeffrey M. & Stacey, D.L. (2004). Unpublished research Anderson C. G. (2005). The Treatment of Arsenic Bearing Ores, Concentrates and Materials with Alkaline Sulfide Hydrometallurgy, Arsenic Metallurgy, TMS, San Francisco California Anderson, C.G.; Dahlgren, E.; Huang, HH; Miranda, PJ; Stacey, D.; Jeffrey, M. & Chandra, I. (2005). Fundamentals and Applications of Alkaline Sulfide Leaching and Recovery of Gold, CIM Gold Symposium, Calgary, Alberta Anderson, C.G. (2007). Piloting of the Alkaline Sulfide Gold Leaching Process, Precious Metals 2007, SME & TMS.. Anderson, C.G. & Twidwell, L.G. (2008a). Antimony, Arsenic, Gold, Mercury and Tin Separation, Recovery, and Fixation by Alkaline Sulfide Hydrometallurgy, Proceedings of Hydrometallurgy 2008, Sixth International Symposium, SME, Phoenix Arizona, Littleton, Colorado, Anderson, C.G. & Twidwell, L.G. (2008b). Hydrometallurgical Processing of Gold Bearing Copper Enargite Concentrates, Canadian Metallurgical Quarterly John Dutrizac Hydrometallurgy Symposium Special Issue, Volume 47, Number 3, Anderson, C.G. & Twidwell, L.G., (2010). Hydrometallurgical Treatment For Separation, Concentration, Recovery and Fixation of Rare Metals From Lead and Zinc Materials, Pb-Zn 2010 CIM, Vancouver October. Anderson, C.G. (2012). The Metallurgy of Antimony, Chemie der Erde-Geochemistry, Volume 72, March.

15 Anderson, C.G., (2014). Alkaline Sulfide Leaching Technology; Just The Facts, Proceedings of Hydrometallurgy 2014, Seventh International Symposium, CIM, Victoria, British, Columbia, Canada. Brooks, W.E.( 2012). Mineral Commodity Summary: Arsenic. U.S. Geological Survey. Conner, K. & Anderson, C.G., (2013)., Enargite Treatments and Pressure Oxidation of Concentrates, Journal of Metallurgical Engineering Cornell, R.M., and Udo Schwertmann. (2003). The Iron Oxides: Structure, Properties, Reactions, Occurrences, and Uses. Wiley-VCH. Demopoulos, G.P. (2005). On the Preparation and Stability of Scorodite. In Arsenic Metallurgy, San Francisco, CA: TMS. Dove, Patricia Martin, and J. Donald Rimstidt. (1985). The Solubility and Stability of Scorodite, FeAsO 4.2H 2 O. American Mineralogist 70 (7-8) (August 1): Edwards, C.R., (1985). Engineering the Equity Concentrate Leach Process, In: Complex Sulfides: Processing of Ores, Concentrates and By-Products, A.D Zunkel, et al, eds. Proceedings Metallurgical Society of AIME and the CIM, TMS-AIME Fall Extractive Meeting, San Diego California, Nov , Glazkov H. & Tseft, A.L. (1961). Solubility of Sulfide Minerals of Antimony and Arsenic in Metal Salts at Elevated Pressure, Sb. Nauch Tr. Irkutsk. Gos. Nauch-Issled. lnst._redk. Metl, 9, 2Q4-B. Gnatyshenko, G.F. & Polyvyanni,T.R. (1961). Arsenic Leaching from Cottrell Dusts by Sodium Sulfide, lav. Ahad. Naub. Kaz. SSR. Ser. Met. Qboaashch. Ogneuporov.. 3, Harris, G.B., and S. Monette. (1988). The Stability of Arsenic-Bearing Residues. In Arsenic Metallurgy Fundamentals and Applications, Phoenix, AZ: TMS. Haynes, W.M., and David R. Lide, ed. (2011). CRC Handbook of Chemistry and Physics. 92nd ed. Boca Raton, FL: CRC Press, Taylor and Francis Group. Huang, H.H. (2007). STABCAL, Metallurgical and Materials Engineering Department, Montana Tech, Butte, Montana Holmes, W.C. (1944). How Electrolytic Antimony is Made at the Sunshine Plant, Eng. Mining J. 145 (3), Holmes, W.C. (1944). Electrolytic Recovery of Antimony From Tetrahedrite Concentrates", U. S. Patent. No. 2,331,375, October 12 Jeffrey, M. & Anderson, C.G. (2002). A Fundamental Study of the Alkaline Sulfide Leaching of Gold, The European Journal of Mineral Processing and Environmental Protection, October. Kaloc. J. (1967). Alkaline Leaching of Tetrahedrite Concentrates, Hutn. Listy. Z2 (9) Lunt, R.R., Modrow D.K. & Roset, G.K. (2003). Adaption of Dilute Mold Lime Dual Alkali Scrubbing at Stillwater Mining Company s PGM Smelter, Hydrometallurgy 2003, Vancouver, B.C. October. Nadkarni, R.M., Kusik & Heissner, H.P. (1975). Method of Removing Arsenic and Antimony From Copper Ore Concentrates, U. S. Patent 3, 911,073, Oct. 7. Nadkarni, R.M., & Kusik, C.L., (1988). Hydrometallurgical Removal of Arsenic from Copper Concentrates, Arsenic Metallurgy Fundamentals and Applications, Edited by R.G. Reddy, J.L. Hendrix and P.B. Queneau, Phoenix, Arizona. Nordwick, S.M. & Anderson, C.G., (1993). Advances In Antimony Electrowinning at the Sunshine Mine, Proceedings of the Fourth International Symposium on Hydrometallurgy, Salt Lake City, Utah.

16 Polyvyanni, I.R.,et al., (1963). Leaching arsenic from arsenic-containing dust by means of sodium sulfide, Tr. Jlnst. Met- Oboeashck. Akad. Mauk Kaz. SSR, Pourbaix, M. (1966). Atlas of Electrochemical Equilibrium, Pergamon Press, p 551. Robins, R.G. (1988). Arsenic Hydrometallurgy. In Arsenic Metallurgy Fundamentals and Applications, Phoenix, AZ: TMS. Robins, R.G. (2000). Computer- Generated Stability Diagrams for Sulfur Minor Element Water Systems: The Sb-S- H 2 O System, Minor Elements 2000 Processing and Environmental Aspects of As, Sb, Se, Te and Bi, Courtney Young Ed., SME, Littleton, Colorado, Schwertmann, U., and R. M. Cornell. (2012). Frontmatter. In Iron Oxides in the Laboratory, i xviii. Wiley VCH Verlag GmbH. Accessed March Tian-cong, Z., (1988). The Metallurgy of Antimony, Central South University of Technology Press, Changsa, the People s Republic of China. Twidwell, L.G., R.G. Robins, and J.W. Hohn. (2005). The Removal of Arsenic from Aqueous Solution by Coprecipitation with Iron (III). In Arsenic Metallurgy, San Francisco, CA: TMS. Twidwell, L.G., and J.W. McCloskey. (2011). Removing Arsenic from Aqueous Solution and Long-term Product Storage. JOM Journal of the Minerals, Metals and Materials Society 63 (8): US EPA, (2012). Arsenic Compounds, Technology Transfer Network Air Toxics Web Site van Stein, P.U. (1971). Selective Separation of Antimony from Concentrates, German Offen. No. 2,020,656 February 25 Young, C.A. & Robins, R.G. (2000). The Solubility of As 2 S 3 in Relation to the Precipitation of Arsenic from Process Solutions, Minor Elements 2000 Processing and Environmental Aspects of As, Sb, Se, Te and Bi, Courtney Young Ed., SME, Littleton, Colorado,